Molecular Regulation of Parturition: The Role of the Decidual Clock Errol R. Norwitz 1,2 , Elizabeth A. Bonney 3 , Victoria V. Snegovskikh 4 , Michelle A. Williams 5 , Mark Phillippe 6 , Joong Shin Park 7 , and Vikki M. Abrahams 8 1 Department of Obstetrics and Gynecology, Tufts Medical Center, Boston, Massachusetts 02111 2 Mother Infant Research Institute (MIRI), Tufts University School of Medicine, Boston, Massachusetts 02110 3 Department of Obstetrics, Gynecology and Reproductive Sciences, University of Vermont, Burlington, Vermont 05405 4 Division of Reproductive Endocrinology, Department of Obstetrics and Gynecology, Women and Infants Hospital of Rhode Island, Providence, Rhode Island 02905 5 Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts 02115 6 Vincent Center for Reproductive Biology, Department of Obstetrics and Gynecology, Massachusetts General Hospital, Boston, Massachusetts 02114 7 Department of Obstetrics and Gynecology, Seoul National University College of Medicine, Seoul 110-799, Korea 8 Department of Obstetrics, Gynecology and Reproductive Sciences, Yale University School of Medicine, New Haven, Connecticut 06510 Correspondence: [email protected]The timing of birth is acritical determinant of perinatal outcome. Despite intensive research, the molecular mechanisms responsible for the onset of labor both at term and preterm remain unclear. It is likely that a “parturition cascade” exists that triggers labor at term, that preterm labor results from mechanisms that either prematurely stimulate or short-circuit this cascade, and that these mechanisms involve the activation of proinflammatory pathways within the uterus. It has long been postulated that the fetoplacental unit is in control of the timing of birth through a “placental clock.” We suggest that it is not a placental clock that regulatesthe timing of birth, but rather a “decidual clock.” Here, we review the evidence in support of the endometrium/decidua as the organ primarily responsible for the timing of birth and discuss the molecular mechanisms that prime this decidual clock. T he timely onset of labor and birth is a critical determinant of perinatal outcome. Both preterm birth (defined as delivery before 37– 0/7 wk of gestation) and postterm pregnancy (failure to deliver before 42–0/7 wk) are asso- ciated with an increased risk of adverse preg- nancy events. Despite intensive research, the molecular mechanisms responsible for the on- set of labor both at term and preterm remain enigmatic. This is due primarily to the lack of an adequate animal model and to the autocrine/ paracrine nature of human parturition, which Editors: Diana W. Bianchi and Errol R. Norwitz Additional Perspectives on Molecular Approaches to Reproductive and Newborn Medicine available at www.perspectivesinmedicine.org Copyright # 2015 Cold Spring Harbor Laboratory Press; all rights reserved Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 1 www.perspectivesinmedicine.org Press on November 30, 2020 - Published by Cold Spring Harbor Laboratory http://perspectivesinmedicine.cshlp.org/ Downloaded from
Transcript
Molecular Regulation of Parturition: The Roleof the Decidual Clock
Errol R. Norwitz1,2, Elizabeth A. Bonney3, Victoria V. Snegovskikh4, Michelle A. Williams5,Mark Phillippe6, Joong Shin Park7, and Vikki M. Abrahams8
1Department of Obstetrics and Gynecology, Tufts Medical Center, Boston, Massachusetts 021112Mother Infant Research Institute (MIRI), Tufts University School of Medicine, Boston, Massachusetts 021103Department of Obstetrics, Gynecology and Reproductive Sciences, University of Vermont,Burlington, Vermont 05405
4Division of Reproductive Endocrinology, Department of Obstetrics and Gynecology, Women and InfantsHospital of Rhode Island, Providence, Rhode Island 02905
5Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts 021156Vincent Center for Reproductive Biology, Department of Obstetrics and Gynecology, Massachusetts GeneralHospital, Boston, Massachusetts 02114
7Department of Obstetrics and Gynecology, Seoul National University College of Medicine, Seoul110-799, Korea
8Department of Obstetrics, Gynecology and Reproductive Sciences, Yale University School of Medicine,New Haven, Connecticut 06510
The timing of birth is a critical determinant of perinatal outcome. Despite intensive research,the molecular mechanisms responsible for the onset of labor both at term and preterm remainunclear. It is likely that a “parturition cascade” exists that triggers labor at term, that pretermlabor results from mechanisms that either prematurely stimulate or short-circuit this cascade,and that these mechanisms involve the activation of proinflammatory pathways within theuterus. It has long been postulated that the fetoplacental unit is in control of the timing of birththrough a “placental clock.” We suggest that it is not a placental clock that regulates thetiming of birth, but rather a “decidual clock.” Here, we review the evidence in support of theendometrium/decidua as the organ primarily responsible for the timing of birth and discussthe molecular mechanisms that prime this decidual clock.
The timely onset of labor and birth is a criticaldeterminant of perinatal outcome. Both
preterm birth (defined as delivery before 37–0/7 wk of gestation) and postterm pregnancy(failure to deliver before 42–0/7 wk) are asso-ciated with an increased risk of adverse preg-
nancy events. Despite intensive research, themolecular mechanisms responsible for the on-set of labor both at term and preterm remainenigmatic. This is due primarily to the lack of anadequate animal model and to the autocrine/paracrine nature of human parturition, which
Editors: Diana W. Bianchi and Errol R. Norwitz
Additional Perspectives on Molecular Approaches to Reproductive and Newborn Medicine available
at www.perspectivesinmedicine.org
Copyright # 2015 Cold Spring Harbor Laboratory Press; all rights reserved
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
1
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
precludes direct investigation. That said, a num-ber of central themes have become clear over thepast few years: (1) That a “parturition cascade”exists that triggers spontaneous labor at term;(2) that preterm labor results from mechanismsthat either prematurely stimulate or short-cir-cuit this cascade; and (3) that these mechanismsinvolve the activation of proinflammatory path-ways within the uterus triggered by such eventsas intrauterine infection, hemorrhage, excessiveuterine stretch, and/or maternal or fetal stress(Norwitz et al. 1999, 2014; Challis et al. 2000;Lockwood and Kuczynski 2001; Gargano et al.2010; Muglia and Katz 2010; Esplin 2014; Ro-mero et al. 2014).
It has long been postulated that the fetus—or more correctly the fetoplacental unit—is incontrol of the timing of birth through a “pla-cental clock” (McLean et al. 1995; Sandman etal. 2006). However, the inner workings of thisputative placental clock have not been elucidat-ed despite many years of investigation. We positthat this is because investigators have been look-ing in the wrong place. It is not a placental clock;it is a “decidual clock.” Here, we review the ev-idence in support of the endometrium/deciduaas the organ primarily responsible for the tim-ing of birth and discuss the molecular, cellular,and immunological mechanisms that prime orset this decidual clock.
WHY DOES THE HUMAN UTERUS ONLYSUPPORT A PREGNANCY FOR NINEMONTHS?
The human uterus exists mostly in the nonpreg-nant state. The normal phenotype of the myo-metrium is contractile. It is responsible eachmonth for actively expelling the endometriallining and compressing the penetrating (radial)arteries so as to minimize menstrual blood loss.During pregnancy, this contractile phenotypehas to be actively suppressed to allow the uterusto expand to 500-fold of its nonpregnant size.It is now well accepted that the myometrial ac-tivity that characterizes labor at term resultsprimarily from withdrawal of mechanisms re-sponsible for maintaining uterine quiescence(such as progesterone), with a smaller contribu-
tion from factors that actively promote uterinecontractility (such as oxytocin) (Norwitz et al.2014). We suggest that this same paradigm istrue also of the endometrium/decidua.
The human endometrium also exists mainlyin the nonpregnant state, during which time itcommunicates directly with the outside envi-ronment. Despite the presence of protectivebarriers (the cervix with its protective mucuscoat and the vagina with its acidic milieu andactive mucosal immunity), the endometrium isexposed constantly to external stimuli, includ-ing sperm, infectious organisms, commensalbacteria, and environmental toxins. These stim-uli have the ability to induce a proinflammatoryresponse within the tissues of the endometrium.Indeed, a robust proinflammatory reaction atthe site of implantation appears to be necessaryfor successful trophoblast invasion and placen-tation (Norwitz et al. 2001; Dekel et al. 2014).How is it then that microorganisms can coexistwithin the endometrium throughout gestationand within the maternal basal plate of the pla-centa in an apparent symbiotic relationship(Stout et al. 2013)? How is it that the blastocystcan survive and thrive within this potentiallyhostile environment?
A numberof different theories exist as to howthese phenomena occur. One opinion suggeststhat the driving force behind the apparent lackofdecidual inflammation in response to intracel-lular organisms and the developing conceptus isactive suppression. The mechanisms responsiblefor this active suppression are not well under-stood, but hormonal factors are likely involved.Levels of progesterone—named because it isthe “progestational steroidal ketone” hormoneof pregnancy—increase 50- to 100-fold in thematernal circulation after ovulation and act onthe endometrium to facilitate the morphologi-cal changes referred to as “decidualization.”
The decidualized endometrium (decidua)can be seen as a specialized stromal tissue thatencapsulates and protects the fetoplacental unitthroughout pregnancy with intrinsic and spe-cific immunological and endocrine functions.It is comprised of endogenous decidual stromalcells as well as a variety of maternal immunecells, many of which track in from the bone
E.R. Norwitz et al.
2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
marrow and are present in concentrations thatvary in a predictable pattern throughout gesta-tion (Vince et al. 1990; Du and Taylor 2007).The function of these cells depends on theirhormonal milieu. Acting through an interleu-kin-8 (IL-8)-dependent mechanism, progester-one induces an influx of polymorphonuclearleukocytes into the endometrium beginningduring the secretory phase of the menstrual cy-cle. Estriol-17b may suppress the inflammatoryfunction of these cells (Abrahams et al. 2003).At the end of the menstrual cycle when estriol-17b levels decrease, however, these cells induceinflammation through the release of reactiveoxygen species to contribute to the tissue break-down associated with menstruation (Evansand Salamonsen 2012). Similarly, endometrialmacrophages, dendritic cells, T regulatory cells(Tregs), and NK cells increase in numbers duringthe proliferative phase in preparation for im-plantation of the blastocyst (Shaw et al. 2011;Mor and Abrahams 2013; Saito et al. 2013). Inthe nonpregnant state, high levels of innate im-munity also exist within the endometrium in theform of secreted antimicrobials and IgA fromthe mucosal surface (Hickey et al. 2011; Wiraet al. 2011). This mucosal immune functionis maintained in the decidua throughout preg-nancy (Gurevich et al. 2003; King et al. 2007).Other factors, such as seminal fluid, also appearto play a role in priming the endometrial epithe-lium, muting the inflammatory reaction, andrecruiting the necessary uterine immune cells tofacilitate reproductive success (Schjenken andRobertson 2014).
An alternative point of view—referred to asthe Matzinger “danger theory” (for review, seePradeu and Cooper 2012)—suggests that intra-cellular organisms (including viruses and bac-teria) do not incite an inflammatory responsewithin the endometrium/decidua because theydo not interfere with the normal functioning ofthe cells in which they live. Similarly, accordingto the danger theory, the conceptus does notincite an inflammatory response because its de-velopment and metabolism do not induce dam-age or stress signals; that is, the presence of theconceptus does not pose a threat to the sur-rounding maternal tissues. In this model, pro-
gesterone is the primarily hormonal mediatorand its dysregulation activates intermediate sig-naling pathways leading to decidual inflamma-tion. No matter the point of view, the criticaland novel hypothesis presented in this mono-graph is that the factors responsible for regulat-ing (dampening) the immune response withinthe endometrium/decidua (and myometrium)are put in place early in pregnancy and have aset time limit. Stated differently, these mecha-nisms are strongest in early pregnancy and waneover time.
According to this developmental model,advancing gestational age is associated with awithdrawal of active suppression and/or an en-hanced ability to induce inflammatory sig-nals within the endometrium/decidua. This, inturn, promotes the production and release of avariety of biologically active inflammatory medi-ators (prostaglandins [PGs], cytokines, growthfactors, chemokines, and reactive oxygen spe-cies) at the maternal–fetal interface leading toregular phasic uterine contractions and cervicalchange, the condition we refer to clinically aslabor (Fig. 1). The timing of this withdrawalhas been closely honed over many millions ofyears of human evolution to optimize the onsetof labor and birth at 280 d (40 wk) of gestationdated from the first day of the last menstrualperiod to maximize offspring survival. If activeanti-inflammatory mechanisms are withdrawntoo soon or if the decidua is metabolicallydysregulated too early in gestation, spontaneouspreterm labor will ensue. Preterm labor mayalso result from active induction of decidualinflammation in the midtrimester of pregnancyleading to the release of these same proinflam-matory mediators. In such cases, however,the inciting event is likely to be known and maywell fall into one of the four well-characteriz-ed pathogenic processes: intrauterine infection(chorioamnionitis), placental abruption (whichrefers to a decidual hemorrhage), excessive uter-ine stretch (as seen in multifetal pregnanciesand pregnancies complicated by polyhydram-nios), and/or excessive maternal or fetal stress(likely mediated through premature activationof the fetal hypothalamic–pituitary–adrenalaxis) (Fig. 2) (Lockwood and Kuczynski 2001;
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 3
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Gargano et al. 2010; Muglia and Katz 2010; Es-plin 2014; Norwitz et al. 2014; Romero et al.2014). A central role for another pathogenic pro-cess has recently been proposed, namely oxida-tive stress (Menon et al. 2014).
A new hypothesis concerning activation ofthe decidual clock has to do with the role of cell-free fetal DNA (cffDNA), which is released intothe maternal circulation by placental apoptosis(Phillippe 2014). Circulating levels of cffDNApeak at the time of parturition, both at term(Lo et al. 1998; Ariga et al. 2001; Birch et al.2005; Mitsunaga et al. 2010) and preterm (Fa-rina et al. 2005; Jakobsen et al. 2012). Because itcontains hypomethylated CpG motifs, cffDNAhas the capacity to activate toll-like receptor-9(TLR9) at the maternal–fetal interface, leadingto the release of proinflammatory mediators(Scharfe-Nugent et al. 2012). Indeed, adminis-tration of TLR9 agonists can induce pretermlabor in mice, a mechanism that appears to beactively suppressed throughout pregnancy byIL-10 (Thaxton et al. 2009; Sun et al. 2013).
Whether this mechanism is active also in hu-mans remains to be confirmed.
An active role for the decidua in the molec-ular mechanisms responsible for parturition,both at term and preterm, is not an entirely novelconcept. As far back as the 1980s, Drs. Paul Mac-Donald and M. Lynette Casey proposed that“decidual activation” was a critical elementrequired for labor in humans (Casey andMacDonald 1988; MacDonald et al. 1991). Inan opinion piece published in 2001 in MedicalHypothesis, Dr. Neil Sebire coined the term“choriodecidual inflammatory syndrome” (Co-DIS) in an effort to “. . . provide a better under-standing of the underlying pathophysiology thatcurrently uses terminology which overempha-sizes the importance of overt intra-amnioticinfection as opposed to localized extra-amnioticinflammation which stimulates uterine evacua-tion” (Sebire 2001). We propose that “decidualreactivation” (or more correctly a withdrawal ofdecidual active suppression) is not simply a con-sequence of labor, but is the primary inciting
Upregulation of contraction associated proteins (CAPs) in the myometrium
Preterm labor(characterized by uterine contractions, cervical change, rupture of membranes)
Release of prostaglandins, cytokines, chemokines, reactive oxygenspecies from the decidua
Figure 2. Proposed molecular mechanisms underlying preterm labor. The five major molecular mechanismsthought to be responsible for preterm labor and birth are shown. All converge on a single common pathway(namely, decidual inflammation) with the release of biologically active inflammatory mediators (prostaglandins[PG], cytokines, growth factors, and chemokines) at the maternal–fetal interface leading to regular phasicuterine contractions and cervical change. CRH, corticotropin releasing hormone.
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 5
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
event in the parturition cascade, both at termand preterm.
Evidence in support of a decidual clock (andagainst a placental clock) is shown in Table 1.
The concept that the decidua is an immu-nologically distinct tissue has been discussedfor many years particularly in light of olderdata showing that skin grafts placed in the uter-us survive far longer than those on the skin(Dodd et al. 1980), a mechanism that is medi-ated in part through the action of progesterone(Hansen et al. 1986). However, the presence ofan endometrium/decidua is not an absoluterequirement for pregnancy, and extrauterineintra-abdominal ectopic pregnancies can go toterm resulting in a live birth. Interestingly, al-though ectopic pregnancies do not have a de-fined endometrium/decidua, there is an influxof unique populations of immune cells into thetissues surrounding the ectopic implantationsite (Shaw et al. 2011; Shaw and Horne 2012).
The decidua is a fascinating organ from animmunological point of view. It is the maternaltissue most intimately in contact with the feto-placental unit, with access to maternal blood onthe one side and to the MHC class I-expressingextravillous trophoblast (fetal) cells that anchorthe placenta to the underlying maternal deciduaon the other. Blood vessels exiting the maternalvasculature pass directly through the deciduaon their way to the intervillous spaces of theplacenta, thereby allowing interaction betweenblood components and resident decidual cells.Immune cells of several types exist within thedecidua (Mor and Abrahams 2013), includingnatural killer (NK) cells that produce cytokines;these play a critical role in vascular remodelingat the maternal–fetal interface (Zhang et al.2011).
Under normal circumstances, the hemi-allogeneic conceptus is not “rejected” by thematernal immune system. This phenomenonhas long been used as a model to understandimmune tolerance in other organ systems (Bill-ingham et al. 1953; Trowsdale and Betz 2006).Classically, immunologic “tolerance” of the fe-tus has been explained in a number of ways,including: (1) That the maternal immune sys-tem is inherently limited in its response during
pregnancy (i.e., that women are systemically im-munosuppressed during pregnancy); (2) thatfetal antigens are not “antigenic” and, as such,do not elicit an immune response; and (3) thatthere is an anatomic separation between themother and fetus such that their immune sys-tems do not interact in any way (Billingham etal. 1953). Recent evidence suggests that none ofthese assertions are true. Although the precisemechanisms responsible for this immunologictolerance of the fetus are not clear, it is likely thatseveral overlapping and redundant mechanismsmay be involved given how critical this process isfor the survival of our species.
Even before the arrival of the blastocyst, im-munological priming of the endometrium maybe critical for optimal implantation and placen-tation (Redman and Sargent 2010). Epidemio-logical studies have shown that women exposedto paternal antigens (i.e., sperm) from the samepartner and for an extended period of time be-fore conception (such as those who are multip-arous, do not use barrier contraception, andhave a long period of cohabitation or long in-ter-pregnancy interval before conception) haveimproved placentation and fewer adverse preg-nancy events (Dekker et al. 1998, Dekker andRobillard 2007). In light of these and other stud-ies in murine models, the prevailing theory isthat exposure to semen promotes a state of ac-tive immune tolerance to paternal allo-antigensthat facilitates maternal acceptance of the hemi-allogeneic conceptus at implantation, and thiseffect appears to be mediated through expan-sion of the Treg cell pool within the endometri-um (for review, see Jiang et al. 2014; Nancy andErlebacher 2014). Treg cells, formerly known assuppressor T-cells, are a heterogeneous subpo-pulation of T-cells that support inherent toler-ance of self-antigens (natural Tregs) and expandwhen challenged by bystander (anti-self ) anti-gens to facilitate immunity to those antigens.Both seminal plasma and sperm componentsof semen appear to elicit the expanded Tregcell response, likely by exposing the endometri-um to immune-modulating cytokines (such asTGFb) and male allo-antigens, respectively. In amurine model, uterine dendritic cells were ca-pable of cross-presenting seminal fluid antigens
E.R. Norwitz et al.
6 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Table 1. Evidence in support of a decidual clock for the timing of parturition
Evidence Key Points Key References
(1) The decidua is animmunologicalorgan distinct fromthe placenta andother maternaltissues
The decidua is the maternal tissue most intimately incontact with the feto-placental unit.
Immunological priming of the endometriumappears to be critical to prevent rejection of thehemi-allogeneic conceptus, and this effect ismediated through expansion of a specific subsetof T-cells (Treg cells) within the endometrium.
Once pregnancy is established, selective epigeneticsilencing of key T-cell-attracting inflammatorychemokine genes in decidual stromal cells appearsto be critical for maintaining pregnancy.
Redman et al. 2010;Nancy et al. 2012;Nancy andErlebacher, 2014
(2) Suppression ofdecidualprostaglandin (PG)synthesis is criticalfor pregnancysuccess
Endogenous levels of PGs in the decidua are 200-fold lower in pregnancy than in the endometriumat any stage of the menstrual cycle.
Failure to suppress PG production in theendometrium around the time of implantation isassociated with spontaneous abortion.
The administration of exogenous PGs, by any route,at any stage of gestation, and in all speciesexamined, has the ability to induce abortion.
Levels of PGs increase in maternal plasma, urine,and amniotic fluid before the onset of uterinecontractions, suggesting that this PG surge is thecause and not simply a consequence of labor.
Norwitz et al. 1992;Romero et al. 1996;Challis et al. 2002
(3) There is a geneticpredisposition forpreterm birth that iscarried primarily inthe maternal lineage
Familial clustering, racial disparities, the highincidence of recurrent preterm birth, and studiesin twins all suggest an important role for maternalgenetic factors in the timing of labor.
A few target maternal genes of interest have beenidentified.
Epigenetic and gene-environmental factors are likelyalso involved.
Clausson et al. 2000;Varner and Esplin,2005; Murray et al.2010
The final common pathway in the preterm birthcascade likely involves a dysregulation (de-repression) of decidual inflammation within theuterus.
A two-hit hypothesis has been proposed in which agenetic predisposition (the first hit) primes thedecidua for an exaggerated inflammatoryresponse to a given environmental stimulus (thesecond hit, often an ascending infectious insult).
Evidence suggests that the first hit may be anunderlying genetic predisposition (such as agenetic variant in a gene coding for a keyproinflammatory mediator), but may also be anearly infection (bacterial or viral) or anunderlying medical condition (PCOS,endometriosis, obesity).
Nordling, 1953;Cardenas et al.2011a; Iams et al.2011; Cha et al.2013
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 7
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
to activate both CD8þ and CD4þ T-cells, andmating events deficient in either sperm or sem-inal plasma resulted in diminished CD4þ,CD25þ, Foxp3þTreg cells at the time of implan-tation (Robertson et al. 2009; Robertson andMoldenhauer 2014). Components of seminalfluid are also needed to prime the endometriumfor the recruitment of immune (including mac-rophages and dendritic) cells to the implan-tation site (Schjenken and Robertson 2014).However, experiments involving incapacitationof the maternal systemic immune response tofetal and/or paternal antigen show that Tregcells do not provide a complete explanation offetal tolerance.
The suggestion that fetal antigens do notelicit a maternal immune response has long agobeen disproven. Indeed, immunization of themother against fetal antigens in animal modelsresults in a robust systemic immune response(Bonney 2001; Bonney and Onyekwuluje 2003).Moreover, evidence in humans shows that thematernal immune system can be primed to fetalantigens early in gestation and throughout thecourse of pregnancy to produce both cytotoxicT-cells (Bonney and Matzinger 1997; Lissaueret al. 2012) and antibodies (for review, see Par-ham et al. 2012).
Although fetal cells can and do leave theuterus and may thus be processed and presentedby distant maternal lymph nodes, the level ofsystemic immunity to fetal antigens appears tobe regulated primarily by the decidua. One ofthe major roles of the decidua, in this regard, isto limit the extent to which fetal antigens arepresented to T-cells within the draining lymphnodes (i.e., to limit the extent to which the fetusis seen as antigenic) (Erlebacher et al. 2007; Bi-zargity et al. 2009). Exactly how this occursis not clear. One suggestion is that the decidualimmune cells themselves are poor antigen pre-senting cells and that, as long as the conceptusis developing normally, resident decidual den-dritic (professional antigen presenting) cells donot become activated to pick up and processfetal antigen and move to the draining node toinitiate activation of naıve T-cells (Matzingerand Kamala 2011). An alternative explanationis that pregnancy itself decreases the ability of
resident dendritic cells within the decidua toleave the uterus to present antigen (Taglianiand Erlebacher 2011).
The decidua shows other immunoregulato-ry functions as well. For example, if a potentiallyharmful immune response occurs in the nodesdraining the uterus or in the systemic circula-tion, specific molecules expressed by decidualcells appear to inhibit the trafficking of effectorT-cells through this tissue thereby limiting theiraccess to the fetal cells of the placenta (Kruseet al. 1999a,b, 2002; Collins et al. 2009). Theunderlying mechanism appears to be a marked-ly reduced capacity of the CD45 – decidual stro-mal cells, the dominant cell population withinthe decidua, to produce T-cell chemoattractantsduring normal pregnancy (Nancy et al. 2012;Erlebacher 2013). It has long been knownthat T-cells are relatively scarce in the decidua(Bulmer et al. 2010), although the explanationhas thus far remained unclear. Recent studiessuggest that this phenomenon is due in partto selective epigenetic silencing of key T-cell-attracting inflammatory chemokine genes indecidual stromal cells (Nancy et al. 2012; Erle-bacher 2013). As pregnancy is established andendometrial stromal cells are transformed intodecidual stromal cells under the influence ofprogesterone, chromatin alterations occur asevidenced by an increase in levels of the repres-sive histone marker, H3 trimethyl lysine 27(H3K27me3), in the promoter regions of selectproinflammatory and chemoattractant genes.The end result is a suppression that is intrinsic,gene-specific, and independent of inflammato-ry stimuli (Nancy et al. 2012). This same mech-anism may explain the suppression in PG syn-thesis that occurs in the endometrium/deciduaaround the time of implantation (discussedbelow), suggesting that this effect targets notjust PGs but also many other proinflammatorymediators and chemokines.
A number of other factors also appear toaffect the trafficking of effector T-cells throughthe decidua, including cytokines (such asTGFb, IL-10, CCL2), hormones (progesterone,corticotropin-releasing hormone, PGs), Fas-Fasligand signaling, HLA-G, Programmed Death-1(PD-1), and other molecules that affect T-cell
E.R. Norwitz et al.
8 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
metabolism (such as tryptophan) (Bonney andMatzinger 1998; Kalantaridou et al. 2007; Xionget al. 2010; Silasi and Mor 2012; Shepard andBonney 2013; Nancy and Erlebacher 2014; Wuet al. 2014). Tryptophan is an essential aminorequired for T-cell activation. The enzyme, in-doleamine 2,3-dioxygenase (IDO), degradestryptophan through the kynurenine metabolicpathway. By producing IDO and catabolizingtryptophan at the site of implantation, the de-veloping mammalian conceptus fails to activatean anti-fetal immune response (Munn et al.1998). Two theories have been proposed toexplain how tryptophan catabolism facilitatesimmune tolerance. One theory posits that tryp-tophan breakdown suppresses T-cell prolif-eration by dramatically reducing the supply ofthis essential amino acid. The other postulatesthat the downstream metabolites from trypto-phan catabolism suppress immune cell func-tion, probably through proapoptotic mecha-nisms (Moffett and Aryan Namboodiri 2003).Either way, it is clear that the conceptus is notmerely a passenger in this process, but is activelyinvested in securing its own survival.
SUPPRESSION OF DECIDUALPROSTAGLANDIN SYNTHESISIS CRITICAL FOR PREGNANCYSUCCESS
Endogenous levels of PGs in the decidua are 200-fold lower in pregnancy than in the endometri-um at any stage of the menstrual cycle (Abel andKelley 1979; Norwitz et al. 1992; Norwitz andWilson 2000). This is true also of other uterinetissues. The prevailing evidence shows that, atleast as far as the decidua is concerned, this iscaused by a decrease in PG synthesis and not anincrease in PG catabolism (Norwitz et al. 1992).Moreover, concentrations of unesterified (free)arachidonic acid, the precursor of the primary(biologically active) PGs and the rate-limitingstep in the eicosanoid biosynthetic cascade, re-main high in the decidua throughout gestation.Furthermore, the administration of exogenousPGs, by any route, at any stage of gestation, andin all species examined, has the ability to induceabortion (Embrey 1971; Casey and MacDonald
1988; Gibb 1998). Taken together, these findingssupport the hypothesis that pregnancy is main-tained by a mechanism that tonically suppressesdecidual PG synthesis throughout gestation.Indeed, failure to suppress PG production inthe endometrium around the time of implanta-tion is associated with spontaneous abortion(Jaschevatzky et al. 1983). Interestingly, one ofthe theories as to how intrauterine contracep-tive devices (IUCDs) work is to recruit residentmacrophages and T-cells from the endometri-um, which then become activated and producePGs that create a hostile proinflammatory envi-ronment within the uterus for both sperm andconceptuses (Myatt et al. 1977).
Suppression of PG production withinthe endometrium/decidua persists throughoutmost of pregnancy, and—in keeping with ouroverall hypothesis—withdrawal of this suppres-sion appears to be a prerequisite for parturition.There is now overwhelming evidence that PGsare involved in the onset of labor, both at termand preterm (Norwitz et al. 1999; Challis et al.2000), which is probably common to all mam-malian viviparous species. For example, micelacking functional PGF2a receptors, cytosolicphospholipase A2 (PLA2), or PGH2 synthasetype 1 (PGHS-1, also known as cyclooxygen-ase-1 [COX-1]), all show a delay in the onsetof labor (Muglia 2000). In humans, exogenousPGs stimulate uterine contractility both in vitroand in vivo (Olson et al. 1995), and drugs thatblock PG synthesis can inhibit uterine contrac-tility and prolong gestation at least for severaldays (King et al. 2005). Levels of PGs increase inmaternal plasma, urine, and amniotic fluid be-fore the onset of uterine contractions (Keirseand Turnbull 1973; Fuchs 1995; Romero et al.1996), suggesting that this PG surge is the causeand not simply a consequence of labor.
All uterine tissues produce PGs and theirproduction is carefully compartmentalizedwithin the uterus. The fetal membranes pro-duce almost exclusively PGE2, the decidua syn-thesizes mainly PGF2a but also small amountsof PGE2 and PGD2, and the myometrium pro-duces primarily prostacyclin (PGI2). This is be-cause, although these compounds are structur-ally similar, they can have different and often
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 9
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
antagonistic biological actions. For example,PGF2a, thromboxane, and PGE1 promote my-ometrial contractility by increasing calcium in-flux into myometrial cells and enhancing gapjunction formation; whereas PGD2 and PGI2
have the opposite effects and inhibit contrac-tions (Fuchs 1995). This compartmentalizationof the primary PGs within the uterus has dis-tinct functional implications. PGE2 is primarilyof fetal origin. It is derived mainly from the fetalmembranes and is rapidly degraded by the en-zyme, PG dehydrogenase (PGDH), into its pri-mary metabolite, 13,14-dihydro-15-keto-PGE2
(PGEM), which has markedly reduced biologi-cal activity. PGDH is highly expressed in thechorion. In this way, the chorion serves as aprotective barrier, preventing the transfer of fe-toplacental PGE2 to the underlying myome-trium and thereby precluding myometrial con-tractions. At term, the changing hormonalmilieu (increase in estrogen and cortisol; de-crease in progesterone) leads to both an increasein COX expression and decrease in PGDH ac-tivity (Challis et al. 2002; Patel and Challis2002). The resultant surge in PGE2 within theamniotic cavity at the end of pregnancy pro-motes cervical effacement and weakening ofthe fetal membranes leading to membrane rup-ture. A similar surge in PGF2a occurs in thedecidua. This PGF2a then diffuses to the adja-cent myometrium where it stimulates the ex-pression of: (1) the contraction-associated pro-tein (CAP) genes, including ion channels thatpromote myocyte excitability; (2) receptors forthe uterotonic agonists oxytocin and the stim-ulatory PGs (PGE2 and PGF2a); and (3) gapjunction proteins (such as connexin 43) thatincrease the synchronization of contractions(Norwitz et al. 1999; Challis et al. 2000). Espe-cially critical for labor is the action of oxytocinon the uterus. Levels of oxytocin in the mater-nal circulation remain unchanged throughoutpregnancy and the first and second stages oflabor. The increased sensitivity of the uterus tocirculating levels of oxytocin that occurs at termis mediated instead by an increase in the num-ber of oxytocin receptors in the myometrium(Fuchs and Fuchs 1984; Fuchs 1995; Zeemanet al. 1997). Oxytocin cannot increase its own
receptor number. Rather, it acts on oxytocinreceptors in the decidua to stimulate PGF2a re-lease, and it is this decidual PGF2a that stimu-lates oxytocin receptor gene expression in themyometrium (Husslein et al. 1981; Fuchs et al.1982). Indeed, induction of labor at term issuccessful only when the oxytocin infusion isassociated with an increase in PGF2a produc-tion, despite seemingly adequate uterine con-tractions in both induction failures and success-es (Fuchs et al. 1984). Taken together, these datashow that suppression of PG synthesis and re-lease by the endometrium/decidua startingaround the time of implantation and persistingto term is critical for pregnancy success.
THERE IS A GENETIC PREDISPOSITIONFOR PRETERM BIRTH THAT ISCARRIED PRIMARILY IN THEMATERNAL LINEAGE
Familial clustering (Iams et al. 1998; Winkvistet al. 1998; Varner and Esplin 2005; Svenssonet al. 2009), racial disparities (Blackmore et al.1993; Carmichael et al. 1998; Ekwo and Moa-wad 1998; Blackmore-Prince et al. 1999; Ventu-ra and Bachrach 2000), the high incidence ofrecurrent preterm birth (Mercer et al. 1999; Var-ner and Esplin 2005), and studies in twins(Clausson et al. 2000) all suggest an importantrole for maternal genetic factors in the timing oflabor. Of these, the racial disparity in pretermbirth risk is perhaps best documented.
Black women in the United States (includ-ing African–American, African, and Caribbe-an–American) have a preterm birth rate that istwofold higher than that observed in Caucasians(Blackmore et al. 1993; Carmichael et al. 1998;Ekwo and Moawad 1998; Blackmore-Princeet al. 1999; Ventura and Bachrach 2000). Evenafter adjusting for potential confounding de-mographic and behavioral variables, the rate ofpremature deliveries in black women remainssignificantly higher than that for white women,and this is especially true for spontaneouspreterm birth at 20–28 wk in which the risk isincreased almost fivefold (Blackmore-Princeet al. 1999). Interestingly, one retrospective co-hort analysis of 21,005,786 singleton deliveries
E.R. Norwitz et al.
10 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
showed that interracial (black–white) coupleshave a risk of preterm birth that is significantlydifferent and intermediate between that ofwhite–white and black–black couples (Geta-hun et al. 2005). There is also a striking discrep-ancy between black and white populations interms of recurrence risk, especially of very earlypreterm births. For example, in black and whitewomen with a first delivery at 20–31 wk of ges-tation, the incidence of a second delivery at thesame gestational age range is 13.4% and 8.2%,respectively. Among black and white womenwho had a first delivery at 32–36 wk, the inci-dence of a second delivery at the same gestation-al age range was 3.8% and 1.9%, respectively(Iams et al. 1998; Mercer et al. 1999).
However, not all black women deliver pre-term. Although independently associated withpreterm delivery, race/ethnicity may not becausally related to preterm birth but rather amarker of such factors as social stress, environ-mental exposures, and/or medical conditionsthat disproportionately impact women of color(Blackmore et al. 1993; Wadhwa et al. 2001; Var-ner and Esplin 2005; Menon et al. 2006). Datashowing an independent association betweenrace/ethnicity and pregnancy outcome suggestthat genetic factors likely complicate the racialargument. Moreover, gene–environment inter-actions may be more important in determiningwhether or not the underlying genetic predis-position manifests in a clinical phenotype. Forexample, maternal carriers of the IL1RN�2 SNPin intron 2 of the IL-1 receptor antagonist(IL-1ra) gene show a blunted proinflammatoryIL-1b response to abnormal vaginal flora and alower rate of spontaneous preterm delivery (6%vs. 18%, P ¼ 0.02) (Genc et al. 2004a). This istrue also of the 896(A.G) SNP in the TLR4gene, where maternal carriers of this varianthave an increase in vaginal pH, a .10-fold in-crease in vaginal levels of Gardnerella vaginalisand anaerobic Gram-negative rods, and an al-teration in vaginal IL-1b and IL-1ra levels (Gencet al. 2004b). Similarly, maternal carriers of the–308(G . A) polymorphism in the promoterregion of the TNFa gene have an increased riskof spontaneous preterm birth (OR, 2.7; 95% CI,1.7–4.5) (Macones et al. 2004; Genc et al. 2007),
which was further increased in the presence ofbacterial vaginosis (BV) (OR, 6.1; 95% CI, 1.9–21.0) (Macones et al. 2004; Nguyen et al. 2004;Genc et al. 2007). Interestingly, women with thisTNFa gene promoter polymorphism who haveBV and who are black have an even further in-creased risk of spontaneous preterm birth (OR,17) (Nguyen et al. 2004), suggesting that suchfactors may be additive or synergistic in theireffect.
An understanding of these genetic factorsmay explain, at least in part, why antibioticsdo not prevent preterm birth in women withpreterm labor and intact membranes (Subra-maniam et al. 2012; Flenady et al. 2013). Lowergenital tract infections (Trichomonas vaginalis,BV) and periodontal disease are both associatedwith an increased risk of spontaneous pretermbirth, but treatment of these disorders does notabrogate this risk (Klebanoff et al. 2001; Ma-cones et al. 2010; Lamont et al. 2011; Boutinet al. 2013). This is because, although there isan association between such infections and pre-term labor, it is not a causal association. Theinfecting organisms do not travel from the lowergenital tract to the uterus and cause contrac-tions. Rather, it is the underlying DNA sequenc-es in these women, e.g., the specific variants orsingle nucleotide polymorphisms (SNPs) thatthey carry in their proinflammatory genes,which predisposes them to a number of separateinflammatory conditions, including periodon-tal disease, lower genital tract infections, andpreterm birth. The observation that coexistenceof both BVand periodontal disease do not havea synergistic effect on the risk of preterm birth(Harper et al. 2012) supports the conclusionthat these are independent adverse events thatare not causally related. Incidentally, broad-spectrum antibiotics may also adversely affectthe normal uterine microbiome, which is nec-essary for a healthy pregnancy (Aagaard et al.2014).
Taken together, these data suggest an impor-tant role for maternal genetic factors in the tim-ing of labor, both at term and preterm. Targetedand genome-wide association studies (Dolan etal. 2010; Plunkett et al. 2010, 2011; Kim et al.2013), as well as pathway-based genetic analyses
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 11
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
(Uzun et al. 2013), have been performed in ma-ternal–fetal dyads discordant for spontaneouspreterm birth in an effort to identify specificgenetic variants associated with prematurity.A number of candidate genes have been identi-fied. One promising candidate is a variant inPLA2G4C, which codes for a specific phospho-lipase A isoform involved in PG biosynthesis(Plunkett et al. 2010).
Epigenetic factors have also been implicat-ed. In one study, Brown and colleagues showedthat abnormal DNA methylation in the uterinedecidua (but not placenta) likely plays a role inthe CBA/J � DBA/2 murine model of preg-nancy failure (Brown et al. 2013). In addition,elegantly designed ovarian transplant experi-ments in 15 inbred strains of mice showed awide variation of gestation length between(but not within) strains. This variation was de-termined primarily by maternal genetic factorsand was independent of the genetic origins ofthe ovary (Murray et al. 2010), thus focusingattention again on the maternal tissues of theuterus.
DECIDUAL DYSREGULATIONPREDISPOSES TO PRETERM BIRTH:THE TWO-HIT HYPOTHESIS
Spontaneous preterm birth is a complex disease.It represents the final common pathway of mul-tiple genetic, environmental, and immunologi-cal factors as well as gene2environment inter-actions. Environmental factors that have beenimplicated include maternal cigarette smoking,obesity, lower genital tract infections, and expo-sure to environmental toxins (Esplin 2014; Ro-mero et al. 2014). The results of large epidemi-ological and genome-wide association studiesdesigned to identify the underlying causes ofspontaneous preterm birth have been largelydisappointing and have led to a renewed interestin identifying the interactions between geneticsusceptibility and environmental stimuli.
A murine model was recently developedwith a conditional uterine deletion of Trp53, agene encoding the tumor suppressor protein53 (Trp53loxP/loxPPgrCre/þ), which shows a50% incidence of spontaneous preterm birth
(Hirota et al. 2010; Cha et al. 2012). Interesting-ly, these preterm births occurred in the absenceof systemic progesterone withdrawal, whichhad previously been thought to be a prerequi-site for labor in this species. Further investiga-tion showed that these mice show “prematuredecidual senescence” characterized by increasedmTORC1 (mammalian target of rapamycincomplex 1) signaling early in pregnancy. Aspregnancy progresses, aberrantly high expres-sion of the PG synthetic enzymes, COX-2 andPGF synthase, lead to a premature increasein decidual PGF2a production and ultimatelyto preterm labor. mTORC1 is an intracellularprotein complex that functions as a redox sen-sor and controls protein synthesis. Interestingly,this preterm birth phenotype could be rescuedby either administration of an mTORC1 inhib-itor (rapamycin) or a selective COX-2 inhibitor(celecoxib) without any apparent adverse effectson the dams or fetuses (Hirota et al. 2010, 2011).Whether such a mechanism is active in humansis unclear, although preliminary evidence sug-gests that this pathway may well be up-regulatedin the decidua of women with spontaneous pre-term birth (including an increase in mTORC1signaling, COX-2 expression, and immunohis-tological staining for gH2AX, a marker of tissuesenescence and DNA damage) and can be in-duced in cultured human decidual stromalcells in vitro (Cha et al. 2013). Moreover, thismechanism is confined to the uterus and is in-dependent of circulating progesterone concen-trations, which is consistent with preterm birthin humans.
These same investigators then posed the fol-lowing question: Knowing that the geneticallyengineered Trp53loxP/loxPPgrCre/þmice manifestpremature decidual senescence with elevatedlevels of inflammatory mediators within the de-cidua leading to spontaneous preterm birth in50% of cases, what would happen if these micewere also exposed to an exogenous inflamma-tory stimulus? To answer this question, they in-jected timed pregnant Trp53loxP/loxPPgrCre/þ
mice intraperitoneally with a low dose (10 mg)of Toll-like receptor-4 (TLR4)-specific lipo-polysaccharide (LPS) on day 16 of pregnancyand showed preterm birth and/or stillbirth in
E.R. Norwitz et al.
12 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
100% of cases. In contrast, wild-type femalesshowed no effect when exposed to 10 mg LPSand required a dose at least fivefold higher toinduce preterm birth. Moreover, treatment ofthe Trp53loxP/loxPPgrCre/þ mice with a cocktailof anti-inflammatory agents (celecoxib, proges-terone, and rapamycin) was able to completelyrescue the preterm birth phenotype with no ap-parent effect on the viability or growth of theoffspring (Cha et al. 2013). These experimentsprovide an example of how gene–environmentinteractions may predispose to preterm birth.Although elegant, this model is not unique.Mice deficient for IL-10 are sensitized to low-dose bacterial- and LPS-induced preterm birth(Murphy et al. 2005; Robertson et al. 2006).Similarly, TLR4 appears to play an importantrole in LPS-induced preterm birth (Elovitzet al. 2003; Wang and Hirsch 2003) and block-ing TLR4 function prevents high-dose LPS-in-duced preterm uterine contractility in nonhu-man primates (Adams Waldorf et al. 2008). Inall these studies, a mild inflammatory stimulussuperimposed on an underlying genetic predis-position led to a dramatic clinical phenotype,which did not manifest in the absence of thegenetic predisposition. Sometimes referred toas the “two-hit hypothesis” (or “multiple-hithypothesis”), this concept was first put forwardby Carl Nordling in 1953, who suggested thatmultiple “hits” to DNA were necessary to causecancer (Nordling 1953).
Instead of a genetic predisposition, could aninfection in early pregnancy serve as the “firsthit” for preterm birth? The answer appears to beyes. Experiments in several animal model sys-tems suggest that infection may significantlydisrupt normal immune functioning withinthe endometrium/decidua. For example, infec-tion with the intracellular bacterium Listeriamonocytogenes blocks the pregnancy-inducedCD4þ Treg cell expansion and differentiationin the decidua that is required for pregnancysuccess (Xin et al. 2014). Viral infections canalso increase the presence of CD4þ and CD8þ
T-cells within the decidua (Constantin et al.2007). In another murine model, timed preg-nant C57B/6 mice infected with the murine g-herpes virus 68 (MHV-68) (which alone does
not induce inflammation or preterm birth) ap-pears to sensitize the mother to subsequentexposure to low dose LPS (which alone has amild or no effect on inflammation and pretermbirth), leading to increased inflammation at thematernal–fetal interface and preterm deliverywith fetal death in 100% of experimental ani-mals (Cardenas et al. 2010, 2011a). Similarly,intrauterine delivery of viral dsRNA along withbacterial peptidoglycan to pregnant mice am-plifies decidual inflammation, leading to pre-term delivery (Ilievski and Hirsch 2010).
Although there are reports of viral invasionof the amniotic cavity in a small proportion ofwomen in the midtrimester (Baschat et al. 2003;Gervasi et al. 2012), the causal link betweenviral infection and preterm birth in humans isnot yet well established. Nonetheless, the highrate of preterm birth among infected women inthe most recent H1N1 influenza pandemic isintriguing (Carlson et al. 2009; Uchide et al.2012). It has also been suggested that viral in-fection of the cervix during early pregnancymay reduce the capacity of the female reproduc-tive tract to prevent ascending bacterial infec-tion of the uterus later in gestation (Racicotet al. 2013).
The mechanism by which a subclinical viral(or bacterial) infection may prime the immunesystem leading to an exaggerated immunologi-cal response to a subsequent inflammatory in-sult is not well understood, but likely involvesinnate immune pattern recognition receptorsand their regulators (Koga et al. 2009; Cardenaset al. 2011b). Cells of the maternal–fetal inter-face (trophoblast, decidual, endothelial, and fe-tal membranes) recognize and respond to mi-crobes through TLRs and Nod-like receptors(NLRs) (Abrahams 2008, 2011; Hoang et al.2014). In recent years, much attention has fo-cused on the role of TLRs in infection-associat-ed preterm labor. TLR4 has gained particularattention, as it is the receptor responsible forrecognizing the major Gram-negative bacterialcomponent, LPS. However, activation of TLR2(which recognizes bacterial peptidoglycan, li-poteichoic acid, lipoproteins, and fungal zymo-san), TLR3 (which senses viral dsRNA), TLR9(which responds to bacterial CpG DNA), and
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 13
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Nod1 (which senses bacterial g-D-glutamyl-meso-diaminopimelic acid [iE-DAP]) (Carde-nas et al. 2011b) have also been shown to play arole in infection-associated preterm birth, al-though the exact site and type response varies(Koga et al. 2009; Thaxton et al. 2009). Thistells us that, although some of the downstreampathways and outcomes triggered by infectionthrough TLRs and NLRs may be similar, theupstream effector mechanisms can vary de-pending on the tissue involved, the infectiousstimulus, and the specific innate immune recep-tor activated.
Finally, it has been proposed that underly-ing medical conditions (such as endometriosis,polycystic ovarian syndrome, and obesity) mayaffect the ability of progesterone to decidualizethe endometrium leading to reproductive dys-function, including infertility and preterm birth(Schulte et al. 2015). Endometriosis, a majorinflammatory disease affecting women, is asso-ciated with “progesterone resistance” and down-stream changes in endometrial gene expressionin both ectopic and eutopic endometrium (Les-sey et al. 2013). Indeed, gene expression profil-ing of eutopic endometrium in women withendometriosis has shown a distinct “proinflam-matory profile” with dysregulation of selectgenes (including those involved in embryonicattachment, embryo toxicity, immune dysfunc-tion, and apoptotic responses) leading to an in-hospitable environment for implantation (Kaoet al. 2003). In this way, underlying medical con-ditions may also serve as the first hit for pretermbirth.
CONCLUDING REMARKS
Regardless of the gestational age at which it oc-curs, parturition is first and foremost a proin-flammatory event. Our central hypothesis isthat factors that are put in place around thetime of blastocyst implantation tonically inhibitinflammation in the endometrium/deciduathroughout gestation, but that this effect wanesover time. We posit that, as part of this devel-opmental program, advancing gestational age isassociated with a withdrawal of active suppres-sion and/or an enhanced sensitivity of the
decidua to signals capable of inducing inflam-mation. This promotes the release of a varietyof biologically active inflammatory mediators(primarily PGs) leading to the onset of la-bor. If dysregulation of decidual inflammatorysignaling occurs early, spontaneous preterm la-bor will ensue. Preterm labor may also resultfrom active induction of decidual inflamma-tion in the midtrimester of pregnancy due, forexample, to intrauterine infection or placentalabruption.
Numerous risk factors for spontaneous pre-term birth in humans have been identified,including (among others) African–Americanethnicity, cervical shortening, lower genital tractinfection, undernutrition, anemia, and ciga-rette smoking. Rather than being causally relat-ed to preterm birth, there is mounting evidenceto suggest that these risk factors are simply amarker of dysfunctional immunological defensewithin the tissues of the uterus (J Iams, pers.comm.). In the United States, preterm birth isdefined as delivery between 20–0/7 and 36–6/7 wk of gestation. This is largely a definition ofconvenience. Preterm birth is not an isolatedclinical condition, but is part of a continuumof adverse pregnancy events that extendsthroughout the length of gestation and includessubfertility, recurrent pregnancy loss, cervicalinsufficiency, preterm birth, and postterm preg-nancy (Fig. 3) (Iams et al. 2011). The decidualclock hypothesis outlined in this monographcan be seen as a “grand unified theory” becauseit can explain all of these adverse pregnancyevents depending on when in gestation the de-cidual clock is dialed down.
ACKNOWLEDGMENTS
This work was supported in part by the Nation-al Institutes of Health NIH/NICHD-sponsoredReproductive Scientist Development Program(to E.R.N.) and March of Dimes Grants (21-FY05-1250 to E.R.N. and 21-FY06-574 toE.A.B.). E.A.B. is also supported by a P20RR021905 Grant to the Vermont Center forImmunology and Infectious Disease. V.M.A.is also supported by NIH/NICHD Grants(RO1HD049446 and PO1HD054713).
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 15
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Aagaard K, Ma J, Antony KM, Ganu R, Petrosino J, Versa-lovic J. 2014. The placenta harbors a unique microbiome.Sci Transl Med 6: 237ra65.
Abel MH, Kelley RW. 1979. Differential production of pros-taglandins within the human uterus. Prostaglandins 18:821–828.
Abrahams VM. 2008. Pattern recognition at the maternal-fetal interface. Immunol Invest 37: 427–447.
Abrahams VM. 2011. The role of the Nod-like receptor fam-ily in trophoblast innate immune responses. J ReprodImmunol 88: 112–117.
Abrahams VM, Collins JE, Wira CR, Fanger MW, YeamanGR. 2003. Inhibition of human polymorphonuclear celloxidative burst by 17-b-estradiol and 2,3,7,8-tetrachlo-rodibenzo-p-dioxin. Am J Reprod Immunol 50: 463–472.
Adams Waldorf KM, Persing D, Novy MJ, Sadowsky DW,Gravett MG. 2008. Pretreatment with toll-like receptor 4antagonist inhibits lipopolysaccharide-induced pretermuterine contractility, cytokines, and prostaglandins inrhesus monkeys. Reprod Sci 15: 121–127.
Ariga H, Ohto H, Busch MP, Imamura S, Watson R, Reed W,Lee TH. 2001. Kinetics of fetal cellular and cell-free DNAin the maternal circulation during and after pregnancy:Implications for noninvasive prenatal diagnosis. Transfu-sion 41: 1524–1530.
Baschat AA, Towbin J, Bowles NE, Harman CR, Weiner CP.2003. Prevalence of viral DNA in amniotic fluid of low-risk pregnancies in the second trimester. J Matern FetalNeonatal Med 13: 381–384.
Billingham RE, Brent L, Medawar PB. 1953. Actively ac-quired tolerance of foreign cells. Nature 172: 603–606.
Birch L, English CA, O’Donoghue K, Barigye O, Fisk NM,Keer JT. 2005. Accurate and robust quantification of cir-culating fetal and total DNA in maternal plasma from 5 to41 weeks of gestation. Clin Chem 51: 312–320.
Bizargity P, Del Rio R, Phillippe M, Teuscher C, Bonney EA.2009. Resistance to lipopolysaccharide-induced pretermdelivery mediated by regulatory T cell function in mice.Biol Reprod 80: 874–881.
Blackmore CA, Ferre CD, Rowley DL, Hogue CJ, Gaiter J,Atrash H. 1993. Is race a risk factor or a risk marker forpreterm delivery? Ethn Dis 3: 372–377.
Blackmore-Prince C, Kieke B Jr, Kugaraj KA, Ferre C, Elam-Evans LD, Krulewitch CJ, Gaudino JA, Overpeck M.1999. Racial differences in the patterns of singleton pre-term delivery in the 1988 National Maternal and InfantHealth Survey. Matern Child Health J 3: 189–197.
Bonney EA. 2001. Maternal tolerance is not critically depen-dent on IL-4. Immunology 103: 1–13.
Bonney EA, Matzinger P. 1997. How does the maternal im-mune system interact with migrating fetal cells? J Immu-nol 158: 40–47.
Bonney EA, Matzinger P. 1998. Much IDO about pregnancy.Nat Med 4: 1128–1129.
Bonney EA, Onyekwuluje J. 2003. The H-Y response in mid-gestation and long after delivery in mice primed beforepregnancy. Immunol Invest 32: 71–81.
Boutin A, Demers S, Roberge S, Roy-Morency A, Chandad F,Bujold E. 2013. Treatment of periodontal disease and
prevention of preterm birth: Systematic review andmeta-analysis. Am J Perinatol 30: 537–544.
Brown LY, Bonney EA, Raj RS, Nielsen B, Brown S. 2013.Generalized disturbance of DNA methylation in the uter-ine decidua in the CBA/J � DBA/2 mouse model ofpregnancy failure. Biol Reprod 89: 120.
Bulmer JN, Williams PJ, Lash GE. 2010. Immune cells in theplacental bed. Int J Dev Biol 54: 281–294.
Cardenas I, Means RE, Aldo P, Koga K, Lang SM, Booth CJ,Manzur A, Oyarzun E, Romero R, Mor G. 2010. Viralinfection of the placenta leads to fetal inflammation andsensitization to bacterial products predisposing to pre-term labor. J Immunol 185: 1248–1257.
Cardenas I, Mor G, Aldo P, Lang SM, Stabach P, Sharp A,Romero R, Mazaki-Tovi S, Gervasi M, Means RE. 2011a.Placental viral infection sensitizes to endotoxin-inducedpre-term labor: A double hit hypothesis. Am J ReprodImmunol 65: 110–117.
Cardenas I, Mulla MJ, Myrtolli K, Sfakianaki AK, NorwitzER, Tadesse S, Guller S, Abrahams VM. 2011b. Nod1activation by bacterial iE-DAP induces maternal–fetalinflammation and preterm labor. J Immunol 187: 980–986.
Carlson A, Thung SF, Norwitz ER. 2009. H1N1 Influenza inpregnancy: What all obstetric care providers ought toknow. Rev Obstet Gynecol 2: 139–145.
Carmichael SL, Iyasu S, Hatfield-Timajchy K. 1998. Cause-specific trends in neonatal mortality among black andwhite infants, United States, 1980–1995. Matern ChildHealth J 2: 67–76.
Clausson B, Lichtenstein P, Cnattingius S. 2000. Geneticinfluence on birthweight and gestational length deter-mined by studies in offspring of twins. Br J Obstet Gynae-col 107: 375–381.
Collins MK, Tay CS, Erlebacher A. 2009. Dendritic cell en-trapment within the pregnant uterus inhibits immunesurveillance of the maternal/fetal interface in mice. JClin Invest 119: 2062–2073.
Constantin CM, Masopust D, Grayson J, Stricklan OL, Ah-med R, Bonney EA. 2007. Normal establishment of virus-specific memory CD8 T cell pool following primary in-fection during pregnancy. J Immunol 179: 4383–4389.
E.R. Norwitz et al.
16 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Dekel N, Gnainsky Y, Granot I, Racicot K, Mor G. 2014.The role of inflammation for a successful implantation.Am J Reprod Immunol 72: 141–147.
Dekker G, Robillard PY. 2007. Pre-eclampsia: Is the immunemaladaptation hypothesis still standing? An epidemio-logical update. J Reprod Immunol 76: 8–16.
Dekker GA, Robillard PY, Hulsey TC. 1998. Immune mal-adaptation in the etiology of preeclampsia: A review ofcorroborative epidemiologic studies. Obstet Gynecol Surv53: 377–382.
Dodd M, Andrew TA, Coles JS. 1980. Functional behaviourof skin allografts transplanted to rabbit deciduomata.J Anat 130: 381–390.
Dolan SM, Hollegaard MV, Merialdi M, Betran AP, Allen T,Abelow C, Nace J, Lin BK, Khoury MJ, et al. 2010. Syn-opsis of preterm birth genetic association studies: Thepreterm birth genetics knowledge base (PTBGene). Pub-lic Health Genomics 13: 514–523.
Du H, Taylor HS. 2007. Contribution of bone marrow-de-rived stem cells to endometrium and endometriosis.Stem Cells 25: 2082–2086.
Ekwo E, Moawad A. 1998. The risk for recurrence of pre-mature births to African–American and white women.J Assoc Acad Minor Phys 9: 16–21.
Elovitz MA, Wang Z, Chien EK, Rychlik DF, Phillippe M.2003. A new model for inflammation-induced pretermbirth: The role of platelet-activating factor and Toll-likereceptor-4. Am J Pathol 163: 2103–2111.
Embrey M. 1971. PGE compounds for induction of labourand abortion. Ann NY Acad Sci 180: 518–523.
Erlebacher A. 2013. Immunology of the maternal-fetal in-terface. Annu Rev Immunol 31: 387–411.
Erlebacher A, Vencato D, Price KA, Zhang D, Glimcher LH.2007. Constraints in antigen presentation severely restrictT cell recognition of the allogeneic fetus. J Clin Invest 117:1399–1411.
Esplin MS. 2014. Overview of spontaneous preterm birth:A complex and multifactorial phenotype. Clin ObstetGynecol 57: 518–530.
Farina A, LeShane ES, Romero R, Gomez R, Chaiwora-pongsa T, Rizzo N, Bianchi DW. 2005. High levels of fetalcell-free DNA in maternal serum: A risk factor for spon-taneous preterm delivery. Am J Obstet Gynecol 193: 421–425.
Flenady V, Hawley G, Stock OM, Kenyon S, Badawi N. 2013.Prophylactic antibiotics for inhibiting preterm labourwith intact membranes. Cochrane Database Syst Rev 5:12:CD000246.
Fuchs AR. 1995. Plasma membrane receptors regulatingmyometrial contractility and their hormonal modula-tion. Semin Perinatol 19: 15–30.
Fuchs AR, Fuchs F. 1984. Endocrinology of human parturi-tion: A review. Br J Obstet Gynaecol 91: 948–967.
Fuchs AR, Fuchs F, Husslein P, et al. 1982. Oxytocin recep-tors and human parturition: A dual role for oxytocin inthe initiation of labor. Science 215: 1396–1398.
Fuchs AR, Fuchs F, Husslein P, Soloff MS. 1984. Oxytocinreceptors in the human uterus during pregnancy andparturition. Am J Obstet Gynecol 150: 734–741.
Gargano JW, Holzman CB, Senagore PK, Reuss ML, PathakDR, Williams MA, Fisher R. 2010. Evidence of placentalhaemorrhage and preterm delivery. Br J Obstet Gynaecol117: 445–455.
Genc MR, Onderdonk AB, Vardhana S, Delaney ML, Nor-witz ER, Tuomala RE, Paraskevas LR, Witkin SS. 2004a.Polymorphism in intron 2 of the interleukin-1 receptorantagonist gene, local mid-trimester cytokine response tovaginal flora, and subsequent preterm birth. Am J ObstetGynecol 191: 1324–1330.
Genc MR, Vardhana S, Delaney ML, Onderdonk A, Tuo-mala R, Norwitz E, Witkin SS. 2004b. Relationship be-tween a toll-like receptor-4 gene polymorphism, bacterialvaginosis-related flora and vaginal cytokine responses inpregnant women. Eur J Obstet Gynecol Reprod Biol 116:152–156.
Gervasi MT, Romero R, Bracalente G, Chaiworapongsa T,Erez O, Dong Z, Hassan SS, Yeo L, Yoon BH, Mor G, et al.2012. Viral invasion of the amniotic cavity (VIAC) in themidtrimester of pregnancy. J Matern Fetal Neonatal Med25: 2002–2013.
Getahun D, Ananth CV, Selvam N, Demissie K. 2005. Ad-verse perinatal outcomes among interracial couples inthe United States. Obstet Gynecol 106: 81–88.
Gibb W. 1998. The role of prostaglandins in human partu-rition. Ann Med 30: 235–241.
Gurevich P, Elhayany A, Ben-Hur H, Moldavsky M, Szvalb S,Zandbank J, Shperling I, Zusman I. 2003. An immuno-histochemical study of the secretory immune system inhuman fetal membranes and decidua of the first trimesterof pregnancy. Am J Reprod Immunol 50: 13–19.
Hansen PJ, Bazer FW, Segerson EC Jr. 1986. Skin graft sur-vival in the uterine lumen of ewes treated with progester-one. Am J Reprod Immunol Microbiol 12: 48–54.
Harper LM, Parry S, Stamilio DM, Odibo AO, Cahill AG,Strauss JF III, Macones GA. 2012. The interaction effectof bacterial vaginosis and periodontal disease on the riskof preterm delivery. Am J Perinatol 29: 347–352.
Hickey DK, Patel MV, Fahey JV, Wira CR. 2011. Innate andadaptive immunity at mucosal surfaces of the female re-productive tract: Stratification and integration of im-mune protection against the transmission of sexuallytransmitted infections. J Reprod Immunol 88: 185–194.
Hirota Y, Cha J, Yoshie M, Daikoku T, Dey SK. 2011. Height-ened uterine mammalian target of rapamycin complex 1(mTORC1) signaling provokes preterm birth in mice.Proc Natl Acad Sci 108: 18073–18078.
Hoang M, Potter JA, Gysler SM, Han CS, Guller S, NorwitzER, Abrahams VM. 2014. Human fetal membranes gen-erate distinct cytokine profiles in response to bacterial
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 17
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Toll-like receptor and Nod-like receptor agonists. BiolReprod 90: 39.
Husslein P, Fuchs A-R, Fuchs F. 1981. Oxytocin and theinitiation of human parturition. I: Prostaglandin releaseduring induction of labor with oxytocin. Am J ObstetGynecol 141: 688–693.
Iams JD, Goldenberg RL, Mercer BM, Moawad A, Thom E,Meis PJ, McNellis D, Caritis SN, Miodovnik M, et al.1998. The Preterm Prediction Study: Recurrence risk ofspontaneous preterm birth. Am J Obstet Gynecol 178:1035–1040.
Iams JD, Cebrik D, Lynch C, Behrendt N, Das A. 2011. Therate of cervical change and the phenotype of spontaneouspreterm birth. Am J Obstet Gynecol 205: 130.e1–130.e6.
Ilievski V, Hirsch E. 2010. Synergy between viral and bacte-rial Toll-like receptors leads to amplification of inflam-matory responses and preterm labor in the mouse. BiolReprod 83: 767–773.
Jakobsen TR, Clausen FB, Rode L, Dziegiel MH, Tabor A.2012. High levels of fetal DNA are associated with in-creased risk of spontaneous preterm delivery. Prenat Di-agn 32: 840–845.
Jaschevatzky OE, Shalit A, Grunstein S, Kaplanski J, DanonA. 1983. Increased decidual prostaglandin E concentra-tion in human abortion. Br J Obstet Gynaecol 90: 958–960.
Jiang TT, Chaturvedi V, Ertelt JM, Kinder JM, Clark DR,Valent AM, Xin L, Way SS. 2014. Regulatory T cells:New keys for further unlocking the enigma of fetal toler-ance and pregnancy complications. J Immunol 192:4949–4956.
Kalantaridou SN, Zoumakis E, Weil S, Lavasidis LG, Chrou-sos GP, Makrigiannakis A. 2007. Reproductive cortico-tropin releasing hormone, implantation, and fetal immu-notolerance. Crit Rev Clin Lab Sci 44: 461–481.
Kao LC, Germeyer A, Tulac S, Lobo S, Yang JP, Taylor RN,Osteen K, Lessey BA, Giudice LC. 2003. Expression pro-filing of endometrium from women with endometriosisreveals candidate genes for disease-based implantationfailure and infertility. Endocrinology 144: 2870–2881.
Keirse MJNC, Turnbull AC. 1973. E prostaglandins in am-niotic fluid during late pregnancy and labour. J ObstetGynaecol Br Commonw 80: 970–973.
Kim J, Stirling KJ, Cooper ME, Ascoli M, Momany AM,McDonald EL, Ryckman KK, Rhea L, Schaa KL, Cosen-tino V, et al. 2013. Sequence variants in oxytocin pathwaygenes and preterm birth: A candidate gene associationstudy. BMC Med Genet 14: 77.
King J, Flenady V, Cole S, Thornton S. 2005. Cyclo-oxygen-ase (COX) inhibitors for treating preterm labour. Co-chrane Database Syst Rev (2): CD001992.
King AE, Kelly RW, Sallenave JM, Bocking AD, Challis JR.2007. Innate immune defenses in the human uterus dur-ing pregnancy. Placenta 28: 1099–1106.
Klebanoff MA, Carey JC, Hauth JC, Hillier SL, Nugent RP,Thom EA, Ernest JM, Heine RP, Wapner RJ, Trout W, etal. 2001. Failure of metronidazole to prevent pretermdelivery among pregnant women with asymptomaticTrichomonas vaginalis infection. N Engl J Med 345:487–493.
Koga K, Cardenas I, Aldo P, Abrahams VM, Peng B, Fill S,Romero R, Mor G. 2009. Activation of TLR3 in the tro-phoblast is associated with preterm delivery. Am J ReprodImmunol 61: 196–212.
Kruse A, Hallmann R, Butcher EC. 1999a. Specialized pat-terns of vascular differentiation antigens in the pregnantmouse uterus and the placenta. Biol Reprod 61: 1393–1401.
Kruse A, Merchant MJ, Hallmann R, Butcher EC. 1999b.Evidence of specialized leukocyte-vascular homing inter-actions at the maternal/fetal interface. Eur J Immunol 29:1116–1126.
Kruse A, Martens N, Fernekorn U, Hallmann R, Butcher EC.2002. Alterations in the expression of homing-associatedmolecules at the maternal/fetal interface during thecourse of pregnancy. Biol Reprod 66: 333–345.
Lamont RF, Nhan-Chang CL, Sobel JD, Workowski K, Con-de-Agudelo A, Romero R. 2011. Treatment of abnormalvaginal flora in early pregnancy with clindamycin for theprevention of spontaneous preterm birth: A systematicreview and metaanalysis. Am J Obstet Gynecol 205: 177–190.
Lessey BA, Lebovic DI, Taylor RN. 2013. Eutopic endome-trium in women with endometriosis: Ground zero forthe study of implantation defects. Semin Reprod Med31: 109–124.
Lissauer D, Piper K, Goodyear O, Kilby MD, Moss PA. 2012.Fetal-specific CD8þ cytotoxic T cell responses developduring normal human pregnancy and exhibit broadfunctional capacity. J Immunol 189: 1072–1080.
Lo YM, Tein MS, Lau TK, Haines CJ, Leung TN, Poon PM,Wainscoat JS, Johnson PJ, Chang AM, Hjelm NM. 1998.Quantitative analysis of fetal DNA in maternal plasmaand serum: Implications for noninvasive prenatal diag-nosis. Am J Hum Genet 62: 768–775.
Lockwood CJ, Kuczynski E. 2001. Risk stratification andpathological mechanisms in preterm delivery. PaediatrPerinat Epidemiol 15: S78–S89.
MacDonald PC, Koga S, Casey ML. 1991. Decidual activa-tion in parturition: Examination of amniotic fluid formediators of the inflammatory response. Ann NY AcadSci 622: 315–330.
Macones GA, Parry S, Elkousy M, Clothier B, Ural SH,Strauss JF. 2004. A polymorphism in the promoter regionof TNF and bacterial vaginosis: Preliminary evidence ofgene-environment interaction in the etiology of sponta-neous preterm birth. Am J Obstet Gynecol 190: 1504–1508.
Macones GA, Parry S, Nelson DB, Strauss JF, Ludmir J,Cohen AW, Stamilio DM, Appleby D, Clothier B, SammelMD, et al. 2010. Treatment of localized periodontal dis-ease in pregnancy does not reduce the occurrence of pre-term birth: Results from the Periodontal Infections andPrematurity Study (PIPS). Am J Obstet Gynecol 202:147.e1–148.e8.
Matzinger P, Kamala T. 2011. Tissue-based class control: Theother side of tolerance. Nat Rev Immunol 11: 221–230.
McLean M, Bisits A, Davies J, Woods R, Lowry P, Smith R.1995. A placental clock controlling the length of humanpregnancy. Nat Med 1: 460–463.
Menon R, Velez DR, Thorsen P, Vogel I, Jacobsson B, Wil-liams SM, Fortunato SJ. 2006. Ethnic differences in key
E.R. Norwitz et al.
18 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
candidate genes for spontaneous preterm birth: TNF-aand its receptors. Hum Hered 62: 107–118.
Menon R, Boldogh I, Hawkins HK, Woodson M, Polettini J,Syed TA, Fortunato SJ, Saade GR, Papaconstantinou J,Taylor RN. 2014. Histological evidence of oxidative stressand premature senescence in preterm premature ruptureof the human fetal membranes recapitulated in vitro. AmJ Pathol 184: 1740–1751.
Mercer BM, Goldenberg RL, Moawad AH, Meis PJ, Iams JD,Das AF, Caritis SN, Miodovnik M, Menard MK, et al.1999. The preterm prediction study: Effect of gestationalage and cause of preterm birth on subsequent obstetricoutcome. Am J Obstet Gynecol 181: 1216–1221.
Mitsunaga F, Ueiwa M, Kamanaka Y, Morimoto M, Naka-mura S. 2010. Fetal sex determination of macaque mon-keys by a nested PCR using maternal plasma. Exp Anim59: 255–260.
Mor G, Abrahams VM. 2013. The Immunology of Pregnan-cy. In: Creasy and Resnik’s maternal–fetal medicine, 7thedition (ed. Creasy R, Resnik R, et al.), pp. 80–92. Elsev-ier-Saunders, Philadelphia.
Muglia LJ. 2000. Genetic analysis of fetal development andparturition control in the mouse. Pediatr Res 47: 437–443.
Muglia LJ, Katz M. 2010. The enigma of spontaneous pre-term birth. N Engl J Med 362: 529–535.
Munn DH, Zhou M, Attwood JT, Bondarev I, Conway SJ,Marshall B, Brown C, Mellor AL. 1998. Prevention ofallogeneic fetal rejection by tryptophan catabolism. Sci-ence 281: 1191–1193.
Murphy SP, Fast LD, Hanna NN, Sharma S. 2005. UterineNK cells mediate inflammation-induced fetal demise inIL-10-null mice. J Immunol 175: 4084–4090.
Murray SA, Morgan JL, Kane C, Sharma Y, Heffner CS, LakeJ, Donahue LR. 2010. Mouse gestation length is geneti-cally determined. PLoS ONE 5: e12418.
Myatt L, Chaudhuri G, Gordon D, Elder MG. 1977. Prosta-glandin production by leucocytes attached to intrauter-ine devices. Contraception 15: 589–599.
Nancy P, Erlebacher A. 2014. T cell behavior at the mater-nal–fetal interface. Int J Dev Biol 58: 189–198.
Nancy P, Tagliani E, Tay CS, Asp P, Levy DE, Erlebacher A.2012. Chemokine gene silencing in decidual stromal cellslimits T cell access to the maternal-fetal interface. Science336: 1317–1321.
Nguyen DP, Genc MR, Vardhana S, Babula O, Onderdonk A,Witkin SS. 2004. Ethnic differences of polymorphisms incytokine and innate immune system genes in pregnantwomen. Obstet Gynecol 104: 293–300.
Nordling C. 1953. A new theory on cancer-inducing mech-anism. Br J Cancer 7: 68–72.
Norwitz ER, Wilson T. 2000. Secretory component: A po-tential regulator of endometrial-decidual prostaglandinproduction in early human pregnancy. Am J Obstet Gy-necol 183: 108–117.
Norwitz ER, Starkey PM, Lopez Bernal A. 1992. Prostaglan-din D2 production by term human decidua: Cellularorigins defined using flow cytometry. Obstet Gynecol80: 440–445.
Norwitz ER, Robinson JN, Challis JRG. 1999. The control oflabor. N Engl J Med 341: 660–666.
Norwitz ER, Schust DJ, Fisher SJ. 2001. Implantation andthe survival of early pregnancy. N Engl J Med 345: 1400–1408.
Norwitz ER, Mahendroo M, Lye SJ. 2014. Biology of Partu-rition. In: Creasy and Resnick’s maternal–fetal medicine,7th edition (ed. Creasy R, Resnik R, et al.), pp. 66–79.Elsevier-Saunders, Philadelphia.
Olson DM, Mijovic JE, Sadowsky DW. 1995. Control ofhuman parturition. Semin Perinatol 19: 52–63.
Parham P, Norman PJ, Abi-Rached L, Hilton HG, GuethleinLA. 2012. Review: Immunogenetics of human placenta-tion. Placenta 33: S71–S80.
Patel FA, Challis JRG. 2002. Cortisol progesterone antago-nism in the regulation of 15-hydroxy prostaglandin de-hydrogenase activity and mRNA levels in human chorionand placental trophoblast cells at term. J Clin EndocrinolMetab 87: 700–708.
Phillippe M. 2014. Cell-free fetal DNA—A trigger for par-turition. N Engl J Med 370: 2534–2536.
Plunkett J, Doniger S, Morgan T, Haataja R, Hallman M,Puttonen H, Menon R, Kuczynski E, Norwitz E, Snegov-skikh V, et al. 2010. Primate-specific evolution of non-coding element insertion into PLA2G4C and human pre-term birth. BMC Med Genomics 3: 62.
Plunkett J, Doniger S, Orabona G, Morgan T, Haataja R,Hallman M, Puttonen H, Menon R, Kuczynski E, Nor-witz E, et al. 2011. An evolutionary genomic approachto identify genes involved in human birth timing. PLoSGenet 7: e1001365.
Pradeu T, Cooper EL. 2012. The danger theory: 20 yearslater. Front Immunol 3: 287.
Racicot K, Cardenas I, Wunsche V, Aldo P, Guller S, MeansRE, Romero R, Mor G. 2013. Viral infection of the preg-nant cervix predisposes to ascending bacterial infection.J Immunol 191: 934–941.
Redman CW, Sargent IL. 2010. Immunology of pre-eclamp-sia. Am J Reprod Immunol 63: 534–543.
Robertson SA, Moldenhauer LM. 2014. Immunological de-terminants of implantation success. Int J Dev Biol 58:205–217.
Robertson SA, Skinner RJ, Care AS. 2006. Essential role forIL-10 in resistance to lipopolysaccharide-induced pre-term labor in mice. J Immunol 177: 4888–4896.
Robertson SA, Guerin LR, Moldenhauer LM, Hayball JD.2009. Activating T regulatory cells for tolerance in earlypregnancy—The contribution of seminal fluid. J ReprodImmunol 83: 109–116.
Romero R, Munoz H, Gomez R, Parra M, Polanco M, Valve-rde V, Hasbun J, Garrido J, Ghezzi F, Mazor M, et al. 1996.Increase in prostaglandin bioavailability precedes the on-set of human parturition. Prostaglandins Leukot EssentFatty Acids 54: 187–191.
Saito S, Shima T, Inada K, Nakashima A. 2013. Which typesof regulatory T cells play important roles in implantationand pregnancy maintenance? Am J Reprod Immunol 69:340–345.
The Decidual Clock and the Timing of Birth
Advanced Online Article. Cite this article as Cold Spring Harb Perspect Med doi: 10.1101/cshperspect.a023143 19
ww
w.p
ersp
ecti
vesi
nm
edic
ine.
org
Press on November 30, 2020 - Published by Cold Spring Harbor Laboratoryhttp://perspectivesinmedicine.cshlp.org/Downloaded from
Sandman CA, Glynn L, Schetter CD, Wadhwa P, Garite T,Chicz-DeMet A, Hobel C. 2006. Elevated maternal cor-tisol early in pregnancy predicts third trimester levelsof placental corticotrophin releasing hormone (CRH):Priming the placental clock. Peptides 27: 1457–1463.
Scharfe-Nugent A, Corr SC, Carpenter SB, Keogh L, DoyleB, Martin C, Fitzgerald KA, Daly S, O’Leary JJ, O’NeillLA. 2012. TLR9 provokes inflammation in response tofetal DNA: Mechanism for fetal loss in preterm birth andpreeclampsia. J Immunol 188: 5706–5712.
Schjenken JE, Robertson SA. 2014. Seminal fluid and im-mune adaptation for pregnancy—Comparative biologyin mammalian species. Reprod Domest Anim 49: 27–36.
Schulte MM, Tsai J, Moley KH. 2015. Obesity and PCOS:The effect of metabolic derangements on endometrialreceptivity at the time of implantation. Reprod Sci 22:6–14.
Sebire NJ. 2001. Choriodecidual inflammation syndrome(CoDIS) is the leading, and under recognised, cause ofearly preterm delivery and second trimester miscarriage.Med Hypothesis 56: 497–500.
Shaw JL, Horne AW. 2012. The paracrinology of tubal ec-topic pregnancy. Mol Cell Endocrinol 358: 216–222.
Shaw JL, Fitch P, Cartwright J, Entrican G, Schwarze J,Critchley HO, Horne AW. 2011. Lymphoid and myeloidcell populations in the non-pregnant human Fallopiantube and in ectopic pregnancy. J Reprod Immunol 89:84–91.
Shepard MT, Bonney EA. 2013. PD-1 regulates T cell pro-liferation in a tissue and subset specific manner duringnormal mouse pregnancy. Immunol Invest 42: 385–408.
Silasi M, Mor G. 2012. Decidual stromal cells as regulators ofT-cell access to the maternal–fetal interface. Am J ReprodImmunol 68: 279–281.
Stout MJ, Conlon B, Landeau M, Lee I, Bower C, Zhao Q,Roehl KA, Nelson DM, Macones GA, Mysorekar IU.2013. Identification of intracellular bacteria in the basalplate of the human placenta in term and preterm gesta-tions. Am J Obstet Gynecol 208: 226.e1–226.e7.
Subramaniam A, Abramovici A, Andrews WW, Tita AT.2012. Antimicrobials for preterm birth prevention: Anoverview. Infect Dis Obstet Gynecol 2012: 157159.
Sun Y, Qin X, Shan B, Wang W, Zhu Q, Sharma S, Wu J, LinY. 2013. Differential effects of the CpG-Toll-like receptor9 axis on pregnancy outcome in nonobese diabetic miceand wild-type controls. Fertil Steril 99: 1759–1767.
Svensson AC, Sandin S, Cnattingius S, Reilly M, Pawitan Y,Hultman CM, Lichtenstein P. 2009. Maternal effects forpreterm birth: A genetic epidemiologic study of 630,000families. Am J Epidemiol 170: 1365–1372.
Tagliani E, Erlebacher A. 2011. Dendritic cell function at thematernal–fetal interface. Expert Rev Clin Immunol 7:593–602.
Thaxton JE, Romero R, Sharma S. 2009. TLR9 activationcoupled to IL-10 deficiency induces adverse pregnancyoutcomes. J Immunol 183: 1144–1154.
Trowsdale J, Betz AG. 2006. Mother’s little helpers: Mecha-nisms of maternal–fetal tolerance. Nat Immunol 7: 241–246.
Uchide N, Ohyama K, Bessho T, Takeichi M, Toyoda H.2012. Possible roles of proinflammatory and chemoat-tractive cytokines produced by human fetal membranecells in the pathology of adverse pregnancy outcomesassociated with influenza virus infection. Mediators In-flamm 2012: 270670.
Uzun A, Dewan AT, Istrail S, Padbury JF. 2013. Pathway-based genetic analysis of preterm birth. Genomics 101:163–170.
Varner MW, Esplin MS. 2005. Current understanding ofgenetic factors in preterm birth. Br J Obstet Gynaecol112: S28–S31.
Ventura SJ, Bachrach CA. 2000. Nonmarital childbearingin the United States, 1940–99. Natl Vital Stat Rep 48:1–40.
Vince GS, Starkey PM, Jackson MC, Sargent IL, RedmanCW. 1990. Flow cytometric characterisation of cell pop-ulations in human pregnancy decidua and isolation ofdecidual macrophages. J Immunol Methods 132: 181–189.
Wadhwa PD, Culhane JF, Rauh V, Barve SS, Hogan V, Sand-man CA, Hobel CJ, Chicz-DeMet A, Dunkel-Schetter C,Garite TJ, et al. 2001. Stress, infection and preterm birth:A biobehavioural perspective. Pediatr Perinat Epidemiol15: S17–S29.
Wang H, Hirsch E. 2003. Bacterially-induced preterm laborand regulation of prostaglandin-metabolizing enzymeexpression in mice: The role of toll-like receptor 4. BiolReprod 69: 1957–1963.
Winkvist A, Mogren I, Hogberg U. 1998. Familial patterns inbirth characteristics: Impact on individual and popula-tion risks. Int J Epidemiol 27: 248–254.
Wira CR, Patel MV, Ghosh M, Mukura L, Fahey JV. 2011.Innate immunity in the human female reproductive tract:Endocrine regulation of endogenous antimicrobial pro-tection against HIVand other sexually transmitted infec-tions. Am J Reprod Immunol 65: 196–211.
Wu HX, Jin LP, Xu B, Liang SS, Li DJ. 2014. Decidual stromalcells recruit Th17 cells into decidua to promote prolifer-ation and invasion of human trophoblast cells by secret-ing IL-17. Cell Mol Immunol 11: 253–262.
Xin L, Ertelt JM, Rowe JH, Jiang TT, Kinder JM, ChaturvediV, Elahi S, Way SS. 2014. Cutting edge: Committed Th1CD4þ T cell differentiation blocks pregnancy-inducedFoxp3 expression with antigen-specific fetal loss. J Immu-nol 192: 2970–2974.
Xiong M, Lu J, Zhao A, Xu L, Bao S, Lin Q, Yang B. 2010.Therapy with FasL-gene-modified dendritic cells confersa protective microenvironment in murine pregnancy.Fertil Steril 93: 2767–2769.
Zeeman GG, Khan-Dawood FS, Dawood MY. 1997. Oxyto-cin and its receptor in pregnancy and parturition: Cur-rent concepts and clinical implications. Obstet Gynecol89: 873–883.
published online April 27, 2015Cold Spring Harb Perspect Med Joong Shin Park and Vikki M. AbrahamsErrol R. Norwitz, Elizabeth A. Bonney, Victoria V. Snegovskikh, Michelle A. Williams, Mark Phillippe, Molecular Regulation of Parturition: The Role of the Decidual Clock
Subject Collection Molecular Approaches to Reproductive and Newborn Medicine
Information in SpermIntergenerational Transfer of Epigenetic
Oliver J. Rando TechnologiesOutcomes with Assisted Reproductive A Molecular Perspective on Procedures and
Butts, et al.Monica A. Mainigi, Carmen Sapienza, Samantha
Programming: Molecular ApproachesEffects of Maternal Obesity on Fetal
Caterina Neri and Andrea G. Edlow Have Single-Gene DisorderstoSequencing in Critically Ill Neonates Suspected
Whole-Exome Sequencing and Whole-Genome
KingsmoreLaurie D. Smith, Laurel K. Willig and Stephen F.
The Neonatal Salivary TranscriptomeJill L. Maron Blood Group Using Next-Generation Sequencing
Noninvasive Antenatal Determination of Fetal
Morten Hanefeld DziegielKlaus Rieneck, Frederik Banch Clausen and
Tract Development, Adult Function, and Fertility Genes in Female ReproductiveHoxThe Role of
Hongling Du and Hugh S. Taylor Newborn ScreeningGenome-Scale Sequencing to Augment Current Potential Uses and Inherent Challenges of Using
Jonathan S. Berg and Cynthia M. Powell
InterfaceMaternal−Molecular Cross-Talk at the Feto
Gendie E. Lashthe Decidual ClockMolecular Regulation of Parturition: The Role of
Snegovskikh, et al.Errol R. Norwitz, Elizabeth A. Bonney, Victoria V.
PerspectiveMolecular Regulation of Parturition: A Myometrial
Montalbano, et al.Nora E. Renthal, Koriand'r C. Williams, Alina P.
Molecular Mechanisms of Preeclampsia
KarumanchiTammy Hod, Ana Sofia Cerdeira and S. Ananth
of Fetal Single-Gene DisordersGenome-Wide Sequencing for Prenatal Detection
Ignatia B. Van den Veyver and Christine M. Eng Maternal Plasma DNADiseases Using Massively Parallel Sequencing of Noninvasive Prenatal Screening for Genetic
Lyn S. Chitty and Y. M. Dennis LoMicroRNA in Ovarian Biology and Disease
ChristensonLynda K. McGinnis, Lacey J. Luense and Lane K. The Oocyte's Perspective on the Incoming Sperm
Confrontation, Consolidation, and Recognition:
David Miller
http://perspectivesinmedicine.cshlp.org/cgi/collection/ For additional articles in this collection, see
