R&D Systems Inc., 614 McKinley Place, NE, Minneapolis, MN 55413, USA
Accepted 28 July 2000
Abstract
Chemokines, once thought to be functionally restricted to controlling cellular migration to sites ofinflammation, have been recognized more recently to be involved in other biological functions such ashematopoiesis, angiogenesis and oncogenesis. Chemokines have established themselves as importantregulators of the immune system through their actions on T, B and dendritic cell development. Of clin-ical interest is the important role that a number of chemokine receptors, such as CXCR4, CCR2,CCR3, CCR5, CCR8 and CX
Cellular development and migration of cells to different tissues are important elements ofimmunity. Ultimately, these processes determine how effectively the host is able to respondto infection or injury. Efficient migration of cells requires extensive cellular communicationbetween various components of the immune system. Molecules involved in the directional
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
migration of leukocytes include selectins, integrins and chemokines. Chemokines (orchemoattractant cytokines) constitute a large group of low molecular weight (8–20 kDa) se-creted molecules that were initially recognized to play a role in leukocyte communicationand migration. Subsequent studies have expanded their functions, and they are now believedto play additional roles (directly or indirectly) in hematopoiesis, angiogenesis and oncogene-sis. In addition, chemokine receptors are also involved in viral pathologies, the best examplebeing the role that selected chemokine receptors play as HIV co-receptors.
Until recently, research in this field was complicated by an ever-increasing number of re-ported chemokines and chemokine receptors, a lack of consensus in the naming of moleculesand a paucity of reagents which could accommodate various methods of investigation. How-ever, substantial changes are presently under way. First, a greater emphasis is now beingplaced on the full characterization of known molecules; second, a nomenclature scheme hasbeen adopted that will bring uniformity to the field [1]; and third, commercial sources ofchemokine reagents applicable to a variety of investigative techniques are increasingly avail-able. As an introduction to this ever-expanding field, it is our intention to present a short re-view of chemokines, with particular emphasis on chemokine receptors and their expressionpatterns. This review is largely meant for those new to the field, as detailed reviews ofchemokines are readily available [2–5].
2. Chemokine nomenclature
Chemokines are a diverse group of about 40–50 small molecular weight polypeptideswhose general biological activities are focused on the chemoattraction of various cells tospecific tissues.
In vitro
studies suggest that chemokines exhibit some redundancy in theiraction although this phenomenon appears to be less clear in
in vivo
experiments. Chemok-ines are also characterized by their promiscuity of receptor binding. The biological signifi-cance of these properties is not clear. Some have suggested that this may represent a safetymechanism that insures that proper cells are targeted by the appropriate chemokine. Alterna-tively, redundancy in receptor binding may represent a hierarchy of chemoattractants withweak and strong binding affinities for the receptor where different chemokines are able workin concert to better direct leukocyte traffic.
Chemokines have been traditionally classified by an amino acid motif that occurs withinthe first two N-terminal cysteines. Based on this motif, the 40 or so known chemokines arecurrently classified into four groups. The largest group is the
beta
or CC group (Table 1).This group is characterized by adjacent cysteines in the N-terminus, accounting for its name,CC. In terms of function, the CC chemokines target their action on lymphocytes, monocytes,basophils and eosinophils. The second largest group is the
alpha
, or CXC group (Table 2).This group is characterized by a variable amino acid that lies between the two N-terminalcysteines, hence CXC. Within the CXC chemokine group, there are two identifiable sub-groups: one that contains a conserved glutamic acid-leucine-arginine (or ELR) amino acidmotif prior to the CXC identifying motif, and one subgroup that shows no such conservedmotif. The significance of this ELR motif is not simply structural, since non-ELR containingCXC chemokines act principally on lymphocytes, while ELR-containing chemokines show
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
107
strong preferential activity toward neutrophils. The other two groups are the
gamma
or XCchemokine group (Table 3) where a single cysteine is present at the N-terminus, and the
delta
or CX
3
C (Table 3) chemokine group that has three intervening amino acids between thefirst two cysteines.
Chemokines deliver their activity by interacting with cell surface expressed chemokinereceptors that belong to the family of seven transmembrane domain G protein-coupledrhodopsin-like receptors. These chemokine receptors have an extracellular N-terminus,three extracellular loops, three intracellular loops and an intracellular C-terminus. Intracel-lular signaling is delivered through the heterotrimeric, pertussis toxin-sensitive G proteins(
a
,
b
and
g
) which associate with the receptor’s intracellular domains. The engagement of a
Table 1CC chemokine receptor family
Receptor Ligand Respective new chemokine nomenclature
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
receptor by a chemokine ligand results in the activation of phospholipase C which generatesinositol-triphosphate and diacylglycerol. This in turn generates elevated levels of intracellu-lar Ca
2
1
as well as activation of protein kinase C. Following the rise in intracellular Ca
2
1
concentration, the cell undergoes a refractory period during which it is unable to be trig-gered through the same receptor. This temporal ligand-specific desensitization of the recep-tor has been used as an indicator of what additional ligands also signal through the same re-ceptor.
Chemokine receptor nomenclature is simply based on the chemokine group (CC, CXC, Cor CX
3
C) to which its ligand(s) belong. Chemokine receptors can be found mainly on leuko-cytes but have recently also been detected on other tissues such as neuronal cells, endothelialcells, placenta and liver. The increased availability of reagents to monitor chemokine receptorexpression has allowed investigators to phenotypically identify new cell populations with dis-tinct biological functions. The naming of chemokine receptors follows the guidelines adoptedat the Second Gordon Conference on Chemotactic Cytokines (Plymouth, NH, 1997).
3. Alpha or CXC chemokine receptors
3.1. CXCR1
CXCR1, previously known as IL-8RA, was one of the first chemokine receptors to beidentified in the early 1990s [6,7]. Messenger RNA (mRNA) for CXCR1 was found in abun-dance in neutrophils with a more limited expression in lymphocytes [6]. These observationsare supported at the protein level through the use of specific antibodies to the CXCR1 recep-tor. Immunofluorescence staining techniques and flow cytometric analysis have been usefulin demonstrating that 70–90% of neutrophils bear the CXCR1 receptor. Monocytes, and to alesser degree T cells, also express CXCR1 receptors [8,9]. Immature dendritic cells (DC)have also been described to express CXCR1 [10]. Different stimuli have been reported tomodulate CXCR1 expression; following lipopolysaccharide (LPS) mediated activation ofDC, both mRNA levels and surface expression of CXCR1 decrease [10]. A reduction inCXCR1 expression on neutrophils has been observed following ligation of the CD45 struc-ture (a tyrosine phosphatase) with a monoclonal antibody [11]. Unlike CXCR2 (describedbelow), CXCR1 expression is not affected by neutrophil exposure to tumor necrosis factoralpha (TNF-
a
) [12].To date, CXCR1 has been noted to bind two different chemokines: CXCL8/IL-8 (interleu-
kin-8) and CXCL2/GCP-2 (granulocyte chemoattractant protein-2) [2,3,13]. CXCL8/IL-8has been shown to antagonize neutrophil adhesion as well as cause neutrophil infiltration.
Table 3XC and CX
3
C chemokine receptor families
Receptor Ligand Respective new chemokine nomenclature
XCR1 Lymphotactin, SCM-1
b
XCL1, XCL2CX
3
CR1 Fractalkine CX
3
CL1
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Stimulation of neutrophils through their CXCR1 receptors appears to preferentially triggerCa
2
1
flux responses [12].
3.2. CXCR2
CXCR2, also known as IL-8RB, is a neutrophil associated chemokine receptor that shares77% amino acid sequence homology with CXCR1 [7]. Surface expression of CXCR2 is ob-served on a majority of neutrophils, monocytes and, although controversial, on a small pro-portion of lymphocytes [7–9]. Surface expression of the CXCR2 receptor closely parallelsthat of CXCR1. CXCR2 is the most promiscuous of the known alpha group receptors, bind-ing as many as seven different chemokines. CXCR2 has been demonstrated to bind CXCL1/GRO
a
(growth related-oncogene alpha), CXCL2/GRO
b
, CXCL3/GRO
g
, CXCL5/ENA-78(epithelial-derived neutrophil attractant-78), CXCL6/GCP-2, CXCL7/NAP-2 (neutrophil ac-tivating peptide-2) and CXCL8/IL-8 [3,4,14]. CXCL8/IL-8 binding to CXCR2 antagonizesneutrophil adhesion as well as promoting neutrophil migration and infiltration [4]. Treatmentof neutrophils with TNF-
a
causes CXCR2 receptors to be shed from the cell surface with asubsequent reduction in the ability of neutrophils to respond to CXCL8/IL-8 [12]. This re-duced chemotactic response to CXCL8/IL-8 is observable in spite of the fact that CXCR1expression is unaltered by exposure to TNF
a
. This suggests two distinct roles in cellular ac-tivation for the two IL-8 receptors (CXCR1 and CXCR2) [12].
3.3. CXCR3
CXCR3 was cloned from a CD4
1
T cell cDNA library, and upon transfection was demon-strated to generate a Ca
2
1
flux in response to two specific ligands, CXCL10/IP-10 (
g
-inter-feron inducible protein) and CXCL9/MIG (monokine induced by
g
-interferon) [15]. Initialcharacterization studies suggested that CXCR3 expression was restricted to IL-2 activated Tcells and most of these appeared to be of the memory phenotype [15,16]. However, other cir-culating cells have also been demonstrated to express CXCR3 receptors, including naïveCD4
1
and CD8
1
T cells, peripheral blood B cells and NK cells [16]. CXCR3 expression ismore pronounced in Th
1
over Th
2
cells following
in vitro
polarization [16,17]. It is unclear atthis time whether this selective CXCR3 expression on Th
1
cells is evident in an
in vivo
situa-tion. Of clinical diagnostic interest is a report demonstrating the association of CXCR3 ex-pression with B-cell chronic lymphocytic leukemias and splenic marginal zone lymphomas[18]. Furthermore, the demonstration that CXCL9/MIG is produced by some B-cell tumorsthat also express CXCR3 receptors suggests that these tumor cells may be endowed with aproliferative and/or chemotactic advantage [18]. Three chemokines have been described tospecifically bind to the CXCR3 receptor, CXCL9/MIG [19], CXCL10/IP-10 [20] andCXCL11/I-TAC (interferon inducible T cell alpha chemokine) [21]. The fact that the expres-sion of all three chemokines is up-regulated by proinflammatory cytokines, suggests thatthey may play a role in directing activated T-cell traffic in immune responses dominated bymolecules such as interferon gamma (IFN
g
) and IL-2, including inflammatory lesions andtumors. It is interesting that the presence of IFN-
g
and IL-2 have been traditionally associ-ated with Th
1
-type T-cell responses and this may further support the observation thatCXCR3 may be a marker for T cells participating in a Th
1
response.
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3.4. CXCR4
CXCR4, also known as Fusin, LESTR and HUMSTR, is one of the most extensively stud-ied chemokine receptors due to its role as a co-receptor for T-tropic HIV strains. CXCR4was discovered as an orphan receptor prior to studies elucidating its role in the HIV infectionprocess [22–26]. CXCR4 cell surface expression has been demonstrated on mononuclearcells such as T cells, B cells and monocytes [27]. This cell expression pattern has implica-tions for the pathogenesis of AIDS since
in vitro
studies have shown HIV variants can targetall three cell types for infection when CD4 and CXCR4 are coexpressed [27–29]. This isconsistent with the observation that CXCL12/SDF-1 (stromal cell-derived factor-1), the nat-ural ligand for CXCR4, can inhibit HIV entry into T cells [30].
CXCR4 expression on T cells is variable. This variability in cell surface receptor expres-sion levels is increased by the modulation that occurs following cellular activation. It hasbeen observed that CXCR4 is rapidly down-regulated from the cell surface following recep-tor engagement by its ligand, CXCL12/SDF-1, and the receptor is subsequently re-expressedafter an appropriate lag time [31]. Furthermore, exposure of resting T cells to IL-4 has beenreported to result in increased CXCR4 expression on resting T cells [32]. In view of the im-portant role that IL-4 plays in Th
1
/Th
2
polarization, the above finding would suggest thatCXCR4 expression is more likely to be associated with Th
2
type T-cell responses.In addition to its T-cell chemotactic activity, CXCL12/SDF-1 has been shown to be im-
portant in bone marrow associated B-cell development [33]. The importance of the CXCR4interaction with its ligand, CXCL12/SDF-1, is perhaps best highlighted in experimentswhere gene knockouts of either of these molecules results in lethal mutations of the develop-ing mouse embryos [33–35]. Fifty percent of homozygous mutants for the SDF-1 gene die atday 15 of embryogenesis [33]. SDF-1 mutant neonates die within hours after birth withgreatly reduced B lymphopoietic and myelopoeitic activity within the bone marrow as wellas demonstrating signs of abnormal cardiac development [33]. Additionally, CXCR4 knock-out animals exhibited abnormalities in their vascularization of the gastrointestinal tract andmalformations in cerebellar tissue [34,35]. Other cell populations described as expressingCXCR4 include mature DC [10], bone marrow B-cell progenitors and embryonic brain [35].One gene product with two isoforms, CXCL12/SDF-1
a
and
b
, has been described to be ca-pable of binding to CXCR4 [30]. Clearly CXCR4 and its ligand, CXCL12/SDF-1, representa monogamous ligand pair with critical functions in immune responses and in embryonic de-velopment.
3.4. CXCR5
CXCR5, previously known as BLR1 (Burkitt’s lymphoma receptor-1), is mainly ex-pressed on peripheral blood and tonsillar CD19
1
B cells [2,35]. Low levels of expressionhave been reported on memory CD4
1
and CD8
1
T cells, as well as
g
/
d
T cells [36,37]. Theelevated levels of expression on B cells is suggestive of a role for CXCR5 in B-cell matura-tion. This idea is strongly supported by studies using CXCR5 gene knockout mice [38].CXCR5-deficient animals show abnormal follicles and germinal centers in the spleen andPeyer’s patches areas, which are important lymphoid structures for B-cell affinity maturation[38]. Of clinical interest are the observations of aberrant CXCR5 levels in HIV-infected indi-
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111
viduals [37]. Both B and T cells in HIV
1
individuals show decreased levels of CXCR5 ex-pression [37].
CXCL13/BLC/BCA-1 (B-lymphocyte chemoattractant
2
1
) is the only known ligand forCXCR5 [39]. CXCL13/BLC, in association with its receptor CXCR5, appears to be involvedin the homing of memory T cells to lymph nodes, and in the transit of B cells through T-cellrich areas into B-cell follicles for germinal center formation [38].
3.6. CXCR6
A proposal presently exists that the orphan receptor previously known as STRL33 (seventransmembrane domain receptor from lymphocytes clone 33) [40] or Bonzo [41] be giventhe CXCR6 designation. CXCR6 was initially isolated from CD4
1
T cells that infiltrated tu-mors [40]. At the mRNA level CXCR6 expression can be demonstrated in both CD4
1
andCD8
1
tumor infiltrating lymphocytes [40], spleen, thymus, small intestine, activated PBL[40,41], placenta, bone marrow appendix and lymph nodes [40]. By monoclonal antibodystaining, CXCR6 expression has been documented on a fraction of resting and activatedCD4
1
or CD8
1
T cells, naïve T cells (CD4
1
/CD45RA
1
/CD62L
1
), a subpopulation of NKcells (CD3
2
/CD16low/CD566) and B cells [42]. The CXCR6 molecules is capable of actingas a co-receptor for SIV, HIV-2 and some strains of HIV-1 in the infection of T cells [40–42]. Furthermore, several M-tropic HIV strains can use CXCR6 as a cell entry cofactor [41].This is of particular significance because M-tropic HIV strains are thought to be critical inestablishing and maintaining HIV infections. Therefore in the absence of CCR5, as is thecase for those individuals with the CCR5-delta32 mutation, the HIV virus may make use ofalternate receptors, such as CXCR6, to establish infection. This principle has been formallydemonstrated in vitro where HIV strains that normally require either CCR5 or CXCR4 in ad-dition to CD4 can infect cells that express CXCR6 but lack CCR5 while the CXCR4 was in-activated with an inhibitor [42]. These data would suggest that CXCR6 may be a valuablemarker for cells that have a tumor infiltrating capacity, like T cells and NK cells, as well asbeing an additional receptor molecules that could predispose cells to infection by HIV. Stud-ies on the identification of possible chemokine ligands for the CXCR6 receptor are ongoing.
4. Beta or CC chemokine receptors
4.1. CCR1
CCR1 was first identified following attempts to clone the receptor for the two chemokinesCCL3/MIP-1a (macrophage inflammatory protein) and CCL5/RANTES (regulated on acti-vation normally T-cell expressed and secreted) [43]. Subsequently, it was shown that addi-tional chemokines can also interact through this receptor including CCL7/MCP-3 (monocytechemotactic protein), CCL23/MIPF-1 (myeloid progenitor inhibitory factor 1) andCCL14,15,16/HCC-1,2,4 (hemofiltrate CC chemokine) [2,4,43–45]. CCR1 cell surface ex-pression has been demonstrated on monocytes, NK cells as well as various subsets of T lym-phocytes with the aid of polyclonal antibodies to the receptor [46]. The high level CCR1 ex-pression on monocytes is consistent with the chemotactic action that CCL3/MIP-1a has onthis cell population. Expression patterns of CCR1 on different cell lineages has been largely
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addressed though mRNA analysis, and whether other cell types, such as resting T cells, alsoexpress CCR1 remains controversial. Activated T cells have been reported to express CCR1receptors, including Th1 polarized T cells [47,48]. However, Su et al. report CCR1 expres-sion on resting T cells but not on B cells [46], while Gao et al. report no CCR1 expression onnormal T cells but detectable expression in B cells [49]. Since CCL3/MIP-1a can affect theproliferation of erythroid progenitor cells from cord blood and bone marrow, it is not surpris-ing that CCR1 has been found on CD341 stem cells [46]. Low level CCR1 expression canalso be found on eosinophils from individuals who are highly responsive to the chemotacticactivity of CCL3/MIP-1a [50]. Finally, CCR1 is also expressed on immature DC [10]. Expo-sure of immature DC to LPS or TNFa lead to down regulation of CCR1 surface expressionon mature DC [10]. Although, CCL2/MCP-1 and CCL3/MIP-1b, have been described tobind CCR1 but only when used at relatively high concentrations, and therefore they are notconsidered true CCR1 ligands [43].
4.2. CCR2
The CCR2 receptor exists in two forms, CCR2A and CCR2B. The two forms differ onlyin the amino acid sequence of their intracellular C-terminus [51]. No differences in mRNAexpression are observed for CCR2A and CCR2B in those cells that are responsive to CCL2/MCP-1, which is one of the CCR2 primary ligands [51]. CCR2 is expressed on monocytes, Bcells and activated T cells [52,53]. CCR2 expression can be modulated by exposure to LPSresulting in reduced levels of CCR2 expression on monocytes [53]. Monocytes that have lostCCR2 expression demonstrate a reduced ability to migrate and show decreased Ca21 flux inresponse to stimulation with CCL2/MCP-1 [51,53].
CCR2 binds a variety of chemokine ligands including CCL2/MCP-1, CCL7/MCP-3,CCL8/MCP-2 and CCL13/MCP-4 [2–4]. CCL2/MCP-1 has been implicated in a number ofpathologies including asthma, arthritis, atherosclerosis, gingivitis and glomerulonephritis[43,51]. Furthermore, a role for CCR2 in viral pathogenesis is supported by in vitro observa-tions showing that CCR2B can act as a fusion cofactor for HIV-1 [54].
4.3. CCR3
CCR3, first identified as the receptor for eotaxin [55], has generated greater interest sinceits description of being associated with Th2 polarized T cells [56]. Quantitative flow cytomet-ric analysis revealed that both eosinophils and basophils express comparable levels of CCR3receptor, with approximately 20,000 receptors per cell [57]. CCR31 T cells can be found atsites of allergic inflammation, such as contact dermatitis and ulcerative colitis, where eosino-phils have also been recruited [58]. Furthermore, CCR31 T cells recruited to inflammatorysites demonstrate a cytokine secretion profile consistent with Th2 type T cells. These inflam-matory responses are typically dominated by high levels of IL-4 [58]. Expression of CCR3 isnot observed on granulocytes, monocytes or non-activated lymphocytes. In addition, T cellsisolated from rheumatoid arthritis synovium do not appear to express CCR3 [58].
CCR3 can bind a multitude of ligands; CCL11/Eotaxin and CCL13/MCP-4 are probably thebest known of the CCR3 ligands since they attract eosinophils and basophils and induce the re-lease of histamines and leukotrienes. Other chemokines capable of binding to CCR3 include:
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CCL5/RANTES, CCL7/MCP-3, CCL8/MCP-2, CCL15/HCC-2, CCL24/Eotaxin-2 andCCL26/Eotaxin-3 [2,3,44,58,59]. Of clinical interest is the demonstration that CCR3 can facil-itate infection by HIV-1 isolates, and that eotaxin can inhibit viral entry into cells [54,60].
4.4. CCR4
CCR4 was originally cloned from a basophilic cell line and was demonstrated to be ex-pressed on both peripheral blood lymphocytes and thymocytes [61]. At the mRNA level,CCR4 is expressed in CD41 T cells and in cell lines of basophilic and megakaryocytic lin-eage, including platelets [62]. No CCR4 mRNA expression could be demonstrated in B cells,NK cells, monocytes or neutrophils [62]. However, a more recent report suggests that IL-2activated NK cells do express CCR4 receptors [63]. Increasing evidence suggests that CCR4expression is associated with Th2 type T cells [17,64]. However, it should be pointed out thatTh1 cells have also been documented to express low levels of CCR4 mRNA [17]. Activationof T cells through their T-cell receptor appears sufficient to increase mRNA expression lev-els of CCR4 on T cells [65]. This elevated CCR4 expression lasts 1–2 days before returningto basal levels [65]. Although immature DC show no evidence of CCR4 expression, LPS-induced maturation of DC rapidly up-regulates the level of CCR4 mRNA [10].
The two ligands of CCR4 are CCL17/TARC (thymus and activation regulated chemok-ine) and CCL22/MDC (monocyte-derived chemokine) [2,62,66,67]. Both CCL17/TARCand CCL22/MDC appear to be expressed at high levels in the thymus [66,67]. CCL22/MDC,also known as STCP-1 (stimulated T-cell chemotactic protein), is expressed in both imma-ture and mature DC [10,66]. Of clinical interest are the recent findings that the Kaposi’s sar-coma-associated herpes virus, human herpes virus 8, encodes an agonist of the CCR4 recep-tor vMIP-III [68]. In addition to the role that CCR4 plays in T-cell responses, it also mayplay a role in the progression of Kaposi’s sarcoma, a malignancy commonly associated withHIV infections.
4.5. CCR5
CCR5 was first identified as the receptor for CCL3/MIP-1a, CCL4/MIP-1b and CCL5/RANTES [69]. CCR5 also acts as a co-receptor for macrophage (M) and dual (T cell andmacrophage)-tropic immunodeficiency viruses [60,68,70–72]. Evidence that CCR5 plays arole in HIV infection came with the demonstration that all three ligands for the CCR5 recep-tor, CCL3/MIP-1a, CCL4/MIP-1b and CCL5/RANTES, were potent inhibitors of viral en-try into cells [73]. Notably, a fraction of the Caucasian population are resistant to infectionby M-tropic HIV. A 32-base-pair mutation in the CCR5 gene structure results in a loss of ex-pression of functional CCR5 receptors [74–77]. Individuals who are homozygous for thismutation (a frequency of 1%) have a high degree of protection from infection by M-tropicHIV. The fact that HIV variants develop during the course of infection [78], suggests thatHIV can make use of alternate co-receptors to gain entry into cells. This is supported by sev-eral in vitro studies showing that CXCR4, STRL33 (see CXCR6 section), CCR2 and CCR3can act as HIV co-receptors to infect cells [26,42,54,60]. Furthermore, peripheral bloodmononuclear cells from individuals with the homozygous CCR5 mutation can be infected invitro by dual and T tropic HIV strains [75].
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CCR5 surface expression is restricted to monocytes and some subsets of T cells with noexpression on neutrophils [69,70]. Among T cells, CCR5 expression is restricted to memoryT cells and IL-2 cultured T cells [27,69]. Studies using Th1 and Th2 cell lines establishedfrom cord blood lymphocytes support the claim that CCR5 is preferentially expressed on Th1
type T cells [18,64]. CCR5 expression is also seen on immature DC, however this expressionis dramatically reduced following LPS-induced maturation of DC [10]. CCR5 and its ligandsare important molecules in viral pathogenesis. Furthermore, the ability of various cell stimulito modulate chemokine receptor expression on T cells [79] can ultimately impact both theT-cell migration patterns and the type of T-cell responses [16,79,80].
4.6. CCR6
CCR6, previously described as GPR-Cy4, STRL22, DRY6 and CKR-L3 [81,82], isknown to specifically interact with CCL20/MIP-3a, also referred to as Exodus and LARC(liver and activation regulated chemokine) [83–85]. Consistent with the chemoattractant ac-tion of CCL20/MIP-3a on lymphocytes, CCR6 surface expression is observed on bothCD41 T cells and CD81 T cells, B cells and DC [81,86,87]. No CCR6 surface expression isobserved on monocytes, eosinophils, granulocytes and NK cells [81,86]. CCR6 mRNA hasbeen specifically detected in spleen, lymph node, thymus, appendix and in CD341 bone mar-row progenitor cells, [2,82]. CD34 derived immature DC also express CCR6 receptors[87,88]. Following DC maturation in the presence of TNF-a and IL-4, CCR6 expression isdown-regulated [87,88]. The loss of CCR6 expression on mature DC correlates with a loss ofresponsiveness to CCL20/MIP-3a [88]. However, mature DC acquire another chemokine re-ceptor, CCR7, which imparts responsiveness to CCL19/MIP-3b [88]. Of particular interestin this coordinated regulation of CCR6 and CCR7 on DC is the finding that monocyte-de-rived, as opposed to CD341-derived, immature DC never express CCR6; however, their ma-ture counterparts express CCR7 [87,88]. The single ligand for CCR6 is CCL20/MIP-3a/LARC, which is expressed in fetal liver, fetal lung, appendix, lung, pancreatic islets, lymphnodes, thymus and PBL [2,81,83–86]. The action of CCL20/MIP-3a is directed primarily onDC, lymphocytes and on hematopoietic progenitors [84,85]. The specific actions of CCL20/MIP-3a for lymphoid and DC suggests that CCR6 and CCL20/MIP-3a are important mole-cules that can affect antigen presentation and ultimately T-cell activation.
4.7. CCR7
CCR7, previously known as the orphan receptor EBI1 (EBV-induced gene 1) [89], hasbeen identified as the receptor for two chemokines: CCL19/MIP-3b/Exodus-3/ELC (EBI-1ligand chemokine) and CCL21/SLC (secondary lymphoid tissue chemokine)/6Ckine/Exo-dus-2 [83,90,91]. Consistent with the chemotactic action of CCL19/MIP-3b and CCL21/SLC on lymphocytes and DC, CCR7 expression can be demonstrated on T cells and on ma-ture DC [10,90,92]. CCR7 expression is augmented following T-cell activation with anti-CD3 [93]. Both memory (CD45RO1 ) and naïve (CD45RA1) CD41 and CD81 T cells havebeen shown to express CCR7 receptors [94]. The CCR7 marker appears to be of even greaterutility in discriminating between two functional subsets of memory T cells: CCR72 memoryT cells which have effector function and can migrate to inflamed tissues, and CCR71 T cells
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125 115
which require a secondary stimulus before displaying effector function [94]. These twomemory T cell-populations, CD45RA2/CCR71 and CD45RA2/CCR72, have been referredto as central memory T cells and effector memory T cells, respectively [94]. Both of thesepopulations are thought to play important roles in maintaining immunological memory. Incontrast to immature DC, mature DC acquire responsiveness to CCL19/MIP-3b, which isconsistent with the CCR7 expression observed on mature DC [10,88].
CCR7 and its ligands appear to facilitate recruitment and retention of cells to secondarylymphoid organs which is a critical step for efficient exposure of T cells to antigens.Chemokines such as CCL19/MIP-3b and CCL21/SLC can trigger b2-integrin activation oncirculating lymphocytes and allow these cells to adhere to tissue sites [95]. The importanceof these interactions is best exemplified in CCR7 knockout experiments that result in animalswith poorly developed secondary lymphoid organs [96]. CCR7- deficient animals exhibit anirregular distribution of both T and B cells within lymph nodes, Peyer’s patches and in thesplenic periarteriolar lymphoid sheath [96]. Compared to control CCR7 animals, CCR7-defi-cient animals show impaired migration of both T and B cells to lymph nodes which in turnresults in inappropriate B-cell activation [96]. Additionally, primary T-cell responses, suchas delayed type hypersensitivity and contact sensitivity, are severely impaired in CCR7-defi-cient mice, largely the result of the inability of interdigitating dendritic cells to migrate intolymph nodes [96]. The important role that the CCR7-CCL21/SLC ligand pair plays is alsohighlighted by studies on a natural mouse mutant, plt (paucity of lymph node T cells), whichis unable to express CCL21/SLC [97]. This chemokine defect manifests itself in reducednumbers of lymph node T cells that is likely the result of the inability of T cells to migrateinto secondary lymphoid organs [96,97]. In total, CCR7 and its two ligands appear to be oneof the more important sets of molecules that regulate interactions between circulating T cells,B cells and DC.
4.8. CCR8
CCR8, previously known as TER1, ChemR1 and CKR-L1, is the receptor for the chemok-ine CCL1/I-309 [98,99]. In addition, three virally encoded ligands are also known to interactwith CCR8. Specifically, vMIP-I and vMIP-II encoded by the HHV8 herpes virus andvMCC-I encoded by the Molluscum contagiosum poxvirus bind CCR8 [100]. Notably not allviral ligands are agonists for the receptor. Accordingly, like CCL1/I-309, vMIP-I is an ago-nist of the CCR8 receptor; however, vMIP-II is an antagonist of CCR8 [100]. Consistentwith the chemoattractant action of I-309 on monocytes [101], and its anti-apoptotic activityon thymocytes [102], CCR8 expression has been detected on monocytes and thymocytes[99,103,104]. Among peripheral T cells, only those with a Th2 cytokine profile have beendemonstrated to express CCR8 [65,105].
The preferential expression of CCR8 on Th2 T cells, and the virally encoded nature ofthree of its ligands, suggests that viruses may find it advantageous to target the host’s Th2 re-sponse during their infection process [99]. Additional ligands for CCR8 have been proposed,including CCL17/TARC, CCL4/MIP-1b [106] and CCL16/HCC-4 [45]; however the inabil-ity of these ligands to induce a calcium signal upon binding has generated debate on whetherthese molecules should be considered true CCR8 ligands [45,107].
116 J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
4.9. CCR9
CCR9 was first known as the promiscuous D6 receptor that appeared to bind a variety ofchemokines but was incapable of delivering a signal [108]. Following a meeting of thechemokine nomenclature committee in Keystone, Colorado (January 1999), it was agreedthat the designation CCR9 be assigned to a molecule previously described as GPR-9-6 [109].D6 currently does not have a beta chemokine receptor designation. CCR9 (GPR-9-6) is thereceptor for CCL25/TECK (thymus expressed chemokine) [110] and exists as two isoforms(CCR9A & CCR9B) [111]. The “A” isoform contains an additional 12 amino acids at theN-terminus, and appears to be expressed in thymocytes, lymphoid cell lines and PBMC at a10-fold greater level than CCR9B [111].
CCR9 mRNA levels are present in both mature and immature thymocytes, with doublepositive (CD41/CD81) thymocytes expressing higher levels than single positive thymocytes[111–113]. Low levels of CCR9 mRNA have been observed in peripheral blood lympho-cytes, and in some T-cell lines such as MOLT-4, MOLT-13 and SUP-T1 [111,113]. Lowlevels of CCR9 surface expression have been demonstrated on CD191 B cells, CD41 mem-ory T cells and CD81 memory T cells [113]. CCR9 expression identifies a4b71 CD41 andCD81 memory T cells that home to the gut [113]. Alternatively, the lack of CCR9 expressionand the presence of CCR4 may help better define another subset of memory CD41 T cellswhich are CLA1 (cutaneous lymphocyte-associated antigen) and appear to home to lym-phoid organs other than the gut [113]. No cell surface expression of CCR9 is observed onCD561 NK cells, monocytes, neutrophils, basophils, eosinophils and DC [113]. The onlyknown ligand for CCR9 is CCL25/TECK, which is preferentially expressed in the fetalspleen, fetal and adult small intestine and adult thymus [110,111,113].
4.10. CCR10
CCR10 is one of the more recent chemokine receptors to be described. In spite of this, ithas not been spared the confusion in nomenclature that has generally plagued this field. TheD6 receptor, temporarily known as CCR9, was initially renamed CCR10 [108,114]. Subse-quently, the CCR10 designation was rescinded and a recent report provisionally assignsCCR10 to an orphan receptor with homology to the bovine gustatory receptor PPR1 [115].Shortly after this report, two additional papers were published that assigned the CCR10 des-ignation to another orphan receptor previously known as GPR-2 that binds a recently de-scribed chemokine named CCL/27CTACK (cutaneous T-cell attracting chemokine), alsoknown as ESkine, ALP [116–118] and the newly identified CCL28 [119]. This latterCCR10/GPR-2 is known to be expressed on melanocytes, dermal fibroblasts, CLA1 memoryT cells and skin derived Langerhans cells [116,118,119].
4.11. CCR11
Recently, a human chemokine receptor with homology to the bovine gustatory receptorPPR1 was identified, whose designation has become CCR11 [115,120]. An earlier study re-ports a seven transmembrane receptor with an identical amino acid sequence to CCR11, butwas given the name CCRL1 for CC chemokine receptor-like 1 [121]. All three above papersagree on the fact that CCR11 mRNA is expressed in the heart and kidney [115,120,121]. Other
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tissues reported to express CCR11 include spleen [115,121], lung, liver, and small intestine[120,121], pancreas [121], lymph node, T cells, immature DC, placenta and brain [115].
There is substantial disagreement on the chemokines that interact with the CCR11 recep-tor. A report on CCR11 binding studies suggests that CCL19/ELC, CCL21/SLC and CCL25/TECK all interact with CCR11 [115]. Another report identifies CCL13/MCP-4 and CCL8/MCP-2 through binding studies as the most likely ligands of CCR11 [120]. What appears ob-vious is that none of the above ligands elicit a strong calcium response [115,120,121]. Theseresults could be due to limitations in the signaling pathways of the cells being used for exper-imentation or it is possible that CCR11 is simply a non-signaling receptor. The precise roleof CCR11 and any of its potential ligands remains to be elucidated.
5. Gamma or C chemokine receptors
5.1. XCR1
XCR1, previously known as GPR5, is the receptor for the two-member g-chemokine fam-ily [122]. High levels of XCR1 mRNA have been reported in the placenta, while low levelshave been observed in the thymus, spleen and in peripheral blood lymphocytes [122]. Con-sistent with these observations, CD81 T cells, CD41 T cells, resting and IL-2 activated NKcells have been shown to respond to the chemotactic action of the XCR1 ligand, lymphotac-tin [123–125].
As noted, the primary ligand for XCR1 is XCL1/lymphotactin, also referred to as SCM-1(single C motif-1). In humans, two highly homologous forms of SCM-1 are produced,XCL1/SCM-1a and XCL2/SCM-1b [123]. Although, produced by two distinct genes, theirsequences vary by only two amino acids [2,124]. Recently, the virally encoded proteinvMIP-II has been demonstrated to also interact with the XCR1 receptor [126]. The biologicalrole for XCR1 and XCL1/lymphoctactin interaction is not well defined. However, throughmouse studies evidence is emerging of lymphoctactin’s ability to recruit T cells to tumorsites, which is suggestive of an important role in controlling tumor progression [127].
6. Delta or CX3CR chemokine receptors
6.1. CX3CR1
CX3CR1, previously known as V28 and CMK-BRL1, is the receptor for CX3CL1/fracta-lkine [128–130]. CX3CR1 receptor expression has been reported in brain, skeletal muscle,peripheral blood [128–130], neutrophils, monocytes, T cells and NK cells [2,128]. Among Tcells, CX3CR1 can be found on CD45RO1 memory cells as well as HLA-DR1 activated Tcells [131]. Of clinical interest is the observation that CX3CR1 can act as an alternate co-receptor for HIV-1, further reinforcing the important role that chemokines may play in viralimmunity [132].
The ligand for CX3CR1, CX3CL1/fractalkine, is unique in being a membrane-anchoredchemokine that is structurally composed of an N-terminal chemokine domain attached to amucin-like stalk that is tethered to the cell membrane [133]. Soluble forms of CX3CL1/fracta-
118 J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
lkine have been detected in in vitro transfection studies as a result of the likely action of pro-teinases on the membrane-anchored form of the chemokine [133]. CX3CL1/fractalkine is ex-pressed in a variety of tissues including: endothelial cells, heart, brain, lung, skeletal muscle,kidney, pancreas, colon, small intestine, prostate and testis [133]. Very low levels of CX3CR1expression have been reported in T cells, B cells and NK cells [133]. CX3CL1/fractalkine ex-pression in B cells can be up-regulated following activation with either anti-m or anti-CD40antibodies [131]. Exposure of HUVEC cells to either TNFa or IL-1 induces CX3CL1/fracta-lkine expression that can be demonstrated by both mRNA analysis and antibody staining[133]. Lastly, interdigitating DC and follicular DC have also been reported to expressCX3CL1 mRNA [133]. Interactions between CX3CR1 and CX3CL1/fractalkine promotes celladhesion and this is facilitated by the unique membrane-anchored feature of CX3CL1/fracta-lkine which may have the additional function of focusing chemokine expression on selectedcell surfaces [2,128,133]. This action may in turn promote the necessary communications be-tween activated T and B cells, DC and endothelial cells in order to achieve maturation of theimmune response and/or impart trafficking constraints on cells of the immune system.
7. Conclusion
It should be evident from the above presentation that chemokines and their receptors, oncethought to have very limited biological function, have become important players in hemato-poiesis, cell differentiation, dendritic cell development, viral pathogenesis and immunity.With additional research, it is likely that stronger links will emerge between selected pathol-ogies and chemokine production or chemokine receptor expression patterns. New infiltratingcell populations, with distinct bioactivity, may ultimately be defined by the array of chemok-ine receptors they express on their surfaces or the chemokines they may secrete. These andother findings will promote the introduction of chemokines and their receptors into the clini-cal laboratory as diagnostic tools for diseases. Ultimately chemokine receptors may prove tobe ideal targets for therapeutic intervention through the use of small synthetic molecules thatcan act as chemokine agonists or antagonists. The search for bioactive small molecules thatcan interact with chemokine receptors is the focus of ongoing research.
Acknowledgments
We apologize to our colleagues who extensively contributed to the development of this fieldbut unfortunately were not cited in our review. This was largely due to space limitations. Wethank Tracey Crisp, Joanna Guann and Mary M. Bartik for their help with this manuscript.
References
[1] Zlotnik A, Yoshie O. Chemokines: a new classification system and their role in immunity. Immunity 2000;12:121–7.
[2] Zlotnik A, Morales J, Hedrick JA. recent advances in chemokines and chemokine receptors. Crit Rev Im-munol 1999;19:1–47.
[3] Rollins BJ. Chemokines. Blood 1997;90:909–28.
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125 119
[4] Taub DD. Chemokine-leukocyte interactions. Cytokine Growth Factor Rev 1996;7:355–76.[5] Miller MD, Krangel MS. Biology and biochemistry of the chemokines: a family of chemotactic and inflam-
matory cytokines. Crit Rev Immunol 1992;12:17–46.[6] Holmes WE, Lee J, Kuang WJ, Rice GC, Wood WI. Structure and functional expression of a human inter-
leukin-8 receptor. Science 1991;253:1278–80.[7] Murphy P, Tiffany H. Cloning of complementary DNA encoding a functional human interleukin-8 receptor.
Science 1991;253:1280–3.[8] Gerszten RE, Garcia-Zepeda EA, Lim YC, Yoshida M, Ding HA, Gimbrone MA Jr, et al. MCP-1 and IL-8
trigger firm adhesion of monocytes to vascular endothelium under flow conditions. Nature 1999;398:718–23.
[9] Chuntharapai A, Lee J, Hebert CA, Kim KJ. Monoclonal antibodies detect different distribution pattern ofIL-8 receptor A and IL-8 receptor B on human peripheral blood leukocytes. J Immunol 1994;153:5682–8.
[10] Sallusto F, Schaerli P, Loetscher P, Schaniel C, Lenig D, Mackay CR, et al. Rapid and coordinated switchin chemokine receptor expression during dendritic cell maturation. Eur J Immunol 1998;28:2760–9.
[11] Mitchell GB, Khandaker MH, Rahimpour R, Xu L, Lazarovits AI, Pickering JG, Suria H, et al. CD45 mod-ulation of CXCR1 and CXCR2 in human polymorphonuclear leukocytes. Eur J Immunol 1999;29:1467–76.
[12] Asagoe K, Yamamoto K, Takahashi A, Suzuki K, Maeda A, Nohgawa M, et al. Down regulation of CXCR2expression on human polymorphonuclear leukocytes by TNFa. Immunol 1998;160:4518–25.
[13] Wuyts A, Osselaer NV, Haelens A. Characterization of synthetic human granulocyte chemotactic protein 2:usage of chemokine receptors CXCR1 and CXCR2 in in vivo inflammatory properties. Biochem 1997;36:2716–23.
[14] LaRosa G, Thomas KM, Kaufmann ME, Mark R, White M, Taylor L, et al. Amino terminus of the interleu-kin-8 receptor is a major determinant of receptor subtype specificity. J Biol Chem 1992;267:25,402–26.
[15] Loetscher M, Gerber B, Loetscher P, Jones SA, Piali L, Clark-Lewis I, et al. Chemokine receptor specificfor IP10 and Mig: structure, function, and expression in activated T-lymphocytes. J Exp Med 1996;184:963–9.
[16] Qin S, Rottman JB, Myers P, Kassam N, Weinblatt M, Loetscher M, et al. The chemokine receptorsCXCR3 and CCR5 mark subsets of T cells associated with certain inflammatory reactions. J Clin Invest1998;101:746–54.
[17] Sallusto F, Lenig D, Mackay CR, Lanzavecchia A. Flexible programs of chemokine receptor expression onhuman polarized T helper 1 and 2 lymphocytes. J Exp Med 1998;187:875–83.
[18] Jones D, Benjamin RJ, Shahsafaei A, Dorfman DM. The chemokine receptor CXCR3 is expressed in a sub-set of B-cell lymphomas and is a marker of B-cell chronic lymphocytic leukemia. Blood 2000;95:627–32.
[19] Farber JM. A macrophage mRNA selectively induced by g-interferon encodes a member of the platelet fac-tor-4 family of cytokines. Proc Natl Acad Sci USA 1990;87:5238–42.
[20] Luster AD, Unkeless JC, Ravetch JV. g-Interferon transcriptionally regulates an early-response gene con-taining homology to platelet proteins. Nature 1985;315:672–6.
[21] Cole KE, Strick CA, Paradis TJ, Ogborne KT, Loetscher M, Gladue RP, et al. Interferon-inducible T cell al-pha chemoattractant (I-TAC): a novel non-ELR CXC chemokine with potent activity on activated T cellsthrough selective high affinity binding to CXCR3. J Exp Med 1998;187:2009–21.
[22] Federsppiel B, Melhado IG, Duncan AM, Delaney A, Schappert K, Clark-Lewis I, et al. Molecular cloningof the cDNA and chromosomal localization of the gene for a putative seven-transmembrane segment (7-TMS) receptor isolated from human spleen. Genomics 1993;16:707–12.
[23] Herzog H, Hort YJ, Shine J, Selbie LA. Molecular cloning, characterization, and localization of the humanhomologue to the reported bovine NPY Y3 receptor: lack of NPY binding and activation. DNA Cell Bio1993;12:465–71.
[24] Jazin EE, Yoo H, Blomqvist AG, Yee F, Weng G, Walker MW, et al. A proposed bovine neuropeptide Y(NPY) receptor cDNA clone, or its human homologue, confers neither NPY binding sites nor NPY respon-siveness on transfected cells. Regul Pept 1993;47:247–58.
[25] Nomura H, Nielsen BW, Matsushima K. Molecular cloning of cDNAs encoding a LD78 receptor and puta-tive leukocyte chemotactic peptide receptors. Int Immunol 1993;5:1239–49.
120 J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
[26] Feng Y, Broder CC, Kennedy PE, Berger EA. HIV-1 Entry cofactor: functional cDNA cloning of a seven-transmembrane, G protein-coupled receptor. Science 1996;272:872–7.
[27] Bleul CC, Wu L, Hoxie JA, Springer TA, Mackay CR. The HIV coreceptors CXCR4 and CCR5 are differ-entially expressed and regulated on human T lymphocytes. Proc Natl Acad Sci USA 1997;94:1925–30.
[28] Moir S, Lapointe R, Malaspina A, Ostrowski M, Cole CE, Chun TW, et al. CD40-Mediated induction ofCD4 and CXCR4 on B lymphocytes correlates with restricted susceptibility to human immunodeficiencyvirus type 1 infection: potential role of B lymphocytes as a viral reservoir. J Virol 1999;73:7972–80.
[29] Poulin L, Paquette N, Moir S, Lapointe R, Darveau A. Productive infection of normal CD40-activated hu-man B lymphocytes by HIV-1. AIDS 1994;8:1539–44.
[30] Oberlin E, Amara A, Bachelerie F, Bessia C, Virelizier JL, Arenzana-Seisdedos F, et al. The CXC chemok-ine SDF-1 is the ligand for LESTR/fusin and prevents infection by T-cell-line-adapted HIV-1. Nature1996;382:833–5.
[31] Broxmeyer HE, Kim CH. Regulation of hematopoiesis in a sea of chemokine family members with a pleth-ora of redundant activities. Exp Hematol 1999;7:1113–1123.
[32] Jourdan P, Abbal C, Noraz N, Hori T, Uchiyama T, Vendrell JP, et al. IL-4 induces functional cell-surfaceexpression of CXCR4 on human T cells. J Immunol 1998;160:4153–7.
[33] Nagasawa T, Hirota S, Tachibana K, Takakura N, Nishikawa S, Kitamura Y, et al. Defects of B-cell lym-phopoiesis and bone marrow myelopoeisis in mice lacking the CXC chemokine PBSF/SDF-1. Nature1996;382:635–8.
[34] Tachibana K, Hirota S, Iizasa H, Yoshida H, Kawabata K, Kataoka Y, et al. The chemokine receptorCXCR4 is essential for vascularization of the gastrointestinal tract. Nature 1998;393:591–4.
[35] Zou YR, Kottmann AH, Kuroda M, Taniuchi I, Littman DR. Function of the chemokine receptor CXCR4 inhaematopoiesis and in cerebellar development. Nature 1998;393:595–9.
[36] Forster R, Emrich T, Kremmer E, Lipp M. Expression of the G-protein-coupled receptor BLR1 defines ma-ture, recircilating B cells and a subset of T-helper memory cells. Blood 1994;84:830–40.
[37] Forster R, Schweigard G, Johann S, Emrich T, Kremmer E, Nerl C, et al. Abnormal expression of the B-cellhoming chemokine receptor BLR1 during the progression of acquired immunodeficiency syndrome. Blood1997;90:520–5.
[38] Forster R, Mattis AE, Kremmer E, Wolf E, Brem G, Lipp M. A putative chemokine receptor, BLR1, directsB cell migration to defined lymphoid organs and specific anatomic compartments of the spleen. Cell 1996;87:1037–47.
[39] Legler DF, Loetscher M, Roos RS, Clark-Lewis I, Baggiolini M, Moser B. B cell attracting chemokine 1, ahuman CXC chemokine expressed in lymphoid tissues, selectively attracts B lymphocytes via BLR1/CXCR5. J Exp Med 1998;187:655–60.
[40] Liao F, Alkhatib G, Peden KWC, Sharma G, Berger EA, Farber JM. STRL33, a novel chemokine receptor-like protein, functions as a fusion cofactor for both macrophage-tropic and T cell line-tropic HIV-1. J ExpMed 1997;185:2015–23.
[41] Deng HK, Unutmaz D, KewlRamani VN, Littman DR. Expression cloning of new receptors used by simianand human immunodeficiency viruses. Nature 1997;388:296–300.
[42] Sharron M, Pohlmann S, Price K, Lolis E, Tsang M, Kirchhoff F, et al. Expression and coreceptor activityof STRL33/Bonzo on primary peripheral blood lymphocytes. Blood 2000;96:41–9.
[43] Neote K, DiGregorio D, Mak JY, Horuk R, Schall TJ. Molecular cloning, functional expression, and signal-ing characteristics of a CC chemokine receptor. Cell 1993;72:415–25.
[44] Patel VP, Kreider BL, Li Y, Li H, Leung K, Salcedo T, et al. Molecular and functional characterization oftwo novel human C-C chemokines as inhibitors of two distinct classes of myeloid progenitors. J Exp Med1997;185:1163–72.
[45] Zack Howard OM, Dong HF, Shirakawa AK, Oppenheim JJ. LEC induces chemotaxis and adhesion by in-teracting with CCR1 and CCR8. Blood 2000;96:840–5.
[46] Su SB, Mukaida N, Wang J, Nomura H, Matsushima K. Preparation of specific polyclonal antibodies to aC-C chemokine receptor, CCR1, and determination of CCR1 expression on various types of leukocytes. JLeuko Biol 1996;60:658–66.
[47] Colantonio L, Iellem A, Clissi B, Pardi R, Rogge L, Sinigaglia F, et al. Upregulation of integrin a6/b1 and
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125 121
chemokine receptor CCR1 by interleukin-12 promotes the migration of human type 1 helper T cells. Blood1999;94:2981–9.
[48] Cipriani B, Borsellino G, Poccia F, Placido R, Tramonti D, Bach S, et al. Activation of C-Cb-chemokinesin human peripheral blood gd T cells by isopentenyl pyrophosphate and regulation by cytokines. Blood2000;95:39–47.
[49] Gao JL, Kuhns DB, Tiffany HL, McDermott D, Li X, Francke U, et al. Structure and functional expressionof the human macrophage inflammatory protein 1a/Rantes receptor. J Exp Med 1993;177:1421–7.
[50] Sabroe I, Hartnell A, Jopling LA, Bel S, Ponath PD, Pease JE, et al. Differential regulation of eosinophilchemokine signaling via CCR3 and non-CCR3 pathways. Immunol 1999;162:2946–55.
[51] Charo IF, Myers SJ, Herman A, Franci C, Connolly AJ, Coughlin SR. Molecular cloning and functional ex-pression of two monocyte chemoattractant protein receptors reveals alternative splicing of the carboxyl-ter-minal tails. Proc Natl Acad Sci USA 1994;91:2752–6.
[52] Frade JM, Mellado M, del Real G, Gutierrez-Ramos JC, Lind P, Martinez-A C. Characterization of theCCR2 chemokine receptor: function CCR2 expression in B cells. Immunol 1997;159:5576–84.
[53] Xu L, Khandaker MH, Barlic J, Ran L, Borja ML, Madrenas J, et al. Identification of a novel mechanismfor endotoxinmediated down-regulation of CC chemokine receptor expression. Eur J Immunol 2000;30:227–35.
[54] Doranz BJ, Rucker J, Yi Y, Smyth RJ, Samson M, Peiper SC, et al. A dual-tropic primary HIV-1 isolate thatuses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors. Cell 1996;85:1149–58.
[55] Kitaura M, Nakajima T, Imai T, Harada S, Combadiere C, Tiffany HL, et al. Molecular cloning of humaneotaxin, an eosinophil-selective CC chemokine, and identification of a specific eosinophil eotaxin receptor,CC chemokine receptor 3. J Biol Chem 1996;271:7725–30.
[56] Sallusto F, MacKay CR, Lanzavecchia A. Selective expression of the eotaxin receptor CCR3 by human Thelper 2 cells. Science 1997;277:2005–7.
[57] Uguccioni M, Mackay CR, Ochensberger B, Loetscher P, Rhis S, LaRosa GJ, et al. High expression of thechemokine receptor CCR3 in human blood basophils. J Clin Invest 1997;100:1137–43.
[58] Gerber BO, Zanni MP, Uguccioni M, Loetscher M, Mackay CR, Pichler WJ, et al. Functional expression ofthe eotaxin receptor CCR3 in T lymphocytes co-localizing with eosinophils. Curr Biol 1997;7:836–43.
[59] Kitaura M, Suzuki N, Imai T, Takagi S, Suzuki R, Nakajima T, et al. Molecular cloning of a novel humanCC chemokine (Eotaxin-3) that is a functional ligand of CC chemokine receptor 3. J Biol Chem 1999;274:27,975–80.
[60] Choe H, Farzan M, Sun Y, Sullivan N, Rollins B, Ponath PD, et al. The beta-chemokine receptors CCR3and CCR5 facilitate infection by primary HIV-1 isolates. Cell 1996;85:1135–48.
[61] Power CA, Meyer A, Nemeth K, Bacon KB, Hoogewerf AJ, Proudfoot AE, et al. Molecular cloning andfunctional expression of a novel CC chemokine receptor cDNA from a human basophilic cell line. J BiolChem 1995;270:19,495–500.
[62] Imai T, Baba M, Nishimura M, Kakizaki M, Takagi S, Yoshie O. The T cell directed chemokine TARC is ahighly specific biological ligand for CC chemokine receptor 4. J Biol Chem 1997;272:15,036–42.
[63] Inngjerdingen M, Damaj B, Maghazachi AA. Human NK cells express CC chemokine receptors 4 and 8 andrespond to thymus and activation-regulated chemokine, macrophage-derived chemokine, and I-309. J Im-munol 2000;164:4048–54.
[64] Bonecchi R, Bianchi G, Bordignon PP, D’Ambrosio D, Lang R, Borsatti A, et al. Differential expression ofchemokine receptors and chemotactic responsiveness of type 1 T helper cells (Th1s) and Th2s. J Exp Med1998;187:129–34.
[65] D’Ambrosio D, Iellem A, Bonecchi R, Mazzeo D, Sozzani S, Mantovani A, et al. Cutting edge: selectiveup-regulation of chemokine receptors CCR4 and CCR8 upon activation of polarized human type 2 Th cells.J Immunol 1998;161:5111–5.
[66] Godiska R, Chantry D, Raport CJ, Sozzani S, Allavena P, Leviten D, et al. Human macrophage-derivedchemokine (MDC), a novel chemoattractant for monocytes, monoctye-derived dendritic cells, and naturalkiller cells. J Exp Med 1997;185:1595–604.
[67] Imai T, Yoshida T, Baba M, Nishimura M. Molecular cloning of a novel T cell-directed CC chemokine
122 J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
expressed in thymus by signal sequence trap using Epstein-Barr virus vector. J Biol Chem 1996;271:21514–21.
[68] Stine JT, Wood C, Epp A, Raport CJ, Schweickart VL, et al. KSHV-encoded CC chemokine vMIP-III is aCCR4 agonist, stimulates angiogenesis, and selectively chemoattracts Th2 cells. Blood 2000;95:1151–7.
[69] Samson M, Labbe O, Mollereau C, Vassart G, Parmentier M. Molecular cloning and functional expressionof a new human CC-chemokine receptor gene. Biochem 1996;35:3362–7.
[70] Dragic T, Litwin V, Allaway GP, et al. HIV-1 entry into CD41 cells is mediated by the chemokine receptorCC-CKR-5. Nature 1996;381:667–73.
[71] Deng H, Liu R, Ellmeier W, Choe S, Unutmaz D, Burkhart M, et al. Identification of a major co-receptor forprimary isolates of HIV-1. Nature 1996;381:661–6.
[72] Alkhatib G, Combadiere C, Broder CC, Feng Y, Kennedy PE, Murphy PM, et al. CC CKR-5: a RANTES,MIP-1alpha, MIP-1beta receptor as a fusion cofactor for macrophage-tropic HIV-1. Science 1996;272:1955–8.
[73] Cocchi F, DeVico AL, Garzino-Demo A, Arya SK, Gallo RC, Lusso P. Identification of RANTES, MIP-1aand MIP-1b as the major HIV-suppressive factors produced by CD81 T cells. Science 1995;270:1811–5.
[74] Liu R, Paxton WA, Choe S, Ceradini D, Martin SR, Horuk R, et al. Homozygous defect in HIV-1 corecep-tor accounts for resistance of some multiply-exposed individuals to HIV-1 infection. Cell 1997;86:367–77.
[75] Samson M, Libert F, Doranz BJ, Rucker J, Liesnard C, Farber CM, et al. Resistance to HIV-1 infection inCaucasian individuals bearing mutant alleles of the CCR-5 chemokine receptor gene. Nature 1996;382:722–5.
[76] Dean M, Carrington M, Winkler C, Huttley GA, Smith MW, Allikmets R, et al. Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene. Hemophilia Growthand Development Study, Multicenter AIDS Cohort Study, Multicenter Hemophilia Cohort Study, SanFrancisco City Cohort, ALIVE Study. Science 1996;273:1856–62.
[77] Zimmerman PA, Buckler-White A, Alkhatib G., Spalding T, Kobofcik J, Combadiere C, et al. Inherited resis-tance to HIV-1 conferred by an inactivating mutation in CC chemokine receptor 5: studies in populations withcontrasting clinical phenotypes, defined racial background, and quantified risk. Mol Med 1997;3:23–36.
[78] Fauci AS. Host factors and the pathogenesis of HIV-induced disease. Nature 1996;384:529–34.[79] Loetscher P, Seitz M, Baggiolini M, Moser B. Interleukin-2 regulates CC chemokine receptor expression
and chemotactic responsiveness in T lymphocytes. J Exp Med 1996;184:569–77.[80] Sallusto F, Kremmer E, Palermo B, Hoy A, Ponath P, Qin S, et al. Switch in chemokine receptor expression
upon TCR stimulation reveals novel homing potential for recently activated T cells. Eur J Immunol 1999;29:2037–45.
[81] Baba M, Imai T, Nishimura M, Kakizaki M, Takagi S, Hieshima K, et al. Identification of CCR6, the spe-cific receptor for a novel lymphocyte-directed CC chemokine LARC. J Biol Chem 1997;272:14893–14898.
[82] Liao F, Lee HH, Farber JM. Cloning of STRL22, a new human gene encoding a G-protein-coupled receptorrelated to chemokine receptors and located on chromosome 6q27. Genomics 1997;40:175–80.
[83] Rossi DL, Vicari AP, Franz-Bacon K, McClanahan TK, Zlotnik A. Identification through bioinformatics oftwo new macrophage proinflammatory human chemokine MIP-3a and MIP-3b. J Immunol 1997;158:1033–6.
[84] Hieshima K, Imai T, Openakker G, Van Damme J, Kusuda J, Tei H, et al. Molecular cloning of a novel hu-man CC chemokine liver and activation-regulated chemokine (LARC) expressed in liver. J Biol Chem1997;272:5846–53.
[85] Hromas R, Gray PW, Chantry D, Godiska R, Krathwohl M, Fife K, et al. Cloning and characterization ofexodus, a novel beta-chemokine. Blood 1997;89:3315–22.
[86] Liao F, Rabin RL, Smith CS, Sharma G, Nutman TB, Farber JM. CC-Chemokine receptor 6 is expressed ondiverse memory subsets of T cells and determines responsiveness to macrophage inflammatory protein 3alpha. J Immunol 1999;62:186–94.
[87] Carramolino L, Kremer L, Goya I, Varona R, Buesa JM, Gutierrez J, et al. Down-regulation of the beta-chemokine receptor CCR6 in dendritic cells mediated by TNFa and IL-4. Leukoc Biol 1999;66:837–44.
[88] Dieu MC, Vanbervliet B, Vicari A, Bridon JM, Oldham E, Ait-Yahia S, et al. Selective recruitment of im-
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125 123
mature and mature dendritic cells by distinct chemokines expressed in different anatomic sites. J Exp Med1998;188:373–86.
[89] Birkenbach M, Josefsen K, Yalamanchili R, Lenoir G, Kieff E. Protein, nucleotide, Epstein-Barr virus-induced genes: first lymphocyte-specific G protein-coupled peptide receptors. J Virol 1993;67:2209–20.
[90] Yoshida R, Imai T, Hieshima K, Kusuda J, Baba M, Kitaura M, et al. Molecular cloning of a novel humanCC Chemokine EBI1-ligand chemokine that is a specific functional ligand for EBI1, CCR7. J Biol Chem1997;272:13,803–9.
[91] Yoshida R, Nagira M, Kitaura M, Imagawa N, Imai T, Yoshie O. Secondary lymphoid-tissue chemokine isa functional ligand for the CC chemokine receptor CCR7. J Biol Chem 1998;273:7118–22.
[92] Willimann K, Legler DF, Loetscher M, Roos RS, Delgado MB, Clark-Lewis I, et al. The chemokine SLC isexpressed in T cell areas of lymph nodes and mucosal lymphoid tissues and attracts activated T cells viaCCR7. Eur J Immunol 1998;28:2025–34.
[93] Sallusto F, Kremmer E, Palermo B, Hoy A, Ponath P, Qin S, et al. Switch in chemokine receptor expressionupon TCR stimulation reveals novel homing potential for recently activated T cells. Eur J Immunol 1999;29:2037–45.
[94] Sallusto F, Lenig D, Forster R, Lipp M, Lanzavecchia A. Two subsets of memory T lymphocytes with dis-tinct homing potentials and effector functions. Nature 1999;401:708–12.
[95] Campbell JJ, Hedrick J, Zlotnik A, Siani MA, Thompson DA, Butcher EC. Chemokines and the arrest oflymphocytes rolling under flow conditions. Science 1998;279:381–4.
[96] Forster R, Schubel A, Breitfeld D, Kremmer E, Renner-Muller I, Wolf E, et al. CCR7 coordinates the pri-mary immune response by establishing functional microenvironments in secondary lymphoid organs. Cell1999;99:23–33.
[97] Nakano H, Mori S, Yonekawa H, Nariuchi H, Matsuzawa A, Kakiuchi T. A novel mutant gene involved inT-lymphocyte-specific homing into peripheral lymphoid organs on mouse chromosome 4. Blood 1998;91:2886–95.
[98] Roos RS, Loetscher M, Legler DF, Clark-Lewis I, Baggiolini M, Moser B. Identification of CCR8, the re-ceptor for the human CC chemokine I-309. J Biol Chem 1997;272:17,251–4.
[99] Tiffany HL, Lautens LL, Gao JL, Pease J, Locati M, Combadiere C, et al. Identification of CCR8: a humanmonocyte and thymus receptor for the CC chemokine I-309. J Exp Med 1997;186:165–70.
[100] Dairaghi DJ, Fan RA, McMaster BE, Hanley MR, Schall TJ. HHV8-encoded vMIP-i selectively engageschemokine receptor CCR8. Agonist and Antagonist profiles of viral chemokines. J Biol Chem 1999;274:21,569–74.
[101] Miller MD, Krangel MS. The human cytokine I-309 is a monocyte chemoattractant. Proc Natl Acad SciUSA 1992;89:2950–4.
[102] Van Snick J, Houssiau F, Proost P, Van Damme J, Renauld JC. I-309/T cell activation gene-3 chemokineprotects murine T cell lymphomas against dexamethasone-induced apoptosis. J Immunol 1996;157:2570–6.
[103] Napolitano M, Zingoni A, Bernardini G, Spinetti G, Nista A, Storlazzi CT, et al. Molecular cloning ofTER1, a chemokine receptor-like gene expressed by lymphoid tissues. J Immunol 1996;157:2759–63.
[104] Zaballos A, Varona R, Gutierrez J, Lind P, Marquez G. Molecular cloning and RNA expression of two newhuman chemokine receptor-like genes. Biochem Biophys Res Commun 1996;227:846–53.
[105] Zingoni A, Soto H, Hedrick JA, Stoppacciaro A, Storlazzi CT, Sinigaglia F, et al. The chemokine receptorCCR8 is preferentially expressed in Th2 but not Th1 cells. J Immunol 1998;161:547–51.
[106] Bernardini G, Hekrick J, Sozzani S, Luini W, Spinetti G, Weiss M, et al. Identification of the CC chemok-ine TARC and macrophage inflammatory protein-1b as novel functional ligands for the CCR8 receptor.Eur J Immunol 1998;28:582–8.
[107] Garlisi CG, Xaio H, Tian F, Hedrick JA, Billah MM, Egan RW, et al. The assignment of chemokine-chemokine receptor pairs: TARC and MIP-1b are not ligands for human CC-chemokine receptor 8. Eur JImmunol 1999;29:3210–5.
[108] Nibbs RJ, Wylie SM, Yang J, Landau NR, Graham GR. Cloning and characterization of a novel promiscu-ous human b-chemokine receptor D6. J Biol Chem 1997;272:32,078–83.
[109] Zaballos A, Gutierrez J, Varona R, Ardavin C, Marquez G. Cutting edge: identification of the orphan chemok-ine receptor GPR-9-6 as CCR9, the receptor for the chemokine TECK. J Immunol 1999;162:5671–5.
124 J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125
[110] Vicari AP, Figueroa DJ, Hedrick JA, Foster JS, Singh KP, Menon S, et al. TECK: a novel CC chemokinespecifically expressed by thymic dendtritic cells and potentially involved in T cell development. Immunity1997;7:291–301.
[111] Yu CR, Peden KW, Zaitseva MB, Golding H, Farber JM. CCR9A and CCR9B: two receptors for thechemokine CCL25/TECK/Ck-15 that differ in their sensitivities to ligand. J Immunol 2000;164:1293–305.
[112] Youn BS, Kim CH, Smith FO, Broxmeyer HE. TECK, an efficacious chemoattractant for human thy-mocytes, uses GPR-9-6/CCR9 as a specific receptor. Blood 1999;94:2533–6.
[113] Zabel BA, Agace WW, Campbell JJ, Heath HM, Parent D, Roberts AI, et al. Human G protein-coupled recep-tor GPR-9-6/CC chemokine receptor 9 is selectively expressed on intestinal homing T lymphocytes, muscosallymphocytes, and thymocytes and is required for thymus-expressed chemokine-mediated chemotaxis. J ExpMed 1999;190:1241–55.
[114] Bonini JA, Martin SK, Dralyuk F, Roe MW, Philipson LH, Steiner DF. Cloning, expression, and chromo-somal mapping of a novel human CC-chemokine receptor (CCR10) that displays high-affinity binding forMCP-1 and MCP-3. DNA Cell Biol 1997;16:1249–56.
[115] Gosling J, Dairaghi DJ, Wang Y, Hanley M, Talbot D, Miao Z, et al. Identification of a novel chemokine re-ceptor that binds dendritic cell and T cell active chemokines including ELC, SLC and TECK. J Immunol2000;164:2851–6.
[116] Homey B, Wang HB, Soto WW, Buchanan ME, Wiesenborn A, Catron D, et al. The orphan receptor G-pro-tein coupled receptor-2 (GPR-2,CCR10) binds the skin associated chemokine CCL27 (CTACK/ALP/ILC).J Immunol 2000;164:3465–70.
[117] Jarmin DI, Rits M, Bota D, Gerard NP, Graham GJ, Clark-Lewis I, et al. Identification of the orphan recep-tor G-protein-coupled receptor 2 as CCR10, a specific receptor for the chemokine ESkine. J Immunol2000;164:3460–4.
[118] Morales J, Homey B, Vicari AP, Hudak S, Oldham E, Hedrick J, et al. CTACK, a skin-associated chemok-ine that preferentially attracts skin-homing memory T cells. Proc Natl Acad Sci USA 1999;96:14,470–5.
[119] Wang W, Soto H, Oldham ER, Buchanan ME, Homey B, Catron D, et al. Identification of a novel chemok-ine (CCL28), which binds CCR10(GPR2). J Biol Chem 2000;275:22,313–23.
[120] Schweickart VL, Epp A, Raport CJ, Gray PW. CCR11 is a functional receptor for the monocyte chemoat-tractant protein family of chemokines. J Biol Chem 2000;275:9550–6.
[121] Khoja H, Wang G, Hg CL, Tucker J, Brown T, Shyamala V. Cloning of CCRL1, an orphan seven trans-membrane receptor related to chemokine receptors, expressed abundantly in the heart. Gene 2000;246:229–38.
[122] Yoshida T, Imai T, Kakizaki M, Nishimura M, Takagi S, Yoshie O. Identification of Since C Motif-1/Lym-photactin Receptor XCR1. Biol Chem 1998;273:16,551–4.
[123] Yoshida T, Imai T, Takagi S, Nishimura M, Ishikawa I, Yaoi T, et al. Structure and expression of twohighly related genes encoding SCM-1/human lymphotactin. FEBS Lett 1996;395:82–8.
[124] Kennedy J, Kelner GS, Kleyensteuber S, Schall TJ, Weiss MC, Yssel H, et al. Molecular cloning and func-tional characterization of human lymphotactin. J Immunol 1995;155:203–9.
[125] Bianchi G, Sozzani S, Zlotnik A, Mantovani A, Allavena P. Migratory response of human natural killercells to lymphotactin. Eur J Immunol 1996;26:3238–41.
[126] Shan L, Qiao X, Oldham E, Catron D, Kaminski H, Lundell D, et al. Identification of viral macrophage in-flammatory protein (vMIP)-II as a ligand for GPR5/XCR1. Biochem Biophys Res Commun 2000;268:938–41.
[127] Dilloo D, Bacon K, Holden W, Zhong W, Burdach S, Zlotnik A, et al. Combined chemokine and cytokinegene transfer enhances antitumor immunity. Nat Med 1996;2:1090–5.
[128] Imai T, Hieshima K, Haskell H, Baba M, Nagira M, Nishimura M, et al. Identification and molecular char-acterization of fractalkine receptor CX3CR1, which mediates both leukocyte migration and adhesion. Cell1997;91:521–30.
[129] Raport CJ, Schweickart VL, Eddy RL, Shows TB, Gray PW. The orphan G-protein-coupled receptor-encoding gene V28 is closely related to genes for chemokine receptors and is expressed in lymphoid andneural tissues. Gene 1995;163:295–9.
J. David, F. Mortari/Clin. Applied Immunol. Rev. 1 (2000) 105–125 125
[130] Combadiere C, Ahuja SK, Murphy PM. Cloning, chromosomal localization, and RNA expression of a hu-man beta chemokine receptor-like gene. DNA Cell Biol 1995;14:673–80.
[131] Foussat A, Coulomb-L’Hermine A, Gosling J, Krzysiek R, Durand-Gasselin I, Schall T, et al. Fractalkinereceptor expression by T lymphocyte subpopulations and in vivo production of fractalkine in human. Eur JImmunol 2000;30:87–97.
[132] Combadiere C, Salzwedel K, Smith ED, Tiffany HL, Berger EA, Murphy PM. Identification of CX3CR1. Achemotactic receptor for the human CX3C chemokine fractalkine and a fusion receptor for HIV-1. J BiolChem 1998;273:23,799–804.
[133] Bazan FJ, Bacon KB, Hardiman G, Wang W, Soo K, Rossi D, et al. A new class of membrane-boundchemokine with a CX3C motif. Nature 1997;385:640–4.
