Optimal Dendritic CellDifferentiation in RPMI MediaRequires the Absence of HEPESBuffer
Urban Švajger1,2 and Matjaž Jeras3,4
1Blood Transfusion Centre of Slovenia, Slajmerjeva 6, Ljubljana, Slovenia2Faculty of Pharmacy, Department of Pharmaceutical Biology, Aškerceva 7, SI-1000,Ljubljana, Slovenia3Faculty of Pharmacy, Department of Clinical Chemistry, Aškerceva 7, SI-1000,Ljubljana, Slovenia4Celica, Biomedical Center, Technology Park 24, SI-1000 Ljubljana, Slovenia
Monocyte-derived dendritic cells (DCs) are considered an indispensible and one of pri-mary tools for in vitro DC-based studies. For majority of in vitro DC-based studiesthe medium of choice is supplemented RPMI, with certain variable ingredients suchas HEPES buffer or Phenol Red (PHR). In effort to identify potential obstruction ofDC differentiation process due to presence of mentioned additives, we differentiatedDCs using RPMI either with or without HEPES or PHR. Although PHR caused acertain down-regulation of immature DCs (iDCs) differentiation markers and lowerexpression of co-stimulatory molecules on mature DCs, these changes were not sig-nificant. In contrast, use of RPMI also containing HEPES resulted in significantlylower CD1a and DC-SIGN expression on iDCs and extensively lowered co-stim-ulatory molecule expression after DC activation (HEPES-DCs). Furthermore, DCsdifferentiated in HEPES-free RPMI possessed more genuine immature/mature DCcharacteristics in context of Th1 polarization. Additionally, during classical differentia-tion procedure, fewer DCs remained adherent and possessed better overall morphologyin HEPES-free medium. In summary our study clarifies a seemingly minor, but a veryimportant issue, that will most likely facilitate lab work for many scientists dealingwith monocyte-derived DCs.
Keywords Dendritic cells, RPMI, Monocytes.
Address correspondence to Urban Švajger, Tissue Typing Centre, Blood TransfusionCentre of Slovenia, Šlajmerjeva 6, SI-1000, Ljubljana, Slovenia; E-mail: [email protected]
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INTRODUCTION
Dendritic cells (DCs) are defined as professional antigen-presenting cells(APCs) with the main task of capturing and presenting antigens from the out-side world (as well as endogenous antigens) and transferring this informationto other cells of the adaptive immune system. Spectrum of DC activity is notlimited only to induction of effector immune responses, but they also largelycontribute to tolerance induction and to regulation of T cell-mediated immuneresponse (Banchereau et al., 2000). In humans three basic subsets of DCs canbe defined: Dermal or Interstitial DCs (IDCs), Langerhans DCs (LCs) – both ofmyeloid origin and Plasmacytoid DCs (pDCs) – lymphoid origin. DCs producedin vitro most closely resemble interstitial or dermal DCs (Grassi et al., 1998).
Although high expression of CD1a is characteristic for all myeloid DCtypes, namely monocyte-derived DCs, IDCs and LCs, only monocyte-derivedDCs and IDCs express high levels of surface C-type lectin DC-SIGN (Rossiand Young, 2005). Thus, high expression of both CD1a and DC-SIGN is centralfor determining the quality of in vitro differentiated DCs from monocytes. DCsdisplay broad functional and phenotypical plasticity, and addition of variouscytokines and other factors to DC cultures can crucially influence the out-come of differentiation process from monocytes. For example, the presence ofIL-6 switches differentiation process from DCs to macrophages (Chomarat etal., 2000). A similar observation was seen with IL-10 (Allavena et al., 1998).An opposite effect, preferred DC instead of macrophage differentiation, wasproven for TNF-α (Chomarat et al., 2003).
Addition of TGF-β along with GM-CSF and IL-4 to monocyte cultures wasshown to result in differentiation of cells resembling LCs (Geissmann et al.,1998). Factors other than cytokines were also shown to influence monocyte-DCdifferentiation. We have shown that niflumic acid, a non-steroidal anti-inflammatory agent with additional chloride channel inhibitory potential,greatly influences DC differentiation and activation resulting in alternativelyactivated or tolerogenic DCs (Svajger et al., 2008).
The most popular cell culture medium used for DC culture in in vitro DC-associated studies is RPMI, frequently supplemented with a buffering agentHEPES and PHR as a pH indicator. Although commonly used, HEPES hasbeen reported with certain pharmacological activity, particularly as a potentialblocker of chloride channels (Yamamoto et al., 1987). PHR on the other hand,has been reported to possess weak estrogenic activity (Berthois et al., 1986).As such, HEPES or PHR could as well influence DC differentiation in HEPES-containing media, resulting in suboptimal DC differentiation.
In the present study, we report for the first time that monocyte-derivedDCs, generated using HEPES-free RPMI display more genuine DC charac-teristics than DCs generated in RPMI containing HEPES, while PHR hadinsignificant effects. DCs generated in HEPES-free RPMI ((-)HEPES-DCs)express greater levels of differentiation markers than DCs generated in
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HEPES Affects Dendritic Cell Differentiation 415
HEPES-containing RPMI ( (+)HEPES-DCs). (-)HEPES-DCs also display bet-ter activation potential by LPS, expressing greater levels of co-stimulatorymolecules and HLA-DR after activation than (+)HEPES-DCs. Additionally,(-)HEPES-DCs were more proficient than (+)HEPES-DCs at polarizing Th1immune responses.
MATERIALS AND METHODS
Cell Preparation and CultureBuffy coats from the venous blood of normal healthy volunteers were
obtained from the Blood Transfusion Centre of Slovenia, according to insti-tutional guidelines. Peripheral blood mononuclear cells (PBMCs) were iso-lated using Lympholyte
®-H (Cedarlane laboratories, Ontario, Canada). The
cells were washed twice with Dulbecco’s phosphate-buffered saline (DPBS),counted, and used as a source for immunomagnetic isolation of CD14-positivecells (CD14 Microbeads, Miltenyi Biotec GmbH, Bergisch Gladbach, Germany).CD14-positive cells (purity of CD14+ cells was always greater than 95%,as determined by flow cytometry) were cultured in RPMI 1640 (Cambrex)medium either containing both PHR and HEPES, only PHR, or without eitherPHR or HEPES.
All cultures were performed in cell incubators at 37◦C in 5% CO2 atmo-sphere. By carefully examining the formulation of different RPMI media, aspresented by the manufacturer, all other ingredients besides HEPES and PHRwere present in the exact same amounts in all media used. For DC differenti-ation, the media were additionally supplemented with 10% fetal bovine serum(FBS), gentamicin (50 µg/ml; Gibco, Paisley, UK), 800 U/ml of rhGM-CSF and1000 U/ml of rhIL-4 (both Peprotech EC, London, UK).
On day 2, half of the medium was exchanged with the addition of startingquantities of rhGM-CSF and rhIL-4. After 5 days, non-adherent immature DCswere harvested and characterized by flow cytometry as CD1ahi, CD80−, CD83−,CD86low and HLA-DRlow. For maturation, DCs were activated with 20 ng/mlof lipopolysaccharide (LPS) along with addition of 800 U/ml of rhGM-CSF for48 h, using the same type of RPMI medium as for the differentiation process.
T-cells were purified from human buffy coats. Naive CD4+CD45RA+ wereisolated using the naive CD4+ T cell isolation kit from Milteny Biotec (BergischGladbach, Germany), strictly following the manufacturer’s protocol. The purityof naive CD4+ T cells was always greater than 98% as determined by flowcytometry.
Flow Cytometry AnalysisThe levels of membrane markers were determined by flow cytometry
using fluorescently labeled antibodies. Non-adherent cells were harvested
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416 U. Švajger and M. Jeras
on day 5 (immature DCs) or 7 (mature DCs) and collected by centrifu-gation. Antibody was added and the cells incubated at room temperaturefor 15 min in the dark. They were then washed twice with DPBS andresuspended in 2% paraformaldehyde. The following monoclonal antibodieswere used: FITC labeled anti-CD1a, anti-CD14, anti-CD80, anti-DCSIGN(all from Biolegend, CA, USA), FITC anti-CD83 (IQ Products, NL), FITCanti-CD86 (DakoCytomation, Denmark), and PE anti-HLA-DR (Santa CruzBiotechnology, CA, USA). Flow cytometry was performed using a FACSCalibursystem (Becton Dickinson, Inc.).
Endocytosis AssayTo analyze the endocytic capacity of the DCs mediated by the mannose
receptor, 1x105 cells were incubated at 37◦C for 1 h with 1 mg/ml of FITClabeled dextran (Sigma). After incubation, the cells were washed twice withDPBS, resuspended in 2% paraformaldehyde and analyzed by flow cytome-try. For controls, parallel experiments were performed by incubating cells withFITC-dextran at 4◦C.
Morphological AnalysisMicroscopy was performed in parallel to all other analyses to assess cell
morphology by using the inverted light optical microscope (Nikon EclipseTE300, Tokyo, Japan).
Intracellular Cytokine AnalysisTo determine cytokine production by CD4+ T cells on a single cell level
we stained the cells intracellularly using FITC-anti-IFN-γ and PE-anti-IL-10Abs (both from Invitrogen, San Diego, CA). For gating purposes we used Cy5-PE-anti-CD4 Ab (Biolegend, CA, USA). Briefly, 1x105 MoDCs were co-culturedwith 1x106 naïve CD4+CD45RA+ T cells in 24-well tissue culture plates usingeither HEPES-free or HEPES-containing RPMI medium (same as for differen-tiation of either (-)HEPES-DCs or (+)HEPES-DCs), supplemented as describedhere. After 7 days of co-culture, CD4+ T cells were collected and fixed with 4%PFA for 1 h.
After fixation, the cells were permeabilized using 0.1% Triton X-100 for10 minutes and washed twice with DPBS. To prevent non-specific staining,the cells were incubated for 45 min DPBS containing 3% BSA. Cells werestained with FITC-anti-IFN-γ and PE-anti-IL-10 Abs for 45 min in the darkand afterwards washed twice with DPBS and analyzed on a flow cytome-ter. Flow cytometry was performed using a FACSCalibur system (BectonDickinson, Inc.).
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RESULTS
HEPES, Not PHR, Significantly Lowers the Expression of CD1a andDC-SIGN Differentiation Markers on DCsTo determine the effect of PHR and HEPES in culture media on phe-
notype of differentiated DCs, we performed flow cytometric analysis of keysurface markers denominating proper DC differentiation from monocytes.To determine the effects of either PHR or HEPES on DC culture, we usedHEPES-free, PHR-free RPMI, RPMI containing only PHR and RPMI contain-ing both PHR and HEPES. At their immature stage, (-)HEPES-DCs expressedsignificantly greater levels of CD1a and DC-SIGN than (+)HEPES-DCs(Fig. 1A). The expression of CD14 was negative on all cell types, as expected(Table 1).
On immature cells, the expression of CD80, CD83 and CD86 was simi-lar for both (-)HEPES-DCs and (+)HEPES-DCs (Fig. 1B). However, expressionof HLA-DR was greater on DCs, generated in HEPES-free RPMI (Fig. 1B).When only PHR was present in RPMI medium, the differentiated iDCs didshow slightly down-regulated CD1a expression, although this was not signif-icant compared to controls (Table 1). Surprisingly, DCs differentiated in PHRcontaining RPMI had increased surface expression of DC-SIGN (Table 1).
After a 5-day differentiation period, non-adherent DCs were collected andcounted and the results compared between DCs generated in HEPES-freeor HEPES-containing RPMI. Interestingly, DCs generated using HEPES-freeRPMI were obtained in significantly greater yields than DCs cultured inHEPES-containing RPMI. In four independent samples analyzed, DCs weregenerated with 30.3 ± 11.7% greater yield in RPMI without HEPES comparedto RPMI with HEPES. The presence of PHR in RPMI caused inconstant vari-ations in DC yield; however, the differences were never greater than 5%.
Greater Activation Potential of DCs Generated in HEPES-free RPMITo see if the above-mentioned effect of HEPES on DC differentiation also
reflects on the ability of DCs to respond to LPS-induced activation, we cul-tured (-)HEPES-DCs and (+)HEPES-DCs for another 2 days in the presence ofstrong maturation factor LPS. DCs generated in HEPES-free RPMI expressedsignificantly greater levels of both CD80 and CD86 upon LPS activation thantheir DC counterparts generated in HEPES-containing RPMI. Additionally,LPS activation induced more than 2-fold greater expression of surface HLA-DR on (-)HEPES-DCs than it did on (+)HEPES-DCs (Fig. 1B). Although lowerexpression of co-stimulatory molecules and HLA-DR was found on matureDCs differentiatied in RPMI containing only PHR, these differences were notsignificant (Table 1).
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HEPES Affects Dendritic Cell Differentiation 419
Table 1: Shown are mean fluorescence intensities (MFI) for various differentiation andmaturation markers of DCs cultured in either HEPES- and PHR-free RPMI (-HEPES, -PHR), HEPES-free and PHR-containing RPMI (-HEPES, +PHR) or RPMI containing bothHEPES and PHR (+HEPES, +PHR). Statistical comparison between individual pairs(specific marker of either +HEPES, +PHR or –HEPES, +PHR compared to –HEPES, -PHR)was done using Student’s unpaired t- test.
Results are representative of four independent experiments (∗ - p<0.05, ns – non-significant).
To further determine any possible the effects exerted by PHR, we addedPHR to HEPES-free, PHR-free medium in doses 10-fold greater than thatof original formulation. PHR exerted no greater effects even at such highconcentrations (data not shown). Since the additional presence of HEPES inRPMI so extensively affects DC differentiation and activation, further studiesconcerning DC function were performed using only medium containing PHRand HEPES in comparison to HEPES-free, PHR-free RPMI. A formulationof RPMI containing only HEPES without PHR is not commercially available.However, to determine distinct effects of HEPES, this did not present a majorinconvenience, due to relatively minor effects of PHR.
(-)HEPES-DCs Possess Better Morphology than (+)HEPES-DCsBy using inverted light microscopy, we assessed morphology of day 5 imma-
ture DCs generated in either HEPES-free or HEPES-containing RPMI. A clearsign of optimal DC differentiation is their detachment from the surface within24 h, due to podosome disassembly, cytoskeletal reorganization and inactiva-tion of specific adhesion receptors (11). However, frequently a certain percent ofcells remains attached throughout the differentiation process and are usuallynot collected at the end, since they can resemble a more macrophage-like celltype. When assessing and comparing morphology of DCs generated in RPMIeither with or without HEPES, the ones generated in RPMI without HEPESdisplayed much better overall morphology. In cultures without HEPES, prac-tically all DCs were non-adherent after 24 h. On the other hand, in cultureswhere RPMI with HEPES was used, several cells were attached and displayedfaulty morphology (Fig. 2).
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Figure 2: DCs generated in HEPES-free RPMI (-HEPES) possess better overall morphology thantheir counterparts differentiated in RPMI with HEPES (+HEPES). Inverted light microscopy wasused to assess the overall morphology of day 5 immature DCs for basic DC morphologicalcharacteristics. (-)HEPES-DCs were practically 100% non-adherent (right picture) on day 5 andpossessed better morphology than (+)HEPES-DCs, where at the end of differentiation period,still a notable percentage of cells remained attached with faulty morphology (left picture).One representative experiment out of 3 performed is shown.
Similar Endocytotic Capacity for (-)HEPES-DCs and (+)HEPES-DCsBecause the presence of PHR in culture media had significant effect
on DC differentiation markers and co-stimulatory molecules, we wanted tosee if it can also affect DCs’ mannose receptor-mediated endocytotic capac-ity. DCs generated in either HEPES-free or HEPES-containing RPMI wereincubated with FITC-dextran and analyzed by flow cytometry to determinedextran uptake. Although slightly greater endocytotic potential was observedwith (-)HEPES-DCs compared to (+)HEPES-DCs, these differences were notsignificant neither for immature or mature DCs (Fig. 3).
Figure 3: DCs differentiated in either HEPES-free or HEPES-containing RPMI have similar endocy-totic capacity. DCs differentiated in RPMI either with (+HEPES) or without HEPES (-HEPES) wereassessed for their capacity to endocytose via mannose receptor. Although a slightly greaterendocytotic capacity of immature (-)HEPES-DCs was noted in comparison to (+)HEPES-DCs,these differences were not statistically significant (Student’s t-test between individual pairs).Data shown is representative of three independent experiments.
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(-)HEPES-DCs Possess More Profound FunctionalImmature/Mature DC Characteristics than (+)HEPES-DCsAlthough fully mature DCs are known to have the ability to induce strong
Th1 polarized immune responses, immature DCs on the other hand, inducea more Th0 response (intermediate production of IFN-γ , IL-4 and IL-10) inshort-term co-cultures (multiple stimulations with immature DCs are knownto generate Tr1-like regulatory T cells (Jonuleit et al., 2000)). When DCswere cultured with allogeneic naive CD4+ T cells, immature (-)HEPES-DCsinduced CD4+ T cells that produced less IFN-γ upon activation than CD4+
T cells co-cultured with (+)HEPES-DCs (Fig. 4). On the other hand, a simi-lar but inverse observation was made when using LPS-activated DCs. Mature(-)HEPES-DCs induced a more extensive Th1-like response in respondingallogeneic naive CD4+ T cells than did mature (+)HEPES-DCs. Strikingly,mature (-)HEPES-DCs were able to generate approximately 2-times moreIFN-γ -producing CD4+ T cells than did mature (+)HEPES-DCs (Fig. 4).
DISCUSSION
In the present study we have addressed for the first time the issue of DC cul-ture in classical and widely used cell culture medium RPMI either containingor not, a well known pH indicator PHR and HEPES, a buffering agent. PHRhas been shown to possess weak estrogenic activity in a study conducted morethan 20 years ago by Berthois et al. (1986) where they warned about the futureuse of PHR in culturing of cells, responsive to estrogens.
Estrogen in particular, has been shown to exert immunomodulating effectson many immune cell types including effector T cells (Polanczyk et al., 2006),regulatory T cells (Tai et al., 2008), B cells (Grimaldi, 2006), macrophages(Lambert et al., 2005), as well as DCs (Mao et al., 2005; Pettersson et al.,2004; Zhu et al., 2007). On the other hand, data on pharmacological activity ofHEPES are scarce, although it has been shown a while ago that HEPES cancause chloride channel blockage (Yamamoto and Suzuki, 1987).
In this article, we show that particularly HEPES, present in cell culturemedia, significantly influences DC differentiation from monocytes. DCs gener-ated from monocytes in the presence of GM-CSF and IL-4 are the most widelyused model for in vitro studies associated with DCs and RPMI, frequently con-taining HEPES and PHR, is still the medium of choice in most cases. Thequality of DCs differentiated from monocytes in vitro, can be checked duringand after DC culture by certain indicators. During the differentiation process,monocytes lose surface CD14 and begin to express CD1a.
Phenotypically, only DCs that are CD14−, CD1a+ are considered fully dif-ferentiated. Another indicator is the suspension state of DCs in cell culture.This is not the case for the whole starting population of monocytes, since
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during the differentiation process, some cells can remain attached through-out the differentiation period. Such attached cells are sometimes referred toas macrophage-like cells, show CD14 expression and are usually not collectedtogether with properly differentiated DCs, since they would not possess simi-lar functional and phenotypical characteristics needed for the study intended.Both DCs generated in either RPMI with or without PHR or HEPES/PHRdid not show CD14 expression on day 5. However, (-)HEPES-DCs showedsignificantly higher expression of CD1a than (+)HEPES-DCs (Fig. 1A).
Furthermore, the expression of DC-specific surface lectin DC-SIGNon immature (-)HEPES-DCs was more than 3-fold greater than that of(+)HEPES-DCs (Fig. 1A). Due to extremely low number of reports on HEPESaltering cellular functions, it is very difficult to explain the observed effects onDC phenotype. However, it is known that, due to DC functional and phenotyp-ical plasticity, the differentiation of DCs is an extremely delicate process, andit can be accepted that any potential cellular activity exerted by HEPES, espe-cially due to its high content in RPMI (approx. 25 mM), could lead to affect DCdifferentiation.
Both CD1a and DC-SIGN are markers expressed at high levels onmonocyte-derived DCs and IDCs, but not on LCs, which express e-cadherinand langerin (Rossi et al., 2005). In this context, as an estrogen agonist, effectsof PHR on DC differentiation could work in the way of directing the differenti-ation process slightly towards LC cell type, since the same effect but in greatermagnitude, has been shown for estrogen (Mao et al., 2005). However, use ofRPMI containing only PHR resulted in minor down-regulation of CD1a, whileDC-SIGN expression remained high indicating that PHR by its own affects DCdifferentiation only slightly compared to HEPES.
Furthermore, since PHR exerted a certain, although minor effect on DCphenotype, we decided to supplement PHR-free, HEPES-free RPMI with PHRdoses exceeding the general formulation by 10-fold. Even in this case, PHRwas unable to cause any significant effect on DC maturation in terms of CD1a,DC-SIGN and CD14 expression, as well as DC morphology and yield (data notshown).
When analyzing the morphology of differentiating DCs, (-)HEPES-DCspossessed better overall appearance than (+)HEPES-DCs. Cells in cultureswith HEPES-free RPMI were practically 100% non-adherent in all cases, asopposed to DCs in RPMI with HEPES where a notable amount of cells wasattached to bottom surface with faulty morphology (Fig. 2).
The high capacity of immature DCs to endocytose and process Ags is theirprimary functional feature and emphasizes their role as professional Ag pre-senting cells (Sallusto et al., 1995). To evaluate whether PHR present in cul-ture media during DC differentiation also influences this aspect of DC feature,we studied the ability of DCs to endocytose FITC-dextran. No significant differ-ences in FITC-dextran endocytosis was measured between (-)HEPES-DCs and
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(+)HEPES-DCs (Fig. 3), although a slightly greater dextran uptake ability canbe seen for (-)HEPES-DCs.
The most general feature of fully mature DCs activated with TLR agonistssuch as LPS, is the strong up-regulation of co-stimulatory molecules CD80and CD86 along with induced expression of MHC class II molecules. Due toincreased production of pro-inflammatory and Th1-polarizing cytokines suchas IL-12, activated DCs have the ability to induce strong Th1 effector immuneresponses. When activated with LPS for 48 hours, (-)HEPES-DCs displayedsignificantly greater up-regulation of both CD80 and CD86 (approx. 2-fold)compared to (+)HEPES-DCs (Fig. 1B).
Additionally, HLA-DR expression on mature (-)HEPES-DCs was alsogreater than that of mature (+)HEPES-DCs (Fig. 1B). To evaluate if therealso exist differences in the ability of both DC types to induce Th1 polar-ization in responding CD4+ T cells, we analyzed intracellular cytokine pro-duction in CD4+ T cells after 7 day allogeneic co-culture with either DCtype. Interestingly, both immature and mature DCs generated in HEPES-freemedium displayed a more genuine characteristic known for either immatureor mature DC type.
In this manner, immature (-)HEPES-DCs generated less IFN-γ -secreting Tcells than immature (+)HEPES-DCs, as with mature cells, (-)HEPES-DCs gen-erated more IFN-γ -secreting T cells than (+)HEPES-DCs (Fig. 4). This leadsto conclude that DCs differentiated in HEPES-free RPMI are more immaturethan DCs generated in HEPES-containing RPMI. The latter could show tobe useful in such cases as regulatory T cell generation protocols, where lowactivation status of DCs is important (Roncarolo et al., 2001).
In summary, it is difficult to speculate the mechanisms behind theoverall effect of HEPES on the quality of DC cultures. However, HEPEShas been shown to act as a chloride channel blocker (Yamamoto and Suzuki,1987). Furthermore, we have shown previously that a non-steroidal anti-inflammatory agent, niflumic acid, which also possesses chloride channel-blocking properties, influences DC phenotype and functions in a great way(Svajger et al., 2008).
To conclude, generation of DCs from monocytes in vitro remains to this daythe most widely used model for studies dedicated to these important APCs. Wehave shown that the use of HEPES-free RPMI medium is far superior for gen-eration of CD1ahigh, DC-SIGNhigh DCs than medium containing HEPES. SinceDCs possess great heterogeneity and functional plasticity, it is natural thattheir differentiation from DC precursors presents a delicate matter, influencedby many factors that can skew this differentiation towards various DC types.Although the presence of PHR in culture medium caused only minor effectson DC differentiation and activation, we suggest it to be avoided in DC cul-tures, due to possible additive or synergistic effects. In light of this simple butuseful study, we suggest the use of HEPES-free, PHR-free RPMI for studies
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dealing with any aspect of DC research, where optimal differentiation of aspecific DC-type is required.
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