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Institute of Molecular Medicine and Experimental Immunology (IMMEI), Friedrich-Wilhelms-Universität, Bonn, Germany
| Abstract |
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| Introduction |
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-glucuronidase and procollagen, which are up-regulated in the blood serum during inflammation (2). The MR consists of an N-terminal cysteine-rich domain, a fibronectin type II repeat domain, eight carbohydrate recognition domains (CRD), a transmembrane domain, and a short intracellular region (3). The cysteine-rich region mediates binding to sulfated sugar moieties, whereas the CRD bind glycoproteins bearing (for instance) terminal mannose, fucose, and, with a lower affinity, glucose residues (4). Most studies addressing the function of the MR have used macrophages (5), which use it for uptake of mannosylated structures such as dextrans (1). In addition to these cells, the MR has also been detected in liver endothelial cells, dermal microvascular endothelial cells, monocytes, Langerhans cells, and dendritic cells (DC) (6).
DC play a central role in the induction of adaptive immune responses (7). After capturing and internalizing Ag in peripheral organs, they migrate toward the draining lymph nodes, where they can activate naive T cells. For activation of CD8+ T cells, captured extracellular Ag are presented on MHC class I molecules (8)a process termed cross-presentationwhich contributes to the induction of cytotoxicity against many viruses and tumors (9). The murine DC subpopulation expressing the CD8
homodimer has been shown to be particularly relevant for cross-presentation of foreign Ag (10) and of self Ag under homeostatic conditions (11, 12). In the presence of inflammatory stimuli, also CD8
-deficient DC were able to cross-present (13, 14).
Several receptors mediating Ag uptake in DC have been identified, such as Fc-receptors, DC-SIGN and DEC205. A role of the MR in Ag uptake and presentation by DC has been proposed based on the finding that mannosylated proteins are presented more efficiently than nonmannosylated ones (15, 16). It is unclear, however, whether this uptake was due to the MR, because DC express other receptors, such as DC-SIGN, with affinity for mannosylated proteins (6, 17). For the same reason, mannan, a polymer of mannose, which competitively blocks endocytosis of mannose-rich structures, cannot be considered a specific inhibitor of the MR. To overcome these limitations, we have used MR/ mice to elucidate the role of the MR in the uptake and presentation of soluble vs cell-associated OVA.
| Materials and Methods |
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MR/ mice on a C57BL/6 (B6) background were generated and provided by Dr. M. C. Nussenzweig (Rockefeller University, New York, NY) (2). B6 mice bearing the Kb mutant bm1 (bm1 mice) and OT-I Rag-1/ mice on a B6 background were provided by Dr. W. R. Heath (Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia) (18). For all experiments, mice between 8 and 16 wk of age bred under specific pathogen-free conditions were used in accordance with local animal experimentation guidelines.
Abs and reagents
All mAb used were purchased from BD Biosciences, except anti-MR Ab (Serotec) and SF1, which were purified from hybridoma supernatant (American Type Culture Collection) and used after conjugation with alexa488. All reagents, if not specified otherwise, were obtained from Sigma-Aldrich.
Generation of bone marrow-derived dendritic cells (BMDC)
BMDC were generated using GM-CSF as described previously (19). At day 7, CD11c+ cells isolated by magnetic separation with the autoMACS system (Miltenyi Biotec) were used for all in vitro experiments. Purity of CD11c+ cells was typically higher than 98%.
Preparation of fluorescent soluble and cell-associated OVA
Soluble OVA was conjugated to a fluorochrome using an alexa647 labeling kit (Invitrogen Life Technologies) according to the manufacturers guidelines. The labeling procedure involved gel filtration as a final step for removal of low molecular mass molecules such as unbound fluorochrome. For cell-associated OVA, splenocytes from bm1 mice (2 x 108 cells/ml) were incubated with 10 mg/ml OVA-FITC for 10 min at 37°C, UV-irradiated with 15 mJ for 5 min, and washed extensively. For cell culture experiments, 106 splenocytes were coincubated with 4 x 105 BMDC for 18 h.
Cross-presentation assays
OT-I cells were isolated from OT-I rag/ mice as previously described (18) and further purified by a nanobead-based CD8 T cell isolation kit (Miltenyi Biotec). Purity was typically higher than 96% of viable cells; contaminating CD11c+ cells were typically rarer than 0.2%, NK1.1+ cells rarer than 0.03%, and CD4+ cells below 1%. For presentation of soluble OVA, 4 x 105 BMDC were stimulated with 10 µg/ml LPS for 2 h and incubated with 500 µg/ml OVA or 20 nM SIINFEKL (OVA peptide). After another 3 h, cells were washed, fixed with 0.008% glutaraldehyde for 3 min, and coincubated with 2 x 105 OT-I cells. IL-2 concentrations were determined after 18 h by ELISA. For the analysis of cell-associated Ag, bm1 splenocytes were coated with OVA as described above. Splenocytes (106) were coincubated with 4 x 105 BMDC and 2 x 105 OT-I cells. IL-2 concentrations in the supernatant were determined by ELISA after 40 h.
Isolation of DC from experimental animals
Cells were isolated from spleen and cutaneous lymph node (cLN) as described before (12). For the preparations from bone marrow, cells were collected by flushing femurs and tibias with PBS. CD11c+ cells from all organs were enriched by magnetic separation using MS25 columns (Miltenyi Biotec). Purity was typically higher than 85%.
Flow cytometry, data analysis, and statistics
Flow cytometry was performed on an LSR (BD Biosciences). Dead cells were excluded by Hoechst-33342 dye. Data were analyzed using Flow-Jo software (Tristar), including calculation of division indices, which indicate the average number of cell divisions. Statistical analysis was done using Excel (Microsoft). All experiments reported here have been reproduced at least twice.
| Results |
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When we studied uptake of OVA in BMDC, we noticed that only some of the CD11c+ cells took up this model Ag in vitro (Fig. 1, A and B), as others have noted recently (20). We reasoned that this might be explained by selective expression of an endocytic receptor that mediated uptake of OVA. In support of this hypothesis, we observed a close correlation between the uptake of OVA and the extent of MR expression (Fig. 1A). To investigate whether this association was due to a functional role of the MR in Ag uptake, we preincubated the DC with mannan, which competitively inhibits MR-mediated endocytosis (21). This agent blocked the uptake of OVA completely (Fig. 1B). To exclude an influence of other receptors with affinity to mannan, we performed this experiment also with BMDC from MR/ mice (2). These cells did not show any uptake (Fig. 1, B and C), indicating that the MR was indispensable for endocytosis of soluble OVA by BMDC in vitro. Remarkably, under these experimental conditions, no other receptor appeared to compensate even partially for the absence of the MR.
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(Fig. 1D). CD11c and CD11b expression was identical in both BMDC populations (Fig. 1D), as was constitutive and LPS-induced expression of costimulatory molecules (Fig. 1E), suggesting equivalent states of maturation. Next, we investigated whether the MR was important also for the uptake of cell-associated OVA, using splenocytes loaded with fluorochrome-labeled OVA (Fig. 2A). To demonstrate intracellular uptake, loading was performed at 4°C and at 37°C, because intracellular uptake is energy-dependent, as opposed to extracellular coating. Indeed, uptake at 37°C was significantly higher than at 4°C, suggesting that some of the Ag was transported into the spleen cells. We then induced apoptosis by UV irradiation and cocultured these cells with BMDC from MR/ mice or controls. After 18 h, the uptake of cell-associated OVA by wild-type and MR/ cells was indistinguishable (Fig. 2B), indicating that receptors other than the MR had mediated endocytosis of cell-associated OVA.
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The requirement of the MR for OVA uptake suggested a role in Ag presentation. To address this hypothesis, we studied cross-presentation of OVA in coculture experiments with BMDC prepared from B6 or MR/ donor mice and OVA-specific CD8 T cells isolated from transgenic OT-I mice. Their activation was monitored by measuring IL-2 release into the culture supernatant. This release correlated with the Ag amount in a dose-dependent fashion (Fig. 3A). To ensure that OT-I cell activation was in fact due to cross-presentation, and not to coating of the BMDC with peptide fragments present in the OVA solution, we performed incubation with soluble OVA in the presence of the proteasome inhibitor MG132, which inhibits intracellular generation of peptides for loading onto MHC class I molecules (24). This inhibitor prevented IL-2 release nearly completely (Fig. 3A), indicating that OT-I cells were indeed activated by OVA peptides generated intracellularly. MG132 did not affect OT-I cell activation by BMDC coated with the OVA peptide SIINFEKL, which is recognized by OT-I cells, demonstrating that in this experimental setting, the ability of OT-I cells to produce IL-2 was not compromised (Fig. 3A).
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Because the uptake of cell-associated OVA was not reduced in MR-deficient DC, we tested whether OT-I cell activation remained operative as well. To this end, we loaded splenocytes from bm1 mice with OVA. These mice bear a mutant H2-Kb protein that cannot present OVA to OT-I cells (18). After coating with OVA, apoptosis was induced by UV irradiation. These cells were then cocultured with wild-type and MR-deficient BMDC and OT-I cells. No differences in T cell activation between the two DC types could be observed in vitro (Fig. 3C), indicating that the MR not only was dispensable for the uptake but also for intracellular processing of cell-associated OVA for cross-presentation.
The MR contributes to in vivo uptake of soluble but not of cell-associated OVA by DC
Next, we decided to investigate the in vivo role of the MR in Ag uptake. To this end, we injected fluorochrome-labeled soluble OVA into B6 mice. Consistent with our in vitro findings, a close correlation between the uptake of soluble OVA and the expression of the MR in DC was found (Fig. 4, A and B). DC from MR/ mice showed significant but incomplete reduction of uptake of soluble OVA (Fig. 4, C and D). This was most evident in the spleen and in the bone marrow, which have been described as locations in which cross-presentation takes place (8, 25). In the cLN, only moderate uptake was observed, which was further reduced in MR/ mice, albeit not significantly (Fig. 4, C and D). This reduction was not due to changes of the DC subpopulations present in MR/ mice, because these were indistinguishable from those in wild-type mice (Fig. 4E). These findings demonstrated that the MR was involved also in the in vivo uptake of soluble OVA, but it was not essential, as opposed to its role in vitro. Indeed, in wild-type mice we found some DC that had taken up OVA but did not express the MR (Fig. 4B). The mechanisms that partially compensated for the absence of the MR in vivo remain to be identified.
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To assess the in vivo role of the MR for CD8 T cell activation by cross-presentation, we injected CFSE-labeled OT-I transgenic T cells into wild-type or MR-deficient recipient mice. After priming with soluble OVA, we analyzed the proliferation of OT-I cells in spleen, bone marrow, and cLN. In MR/ mice, proliferation of these T cells was substantially reduced (Fig. 6A). Also, CD69 expression on OT-I cells was substantially diminished on day 1 in all organs tested in MR/ mice (data not shown), demonstrating that the diminished proliferation of OT-I cells was due to decreased activation at the priming site, rather than to reduced recirculation of OT-I cells activated elsewhere.
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| Discussion |
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+ DC, which have been reported to mediate in vivo cross-presentation of soluble and cell associated OVA (10, 14).
As opposed to the clear-cut in vitro situation, uptake and cross-presentation of soluble OVA in vivo was not completely abolished in MR/ mice, yet it was significantly reduced. This may indicate that BMDC do not mimic all mechanisms involved in Ag uptake by DC in vivo. These mechanisms may involve the generation of OVA peptides by endogenous proteases, or Ag uptake by additional cellular receptors, such as the lectins DC-SIGN and DEC205, whose expression might differ between BMDC and DC in living animals (17, 27). This may be the case for CD8
+ DC that were shown to mediate cross-presentation in vivo (10, 14), because these are known to be absent from BMDC generated by standard methodology (22).
Uptake of cell-associated OVA and the resulting activation of OT-I cells were not impaired by the absence of the MR, either in vivo or in vitro. The cell-associated Ag used in the present study was located intracellularly, at least in part. Such Ag may be available for endocytosis by one or several distinct receptors. Reports implicating CD36 in uptake of Ag borne by apoptotic cells are controversial (28). Thus, the mechanisms mediating Ag uptake for classical cross-priming, as described by Bevan (8), remain to be resolved. Nevertheless, our findings demonstrated that additional pathways exist for cross-presentation of Ag carried by microorganisms, or of viral Ag within infected host cells.
The MR is mostly known for its role as a scavenger receptor of macrophages. A role in Ag presentation has been proposed by others based on its expression by DC and on its role in the uptake of mannosylated structures such as dextrans or horseradish peroxidase in a mannan-blockable fashion into cellular compartments that contained MHC class II molecules (21). Such uptake could result in stimulation of CD4 T cell clones in vitro (16), suggesting a role for the MR in adaptive immunity. The findings reported here support such a role by demonstrating MR-mediated activation of naive CD8 T cells, and by providing evidence for in vivo relevance of such activation. Furthermore, the use of MR/ mice in the present study precluded the specificity concerns that apply to mannan blockade of the MR (6).
Our findings suggest that targeting of the MR may be useful for introducing extracellular Ag into the MHC class I-restricted Ag presentation pathway. Targeting of a related lectin, DEC205, has been shown to result in tolerogenic CD4 and CD8 T cell activation (27). It will be interesting to investigate whether Ag uptake by the MR would result in immunogenic or tolerogenic CTL priming, or both, depending on the absence or presence of inflammatory stimuli. Finally, the indispensability of the MR for uptake of soluble OVA by BMDC may allow further in vitro dissection of intracellular mechanisms governing cross-presentation.
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This work was supported by was supported by a junior research group grant from the German state of Nordrhein-Westfalen (to C.K.). S.B. was supported by BONFOR Grant O-173.0009 of the University of Bonn Clinic, Bonn, Germany. ![]()
2 S.B. and V.L-K. contributed equally to this work. ![]()
3 Address correspondence and reprint requests to Dr. Christian Kurts, Institute of Molecular Medicine and Experimental Immunology, Friedrich-Wilhelms-Universität, 53105 Bonn, Germany. E-mail address: ckurts{at}web.de ![]()
4 Abbreviations used in this paper: MR, mannose receptor; DC, dendritic cell; OT-I cell, OVA-specific CD8 T cell; CRD, carbohydrate recognition domain; bm1 mice, C57BL/6 mice bearing the Kb mutant bm1; BMDC, bone marrow-derived DC; cLN, cutaneous lymph node; MFI, mean fluorescence intensity. ![]()
Received for publication January 26, 2006. Accepted for publication March 15, 2006.
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