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Institute of Medical Microbiology, Immunology and Hygiene, Technical University Munich, Munich, Germany
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Abstract |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In contrast to DCs, the role of B cells in T cell priming has been controversial. For example, elegant experiments implied that both resting and activated H-Y-presenting B cells induce T cell tolerance, yet memory T cells become reactivated (8, 9). In contrast, Ag specifically bound to surface Ig (sIg) on B cells was shown to induce CD4-T cell priming (10, 11), presumably due to efficient Ig-mediated endocytosis of Ag. Furthermore, engagement of Ig receptors by specific Ag was found to activate B cells and thus trigger up-regulation of costimulatory molecules on B cells both in vitro (12) and in vivo (13). Thus, at least two criteria required for the activation of naive CD4 T cells, e.g., TCR ligation and costimulation, appear to be met by Ag-specific B cells. However, little is known as to whether B cells can cross-present Ag via MHC class I to naive CD8+ T cells, and whether cross-presentation leads to cross-priming.
Covalent linkage of immunostimulatory CpG-DNA to proteinaceous Ag such as OVA results in DNA receptor-mediated endocytosis of Ag, Ag cross-presentation, and cross-priming of CD8 (14, 15) and CD4 T cells (16) by DCs. In this system, the function of Toll-like receptor 9 (TLR9) is restricted to the activation of immature DCs into professional APCs, while cross-presenting OVA. However, TLR9 is not involved in CpG-DNA-aided cellular uptake (15, 17).
Murine as well as subsets of human B cells express TLR9 and thus are sensitive to immunostimulatory CpG-DNA (18). Because TLR9 is not detectable at the cell surface, but at cytoplasmatic endosome-like organelles (19), we argued that CpG-DNA needs to be endocytosed to activate B cells via TLR9. It follows that Ag loading of B cells (20) via DNA receptor-mediated endocytosis with OVA covalently linked to CpG-DNA (CpG-DNA-OVA complex) may be possible and unravel whether B cells are able to route internalized OVA into the MHC class I pathway. In this study, we describe the ability of B cells to cross-present OVA and to cross-prime CD8+ T cells.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Cell lines and in vitro culture medium
EL-4 (H-2b) thymoma cells were purchased from the American Type Culture Collection (Manassas, VA). B3Z, a somatic T cell hybrid generated by fusing the OVA/Kb-specific cytotoxic clone, B3, with a lacZ-inducible derivative of BW5147 fusion partner (21), was kindly provided by B. L. Kelsall (National Institutes of Health, Bethesda, MD). Cells were cultured in RPMI 1640 supplemented with 10% (v/v) heat-inactivated FBS, 100 IU/ml penicillin G, 100 IU/ml streptomycin sulfate (all: Biochrom KG, Berlin, Germany), and 50 µM of 2-ME (Invitrogen, Karlsruhe, Germany) at 37°C/5% CO2.
Reagents
Chicken egg albumin (OVA) was obtained from Sigma-Aldrich (Seelze, Germany). Its dominant CD8 T cell epitope, the peptide SIINFEKL (OVA peptide 257–264), was custom synthesized by Research Genetics (Huntsville, AL). FITC-labeled OVA was purchased from Molecular Probes (Leiden, The Netherlands).
Phosphothioate-modified immunostimulatory CpG-ODN were custom synthesized by MWG Biotec (Ebersberg, Germany). The phosphothioated sulfhydryl-modified ODN (TriLink Biotechnologies, La Jolla, CA) used throughout this study consisted of 20 bases and contained a CpG motif (1668: 5'-S-TCCATGACGTTCCTGATGCT-3'); as control, the 20-mer non-CpG motif (1720: 5'-S-TCCATGAGCTTCCTGATGCT-3') was used. The ODNs were conjugated to OVA by incubation with the cross-linker sulfo-maleimidobenzoyl-N-hydroxysuccinimide ester (S-MBS; Pierce, Bonn, Germany) in a 50 mM EDTA-PBS buffer, pH 7.0, at a molar ratio of 1:10 for 1 h at room temperature. The sulfhydryl-modified ODN were reduced in a 50 mM 1,4-DTT-PBS solution. Subsequently, unbound S-MBS and 1,4-DTT were removed by chromatography on a Biorade P-6 gel column (Bio-Rad, Muenchen, Germany). Then the activated ODN were incubated with the linker-modified OVA at a molar ratio of 5:1 for 2.5 h at room temperature, and thereafter L-cysteine was added to quench reactive S-MBS. Free ODN were removed by FPLC on a Superdex 75HR column (Amersham Biosciences, Freiburg, Germany). Purified conjugates were analyzed on a 6–20% gradient SDS-PAGE and consecutively silver stained. To determine ratio of bound ODN on OVA, a 4–15% gradient nondenaturing, nonreducing PAGE was run and visualized using ethidium bromide staining. Protein concentration was determined by the Lowry method (Pierce), as described previously (14). The batches of all CpG-OVA conjugates used in this study had a ratio of 2.5 CpG-DNA molecules linked to 1 OVA molecule. That means that 50 µg of used 1668-OVA conjugate contained 50 µg of OVA linked to 2.85 nmol of 1668 CpG-DNA. The FITC-labeled conjugates were synthesized similarly and contained the same ratio of OVA-FITC to ODN. Escherichia coli-derived LPS was purchased from Sigma-Aldrich.
Generation of GM-CSF-cultured DC from bone marrow
Generation of GM-CSF-induced, bone marrow-derived DC cultures was performed, as previously described (14). Bone marrow cells were flushed out of femurs and tibiae, centrifuged, and after washing, cells were plated at 5 x 106 cells/10 ml in medium complemented with 18 ng/ml GM-CSF and cultured at 37°C/5% CO2. Cells were used after 6 days of culture.
Preparation of purified B cells
For preparation of highly purified, negatively selected, and thus untouched murine B cells, the MACS CD43 MicroBead system (Miltenyi Biotec, Bergisch Gladbach, Germany) was used. To this, spleens were minced, and after lysis of RBC, the cell suspensions were incubated with anti-CD43 (Ly-48) Microbeads for 30 min on ice. After washing, the cells were negatively selected by running through LS columns (Miltenyi Biotec). Negative selected cells were collected, and purity of each batch prepared was analyzed by FACS. Cells were used when no CD11c-positive cells and >96% B cells could be scored. To further rule out a role of potentially contaminating DCs, the MACS CD43 Microbead system negatively selected B cells were first stained with APC-labeled anti-CD11c (clone HL3) and PE-labeled anti-CD19 (clone 1D3), and stained cells were sorted for cells staining only with anti-CD19 PE using the high speed MoFlo cytometer (DAKO Cytomation, Hamburg, Germany). After sorting, the purity of B cells was >99%.
Preparation of CD8 T cells
To enrich CD8 T cells from LNs or spleen, the organs were minced and the cell suspension was incubated with PE-labeled Abs against CD4, MHC class II, and CD19 for 30 min on ice. Cells were then washed and incubated with anti-PE-Microbeads (Miltenyi Biotec) for additional 20 min. After washing, the labeled cells were negatively selected via a LS column (Miltenyi Biotec) and were used as CD8-positive cell fraction.
Listeria monocytogenes infection
Six- to 8-wk-old C57BL/6 mice were i.v. injected in the tail veins with recombinant L. monocytogenes-secreting OVA (L. monocytogenes-OVA, kindly provided by H. Shen (University of Pennsylvania School of Medicine, Philadelphia, PA) (22). L. monocytogenes-OVA were grown in brain heart medium. For primary infection, a sublethal dose of 
0.1 x LD50 (2000 bacteria for C65BL/6 mice) was injected. Reimmunization was performed by i.v. injection of 5 x LD50 bacteria. Mice were used 5 days after second infection.
Cross-presenting B cells; immunization of mice; chromium release assay
Highly purified B cells (1 x 107) were incubated with 50 µg/ml of 1668-OVA conjugate or OVA for 90 min. B cells were washed twice, and 2 x 107 cells were injected into either both hind footpads (s.c.) or the tail vein (i.v.) of 6- to 12-wk-old C57BL/6 mice. Draining LN and spleen were removed 7 days later, and single-cell suspensions were prepared. LN cells (3 x 106 cells) or spleen cells (5 x 106 cells) were cultured for additional 7 days in medium conditioned with 5 IU/ml rIL-2 and syngenic, irradiated, SIINFEKL (1 µM) pulsed or unpulsed (2 x 106 cells) spleenic feeder cells. Cytolytic activity was assayed via 51Cr release assay, essentially as described (14). SIINFEKL peptide-untreated EL-4 cells served as specificity control. Specific lysis was calculated according to the formula: percent specific lysis = (cpm (sample) – cpm(spontaneous release)/(maximum release) – cpm (spontaneous release)) x 100.
OVA uptake, B cell activation analysis, and mAbs used
To examine uptake of FITC-labeled CpG-OVA in vitro, purified B cells were exposed to FITC-labeled OVA (10 µg/ml), mixed with1668 CpG-DNA (10 µg/ml) or 1668-OVA-FITC conjugates (10 µg/ml) or with medium (30 min at 37°C), washed twice with ice-cold 3% FCS-PBS, and stained with APC-labeled anti-CD11c (clone HL3) and PE-labeled anti-CD19 (clone 1D3). Analysis was performed in a FACSCalibur flow cytometer (BD Biosciences, Heidelberg, Germany) (50,000 events/sample).
To analyze B cell activation by CpG-OVA complexes, purified B cells were incubated with 50 µg/ml OVA conjugated with 2.85 nmol of CpG-ODN. Cells were cultured for 24 h, and thereafter washed twice and stained with APC-labeled anti-CD11c (clone HL3), PE-labeled anti-CD19 (clone 1D3), FITC-labeled anti-CD40 (clone 3/23), and anti-CD86 (clone GL1). FACS analysis was performed using a FACSCalibur flow cytometer (BD Biosciences) acquiring at least 50,000 events per sample. FACS data were analyzed using CellQuest software (BD Biosciences). mAbs and corresponding isotype controls were purchased from BD Biosciences.
Cross-presentation assay
Cross-presentation of SIINFEKL after CpG-DNA-aided uptake of OVA by purified B cells was assayed, as previously described (14, 15), by measuring induction of lacZ activity in SIINFEKL/Kb-specific T cell hybridome B3Z (21). To this, 1 x 105 purified B cells were incubated with the indicated reagents for 18 h at 37°C, followed by washing twice, and transferred into 96-well plates in triplets. B3Z cells (1 x 105) were added to each well. After additional incubation at 37°C overnight, the cells were fixed with 0.5% glutaraldehyde for 10 min and incubated with X-Gal solution (23) at 37°C. After 6–8 h, blue B3Z cells were counted under the microscope.
B and T cell proliferation assay
Purified B cells (3 x 106 cells/ml), CpG-OVA conjugate (50 µg/ml), or OVA alone (50 µg/ml) loaded for 3 h were labeled with 5 mM CFSE for 10 min at 37°C. To stop the CSFE reaction, the cells were taken up in 10 ml of medium containing FCS for 5 min on ice. Then CSFE-labeled B cells were incubated in a 12-well plate at 37°C for 3 days. After washing twice, the cells were analyzed. For CD19-positive B cells, additionally stained with anti-B220 APC Ab (clone RA3-6B2; BD Biosciences), proliferation was analyzed using a FACSCalibur flow cytometer (BD Biosciences).
T cell proliferation was assayed using OVA (50 µg/ml) or CpG-OVA conjugate (50 µg/ml)-loaded, irradiated (3000 rad), purified B cells as stimulator cells via [3H]thymidine incorporation. To this, purified B cells were loaded with the relevant reagents for 1.5 h at 37°C, washed twice, and coincubated at a defined responder to stimulator ratio in 96-well round-bottom plates in triplets with murine CD8 T cells (5 x 104 cells/well) from either OT-1 (expressing a SIINFEKL-specific transgenic TCR) mice (responder to stimulator ratio 1:1) or L. monocytogenes (OVA-secreting)-primed mice (responder to stimulator ratio 1:4). After 48 h of incubation, 1 mCi [3H]thymidine was added to each well, followed by an 18-h incubation at 37°C. The assay was harvested using a Molecular Devices (Sunnyvale, CA) Micro 96 Harvester and analyzed with a gamma counter (Matrix 9600; Packard Instrument, Meriden, CT).
Ex vivo tetramer staining of primed SIINFEKL-specific, CD8-positive, CD62L low T cells
A total of 2 x 107 purified B cells was loaded for 1.5 h with either CpG-OVA conjugate (50 µg/ml) or OVA alone (50 µg/ml), washed twice, and injected either s.c. (hind footpad) or i.v. (tail vein) into C57BL/6 mice. Seven days later, the LN or spleen was analyzed for presence of SIINFEKL tetramer binding, and thus peptide-specific CD8-positive, CD62L low T cells, as described (24). In short, whole cell suspensions were depleted of RBC, followed by a triple staining with anti-CD8 APC (clone CD8
; Caltag, Hamburg, Germany), anti-CD62L FITC (clone MEL-14), and MHC SIINFEKL tetramer PE (H-2Kb/SIINFEKL (257–264/murine 
2-microglobulin streptavidin-PE)) for 1 h at 4°C. Additionally, an Fc block (CD16/CD32, 2.4G2; BD Biosciences) was used to avoid unspecific Ab binding. For life/death cell discrimination, cells were subsequently incubated with ethidium monoazide bromid (Molecular Probes, Eugene, OR). At least 1 x 105 CD8-positive events were acquired on a FACSCalibur flow cytometer (BD Biosciences) and analyzed with FlowJo software (Tree Star, Ashland, OR)
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Results |
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Previously, we have shown (14, 15) that immunostimulatory CpG-DNA motifs linked to protein (OVA) fulfill a dual role when exposed to DCs: first, CpG-DNA-aided, but TLR9-independent enhancement of OVA uptake, coupled with efficient Ag cross-presentation; second, TLR9-dependent activation of DCs into professional APCs, able to cross-prime naive CD8 T cells. Because murine B cells express TLR9 (25), and thus are CpG-DNA responsive, we first analyzed whether B cells exposed to CpG-OVA complexes display CpG-DNA-aided enhancement of OVA uptake. To this we conjugated CpG-DNA to FITC-labeled OVA (CpG-OVA-FITC conjugate) and exposed highly purified B cells to either CpG-OVA conjugate, a mixture of CpG-DNA plus FITC-marked OVA, or FITC-OVA alone. As shown in Fig. 1, purified B cells exposed to either OVA-FITC (10 µg/ml) or OVA-FITC (10 µg/ml) mixed with stimulatory CpG-DNA yielded in 1% FITC-positive B cells. In contrast, CpG-OVA conjugates (10 µg/ml) enhanced OVA uptake by B cells >40-fold. Next, we also compared the loading efficacy of B cells with that of bone marrow-derived DCs. When exposed to graded concentrations of FITC-labeled CpG-OVA conjugates, DCs turned out to be 10- to 20-fold more efficient to internalize CpG-OVA conjugates (Fig. 2).
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Because B cells loaded with CpG-OVA conjugates could generate the CD8 T cell epitope SIINFEKL complexed to MHC class I and in parallel became activated to display costimulatory molecules and to produce cytokines, we measured whether they can function as APCs able to cross-prime CD8 T cells. To this, highly purified (>99%) B cells from C57BL/6 mice devoid of DCs, as judged by FACS analysis, were first loaded with CpG-OVA conjugates. These loaded B cells (1 x 107) were injected s.c. into C57BL/6 recipients. After 7 days, the draining lymph nodes (LN) and spleen were harvested, and dissociated cells were restimulated in vitro. As shown in Fig. 6a, both the draining LN as well as the spleen of challenged mice contained primed cells that upon restimulation differentiated into SIINFEKL-specific CTL. These results in turn raised the question as to whether, upon adoptive transfer, B cells loaded with CpG-OVA complexes might undergo apoptosis and CpG-OVA complexes might in fact be represented by host DCs, known to ingest apoptotic cells (26). If so, T cells of MyD88-deficient C57BL/6 mice challenged with wt CpG-OVA-loaded B cells ought not to be cross-primed because TLR9-initiated signaling is defective in MyD88 knockout mice (27, 28, 29). As shown in Fig. 6b, adoptive transfer of CpG-OVA complex-loaded wt B cells (1 x 107) in MyD88-deficient mice primed OVA-specific T cells as effectively as in wt mice. These data excluded re-presentation by host DCs.
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0.07%), CpG-OVA conjugate-loaded B cells raised the frequency of SIINFEKL tetramer-binding CD8 T cells up to 0.3–0.5% (Fig. 7).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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To date, the role of B cells as APCs has been controversial. For example, both naive and activated B cells presented cell-associated H-Y Ag in a tolerogenic form to naive CD8 T cells (6, 8). In contrast, B cells have been shown to serve as APCs for CD4 T cells when surface Igs (sIgs) promote both the capture of Ags (10, 11) as well as B cell activation (12, 13). In addition, receptor-mediated endocytosis of Ag that is independent of sIg specificity on B cells has been described, an example being CD21 (complement receptor type 2) (31). In this study, we use a novel approach for enhanced Ag uptake by B cells, that is, exposure of B cells to CpG-Ag conjugates. Basically, we took advantage of the previously described dual role of CpG-OVA complexes (14, 15, 20), that is, enhanced endocytosis via a sequence-nonspecific DNA receptor (15, 17, 32) as well as sequence-specific activation via endocytosed CpG-DNA that meets TLR9 at cytoplasmatic endosome-like organelles (28, 29). This approach makes use of CpG-DNA as a vehicle for endocytosis of linked Ag and, upon endocytosis, as activator of Ag-loaded cells. As shown in Figs. 1–3, this sIg-independent mechanism allows Ag loading of 50% of B cells, which subsequently cross-present SIINFEKL epitope, as read out in vitro. Thus, under the conditions used, B cells shuttle exogenous Ag into the class I MHC presentation pathway.
In general, exogenous Ags enter the class II pathway and are selectively associated with class II MHC molecules (33, 34). The presentation of exogenous Ag in association with MHC class I molecules has been considered as a unique characteristic of DCs (5, 35). The data presented in this work provide compelling evidence that, besides DCs (14, 15), B cells also can cross-present endocytosed OVA linked to CpG-DNA. Although linked DNA of the CpG-OVA complex triggers endocytosis in an sIg-independent fashion, endocytosed CpG-DNA in addition activates via TLR9 the cross-presenting B cells to up-regulate costimulatory molecules, to proliferate, and to produce cytokines, including IL-12. It follows that B cells loaded with CpG-OVA conjugates disclose some features that hitherto were attributed to DCs. Furthermore, OVA cross-presenting B cells appear to be immunogenic. Following adoptive transfer of CpG-OVA complex-loaded purified B cells into syngenic recipients, CD8 T cells became primed and thus differentiated into CTL upon in vitro restimulation. In fact, SIINFEKL-specific naive CD8 T cells of recipient mice had undergone clonal expansion, as reflected by the increase in frequencies, of SIINFEKL tetramer-specific T cells in draining lymph nodes and spleen.
To rule out the possibility that adoptively transferred CpG-OVA complex-loaded B cells succumb to apoptosis and thus allow CpG-OVA complexes to be (re-)presented by host DCs ingesting apoptotic cells, we used as recipients mice incapable of MyD88 signaling. The fact that CpG-DNA refractory (27, 28) MyD88-/- mice effectively generated primed CD8 T cells when challenged with CpG-OVA-loaded wt B cells excluded this contention. Clearly, the availability of conditional DC knockout mice (36) will help to answer more definitively the question as to whether CpG-Ag complex-loaded B cells function in the absence of DCs as professional APCs.
Overall, these data raise several questions. For example, CpG-OVA complex-mediated loading of B cells may be unique both in terms of Ag quantity and Ag load. If so, cross-presentation and cross-priming by B cells may not occur under physiological conditions. Furthermore, the ability of CpG-OVA complex-loaded B cells to cross-present/cross-prime appears lower compared with DCs. Dose-response curves of B cells and DCs exposed to CpG-OVA-FITC revealed that DCs are 10- to 20-fold more efficient in CpG-OVA conjugate uptake (Fig. 2), which appears to translate in superior cross-presentation efficacy by DCs. Previous work established that compared with B cells, DCs are more efficient in presenting endogenous, naturally processed self epitopes to class II-restricted T cells, a hierarchy that does not depend on differential processing capacity, but correlates with expression of CTLA-4 ligands and ICAM-1 molecules (37). Perhaps the rate-limiting step of B cells to cross-present exogenous Ag is cellular uptake of exogenous Ag rather than limited processing capacity to generate MHC class I T cell epitopes. Although CpG-DNA linked to OVA was efficient in B cell activation, cellular uptake (loading) of B cells with linked OVA occurred only in 50% of B cells (Fig. 1), and was 10- to 20-fold less efficient as in DCs (Fig. 2). It follows that a quantitative comparison of B cell- and DC-mediated cross-prriming will require use of B cells and DCs equally loaded with CpG-OVA complexes. Our results also raise questions in regard to recent data implying that chromatin-Ig complexes activate autoreactive B cells by dual (linked) engagement of IgM and TLR9 (38). To date, all evidence available (14, 15) implies that DNA-protein particles need first to be endocytosed to engage intracellularily TLR9, while Ag receptors of B cells are membrane bound. As a consequence, we favor unlinked recognition of DNA-protein conjugates by Ag-specific B cells (38).
In conclusion, our data suggest that B cells have the basic ability to route exogenous Ag in the MHC class I presentation pathway. Furthermore, we provide forceful evidence that cross-presenting B cells can cross-prime CD8 cells in mice that have not been exposed to the Ag.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Hermann Wagner, Institute of Medical Microbiology, Immunology and Hygiene, Trogerstr. 9, 81675 Munich, Germany. E-mail address: h.wagner{at}lrz.tu-muenchen.de
3 Abbreviations used in this paper: DC, dendritic cell; LN, lymph node; S-MBS, sulfo-maleimidobenzoyl-N-hydroxysuccinimide ester; sIg, surface Ig; TLR, Toll-like receptor; wt, wild type.
Received for publication April 1, 2003. Accepted for publication November 24, 2003.
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receptor-mediated induction of dendritic cell maturation and major histocompatibility complex class I-restricted antigen presentation after immune complex internalization. J. Exp. Med. 189:371.This article has been cited by other articles:
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