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CUTTING EDGE |

Departments of
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Pathobiology and
Biology, University of Pennsylvania, Philadelphia, PA 19104
| Abstract |
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| Introduction |
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In this study, we investigate the ability of DC exposed to a combination of SEA plus Pa to mature and generate SEA- and Pa-specific Th responses in vivo. Interestingly we found that: 1) these copulsed DC were capable of inducing nonoverlapping Th1 and Th2 responses to Pa and SEA, respectively; 2) SEA inhibits the ability of Pa to induce IL-12 production by DC; and 3) SEA and Pa are segregated into distinct intracellular compartments. These results suggest that differential processing of SEA and Pa, along with SEA-mediated modifications of the Pa-induced DC maturation pathway, explain the ability of DC to interpret signals inherent to SEA and Pa and to simultaneously induce Th responses appropriate to each Ag.
| Materials and Methods |
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Female wild-type C57BL/6 (B6) mice were purchased from The Jackson Laboratory (Bar Harbor, ME). Pa was obtained from The Van Kampen Group (Hoover, AL) and endotoxin-free SEA was prepared as described previously (1, 10).
DC preparation
DC were cultured from bone marrow in the presence of GM-CSF (PeproTech, Rocky Hill, NJ) as described elsewhere (1, 11). For activation, DC were pulsed with 50 µg/ml SEA, 10 or 50 µg/ml Pa, or a mixture of these Ag at these concentrations for the final 18 h of culture.
Confocal microscopy and flow cytometry.
SEA was labeled with Alexa Fluor 647 (Molecular Probes, Eugene, OR) and filter-sterilized before use. Pa was stained with Syto-9, using the Live/Dead BacLight Bacterial Viability kit (Molecular Probes). Alexa Fluor 488 and 594 transferrin (Tf) conjugates (Molecular Probes) were used at 25 µg/ml. FITC-labeled rat anti-lysosome-associated membrane protein 2 (LAMP2) and unlabeled rat anti-LAMP2 Ab (BD PharMingen, San Diego, CA) were used at 2.5 µg/ml. Alexa Fluor 546-labeled goat anti-rat IgG (Molecular Probes) was used at 1 µg/ml. For microscopy, DC were plated in eight-well chamber slides (Lab-Tek, Naperville, IL) and cultured for 2 or 18 h in the presence of SEA, Pa, or a mix of both Ag. For Tf staining, DC were incubated at 37°C for 1.5 h with SEA, Pa, or both Ag, washed twice with, and resuspended in, phenol red and FCS-free RPMI (RPMI*), and pulsed with labeled Tf for 0.5 h at 37°C. After two RPMI* washes, cells were fixed in 3% paraformaldehyde in PBS and mounted in Fluoromount G (Electron Microscopy Sciences, Fort Washington, PA). For Ab staining, cells were fixed in paraformaldehyde in PBS for 20 min at room temperature and permeabilized by incubation in PBS, 0.075% saponin, 1.5% FCS plus the relevant Ab, for 45 min at room temperature. Using the same buffer, cells were subsequently washed, labeled with secondary Ab in PBS, washed again, mounted in Fluoromount G (Electron Microscopy Sciences), and analyzed using a Nikon Eclipse E-600 microscope/Bio-Rad (Hercules, CA) confocal system. Each image reported represents the computer-generated coalescence of consecutive sections collected at 0.5-µm interval Z-stacks throughout individual cells. Flow cytometric analysis of DC was performed using a FACSCalibur flow cytometer (BD Biosciences, Mountain View, CA) with CellQuest (BD Biosciences) and FlowJo (Tree Star, Ashland, OR) software.
Determination of DC maturation state and Th response-priming ability
Expression of surface molecules on DC was quantified by flow cytometry using FITC-, allophycocyanin-, or PE-conjugated mAb specific for CD11c, MHC class II (MHCII), and CD80 (BD PharMingen). Cytokine ELISAs were performed on 18-h culture supernatants using paired mAb in combination with recombinant cytokine standards (BD PharMingen) as described previously (1).
Mice were injected i.p. with 5 x 105 DC that had been pulsed with SEA, Pa, both Ag (SEA/Pa), or neither Ag, as described elsewhere (1). Splenic T cell responses were measured by restimulating splenocytes with Ag and quantitating IL-5 or IFN-
in 72-h culture supernatants using ELISAs (1).
| Results |
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and IL-5. As expected (1), Pa-pulsed DC induced a Th1 response (Fig. 2A) and SEA-pulsed DC induced a Th2 response (Fig. 2B). Unexpectedly, DC pulsed with both Ag induced concurrent Pa-specific Th1 and SEA-specific Th2 responses (Fig. 2). There was no evidence of a Pa-specific Th2 response or SEA-specific Th1 response in animals that had received SEA/Pa-pulsed DC (Fig. 2). However, the intensity of the Th2 response induced by the copulsed DC was consistently diminished compared with that induced by DC pulsed with SEA alone (Fig. 2B); this reduction was not observed when mice were injected with a 1:1 mixture of DC pulsed with Pa alone and DC pulsed with SEA alone (data not shown), indicating that it was related to coacquisition of both Ag by DC.
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Since DC maturation signals induce activation of lysosomal function and accumulation of peptide-MHCII complexes (15) and increased Ag dose favors Th1 response development, we examined whether increasing the strength of the Pa maturation signal given to copulsed DC from 10 to 50 µg/ml, while retaining the SEA concentration at 50 µg/ml, would change the pattern of Ag localization and/or the ability of the DC to induce a SEA-specific Th2 response. Compared with DC copulsed with SEA/Pa (10 µg/ml), DC copulsed with SEA/Pa (50 µg/ml) exhibited greater increases in surface expression of MHCII and CD80 (data not shown), indicating that they were more mature. In contrast to the situation in copulsed DC exposed to the low concentration of Pa, in DC copulsed with SEA/Pa (50 µg/ml) SEA localized extensively to a LAMP2-positive compartment (Fig. 4A, jl cf with 4A, gi) and in part colocalized with Pa (Fig. 4A, df cf with 4A, ac). This change in the final localization of SEA that accompanied more extensive Pa-induced DC maturation was not reflected in a change in the way that the two Ag were acquired by the cells since SEA and Pa entered TfR-negative and -positive compartments regardless (data not shown). We next assessed the consequences of the altered localization of SEA within the SEA/Pa (50 µg/ml) copulsed DC on the ability of these DC to induce SEA-specific Th responses. In marked contrast to the situation in mice injected withDC copulsed with SEA/Pa (10 µg/ml) animals injected with DCcopulsed with SEA/Pa (50 µg/ml) failed to develop a SEA-specific Th2 response (Fig. 4C), but rather mounted a weak Th1 response to this Ag (Fig. 4B). Strong Pa-specific Th1 responses were induced in these animals (Fig. 4A).
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| Discussion |
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We have compared how DC take up and are activated by SEA and Pa, Ag that inherently induce oppositely polarized Th responses. We expected that if SEA has little effect on DC, acting only as a passive Ag, Pa-induced activation should dominate when cells are simultaneously exposed to both Ag. Our data show clearly that SEA/Pa-copulsed DC undergo changes associated with TLR ligand-induced maturation, including up-regulation of MHCII, and CD80. However, the ability of SEA/Pa-pulsed DC to make IL-12 was impaired compared with that of DC pulsed with Pa alone. This result supports the recent report that SEA can have significant suppressive effects on the ability of DC to make IL-12 in response to the TLR4 ligand LPS (12). It is tempting to believe that the ability of SEA to suppress Pa-stimulated IL-12 production is linked to its ability to induce Th2 responses. This situation would be consistent with the observed absence of any effect of SEA on Pa-induced Th1 responses, since we know that DC need not make IL-12 to prime a Pa-specific Th1 response (5). Preliminary microarray analyses of transcript profiles of DC pulsed with SEA, Pa, or SEA/Pa have revealed a >3-fold repression by SEA of expression of a group of 17 genes induced by Pa (J. Sun, L. Cervi, A. Straw, and E. J. Pearce, unpublished data), indicating that the regulatory effects of SEA on Pa are broad ranging and suggesting that the suppression of the production of factors in addition to IL-12 could also play a role in permitting Th2 response development.
At this time it is unclear how SEA suppresses the ability of DC to make IL-12. However, recent reports have shown that SEA binds to DC-SIGN via core fucosylated glycans (21). This is of interest since DC-SIGN ligation by Mycobacterium tuberculosis-secreted mannose-capped lipoarabinomannan was recently shown to block maturation of mycobacteria-infected DC (22). Thus, a role for DC-SIGN in the SEA-mediated suppression of Pa-induced IL-12 production is plausible.
Our analysis of SEA and Pa uptake into DC revealed that these Ag are acquired via different pathways. Pa enters early endosomes that stain positively for TfR, whereas SEA does not. Thereafter, the Ag remain in nonoverlapping compartments. The compartment that Pa enters is LAMP2 bright, which is consistent with the bacteria being in an Ag-processing compartment. In contrast, some SEA within individual cells enters a LAMP2-dull environment, while the majority of the Ag appear to be in a LAMP2-negative compartment. Thus, the ways that SEA and Pa are trafficked intracellularly are quite different, and we speculate that this difference is linked to the contrasting antigenicity of the two Ag. One possibility, based on observed patterns of Ag/LAMP2 colocalization, is that processing of SEA is less extensive than that of Pa. Such a difference could lead to a higher density display of Pa-derived vs SEA-derived peptides, a situation that could be consistent with the observed abilities of Pa-pulsed DC to induce Th1 responses, SEA-pulsed DC to induce Th2 responses, and of SEA/Pa-copulsed DC to induce Ag-appropriate Th1 and Th2 responses. In accordance with these observations, our data show that, when copulsed DC are stimulated to an increased level of maturation by exposure to a higher concentration of Pa, SEA localizes to LAMP2-bright compartments, suggesting more extensive processing of this Ag. This could result in greater surface expression of MHCII complexed with SEA-derived peptides.
The ability of DC to mature in response to TLR ligands is clearly important for their ability to induce Th1 responses, but may not be important for Th2 response induction (1, 3, 23). It seems likely that the ability of SEA to suppress aspects of DC maturation plays a role in its inherent Th2 response-inducing characteristics. However, the fact that SEA/Pa-copulsed DC do not induce Pa-specific Th2 responses emphasizes that the segregated intracellular localization of SEA plays an equally, if not more important role in this regard. The observations that in highly matured DC (pulsed with SEA plus high concentrations of Pa) SEA is forced to localize to LAMP2-positive, Pa-positive compartments and that these DC fail to induce a strong SEA-specific Th2 response but rather induce a weak SEA-specific Th1 response lend support to this view.
| Acknowledgments |
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| Footnotes |
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2 Current address: University of Edinburgh, Institute of Cell, Animal and Population Biology, 212B Ashworth Laboratories, Kings Buildings, West Mains Road, Edinburgh EH9 3JT, U.K. ![]()
3 Address correspondence and reprint requests to Dr. Edward J. Pearce, Department of Pathobiology, University of Pennsylvania, 203D Johnson Pavilion, 3610 Hamilton Walk, Philadelphia, PA 19104-6076. E-mail address: ejpearce{at}mail.med.upenn.edu ![]()
4 Abbreviations used in this paper: DC, dendritic cell; Pa, Proprionibacterium acnes; SEA, schistosome egg Ag; TLR, Toll-like receptor; Tf, transferrin; TfR, Tf receptor; LAMP2, lysosome-associated membrane protein 2; MHCII, MHC class II. ![]()
Received for publication October 15, 2003. Accepted for publication December 22, 2003.
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