General features of dendritic cells (DCs)
Introducing general features of dendritic cells (DCs)
DCs have been first described in 1973 by Ralph M. Steinman (1). For his finding he received the Nobel Price in 2011. DCs are well established as antigen presenting cells that can initiate CD4+ and CD8+ T cell immune responses against pathogens (2). They are specialized for this by their capacity to recognize and capture pathogens or their products during their immature stage at the infection site and transport them to the draining lymph nodes (3). There the processed pathogen proteins will be presented by the mature stage on MHC II or cross-presented on MHC I molecules to CD4+ or CD8+ T cells in an immunogenic context (also providing costimulation and proinflammatory cytokines) (4, 5). This old model on general DC biology is still valid but many more aspects of DC biology have been added in the meantime.
It is also well established but less known that DCs also present glycolipids on CD1a,b,c,d (human) or CD1d (mouse) molecules to type I and type II NKT cells. Depending on the type of DC maturation, also NKT cells can be directed to produce selectively IFN-g or IL-4, similar to Th1 or Th2 cells (6). In addition, DCs can transport protein antigens in their native form in storage compartments at neutral pH (7) for "presentation" to B cells in the lymph nodes (8).
Besides these immunogenic functions it became also clear during the last years that immature DCs are not only resting cells waiting for pathogens, but that immature and semi-mature DC maturation stages (see below) are constantly involved to mediate immune tolerance to peripheral antigens (9, 10). These constitutively active DC-mediated peripheral tolerance mechanisms help to avoid autoimmunity in healthy individuals. In addition, they can be used for therapy to revert transplant rejections or to treat autoimmune diseases, allergy and overshooting immune responses against pathogens (11-14).
One of our major research interests is to understand the tolerogenic activities of DCs in healthy individuals and the underlying molecular mechanisms. The methods used are various gene-deficient mice, gene expression profiling and cellular therapy by adoptive transfered DCs into murine disease models. We also try to extend our findings to therapeutic interventions in horses (15) and humans (16, 17).
1. Tolerogenic DC maturation stages (immature, semi-mature, fully mature)
Tolerogenicity of DCs is mainly directed by their maturation stage. For every major subset in the mouse (CD8a+, CD4+, monocyte-derived BM-DCs, plasmacytoid DCs) immature stages have been shown to act tolerogenic on T cells and only after maturation convert to immunogenic cells (18).
We found that DC maturation can lead to semi-mature or fully mature stages. They are distingishable mainly by their presence or absence of proinflammatory cytokine production, while the expression of MHC II- and costimulatory molecules aas well as the expression of the CCR7 homing receptor remain similar. Such semi-mature DCs appeared tolerogenic in a Th1/Th17-driven autoimmune model (EAE)(19) but had no effect in Th2 models (asthma, BALB/c-Leishmania infection)(20) and could even induce autoimmunity of CD8+ T cells in a Type I diabestes model (21). As mechansims we identified Th2- and NKT-cell-mediated immune deviation and IL-10-producing regulatory Tr1 cells.
2. Instruction of tolerogenic CD4+ T cell phenotypes by dendritic cells
2.1 T cell anergy
Anergy can be induced by immature DCs in the absence of TGF-b.In this way low level antigen presentation occurs on MHC II molecules (Signal 1) with absent to low costimulation (Signal 2). Anergy induction represents a tolerance mechanism to functionally inactivate T cells for further antigen-specific T cell receptor activation (18, 22).
2.2 Regulatory T cells
Regulatory T cells emerge either directly from the thymus (natural Tregs, nTregs) or can be induced in peripheral lymphoid organs (23). In the murine spleen immature DCs of the CD4+ or CD8a+ can convert naive CD4+ T cells into Foxp3+ induced Tregs (iTregs) if TGF-beta (24). Most likely the capture of soluble self-antigens initiates this process (25). In addition Foxp3- IL-10+ Tregs, called Tr1, can be induced upon repetitive stimulation of Th1 or Th2 cells or by stimulation of naive T cells in the presence of IL-10 (26-28).
In peripheral lymph nodes steady state migratory DCs constitutively transport tissue-associated self-antigens from peripheral tissues to the draining lymph node (29). They appear at a semi-mature stage (9), e.g. express RelB/p52 but not RelA/p50 as compared with fully mature/immunogenic DCs. They also express the lymph node homing receptor CCR7 at their surface which enables their migration into the paracortical T cell areas of lymph nodes. They carry LAP/TGF-b complexes on their surface leading to Foxp3+ iTreg conversion (30). In contrast, self-antigen-specific CD8+ T cells are deleted (31).
2.3 Instruction of CD4+ T helper cell subsets by DCs
DCs are not only engaged to prime T cell responses via signal 1 and 2 (see above) but also contribute to further differentiation of T cells (signal 3)(32). For the CD4+ T cell subset this process had been termed polarization into T helper cell (Th) subsets (33, 34).
We are especially interested in the question which pathogen-, danger- or inflammatory signals and signaling receptors instruct DCs for further instruction of T cell polarization into specific Th1, Th2, Th17 or CD4+ regulatory T cell subtypes.
We found that Th2 instruction potential by DCs is characterized by semi-maturation and reflected by a genetic pattern (detected by mRNA microarrays) that encompasses only an inflammatory signature but no specific instructive signals. In this respect TNFa and Trypanosoma brucei antigens were highly overlapping. In contrast, the DC maturation by LPS leading to Th1 instruction induces the same inflammatory plus many more additional genes (full maturation), including IL-12 (28). These data indicate that quantitative differences in DC maturation can direct Th1/Th2 polarization (35).
Interstingly, the gene pattern induced by cholera toxin shows almost no qualitative overlap with the Th-1 instructing LPS or Th2-instructing TNFa or Trypanosoma stimuli. Instead, cholera toxin matured DCs polarize into Th17 responses. The underlying mechanisms are currently investigated.
4. Sallusto, F., and A. Lanzavecchia. 1994. Efficient presentation of soluble antigen by cultured human dendritic cells is maintained by granulocyte/macrophage colony-stimulating factor plus interleukin 4 and downregulated by tumor necrosis factor alpha. J Exp Med 179: 1109-1118.
6. Wiethe, C., M. Schiemann, D. Busch, L. Haeberle, M. Kopf, G. Schuler, and M. B. Lutz. 2007. Interdependency of MHC class II/self-peptide and CD1d/self-glycolipid presentation by TNF-matured dendritic cells for protection from autoimmunity. J Immunol 178: 4908-4916.
7. Lutz, M. B., P. Rovere, M. J. Kleijmeer, M. Rescigno, C. U. Assmann, V. M. Oorschot, H. J. Geuze, J. Trucy, D. Demandolx, J. Davoust, and P. Ricciardi-Castagnoli. 1997. Intracellular routes and selective retention of antigens in mildly acidic cathepsin D/lysosome-associated membrane protein-1/MHC class II-positive vesicles in immature dendritic cells. J. Immunol. 159: 3707-3716.
15. Dietze, B., E. Cierpka, M. Schafer, W. Schill, and M. B. Lutz. 2008. An improved method to generate equine dendritic cells from peripheral blood mononuclear cells: Divergent maturation programs by IL-4 and LPS. Immunobiology 213: 751-758.
16. Berger, T. G., H. Schulze-Koops, M. Schafer, E. Muller, and M. B. Lutz. 2009. Immature and maturation-resistant human dendritic cells generated from bone marrow require two stimulations to induce T cell anergy in vitro. PLoS One 4: e6645.
17. Baur, A. S., *, M. B. Lutz, *, S. Schierer, L. Beltrame, G. Theiner, E. Zinser, C. Ostalecki, G. Heidkamp, I. Haendle, M. Erdmann, M. Wiesinger, W. Leisgang, S. Gross, A. J. Pommer, E. Kämpgen, D. Dudziak, A. Steinkasserer, D. Cavalieri, B. Schuler-Thurner, and G. Schuler. 2013. Denileukin diftitox (ONTAK) induces a tolerogenic phenotype in Dendritic Cells and stimulates survival of resting Treg. Blood (in press) * equal contribution with first author.
18. Pletinckx, K., A. Döhler, V. Pavlovic, and M. B. Lutz. 2011. Role of dendritic cell maturity/costimulation for generation, homeostasis and suppressive activity of regulatory T cells. Frontiers in Immunology 2 (article 39) 1-15.
19. Menges, M., S. Rossner, C. Voigtlander, H. Schindler, N. A. Kukutsch, C. Bogdan, K. Erb, G. Schuler, and M. B. Lutz. 2002. Repetitive injections of dendritic cells matured with tumor necrosis factor alpha induce antigen-specific protection of mice from autoimmunity. The Journal of experimental medicine 195: 15-21.
20. Wiethe, C., A. Debus, M. Mohrs, A. Steinkasserer, M. B. Lutz, *,, and A. Gessner. 2008. Dendritic cell differentiation state and their interaction with NKT cells determine Th1/Th2 differentiation in the murine model of Leishmania major infection. J Immunol 180: 4371-4381 *equal contribution with last author.
21. Kleindienst, P., C. Wiethe, M. B. Lutz, and T. Brocker. 2005. Simultaneous induction of CD4 T cell tolerance and CD8 T cell immunity by semimature dendritic cells. J Immunol 174: 3941-3947 *equal contribution with last author.
24. Yamazaki, S., D. Dudziak, G. F. Heidkamp, C. Fiorese, A. J. Bonito, K. Inaba, M. C. Nussenzweig, and R. M. Steinman. 2008. CD8+ CD205+ splenic dendritic cells are specialized to induce Foxp3+ regulatory T cells. J Immunol 181: 6923-6933.
25. Sixt, M., N. Kanazawa, M. Selg, T. Samson, G. Roos, D. P. Reinhardt, R. Pabst, M. B. Lutz, and L. Sorokin. 2005. The conduit system transports soluble antigens from the afferent lymph to resident dendritic cells in the T cell area of the lymph node. Immunity 22: 19-29.
28. Pletinckx, K., B. Stijlemans, V. Pavlovic, R. Laube, C. Brandl, S. Kneitz, A. Beschin, P. De Baetselier, and M. B. Lutz. 2011. Similar inflammatory DC maturation signatures induced by TNF or Trypanosoma brucei antigens instruct default Th2-cell responses. Eur. J. Immunol. 41: 3479-3494.
30. Azukizawa, H., A. Dohler, N. Kanazawa, A. Nayak, M. Lipp, B. Malissen, I. Autenrieth, I. Katayama, M. Riemann, F. Weih, F. Berberich-Siebelt, and M. B. Lutz. 2011. Steady state migratory RelB+ langerin+ dermal dendritic cells mediate peripheral induction of antigen-specific CD4+ CD25+ Foxp3+ regulatory T cells. Eur. J. Immunol. 41: 1420-1434.
31. Waithman, J., R. S. Allan, H. Kosaka, H. Azukizawa, K. Shortman, M. B. Lutz, W. R. Heath, F. R. Carbone, and G. T. Belz. 2007. Skin-derived dendritic cells can mediate deletional tolerance of class I-restricted self-reactive T cells. J. Immunol. 179: 4535-4541.