Dendritic cells (DC) are major antigen-presenting cells consisting of numerous heterogeneous subtypes. In humans, several subtypes of DCs have been identified in different tissues including peripheral blood, secondary lymphoid organs, and in the skin. In peripheral blood mononuclear cells (PBMC) and secondary lymphoid organs, these subtypes include BDCA1+, BDCA3+, and plasmacytoid DCs which differ functionally and in expression of various markers. So how do these many human DC subsets differ functionally?
Antigen cross-presentation is a DC-specialized mechanism by which antigens are taken up through endocytic and phagocytic pathways but presented in the context of MHC-class I, to activate antigen-specific cytotoxic CD8 T cells. Recent studies have sought to characterize the differences between the many tissue-associated DC subsets including their ability to cross-present antigen.
In PBMC, DCs are quite rare, comprising only 1 – 2% of PBMCs. In previous blog posts, the generation of dendritic cells from PBMC monocytes and maturing and assaying monocyte-derived dendritic cells have been discussed. However, blood DCs and in vitro generated DCs may not represent the true physiological state of DCs present in secondary lymphoid organs where natural antigen cross-presentation and T cell activation occur in vivo.
In a recent study in The Journal of Experimental Medicine, Segura et. al explored the antigen cross-presentation versus phagocytic capabilities of human BDCA1+, BDCA3+, and plasmacytoid DC subsets compared with CD11c+HLADR+CD14+ macrophages that were all freshly isolated from healthy donor tonsils. The cross-presentation capabilities of different types of antigens, including necrotic dead cell antigens and soluble antigens were assessed.
For necrotic dead cell antigens, such as dead tumor cells, BDCA1+ and BDCA3+ DC subsets both took up antigens to a similar extent and were the most efficient in activating CD8+ T cell responses which were measured by IFN-gamma production in allogeneic mixed leukocyte reaction assays. Macrophages far exceeded the ability of any DC subsets in dead cell phagocytosis, but were extremely poor at cross-presentation and CD8+ T cell activation. Plasmacytoid DCs were the poorest at phagocytosis of dead cells, and were also unable to cross-present these antigens. For soluble antigens however, all three DC subsets (BDCA1+, BDCA3+, and plasmacytoid DC) efficiently cross-presented both shorter and longer soluble peptides, while macrophages continued to be poor at cross-presentation.
In an additional set of assays, the authors explored mechanisms that may contribute to the differential phagocytosis versus antigen cross-presentation of DC subsets and macrophages. Compared with macrophages which did not cross-present antigens, the endocytic compartments of cross-presenting DCs kept an alkaline pH and contained reactive oxygen species, and these DCs further were able to export internalized antigens to the cytosol where they can be loaded onto MHC-class I. Thus, while macrophages are efficient phagocytes, they are unable to process antigens to allow for cross-presentation.
In conclusion, understanding the capabilities of immune cells in different tissues is critical to discovering the full spectrum of cellular functions. DCs are a major target for vaccinations and immunotherapeutic strategies, and describing and understanding these subsets in vivo will lead to maximized success in immune modulating modalities.
Similar antigen cross-presentation capacity and phagocytic functions in all freshly isolated human lymphoid organ-resident dendritic cells. Segura E, Durand M, Amigorena S. J Exp Med. 2013 Apr 8.
Cross-presentation by dendritic cells. Joffre OP, Segura E, Savina A, Amigorena S. Nat Rev Immunol. 2012 Jul 13;12(8):557-69. doi: 10.1038/nri3254. Review.
BDCA-2, BDCA-3, and BDCA-4: three markers for distinct subsets of dendritic cells in human peripheral blood. Dzionek, A., A. Fuchs, P. Schmidt, S. Cremer, M. Zysk, S. Miltenyi, D.W. Buck, and J. Schmitz. 2000. J. Immunol. 165:6037–6046.
Characterization of resident and migratory dendritic cells in human lymph nodes. Segura, E., J. Valladeau-Guilemond, M.H. Donnadieu, X. Sastre-Garau, V. Soumelis, and S. Amigorena. 2012. J. Exp. Med. 209:653– 660.
Gene family clustering identifies functionally associated subsets of human in vivo blood and tonsillar dendritic cells. Lindstedt, M., K. Lundberg, and C.A. Borrebaeck. 2005. J. Immunol. 175:4839–4846.
Functional specializations of human epidermal Langerhans cells and CD14+ dermal dendritic cells. Klechevsky, E., R. Morita, M. Liu, Y. Cao, S. Coquery, L. Thompson- Snipes, F. Briere, D. Chaussabel, G. Zurawski, A.K. Palucka, et al. 2008. Immunity. 29:497–510.
Characterization of dermal dendritic cells obtained from normal human skin reveals phenotypic and functionally distinctive subsets. Nestle, F.O., X.G. Zheng, C.B. Thompson, L.A. Turka, and B.J. Nickoloff. 1993. J. Immunol. 151:6535–6545.