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Resolution of virus infections depends on the priming of virus-specific CD8 + T cells by dendritic cells (DC). While this process requires major histocompatibility complex (MHC) class I-restricted antigen presentation by DC, the relative contribution to CD8 + T cell priming by infected DC is less clear. We have addressed this question in the context of a peripheral infection with herpes simplex virus 1 (HSV). Assessing the endogenous, polyclonal HSV-specific CD8 + T cell response, we found that effective in vivo T cell priming depended on the presence of DC subsets specialized in cross-presentation, while Langerhans cells and plasmacytoid DC were dispensable. Utilizing a novel mouse model that allows for the in vivo elimination of infected DC, we also demonstrated in vivo that this requirement for cross-presenting DC was not related to their infection but instead reflected their capacity to cross-present HSV-derived antigen. Taking the results together, this study shows that infected DC are not required for effective CD8 + T cell priming during a peripheral virus infection. IMPORTANCE The ability of some DC to present viral antigen to CD8 + T cells without being infected is thought to enable the host to induce killer T cells even when viruses evade or kill infected DC. However, direct experimental in vivo proof for this notion has remained elusive. The work described in this study characterizes the role that different DC play in the induction of virus-specific killer T cell responses and, critically, introduces a novel mouse model that allows for the selective elimination of infected DC in vivo Our finding that HSV-specific CD8 + T cells can be fully primed in the absence of DC infection shows that cross-presentation by DC is indeed sufficient for effective CD8 + T cell priming during a peripheral virus infection., (Copyright © 2018 American Society for Microbiology.)

Adaptive cellular immunity is initiated by antigen-specific interactions between T lymphocytes and dendritic cells (DCs). Plasmacytoid DCs (pDCs) support antiviral immunity by linking innate and adaptive immune responses. Here we examined pDC spatiotemporal dynamics during viral infection to uncover when, where, and how they exert their functions. We found that pDCs accumulated at sites of CD8 + T cell antigen-driven activation in a CCR5-dependent fashion. Furthermore, activated CD8 + T cells orchestrated the local recruitment of lymph node-resident XCR1 chemokine receptor-expressing DCs via secretion of the XCL1 chemokine. Functionally, this CD8 + T cell-mediated reorganization of the local DC network allowed for the interaction and cooperation of pDCs and XCR1 + DCs, thereby optimizing XCR1 + DC maturation and cross-presentation. These data support a model in which CD8 + T cells upon activation create their own optimal priming microenvironment by recruiting additional DC subsets to the site of initial antigen recognition., (Published by Elsevier Inc.)

Antigen-presenting cells survey their environment and present captured antigens bound to major histocompatibility complex (MHC) molecules. Formation of MHC-antigen complexes occurs in specialized compartments where multiple protein trafficking routes, still incompletely understood, converge. Autophagy is a route that enables the presentation of cytosolic antigen by MHC class II molecules. Some reports also implicate autophagy in the presentation of extracellular, endocytosed antigen by MHC class I molecules, a pathway termed "cross-presentation." The role of autophagy in cross-presentation is controversial. This may be due to studies using different types of antigen presenting cells for which the use of autophagy is not well defined. Here we report that active use of autophagy is evident only in DC subtypes specialized in cross-presentation. However, the contribution of autophagy to cross-presentation varied depending on the form of antigen: it was negligible in the case of cell-associated antigen or antigen delivered via receptor-mediated endocytosis, but more prominent when the antigen was a soluble protein. These findings highlight the differential use of autophagy and its machinery by primary cells equipped with specific immune function, and prompt careful reassessment of the participation of this endocytic pathway in antigen cross-presentation.

Skin-derived dendritic cells (DCs) include Langerhans cells, classical dermal DCs and a langerin-positive CD103(+) dermal subset. We examined their involvement in the presentation of skin-associated viral and self antigens. Only the CD103(+) subset efficiently presented antigens of herpes simplex virus type 1 to naive CD8(+) T cells, although all subsets presented these antigens to CD4(+) T cells. This showed that CD103(+) DCs were the migratory subset most efficient at processing viral antigens into the major histocompatibility complex class I pathway, potentially through cross-presentation. This was supported by data showing only CD103(+) DCs efficiently cross-presented skin-derived self antigens. This indicates CD103(+) DCs are the main migratory subtype able to cross-present viral and self antigens, which identifies another level of specialization for skin DCs.

Mouse spleens contain three major dendritic cell (DC) populations: plasmacytoid DC, conventional CD8(+)CD24(+) DC (CD8(+) DC), and conventional CD8(-)CD24(-) DC (CD8(-) DC). We have previously shown that CD8(+) DC are the major cross-presenting subtype in vivo and are the main inducers of antiviral cytotoxic T lymphocyte responses. Here we show that after depletion of CD8(+) DC, the only DC capable of viral Ag presentation was a small subset that expresses CD24 but not CD8. This CD8(-)CD24(+) DC population is greatly expanded in mice treated with the DC growth factor FMS-like tyrosine kinase 3 ligand. The CD8(-)CD24(+) DC represent an immediate precursor of CD8(+) DC, as demonstrated by their expression pattern of characteristic markers of CD8(+) DC, their capacity to cross-present in vitro, and their conversion into CD8(+) DC upon adoptive transfer into recipient mice. Accordingly, the lifespan of transferred CD8(-)CD24(+) DC in vivo was greatly enhanced as compared with terminally differentiated CD8(+) DC. Moreover, in a vaccination protocol, CD8(-)CD24(+) DC induced stronger T cell responses and accelerated viral clearance of HSV-1 compared with CD8(+) DC. Our results demonstrate that the ability to cross-present first appears in an immediate precursor population of CD8(+) DC that does not yet express CD8. The enhanced capacity of CD8(-)CD24(+) DC to induce immune responses upon adoptive transfer makes them an attractive novel tool for DC-based immunotherapies.

Skin dendritic cells (DCs) are thought to act as key initiators of local T cell immunity. Here we show that after skin infection with herpes simplex virus (HSV), cytotoxic T lymphocyte (CTL) activation required MHC class I-restricted presentation by nonmigratory CD8(+) DCs rather than skin-derived DCs. Despite a lack of direct presentation by migratory DCs, blocking their egress from infected skin substantially inhibited class I-restricted presentation and HSV-specific CTL responses. These results support the argument for initial transport of antigen by migrating DCs, followed by its transfer to the lymphoid-resident DCs for presentation and CTL priming. Given that relatively robust CTL responses were seen with small numbers of skin-emigrant DCs, we propose that this inter-DC antigen transfer functions to amplify presentation across a larger network of lymphoid-resident DCs for efficient T cell activation.