Effective priming of herpes simplex virusspecific CD8+ T cells in vivo does not require infected dendritic cells

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.Our research is supported by the National Health and Medical Research Council of Australia. P. Whitney is supported by an Overseas Biomedical Fellowship (NHMRC) and a MDHS Faculty Fellowship (University of Melbourne). T. Gebhardt is supported by a fellowship from the Sylvia and Charles Viertel Charitable Foundation. D. Tscharke is supported by a Senior Research Fellowship (NHMRC)

Characterisation of CD4+ T cell help in CD8+ T cell priming

Dendritic cells (DCs) often require stimulation from CD4+ T cells to propagate CD8+ T cell responses, but precisely how T cell help optimizes the priming capacity of DCs and why this differs between varying types of CD8+ T cell immunity remains unclear. Work summarized in this thesis aimed to gain new insights into the mechanisms by which CD4+ T cell help optimises the capacity of DCs to elicit potent primary CD8+ T cell responses. Using Herpes simplex virus type I skin infection, the findings presented herein demonstrate that HSV-1 specific CD8+ T cell priming depended on DCs receiving stimulation from both IFNα/β and CD4+ T cells to provide IL-15. This was not an additive effect, but resulted from CD4+ T cells amplifying DC production of IFNα/β-induced IL-15. To test if CD4+ T cell help as an amplifier of suboptimal IFNα/β stimulation also applies to other helper-dependent models, mice were immunized with cell-associated ovalbumin (OVA) in the presence of grading concentrations of the inflammatory mediator poly(I:C). It was observed that the helper-dependence of OVA-specific CTL priming could be overcome in vivo by escalating the dose of poly(I:C). Importantly, the OVA- specific CD8+ T cell response depended on IL-15 regardless of whether it required T cell help or not. Our results further demonstrate that altering the innate stimulus delivered alongside the same source of antigen changes the cytokine requirement of the ensuing helper-dependent OVA-specific CD8+ T cell response. These findings reveal a complex interplay between IFNα/β and CD4+ T cell help in DC activation and CD8+ T cell priming. Importantly, these findings suggest T cell help-mediated amplification as a flexible mechanism applicable to varying innate signals in DCs. It was further evaluated how the CD4+ T cell-dependent provision of IL- 15 optimised the ensuing HSV-1 specific CD8+ T cell response. The role for IL-15 in regulating initial CD25 expression on HSV-1 specific CD8+ T cells prior to their proliferation and likely autocrine provision of IL-2 was excluded. Our data indicated a link between IL-15 and CD8+ T cell retention in the priming lymph node, suggesting that IL-15 prevents premature lymph node departure. Further experiments are under way to test this notion and further address the underlying molecular mechanisms

T Cell Help Amplifies Innate Signals in CD8+ DCs for Optimal CD8+ T Cell Priming

DCs often require stimulation from CD4+ T cells to propagate CD8+ T cell responses, but precisely how T cell help optimizes the priming capacity of DCs and why this appears to differ between varying types of CD8+ T cell immunity remains unclear. We show that CD8+ T cell priming upon HSV-1 skin infection depended on DCs receiving stimulation from both IFN-α/β and CD4+ T cells to provide IL-15. This was not an additive effect but resulted from CD4+ T cells amplifying DC production of IL-15 in response to IFN-α/β. We also observed that increased innate stimulation reversed the helper dependence of CD8+ T cell priming and that the innate stimulus, rather than the CD4+ T cells themselves, determined how “help’” was integrated into the priming response by DCs. These findings identify T cell help as a flexible means to amplify varying suboptimal innate signals in DCs