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CD8 T cells have been demonstrated to be critical in the resolution of acute influenza A virus (IAV) infections. Previously our laboratory has demonstrated that the magnitude of the IAV-specific CD8 T cell response is inversely proportional to the initial IAV inoculum. The decrease in CD8 T cells observed during lethal dose IAV infections was shown tobe a result of apoptosis driven by FasL expressed on lymph node dendritic cells (LNDC) during lethal, but not sublethal, dose IAV infections. However the specific LNDC subset(s) responsible for FasL:Fas mediated apoptosis of IAV-specific CD8 T cells remains to be identified. The existence of multiple subsets of dendritic cells (DC) within the lymph node (LN) with distinct functions during viral infections suggest the possibility that a specific subset(s) may be responsible for this effect. Furthermore, the regulation of FasL expression on LNDC during lethal versus sublethal dose IAV infections was shown to be dependent on the levels of IL-12p40. However, the specific IL-12p40 containing cytokine, as well as which cells produce this cytokine within the LN remain unknown. Finally, whether or not the expression of FasL expression induced during infections is unique to IAV or if other pulmonary insults can mediate this effect is as of yet undetermined.
Here we demonstrate that plasmacytoid DC (pDC), which accumulate in the lung draining LN, are the only LNDC subset eliminating IAV-specific CD8 T cells through FasL:Fas dependent mechanism both in vitro and in vivo during lethal dose IAV infections despite FasL expression by all LNDC subsets. Further we demonstrate that this disparity in LNDC FasL induced apoptosis is related to the individual DC subsets ability to present IAV antigen as pDC are the only LNDC subset incapable of viral antigen presentation via MHC class I during IAV infections.
This dissertation further demonstrates that IL-12p40 homodimer (p402), which is produced by respiratory DC (rDC) and LNDC, and not IL-12p40 monomer controls FasL expression on LNDC during lethal dose IAV infections. Additionally I also demonstrate that IL-12Rβ1, which binds IL-12p40, is important for p402 mediated LNDC FasL expression. We further go on to demonstrate that rDC migration from the lungs to the LN is required for both LNDC p402 production and FasL expression. However, LNDC in isolation are unable maintain FasL expression suggesting their production of p402 may not me sufficient for FasL expression. Conversely, rDC are sufficient to induce FasL expression on LNDC from IL-12p40 deficient LN. Together these data suggest that rDC are a critical component of p402 mediated LNDC FasL expression. Finally, we demonstrate that the differential production of p402, and downstream LNDC FasL expression, observed during lethal versus sublethal dose IAV infections is not unique to IAV as intranasal stimulation with diverse TLR agonists also results in differential rDC p402 production and LNDC FasL expression. These data suggest that in addition to IAV, a multitude of pulmonary pathogens may regulate antigen-specific CD8 T cells through this pathway.
Taken together the results present herein detail a mechanism of CD8 T cell regulation during IAV infections mediated through p402 induction of FasL expression on pDC within the LN. FasL expressing pDC then induce apoptosis of activated Fas+ IAV-specific CD8 T cells within the LN during lethal a IAV infections leading to a reduction in the number of IAV-specific CD8 T cells that reach the lung and as a result death of the host.