Cell-intrinsic role of caspase-1 in regulating antigen-specific CD8+ T cell responses

ABSTRACT Caspase-1 is an essential component of the inflammasome complex and is responsible for cleaving pro- inflammatory cytokines IL-1β and IL-18 and initiating pyroptosis through cleavage of Gasdermin D. Using a murine model of West Nile virus (WNV) infection, we and others have previously demonstrated the importance of inflammasome signaling for promoting IL-1β-mediated protective immunity. We have now extended these observations and discovered a novel, cell-intrinsic role for caspase-1 in regulating antigen-specific CD8+ T cells responses during WNV infection. The absence of CD8+ T cells leads to uncontrolled WNV replication and significant neuronal injury within the brain. To understand how environmental cues within the brain impact T cell responses during WNV infection, we performed transcriptional profiling on antigen-specific CD8+ T cells isolated from the spleen and brain at early and late times post-WNV infection. Through this analysis, we found that caspase-1, as well as other inflammasome signaling components, were enriched within brain-resident antigen- specific CD8+ T cells as compared to antigen-specific CD8+ T cells isolated from the spleen. We confirmed that active caspase-1 is more abundantly expressed within brain-resident CD8+ T cells as compared to splenic CD8+ T cells. Using a co-adoptive transfer model, we observed increased antigen-specific Casp1-/- CD8+ T cells as compared to WT CD8+ T cells within the spleen and brain during infection. Of note, CD8+ T cells are not infected by WNV, suggesting that the activation of caspase-1 is independent of virus infection. Next, we characterized caspase-1 function in vitro using an in vitro T cell receptor (TCR) stimulation assay and found that caspase-1 is autoproteolytically cleaved following CD3/CD28 stimulation. Moreover, we found that ASC, but not NLRP3 or IL- 1β, is expressed in naïve and stimulated CD8+ T cells. Lastly, recent studies have linked caspase-1 with mitochondrial function. Consistent with these observations, we found that αCD3/αCD28 primed Casp1-/- CD8+ T cells displayed enhanced oxidative phosphorylation as compared to WT CD8+ T cells, suggesting that caspase-1 may regulate CD8+ T cell function by reprogramming mitochondrial dynamics and metabolism. Based on these findings, we hypothesize that TCR stimulation of CD8+ T cells triggers caspase-1 activation, which functions to limit T cell activation, mitochondrial bioenergetics, and cell expansion in a tissue-specific manner. In turn, we believe that active caspase-1 is critically required for controlling virus replication within the CNS and minimizing consequential neuronal damage by preventing an over-exuberant T cell response during viral infection. To address this hypothesis, we have two specific aims: 1) How does caspase-1 mediate CD8+ T cell responses during WNV infection? and 2) Aim 2. What are the molecular mechanisms of caspase-1 function in activated CD8+ T cells? The completion of these aims will provide us with a broader understanding of the underlying mechanisms by which caspase-1 regulates CD8+ T cells responses during virus infection.