Herpes simplex virus
encephalitis (HSE) is the most common cause of sporadic
viral encephalitis, and despite targeted
antiviral therapy, outcomes remain poor. Although the innate immune system is critical for restricting herpes simplex virus type I (HSV-1) in the brain, there is evidence that prolonged
neuroinflammation contributes to HSE pathogenesis. In this study, we investigated the contribution of
inflammasomes to disease pathogenesis in a murine model of HSE.
Inflammasomes are signaling platforms that activate the pro-inflammatory
cytokines interleukin-1β (IL-1β) and
IL-18. We found that mice deficient in the
inflammasome adaptor
protein, apoptosis-associated speck-like
protein containing a caspase activation and recruitment domain (ASC), had significantly improved survival and lower levels of IL-1β and
IL-18 in the brain. Importantly, this difference in survival was independent of viral replication in the central nervous system (CNS). We found that microglia, the resident macrophages of the CNS, are the primary mediators of the ASC-dependent
inflammasome response during
infection. Using in vitro glial
infections and a murine HSE model, we demonstrate that
inflammasome activation contributes to the expression of
chemokine (C-C motif)
ligand 6 (CCL6), a leukocyte
chemoattractant. The lower concentration of CCL6 in the brains of ASC-/- mice correlated with lower numbers of infiltrating macrophages during
infection. Together, these data suggest that
inflammasomes contribute to pathogenic
inflammation in HSE and provide a mechanistic link between glial
inflammasome activation and leukocyte infiltration. The contribution of
inflammasomes to survival was independent of viral replication in our study, suggesting a promising new target in combating harmful
inflammation in HSE.