Herpes simplex encephalitis (HSE), caused by HSV type 1 (HSV-1) infection, is an acute neuroinflammatory condition of the CNS and remains the most common type of sporadic viral encephalitis worldwide. Studies in humans have shown that susceptibility to HSE depends in part on the genetic make-up of the host, with deleterious mutations in the TLR3/type I IFN axis underlying some cases of childhood HSE. Using an in vivo chemical mutagenesis screen for HSV-1 susceptibility in mice, we identified a susceptible pedigree carrying a causal truncating mutation in the Rel gene (Rel^C307X ), encoding for the NF-κB transcription factor subunit c-Rel. Like Myd88^-/- and Irf3^-/- mice, Rel^C307X mice were susceptible to intranasal HSV-1 infection. Reciprocal bone marrow transfers into lethally irradiated hosts suggested that defects in both hematopoietic and CNS-resident cellular compartments contributed together to HSE susceptibility in Rel^C307X mice. Although the Rel^C307X mutation maintained cell-intrinsic antiviral control, it drove increased apoptotic cell death in infected fibroblasts. Moreover, reduced numbers of CD4⁺CD25⁺Foxp3⁺ T regulatory cells, and dysregulated NK cell and CD4⁺ effector T cell responses in infected Rel^C307X animals, indicated that protective immunity was also compromised in these mice. In the CNS, moribund Rel^C307X mice failed to control HSV-1 viral replication in the brainstem and cerebellum, triggering cell death and elevated expression of Ccl2, Il6, and Mmp8 characteristic of HSE neuroinflammation and pathology. In summary, our work implicates c-Rel in both CNS-resident cell survival and lymphocyte responses to HSV-1 infection and as a novel cause of HSE disease susceptibility in mice.

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