Endogenous retroviruses mediate IFN-independent protection against HSV-2 infection

Endogenous retroviruses (ERVs) are genomic sequences that originated from retroviruses and are present in most eukaryotic genomes. Both beneficial and detrimental functions are attributed to ERVs, but whether ERVs contribute to antiviral immunity is not well studied. Here, we used herpes simplex virus type 2 (HSV-2) infection as a model and found that mice deficient in Toll-like receptor 7 (Tlr7-/-) that have high systemic levels of infectious ERVs are resistant to intravaginal HSV-2 infection, compared with wildtype C57BL/6 (B6) mice. We deleted the endogenous ecotropic murine leukemia virus (Emv2) locus on the Tlr7-/- background (Emv2-/-Tlr7-/-) and found that Emv2-/-Tlr7-/- mice lose protection against HSV-2 infection. Intravaginal application of purified ERVs prior to HSV-2 infection delays disease in both B6 and highly susceptible interferon-alpha receptor-deficient (Ifnar1-/-) mice. We did not observe enhanced type I interferon (IFN-I) signaling in the vaginal tissues from Tlr7-/- mice or B6 mice treated with purified ERVs, and instead found enrichment in genes associated with extracellular matrix organization. Together, our results revealed an IFN-independent modulation of the vaginal epithelium by ERVs that protects mice against vaginal HSV-2 infection and delays disease. Overall design: Bulk RNA-seq of polyA RNA obtained from whole vaginal tissues of uninfected or HSV-2 186 Syn+ infected wildtype B6, Tlr7-/-, Emv2-/-, and Emv2-/-Tlr7-/- mice.