Cytomegalovirus (CMV) establishes a lifelong infection facilitated, in part, by circumventing immune defenses mediated by tumor necrosis factor (TNF)-family cytokines. An example of this is the mouse CMV (MCMV) m166 protein, which restricts expression of the TNF-related apoptosis-inducing ligand (TRAIL) death receptors, promoting early-phase replication. We show here that replication of an MCMV mutant lacking m166 is also severely attenuated during viral persistence in the salivary glands (SG). Depleting group I innate lymphoid cells (ILCs) or infecting Trail-/- mice completely restored persistent replication of this mutant. Group I ILCs are comprised of two subsets, conventional natural killer cells (cNK) and tissue-resident cells often referred to as innate lymphoid type I cells (ILC1). Using recently identified phenotypic markers to discriminate between these two cell types, their relative expression of TRAIL and gamma interferon (IFN-γ) was assessed during both early and persistent infection. ILC1 were found to be the major TRAIL expressers during both of these infection phases, with cNK expressing very little, indicating that it is ILC1 that curtail replication via TRAIL in the absence of m166-imposed countermeasures. Notably, despite high TRAIL expression by SG-resident ILC1, IFN-γ production by both ILC1 and cNK was minimal at this site of viral persistence. Together these results highlight TRAIL as a key ILC1-utilized effector molecule that can operate in defense against persistent infection at times when other innate control mechanisms may be muted and highlight the importance for the evolution of virus-employed countermeasures.IMPORTANCE Cytomegalovirus (a betaherpesvirus) is a master at manipulating immune responses to promote its lifelong persistence, a result of millions of years of coevolution with its host. Using a one-of-a-kind MCMV mutant unable to restrict expression of the TNF-related apoptosis-inducing ligand death receptors (TRAIL-DR), we show that TRAIL-DR signaling significantly restricts both early and persistent viral replication. Our results also reveal that these defenses are employed by TRAIL-expressing innate lymphoid type I cells (ILC1) but not conventional NK cells. Overall, our results are significant because they show the key importance of viral counterstrategies specifically neutralizing TRAIL effector functions mediated by a specific, tissue-resident subset of group I ILCs.
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