The ß-herpesvirus human cytomegalovirus (HCMV) infects around 50% of the world population. Although it remains asymptomatic in healthy individuals, it is never completely cleared from the host and can cause recurrent infections, especially at times of immunosuppression. HCMV poses the biggest threat to pregnant women, neonates, and immunocompromised individuals, whose symptoms tend to be more severe. The mouse homolog of HCMV, murine cytomegalovirus (MCMV), is a well-characterized animal model of viral infection. Like HCMV, MCMV has a tropism for the salivary glands (SMG) and persists in this organ for several months after primary infection. We have demonstrated that NK cells, which are critical in the spleen and blood, are hyporesponsive in the SMG, conceivably explaining the MCMV persistence in this organ. Interestingly, our preliminary data show that salivary gland NK cells regain normal effector functions against MCMV when relocated to different tissues, indicating that the salivary gland environment regulates NK cell function. MCMV exploits the SMG as the key site of viral persistence, which, similar to tumors, creates an immune environment with mechanisms to preclude excessive inflammation and/or tissue damage. Because the immune response to cancer and infection exhibit commonalities, we reasoned that the balance between pathogen clearance and immunopathology is likely to be regulated in part by checkpoint inhibitors within the salivary glands, especially during the persistence phase. Our data demonstrate that targeting KLRG1 improves the effector immune response to MCMV and survival at certain doses, but can lead to immunopathology depending on the dose. Our data also suggest that NK cells may inhibit CD8+ T cell recruitment, proliferation and effector functions in the salivary glands via the PD-1 pathway. This is supported by CITE-seq/TotalSeq on sorted T cells, showing expansion of unique CD8+ T cell clusters. Based on this preliminary data, we hypothesize that NK cells contribute to CMV control while preventing immunopathology by limiting the expansion of CD8+ T cells in the salivary glands. The overall goal of this proposal is to reveal the underlying mechanisms leading to MCMV persistence. In Specific Aim 1, we will investigate how the equilibrium between pathogen clearance and immunopathology is regulated. In Specific Aim 2, we will investigate how MCMV persistence is regulated in the salivary glands. In Specific Aim 3, we will investigate how NK cells inhibit T cell accumulation possibly limiting immunopathology in the salivary glands. We have generated a large number of tools to investigate the questions asked in this proposal. We are now in an excellent position to conduct the proposed studies. The results from these studies are likely to have relevance to infections caused by other viruses and aid in the design of strategies and therapeutics directed at manipulating effector cells during viral infection.