PROJECT SUMMARY/ABSTRACT Viral infection outcomes are sex-biased, with males generally more susceptible to human cytomegalovirus (HCMV) and other viral infections compared to females. These differences may reflect sexual dimorphism in immune cell composition and function. As such, it is surprising that numbers of natural killer (NK) cells, a first line of defense against HCMV, are increased in males compared to females. Here we show in mouse models and human samples that while males harbor increased NK cell numbers, they produce less IFN-γ, a critical pro- inflammatory cytokine for NK-mediated anti-viral responses. This difference is not due to divergent levels of gonadal hormones, since these differences are still present in gonadectomized mice. Instead, a screen for X chromosome genes that escape inactivation and demonstrate sexually dimorphic expression in NK cells identified UTX, an epigenetic regulator that alters transcriptional programs through reorganization of chromatin. NK cell-specific UTX deletion phenocopies multiple features of male NK cells, which include increased NK numbers and reduced IFN-γ production. Thus, we hypothesize that NK cell sex differences can be attributed to differential expression of X-linked UTX, which reorganizes chromatin at loci important in NK cell fitness and function. In Aim 1, we will define UTX-mediated temporal control of NK cell numbers and determine whether differences in cellular fitness underlie differences in NK cell homeostasis in males compared to females. In Aim 2, we will delineate UTX’s role in promoting NK cell cytotoxic activity and whether lower UTX levels in male NK cells also impairs their cytotoxic capacity. In Aim 3, we will delineate molecular mechanisms by which UTX controls chromatin accessibility and gene expression at loci important for NK cell homeostasis and function. Knowledge gained from completion of these studies will contribute to our basic understanding of sex differences in anti-viral responses and NK cells. A deeper understanding of sex differences will benefit human health overall by revealing new targets for immunotherapy and guiding interventions for optimizing anti-viral immunity.