Abstract: CMV infection in utero leads to prolonged viremia and often devastating clinical sequelae. In adults, it is established that the T cell response to CMV is required for containment of viremia. However, the immunologic determinants of viral control and clinical sequelae following congenital CMV infection are not known. In this project, we will study the immune response to CMV as a window into immune ontogeny and the age-related maturation of antiviral T cell function. We will leverage a large cohort of mother-infant pairs with samples banked longitudinally for immunologic studies. This cohort, which includes a large number of infants infected with CMV in utero and others infected during early childhood, affords a unique opportunity to examine the relationship between the age-related maturation of the immune system and control of CMV viremia. Prior studies have shown that two populations of cytotoxic T lymphocytes, CD8 T and gd T cells, expand and differentiate upon CMV infection in utero. We hypothesize that T cells generated during fetal development (including both CD8 and gd T cells) are intrinsically biased toward rapid differentiation into terminal effector cells. We further hypothesize that this effector-biased programming limits the ability of infant CD8 T cells to generate the long-lived memory sub-populations that are required for sustained control of a chronic viral infection. This hypothesis is supported by data from experimental murine models but has not been fully examined in human infants in the context of a natural pathogen. While the CD8 response continues to mature postnatally, gd T cells develop earlier in gestation and exhibit many innate-like qualities that could enable them to act as important antiviral effectors in utero. Remarkably, gd T cells that express CMV-reactive gd TCRs and pre-programmed effector functions are already present in the fetal thymus at mid-gestation. These fetal gd T cells can be rapidly activated to produce IFNg and granzymes upon stimulation. Hence, we hypothesize that fetal gd T cells play an important role in mediating anti-CMV effector functions in utero, while the adaptive ab T cell response matures. In the first two aims, we will compare gd and CD8 T cell responses in congenitally CMV- infected newborns to those of children who acquire primary CMV infection during the second year of life, in order to identify critical pathways of immune maturation. CD8 and gd T cells will be assessed by high- parameter cytometry, functional assays, and paired transcriptional and TCRseq profiling of individual cells in order to identify differences in the response to CMV based on the developmental window during which infection occurred. In Aim 3, we will relate these immunologic parameters to clinical sequelae and the resolution of viremia during infancy in order to identify immune correlates of viral containment. Understanding how variability in the infant immune response to CMV relates to clinical and vir...