Human cytomegalovirus (HCMV) is the most common cause of congenital viral infection and an important cause of morbidity and mortality following solid organ and hematopoietic cell transplantation. Understanding how the immune system responds to HCMV is vital to the efforts to achieve efficient eradication of infection. This proposal aims to identify novel relationships between NK cells and T cells during HCMV infection, with the central hypothesis that a strong T cell response to HCMV infection can abrogate the emergence of an adaptive NK “memory” phenotype. Central to a strong T cell response is the HLA molecule, and T cell response to HCMV may be predicted by HLA genotype. An HCMV-activated NK cell can reciprocally regulate the magnitude of the T-cell response. Using a unique fully autologous in vitro HCMV infection system, regulatory relationships between NK cells and T cells can be investigated and identified. Preliminary studies have generated the novel finding that direct interaction of NK cells with HCMV-infected dendritic cells induces PD-L1 surface expression on NK cells, which directly inhibits antibody-mediated T cell proliferation. To determine the impact of PDL1+ NK cell on HCMV antigen specific T cells, T cell responses to HCMV infection will be stimulated by the use of an artificial antigen-presenting cell pulsed with CMVpp65 peptides. Co-incubation of CMV-activated PDL1+ NK cells and CMV-sensitized T cells will permit elucidation of regulatory effects of one lymphocyte on the other. We will utilize a humanized mouse model for the study of NK and T cell interaction in response to HCMV. The understanding of NK and T cell relationships developed in the in vitro and in vivo studies will be applied to the study of expansion and maintenance of adaptive NKG2C+ NK cells in healthy human donors as well as in patients with CMV reactivation following stem cell transplantation. The availability of blood samples from a cohort of hematopoietic cell transplant patients with CMV reactivation provides the unique opportunity to examine the NK and T cell repertoire in the setting of CMV infection. These studies will describe novel mechanisms that will expand existing knowledge on the balance of the adaptive and innate immune systems during viral infection and will provide further insight to mechanisms that can be targeted to prevent reactivation and chronic viral infection.