Summary Cytomegalovirus (CMV, a b-herpesvirus) establishes a persistent infection that is endemic in humans and mice. CMV causes acute clinical disease only if immunity is naïve or compromised, exemplifying how coevolution with its host over millennia has resulted in a largely non-pathogenic détente. However, this `percolating' infection imposes a huge footprint on the immune system, especially the T cell memory pool, and growing evidence suggests this may ultimately be detrimental. Mounting a rapid CD4 T cell response correlates with protection from CMV-associated disease and reduced viral shedding. Our lab identified the first mouse CMV (MCMV) epitope-specific CD4 T cells. Of these, one response against the viral M09 protein expands long after systemic viral infection is controlled, and is critical for resolving the persistent-phase of CMV replication. Our new data show these `late-rising' M09 cells are unique with regards to their phenotype, effector function and transcriptome when compared to conventional CD4 T cells that expand early during infection. Additionally, vaccine-induced late-rising CD4 T cells provide potent protection against CMV challenge. Our overarching hypothesis is that distinct immune environments during the acute and persistent phases of CMV infection promote the expansion/differentiation of these late-rising cells that ultimately resolve persistence and promote latency establishment. We will determine why these cells show such delayed expansion, determine whether they use unique mechanisms of cytolysis to kill infected cells and resolve persistence and initiate studies to determine whether similar CD4 T cells exist in CMV infected people. Together this proposal represents a comprehensive plan using unique viral tools and omics-based approaches to elucidate how newly identified late-rising CD4 T cells function to resolve persistent CMV replication, a question that has remained largely unaddressed to date.