Abstract HIV infection causes a rapid loss of CD4+ T cells that is attributed to both direct cytotoxicity of infected CD4+ T cells and bystander loss due to high immune activation. In addition to the absolute decrease of CD4+ T cells, a significant decrease in both frequency and number of long-lived memory CD4+ T cells is observed in untreated HIV infection. Antiretroviral therapies (ART) now reliably achieve viral control (<50 copies/ml) and significantly restore CD4+ T cell levels in people living with HIV (PLWH) within months of treatment initiation. Similarly, ART restores CD4 T cell memory formation as evidenced by an increase in frequency and number of long-lived memory CD4+ T cells and vaccine-induced immunity. However, to date studies have provided limited data on when and how quickly CD4+ T cell memory is restored upon ART initiation particularly in the days to weeks following ART. A greater understanding of CD4+ T cell memory formation following ART initiation may inform the formation of the HIV latent reservoir, the majority of which is harbored in long-lived memory CD4+ T cells. The goal of this study is to characterize in detail CD4 + T cell memory restoration following ART initiation. To do this, we have obtained samples from PLWH during untreated infection, 2, 48 and 96 weeks post ART initiation and used the mass cytometry Maxpar Direct Immune Profiling Assay (Fluidigm) to phenotype these samples. Data to date show that CD4+ T cell memory restoration occurs very rapidly, within 2 weeks of ART initiation.