CD4+ T cells represent a major cell component of the adaptive immune system. Naive CD4+ T cells differentiate into different types of effectors in periphery upon encounter with antigen-presenting dendritic cells. Effectors are composed of fairly diverse cell populations called subsets. During the initial differentiation process, signals via various receptors on T cells, e.g., T cell receptor, co-stimulatory molecules, and cytokine receptors, affect the differentiation fate. While the function of CD4+ T cell subsets is relatively conserved among mammals, the major pathways involved in the differentiation process are different in many instances. Furthermore, even within human-derived naïve CD4+ T cells, cells isolated from different human organs, e.g., cord blood, adult peripheral blood, tonsils, and spleen; and isolated from subjects with different ages, e.g., infants, young adults, and elderly, display different features and differentiation trajectory. The molecular and epigenetic mechanisms underlying the differences in the CD4+ T cell differentiation among species, anatomical sites, and ages remain ill defined. In this project, we aim to define the genetic, transcriptomic, and epigenetic factors that regulate the differentiation of naïve CD4+ T cells that differ in origins. In parallel, we will analyze monocytes as a representative cell type within the innate immune system. We anticipate to reveal the molecular mechanism by which immune cells respond differently among species; mouse, macaques, and humans (Aim1); among anatomical sites in human organs; peripheral blood, tonsils, spleen, and gut (Aim 2); among different ages (Aim 3).