The objective of the proposed studies is to gain a comprehensive understanding of the lineal relationships, activation requirements, and functional capabilities of subpopulations of human CD4+ T cells. These studies are designed to test the hypothesis that the immunoregulatory functions of CD4+ cells are performed by distinct subsets that are recognizable by the presence or absence of membrane antigens variably expressed at sequential stages of post- thymic differentiation. We further hypothesize that the appearance or loss of these antigens is regulated by lymphokines, particularly IL-4 and IL-2. Our specific aims are to determine whether immature CD4+CD45R+ "virgin" T cells can be induced to differentiate in vitro into two distinct subsets of CD45R"-memory" T cells which have either helper or suppressor-inducer functions, and which can be distinguished by the absence or presence, respectively, of Leu 8 antigen expression. Subsets of CD4+ cells purified by panning or fluorescence activated cell sorting will be activated under different conditions to determine the induction stimuli and lymphokines required for their phenotypic and functional maturation. The kinetics and specificity of these phenotypic changes will be determined using three-color flow cytometry techniques. Functional correlation will be obtained by examining the immunoregulatory capabilities of the evolving CD4+ subsets at serial intervals during their in vitro maturation. Findings from these studies will then be used to analyze and interpret the pattern of CD4+ cell repopulation after different types of bone marrow transplantation, with the goal of determining the role of abnormal CD4+ cell differentiation in post-transplant immune deficiency. Studies characterizing the generation of CD3+CD45R+ thymocytes by IL-4 will also be performed, using an in vitro culture system and three-color flow cytometry. In addition, the relationship of these CD45R+ thymocytes to a population of IL-4- responsive cells uniquely found in neonatal cord blood will be investigated. Finally, the maturation and function of CD4+ calls in patients with immunodeficiency or immunoregulatory diseases will be assessed. Definition of the ontogeny and functions of immunoregulatory CD4+ subpopulations and the ability to assess post-thymic CD4+ cell maturation in humans should enhance our understanding of the mechanisms by which quantitative or functional abnormalities of CD4+ subsets accompany or contribute to immunological disorders.