The concept that plasma lipoproteins can regulate cellular activation and proliferation is well established. Among the numerous regulatory phenomena that are mediated by either native or modified lipoproteins is the suppression of lymphocyte function. A biologically active moiety of the human immunosuppressive lipoprotein, LDL-In, that possesses regulatory activity characteristic of the native lipoprotein, has been identified as apoprotein (apo) E. Apo E is also one of the major proteins synthesized and secreted by macrophages. This synthesis is enhanced by plasma lipoprotein-induced cholesterol or cholesteryl ester loading and inhibited by immunologic activation of the cells. An essential biologic theme of the immune response is the use of cellular collaboration via the synthesis of soluble effector molecules to regulate specific responses to relevant stimuli. Therefore, it is both timely and pertinent to investigate the relationship between the biologic properties of macrophage-secreted apo E and plasma lipoprotein-associated apo E. Because we macrophage-secreted apo E has potent inhibitory activity for lymphocyte activation, the major objective of this renewal is to examine the hypothesis that a regulatory network involving macrophage-derived apo E exists between lymphocytes and macrophages. This hypothesis will be examined by pursuing four specific aims. The first specific aim is to identify what factors influence the amount of apo E secreted by macrophages including the roles played by lipoprotein- and platelet-induced cholesterol and cholesteryl ester loading as well as various immunologic and/or differentiation stimuli. The second specific aim is to define conditions for optimal production and isolation of macrophage- secreted apo E and to biochemically characterize the secreted product. The third aim is to identify the fate of macrophage- secreted apo E and to determine if it interacts directly with lymphocytes or indirectly via the formation of stable lipoproteins. Finally, the structural regions of apo E that are responsible for its inhibitory activity will be identified using monoclonal antibodies and synthetic peptides.