The long-term objective of the proposed studies is to elucidate the molecular mechanisms by which HIV gp4l exerts it s inhibitory effects on cells of the immune system. Recent data from this laboratory have shown that a synthetic peptide sharing sequence identity with the transmembrane protein gp4l of HIV (amino acid 581-597) inhibits human immune function in vitro in a dose dependent manner. The amino acid sequence of this synthetic peptide is homologous to a highly conserved region of the animal and human retroviral transmembrane proteins shown to mediate immunosuppressive effects. The specific aims of the present proposal are to: 1) Delineate the minimum sequence of the HIV gp4l aa581-597 required for mediating immunosuppression, by testing candidate synthetic peptides in vitro for their ability to inhibit a) proliferation of cloned T to the H power and T to the C power cells of human origin, b) NK cell function and c) Ig secretion. The physiological relevance of the immunosuppression observed will be confirmed by similar evaluation of the purified native gp4l. 2) Determine whether highly immunogenic B- and T to the H power - cell epitopes mapped to the aa584-609 region of HIV gp4l overlap with the minimum sequence required for immunosuppression. Radiolabeled HIV gp4l will be used to characterize its receptor with respect to specificity, affinity and number of receptors per cell and to identify the molecular size of the receptor. Competition inhibition assays will be used to establish whether the gp4l receptor is shared by pl5E or known cytokines. 4) Characterize, at the molecular level, the receptor involved in HIV gp4l-mediated immunosuppression. A human T cell cDNA library will be screened with antibodies generated against purified receptor in order to identify, clone and sequence the gene encoding the receptor. The deduced amino acid sequence of the receptor protein will be compared to those of known surface proteins expressed on cells of the immune system. It can be anticipated that these experiments will lead to an understanding of a functionally important receptor involved in immunoregulation. The structural characterization of such a receptor is of relevance for the rational design of antagonists and the development of safe and effective subunit vaccines against AIDS.