The objective of this proposal is the biochemical isolation and characterization of immunoregulatory molecules controlled by genes in the I-C subregion of the murine H-2 gene complex. Secreted I-C plus products of alloantigen activated spleen T cells suppress alloreactive T cell proliferative responses in mixed leukocyte reactions (MLR). Three components of the MLR suppressor system will be analyzed: (1) the secreted I-C plus suppressor factor, MLR-TsF; (2) the target cell receptor for MLR-TsF; and (3) the T suppressor cell surface I-C alloantigen. MLR-TsF will be isolated and characterized using established biochemical and immunochemical methodologies. Purified MLR-TsF will be analyzed for antigenic and specificity characteristics, and will be used for production of MLR-TsF specific antisera. In addition, receptor structures for MLR-TsF will be characterized on target responding cells and cell lysates; these receptors complexed with MLR-TsF will be isolated from target cell membranes and characterized. Finally, MLR suppressor T cell I-C alloantigens will be isolated by immunoprecipitation from selected populations of radiolabeled cells and structurally analyzed. In support of various of these studies, continuous T cell lines which express and/or secrete I-C plus products will be established. The studies described in this proposal will provide important new information concerning both the gene products of the I-C subregion and immune regulation of alloreactive T cell resposes.