B lymphocytes are responsible for the production and secretion of antibodies which are an essential component of the immune response. In order to effect this function. B cells must recognize and bind specific antigen and mount an appropriate response. An appropriate response involves activation, proliferation and differentiation to plasma cells or persistence in the circulation as memory cells. Many of the mechanisms that regulate these events are controlled by molecules which interact with B cell-specific surface receptors. The long term objective of this research is to understand in precise molecular terms how B cell surface receptors mediate signals which activate the cells, drive them through the cell cycle and induce them to differentiate. To address this problem, the genes encoding a B cell surface glycoprotein CDw40, which plays an important role in B cell activation will be cloned, expressed in mammalian cells and studied by a new genetic technique which employs saturation mutagenesis and a selection procedure to yield mutant receptors showing altered affinity and specificity. Because the characterization of a receptor can provide only a partial understanding of its role in cell function, the isolated clones will be used to create soluble forms of the encoded receptor in order to identify and isolate its physiologic ligand(s). Isolation of ligands will provide a means for the precise study of receptor-ligand interaction. To facilitate this endeavor, cDNA clones encoding CDw40 will be re-introduced into mammalian cell lines that do not constitutively express them, or mutant cell lines that have lost their expression. Subsequent introduction of mutant cDNA clones encoding the putative ligand(s) will enable an assessment of how modifications in receptor-ligand binding affinity may influence B cell function.