Bacterial lipopolysaccharide (LPS) is one of a number of cell wall constituents of microbial origin which are polyclonal B cell activators (PBA's) and/or immunomodulatory agents. LPS is also known to interact with many other cellular and humoral host defense systems with pathological consequences to the host. The objective of the proposed research is to further our understanding of the nature of the interaction of immunomodulators with non-Ig receptors on B cells. Using LPS as the prototype ligand, we will examine the binding of LPS to target cells using immunofluorescent and radiobinding techniques. We will characterize the specific and non-specific binding, the membrane determinants of LPS-binding B cells, and the genetic control of LPS binding sites to confirm initial observations that specific LPS binding is limited to B cells in genetically LPS-responsive strains of mice. To evaluate the role of membrane phospholipids in non-specific binding, we will study the interaction of LPS with phospholipid in non-specific binding, we will study the interaction of LPS with phospholipid vesicles, examining the effect of LPS on redistribution of phospholipids and the effect of changes in membrane fluidity on LPS binding. We will produce a monoclonal antibody to the postulated specific LPS receptor and use this to characterize the receptor by PAGE analysis of immunoprecipitates of solubilized membranes of surface radiolabelled target cells. Affinity chromatography using the anti-receptor antibody will be used to purify this receptor for biochemical analysis and sequencing. The nature of LPS binding to amphibian lymphocytes will be explored using similar techniques to develop some perspective on the biological significance of mitogen receptors in the evolution and expression of humoral immunity.