Selective Binding of Lipid A to Responder and Nonresponder

125I-Iabeled lipid A is selectively bound to B cell-enriched populations. A combined method that employs autoradiography to detect binding of 125I-Iabeled lipid A and im-munofluorescence to detect B cells is described. With this analytic technique it was demonstrated that 25070-30070 of B cells in the spleen exhibit binding and that the bind-ing by splenocytes from lipopolysaccharide responder mice cannot be distinguished from that of nonresponder mice. Lipopolysaccharide (LPS) induces a wide variety of effects on biologic systems [1, 2]. Perhaps the most frequently studied of these phenomena is the capacity of LPS to function as a B cell-specific mitogen [3-6]. Other studies have established that the lipid A moiety of LPS is responsible for the mitogenic properties of LPS [7, 8]. Several strains of mice have been identified that do not exhibit the usual mitogenic response to LPS [9-11], and genetic tests have identified the chro-mosomallocation of the defective genes [12, 13]. It is important to note that nonresponder mice are simultaneously defective in cellular responses to LPS other than that of B cell mitogenesis [14, 15]. Furthermore, at least three different kinds of cells- B cells [4], macrophages [16], and fibro-blasts [17]- have been reported to exhibit reactiv-ity to LPS. The possibility of another reactive cell type or subtype might be considered in light of the finding that LPS enhances the secretion of non-immunoglobulin proteins by cells in bone marrow but not by cells in a variety of other tissues [18]. Earlier attempts to investigate LPS binding by cells employed biosynthetically labeled LPS [19-21]. With such products the binding by thymocytes and splenocytes was essentially equivalent. Clearly, the possibility of studying the binding of LPS in relation to its activity as a B cell mitogen becomes remote under such circumstances. For example, no meaningful information could be derived by th