Regulation of lipopolysaccharide-induced B-cell activation: Evidence that surface immunoglobulin mediates two independently regulated signals

An antigen-specific B cell response can be induced by low concentrations of haptenated lipopolysaccharide (LPS), whereas high concentrations are inhib itory Two explanations have been proposed for the latter phenomenon. In the first, specific surface Tg focuses LPS to the B cell membrane, where high local concentrations of the mitogen become paralytic for B cell responses. In the alternative, transmission of an antigen signal at higher concentrati ons of hapten LPS actively inhibits the development of antibody secreting c ells (ASC). In the present paper, the immunosuppressant cyclosporine A (CsA ) was used to attempt to distinguish between these two models. Cyclosporine A did not block the inhibitory effects of goat anti-IgM (g alpha IgM) on d evelopment of ASC induced by LPS and therefore was unsuitable for testing b etween the two models. However, surprisingly, in the presence but not the a bsence of CsA, even low concentrations of g alpha IgM became inhibitory for LPS-induced B cell proliferation. Thus, a CsA-insensitive signal could inh ibit both B cell proliferation and the development of ASC. In contrast, the CsA-sensitive signal induced by sig required high concentrations of g alph a IgM for triggering and enhanced the LPS proliferative response without af fecting development of ASC. Evidence is presented that these two signals ar e regulated independently, suggesting that together they may transmit infor mation about the physical form of an antigen to the B cell.