TERMINAL MATURATION OF RESTING B-CELLS INDUCED BY MACROPHAGE FACTORS

Mouse splenic macrophages from BALB/c nude mice (purified by plastic a dherence) or cloned macrophage hybridomas stimulated with jacalin (12. 5 mu g/ml), a D-Gal binding lectin, produce one or more B-cell stimula tory factors which cause splenic B cells from BALB/c or C3H/HeJ mice t o secrete immunoglobulin in a polyclonal manner as detected by reverse protein A plaque assays. Jacalin-stimulated macrophage supernatants ( JacSup) activate both normal and Percoll gradient-purified small high- density (resting) B cells, Supernatants from total or resting BALB/c s pleen cells cultured for 7 days in the presence of JacSup (derived fro m splenic BALB/c nude mice macrophages) were assayed for immunoglobuli n isotypes by ELISA. Resting B cells produce only IgG3 and IgM, wherea s total B cells secrete IgG3 and IgM as well as IgG1, IgG2a, IgG2b and IgA. Resting and total B cells from BALB/c nude mice are also stimula ted by macrophage supernatants to secrete immunoglobulin, thus indicat ing that this activity is likely to be T cell independent, Moreover, j acalin-stimulated macrophage supernatants did not induce spleen cells or purified B cells to proliferate. Fractionation of factor-rich super natants on a Sephacryl S-200 column revealed that the factor activity is located in fractions corresponding to a molecular mass of 25-27 kDa . Taken together, these results suggest that upon the action of a macr ophage factor(s) resting B cells undergo terminal differentiation with out proliferation in the absence of T cells.