Functional role of self IA molecules in polyclonal B cell activation using an autoreactive B cell clone derived from...

The mechanism of polyclonal B cell activation in autoimmune diseases was investigated by using an autoreactive B cell clone established by somatic hybridization with B cells derived from NZB X NZW (B/W) F1 mice. Briefly, splenic B cells from B/W F1 mice were fused with M12.4.1, a mutant of a B cell line, in the presence of polyethylene glycol and DMSO. NW47.7, a subclone of a resulting hybridoma, expresses B cell surface antigens on the cell membrane, namely IAd, IgM, B220, the receptors for the C3 fragment of complement (C3R), and the Fc portion of IgG (Fc gamma R). It also possesses a receptor molecule for mouse red blood cells treated with bromelain (Br-MRBC) on its surface, by rosette-forming assay with Br-MRBC. In contrast, parental M12.4.1 does not express IAd and IgM on the cell membrane, and does not bind to Br-MRBC under the same conditions. Thus, it is likely that NW47.7 may be an autoreactive B cell clone specific for Br-MRBC. Interestingly, NW47.7 was induced to generate a significant number of IgM-secreting cells when treated with Br-MRBC and rIL-5. Furthermore, mAb against IAd molecules, but not IAk and KdDd, markedly inhibited the differentiative effect of polyclonal activators such as LPS and rIL-5. Also, when MHC identical irradiated B cells were added to the culture of NW47.7 as a stimulator, the induction of IgM-producing cells was greatly augmented, but this augmenting effect was lost by interfering with direct contact of NW47.7 cells with stimulator B cells using a semipermeable membrane, as well as by the addition of mAb against IAd molecules. In addition, irradiated NW47.7, but not M12.4.1, by itself could enhance the secretion of IgM by NW47.7 as a stimulator, but this enhancing effect markedly decreased in the presence of anti-IAd mAb. The present results suggest that surface IA molecules on B cells are involved during the differentiative response to polyclonal activators, and may directly provide a differentiative signal for maturation of B cells into IgM-secreting cells.