Signaling to a CD5+ B-cell clone through surface Ig and MHC class II molecules.

Cells of the CD5+ mouse B-cell clone CH12.LX are induced to become antibody-secreting cells by costimulatory signals delivered by binding of their surface Ig and MHC class II molecules. Class II-mediated signals can be delivered by the binding of either T cells or class II-specific monoclonal antibodies. Divalent, but not monovalent antigen-binding fragments of mAbs are effective in signalling, and cross-linking intact anti-class II mAbs with isotype-specific Ab enhance class II-mediated signaling. Class II-mediated signaling is accompanied by a rise in cAMP and is blocked by an adenyl cyclase inhibitor. The cAMP analogue dibutyryl cAMP, can partially but not completely substitute for the class II-mediated signal. The costimulatory Ig-mediated signal can be delivered by binding of either antigen or antiidiotype Ab, but anti-IgM Abs are much less effective. Antibodies specific for Ig constant regions are much more effective, however, if they engage both the mIgM and mIgD molecules; anti-kappa Abs or a combination of anti-IgM and anti-IgD Abs were more effective in signaling.