CROSS-LINKING CELL-SURFACE CLASS-II MOLECULES STIMULATES IG-MEDIATED B-CELL ANTIGEN-PROCESSING

Th cells bind to peptide-class II complexes presented on B cell surfac es. Recent evidence indicates that upon cross-linking, class II molecu les transduce signals that modulate a variety of B cell functions. One possible function of class II signaling is to regulate the assembly o f processed Ag-class II complexes. Here we show that cross-linking B c ell surface class II molecules augments the processing and presentatio n of an E(k)-restricted Ag to a specific T cell hybrid. Significantly, class II cross-linking only affects processing initiated by Ag bindin g to the surface Ig. The processing of Ag taken up by fluid phase pino cytosis is not affected by class II cross-linking, nor is the presenta tion of an antigenic peptide that does not require processing. Augment ation of Ag processing is enhanced by treatment of B cells with dibuty ryl cAMP, a second messenger in the class II signaling pathway. The cr oss-linking of class II molecules does not alter the rate or number of Ig molecules internalized or the biosynthesis or expression of E(k) m olecules. Moreover, changes in the expression of the B7 family of cost imulatory molecules or the adhesion molecule LFA-1 (CD11a/CD18) induce d by class II cross-linking do not appear to account for the augmentat ion of processing observed here. Thus, the cross-linking of class II m olecules on B cell surfaces selectively stimulates Ig-mediated Ag proc essing, indicating that a step in this pathway is a target of class II -mediated signaling events.