B cell responses to a peptide epitope. IX. The kinetics of antigen binding differentially regulates costimulatory capacity of activated B cells

We explore the possible mechanism by which association rates of Ag with activated B cells influences the ability of the latter to selectively recruit Th subsets. Our system used cocultures of Ag-activated B and T cells, where the Ag was a synthetic peptide, G41CT3. Restimulation was with either peptide G41CT3 or its analogue, G28CT3. Peptide G28CT3 has been previously shown to display a higher on rate, relative to the homologous peptide G41CT3, of binding to G41CT3-activated B cells. This difference in on rates was eventually exerted at the level of IFN-gamma, but not of IL-10, induction from T cells, with peptide G28CT3 proving more effective. However, various treatment regimens rendered peptide G41CT3 as potent as peptide G28CT3 at eliciting IFN-gamma responses from the above cultures. This included simultaneous treatment of B cells with peptide G41CT3 and the protein tyrosine kinase inhibitor tyrphostin. Alternatively, pretreatment of B cells with a peptide representing only the B cell epitope constituent of peptide G28CT3 (G28) was also equally effective. Subsequent experiments revealed that IFN-gamma production from activated T cells resulted from an engagement of CD28 by B7-1 on the B cell surface. Finally, the extent of cell surface B7-1 up-regulation on activated B cells was dependent on the on rate of Ag binding to the membrane-bound Ig receptor. Thus, cumulative results suggest that the kinetics of Ag binding to activated B cells can differentially regulate intracellular signaling. This influences selective costimulatory molecule expression, with its consequent effects on relative Th subset activation.