LIGATION OF CD40 INFLUENCES THE FUNCTION OF HUMAN IG-SECRETING B-CELLHYBRIDOMAS BOTH POSITIVELY AND NEGATIVELY

The effect of ligation of CD40 on the proliferation and Ig secretion o f a battery of human Ig-secreting hybridomas was examined to determine the regulatory activity of this surface molecule on B cells after ini tial activation. B cell hybridomas were generated by fusing activated peripheral blood B cells with SPAZ-4, a non-Ig-secreting fusion partne r, and were cloned before analysis, All hybridomas expressed CD40 comp arably. These hybridomas were stimulated with either recombinant bacul ovirus-expressed membrane-bound CD40L or a soluble murine CD40L/CD8 co nstruct in the presence or the absence of various cytokines. Concentra tions of CD40L that saturated 40 to 100% of CD40 induced initial homot ypic aggregation followed by Fas (CD95)-independent apoptosis, with re sultant decreases in growth and Ig secretion. Concentrations of CD40L that saturated 15 to 25% of CD40 also stimulated aggregation of all hy bridomas. However, proliferation and Ig secretion of 9 of 13 IgM-secre ting hybridomas, but none of 14 IgG- or IgA-secreting hybridomas, were enhanced by these concentrations of CD40L, These responses were indep endent of interactions mediated by the adhesion pair CD11a/CD18-CD54. These results indicate that the impact of CD40 ligation on human Ig-se creting hybridomas varies with the extent of CD40 engagement and depen ding on whether the hybridoma derived from an activated B cell that ha d previously undergone switch recombination.