CD40 plays an essential role in the activation of human B cells by murine EL4B5 cells.

A mutant subclone of the murine thymoma EL-4, known as EL4B5, can strongly activate human B cells to proliferate and differentiate in a cell-cell contact-dependent manner. We have investigated whether interaction via CD40 plays a role in this helper activity. For this purpose, three newly generated anti-CD40 monoclonal antibodies (mAb) were used. In contrast with other anti-CD40 mAb described in the literature, these mAb did not co-stimulate proliferation of human B cells. On the other hand, these novel mAb could inhibit the co-stimulatory effect of the previously described anti-CD40 mAb S2C6 on anti-IgM-induced human B-cell proliferation. It was found that addition of these non-stimulatory anti-CD40 mAb could completely inhibit EL4B5-induced human B-cell proliferation. Maximal inhibition occurred already at a mAb concentration of 10 ng/ml. Similarly, a fusion protein, consisting of the extracellular portion of CD40 and human IgM constant domains CH2, CH3 and CH4, could completely inhibit EL4B5-induced human B-cell proliferation. Induction of human B-cell proliferation by EL4B5 cells was also inhibited by anti-CD40 mAb S2C6 and G28.5, but less effectively. In contrast, mAb against B-cell surface antigens CD20 or B7 had no inhibitory effects. It is concluded that interaction via CD40 is essential for the induction of human B-cell proliferation by EL4B5 cells.