A mutant subclone of the murine thymoma EL-4, known as EL4B5, can stro
ngly activate human B cells to proliferate and differentiate in a cell
-cell contact-dependent manner. We have investigated whether interacti
on via CD40 plays a role in this helper activity. For this purpose, th
ree newly generated anti-CD40 monoclonal antibodies (mAb) were used. I
n 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-ce
ll proliferation. It was found that addition of these non-stimulatory
anti-CD40 mAb could completely inhibit EL4B5-induced human B-cell prol
iferation. Maximal inhibition occurred already at a mAb concentration
of 10 ng/ml. Similarly, a fusion protein, consisting of the extracellu
lar portion of CD40 and human IgM constant domains CH2, CH3 and CH4, c
ould completely inhibit EL4B5-induced human B-cell proliferation. Indu
ction of human B-cell proliferation by EL4B5 cells was also inhibited
by anti-CD40 mAb S2C6 and G28.5, but less effectively. In contrast, mA
b against B-cell surface antigens CD20 or B7 had no inhibitory effects
. It is concluded that interaction via CD40 is essential for the induc
tion of human B-cell proliferation by EL4B5 cells.