Anti-CD45 inhibition of human B cell proliferation depends on the nature of activation signals and the state of B cell activation. A study with anti-IgM and anti-CDw40 antibodies

The proliferation of human peripheral and tonsillar B cells stimulated with the anti-CDw40 mAb 626.1 and/or anti-IgM antibody (Ab) in the presence of anti-CD45 mAb A.1.1 was investigated. The anti-CD45 mAb suppressed the anti-CDw40-stimulated proliferation of peripheral blood B cells but had no effect on the proliferation of unfractionated tonsillar B cells stimulated similarly. When tonsillar B cells were separated according to their sizes, the anti-CDw40-induced proliferation of small tonsillar B cells was inhibited by the anti-CD45 mAb, whereas large tonsillar B cells were resistant. In contrast, anti-IgM-induced proliferation of human B cells was always significantly inhibited by the anti-CD45 mAb regardless of cell size and tissue origin. The anti-CD45 mAb also inhibited the anti-IgM-induced initial rise in intracellular [Ca²⁺] and the G₀-G₁ cell cycle transition of small tonsillar B cells. However, co-stimulation with anti-IgM/anti-CDw40 Ab resulted in the resistance to the anti-CD45 inhibitory effect on proliferation of peripheral blood B cells and the majority of tonsillar B cells. In contrast, B cell proliferation co-stimulated with anti-IgM Ab/and B cell growth factors were always suppressed by the anti-CD45 mAb. These results demonstrate that certain activational signal mechanisms utilized by anti-CDw40/anti-IgM Ab and anti-IgM Ab/B cell growth factors are different in that B cells stimulated with these agents differ in their sensitivity to the anti-CD45 mAb. Moreover, both the activational state of human B cells and the nature of activation signals given determine their response to the inhibitory signals delivered by the anti-CD45 mAb.