Stimulation of murine B lymphocytes by isolated C3b

Addition of isolated B3b to murine lymph node cell cultures induced increased DNA synthesis. The stimulation is dependent on the dose of C3b added and is potentiated by fetal calf serum present in the medium. Isolated C3 is less stimulatory than C3b; C3a and C3c had no effect on DNA synthesis in these cultures. 10-20% large immunoglobulin containing blastlike cells developed in lymph node cell cultures stimulated by 50 mug C3b in the presence of 3% fetal calf serum. The stimulation by C3b was observed in cultures of lymph node and spleen cells of several mouse strains including C3H/HeJ and congenitally thymus-deficient (nu/nu) mice. The results suggest that B lymphocytes are the main target of the stimulatory effect of C3b. Two mechanisms which may be involved in the stimulation of lymphocytes by C3b are discussed: (a) cross-linking of receptors on the cell surface or between cells, and (b) the binding of C3b to receptors of B lymphocytes and the formation of the complement enzyme C3bB. The results are compatible with the suggestion that activation of C3 is part of the events triggering the B lymphocytes.