Induction or suppression of B cell proliferation and differentiation by phytohemagglutinin or concanavalin A in mouse spleen cell cultures.

Cultures of mouse spleen cells or various mixtures of mouse T and B cells were stimulated with PHA or Con A, and the T, B, or plasmablast nature of the transformed cells, was determined by immunofluorescence 2 to 4 days later. The lectins enhanced B cell proliferation and plasmablast differentiation ("helper" effect) provided one of the following conditions was fultilled: a) suboptimal doses of lectin were used, b) cultures were performed at low cell concentration, c) cultures were made of spleen cells containing a small percentage of T cells, d) the cultures contained a mixture of T-depleted spleen cells and T cells rendered unable to proliferate by irradiation. In contrast, cultures performed with 1.5 10(6) or more spleen cells/ml and optimal doses of lectin contained almost exclusively T blasts, as did cultures stimulated in the same conditions with both PHA and LPS. This last observation idicates the existence of a lectin-induced "suppressor" effect, since LPS, a B cell mitogen, induces, in the absence of PHA, a marked B cell proliferation and differentiation into plasmablasts. These helper and suppressor effects were entirely mediated by T cells, since they were not observed in spleen cell cultures depleted in T cells by anti-thets + C. Analysis of the cultures by immunofluorescence and radioautography after pulses of 3H-thymidine showed that these antagonistic effects could be related to the number of T blasts present in the culture and to their proliferative behavior. Heoper effect is observed in cultures containing a relatively low number of T blasts (or none in cultures made with irradiated T cells), whereas suppressive effect is observed in cultures containing a high number of T blasts, a large proportion of them having left the proliferative cell cycle. It is proposed that when a critical concentration of T blasts is reached ("saturation density"), further proliferation and differentiation is prevented, resulting in a suppressive effect on the generation of plasmablasts. The helper effect of lectin-activated T cells seems to be exerted on a subpopulation of B cells which was, at least in part, already proliferating in vivo, and to result in a polyclonal IgM plasmablast differentiation.