Synergistic interaction between interleukin 4 and anti-Bp50 (CDw40) revealed in a novel B cell restimulation assay

Highly purified resting B lymphocytes stimulated for 3 days to high-rate DNA synthesis by a synergistic combination of phorbol dibutyrate and ionomycin soon returned to quiescence once those signals had been removed. The maintenance of DNA synthesis in such cultures was found to provide a sensitive assay for revealing the factors that interact with cycling B cells. Whereas several activities-namely, interleukin 4, anti-Bp50 and a low molecular weight B cell growth factor-were, by themselves, capable of prolonging DNA synthesis over a further day or so, no single factor was capable of sustaining the replication cycle out to day 6 of culture. By contrast, certain combinations of activities displayed significant synergy in the restimulation assay. The most striking observed was that between interleukin 4 and anti-Bp50 where, by day 6, their combined effect on maintaining DNA synthesis in 3-day stimulated cells was the same as having kept phorbol dibutyrate and ionomycin in the culture system. The implications of these findings are discussed.