B cell growth factor activity of immunoaffinity-purified and recombinant human interleukin 2

We investigated the effect of recombinant and affinity-purified human interleukin 2 (IL 2) on human B cell proliferation. Five x 10⁴ nonadherent spleen cells that had been depleted twice of T cells were activated by 3-day culture with formaldehyde-killed Staphylococcus aureus Cowan strain I (SAC) prior to addition of tested growth factors. Cultures were harvested 72 h later. It was found that both IL 2 preparations led to optimal cell proliferation compared with a control supernatant obtained by 36 h phytohemagglutinin stimulation of spleen mononuclear cells. Moreover, the effect of such spleen supernatant on B cell proliferation correlated with the IL 2 activity since its B cell growth factor activity (BCGF) was not greater than that of purified IL 2 and no residual BCGF activity could be detected after absorption of all IL 2 activity by the IL 2-dependent cytotoxic T lymphocyte line cells. T cells, enumerated as E-rosetting cells as well as T3⁺ cells, represented 0.2 to 2% of the cells recovered at termination of the cultures (day 6) and there were <1% E-rosetting cells in freshly purified or SAC-activated (day 3) B cell populations. Therefore, we conclude that IL 2 is a growth factor not only for activated T cells but also for activated human B cells.