Human helper T cell factor(s). III. Characterization of B cell differentiation factor I (BCDF I)

Staphylococcus aureus Cowan I (SAC) induced proliferation of human B cells but did not lead the cells to differentiate to immunoglobulin (Ig)-producing cells. The differentiation of SAC-stimulated B cells to IgG-producing cells was totally dependent on T cells or T cell factor, designated B cell differentiation factor I (BCDF I). BCDF I exerted its effect on B cells between 48 and 72 hr after the initiation of the culture, at which time the 3H-TdR uptake of SAC-stimulated B cells had already reached a maximal level. Highly purified human IL 2 did not induce the differentiation of SAC-stimulated B cells for IgG-producing cells. Therefore the differentiation-inducing ability of BCDF I was not due to the residual T cells in a purified B cell preparation, which might have been activated by a possible contamination of IL 2 in the BCDF I preparation. The apparent m.w. of BCDF I was estimated to be 20,000 by gel filtration. Isoelectric focusing analysis showed that its activity was distributed in three fractions corresponding to pI 6.5, 7.0, and 8.0. In contrast, BCDF II, which induced IgG production in an Epstein Barr virus-transformed B lymphoblastoid cell line, was focused in the fractions with pI range of 4.8 to 5.5. Thus there are at least two different kinds of BCDF for human B cells, designated here as BCDF I and BCDF II.