B-cell stimulatory factor-1 (BSF-1), formerly designated B-cell growth factor, is a T-cell-derived factor required for entry into the S phase of the cell cycle by B cells stimulated with low concentrations of anti-IgM antibodies. BSF-1 acts directly on resting B cells to prepare them to synthesize DNA more promptly on subsequent exposure to competent stimuli and to strikingly enhance their expression of class II molecules of the major histocompatibility complex. Previous studies have shown that murine BSF-1 can be separated physically from interleukin-2 (IL-2) and that the molecule has an apparent relative molecular mass (Mr) of approximately 15,000 and pI values of 6.4-6.7 and 7.4. Here, we report the production of a monoclonal antibody to BSF-1, its use in characterizing BSF-1, and functional studies demonstrating that this molecule is distinct from IL-1, IL-2 and IL-3.

The present studies demonstrate that both T-cell-derived supernatants containing B-cell growth factor (BCGF or BSF) and a partially purified preparation of the B-cell growth factor (BSF-p1) induce an increase in the expression of IA and IE-encoded antigens on small resting B cells. This increase is detectable by 6-8 hr after initiation of culture and is relatively selective, since levels of surface immunoglobulin and H-2 antigens do not increase to the same extent. Although interferon-gamma induces increased expression of Ia antigens on macrophages and dividing neoplastic B cells, it does not induce an increase in the expression of Ia antigens on resting B cells. These results demonstrate that BSF-p1 may play two roles: (i) it acts on resting B cells to increase the levels of Ia antigen expression; and (ii) it sustains the growth of B cells that have been previously activated with mitogens, antigens, or anti-Ig.

By three criteria, we have demonstrated that B cell stimulatory factor (BSF-1) and B cell differentiation factor (BCDF-gamma) are the same lymphokine. Highly purified preparations of high performance liquid chromatography-purified or affinity-purified BSF-1 had BCDF-gamma activity but not BCDF-mu activity. A monoclonal anti-BSF-1 antibody coupled to Sepharose depleted both BSF-1 and BCDF-gamma activity but not BCDF-mu activity from two different T cell supernatants. Soluble monoclonal anti-BSF-1 blocked the BSF-1 and BCDF-gamma but not the BCDF-mu responses. These results suggest that BSF-1 acts on both resting and activated B cells to induce different effects.

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