Cell Lines By KENNETH GRABSTEIN , JUNE EISENMAN

The differentiation of resting B lymphocytes to Ig secretion involves several sequential steps regulated by antigen, T lymphocytes, and macrophages. The requirement for T cells may in some cases be replaced by lymphokines secreted upon T cell activation (1, 2). A number of distinct T cell-derived lymphokines have been described (3-9) that directly regulate the growth and maturation of B cells. These include IL-2 (3, 4), IFN-3, (5, 6), and at least two additional, yet less well-characterized molecules, B cell stimulating factor, (also known as B cell growth factor, BCGF or BSF-1) ~ (7, 8), and B cell differentiation factor (BCGFII or BCDF) (9). BSF-1 was originally (7) described as a factor that stimulated proliferation of B cells in conjunction with a submitogenic concentration of antiIg. Recent studies (10, 1 1) have shown that semipurified BSF-1 acts on resting B cells, facilitating their entry into S phase upon subsequent interaction with anti-Ig. These studies (1 1) indicate that BSF-1 might actually be a differentiation factor, and they further suggest that BSF-1 may not have growth factor activity. We have purified BSF-1 to homogeneity from culture supernatants of mitogenactivated EL4 thymoma cells. We used the proliferation of highly purified splenic B cells in the presence ofanti-IgM as an assay in our purification procedure. The purification was also monitored for additional lymphokines using the factordependent cell lines CTLL-2 and FDC-P2. Interestingly, fractions containing BSF-1 always contained stimulatory activity for both IL-2and IL-3-dependent cell lines, even though these fractions were devoid of IL-2 and IL-3 protein. Nterminal amino acid sequencing of homogeneous BSF-1 revealed a unique protein sequence completely different from that of murine IL-2 or IL-3. Based on these results, we conclude that BSF-1 is both a growth and differentiation factor that may have biologic effects beyond the B lymphocyte compartment.