Direct effects of a monoclonal B cell differentiation factor and of purified interleukin 1 on B cell differentiation.

We have previously described the presence of factors in mixed lymphocyte culture supernatants that induce activated but not resting human B cells to secrete Ig. In the present study, we describe the effects of a B cell differentiation factor in the supernatant (SN) of a T-T hybridoma (clone 7D5). We find that it induces Ig secretion by human B cells in the absence of T cells or monocytes. It acts only on activated B cells, because small (resting) B cells isolated by centrifugal counterflow elutriation do not respond to it, whereas the same cells do develop into Ig-secreting cells if activated in culture with Staphylococcus aureus Cowan I before exposure to SN 7D5. By using class-specific reagents in enzyme-linked immunosorbent assays, we found SN 7D5 to result in the secretion of significant amounts of IgM, IgG, and IgA. We also studied the effects of highly purified interleukin 1 on this differentiation process. Interleukin 1 by itself failed to induce Ig secretion by activated B cells, and its presence was not required for the induction of Ig secretion by SN 7D5. However, interleukin 1 consistently synergized with SN 7D5 in inducing Ig secretion by purified B cells.