Isotype switching in anti-immunoglobulin-activated B lymphoblasts: differential requirements for interleukin 4 and other lymphokines to elicit membrane vs. secreted IgG1.

Anti-immunoglobulin (Ig)-activated B lymphoblasts, prepared by culturing high-density B cells with anti-Ig-Sepharose for 48 h, can be induced to secrete IgM and IgG1 by a mixture of T cell-derived lymphokines containing interleukin (IL) 4. In this study we have examined the conditions required for lymphokine-mediated induction of IgG1 secretion and membrane (m)IgG1 expression in B lymphoblasts. Resting B cells exposed to IL 4 (10-100 U/ml) during anti-Ig-mediated blast transformation did not secrete IgG1 upon subsequent culture with lipopolysaccharide (LPS) regardless of whether IL 4 was present or absent during the secondary culture. In contrast, B lymphoblasts previously exposed to IL 4 did secrete IgG1 in response to T cell-derived lymphokines [EL 4 supernatant depleted of IL 4; (D)EL 4 SN]. However, optimal IgG1 secretion was obtained when B lymphoblasts were simultaneously exposed to IL 4 and other lymphokines. Pre-exposure to (D)EL 4 SN, which contains IL 5 and IL 2, failed to prepare anti-Ig blasts to secrete IgG1 in response to LPS and IL 4. Inhibition of IL 5 and IL 2 activity in EL 4 SN suppressed IL 4-mediated IgG1 secretion. Together, these data indicate that B lymphoblasts require IL 5 and IL 2 in addition to IL 4 to secrete IgG1, and that the IL 4 signal(s) must precede or accompany those provided by the other lymphokines. As a measure of the fraction of cells capable of switching to IgG1, we assessed expression of mIgG1 on B lymphoblasts by fluorescence flow cytometry. B lymphoblasts cultured for 3 days with (D)EL 4 SN and IL 4 (10-100 U/ml) were 8% to 20% mIgG1+; in the absence of IL 4 blasts did not express detectable mIgG1. Although anti-Ig blasts treated with LPS and IL 4 did not secrete appreciable IgG1, a substantial fraction of B lymphoblasts (4% - 19%) cultured with LPS and IL 4, but not LPS alone, expressed mIgG1. These results suggest that LPS and IL 4 are sufficient to commit B lymphoblasts to mIgG1 expression, but that additional signals provided by T cell-derived lymphokines are required to elicit IgG1 secretion.