Induction of germ-line gamma 1 and epsilon Ig gene expression in murine B cells. IL-4 and the CD40 ligand-CD40 interaction provide distinct but synergistic signals.

The interaction between B cell CD40 and its ligand (CD40L) on activated Th cells provides a critical signal necessary for T cell-dependent isotype switching. Previous studies suggest that this signal might be important in regulating isotype switching at the level of germ-line Ig transcription. To assess the effects of the CD40L-CD40 interaction on germ-line Ig transcript expression in murine B cells, a membrane-bound form of mouse CD40L was expressed in the baculovirus system. We show that stimulation of resting splenic B cells with CD40L-expressing Sf9 cells induces germ-line gamma 1 and epsilon transcripts independently of cytokines. The CD40-mediated induction cannot be blocked by anti-IL-4 Ab and is not mediated by other cytokines secreted endogenously in response to CD40 stimulation. Importantly, stimulation with CD40L and IL-4 together has a significant synergistic effect on germ-line transcript expression. Stimulation of CD40 does not activate the NF-IL-4-gamma 1 DNA binding factor believed to be required for IL-4-dependent germ-line gamma 1 transcription. Moreover, mutation of the NF-IL-4-gamma 1 DNA binding site in a germ-line gamma 1 promoter-luciferase reporter gene construct completely ablates IL-4 responsiveness but has no effect on responsiveness to CD40L in transient transfection assays. These results demonstrate that the CD40L-CD40 interaction and IL-4 activate germ-line Ig gene transcription by distinct but synergistic mechanisms and suggest that multiple signals may be required to induce sufficient germ-line transcription and/or germ-line transcript levels necessary to target switch recombination.