CD40 CROSS-LINKING INDUCES IG-EPSILON GERMLINE TRANSCRIPTS IN B-CELLSVIA ACTIVATION OF NF-KAPPA-B - SYNERGY WITH IL-4 INDUCTION

Transcription of unrearranged (germline) Ig heavy chain C region (C-H genes is required before Ab class switch recombination. Although the c ytokine IL-4 is well known to induce transcription of unrearranged C e psilon and C gamma, genes, it has been shown recently that CD40 signal ing also induces these transcripts in mouse B cells. We report in this study that treatment of mouse M12.4.1 B lymphoma cells with soluble C D40 ligand (CD40L)-CD8 alpha fusion protein modestly induces the promo ter for germline epsilon transcripts, and that this induction synergiz es with IL-4. CD40L induces binding of nuclear factor (NF)-kappa B/Rel proteins to two tandem kappa B sites located immediately 3' to the IL -4-responsive region of the mouse germline epsilon promoter. The epsil on-124/-56 promoter segment containing the IL-4 response region and th e two kappa B sites is sufficient to transfer CD40L and IL-4 inducibil ity to a minimal c-fos promoter when transiently transfected into M12. 4.1 cells. Mutation of the two kappa B sites eliminates induction by C D40L or by IL-4, and treatment of M12.4.1 cells with inhibitors of NF- kappa B activation prevents induction of endogenous germline epsilon t ranscripts in M12.4.1 cells. In addition to the NF-kappa B/Rel complex es induced by CD40L, two nuclear complexes, each which contain both ST AT6 and NF-kappa B/Rel proteins, are induced in splenic B cells by a c ombination of CD40L and IL-4, and bind to the CD40L/IL-4-responsive re gion of the germline epsilon promoter. The presence of these complexes may explain the synergistic induction of transcription by CD4OL and I L-4 mediated through this promoter segment.