The effect of cytokines and mitogens on the induction of C epsilon germline transcripts in a human Burkitt lymphoma B cell line.

IL-4 is known to induce a 1.8 kb constant epsilon (C) transcript in human peripheral blood mononuclear cells (PBMC). This is a C germline or 'sterile' transcript which is encoded by a germline exon (I) located 5' of the epsilon switch (S) region and the four C germline exon including 3' untranslated region. Among the human B cell lines examined, one Burkitt lymphoma B cell line, DND39 (Epstein-Barr virus negative, sIgM+, CD40+), was found to respond to IL-4 and express the C germline transcript. Induction of C germline transcript in DND39 cells was entirely dependent on the presence of IL-4. Phorbol myristate acetate enhanced its expression in DND39 cells as well as in the purified B cells in combination with IL-4. Pokeweed mitogen (PWM) enhanced the expression of C germline transcripts induced by IL-4 in PBMC but the enhancing activity was not observed when purified B cells or DND39 cells were used. Supernatants obtained from PWM-stimulated PBMC strongly enhanced the expression of C germline transcript in DND39 cells when co-stimulated with IL-4. None of the known cytokines tested mimicked this effect, suggesting that PWM may induce in T cells or monocytes a novel cytokine production that works synergistically with IL-4 and enhances the expression of IL-4-induced C germline transcripts. A similar enhancing effect was observed when anti-CD40 mAb was added to the IL-4-stimulated DND39 cells. Interestingly, transforming growth factor-beta strongly suppressed it. Taken together, DND39 cells resemble normal B cells in terms to responsiveness to IL-4 and should be a useful tool to study the molecular mechanisms of regulation of C germline transcription and class switching to IgE.