CD40 ligand exerts differential effects on the expression of I gamma transcripts in subclones of an IgM+ human B cell lymphoma line.

The CD40:CD40 ligand (CD40L) interaction plays a critical role in T cell-dependent isotype switching. To elucidate the role of CD40 signaling in the activation of gamma germline transcription and as an extension, in targeting Cgamma regions for isotype switching, an IgM+ Burkitt lymphoma cell line (Ramos 2G6) was assayed for the up-regulation of germline gamma transcripts after CD40L stimulation. Independent Ramos 2G6 subclones that either expressed (Igamma+) or did not express (Igamma-) basal levels of Igamma transcripts were assessed for their transcriptional response to CD40L signaling by contact with either a Jurkat T cell line (D1.1) or a transfected CD40L-expressing epithelial cell line (293/CD40L) in the presence or absence of IL-4. Both Igamma- and Igamma+ Ramos 2G6 subclones cultured with IL-4 and CD40L markedly up-regulated germline transcription predominantly from the gamma1, gamma2, and gamma3 subclasses over levels obtained with IL-4 alone. In addition, these two signals were required to obtain de novo switch recombination. However, incubation with CD40L alone resulted in a substantial increase in germline transcription only in the Igamma+ and not the Igamma- subclones. Observed basal transcription at the gamma1 locus also correlated with the ability of not only the gamma1 locus, but also the gamma2 and gamma3 loci, to up-regulate germline transcripts in response to CD40 signaling. These data are consistent with CD40:CD40L contact up-regulating germline transcription only after the B cell has received a signal that alters the transcriptional state of the heavy chain locus.