CD40 ligand exerts differential effects on the expression of Iγ 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 γ germline transcription and as an extension, in targeting Cγ, regions for isotype switching, an IgM⁺ Burkitt lymphoma cell line (Ramos 2G6) was assayed for the up-regulation of germline γ transcripts after CD40L stimulation. independent Ramos 2G6 subclones that either expressed (Iγ⁺) or did not express (Iγ^-) basal levels of Iγ 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 Iγ^- and Iγ⁺ Ramos 2G6 subclones cultured with IL-4 and CD40L markedly up-regulated germline transcription predominantly from the γ1, γ2, and γ3 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 Iγ⁺ and not the Iγ^- subclones. Observed basal transcription at the γ1 locus also correlated with the ability of not only the γ1 locus, but also the γ2 and γ3 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.