CD40 LIGAND EXERTS DIFFERENTIAL-EFFECTS ON THE EXPRESSION OF I-GAMMA TRANSCRIPTS IN SUBCLONES OF AN IGM(-CELL LYMPHOMA LINE() HUMAN B)

The CD40:CD40 ligand (CD40L) interaction plays a critical role in T ce ll-dependent isotype switching. To elucidate the role of CD40 signalin g in the activation of gamma germline transcription and as an extensio n, in targeting C gamma regions for isotype switching, an IgM(+) Burki tt lymphoma cell line (Ramos 2C6) was assayed for the up-regulation of germline gamma transcripts after CD40L stimulation. Independent Ramos 2G6 subclones that either expressed (I gamma(+)) or did not express ( I gamma(-)) basal levels of I gamma transcripts were assessed for thei r transcriptional response to CD40L signaling by contact with either a Jurkat T cell line (D1.1) or a transfected CD40L-expressing epithelia l cell line (293/CD40L) in the presence or absence of IL-4. Both I gam ma(-) and l gamma(+) Ramos 2C6 subclones cultured with IL-4 and CD40L markedly up-regulated germline transcription predominantly from the ga mma 1, gamma 2, and gamma 3 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 I gamma( +) and not the I gamma(-) subclones. Observed basal transcription at t he gamma I locus also correlated with the ability of not only the gamm a I locus, but also the gamma 2 and gamma 3 loci, to up-regulate germl ine transcripts in response to CD40 signaling. These data are consiste nt with CD40:CD40L contact up-regulating germline transcription only a fter the B cell has received a signal that alters the transcriptional state of the heavy chain locus.