Quantitative Regulation of Class Switch Recombination by Switch Region Transcription

The isotype specificity of immunoglobulin (Ig) class switching is regulated by a cytokine which induces transcription of a specific switch (S) region, giving rise to so-called germline transcripts. Although previous studies have demonstrated that germline transcription of an S region is required for class switch recombination (CSR) of that particular S region, it has not been shown whether the level of S region transcription affects the efficiency of CSR. We addressed this question by using an artificial DNA construct containing a constitutively transcribed μ switch (Sμ) region and an α switch (Sα) region driven by a tetracycline-responsive promoter. The construct was introduced into a switch-inducible B lymphoma line and the quantitative correlation between Sα region transcription and class switching efficiency was evaluated. The level of Sα transcription was linearly correlated with CSR efficiency, reaching a plateau at saturation. On the other hand, we failed to obtain the evidence to support involvement of either RNA–DNA heteroduplex or trans germline transcripts in CSR. Taken together, it is likely that S region transcription and/or transcript processing in situ may be required for CSR. We propose that because of the unusual properties of S region DNA, transcription induces the DNA to transiently be single stranded, permitting secondary structure(s) to form. Such structures may be recognition targets of a putative class switch recombinase.