Normal isotype switching in B cells lacking the Iμ exon splice donor site: Evidence for multiple Iμ-like germline transcripts

Ig class switch recombination (CSR) in activated B cells is preceded by the generation of 'switch' transcripts from the heavy chain constant region (CH) genes targeted for rearrangement. Switch transcripts include a sterile 'I' exon spliced onto the first CH exon. Targeted mutations disrupting the expression or splicing of I exons severely hamper CSR to all tested CH loci, except μ. However, all μ switch transcript mutations tested so far have left the Iμ exon splice donor site intact. To test the possibility that the residual CSR activity in Iμ mutants could be due to splicing of a truncated Iμ exon, we generated new mutants specifically lacking the Iμ splice donor site. Surprisingly, normal CSR was observed in the Iμ splice donor mutants even in the absence of detectable spliced Iμ transcripts. In a search for potential alternative sources of switch-like transcripts in the μ locus, we identified two novel exons which map just upstream of the Sμ region and splice onto the Cμ1 exon. Their expression is detectable from early B cell developmental stages, and, at least in hybridomas, it does not require the Eμ enhancer. These studies highlight a unique structure for the μ locus I exon region, with multiple nested switch transcript-like exons mapping upstream of Sμ. We propose that all of these transcripts directly contribute to μ class switching activity.