Normal isotype switching in B cells lacking the I mu exon splice donor site: Evidence for multiple I mu-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 (C H) genes targeted for rearrangement. Switch transcripts include a sterile " I" exon spliced onto the first CH exon. Targeted mutations disrupting the e xpression or splicing of I exons severely hamper CSR to all tested CH loci, except mu. However, all mu switch transcript mutations tested so far have left the I mu exon splice donor site intact. To test the possibility that t he residual CSR activity in I mu mutants could be due to splicing of a trun cated I mu exon, we generated new mutants specifically lacking the I mu spl ice donor site. Surprisingly, normal CSR was observed in the I mu splice do nor mutants even in the absence of detectable spliced I mu transcripts. In a search for potential alternative sources of switch-like transcripts in th e Er. locus, we identified two novel exons which map just upstream of the S mu region and splice onto the C mu 1 exon. Their expression is detectable from early B cell developmental stages, and, at least in hybridomas, it doe s not require the E mu enhancer. These studies highlight a unique structure for the Ir locus I exon region, with multiple nested switch transcript-lik e exons mapping upstream of S mu. We propose that all of these transcripts directly contribute to mu class switching activity.