Multiple types of chimeric germ-line Ig heavy chain transcripts in human B cells: evidence for trans-splicing of human Ig RNA.

Germ-line transcripts from Ig heavy chain loci precede the occurrence of isotype switching and are thought to play an important though still controversial role in Ig class switching. In this study, we employed a reverse transcriptase-PCR approach to detect human chimeric Ig germ-line mRNA transcripts. Multiple types of chimeric Ig germ-line transcripts (Imu-Cepsilon, Iepsilon-Cmu, Imu-Cgamma4, Igamma-Cmu, Igamma-Cepsilon, Iepsilon-Cgamma, and Igamma4-Calpha1 transcripts) were readily detected in human B cells stimulated with IL-4 alone. Sequence analysis revealed that all of these chimeric Ig germ-line transcripts represented the I exons from one Ig locus spliced to the CH exons from another locus by using consensus sequences for splicing donor and acceptor sites, indicating that they were generated through splicing machinery. In the case of stimulation of human resting B cells with IL-4 alone, the chimeric Ig germ-line transcripts are likely derived from a trans-splicing mechanism, as the extensive searching did not find evidence that Ig class-switch recombination had occurred, which alternatively could give rise to chimeric Ig mRNA by mechanisms other than trans-splicing. Similarly, an EBV-transformed gamma2 rearranged B cell line, GM1500, which produces IgG2 and contains both gamma2 productive and epsilon germ-line transcripts, also expressed chimeric germ-line RNA (Iepsilon-Cgamma2) and epsilon-productive transcripts (VDJ-Cepsilon). This line had no further sequential Sgamma2-Sepsilon rearrangements, providing evidence that the productive VDJ-Cepsilon mRNA was derived from a transcriptionally active unrearranged epsilon gene locus by trans-splicing. Taken together, these results provide possible evidence that trans-splicing of germ-line Ig RNA transcripts occurs in human B cells.