MULTIPLE TYPES OF CHIMERIC GERM-LINE IG HEAVY-CHAIN TRANSCRIPTS IN HUMAN B-CELLS - EVIDENCE FOR TRANSSPLICING 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 transcr ipts (I mu-C epsilon, I epsilon-C mu, I mu-C gamma 4, I gamma-C mu, I gamma-C epsilon, I epsilon-C gamma, and I gamma 4-C alpha 1 transcript s) were readily detected in human B cells stimulated with IL-4 alone. Sequence analysis revealed that all of these chimeric Ig germ-line tra nscripts represented the I exons from one Ig locus spliced to the CH e xons from another locus by using consensus sequences for splicing dono r and acceptor sites, indicating that they were generated through spli cing machinery. In the case of stimulation of human resting B cells wi th IL-4 alone, the chimeric Ig germ-line transcripts are likely derive d from a trans-splicing mechanism, as the extensive searching did not find evidence that Ig class-switch recombination had occurred, which a lternatively could give rise to chimeric Ig mRNA by mechanisms other t han trans-splicing. Similarly, an EBV-transformed gamma 2 rearranged B cell line, GM1500, which produces lgG2 and contains both gamma 2 prod uctive and epsilon germ-line transcripts, also expressed chimeric germ -line RNA (I epsilon-C gamma 2) and epsilon-productive transcripts (VD j-C epsilon). This line had no further sequential S gamma 2-S epsilon rearrangements, providing evidence that the productive VDJ-C epsilon m RNA was derived from a transcriptionally active unrearranged epsilon g ene locus by trans-splicing. Taken together, these results provide pos sible evidence that trans-splicing of germ-line Ig RNA transcripts occ urs in human B cells.