THE IL-4 INDUCED INCREASE IN THE FREQUENCY OF RESTING MURINE SPLENIC B-CELLS EXPRESSING GERMLINE IG HEAVY-CHAIN GAMMA-1 TRANSCRIPTS CORRELATES WITH SUBSEQUENT SWITCHING TO IGG1

Cytokine induced germline immunoglobulin heavy chain gene transcription appears to signal commitment to an isotype switch and may be the mechanism by which specific switch regions are targeted as the sites for recombination. In this study, the structure and expression of mouse germline gamma1 RNAs are described. The 5'-ends of these transcripts are derived from an exon denoted Igamma1, located upstream of the gamma1 switch region and initiate at multiple sites over a 200 nucleotide region. Sequence analysis of cDNA and genomic clones reveals that these RNAs, unlike other germline C(H) transcripts, may encode a novel Igamma1/Cgamma1 heavy chain protein, of which the N-terminal 27 residues are encoded by Igamma1. In vitro culture of resting or pre-activated splenic B cells in the presence of lipopolysaccharides and interleukin-4 (IL-4) generates clones that secrete both IgM and IgG1 or either isotype alone. IL-4 increases the frequency of clones secreting both IgM and IgG1 and IgG1 alone, suggesting that commitment to IgG1 secretion may be independent of, or associated with, IgM secretion. PCR analysis of gamma1 germline transcript expression in clonal B cell cultures or single pre-activated B cells, shows that the IL-4 induced increase in the frequency of cells expressing gamma1 germline transcripts directly correlates with the increased frequency of cells switching from IgM to IgG1 production. This finding statistically confirms at a clonal level the relationship between cytokine induced germline transcription and isotype switching.