I-alpha exon-replacement mice synthesize a spliced HPRT-C-alpha transcriptwhich may explain their ability to switch to IgA. Inhibition of switching to IgG in these mice

Antibody class switching is regulated by transcription of unrearranged CH g enes to produce germline (GL) transcripts which direct the choice of isotyp e and are required for switching. However, their role is unknown. GL transc ripts are initiated at the I exons located upstream of each switch region. Although deletion of the I exon by gene targeting prevents switch recombina tion to that C-H gene, the I-alpha exon can be replaced by an entirely diff erent DNA segment, a minigene driven by the phosphoglycerate kinase (PGK) p romoter and encoding hypoxanthine phosphoribosyl transferase (HPRT), orient ed in the sense direction, without reducing antibody class switching to IgA . To understand why HPRT substitution of the I-alpha exon does not disrupt switch recombination, we have analyzed the structure of the transcript from the targeted allele in these mice. We identify a spliced transcript in whi ch the HPRT exons are spliced to the C-alpha gene segments, resulting in a structure similar to normal GL transcripts. The abundance of this transcrip t is similar to that of the normal alpha GL RNA. We also demonstrate that s witching to the four IgG subclasses in a cells from these mice is reduced i n comparison to wild-type mice. We discuss the possibility that the strong PGK promoter inserted at the Ig alpha locus may interfere with interaction of the promoters for gamma GL transcripts with the 3' IgH enhancer.