Iα exon-replacement mice synthesize a spliced HPRT–Cα transcript which may explain their ability to switch to IgA. Inhibition of switching to IgG in these mice

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Abstract
Antibody class switching is regulated by transcription of unrearranged CH genes to produce germline (GL) transcripts which direct the choice of isotype and are required for switching. However, their role is unknown. GL transcripts are initiated at the I exons located upstream of each switch region. Although deletion of the I exon by gene targeting prevents switch recombination to that CH gene, the Iα exon can be replaced by an entirely different DNA segment, a minigene driven by the phosphoglycerate kinase (PGK) promoter and encoding hypoxanthine phosphoribosyl transferase (HPRT), oriented in the sense direction, without reducing antibody class switching to IgA. To understand why HPRT substitution of the Iα 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 which the HPRT exons are spliced to the Cα gene segments, resulting in a structure similar to normal GL transcripts. The abundance of this transcript is similar to that of the normal α GL RNA. We also demonstrate that switching to the four IgG subclasses in B cells from these mice is reduced in comparison to wild-type mice. We discuss the possibility that the strong PGK promoter inserted at the Ig α locus may interfere with interaction of the promoters for γ GL transcripts with the 3′ IgH enhancer.