Replacement of Igamma3 germ-line promoter by Igamma1 inhibits class-switch recombination to IgG3.

Class-switch recombination (CSR) enables IgM-producing B cells to switch to the production of IgG, IgE, and IgA. The process requires germ-line (GL) transcription that initiates from promoters upstream of switch (S) sequences and is regulated by the 3' regulatory region (3'RR) located downstream of the Ig heavy chain (IgH) locus. How the 3'RR effect its long-range activation is presently unclear. We generated a mouse line in which Igamma3 GL promoter was replaced by Igamma1. We found that GL transcription could initiate from the inserted Igamma1 promoter and was induced by increased concentrations of IL-4 and that the transcripts were normally spliced. However, when compared with GL transcripts derived from the endogenous Igamma1 promoter in the same stimulation conditions, those from the inserted Igamma1 promoter were less abundant. CSR to Cgamma3 was abrogated both in vivo and in vitro. The results strongly suggest that the endogenous Igamma1 promoter insulates the inserted Igamma1 from the long-range activating effect of the 3'RR. The implications of our findings are discussed in light of the prominent models of long-distance activation in complex loci.