The 3' end of the heavy chain constant region locus enhances germline transcription and switch recombination of the four gamma genes

The switch in immunoglobulin (Ig) heavy chain class is preceded by germline transcription and then mediated by a DNA recombination event. To study germline transcription and class switch recombination we used transgenic mice with a 230-kilobase bacterial artificial chromosome that included a rearranged VDJ gene and the entire heavy chain constant region locus. In addition to several lines with intact transgenes, we identified two lines in which the heavy chain locus transgene lacked Calpha and everything 3' of it, including the regulatory elements HS3a, HS1-2, HS3b, and HS4. B cells from both lines with the truncated transgenes make abundant transgenic (Tg) VDJCmu transcripts and IgM protein. Deletion of the 3' end of the locus results in dramatically reduced expression of both germline transcripts and switched VDJCH transcripts of the gamma3, gamma2b, gamma2a, and epsilon genes. In addition, the transgenes lacking the 3' end of the locus express reduced amounts of gamma1 germline transcripts and 2-3% of the amount of Tg IgG1 in tissue culture compared with intact transgenes. Finally, switch recombination to gamma1 is undetectable in the transgenes lacking the 3' elements, as measured by digestion circularization-polymerase chain reaction or by the expression of VDJCgamma1 transcripts.