Germline transcription and recombination of a murine VDJμδγ1 transgene

To investigate the regulation of Ig switch recombination, we have constructed mice with a 56 kb VDJ(mu delta gamma)1 transgene, This transgene included an anti-nitrophenyl VDJ segment, S-mu, C-mu, C-delta, I(gamma)1, S(gamma)1, C(gamma)1 and the C(gamma)1 membrane exons from the murine Igh(a) haplotype. Two founder lines were produced, with very similar characteristics. Transgenic B cells expressed normal amounts of C-mu (which is >95% transgenic), C-delta and other cell surface markers, and normal amounts of VDJ and C-mu RNA, yl germline transcription of the transgenes is properly regulated since stable transcripts were not expressed in B cells treated with lipopolysaccharide (LPS) alone, nor in thymus or non-lymphoid tissues, but were expressed after treatment of B cells with LPS + IL-4 or CD40L + IL-4, B cells from both lines of transgenic mice expressed transgenic gamma 1(a) after in vitro culture with CD40L + IL-4, but not after culture with CD40L alone. However, the CD40L + IL-4 induced IgG1 precursor frequency is much lower for VDJ(mu delta gamma)1 transgenic B cells (1:240-760) than for nontransgenic B cells (1:9), Analysis of DNA from transgenic hybridomas indicated that switch recombination can take place in switch (S) regions, but can also take place outside S regions. These results indicate that targeting of switch recombinase to S regions must include regulation beyond the S regions themselves and correct germline transcription. This additional regulation might include cis-acting elements or appropriate spacing or arrangement of the recombining elements.