GERMLINE TRANSCRIPTION AND RECOMBINATION OF A MURINE VDJ(MU-DELTA-GAMMA)1 TRANSGENE

To investigate the regulation of Ig switch recombination, we have cons tructed mice with a 56 kb VDJ(mu delta gamma)1 transgene, This transge ne included an anti-nitrophenyl VDJ segment, S-mu, C-mu, C-delta, I(ga mma)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 si milar characteristics. Transgenic B cells expressed normal amounts of C-mu (which is >95% transgenic), C-delta and other cell surface marker s, and normal amounts of VDJ and C-mu RNA, yl germline transcription o f the transgenes is properly regulated since stable transcripts were n ot expressed in B cells treated with lipopolysaccharide (LPS) alone, n or in thymus or non-lymphoid tissues, but were expressed after treatme nt 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 cul ture with CD40L + IL-4, but not after culture with CD40L alone. Howeve r, 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 nontrans genic B cells (1:9), Analysis of DNA from transgenic hybridomas indica ted that switch recombination can take place in switch (S) regions, bu t can also take place outside S regions. These results indicate that t argeting of switch recombinase to S regions must include regulation be yond the S regions themselves and correct germline transcription. This additional regulation might include cis-acting elements or appropriat e spacing or arrangement of the recombining elements.