Ig heavy chain protein controls B cell development by regulating germ-line transcription and retargeting V(D)J recombination.

The membrane-associated form of Ig heavy chain (mu) protein has been implicated as a critical regulator of B cell development. Mutant mice unable to produce the membrane form of mu protein fail to produce mature B cells. Splenic B cells from mice transgenic for a functionally rearranged Ig mu gene show a marked decrease in endogenous heavy chain gene rearrangement. We have analyzed the effects of a human mu transgene on the regulation of V(D)J recombination during B cell development in the mouse fetal liver. We found that mu transgenic and wild-type littermate mice begin kappa light chain gene rearrangement at the same time during development but the transgenic mice show a striking increase in the frequency of kappa gene rearrangement. The transgenic mice also show an increase in the levels of a germ-line kappa gene transcript known to be associated with kappa gene rearrangement. D-to-JH heavy chain gene rearrangement is unaffected throughout development by the presence of the mu transgene. Endogenous heavy chain gene V-to-DJH rearrangement occurs with similar frequency in transgenic and nontransgenic fetal livers during midgestation but is reduced in late gestation mu transgenic fetal liver. We show that this decrease in rearrangement is associated with a decrease in unrearranged VH gene transcription. Furthermore, we show that changes in the frequencies of rearranged kappa and mu genes are accompanied by changes in the frequencies of dsDNA breaks, a V(D)J recombination reaction intermediate associated with each of these loci. We propose that membrane-associated mu protein regulates B cell development by signaling a change in the pattern of unrearranged Ig gene transcriptional activity, thereby retargeting the V(D)J recombinase.