Rearrangement and expression of χ light chain genes can occur without μ heavy chain expression during differentiation of pre-B cells

The kinetics of χ light (χL) chain gene rearrangement and expression on mRNA and protein level has been studied with four stromal cell/IL-7 reactive, long-term in vitro proliferating pre-B cell lines and clones, two from fetal liver of normal mice and two from fetal liver of EμH-bcl-2 transgenic (bcl-2-tg) mice. These pre-B cell lines and clones are DJH-rearranged on both H chain alleles. Two of the clones harbor H chain rearrangements which do not allow the expression of VHDJH rearranged H chain genes as μH chain proteins. Upon removal of IL-7 from the pre-B cell cultures all four cell lines rearrange VH-DJH and VL-JL gene segments, loose the surface expression of c-kit, CD43, and surrogate light chain, as well as the capacity to be clonable on stromal cells in the presence of IL-7. Pre-B cells from normal mice die by apoptosis during differentiation, while those from bcl-2-tg mice do not. All four lines and clones express comparable levels of mRNA for μH and μL chains with the same time kinetics during 3 days of differentiation. However, only two of the four pre-B cell lines and clones express μH chain protein, whereas all four pre-B cell lines and clones express μL chain protein at comparable levels between 2×105 and 1.40×106 μL chain molecules per cell. These results suggest that μH chain expression is not mandatory for rearrangement and normal expression of μL chain genes when pre-B cells differentiate to B cell