Rearrangement and expression of immunoglobulin light chain genes can precede heavy chain expression during normal B cell development in mice.

In mouse mutants incapable of expressing mu chains, VkappaJkappa joints are detected in the CD43(+) B cell progenitors. In agreement with these earlier results, we show by a molecular single cell analysis that 4-7% of CD43(+) B cell progenitors in wild-type mice rearrange immunoglobulin (Ig)kappa genes before the assembly of a productive VHDHJH joint. Thus, mu chain expression is not a prerequisite to Igkappa light chain gene rearrangements in normal development. Overall, approximately 15% of the total CD43(+) B cell progenitor population carry Igkappa gene rearrangements in wild-type mice. Together with the results obtained in the mouse mutants, these data fit a model in which CD43(+) progenitors rearrange IgH and Igkappa loci independently, with a seven times higher frequency in the former. In addition, we show that in B cell progenitors VkappaJkappa joining rapidly initiates kappa chain expression, irrespective of the presence of a mu chain.