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 μ chains, V_κJ_κ 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)κ genes before the assembly of a productive V_HD_HJ_H joint. Thus, μ chain expression is not a prerequisite to Igκ light chain gene rearrangements in normal development. Overall, ∼15% of the total CD43⁺ B cell progenitor population carry Igκ 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 Igκ loci independently, with a seven times higher frequency in the former. In addition, we show that in B cell progenitors V_κJ_κ joining rapidly initiates κ chain expression, irrespective of the presence of a μ chain.