Loss of Precursor B Cell Expansion but Not Allelic Exclusion in VpreB1/VpreB2 Double-Deficient Mice

The pre-B cell receptor consists of immunoglobulin (Ig) μ heavy chains and surrogate light chain, i.e., the VpreB and λ5 proteins. To analyze the role of the two VpreB proteins, mice lacking the VpreB1 and VpreB2 genes were generated. VpreB1^−/−VpreB2^−/− mice were impaired in their B cell development at the transition from pre-BI to large pre-BII cells. Pre-BII cells did not expand by proliferation, consequently 40-fold less small pre-BII and immature B cells were found in bone marrow, and the generation of immature and mature conventional B cells in spleen appeared reduced. In addition, only low numbers of B-1a cells were detected in the peritoneum. Surprisingly, Ig heavy chain allelic exclusion was still active, apparently ruling out a signaling role of a VpreB1/VpreB2–containing receptor in this process.