Pre-B cell receptor assesses the quality of IgH chains and tunes the pre-B cell repertoire by delivering differential signals

It is well understood how a variety of Ig H and L chains, components of BCR, are generated in the DNA level during B cell development. However, it has remained largely unknown whether and how each component is monitored for its quality and selected before the assembly into the BCR. Here we show that mu H chains produced by pre-B cells display a wide spectrum of ability to form the pre-BCR, which is composed of mu H and surrogate light (SL) chains and is crucial for B cell development. The level of surface pre-BCR expression varies among pre-B cells, depending on the ability of their mu H chains to pair with SL chains. The higher the level of pre-BCR expression by pre-B cells, the stronger their pre-BCR signaling, and the better they proliferate and differentiate. Thus, the extent of survival, proliferation, and differentiation of individual pre-B cells is primarily determined by the SL-pairing ability of their mu H chains. Furthermore, IgH chains with higher potential to assemble with IgL chains appear to be positively selected and amplified through the assessment of their ability to pair with SL chains at the pre-BCR checkpoint before the assembly into the BCR. These results indicate that the pre-BCR assesses the quality of mu H chains and tunes the pre-B cell repertoire by driving the preferential expansion and differentiation of cells with the higher quality of mu H chains.