Mechanism for Pre-B Cell Loss in VH-Mutant Rabbits

Abstract

Pre-BCR signaling is a critical checkpoint in B cell development in which B-lineage cells expressing functional IgH μ-chain are selectively expanded. B cell development is delayed in mutant ali/ali rabbits because the a-allotype encoding VH1 gene, which is normally used in VDJ gene rearrangements in wt rabbits, is deleted, and instead, most B-lineage cells use the a-allotype encoding VH4 gene [VH4(a)], which results in a severe developmental block at the pre-B cell stage. We found that VH4(a)-utilizing pre-B cells exhibit reduced pre-BCR signaling and do not undergo normal expansion in vitro. Transduction of murine 38B9 pre-B cells with chimeric rabbit-VDJ mouse-Cμ encoding retroviruses showed VH4(a)-encoded μ-chains do not readily form signal-competent pre-BCR, thereby explaining the reduction in pre-BCR signaling and pre-B cell expansion. Development of VH4(a)-utilizing B cells can be rescued in vivo by the expression of an Igκ transgene, indicating that VH4(a)-μ chains are not defective for conventional BCR formation and signaling. The ali/ali rabbit model system is unique because VH4(a)-μ chains have the capacity to pair with a variety of conventional IgL chains and yet lack the capacity to form a signal-competent pre-BCR. This system could allow for identification of critical structural parameters that govern pre-BCR formation/signaling.