Mechanism for Pre-B Cell Loss in Mutant Ali/Ali Rabbits

Pre-B cell receptor (pre-BCR) signaling is a critical checkpoint in B cell development in which B-lineage cells expressing functional μ-chains undergo selective expansion. My dissertation work used a rabbit model system to study how VH structure affects the early stages of B cell development in the bone marrow (BM). Developing B cells in mutant ali/ali rabbits predominantly utilize VH4 during VDJ gene rearrangement. However, most of the VH4-utilizing B cells encounter a severe block early in B cell development. In contrast, wildtype rabbit B cells predominantly rearrange VH1, a gene \u3e90% identical to VH4, and develop normally. We hypothesized that VH4-utilizing cells are blocked at the pro- to pre-B cell transition due to defects in pre-BCR signaling. The pre-BCR is a complex of the μ-chain and a light chain-like molecule called surrogate light chain (SL). The fact that some VH4-utilizing B cells survive development in BM and undergo expansion in the periphery suggests that VH4-mu chains in the context of conventional BCR are functional. My studies indicate that VH4-utilizing pre-B cells fail to expand normally and that VH4-mu chains do not form pre-BCR. These data are consistent with a cell intrinsic model of VH4-utilizing pre-B cell loss resulting from insufficient pre-BCR signaling. My studies also identified a possible mechanism by which select VH4-utilizing B- lineage cells survive development through expression of conventional IgL during the early B cell stages. These data suggest that early expression of IgL and signaling through the conventional BCR is a plausible mechanism by which VH4-utilizing cells bypass the developmental block at the early pre-B cell stage. My work identified the stage and mechanism by which VH4-utilizing B-lineage cells are lost during development in the BM, and suggests that select VH4-utilizing cells bypass the block at the pre-B cell stage through early rearrangement and expression of IgL. These studies lay the ground work for future experiments aimed at identifying the structural parameters encoded by VH genes that allow for pre-BCR formation and normal pre-B cell expansion