BAFF receptor signaling aids the differentiation of immature B cells into transitional B cells following tonic BCR signaling.

BAFF is an important prosurvival cytokine for mature B cells. However, previous studies have shown that BAFFR is already expressed at the immature B cell stage, and that the prosurvival protein Bcl-2 does not completely complement the B cell defects resulting from the absence of BAFFR or BAFF. Thus, we hypothesized that BAFF also functions to aid the differentiation of nonautoreactive immature B cells into transitional B cells and to promote their positive selection. We found that BAFFR is expressed at higher levels on nonautoreactive than on autoreactive immature B cells and that its expression correlates with that of surface IgM and with tonic BCR signaling. Our data indicate that BAFFR signaling enhances the generation of transitional CD23(-) B cells in vitro by increasing cell survival. In vivo, however, BAFFR signaling is dispensable for the generation of CD23(-) transitional B cells in the bone marrow, but it is important for the development of transitional CD23(-) T1 B cells in the spleen. Additionally, we show that BAFF is essential for the differentiation of CD23(-) into CD23(+) transitional B cells both in vitro and in vivo through a mechanism distinct from that mediating cell survival, but requiring tonic BCR signaling. In summary, our data indicate that BAFFR and tonic BCR signals cooperate to enable nonautoreactive immature B cells to differentiate into transitional B cells and to be positively selected into the naive B cell repertoire.

Receptor editing can lead to allelic inclusion and development of B cells that retain antibodies reacting with high avidity autoantigens.

Receptor editing is a major B cell tolerance mechanism that operates by secondary Ig gene rearrangements to change the specificity of autoreactive developing B cells. In the 3-83Igi mouse model, receptor editing operates in every autoreactive anti-H-2K(b) B cell, providing a novel receptor without additional cell loss. Despite the efficiency of receptor editing in generating nonautoreactive Ag receptors, we show in this study that this process does not inactivate the autoantibody-encoding gene(s) in every autoreactive B cell. In fact, receptor editing can generate allelically and isotypically included B cells that simultaneously express the original autoreactive and a novel nonautoreactive Ag receptors. Such dual Ab-expressing B cells differentiate into transitional and mature B cells retaining the expression of the autoantibody despite the high avidity interaction between the autoantibody and the self-Ag in this system. Moreover, we find that these high avidity autoreactive B cells retain the autoreactive Ag receptor within the cell as a consequence of autoantigen engagement and through a Src family kinase-dependent process. Finally, anti-H-2K(b) IgM autoantibodies are found in the sera of older 3-83Igi mice, indicating that dual Ab-expressing autoreactive B cells are potentially functional and capable of differentiating into IgM autoantibody-secreting plasma cells under certain circumstances. These results demonstrate that autoreactive B cells reacting with ubiquitous membrane bound autoantigens can bypass mechanisms of central tolerance by coexpressing nonautoreactive Abs. These dual Ab-expressing autoreactive B cells conceal their autoantibodies within the cell manifesting a superficially tolerant phenotype that can be partially overcome to secrete IgM autoantibodies.

Receptor editing is the main mechanism of B cell tolerance toward membrane antigens.

Self-reactive B cells specific for ubiquitous membrane-bound autoantigens are eliminated in the bone marrow by two mechanisms of tolerance: receptor editing and clonal deletion. However, the relative contributions of clonal deletion and receptor editing to B cell tolerance in a polyclonal B cell population have not been established. Here we show that tolerance toward a membrane antigen-reactive B cell clone acts by receptor editing with very minimal cell loss. The capacity of receptor editing to rescue almost all autoreactive B cells from deletion relies on the availability of multiple joining light chain gene segments as substrate for secondary immunoglobulin light chain gene rearrangement and is independent of the affinity of the autoantigen and the presence of non-autoreactive B cells. Our data further suggest that clonal deletion is a default pathway that functions only when receptor editing has been exhausted.