Title: The alternative NF-kB pathway in mature B cell development
Description: The nuclear factor-kB (NF-kB) signaling cascade is comprised of two branches, the canonical and alternative NF-kB pathways. Signaling through the alternative NF-kB pathway culminates in the activation of the downstream transcription factor subunits, RELB and NF-kB2. The biological roles of RELB and NF-kB2 within the B cell lineage have been obscured in constitutional knockout mice by the diverse functions of these subunits in non-B cell types. To overcome these limitations, conditional alleles were generated to investigate the roles of RELB and NF-kB2 in B cell development. These alleles allowed the identification of complex functional requirements for RELB and/or NF-kB2 in naïve B cells, germinal center (GC) B cells and plasma cells (PCs). These functional requirements may have implications for B cell malignancies that display mutations that constitutively activate the alternative NF-kB pathway. A large body of work has demonstrated that B cell activating factor (BAFF) signaling is critical for the maintenance of mature B cells.

However, the contribution of the alternative NF-kB subunits that are activated downstream of BAFF remained unclear, especially in regards to their specific target genes. We have identified critical, B cell-intrinsic roles for RELB and NF-kB2 in the maintenance of mature B cells. In response to BAFF, these subunits were found to control the expression of anti-apoptotic genes, genes that ensure correct positioning within the B cell niche, and genes involved in promoting B–T cell interactions that allow effective antigen-mediated activation. During the GC B cell reaction, light zone (LZ) B cells undergo affinity-based selection mediated by T follicular helper (Tfh) cells. A subset of LZ B cells show activation of the NF-kB signaling cascade, suggesting a critical role for NF-kB in the selection of high-affinity GC B cells. We here report that GC B cell development occurred normally in mice with conditional deletion of either relb (RELB) or nfkb2 (NF-kB2) in GC B cells. In contrast, the simultaneous ablation of both subunits caused rapid involution of established GCs, similar to what has been observed for ablation of the canonical NF-kB transcription factor subunit c-REL.

Intriguingly, RNA-sequencing analysis of relb/nfkb2-deleted GC B cells revealed no overlap between the genes controlled by RELB/p52 and c-REL within GC B cells. This suggests that signaling through the separate NF-kB pathways in GC B cells results in the expression of different biological programs that are independently required for the maintenance of the GC reaction. In addition, we observed that human PCs and PC precursors within the LZ showed high protein levels of NF-kB2 compared to surrounding lymphocytes, suggesting a biological role for this subunit in PCs. Indeed, ablation of nfkb2 alone in GC B cells led to a dramatic decrease in antigen-specific serum IgG1 and antigen-specific IgG1-secreting cells. Interestingly however, the mice developed normal frequencies of PCs, suggesting a role for NF-kB2 in PC physiology rather than differentiation.