3, B and C, showing gating strategy for total splenic (B220?CD138?) and bone marrow (B220?CD138?IgM?) nonCB cells, splenic (PC; B220?CD138hi) and bone marrow PCs (FCShiB220?CD138hi), splenic GC B cells (GC; CD138?B220+Fas+), splenic naive B cells (B220+CD138?Fas?IgM+), bone marrow mature B cells (B220hiCD138?), splenic IgM+ and IgG1+ MBCs (B220+CD138?Fas?PD-L2+), and bone marrow IgM+ and IgG1+ MBCs (B220hiCD138?PD-L2+)
May 29, 2021
3, B and C, showing gating strategy for total splenic (B220?CD138?) and bone marrow (B220?CD138?IgM?) nonCB cells, splenic (PC; B220?CD138hi) and bone marrow PCs (FCShiB220?CD138hi), splenic GC B cells (GC; CD138?B220+Fas+), splenic naive B cells (B220+CD138?Fas?IgM+), bone marrow mature B cells (B220hiCD138?), splenic IgM+ and IgG1+ MBCs (B220+CD138?Fas?PD-L2+), and bone marrow IgM+ and IgG1+ MBCs (B220hiCD138?PD-L2+). BAFFR. Here, using inducible genetic ablation, we show that survival of MBCs is critically dependent on the BCR and on signaling through the associated CD79A protein. Unexpectedly, we found that MBCs express BAFFR and that their survival requires BAFF and BAFFR; hence, loss of BAFF or BAFFR impairs recall responses. Finally, we show that MBC survival requires IKK2, a kinase that transduces BAFFR signals. Thus, MBC survival is critically dependent on signaling from BCR and BAFFR. Introduction Immunological memory is characterized by the ability of the immune system to respond more rapidly and more robustly to a recurring infection. In the case of the humoral immune response, such a reexposure to a pathogen results in a secondary antibody response that, in comparison to a primary response, is quicker, larger in magnitude, and typified by higher-affinity antibodies. This humoral immunological memory IBMX arises from reservoirs of memory B cells (MBCs) and long-lived antibody-secreting plasma cells (PCs), which are established during a preceding primary immune response. During a primary T-dependent antibody response, antigen-specific naive B cell clones selectively expand, supported by T cell help. Subsequently, these activated B cells and T helper cells migrate into the follicles of lymphoid organs where they establish germinal centers (GCs). In these structures, the B cells undergo somatic hypermutation of Ig variable regions, leading to an increase in affinity for antigen of the surface-bound Ig that makes up the B cell antigen receptor (BCR). Furthermore, GC B cells undergo class switch recombination, leading to a change from expressing the IgM and IgD forms of the BCR to other isotypes such as IgG1 (Shlomchik and Weisel, 2012; Suan et al., 2017). MBCs are generated from these activated B cells both before and after entry into the GC. Whereas substantial numbers of IgM+ MBCs are made before GC initiation, IgG1+ MBCs are preferentially generated in early GCs. In contrast, there is a continuous output of PCs during the late GC response (Inamine et al., 2005; Taylor et al., 2012; Weisel et al., 2016). In concordance with the timing of their egress from GCs, IgM+ and IgG1+ MBCs typically exhibit less somatically Mouse monoclonal to PRAK mutated Ig variable regions and therefore lower affinity for their cognate antigen than PCs (Kaji et al., 2012; Takahashi et al., 2001; Weisel et al., 2016). Whereas long-lived PCs reside in the bone marrow and maintain systemic levels of high-affinity antibodies (Nutt et al., 2015), MBCs represent a reservoir of quiescent cells bearing BCRs with low affinity for cognate antigen. MBCs are characterized by heterogeneity in both Ig mutation rates and expression levels of surface markers PD-L2, CD73, and CD80, which reflect variability in effector responses of MBCs following reactivation (Anderson et al., 2007; Tomayko et al., 2010). They can be reactivated by a broader selection of related and possibly mutated antigens and subsequently either undergo further affinity maturation in secondary GCs or rapidly secrete protective antibodies as short-lived plasmablasts (PBs; Dogan et al., 2009; Pape et al., 2011; Zuccarino-Catania et al., 2014). To preserve humoral immunological memory, the antigen-specific reservoir of MBCs has to be maintained indefinitely. Studies have shown that MBCs are quiescent and long-lived, with many having a half-life IBMX in mice that is longer than the life span of the animal (Jones et IBMX al., 2015). Thus, pathways regulating MBC survival play a critical role in immunological memory; however, very little is known about them. Inhibition of the anti-apoptotic proteins Bcl-2, Bcl-XL, and Bcl-W revealed that IgG1+ MBC persistence is dependent on Bcl2-family proteins (Carrington et al., 2010; Torcia et al., 1996); however, external signals such as cytokines or receptors required for MBC longevity have not been identified (Weisel and Shlomchik, 2017). Two crucial receptors mediating the survival of naive mature B cells are the BCR and BAFFR (TNFRSF13C; Schweighoffer and Tybulewicz, 2018). Several studies in naive B cells suggest that the BCR transduces a ligand-independent survival signal via.