Altered BCR and TLR signals promote enhanced positive selection of autoreactive transitional B cells in Wiskott-Aldrich syndrome.

Development of the adaptive immune system requires selection of antigen receptors to establish a diverse but self-tolerant lymphocyte repertoire. Mechanisms to prevent selection of autoreactive B lymphocytes include clonal deletion, anergy, and receptor editing (Nemazee, 2006; Meffre and Wardemann, 2008). Alternatively, a growing body of literature also suggests that antigen-dependent positive selection of transitional B cells can occur via increased survival and/or clonal expansion (Hayakawa et al., 1999; Levine et al., 2000; Gaudin et al., 2004; Meyer-Bahlburg et al., 2008; Zikherman et al., 2012). These negative and positive selection mechanisms function in concert to shape the mature naive B cell repertoire. Positive selection of transitional B cells is regulated by tonic B cell receptor (BCR) signaling (Stadanlick et al., 2008), signaling via the cytokine B cell–activating factor (BAFF; Stadanlick and Cancro, 2008), and T cell help via CD40L-CD40 signaling (Schwartz et al., 2014) to promote cell survival. Positive selection may help to select BCR specificities that maintain key homeostatic functions, including apoptotic cell clearance or conserved pathogen recognition (Gronwall and Silverman, 2014). Although positive selection can be beneficial for these important immune functions, enhanced positive selection of autoreactive BCRs, through incompletely defined mechanisms, is also predicted to occur in autoimmune-prone settings; this process is likely to lead to an enrichment in BCR specificities that may facilitate detrimental immune responses (Groom et al., 2002; Wang and Clarke, 2003; Eilat and Wabl, 2012). In addition to BCR specificity, emerging data suggest a role for TLR signals in modulation of B cell selection. Previous data have shown that TLR signaling adapters, including MyD88, IRAK-4, and UNC93b, may operate in conjunction with the BCR to facilitate negative selection of autoreactive B cells (Isnardi et al., 2008). In contrast to promoting negative selection in immature B cells, dual signals mediated via the BCR and TLR pathways in mature B cells (Leadbetter et al., 2002; Groom et al., 2007; Silver et al., 2007; Rawlings et al., 2012) markedly enhance B cell activation and may directly initiate humoral autoimmunity. In this latter setting, loss in B cell tolerance occurs via generation of self-reactive, germinal center responses, leading ultimately to production of class-switched pathogenic autoantibodies (Jackson et al., 2015). Notably, although these combined data implicate TLR/MyD88 signals in both early and late B cell tolerance checkpoints, a potential role of BCR and/or TLR engagement in transitional B cell positive selection into the naive mature B cell compartment has not been defined. Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency that results from mutations within the gene encoding the WAS protein (WASp), a key multiadapter protein linking a broad range of receptor signaling effectors to the actin cytoskeleton. This complex disorder is characterized by multiple alterations in hematopoietic cell surface receptor signal transduction, cell trafficking, and lineage- and developmental subset–specific homeostasis. Notably, up to 70% of WAS patients exhibit autoimmunity, including autoantibody-mediated cytopenias and organ-specific disease (Notarangelo and Ochs, 2003; Ochs and Thrasher, 2006; Bosticardo et al., 2009). In previous work, we have shown that WASp deficiency modestly enhances both BCR and TLR signaling in naive B cells (Becker-Herman et al., 2011). Furthermore, we and others have demonstrated that B cell–intrinsic WASp deficiency is sufficient to alter B cell tolerance and can promote production of class-switched autoantibodies and autoantibody-mediated autoimmune disease (Becker-Herman et al., 2011; Recher et al., 2012). This break in tolerance is associated with spontaneous GC formation and requires both BCR and TLR/MyD88 signaling (Becker-Herman et al., 2011; Jackson et al., 2014). In this study, we hypothesized that increased BCR and TLR signaling in WASp-deficient B cells may also impact establishment of the mature, naive BCR repertoire. In partial support of this idea, previous studies have revealed evidence for skewing of heavy chain usage in both class-switched and bulk naive peripheral blood B cells isolated from WAS subjects (Castiello et al., 2014; O’Connell et al., 2014; Simon et al., 2014). Here, we present a detailed analysis of the impact of WASp deficiency on the selection of the naive B cell repertoire in mice and humans. Our combined data support a model wherein altered BCR and TLR signaling orchestrates increased positive selection of transitional B cells expressing low-affinity self-reactive BCRs, leading to their enrichment within the naive B cell compartment. As multiple human autoimmune disorders are associated with genetic changes potentially impacting B lineage signaling function, our findings have important implications with respect to understanding events that promote altered B cell selection in both WAS subjects and in other autoimmune-prone individuals.