Horizontal bars in BCD represent the mean of the points shown. generated by somatic hypermutation (SHM). This process occurs in germinal centers (GCs), transient structures formed during T cellCdependent immune responses that enable the preferential survival of B cells producing higher affinity antibodies. Ultimately, this competitive selection process preserves GC B cells with improved antigen affinity and eliminates those that drop specificity or gain autoreactivity. The mechanisms responsible for differential survival remain uncertain but involve tripartite interactions between the GC B cells, FO DCs (FDCs), and T FO helper (TFH) cells. How the B cell receptor (BCR) drives this affinity-dependent selection process is usually debated. Although loss of BCR-associated signals disrupt GC kinetics (Wang and Carter, 2005; Huntington et al., 2006), recent findings Saridegib suggest that antigen capture may be its primary function because BCR signaling is usually damped in most GC B cells by unfavorable regulatory mechanisms (Khalil et al., 2012). This is consistent with models whereby GC B cells compete for antigen displayed on FDCs to mediate effective MHCII-restricted antigen presentation, thereby fostering sustained TFH interactions, which in turn promote GC B cell survival (Allen and Cyster, 2008; McHeyzer-Williams et al., 2009; Victora and Nussenzweig, 2012). This idea is further supported by observations indicating that cognate TFH interactions are a limiting factor in affinity maturation (Schwickert et al., 2011). Thus, higher affinity GC B cells can capture and present antigen more effectively, enabling their preferential access to TFH cells and facilitating positive selection (Victora et al., 2010; Schwickert et al., 2011). Despite mounting evidence for this model, the mechanism whereby TFH interactions mediate selective survival of higher affinity GC B cells remains unclear. TCB interactions via receptors such Saridegib as co-stimulatory molecules, death receptor ligands, and soluble survival factors are probably involved. However, the precise identities and relative roles of these molecules remain obscure because most potential candidates also play functions in GC initiation or maintenance on their own. Therefore, separating these Saridegib functions from direct functions in the preferential selection of high affinity clones has proven difficult. For example, the initiation and maintenance of GCs rely on sustained CD40/CD40L signals, and death receptors such as Fas/FasL interactions act to limit GC responses (Foy et al., 1993; Han et al., 1995; Hao et al., 2008). Similarly, soluble mediators Saridegib such as IL-21 are essential for maintenance of GC B cell character as well as fate choices (Linterman et al., 2010; Zotos et al., 2010). The B lineage survival cytokine, B lymphocyte stimulator (BLyS, also termed B cell activating factor [BAFF]), plays a key role in setting thresholds for BCR-mediated selection among naive B cells (Cancro, 2004), making it an attractive candidate for mediating analogous processes in the GC. Consistent with this notion, GC responses prematurely terminate in mice with either global BLyS deficiency or defects in BLyS receptor 3 (BR3, BMP2 also known as BAFFR) signaling (Rahman et al., 2003). Straightforward interpretation of these findings is difficult, because both BLyS-deficient and BR3 mutant mice are severely B lymphopenic (Moore et al., 1999; Schneider et al., 1999; Yan et al., 2001a). Thus, deficits in naive B cell numbers might explain an inability to sustain GC reactions because GCs are resupplied from the naive pools (Schwickert et al., 2007). Moreover, defects in FDC network maturation and TFH function also occur in B lymphopenic environments (Rahman et al., 2003; Johnston et al., 2009). Thus, whether BLyS plays a direct role Saridegib in GC B cell selection and affinity maturation has remained unclear. To better understand how BLyS influences GC function, we investigated the distribution and expression of BLyS and its receptors during GC responses in normal mice. We find that BLyS is usually spatially segregated between the follicles and GCs, as well as within the GCs, where.