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  • br Experimental Procedures Further details and an


    Experimental Procedures Further details and an outline of resources used in this work can be found in Supplemental Experimental Procedures.
    Introduction In response to T cell-dependent (TD) antigen stimulation, antigen-specific DL-AP5 migrate to the periphery of B cell follicles and interfollicular zones of secondary lymphoid organs, where they interact with cognate T helper cells within 1–3 days (Kerfoot et al., 2011, Okada et al., 2005, Qi et al., 2008). After then, B cells can follow one of three alternative fates by differentiating into extrafollicular plasma cells, follicular germinal center (GC) B cells, or recirculating early memory B cells (McHeyzer-Williams et al., 2012). GC B cells undergo massive clonal expansion, somatic hypermutation, and class switch recombination, after which selected clones undergo differentiation into memory B cells and long-lived plasma cells (Allen et al., 2007, Victora and Nussenzweig, 2012). T follicular helper (TFH) cells express the chemokine receptor CXCR5 and costimulatory molecule PD-1 at high levels and specialize in providing help to B cells during the humoral immune response (Crotty, 2011). Antigen-presenting dendritic cells mediate the initiation of TFH cell differentiation at early time points after immunization (Choi et al., 2011, Goenka et al., 2011). Interactions with cognate B cells, especially GC B cells within GCs, are critical for further polarization, maintenance, and function of TFH cells at later stages of the immune response (Choi et al., 2011, Kerfoot et al., 2011, Kitano et al., 2011). Thus, the reciprocal development of GC B cells and TFH cells is crucial for establishment of the GC reaction, including the formation of high-affinity antibody and the generation of long-lived plasma cells. The Bcl6 transcriptional repressor is a master regulator of the GC reaction required for development of both GC B cells and TFH cells, respectively (Dent et al., 1997, Fukuda et al., 1997, Johnston et al., 2009, Nurieva et al., 2009, Ye et al., 1997, Yu et al., 2009). In addition, Bcl6 plays a key role in suppressing inflammatory cytokine expression in macrophages (Toney et al., 2000). Bcl6-deficient (Bcl6−/−) mice in addition to failing to form GCs are sickly and die within weeks from a lethal inflammatory syndrome primarily driven by macrophages with secondary contributions from Th2 and Th17 cells (Mondal et al., 2010, Toney et al., 2000). However, from a mechanistic standpoint the function of Bcl6 is poorly understood outside of the context of already established GC B cells. Bcl6 enables proliferation and tolerance of DNA damage by silencing DNA damage sensing and cell cycle checkpoint genes, and it also delays plasma cell differentiation by repression of critical GC exit and plasma cell differentiation genes (Bunting and Melnick, 2013, Klein and Dalla-Favera, 2008). However, Bcl6−/− mice display a complete loss of GC formation with no evidence of the capacity to establish nascent GC clusters (Dent et al., 1997, Fukuda et al., 1997, Ye et al., 1997), suggesting that Bcl6 might have biological functions prior to GC formation. Indeed, Bcl6 protein is upregulated in early GC-committed B cells (i.e., “pre-GC B cells”) outside GCs 3–5 days after immunization (Kerfoot et al., 2011, Kitano et al., 2011) and plays an essential role in maintaining interactions with TFH cell as well as subsequent migration and clustering into GC structures, at least in part through repressing the expression of Gpr183, encoding the G protein-coupled receptor Ebi2 (Kitano et al., 2011, Shaffer et al., 2000). Bcl6 functions as a transcription repressor via its N-terminal BTB domain and middle “second repression,” or “RD2” domain (Chang et al., 1996, Seyfert et al., 1996). A loss of function of the BCL6 BTB domain markedly impairs survival and proliferation of mature GC B cells in a B cell intrinsic manner, with no effects on T cells or macrophages (Huang et al., 2013). Notably, unlike Bcl6−/− B cells, BTB-deficient B cells still form GCs although their numbers and sizes are markedly reduced. This discrepancy illustrates our incomplete understanding of the molecular underpinnings of Bcl6-mediated GC B cell development and suggests that other functions of Bcl6 are dominantly involved in modulating early pre-GC B cell fate.