This pre GC B cell differentiation deficiency caused by
This pre-GC B cell differentiation deficiency caused by RD2 loss of function was explained at least in part by the finding that the BCL6 RD2 domain is required to repress expression of the key migration factors GRP183 and S1PR1. Downregulation of GRP183 is critically important for B cell migration into the follicle center (Gatto et al., 2009, Pereira et al., 2009). S1PR1 plays a key role in enabling B cell trafficking out of follicles (Cinamon et al., 2004). Our finding that BCL6 directly represses these genes in GC cells suggests a role for BCL6 at an early stage of the GC response, whereby BCL6 can enable “capture” of Amikacin within follicles to enable their clustering. At the same time, S1PR2 upregulation in GC B cells confines them to an S1P-low niche within follicles (Green et al., 2011). We observed that S1PR1 overexpression promoted GC-type B cell migration and antagonized S1PR2 (Figure S5G). Taken together, the data suggest that dynamic regulation and equilibrium of S1PR1 and S1PR2 is a critical event in the early pre-GC phase of the humoral immune response, controlled at least in part through BCL6 direct binding and repression of S1PR1.
From a mechanistic standpoint, the BCL6 RD2 domain represses the GPR183 and S1PR1 loci by recruiting HDAC2, but not MTA3-NuRD, to suppress the enhancer activation mark H3K27ac at their distal regulatory elements. However, these data do not exclude the possibility that other as yet unknown corepressor proteins may bind with BCL6-HDAC2 repression complexes to these key target genes. Taken together with recent findings showing that BCL6 BTB domain corepressors distribute to different sets of genomic loci with unique functions in GC B cells (Hatzi et al., 2013), these data suggest that transcriptional programming by BCL6 is exquisitely compartmentalized through linkage of distinct biochemical functions to genes involved in specific immunological and biological functions.
In contrast to Bcl6−/− T cells, Bcl6RD2MUT T cells were partially impaired in their ability to form GC-TFH cells (Figure S6), which is somewhat similar to a hypomorphic defect observed in Bcl6 T cells (Yu et al., 2009). This defect is partially explained by the fact that RD2 mutant GC-TFH cells exhibited reduced IL-21 expression and increased Blimp1. Repression of Blimp1 by BCL6 is known to critical for GC-TFH cell differentiation and function (Johnston et al., 2009). IL-21 is expressed by GC-TFH cell to promote GC B cell development and maintenance (Linterman et al., 2010, Zotos et al., 2010). Unlike Bcl6−/− mice, inflammatory responses and macrophage regulation were not significantly disrupted in Bcl6RD2MUT mice. Neither loss of the BTB domain lateral groove nor the RD2 domain are sufficient to elicit deregulation of inflammatory signaling, which must instead be more reliant on other functions of Bcl6. The dominant mechanism may be linked at least in part to the previously reported competition with STAT proteins for binding to promoters of genes regulating inflammatory signaling (Dent et al., 1997, Huang et al., 2013) (Figure S6).
Acknowledgments A.M. is supported by NCI R01 104348 and is also supported by the Burroughs Wellcome Foundation and Chemotherapy Foundation. A.H. and D.G.G. are supported by NIH grants R01AI080850 and R21AI101704. This research was initially supported by a March of Dimes Basil O’Connor Scholar Award (A.M.). This work was facilitated by the Sackler Center for Biomedical and Physical Sciences at Weill Cornell Medical College. We thank H. Ye from the Albert Einstein College of Medicine for sharing Bcl6 mice and P. Wade from NIH for providing anti-MTA3.
Introduction The Epstein–Barr virus induced gene 2 (EBI2 also known as GPR183) is a G protein-coupled seven-transmembrane (7TM) receptor that is predominantly expressed in B and T cells [1,2]. It regulates the trafficking of B cells within lymphoid tissues and is highly important for the generation of humoral immune responses [3,4]. EBI2 remained orphan for years; however, two independent studies recently showed that this receptor is activated by oxysterols, most potently by 7α,25-dihydroxycholesterol (7α,25-OHC) [5,6]. Binding of oxysterols to EBI2 induce Gαi activation, β-arrestin recruitment and ultimately migration of EBI2-expressing B and T cells. Thus, EBI2 functions as a chemo-attractant receptor. Interestingly, the main oxysterol generating cells within the lymphoid tissue were recently shown to be of stromal origin and these are required for efficient T cell-dependent plasma cell responses . Moreover, we [8,9] and others  identified residues critical for oxysterol binding to EBI2 showing that the main anchor points are found in TM-II, -III, -VI and ECL2 of which several are located in the minor binding pocket .