The BCL6 RD2 Domain Governs Commitment of Activated B Cells to Form Germinal Centers

Abstract

Summary To understand how the Bcl6 transcriptional repressor functions in the immune system we disrupted its RD2 repression domain in mice. Bcl6RD2MUT mice exhibit a complete loss of GC formation but retain normal extrafollicular responses. Bcl6RD2MUT antigen-engaged B-cells migrate to the interfollicular zone and interact with cognate T helper cells. However, these cells fail to complete early GC-commitment differentiation and coalesce as nascent GC aggregates. Bcl6 directly binds and represses trafficking receptors S1pr1 and Gpr183 by recruiting Hdac2 through the RD2 domain. Deregulation of these genes impairs B-cell migration and may contribute to GC failure in Bcl6RD2MUT mice. The development of functional GC-TFH cells was partially impaired in Bcl6RD2MUT mice. In contrast to Bcl6−/− mice, Bcl6RD2MUT animals experience no inflammatory disease or macrophage deregulation. These results reveal an essential role for RD2 repression in early GC commitment and striking biochemical specificity in Bcl6 control of humoral and innate immune-cell phenotypes.

Mechanisms of action of BCL6 during germinal center B cell development

“…Interestingly, the co-activator p300 can bind and acetylate the RD2 domain at its KKYK motif and inhibit its transcription activity [42]. Recently, a 45-amino acid region compassing KKYK motif was identified to interact with HDAC2 and MTA3/NurD complex [17]. This region displayed a similar repression effect as full-length RD2.…”

“…High-resolution intravital cellular imaging showed that RD2-mutant antigen-engaged B-cells can normally migrate to the interfollicular zone and interact with cognate T helper cells after immunization. However, these cells failed to complete early GC-commitment differentiation and coalesced as nascent GC aggregates [17]. The profound defect in early pre-GC differentiation in RD2-deficent B cells best explains the complete abrogation of GC formation in Bcl6-deficent mice and emphasizes the importance of Bcl6 in early GC B-cell commitment (Figure 2).…”

“…BTB-mutant B cells can still form early GCs [16], indicating that BTB repression function is dispensable for pre-GC B-cell development (Figure 2). However, mice engineered to express a Bcl6 mutant that lost RD2 domain repression function exhibit a complete loss of GC formation but retain normal extrafollicular responses [17]. High-resolution intravital cellular imaging showed that RD2-mutant antigen-engaged B-cells can normally migrate to the interfollicular zone and interact with cognate T helper cells after immunization.…”

“…The profound defect in early pre-GC differentiation in RD2-deficent B cells best explains the complete abrogation of GC formation in Bcl6-deficent mice and emphasizes the importance of Bcl6 in early GC B-cell commitment (Figure 2). RD2 controls migration at least in part through directly suppressing expression of the key migration factor GRP183 [17], down-regulation of which is critically important for B-cell migration into the follicle center. In addition, BCL6 directly represses S1PR1 and indirectly induces S1PR2 expression.…”

“…However, the link between the transcriptional mechanisms of action of BCL6 with its biological actions in the immune system remains to be further characterized. In recent years, two new mouse models designed to dissect the biochemical mechanisms of action of BCL6 have provided critical insight into the ways that BCL6 can control the phenotype of B cells during the GC reaction [16,17].…”