[BK21 Plus Seminar]
▶Subject: Bcl6 controls the follicular helper T cell (Tfh) genetic program via inhibition-of-inhibitor mechanisms and exhibits autoregulation
▶Speaker: Jinyong Choi, Ph.D. (La Jolla Institute for Immunology)
▶Date: 4:30PM/Mar. 18(Mon.)/2019
▶Place: Auditorium(1F), Postech Biotech Center
Bcl6 is the lineage defining transcription factor of follicular helper T cells (Tfh). Bcl6 appears to control multiple categories of Tfh-associated genes, including genes important in Tfh/non-Tfh differentiation, cell localization, and Tfh functions. It has been controversial whether direct binding of Bcl6 to Tfh-associated gene loci results in repression or induction of gene expression. Bcl6 can bind to the Bcl6 promoter. We tested whether Bcl6 negatively or positively regulates Bcl6 expression, first by using a reporter gene driven by the Bcl6 promoter containing an 8-bp deletion of the Bcl6 DNA-binding motif in the Bcl6 promoter (∆BPS1). We found that deletion of the Bcl6 binding motif increased Bcl6 promoter activity in Tfh cells. We then generated ∆BPS1 germline mutated mice via CRISPR-Cas9. ∆BPS1 SMARTA (LCMV gp61-80 IAb specific) CD4 T cells exhibited significantly increased Bcl6 expression and Tfh differentiation in response to LCMV infection. Together, these experiments demonstrate that Bcl6 exhibits strong, direct negative autoregulation in Tfh cells. These data are consistent with Bcl6 being an obligate repressor. We further hypothesized that Bcl6 controls the induction of Tfh-associated genes via inhibition of one or more inhibitory transcription factors. Bcl6 is known to repress Prdm1, the gene encoding Blimp1, and Blimp1 represses Bcl6. Since Blimp1 expression is not repressed by Bcl6 in Bcl6f/f CreCd4 cells, it cannot be determined whether the resulting CD4 T cell gene expression pattern is due to the absence of Bcl6 or to the presence of Blimp1. Thus, we generated Bcl6f/fPrdm1f/f CreCd4 SMARTA mice to test the mechanism of action of Bcl6. Tfh differentiation did not occur in Bcl6f/fPrdm1f/f CreCd4 SMARTA CD4 T cells, demonstrating the requirement for active functions of Bcl6 independent of Blimp1 to achieve Tfh differentiation. To next identify the putative set of anti-Tfh transcription factors inhibited by Bcl6 (in addition to Prdm1), we performed coordinated gene expression profiling and ATAC-Seq utilizing Bcl6fl/fl CreCd4, Bcl6f/flPrdm1f/f CreCd4, Prdm1f/f CreCd4, and WT SMARTA CD4 T cells in response to acute LCMV infection or KLH-gp61 protein immunization. Transcription factors directly inhibited by Bcl6 were expected to have increased expression in Bcl6f/fPrdm1f/f CD4 T cells compared to Prdm1fl/fl CD4 T cells, and a set of such factors were identified that possessed appropriate attributes. K-means and hierarchical clustering analyses readily separated four gene expression patterns. By integrating RNAseq, ATAC-Seq and human GC Tfh BCL6 ChIP-Seq data, and use of ∆BPS1 as an example, we suggest that Bcl6 regulates Tfh and Th1 gene expression by inhibition-of-inhibitor mechanisms.
▶Inquiry: Prof. Jiwon Jang (279-2321)
* This seminar will be given in English.
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