BCL6-Mediated Silencing of PD-1 Ligands in Germinal Center B Cells Maintains Follicular T Cell Population

The programmed cell death protein 1 (PD-1) ligands PD-L1 and PD-L2 on germinal center (GC) B cells deliver coinhibitory signals to follicular T cells. The PD-L1/L2-PD-1 axis modulates the quality and quantity of follicular T cells and has been shown to influence the GC responses. However, the transcriptional control of PD-1 ligands on GC B cells remains largely unknown. In this study, we report that the transcription factor BCL6 is a key negative regulator of the PD-1 ligands PD-L1 and PD-L2 in GC B cells. Acute deletion of Bcl6 in mature GC B cells resulted in marked upregulation of mRNA and protein abundance of PD-1 ligands. Moreover, the expression levels of BCL6 and PD-1 ligands were inversely correlated during GC B cell development and in human GC-derived lymphoma specimens. Mechanically, BCL6 directly bound to the promoter region of PD-L1 and intron 2 of PD-L2 o suppress their transcription. In addition, BCL6 indirectly inhibited the transcription of PD-1 ligands by repressing the expression of STAT1/STAT3 and IRF1. Moreover, BCL6 exerted these effects via its BTB domain. Finally, PD-1 blockade promoted cell survival to sustain the follicular T cell pool in the presence of Bcl6 -deficinet GC B cells. In summary, B cell-specific expression of BCL6 dampens the PD-L1/L2-PD-1 signaling to maintain the size of follicular T cells during GC development.