BCL6 orchestrates Tfh differentiation via multiple distinct mechanisms

Tfh cells are required for T cell help to B cells, and BCL6 is the defining transcription factor of Tfh cells. However, the functions of Bcl6 in Tfh have largely remained unclear. Here we defined the BCL6 cistrome in primary human germinal center Tfh cells to assess mechanisms of BCL6 regulation of CD4 T cells, comparing and contrasting BCL6 function in T and B cells. BCL6 primarily acts as a repressor in Tfh cells, and BCL6 binding was associated with control of Tfh cell migration, TCR signaling, and repression of alternative cell fates. Interestingly, although some BCL6 bound genes possessed BCL6 DNA binding motifs, many BCL6-bound loci were instead characterized by the presence of DNA motifs for AP1 or STAT. AP1 complexes are key positive downstream mediators of TCR signaling and external stimuli. We show that BCL6 can directly bind AP1, and AP1 and BCL6 co-occupied BCL6 binding sites with AP1 motifs, suggesting that BCL6 subverts AP1 activity. These findings reveal that BCL6 has broad and multifaceted effects on Tfh biology, and provide insight into how this master regulator mediates distinct cell-context dependent phenotypes. ChIP-seq experiments to identify the genome wide targets of BCL6 and the profiles of histone modifications H3K4me3, H3K4me1 and H3K27ac in CXCR5hi Tfh cells isolated from human tonsils.