BCL6 Orchestrates Tfh Differentiation Via Multiple Distinct Mechanisms

Follicular Helper CD4 T Cells (Tfh) cells are required for T cell help to B cells and are essential for germinal centers and the development of most high affinity antibody responses that are a hallmark of protective immunity. BCL6 is the defining transcription factor of Tfh cells. However, the functions of Bcl6 in Tfh have largely remained unclear. Intriguingly, Bcl6 is essential in both Tfh cells and germinal center B cells, two cells with very different functions that exist in the same place at the same time and interact extensively. How Bcl6 accomplishes different tasks in these two different cell types has remained unknown. 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 binds over 3,000 genes in GC Tfh cells and predominantly occupies promoters, but also enhancers. In these cells BCL6 primarily acts as a repressor 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.