BCL6 orchestrates Tfh differentiation via multiple distinct mechanisms
ABSTRACT: 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 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 BCL6 depends on AP1 for recruitment to 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. Sorted naïve tonsil cells were activated with anti-CD3+CD28 Ab coated beads and transduced with BCL6 or control lentiviral vectors as described. RNA was isolated at day 5 following LV infection and microarrays performed as discussed herein. RNA was isolated with the RNeasy Micro Kit (QIAGEN). The quantity and quality of the RNA were confirmed with a NanoDrop 2000c (Thermo Fisher Scientific) and an Experion Electrophoresis System (Bio-Rad). Samples (20 ng) were amplified with Illumina MessageAmp II aRNA Amplification Kits (Ambion), hybridized to HumanHT-12_V4 BeadChips (Illumina), and quantified with Genome Studio (Illumina). Normalized data (heatmap of blood cells) or raw data (heatmap of tonsil cells and Volcano plot) were analyzed with the GenePattern software suite.