Local and long-range transcriptional regulation of the Bcl6 locus by Ikaros zinc finger factors

Abstract

Abstract T follicular helper (TFH) cells play a pivotal role in the humoral immune response by providing help to B cells, resulting in germinal center formation and the subsequent generation of pathogen-neutralizing antibodies. The development of TFH cells requires the expression of the transcriptional repressor Bcl-6; however, the mechanisms underlying Bcl6 transcriptional regulation are incompletely understood. Here we identify the Ikaros Zinc Finger (IkZF) transcription factors Aiolos and Ikaros as novel regulators of Bcl6 expression. We demonstrate that the expression of Aiolos and Ikaros is upregulated in antigen-specific TFH cells generated in response to both Listeria monocytogenes and influenza infection. Mechanistically, we find that Aiolos and Ikaros directly associate with the Bcl6 promoter. This association corresponds with chromatin remodeling events consistent with gene activation including the recruitment of chromatin remodeling complexes, increased histone acetylation, and RNA polymerase II binding. Additionally, we find that Aiolos and Ikaros are enriched at a conserved noncoding sequence 273 kb (CNS273) upstream of the Bcl6 transcriptional start site. CNS273 exhibits a covalent histone modification profile indicative of an active enhancer and is capable of enhancing Bcl6 promoter activity in an Aiolos-dependent manner. Collectively, these data provide insight into the multifaceted mechanisms by which Aiolos and Ikaros regulate Bcl6 expression.