Identification of MLL1 as a novel regulator of Tfh cell differentiation

Abstract

Abstract Transcription factors controlling T follicular helper (Tfh) cell differentiation have been extensively studied in recent years. However, there is limited knowledge of the different chromatin regulators involved in the control of Tfh differentiation. Most transcription factors control gene expression with the help of chromatin regulators, which modify chromatin structure to regulate gene expression. We hypothesize that chromatin modifying enzymes are involved in the control of Tfh differentiation. To uncover novel chromatin modifying enzymes essential for Tfh differentiation, we screened a shRNA library targeting all known chromatin regulators using our in vivo RNAi screen. Utilizing this screen, we identified the histone methyltransferase MLL1 as a positive regulator of Tfh differentiation after viral infection. We found MLL1 to be functionally important for Tfh differentiation. Knockdown of MLL1 utilizing multiple shRNAs impaired Tfh differentiation during LCMV infection. The requirement for MLL1 for proper Tfh differentiation after LCMV infection was further confirmed using a CRISPR/Cas9 approach. Additionally, ablation of MLL1 expression selectively impaired production of IL-21 by Tfh cells. To further understand the role of MLL1 in regulating the Tfh differentiation network, we performed RNA-Seq of MLL1-deficient Tfh as well as MLL1-sufficient Tfh. Analysis revealed that the expression of several genes critical for the regulation of Tfh differentiation is dysregulated in the absence of MLL1. Specific examples will be discussed.