Transcription factor Achaete-Scute homologue 2 initiates T follicular helper cell development [array]. Mus musculus

In immune responses, activated T cells migrate to B cell follicles and develop to T follicular helper (Tfh) cells, a new subset of CD4+ T cells specialized in providing help to B lymphocytes in the induction of germinal centers 1-3. Although Bcl6 has been shown to be essential in Tfh cell function, it may not regulate the initial migration of T cells 4 or the induction of Tfh program as exampled by CXCR5 upregulation 5. Here, we show that the Achaete-Scute homologue 2 (Ascl2) gene that encodes a basic helix-loop-helix (bHLH) transcription factor 6, is selectively upregulated in its expression in Tfh cells. Ectopic expression of Ascl2 uniquely upregulates CXCR5 but not Bcl6 and downregulates CCR7 expression in T cells in vitro and accelerates T cell migration to the follicles and Tfh cell development in vivo. Combined transcriptome profiling and genome-wide occupancy analysis indicate that Ascl2 directly regulates Tfh-related genes while inhibits expression of Th1 and Th17 genes. Acute deletion of Ascl2 as well as blockade of its function with the Id3 protein in peripheral CD4+ T cells results in a failure in Tfh cell development and the germinal center response. Conversely, mutation of Id3, known to cause antibody-mediated autoimmunity, greatly enhances Tfh cell generation. Thus, Ascl2 critically and directly initiates Tfh cell development. Overall design: Decide gene expression patterns of CD4+ T cells with overexpressed Ascl2 or Tfh cells