Selvaratnam, J. S.; Barbosa da Rocha, J. D.; Rajan, V.; Wang, H.; Reddy, E. C.; Gams, M. S.; Murre, C.; Guidos, C. J.; Zuniga-Pflucker, J. C.; Anderson, M. K.

{gamma}{delta}T cells that produce IL-17 ({gamma}{delta}T17) play essential roles in barrier immunity, but the gene networks that install their functions are not well understood. We previously linked T cell receptor (TCR) signal strength to the proportional upregulation of Id3 during T cell development, which antagonizes the activity of HEB (encoded by Tcf12), and showed that HEB is required for {gamma}{delta} T17 development. To understand how HEB and Id3 regulate {gamma}{delta}T17 cell development, we conducted single cell RNA-sequencing on fetal thymic {gamma}{delta} T cells from Tcf12-deficient mice. {gamma}{delta} T cells lacking HEB exhibited profound alterations in the expression of the genes encoding the TCR{gamma} and TCR{delta} chains, accompanied by a decrease in expression of genes in the early {gamma}{delta} T cell specification network. Surprisingly, Id3 was among the most severely decreased genes in HEB-deficient {gamma}{delta} T cell precursors, suggesting a requirement for HEB in Id3 expression. Analysis of fetal thymic {gamma}{delta} T cells in Id3-deficient mice revealed that the TCR{gamma} and TCR{delta} repertoires were unaffected, and expression of early {gamma}{delta} T cell genes was unperturbed. However, later stage regulators of {gamma}{delta}T17 cell differentiation were decreased, and Id3-deficient {gamma}{delta} T cells were defective in IL-17 production. Therefore, our findings reveal an interlinked network in which HEB is initially required to upregulate Id3 expression, which in turn enables the later stages of {gamma}{delta}T17 cell development and functional programming.