Single-cell immune profiling reveals a developmentally distinct CD4(+) GM-CSF(+) T-cell lineage that induces GI tract GVHD

Gastrointestinal (GI) tract involvement is a major determinant for subsequent morbidity and mortality arising during graft-versus-host disease (GVHD). CD4(+) T cells that produce granulocyte-macrophage colony stimulating factor (GM-CSF) have emerged as central mediators of inflammation in this tissue site as GM-CSF serves as a critical cytokine link between the adaptive and innate arms of the immune system. However, cellular heterogeneity within the CD4(+) GM-CSF(+) T-cell population due to the concurrent production of other inflammatory cytokines has raised questions as to whether these cells have a common ontology or if a unique CD4(+) GM-CSF(+) subset exists that differs from other defined T helper subtypes. Using single-cell RNA sequencing analysis (scRNAseq), we identified two CD4(+) GM-CSF(+) T-cell populations that arose during GVHD and were distinguishable according to the presence or absence of interferon-γ (IFN-γ) coexpression. CD4(+) GM-CSF(+) IFN-γ(−) T cells, which emerged preferentially in the colon, had a distinct transcriptional profile, used unique gene regulatory networks, and possessed a nonoverlapping T-cell receptor repertoire compared with CD4(+) GM-CSF(+) IFN-γ(+) T cells as well as all other transcriptionally defined CD4(+) T-cell populations in the colon. Functionally, this CD4(+) GM-CSF(+) T-cell population contributed to pathologic damage in the GI tract that was critically dependent on signaling through the interleukin-17 (IL-7) receptor but was independent of type 1 interferon signaling. Thus, these studies help to unravel heterogeneity within CD4(+) GM-CSF(+) T cells that arise during GVHD and define a developmentally distinct colitogenic T helper subtype GM-CSF(+) subset that mediates immunopathology.