| Summary: | CD4 effector lymphocytes (T ) are traditionally classified by the cytokines they produce. To determine the states that T cells actually adopt in frontline tissues in vivo, we applied single-cell transcriptome and chromatin analyses to colonic T cells in germ-free or conventional mice or in mice after challenge with a range of phenotypically biasing microbes. Unexpected subsets were marked by the expression of the interferon (IFN) signature or myeloid-specific transcripts, but transcriptome or chromatin structure could not resolve discrete clusters fitting classic helper T cell (T ) subsets. At baseline or at different times of infection, transcripts encoding cytokines or proteins commonly used as T markers were distributed in a polarized continuum, which was functionally validated. Clones derived from single progenitors gave rise to both IFN-γ- and interleukin (IL)-17-producing cells. Most of the transcriptional variance was tied to the infecting agent, independent of the cytokines produced, and chromatin variance primarily reflected activities of activator protein (AP)-1 and IFN-regulatory factor (IRF) transcription factor (TF) families, not the canonical subset master regulators T-bet, GATA3 or RORγ. + eff eff eff H H
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