Hobit identifies TRM precursors that are regulated by Eomes

Tissue-resident memory CD8+ T cells (TRM) constitute a non-circulating memory T cell subset that provides early protection against re-infection. However, how TRM arise from antigen-triggered T cells has remained unclear. Exploiting the TRM-restricted expression of Hobit, we developed TRM reporter/deleter mice to study TRM differentiation. We found that Hobit was upregulated in a subset of LCMV-specific T cells located within peripheral tissues during the effector phase of the immune response. These Hobit+ effector T cells were identified as TRM precursors, given that their depletion substantially decreased TRM development, but not the formation of circulating memory T cells. Adoptive transfer experiments of Hobit+ effector T cells corroborated their biased contribution to the TRM lineage. Transcriptional profiling of Hobit+ effector T cells underlined the early establishment of TRM properties including downregulation of tissue exit receptors and upregulation of TRM-associated molecules. Importantly, we identified Eomes as a key factor instructing the early bifurcation of circulating and resident lineages. These findings establish that commitment of TRM occurs early in antigen-driven T cell differentiation and reveal the molecular mechanisms underlying this differentiation pathway.