A population of CD4+ T cells with a naïve phenotype stably polarized to the Th1 lineage
T-bet is the lineage-specifying transcription factor for CD4+ T helper type 1 (TH1) cells. T-bet has also been found in other CD4+ T cell subsets, including TH17 cells and TREG, where it modulates their functional characteristics. However, we lack information on when and where T-bet is expressed during T cell differentiation and how this impacts the T cell function. To address this, we traced the ontogeny of T-bet-expressing cells using a fluorescent fate-mapping mouse line. We demonstrate that T-bet is expressed in a subset of naïve CD4+ T cells and that this novel cell population is phenotypically and functionally distinct from conventional naïve CD4+ T cells. These cells are also distinct from previously described populations of memory phenotype or stem cell-like T cells. Naïve T-bet-experienced cells are polarised to the TH1 lineage, predisposed to produce IFN? upon cell activation, and resist repolarisation to other lineages in vitro and in vivo. These results demonstrate that lineage-specifying factors can function to polarise naïve T cells prior to T cell activation and that this has an impact on the subsequent T helper response. Overall design: RNA sequencing data comparing YFP+ and YFP-  Naïve (CD62L+ CD44-) and Memory (CD62L-CD44+) CD4+ T cells purified from T-bet cre/+ Rosa26 YFP/+ mice.