Transient T-bet expression functionally specifies a distinct T follicular helper subset

T follicular helper (Tfh) cells express transcription factor BCL-6 and cytokine IL-21. Mature Tfh cells are also capable of producing IFN-gamma without expressing the Th1 transcription factor T-bet. Whether this IFN-gamma-producing Tfh population represents a unique Tfh subset with a distinct differentiation pathway is poorly understood. By using T-bet fate-mapping mouse strains, we discovered that almost all the IFN-gamma-producing Tfh cells have previously expressed T-bet and express high levels of NKG2D. DNase I hypersensitivity analysis indicated that the Ifng gene locus is partially accessible in this ex-T-bet population with a history of T-bet expression. Furthermore, multicolor tissue imaging revealed that the ex-T-bet Tfh cells found in germinal centers express IFN-gamma in situ. Finally, we found that IFN-gamma-expressing Tfh cells are absent in T-bet-deficient mice, but fully present in mice with T-bet deletion at late stages of T cell differentiation. Together, our findings demonstrate that transient expression of T-bet epigenetically imprints the Ifng locus for cytokine production in this Th1-like Tfh cell subset.