SUMMARY T helper 1 (Th1) cell identity is defined by the expression of the lineage-defining transcription factor T-bet. Here, we examine the influence of T-bet expression heterogeneity on subset plasticity by leveraging cell sorting of distinct in vivo -differentiated Th1 cells based on their quantitative expression of T-bet and interferon-γ. Heterogeneous T-bet expression states were regulated by virus-induced type-I interferons and were stably maintained even after secondary viral infection. Exposed to Th2-polarizing conditions, the sorted subpopulations exhibited graded levels of plasticity: T-bet quantities were inversely correlated with the ability to express the Th2 lineage-specifying transcription factor GATA-3 and Th2 cytokines. Reprogramed Th1 cells acquired graded, but stable mixed Th1+2 phenotypes with a hybrid epigenetic landscape. Continuous presence of T-bet in differentiated Th1 cells was essential to ensure Th1 cell stability. Thus, innate cytokine signals regulate Th1 cell plasticity via an individual cell-intrinsic rheostat to enable T cell subset adaptation to subsequent challenges. HIGHLIGHTS Type-I interferons triggered by infection determine T-bet expression states in Th1 cells T-bet and IFN-γ expression states indicate the plasticity of individual Th1 cells Individual T-bet expression states and plasticity persist after secondary infection Reprogramming yields stable Th1+2 phenotypes and a mixed epigenetic landscape

Plasticity and lineage commitment of individual Th1 cells are determined by stable T-bet expression quantities