TCRαβ thymocytes differentiate to either CD8αβ cytotoxic T lymphocytes or CD4+ T helper cells. This functional dichotomy is controlled by key transcription factors, including the T helper master regulator, ThPOK, which suppresses the cytolytic program in MHC class II-restricted CD4+ thymocytes. ThPOK continues to repress CD8-lineage genes in mature CD4+ T cells, even as they differentiate to T helper effector subsets. Here we show that the T helper-fate was not fixed and that mature antigen-stimulated CD4+ T cells could terminate Thpok expression and reactivate CD8-lineage genes. This unexpected plasticity resulted in the post-thymic termination of the T helper-program and the functional differentiation of distinct MHC class II-restricted CD4+ cytotoxic T lymphocytes.

Mouse CD4+CD8+ double-positive (DP) thymocytes differentiate into CD4+ helper-lineage cells upon expression of the transcription factor Th-POK but commit to the CD8+ cytotoxic lineage in its absence. We report the redirected differentiation of class I-restricted thymocytes into CD4+CD8- helper-like T cells upon loss of Runx transcription factor complexes. A Runx-binding sequence within the Th-POK locus acts as a transcriptional silencer that is essential for Th-POK repression and for development of CD8+ T cells. Thus, Th-POK expression and genetic programming for T helper cell development are actively inhibited by Runx-dependent silencer activity, allowing for cytotoxic T cell differentiation. Identification of the transcription factors network in CD4 and CD8 lineage choice provides insight into how distinct T cell subsets are developed for regulating the adaptive immune system.

Interferon γ (IFNγ) is the hallmark cytokine produced by T helper type 1 (Th1) cells, whereas interleukin (IL)-4 is the hallmark cytokine produced by Th2 cells. Although previous studies have revealed the roles of cytokine signaling and of transcription factors during differentiation of Th1 or Th2 cells, it is unclear how the exclusive expression pattern of each hallmark cytokine is established. The DNaseI hypersensitivity site IV within the mouse Il4 locus plays an important role in the repression of Il4 expression in Th1 cells, and it has been named the Il4 silencer. Using Cbfβ- or Runx3-deficient T cells, we show that loss of Runx complex function results in derepression of IL-4 in Th1 cells. Binding of Runx complexes to the Il4 silencer was detected in naive CD4+ T cells and Th1 cells, but not in Th2 cells. Furthermore, enforced expression of GATA-3 in Th1 cells inhibited binding of Runx complexes to the Il4 silencer. Interestingly, T cell–specific inactivation of the Cbfβ gene in mice led to elevated serum immunoglobulin E and airway infiltration. These results demonstrate critical roles of Runx complexes in regulating immune responses, at least in part, through the repression of the Il4 gene.