The Duration of T Cell Stimulation Is a Critical Determinant of Cell Fate and Plasticity

The duration of T cell receptor contact with antigen modulates the pattern of downstream signaling to determine CD4+ T cell fate. Signal Duration Controls T Cell Fate CD4+ T cells are broadly classified into two groups: T helper (TH) cells, which mediate the immune response, and regulatory T (Treg) cells, which suppress immunity. Both cell types can arise from the differentiation of naïve CD4+ T cells stimulated through the T cell receptor (TCR) by antigen. Cell fate is thought to arise from differences in the intensity of the TCR signal, with strong TCR signals generating TH cells and weaker TCR signals generating Treg cells. Understanding how to manipulate T cell fate has implications for immunotherapies; for example, generating too few Treg cells can lead to autoimmunity, whereas generating too many Treg cells can suppress antitumor immunity. Miskov-Zivanov et al. developed a logical mathematical model of T cell differentiation focused on early stages of TCR signaling and found that differential activation of TCR signaling pathways by low and high doses of antigen was better modeled as changes in the duration, rather than the intensity, of TCR signaling. Their model predicted, and experiments confirmed, that there is a time window for signaling during which T cell fate is plastic and that varying the duration of stimulation controls the relative proportion of Treg and TH cells produced. The model also predicts that the TH cell phenotype stabilized more rapidly than did the Treg cell phenotype. Together, these data suggest that differential timing of TCR signaling events plays an important role in determining T cell fate. Variations in T cell receptor (TCR) signal strength, as indicated by differential activation of downstream signaling pathways, determine the fate of naïve T cells after encounter with antigen. Low-strength signals favor differentiation into regulatory T (Treg) cells containing the transcription factor Foxp3, whereas high-strength signals favor generation of interleukin-2–producing T helper (TH) cells. We constructed a logic circuit model of TCR signaling pathways, a major feature of which is an incoherent feed-forward loop involving both TCR-dependent activation of Foxp3 and its inhibition by mammalian target of rapamycin (mTOR), leading to the transient appearance of Foxp3+ cells under TH cell–generating conditions. Experiments confirmed this behavior and the prediction that the immunosuppressive cytokine TGF-β (transforming growth factor–β) could generate Treg cells even during continued Akt-mTOR signaling. We predicted that sustained mTOR activity could suppress FOXP3 expression upon TGF-β removal, suggesting a possible mechanism for the experimentally observed instability of Foxp3+ cells. Our model predicted, and experiments confirmed, that transient stimulation of cells with high-dose antigen generated TH, Treg, and nonactivated cells in proportions depending on the duration of TCR stimulation. Experimental analysis of cells after antigen removal identified three populations that correlated with these T cell fates. Further analysis of simulations implicated a negative feedback loop involving Foxp3, the phosphatase PTEN, and Akt-mTOR in determining fate. These results suggest that there is a critical time after TCR stimulation during which heterogeneity in the differentiating population of cells leads to increased plasticity of cell fate.