Coupled IL-2–Dependent Extracellular Feedbacks Govern Two Distinct Consecutive Phases of CD4 T Cell Activation

T cells integrate cell-specific Ag receptor signaling with shared signals mediated by secreted cytokines, which often involve regulatory feedback loops. IL-2 signaling, for example, reduces the synthesis of IL-2 and increases the synthesis of IL-2Rα-chain, whereas both genes require TCR signaling for their activation. The ways by which T cells dynamically integrate these private and public signals during activation are not well understood. We combined robotics, multiparameter flow cytometry, and real-time quantitative PCR to analyze T cell activation at high temporal resolution over several days. Two distinct temporal phases of T cell activation were evident. First, Ag-dependent signals activated low IL-2Rα and high IL-2 production, independent of IL-2 signaling. Subsequently, secreted IL-2 acted as a shared resource driving high IL-2Rα expression, reduced IL-2 synthesis, and cell proliferation. This transition was independent of continued TCR signaling. Our data allowed the determination of the parameters of the IL-2-mediated extracellular positive and negative feedback circuits and demonstrated that the two loops are coupled and become activated at a similar level of IL-2 signaling. We propose that temporal separation of private and shared signals allows T cells to first integrate Ag-specific responses and subsequently share information leading to collective decision making.