Figure 1

(A) Quorum-sensing. The presence of IL-2 and the ability of the Treg cells to detect its levels are crucial to the homeostasis of the immune system. Quorum-sensing in this case is defined as an indirect feedback loop where the IL-2 produced by a subpopulation of activated T cells (among others) is detected (sensed) by a subpopulation of CD4⁺ Treg cells expressing the high-affinity IL-2Rα-chain; these cells contribute to controlling the number of CD4⁺ T cells. In other words, the overall CD4⁺ T cell populations sense the produced quantities of IL-2 and adapt their behavior accordingly. (B) Failure of quorum-sensing by defective sensor molecule. The inability to detect IL-2 because of defects in IL-2R expression (in IL-2Rα^−/− or IL-2Rβ^−/− mice) or signaling (in STAT5^−/− mice) leads to lymphoid hyperplasia and autoimmune disease. (C) Failure of quorum-sensing due to absence of the sensed molecule. In the absence of IL-2, Treg cells do not survive, which causes lymphoid hyperplasia and autoimmune pathology.