Figure 1.

Schematic representation of the fate of CD4 T cells at a localized site of inflammation. Naïve or memory CD4⁺CD25^- T cells are recruited to the site, then become activated upon exposure to antigen and co-stimulation. These cells proliferate and become CD4 T effectors. Activation also induces a subset of CD4⁺CD25^- T cells to upregulate CD25 and FoxP3 and acquire CD25⁺CD4⁺ (T_R) cell function. These cells may result from activation at the initiation of the response or, more probably, as the response matures. As antigen and IL-2 is depleted, effector T cells undergo activation induced cell death, T_R cells lead to the induction of T regulator 1 (Tr1) cells and Th3 cells, which feed back to inhibit inflammation, and the T_R cells inhibit proliferation of antigen-specific and bystander T cells. This results in a small number of CD4⁺ T cells surviving, which persist as memory T cells.