Induction of cytotoxic T lymphocyte antigen 4 (CTLA-4) restricts clonal expansion of helper T cells

Cytotoxic T lymphocyte antigen (CTLA)-4 plays an essential role in immunolo gic homeostasis. How this negative regulator of T cell activation executes its functions has remained controversial. We now provide evidence chat CTLA -4 mediates a cell-intrinsic counterbalance to restrict the clonal expansio n of proliferating CD4(+) T cells. The regulation of CTLA-4 expression and function ensures that, after similar to3 cell divisions of expansion, most progeny will succumb to either proliferative arrest or death over the ensui ng three cell divisions. The quantitative precision of the counterbalance h inges on the graded, time-independent induction of CTLA-4 expression during the first three cell divisions. In contrast to the limits imposed on unpol arized cells. T helper type 1 (Th1) and Th2 effector progeny may be rescued from proliferative arrest by interleukin (IL)-12 and IL-4 signaling, respe ctively, allowing appropriately stimulated progeny to proceed to the stage of tissue homing. These results suggest that the cell-autonomous regulation of CTLA-4 induction may be a central checkpoint of clonal expansion of CD4 (+) T cells, allowing temporally and spatially restricted growth of progeny to be dictated by the nature of the threat posed to the host.