Abstract The CTLA-4 pathway is a key regulator of T cell activation and a critical failsafe against autoimmunity. Although early models postulated that CTLA-4 transduced a negative signal, in vivo evidence suggests that CTLA-4 functions in a cell-extrinsic manner. That multiple cell-intrinsic mechanisms have been attributed to CTLA-4, yet its function in vivo appears to be cell-extrinsic, has been an ongoing paradox in the field. Although CTLA-4 expressed on conventional T cells (Tconv) can mediate inhibitory function, it is unclear why this fails to manifest as an intrinsic effect. In this study, we show that Tconv-expressed CTLA-4 can function in a cell-extrinsic manner in vivo. CTLA-4+/+ T cells, from DO11/rag−/− mice that lack regulatory T cells, were able to regulate the response of CTLA-4−/− T cells in cotransfer experiments. This observation provides a potential resolution to the above paradox and suggests CTLA-4 function on both Tconv and regulatory T cells can be achieved through cell-extrinsic mechanisms.