African green monkeys (AGMs; genus Chlorocebus) are a natural host of simian immunodeficiency virus (SIVAGM). As they do not develop simian AIDS, there is great interest in understanding how this species has evolved to avoid immunodeficiency. Adult African green monkeys naturally have low numbers of CD4+ T cells and a large population of major histocompatibility complex class II-restricted CD8αdim T cells that are generated through CD4 downregulation in CD4+ T cells. Mechanisms that drive this process of CD4 downregulation are unknown. Here, we show that juvenile AGMs accelerate CD4-to-CD8αα conversion upon SIV infection and avoid progression to AIDS. The CD4 downregulation induced by SIV infection is not limited to SIV-specific T cells, and vaccination of an adult AGM who had a negligible number of CD4 T cells demonstrated that CD4 downregulation can occur without antigenic exposure. Finally, we show that the T cell homeostatic cytokines interleukin-2 (IL-2), IL-7, and IL-15 can induce CD4 downregulation in vitro. These data identify a mechanism that allows AGMs to generate a large, diverse population of T cells that perform CD4 T cell functions but are resistant to SIV infection. A better understanding of this mechanism may allow the development of treatments to induce protective CD4 downregulation in humans.