Abstract
T cell proliferation and survival are regulated by the cytokine receptor common gamma-chain-associated cytokines IL-2, IL-7, and IL-15, while IL-4, another gamma-chain-associated cytokine, is thought to primarily affect T cell quality rather than quantity. In contrast, our experiments reveal that endogenously produced IL-4 is a direct, nonredundant, and potent stimulator of CD8(+) T cell proliferation in Ag- and pathogen-induced CD8(+) T cell responses. These stimulatory effects of IL-4 are observed in both BALB/c and C57BL/6 mice and activate both naive and memory/activated phenotype CD8(+) T cells, although the former are stimulated less than are the latter. IL-4 effects are IL-7- and IL-15-independent, but MHC class I-dependent stimulation appears to be required for the mitogenic effect of IL-4 on naive phenotype CD8(+) T cells. Thus, endogenously produced IL-4 is an important regulator of quantitative as well as qualitative aspects of T cell immunity.