IL-7 and IL-15: Biology and Roles in T-Cell Immunity in Health and Disease

Cytokines IL-7 and IL-15 are essentially involved in T-cell homeostasis. IL-7 is required for developing mature T cells in the thymus, whereas in the periphery, it promotes the survival of naive and memory T cells by upregulating the antiapoptotic molecule Bcl-2. IL-15 potently induces the proliferation of memory CD8(+) T cells independently of antigen and augments their effector function. Although IL-7 and IL-15 may help to defend the host against microorganisms and tumors by promoting T-cell immunity, dysregulated production of IL-7 and IL-15 can be harmful. In fact, increased levels of IL-15 in the circulation and inflamed tissues have been reported in various autoimmune diseases, including rheumatoid arthritis (RA), possibly contributing to the pathogenesis. In addition, IL-7, which may induce the production of inflammatory cytokines from T cells and monocytes, are found to be elevated in the joints of patients with RA. Here, we review what is currently known about the roles of these cytokines in T-cell immunity, in general, as well as in RA, in particular, focusing on recent discoveries.