HuMax-IL15 is a high affinity fully humanized IgG1 anti-IL-15 monoclonal antibody designed to neutralize IL-15 by binding receptor-bound IL-15 at the site where it interacts with the common γ-chain. This thereby interferes with its signaling, putatively involving the recruitment and activation of T cells, maintenance of T cell memory, activation of neutrophils, and immortalization of fibroblast-like synoviocytes and endothelial cells. IL-15 also appears to play a significant role in natural killer cell ontogeny and effector function, at least in mice. IL-15 appear to be expressed on macrophages, endothelial cells, and fibroblast-like synoviocytes in rheumatoid arthritis synovial tissues, and this is, in part, the basis from which a Phase I/II study was conducted . HuMax-IL15 appears tolerable with a favorable side effect profile and even an encouraging efficacy profile in terms of improvement of symptom scores.