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The role of T cells in controlling human cancers is well known. Their success requires continued persistence in vivo and efficient trafficking to tumor sites, requirements shared by other effectors such as Natural Killer (NK) cells. To date, cytokine IL2 remains the only clinically approved cytokine therapy available to expand, maintain, and activate these effector lymphoid cells, but toxicities can be severe. Cytokine IL15 offers similar T cell proliferation and activation properties, but without the unwanted side-effects seen with IL2. Several IL15-cytokine fusion proteins have been developed to improve their in vivo function, typically exploiting the IL15Rα to complex with IL15, to extend serum half-life and increase affinity for IL15β receptor on immune cells. Here we describe a novel IL15 complex incorporating the full-length IL15Rα to complex with wild type IL15 to form spontaneous trimers of dimers (6 IL15 + 6 IL15Rα) during co-expression, resulting in a substantial increase in serum half-life and enhancement of in vivo cytokine effect on IgG or T cell engaging antibody-dependent cell-mediated cytotoxicities, when compared to alternative strategies.