Interleukin-2 (IL-2) is a 133 amino acid glycoprotein with one intramolecular disulfide bond and variable glycosylation. It is secreted by activated T cells and induces proliferation and maturation of activated T cells, natural killer cells and lymphokine activated killer cells. IL-2 also stimulates proliferation of antibody-producing B cells, activates neutrophils and induces mononuclear cells to secrete IFN-γ and TNF-α and -β. Moreover, studies have shown that IL-2 is required for activation-induced apoptosis, an important homeostatic mechanism in the immune system, which is involved in the maintenance of peripheral tolerance to self-antigens.

IL-2 promotes T cell proliferation and particularly naive T cells. IL-2 signaling on activated T cells is effected through a quaternary high-affinity receptor complex consisting of IL-2, IL-2Rα (CD25), IL-2Rβ and IL-2Rγ. Naive T cells are relatively insensitive to IL-2 as they only express small amounts of IL-2Rβ and IL-2Rγ. They only acquire sensitivity after CD25 expression, which captures the cytokine and presents it to the IL-2Rβ and IL-2Rγ receptors. IL-2 Superkine (Fc) is an artificial variant of IL-2 containing mutations at positions L80F / R81D / L85V / I 86V / I92F. These mutations are located in the molecule's core that acts to stabilize the structure and to give it a receptor-binding conformation mimicking native IL-2 bound to CD25. These mutations effectively eliminate the functional requirement of IL-2 for CD25 expression and elicit proliferation of T cells. Compared to IL-2, the IL-2 superkine induces superior expansion of cytotoxic T cells, leading to improved antitumour responses in vivo, and elicits proportionally less toxicity by lowering the expansion of Tregulatory cells and reducing pulmonary oedema.