Interleukin-15 (IL15) is one of the most important cytokines currently being considered for
cancer therapy applications. It is thought that by administering IL15 in complex with its cognate
receptor alpha chain (IL15Ra) its biological activity could be increased manifold. We
produced a fusion protein of mouse IL15Ra and the F8 antibody, that targets the alternatively-
spliced extra-domain A (EDA) of fibronectin, which is overexpressed in many types of
cancer. The fusion protein F8IL15Ra was cloned, expressed and characterized in vitro and
its ability to bind to mouse IL15 was assessed with both size exclusion chromatography
(SEC) and surface plasmon resonance (SPR) experiments. Furthermore, mouse and
human IL15 and their corresponding Fc fused IL15Ra subunits were purchased, characterized
and used to compare the capacity of F8IL15Ra to generate complexes. Surprisingly,
none of the IL15Ra fusion proteins showed IL15 complexation on SEC. However, on SPR,
F8IL15Ra displayed the ability to bind IL15. In a cell-based activity assay none of the
IL15Ra fusions were able to increase cellular proliferation in combination with IL15 compared
to IL15 alone. A better understanding of the molecular requirements for effective IL15
signalling are likely to be important for the development of IL15-based biopharmaceuticals.