Interleukin-15Rα-Sushi-Fc Fusion Protein Co-Hitchhikes Interleukin-15 and Pheophorbide A for Cancer Photoimmunotherapy.

Background: Interleukin-15 (IL-15) stimulates the proliferation of natural killer cells or T cells, which, in combination with photodynamic therapy (PDT), has emerged as an effective strategy for cancer photoimmunotherapy. Instead of direct cytokine receptor activation, IL-15 necessitates first binding to the IL-15 receptor α chain subunit (IL-15Rα), followed by trans-presentation to the IL-15 receptor β/γ chain subunit on the effector cells for pharmacologic activation. Therefore, the delivery of IL-15 remains a major challenge owing to its short half-life, its lack of targeting activity, and the limited availability of IL-15Rα. Methods: A co-hitchhiking delivery approach using recombinant IL-15 (rIL-15) and a photosensitizer, pheophorbide A (PhA), is developed for enhanced combinatorial cancer immunotherapy with PDT. A recombinant IL-15Rα-sushi-Fc fusion protein (rILR-Fc) is designed to load rIL-15 through the IL-15Rα sushi domain, which mimics its trans-presentation. Moreover, the Fc moiety of rILR-Fc can load PhA based on its high binding affinity. Results: Through self-assembly, rILR-Fc/PhA/rIL-15 nanoparticles (NPs) are formulated to co-hitchhike PhA and rIL-15, which improves the tumor accumulation of PhA and rIL-15 through receptor-mediated transcytosis. Moreover, the nanoparticles prolong the blood half-life of rIL-15 but do not alter the elimination rate of PhA from the blood. The rILR-Fc/PhA/rIL-15 NPs effectively elicit potent systemic antitumor immunity and long-lasting immune memory against tumor rechallenge in model mice bearing orthotopic colon tumors. Conclusions: The enhanced antitumor therapeutic effect demonstrates that the co-hitchhiking delivery strategy, optimizing the pharmacokinetics of both the photosensitizer and IL-15, provides a promising strategy for combinatorial photodynamic and IL-15 immunotherapy.