Abstract A59: Expression of membrane-bound IL-15/IL-15Rα complex in chimeric antigen receptor-engineered natural killer cells for enhanced efficacy against solid tumors

The natural killer cell line NK92 has shown exciting potential as an anticancer immunotherapeutic tool in both preclinical models and in clinical trials. Preclinical studies have demonstrated improved antitumor responses after modifying NK92 cells with chimeric antigen receptors (CARs), which redirect immune cell activity to target cancer cells. CARs typically contain an antibody-derived extracellular domain, which binds to the desired tumor-associated antigen (TAA) and triggers an intracellular signaling cascade to activate the immune cell against the target cell. While CAR-based therapies have had remarkable success in treating hematological cancers, treatment of solid tumors has encountered many obstacles, including a lack of persistence and immunosuppression within the tumor microenvironment. Researchers have tried to circumvent these issues through the systemic delivery of cytokines such as IL-2 and IL-15. However, the delivery of supraphysiological cytokine concentrations is inefficient and has led to toxicities and unwanted side effects in the clinic. More recently, immune cells have been modified to ectopically express cytokines such as IL-15 to provide autocrine stimulation. IL-15 has been shown to activate and promote the proliferation of immune cells such as T and NK cells upon binding to its receptor, IL-15Rα. Recombinant IL-15 bound to IL-15Rα has been tested in clinical trials to boost antitumor immunity, as it imitates the physiological transpresentation of endogenous IL-15. We have transduced NK92 cells with either a CAR targeted against the human TAA mesothelin (αmeso.NK) or a bicistronic vector containing the αmeso CAR and a membrane-bound human IL-15/IL-15Rα complex (αmeso.mbIL15.NK). Both retroviral vectors demonstrated efficient transduction of NK92 cells, with 55% and 40% transduction efficiency, respectively. CAR+ populations were further selected via fluorescence activated cell sorting. The inclusion of the mbIL15 complex allowed the αmeso.mbIL15.NK cells to continue to proliferate in vitro in the absence of exogenous IL-2 and further enriched CAR expression when tested in an unsorted population. In contrast, αmeso.NK cells without the mbIL15 complex were unable to proliferate without exposure to IL-2 in culture, resulting in cell death and loss of CAR expression within five days. We will compare αmeso.NK and αmeso.mbIL15.NK cells to determine if the mbIL15 complex enhances cytokine secretion and cytotoxicity in the presence of mesothelin-expressing cancer cells in vitro. We further hypothesize that αmeso.mbIL15.NK cells will demonstrate superior proliferation and persistence in a mouse model without additional cytokine supplementation when compared to αmeso.NK cells. NK92 cells represent a viable allogenic, off-the-shelf anticancer immunotherapy, and CAR-engineered NK92 cells have shown promise in preclinical solid tumor models. The addition of membrane-bound IL-15/IL-15Rα complex may further enhance cytotoxic effects as well as persistence and resistance to immune exhaustion within the tumor microenvironment. Citation Format: Elizabeth L. Siegler, Pin Wang. Expression of membrane-bound IL-15/IL-15Rα complex in chimeric antigen receptor-engineered natural killer cells for enhanced efficacy against solid tumors [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr A59.