IMST-02. TRANSGENIC EXPRESSION OF IL15 IMPROVES ANTIGLIOMA ACTIVITY OF IL13Rα2-CAR T CELLS BUT HIGHLIGHTS THE NEED TO TARGET MULTIPLE GLIOMA-ASSOCIATED ANTIGENS

Abstract

The goal of this project is to develop T cells expressing IL13Rα2-specific chimeric antigen receptor (CAR) as an effective immunotherapy for glioblastoma (GBM), the most aggressive, primary brain tumor in humans, in which outcome remains poor. We recently demonstrated that IL13Rα2-CARs with a CD28.z endodomain had potent anti-GBM activity in preclinical models. However, IL13Rα2-CAR T-cell persistence was limited, resulting in recurrence of IL13Rα2-positive GBMs. Next we wanted to evaluate if transgenic expression of IL-15 enhances persistence and anti-tumor activity of IL13Rα2-CAR.CD28.ζ T cells. We generated IL13Rα2-CAR.CD28.ζ T cells expressing IL-15 (IL13Rα2-CAR.IL15 T cells) and determined the effector function of IL13Rα2-CAR.IL15 T cells in vitro and in vivo. Double transduction of CD3/CD28-activated T cells resulted in T-cell lines that expressed both transgenes in 45-50% of T cells. At base line IL13Rα2-CAR.IL15 T cells produced on average 69.5 pg/ml of IL15. Production was significantly increased after CD3 or antigen-specific T-cell stimulation (176.7 pg/ml; n=6; p<0.001). IL13Rα2-CAR.IL15 T cells were as efficient as IL13Rα2-CAR T cells in killing IL13Rα2-positive GBMs in vitro. After intratumoral injection into U373 glioma-bearing mice IL13Rα2-CAR.IL15 T cells persisted significantly longer than IL13Rα2-CAR T cells (p<0.05). This resulted in a significant increase in progression free (84 vs 49 days; p=0.008) and overall survival (p=0.02) of treated mice. 3/5 examined, recurring U373 gliomas post IL13Rα2-CAR. IL15 T-cell therapy had down regulated IL13Rα2 expression, indicating immune escape. While all recurring GBMs expressed the glioma-associated antigen EphA2, 1/5 GBMs had also down regulated the expression of the glioma-associated antigen HER2. In conclusion, transgenic expression of IL15 enhances the in vivo persistence of IL13Rα2-CAR T cells resulting in improved anti-glioma activity. However, enhanced in vivo T-cell persistence resulted in the development of antigen-specific and -unspecific loss variants highlighting the need to target multiple glioma-associated antigens in tumors with heterogeneous antigen expression such as GBM.