Introduction: Interleukin-15 (IL-15), provides strong activation of both CD8+ T-cells and NK cells, without regulatory T-cells activation, making it an attractive immune modulator in cancer therapy. Systemic delivery of IL-15 to patients has revealed dose-limited toxicities resulting primarily in expansion of NK cells. Preclinical data suggest that IL-15 immunotoxicity is mediated by hyperproliferation and activation of NK cells (Guo Y, J Immunol 2015). In this study, we investigate safety and efficacy of T-cells loaded with Deep IL-15 (Deep IL-15 Primed T-cells), in a syngeneic mouse model. Deep IL-15 is a multimer of chemically crosslinked IL-15/IL-15 Rα/Fc heterodimers (IL15-Fc) that is designed for T-cell loading prior to adoptive cell transfer with the aim of improving the therapeutic window by autocrine signaling to the primed cells without causing the immunotoxicologic effects normally associated with IL-15. Deep IL-15 is loaded on the T-cells and, upon crosslinker cleavage, releases IL15-Fc to stimulate the primed cells. This novel T-cell-based therapeutic approach enables autocrine T-cell activation and expansion, and limits systemic exposure to IL15-Fc, thus reducing associated toxicities. Methods: Deep IL-15 was synthesized by incubation of IL15-Fc with a crosslinking reagent. PMEL CD8+ T (PMEL) cells were isolated from B6.Cg-Thy1a/Cy Tg(TcraTcrb)8Rest/J mice. PMEL cells carry a transgenic T-cell receptor specific for gp100, a protein expressed by B16-F10 melanoma cells. PMEL cells were activated, expanded, and loaded with Deep IL-15 to generate Deep IL-15 Primed PMEL (Deep-15 PMEL) cells. Deep-15 PMEL cells were transferred into naïve or B16-F10 tumor-bearing mice (10 x 106; 15 ug Deep IL-15/106 cells), and the toxicity of Deep-15 PMEL was compared with PMEL (10 x 106) co-injected with soluble IL15-Fc at the maximum tolerated dose of 10 μg/mouse (PMEL + IL15-Fc). Readouts included IL15-Fc systemic exposure (ELISA), cytokine release (Luminex), and changes in endogenous T-cells (complete blood counts, CBC; flow cytometry). In addition, the biodistribution and the antitumor activity of Deep-15 PMEL was evaluated in B16-F10 tumor-bearing mice. Results: Deep-15 PMEL cells, carrying 15-fold more IL15-Fc than PMEL + IL15-Fc, resulted in >300-fold lower systemic exposure to IL15-Fc and 30-fold lower circulating IFN-γ. Deep-15 PMEL did not affect CBCs and did not expand endogenous CD8+ nor NK cells. Conversely, IL15-Fc induced significant changes in CBCs and promoted expansion of both transferred and endogenous CD8+ (6.7-fold) and NK (18.2-fold) cells. Similar results were observed in both naïve and tumor-bearing mice. Deep-15 PMEL improved persistence of transferred cells across multiple tissues (15-44-fold rel. to PMEL co-injected with IL15-Fc; day 16): blood, spleen, lymph nodes (tumor draining and non-draining) and tumor. The tumor presence affected the biodistribution of Deep-15 PMEL cells, resulting in lower (0.5-fold) accumulation of cells in the spleen, and increased (2.1-fold) cell numbers in the tumor-draining lymph node. Furthermore, Deep-15 PMEL showed significantly improved anti-tumor activity compared to PMEL in the B16-F10 model. Conclusions: The Deep Priming technology developed by Torque offers the advantage of loading Deep IL-15 prior to cell infusion in ACT, at concentrations unachievable with systemic injection of IL15-Fc, thus resulting in controlled, local delivery and cell-specific activation and proliferation. In vivo transfer of Deep-15 PMEL cells was well tolerated. Importantly, Deep-15 PMEL cells persisted longer across multiple organs compared to PMEL cells exposed to systemic IL15-Fc, while not inducing expansion of endogenous cells, including NK cells, which are reported to be the primary mediators of IL-15 toxicity. Torque is preparing to initiate clinical trials for Deep IL-15 Primed multitargeted human T-cells, TRQ15-01, in both hematologic and solid tumors.

Citation Format: Elena Geretti, Philip Bardwell, Xiaoyan Liang, Santina Caruso, De-Kuan Chang, Jesse Lyons, Austin Boesch, Aaron Handler, Carlos Tassa, Sanela Bilic, Janice Lancita, Becker Hawes, Jonathan Fitzgerald, Thomas Andresen. T-cell priming with Deep IL-15 improves preclinical safety compared to systemic IL-15, and increases in vivo persistence and activity [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A188.