Abstract 5694: Complete regression of murine breast tumors by hetIL-15FC monotherapy correlates with infiltration by T, NK, cDC1 cells and a novel population of dendritic cells

Introduction: IL-15 cytokine stimulates the proliferation and cytotoxic function of CD8+ T and NK cells. Several preclinical models have supported the anti-tumor activity of IL-15. We have produced the native heterodimeric IL-15 (hetIL-15) of human, macaque or mouse and also fusions to the Fc antibody fragments (hetIL-15FC). Experimental procedures: We studied the therapeutic efficacy of hetIL-15 or hetIL-15FC immunotherapy in the murine EO771 orthotopic breast cancer model in syngeneic C57BL/6 mice. The effects of hetIL-15FC on immune cells were compared in tumors, lymph nodes and spleen by flow cytometry, immunohistochemistry (IHC) and gene expression profiling (Nanostring). We also assessed the metabolic profile of tumor infiltrating T cells. Results: hetIL-15 therapy delayed the primary tumor growth and increased overall survival. hetIL-15 or hetIL-15FC peritumoral administration resulted in tumor regression in 40% of the treated animals and resistance to tumor re-challenge. Therefore, monotherapy induced long term immunological memory able to block tumor growth. Both CD8+ T and NK cells were increased in hetIL-15FC treated tumors; phenotypic analysis showed they were activated and in an active proliferation state. Transcriptomic analysis confirmed the activated state of the T and NK cells, whereas metabolic flux analysis of the infiltrated CD8+T cells from treated mice confirmed a rise in oxygen consumption rate (OCR) with substantial increase of spare respiratory capacity (as a measure of mitochondrial reserve), which supports an activated/non exhausted phenotype of these hetIL-15 treated effector cells. In addition, peritumoral hetIL-15FC administration resulted in an increase of infiltrating, conventional type 1 dendritic cells (cDC1s), suggesting mechanisms of DC-lymphocyte interactions in the tumor. We also observed a new population of intra-tumoral DCs induced by hetIL-15, CD24+ F4/80high cDC2s (F4/80high DCs). The new DC population was negatively correlated with the tumor size of EO771 tumor bearing mice. Phenotypic profiling of F4/80high DCs identified expression of several cDC1 specific markers, including CD103 and IRF8 and t-SNE analysis clustered F4/80high DCs close to cDC1s and macrophages. Conclusion: Our results show complete eradication of EO771 tumors by hetIL-15FC treatment, correlating with different tumor infiltrating immune cell types. We propose that the anti-cancer activity of hetIL-15 in primary tumors is orchestrated by the interplay of CD8+T cells, cDC1s and a novel subset of antigen presenting cells with a distinct phenotypic profile. hetIL-15 supported a favorable metabolic profile of intra-tumoral effector lymphocytes, important for their function. Effective hetIL-15 treatment leads to development of long-term anti-tumor memory. Citation Format: Sevasti Karaliota, Dimitris Stellas, Vasiliki Stravokefalou, Bethany Nagy, Cristina Bergamaschi, Barbara K. Felber, George N. Pavlakis. Complete regression of murine breast tumors by hetIL-15FC monotherapy correlates with infiltration by T, NK, cDC1 cells and a novel population of dendritic cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5694.