Abstract 3259: heterodimeric IL-15 monotherapy results in complete regression of EO771 murine breast tumors through cDC1-lymphocyte interactions and induction of antitumor immunity
Authors: Stellas Dimitris , Karaliota Sevasti , Stravokefalou Vasiliki , Nagy Bethany A. , Felber Barbara K. , Pavlakis George N. 
Published: August 2019

Abstract
Introduction: IL-15 is a cytokine important for the maintenance, proliferation and activation of lymphocytes, including CD8+ T and natural killer (NK) cells. The native form of IL-15 is a heterodimer either embedded in the plasma membrane or released in a soluble form. Several preclinical cancer models have supported the anti-tumor activity of heterodimeric IL-15, which is presently in clinical trials. We have produced heterodimeric IL-15 (hetIL-15) and also fusions to the Fc fragments (hetIL-15FC) of human, macaque or mouse antibodies. We assessed the effects of hetIL-15 monotherapy after systemic and locoregional administration in the proximity of EO771 orthotopically implanted breast tumors.
Experimental procedures: We evaluated the therapeutic efficacy of hetIL-15 and hetIL-15FC immunotherapy in the murine EO771 triple negative breast cancer orthotopic model using both syngeneic C57BL/6 mice and RAG-1 immunodeficient mice. We monitored the effects of hetIL-15 treatment on Tumor Infiltrating Lymphocytes (TILs), lymphoid organs and blood by Flow cytometry and Immunohistochemisty (IHC). Using Seahorse technology, we also evaluated the metabolic profile of the CD8+ T cells, reflecting their activation status.
Results: Monotherapy with hetIL-15FC peritumorally in the area of mammary pad resulted in complete tumor regression in half of the treated mice. hetIL-15 treatment resulted in an increased survival accompanied with decreased or completely eradicated metastatic disease, in C57BL/6 mice. Significant tumor delay was also observed after peritoneal injection of hetIL-15 in C57BL/6 as well as in RAG-1 mice with hetIL-15FC, but complete tumor regression was not achieved. hetIL-15FC peritumoral administration reshaped the tumor microenvironment and caused an increased infiltration of NK cells and CD8+ T cells with effector phenotype. Both cytotoxic T cells and NK were activated and proliferating, as shown by Flow, including Granzyme B and Ki67 staining. hetIL-15FC treatment resulted in increased tumor infiltration of both CD8+ and CD4+ T memory cells and provided strong protection to these mice against subsequent re-challenges with EO771 tumor.
Furthermore, peritumoral hetIL-15FC administration resulted in an increase of intratumoral conventional Dendritic cells (cDC1), suggesting a mechanism for the intratumoral attraction of lymphocytes.
Conclusion: Monotherapy with hetIL-15FC locoregional administration resulted in complete regression of EO771 primary breast cancer tumors, activating both cytotoxic arms of the immune system, and providing persisting protective immunity. Mechanism of function of hetIL-15 is through cDC1 and lymphocyte interactions that lead to the reciprocal attraction of cDC1 and activated cytotoxic lymphocytes into the tumors and the generation of long-term anti-tumor immunity.
Citation Format: Dimitris Stellas, Sevasti Karaliota, Vasiliki Stravokefalou, Bethany A. Nagy, Barbara K. Felber, George N. Pavlakis. heterodimeric IL-15 monotherapy results in complete regression of EO771 murine breast tumors through cDC1-lymphocyte interactions and induction of antitumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3259.