Abstract 3259: heterodimeric IL-15 monotherapy results in complete regression of EO771 murine breast tumors through cDC1-lymphocyte interactions and induction of antitumor immunity

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 brea