P-02.07 The combination of a STING agonist and IL15 efficiently reverses the immunosuppressive tumor microenvironment in a complex primary human organoid model

<h3>Background</h3> STING (stimulator of interferon genes)-pathway agonists have emerged to induce T cell and NK cell-mediated antitumor effects in preclinical mice studies and are now being evaluated in clinical trials. Harnessing their immediate and powerful cytotoxic tumor activity independent of neoantigens, together with an enhanced migration of NK cells into solid tumors would be a promising strategy to combat colorectal cancer. Thus, we hypothesize that a combination of a STING agonist and IL-15 can remodel the immunosuppressive tumor microenvironment (TME) and additively supports NK cell-mediated tumor killing. <h3>Materials and Methods</h3> To recreate features of the actual TME we established a complex primary human air-liquid-interface co-culture consisting of colorectal cancer tumor organoids, matched primary cancer-associated fibroblasts (CAFs) and innate immune cells (NK cells and monocytes). Upon treatment with IL-15, a STING agonist (ADU-S100) or a combination of both for 48 hours immune cell responses and tumor cell death were analyzed by flow cytometry. Further, changes in secreted immunomodulatory cytokines were measured by multiplexed immunoassays. <h3>Results</h3> Monotherapy of a STING agonist resulted in repolarization of immunosuppressive CAF-induced monocytes towards a pro-inflammatory phenotype and in an enhanced activation and degranulation of NK cells followed by a significantly increased tumor cell killing. Combination treatment with IL-15 further supported NK cell activation and degranulation. However, IL-15 monotherapy failed to induce tumor cell death, whereas combination therapy exerted a synergistic antitumor effect. Mechanistically, NK cell activation in response to STING pathway stimulation by ADU-S100 was dependent on the presence of monocytes. Monocyte-derived TNFα released upon STING agonist treatment was shown to be involved in NK cell activation and tumor cell death induction. Further, confocal imaging revealed drastically enhanced infiltration of NK cells into organoids 24 hours after STING agonist treatment and combination with IL-15. Tumor cell killing in response to STING pathway stimulation and combination treatment was partially dependent on cell-cell contact as spatial separation of NK cells resulted in a significantly reduced degranulation and tumor cell death. <h3>Conclusions</h3> Combination treatment of a STING agonist and IL-15 modulates the microenvironment into a pro-inflammatory state and synergistically supports immune cell-mediated tumor cell killing in a TME mimicking 3D model of tumor organoids, CAFs and innate immune cells. <b>D. Ammon:</b> None. <b>M. Fabits:</b> None. <b>J. Kabiljo:</b> None. <b>A. Theophil:</b> None. <b>J. Homola:</b> None. <b>B. Wolf:</b> None. <b>A. Kulu:</b> None. <b>H. Dolznig:</b> B. Research Grant (principal investigator, collaborator or consultant and pending grants as well as grants already received); Significant; Boehringer Ingelheim. <b>M. Bergmann:</b> B. Research Grant (principal investigator, collaborator or consultant and pending grants as well as grants already received); Significant; Boehringer Ingelheim.