Introduction & Objectives: Prostate cancer (PC) is the second most commonly diagnosed cancer in men and the eighth leading cause of cancer-related mortality in the world. The only successful immunotherapy for the disease (ProvengeTM) only extends life by a few months. Therefore the finding of immunotherapeutic agents for the treatment of PC is of major interest. It has been previously shown by our group that Interleukin-15 (IL-15), unlike other therapeutic cytokines such as IL-2 and IL-12, can stimulate expansion and activity of CD8 and NK cells in vitro when they are exposed to prostate cancer cells, while studies in mice have shown a reduction in tumor size by 50% with no apparent toxicity. In this study, we aim to examine potencies of IL-15 in combination with a cyclic dinucleotide (CDN) that activates the Stimulator of Interferon-Gene (STING) receptor. CDNs have previously been shown to activate both T cells and Dendritic cells through STING. We hypothesize that combination of CDNs and IL-15 can additively increase NK and T cell activity as they act to increase type 1 interferons through STING activation and IFN gamma through IL-15.
Material & Methods: Peripheral blood mononuclear cells (PBMCs) were isolated and co-cultured with cancer cells (LNCaP, ATCC® CRL-1740™) in the presence of either IL-15, the CDN 2’3’-c-di-AM(PS)2, or a mixture of IL-15 with CDN. Control experiments were also prepared in which IL-15 was replaced by PBS and the CDN by a linear dinucleotide. Co-cultures were incubated for 48 hours at 37°C and 5% CO2. After this period, all cells from each well were collected and stained with fluorophore conjugated anti-CD3, -CD8 and -CD56 to identify CD8 T cells and NK cells, and anti-NKG2D and -perforin to analyze activation of NK and T lymphocytes. Tumor cell death was also evaluated and quantified using Live/Dead staining. All samples were acquired on FACS Canto. Results We observed that 1µg/ml of 2’3’-c-di-AM(PS)2 in combination with 2.5 ng/ml of IL-15 caused 81% of LNCaP killing in prostate cancer-lymphocyte co-cultures, while IL-15 by itself led to approximately 45% of cancer cell death. Little or no cancer cell killing was seen without IL-15 or CDN. Interestingly, the mixture of IL-15 with the CDN did not expand NK cells more than IL-15 alone. This suggests that the activity of NK cells is increased despite lack of their expansion. Preliminary data suggests that the level of perforin is increased by up to 35% in NK cells cultured with the mixture of both therapeutic drugs when compared to IL-15 alone in the cocultures. Of note, cell culture of either LNCaP or PMBCs with 1µg/ml of CDN did not affect cell growth.
Conclusion: The combination of two immunotherapeutic drugs, IL-15 and 2’3’-c-di-AM(PS)2, lead to an increase in the activity of NK cells, with an apparent increase in the levels of perforin and doubling of cell death mediated by immune effector cells compared with IL-15 alone.
Citation Format: Ana M. Esteves, Efthymia Papaevangelou, Prokar Dasgupta, Richard A.G. Smith, Christine Galustian. Combination of Interleukin-15 with a STING (Stimulator of Interferon Gene) agonist: A potential immunotherapy for prostate cancer [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 4070.