Abstract 523: Modulation of immune suppressive cells by toll-like 7 agonist DSP-0509 which leads to potentiate anti-tumor activity of radiotherapy (2021)

Abstract Toll-like receptors (TLRs) are a family of pattern-recognition receptors that recognize pathogen-associated molecular patterns. TLR7 stimulation in pDCs (plasmacytoid dendritic cell) induces type I interferon, which results in innate immune activation. We previously showed that intravenously injectable DSP-0509 activated anti-tumor immunity in combination with immune checkpoint inhibitors. In this study, we present that DSP-0509 can enhance anti-tumor activity of radiation therapy (RT). RT is generally known to induce not only anti-tumor effector T cells through immunogenic cell death, but also cause immune suppression. We assessed the modulatory activity of DSP-0509 on immune suppressive cells. In CD8+ T cell suppression assay in which CD8+ T cells were cocultured with mMDSC (monocytic myeloid-derived suppressor cell, CD11b+Gr-1low), DSP-0509 restored the proliferation of CD8+ T cell suppressed by mMDSC. In Treg differentiation assay, in which Treg (CD4+CD25+Foxp3+) was differentiated from naïve CD4+ T cell in the presence of TGF-β, DSP-0509 suppressed the Treg differentiation. Treg function was assessed in coculture system, in which proliferation of CTV-labeled (CellTrace™ Violet) CD4+ T cell was suppressed by Treg. DSP-0509 restored the proliferation of CD4+ T cell. As a next step, to confirm a combination effect with RT, DSR-0509 (5 mg/kg, weekly) was administered with RT (5 Gy x 5 consecutive day). The combination administration extended the survival of CT26-bearing mice. The ratio of effector memory T cell (CD8+CD62L-CD127+) in CD8+ T cell was increased after administration of combination of DSP-0509 with RT. ELISpot assay using splenocytes collected from CT26-bearing mice after the administration of combination of DSP-0509 with RT was conducted. As a result, the number of IFNγ secreting cytotoxic T lymphocytes (CTLs) that responded to gp70 peptide was increased in combination of DSP-0509 with RT. When CT26 cells were re-challenged to the mice that completely rejected tumor after combination of DSP-0509 with RT, no tumor growth was observed in all mice. Anti-CD8 antibody treatment prior to re-challenge of CT26 cells cancelled this memory effect. qPCR analysis using mRNA isolated from treated tumor was conducted. RT monotherapy increased the expression of Cd8b1, but addition of DSP-0509 further enhanced the expression of Gzmb and Il12b in tumor. We showed two novel effects of DSP-0509 in this study, 1) modulation of immune suppressive cells and 2) extension of the survival of tumor-bearing mice in combination with RT, which would be explained by the additive CTL activation and memory T cell induction. Taken together, these results supported the hypothesis that combination of DSP-0509 with RT may enhance the anti-tumor immune activity by modulating CTL activity as well as immune suppressive cells. Citation Format: Yosuke Ota, Yasuhiro Nagai, Masashi Goto, Jun Oishi, Setsuko Yamamoto. Modulation of immune suppressive cells by toll-like 7 agonist DSP-0509 which leads to potentiate anti-tumor activity of radiotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 523.