Abstract IA012: Intratumoral activation of double-stranded RNA sensors induces systemic immune response to irradiated sarcoma
Authors: Kalbasi Anusha 
Published: September 2022

Abstract
Background: Undifferentiated pleomorphic sarcoma (UPS) is a common aggressive sarcoma subtype for which existing therapies are ineffective. Immune checkpoint blockade (ICB) is effective in a minority of patients. Most UPS tumors have a predominantly myeloid infiltrate, a highly plastic cellular population with the potential to serve as a substrate for immunotherapy. Radiotherapy (RT), a standard therapy for UPS, can also drive myeloid cells into the tumor. Based on abundant expression of pattern recognition receptors (PRRs) by myeloid cells, we hypothesized that intratumoral injection of a PRR agonist would leverage pre-existing and RT-associated myeloid cells by redirecting them to prime adaptive anti-tumor immunity. Methods: We used BO-112, a nanoplexed double-stranded ribonucleic acid that activates PRRs, most of which are not typically activated by RT. We used an ICB-resistant mouse model, in which tumor cells derived from a KrasG12D/−P53−/− model of UPS are used to establish large tumors in flanks of C57BL/6J mice. Tumor-bearing mice were treated with RT (8 Gy x 3 fractions daily) ± BO-112 (30 ug/dose). Tumor volume and survival between groups were compared by ANOVA and log-rank test, respectively. To test the role of lymphocytes, the study was repeated in RAG−/−gc−/− mice. Kinetic analysis of myeloid and T cells was performed in the tumors and lymph nodes (LN) on days 15, 21, 26 using high-dimensional flow cytometry. To evaluate the fate of monocytes, LysM-eGFP monocytes were intratumorally transferred following treatment, with subsequent analysis of tumors and LNs on days 16, 18, 21. Results: We detected significantly improved tumor control starting from day 16 with mean tumor volume of 240 mm3 in the combination group compared to 490 mm3 in BO112, 790 mm3 in RT, and 875mm3 in mock groups (p&lt;0.0001) which translated into a maximal survival advantage in the combination group (p&lt;0.005), which was associated with both local and systemic increase in IFNg+ CD8 T cells. In RAG−/−gc−/− mice, the benefit of combination therapy was completely negated. Despite the lymphocyte-dependent effect, myeloid cells were preferentially targeted by BO-112. In the combination therapy group (compared to mock), we observed an early reduction in Ly6ChiMHCII+ cells (p&lt;0.05) within the tumor followed by a progressive increase of the same population in the draining LN over time (p=0.01). In parallel, we observed trafficking of adoptively transferred intratumoral eGFP+ monocytes from the tumor to dLNs in response to BO-112 and RT. Conclusions: Local-only therapy of BO-112 and RT yields a lymphocyte-dependent anti-tumor immune response in an immunotherapy-resistant model of UPS. Longitudinal analysis of locoregional immune responses and adoptive transfer experiments indicate that combination BO-112 and RT induces tumor-to-dLN myeloid trafficking that may bridge an adaptive response and confer therapeutic effect. An ongoing phase 1 dose escalation study testing the combination of RT, BO-112 and nivolumab is currently accruing.
Citation Format: Anusha Kalbasi. Intratumoral activation of double-stranded RNA sensors induces systemic immune response to irradiated sarcoma [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr IA012.