Targeting Pros1 to improve chemotherapy efficacy and immune anti-tumor response

Introduction/Rationale: Resistance to chemotherapy is a major challenge in the treatment of advanced melanoma. Targeting the innate immune response to improve chemotherapy efficacy is a promising yet understudied area. We identified Pros1 as an innate immune checkpoint utilized by tumors to suppress the macrophage pro-inflammatory response. Secreted Pros1 prevents macrophage response to tumor damage-associated molecular patterns (DAMPs) released during chemotherapy. However, the effect of Pros1 on other immune system cells and the molecular mechanism that triggers its expression remain unclear.

Methods: To determine how Pros1 affects chemotherapy efficacy and immune activation, we generated a melanoma cell line (B16F10) lacking Pros1 using CRISPR-Cas9 and tested it in a syngeneic murine preclinical model. Immune infiltration was assessed by single-cell RNA-seq in end-stage tumors.

Results: Mice bearing Pros1-deficient tumors showed a 63±10% (mean ± SEM) decrease in tumor growth after cisplatin treatment and a 5-fold increase in immune infiltration when compared to wild type. B cells were the most enriched group in Pros1-deficient tumors, with increased communication with T cells. Using bulk RNA-seq, we determined that Ifnγ stimulates Pros1 expression in melanoma cells as well as other immune suppressors, including Ido1, Pd-l1 and Tgfb1. Ifnγ-dependent Pros1 expression was conserved among several cancer cell types, including melanoma, lung, breast, and pancreatic cancer, suggesting its role as a global immune suppressor. By using siRNA, we determined that Ifngr1/Ifngr2 and Stat1 are key modulators of Ifnγ-dependent Pros1 expression, but not Stat3.

Conclusion: We identified a novel mechanism through which cancer cells promote Pros1 expression in response to chemotherapy. A better understanding of the mechanisms that regulate Pros1 expression may allow us to increase the efficacy of conventional therapies in melanoma patients.