Protein and cell profiling of peripheral blood from patients with metastatic soft tissue sarcoma to predict immunotherapy response

<p dir="ltr">Chemotherapy remains the standard-of-care for patients with locally advanced or metastatic soft tissue sarcoma (STS), but this typically produces short response duration and poor overall prognosis. Immune checkpoint inhibitors (ICIs) have revolutionized the treatment of many cancers, and recent evidence suggests that certain STS subtypes may be sensitive to programmed cell death protein 1 (PD-1) ICIs. Despite this, there currently exists no means of identifying patients with STS that will respond to ICIs. In this project, we explored blood-based biomarkers in 13 patients with metastatic STS that may serve as predictors of ICI response and inform on patterns regulating this response. Peripheral blood mononuclear cell (PBMC) and plasma samples collected before therapy and at 2–9, 10–16, 21–24, and 33–37 weeks ontreatment were analysed using flow cytometry and a multiplex cytokine and chemokine assay, respectively. PBMC samples from a single patient with prolonged stable disease (SD; > 6 months) were also single-cell RNA sequenced before therapy and at 5, 12, and 24 weeks ontreatment to assess transcriptomic changes.</p><p dir="ltr">Of the 13 patients, seven were categorised as responders (RECIST 1.1 complete response (CR), n=1 and SD, n=6) and six patients were considered non-responders (progressive disease (PD), n=6). Flow cytometry analysis of pre-treatment PBMCs identified significantly higher frequencies of circulating lymphocytes, and significantly lower frequencies of TIM-3- expressing memory CD4 T cells, total monocytes, and classical monocytes in responders compared to non-responders. As such, relative frequencies of these immune cells may serve as potential biomarkers of ICI response. Single-cell RNA analysis of pre- and on-treatment PBMCs from a single patient with prolonged SD identified unique T cell and monocyte populations associated with ICI treatment resistance. These findings warrant further investigation in larger cohorts of patients with STS to validate the predictive utility of these biomarkers and uncover mechanisms of ICI treatment response and resistance.</p>