521 Circulating PD-L1-exosomes to monitor tumor response in melanoma patients

Abstract

In the era of effective molecular targeted treatments and immunotherapies, there is an urgent need to implement the use of circulating biomarkers in the clinic to facilitate personalized therapy and predict treatment response. We conducted a retrospective study to demonstrate the involvement of circulating PD-L1 exosomes in melanoma patients. One hundred melanoma patients were included. Exosomes were isolated by ultracentrifugation. PD-L1 expression was mesured in tumor tissue, in plasma and in melanoma plasma-derived exosomes (ExoPD-L1). First, ExoPD-L1 was assessed in melanoma cell lines. Exosomes may have a role in cancer immunosuppression mediated by T-cells since they were as efficient as cancer cells to inhibit T-cells activation. In melanoma patients, ExoPD-L1 (median 64,26 pg/mL) was significantly higher than free PD-L1 in the plasma which was barely detectable (0,1 pg/mL). Furthermore, ExoPD-L1 was detected in all patients whereas only 67 % were PD-L1 positive in tumor. Although baseline ExoPD-L1 levels were not associated with clinicopathologic characteristics and tumour burden, ExoPD-L1 variations (ΔExoPD-L1) after treatment correlated with tumor response and survival. A ΔExoPD-L1 cut-off of > 100 was defined, yielding a 83% sensitivity, a 70% specificity, a 91% positive predictive value and a 54% negative predictive values for disease progression. The use of this cut-off allowed stratification in two groups of patients statistically different in terms of overall survival and progression free survival. Monitoring of circulating exosomal PD-L1 may be a promising biomarker to predict tumor response and correlate with survival. Further studies are needded to confirm the use of circulating Exo-PD-L1 in melanoma patients.