Clinical Significance of PD-L1+ Exosomes in Plasma of Head and Neck Cancer Patients

Purpose: The microenvironment of head and neck squamous cell carcinomas (HNSCC) is highly immunosuppressive. HNSCCs expressing elevated levels of PD-L1 have especially poor outcome. Exosomes that carry PD-L1 and suppress T-cell functions have been isolated from plasma of patients with HNSCC. The potential contributions of PD-L1(+) exosomes to immune suppression and disease activity are evaluated. Experimental Design: Exosomes isolated from plasma of 40 HNSCC patients by size exclusion chromatography were captured on beads using anti-CD63 Abs, stained for PD-1 and PD-L1 and analyzed by flow cytometry. The percentages and mean fluorescence intensities (MFI) of PD-L1(+) and PD-1(+) exosome/bead complexes were correlated with the patients' clinicopathologic data. PD-L1(high) or PD-L1(low) exosomes were incubated with activated CD69(+) human CD8(+) T cells +/- PD-1 inhibitor. Changes in CD69 expression levels on T cells were measured. Patients' plasma was tested for soluble PD-L1 (sPD-L1) by ELISA. Results: Levels of PD-L1 carried by exosomes correlated with patients' disease activity, the UICC stage and the lymph node status (P = 0.0008-0.013). In contrast, plasma levels of sPD-L1 or exosome PD-1 levels did not correlate with any clinicopathologic parameters. CD69 expression levels were inhibited (P < 0.03) by coincubation with PD-L1(high) but not by PD-L1(low) exosomes. Blocking of PD-L1(+) exosome signaling to PD-1(+) T cells attenuated immune suppression. Conclusions: PD-L1 levels on exosomes, but not levels of sPD-L1, associated with disease progression in HNSCC patients. Circulating PD-L1(+) exosomes emerge as useful metrics of disease and immune activity in HNSCC patients. Significance: Circulating PD-L1(high) exosomes in HNC patients' plasma but not soluble PD-L1 levels associate with disease progression. (C) 2017 AACR.