Plasma‑derived CD16 exosomes and peripheral blood monocytes as correlating biomarkers in head and neck cancer

Exosomes play an important role in the individual immune regulation in patients with head and neck squamous cell carcinoma (HNSCC) as part of the tumor microenvironment. Patients with HNSCC with advanced tumor stages reveal significantly increased levels of plasma derived CD16⁺ (FcRIIIA) total exosomes as demonstrated in our previous study. Furthermore, increased individual abundances of peripheral blood CD16⁺ non-classical monocytes have been shown to correlate with increased monocytic programmed death ligand 1 (PD-L1) and CD4⁺ T cell disturbances in oropharyngeal cancer. However, the context of plasma derived CD16⁺ exosomes in patients with HNSCC and their role in the immune-regulation of circulating monocyte subsets has not been investigated so far. In the present study, exosomes were isolated from plasma samples of healthy donors and patients with HNSCC and evaluated for their morphology, size and protein composition using transmission electron microscopy, western blotting and bead-based flow cytometry. Monocyte subset abundances were analyzed in whole blood measurements in terms of the CD14/CD16 cell surface expression patterns, different monocytic adhesion molecules and checkpoint molecule PD-L1 using flow cytometry. Isolated exosomes were positive for the tetraspanins CD63 and CD9 as well as the endosomal marker TSG101, but negative for the non-exosomal marker glucose-regulated protein 94 and apolipoprotein ApoA1. Plasma derived CD16⁺ exosomes and exosome size distribution were significantly correlated with abundances of CD16⁺ non-classical monocytes and CD16⁺ intermediate monocytes, respectively. Furthermore, the data revealed significant correlations between CD16⁺ plasma derived exosomes and adhesion molecules CD29 (integrin β1) and CX3CR1 on certain monocyte subsets. These data suggested that CD16 positive exosomes and exosome size distribution were potential surrogates to characterize the composition of monocyte subsets in patients with HNSCC. Overall, both CD16-positive exosomes and CD16-positive monocyte subsets are potential liquid biomarkers that could be used to characterize the individual immunological situation of patients with HNSCC.