Abstract
The adenosinergic pathway converting endogenous ATP to adenosine (ADO) is a major immunosuppressive pathway in cancer. Emerging data indicate that plasma small extracellular vesicles (sEV) express CD39 and CD73 and produce ADO. Using a noninvasive, highly sensitive newly developed assay, metabolism of N(6)-etheno-labeled eATP, eADP or eAMP by ecto-nucleotidases on the external surface of sEV was measured using high pressure liquid chromatography with fluorescence detection. Ecto-nucleotidase activity in sEV isolated from plasma of randomly selected cancer patients and healthy donors (HDs) was compared. Relative to sEV of HDs, sEV from the plasma of melanoma patients metabolized eATP to eADP and eAMP to eADO with significantly greater efficiency. Activities of both CD39 and CD73 were elevated, as determined by the use of pharmacologic inhibitors selective for these enzymes. In contrast, metabolic activity of CD39 and CD73 on sEV isolated from plasma of patients with head and neck cancer was comparable to that of HDs, suggesting that the activity of ecto-nucleotidases on sEV may differ depending on the cancer type or cancer stage. The N(6)-etheno-purine assay measuring contributions of ecto-nucleotidases residing on the surface of sEV to the extracellular ATP to ADO pathway can discriminate cancer patients from HDs, differentiate among different cancer types, and potentially identify patients most likely to benefit from anti-adenosinergic therapy designed to inhibit the adenosine-mediated immune suppression.