Serum CD73 activity as a biomarker of hypoxemia in COVID-19 patients

Ectonucleotidases regulate extracellular purine signaling and play a key role in inflammation and thrombosis, two major components of COVID-19 pathophysiology. We investigated the enzymatic activities of CD39 (NTPDase1) and CD73 (ecto-5’-nucleotidase) as potential biomarkers of disease severity. Patients with RT-PCR-confirmed COVID-19 were enrolled between August 1st, 2020, and August 1st, 2021, and stratified into mild to moderate (MM, n = 55) or severe to critical (SC, n = 30) groups according to WHO criteria. Serum samples were collected on days 1, 3, and 7 following Emergency Department (ED) admission. CD39 and CD73 activities were measured by HPLC using etheno-fluorescent nucleotide substrates and compared to healthy volunteers (HV, n = 30) matched for age and sex. Serum CD73 activity was significantly elevated in COVID-19 patients compared to HV (MM: 76 ± 9.4; SC: 99 ± 21 vs. HV: 25 ± 2.7 pmol/min/µL; p < 0.0001), particularly in patients requiring ≥ 6 L/min oxygen (p = 0.005) or with a respiratory rate > 20/min (p = 0.02). CD73 activity remained elevated throughout hospitalization. Conversely, febrile patients (≥ 38 °C) and those with CRP ≥ 100 mg/L exhibited significantly lower CD73 activity (p = 0.023 and p = 0.004, respectively). CD39 activity was undetectable in 28.2% of patients and showed no association with disease severity or outcome. The lowest CD73 levels were observed in patients requiring oxygen therapy for ≥ 30 days and in those who died before day 7. Serum CD73 activity is associated with hypoxemia and oxygen requirements upon ED admission. Conversely, low CD73 activity reflects a hyperinflammatory state and worse outcomes, supporting its potential use in biomarker-based risk stratification for COVID-19.