Case study observational research: inflammatory cytokines in the bronchial epithelial lining fluid of COVID-19 patients with acute hypoxemic respiratory failure

In this study, the concentrations of inflammatory cytokines were measured in the bronchial epithelial lining fluid (ELF) and plasma in patients with acute hypoxemic respiratory failure (AHRF) secondary to severe coronavirus disease 2019 (COVID-19). We comprehensively analyzed the concentrations of 25 cytokines in the ELF and plasma of 27 COVID-19 AHRF patients. ELF was collected using the bronchial microsampling method through an endotracheal tube just after patients were intubated for mechanical ventilation. Compared with those in healthy volunteers, the concentrations of interleukin (IL)-6 (median 27.6 pmol/L), IL-8 (1045.1 pmol/L), IL-17A (0.8 pmol/L), IL-25 (1.5 pmol/L), and IL-31 (42.3 pmol/L) were significantly greater in the ELF of COVID-19 patients than in that of volunteers. The concentrations of MCP-1 and MIP-1β were significantly greater in the plasma of COVID-19 patients than in that of volunteers. The ELF/plasma ratio of IL-8 was the highest among the 25 cytokines, with a median of 737, and the ELF/plasma ratio of IL-6 (median: 218), IL-1β (202), IL-31 (169), MCP-1 (81), MIP-1β (55), and TNF-α (47) were lower. The ELF concentrations of IL-6, IL-8, IL-17A, IL-25, and IL-31 were significantly increased in COVID-19 patients. Although high levels of MIP-1 and MIP-1β were also detected in the blood samples collected simultaneously with the ELF samples, the results indicated that lung inflammation was highly compartmentalized. Our study demonstrated that a comprehensive analysis of cytokines in the ELF is a feasible approach for understanding lung inflammation and systemic interactions in patients with severe pneumonia.