Comprehensive evaluation of bronchoalveolar lavage from patients with severe COVID-19 and correlation with clinical outcomes.

Information on bronchoalveolar lavage (BAL) in patients with COVID-19 is limited, and clinical correlation has not been reported. This study investigated the key features of BAL fluids from COVID-19 patients and assessed their clinical significances. A total of 320 BAL samples from 83 COVID-19 patients and 70 non-COVID-19 patients (27 patients with other respiratory viral infections) were evaluated, including cell count/differential, morphology, flow cytometric immunophenotyping and immunohistochemistry. The findings were correlated with clinical outcomes. Compared to non-COVID-19 patients, BAL from COVID-19 patients was characterized by significant lymphocytosis (p<0.001), in contrast to peripheral blood lymphopenia commonly observed in COVID-19 patients, and the presence of atypical lymphocytes with plasmacytoid/plasmablastic features (p<0.001). Flow cytometry and immunohistochemistry demonstrated that BAL lymphocytes, including plasmacytoid and plasmablastic cells, were composed predominantly of T cells with a mixture of CD4+ and CD8+ cells. Both populations had increased expression of T-cell activation markers, suggesting important roles of helper and cytotoxic T-cells in immune response to SARS-CoV-2 infection in the lung. More importantly, BAL lymphocytosis was significantly associated with longer hospital stay (p<0.05) and longer requirement for mechanical ventilation (p<0.05), whereas the median atypical (activated) lymphocyte count was associated with shorter hospital stay (p<0.05), shorter time on mechanical ventilation (p<0.05) and improved survival. Our results indicate that BAL cellular analysis and morphologic findings provide additional important information for diagnostic and prognostic work-up, and potential new therapeutic strategies for patients with severe COVID-19.