Single-cell landscape of bronchoalveolar immune cells in patients with COVID-19.

Respiratory immune characteristics associated with Coronavirus Disease 2019 (COVID-19) severity are currently unclear. We characterized bronchoalveolar lavage fluid immune cells from patients with varying severity of COVID-19 and from healthy people by using single-cell RNA sequencing. Proinflammatory monocyte-derived macrophages were abundant in the bronchoalveolar lavage fluid from patients with severe COVID-9. Moderate cases were characterized by the presence of highly clonally expanded CD8+ T cells. This atlas of the bronchoalveolar immune microenvironment suggests potential mechanisms underlying pathogenesis and recovery in COVID-19. Single-cell transcriptome and T cell receptor analysis of bronchoalveolar lavage fluid suggests enrichment of proinflammatory macrophages in patients with severe COVID-19 and the presence of clonally expanded CD8+ T cells in patients with moderate COVID-19.