Bronchoalveolar lavage of healthy human adults
Rationale: Airspace macrophages are the most abundant cell in airspaces and are viewed as a homogeneous population during health. Single cell RNA sequencing allows for examination of transcriptional heterogeneity between cells and between individuals. Understanding the conserved repertoire of airspace leukocytes during health is essential to understanding cellular programing during disease.  Objective: We sought to determine the transcriptional heterogeneity of human bronchoalveolar lavage cells in healthy adults. Methods: Ten healthy subjects underwent bronchoscopy. Cells obtained from lavage fluid were subjected to single cell RNA sequencing. Unique cell populations and putative functions were identified. Transcriptional profiles were compared across individuals. Measurements and Main Results:  Based on transcriptional profiling we identify highly conserved macrophage, monocyte-like, lymphocyte, dendritic cell, and cycling cell populations. We define two unique subgroups of resident airspace macrophages - one defined by a pro-inflammatory profile and one by metallothionein gene expression. We identify distinct subsets of monocyte-like cells and directly compared them to peripheral blood mononuclear cells. Finally, we compare global macrophage and monocyte programing between male and female subjects. Conclusions: Healthy human airspaces contain multiple populations of leukocytes that are highly conserved between individuals and between the sexes. Resident macrophages comprise the largest population and include novel subsets defined by inflammatory and metal-binding gene signatures. Monocyte-like cells within the airspaces are transcriptionally distinct from circulating blood cells and include a rare population defined by expression of cell-matrix interaction genes. This study is the first to define airspace immune cell heterogeneity and identifies three previously unrecognized myeloid cell subsets. Overall design: Fifteen  bronchoalveolar lavage samples from ten healthy adults were included.