Characterization of Distinct Macrophage Subpopulations during Nitrogen Mustard–Induced Lung Injury and Fibrosis

Nitrogen mustard (NM) is an alkylating agent known to cause extensive pulmonary injury progressing to fibrosis. This is accompanied by a persistent macrophage inflammatory response. In these studies, we characterized the phenotype of macrophages accumulating in the lung over time following NM exposure. Treatment of rats with NM (0.125 mg/kg, intratracheally) resulted in an increase in CD11b+ macrophages in histologic sections. These cells consisted of inducible nitric oxide synthase+ (iNOS) proinflammatory M1 macrophages, and CD68+, CD163+, CD206+, YM-1+, and arginase-II+antiinflammatory M2 macrophages. Although M1 macrophages were prominent 1–3 days after NM, M2 macrophages were most notable at 28 days. At this time, they were enlarged and vacuolated, consistent with a profibrotic phenotype. Flow cytometric analysis of isolated lung macrophages identified three phenotypically distinct subpopulations: mature CD11b−, CD43−, and CD68+ resident macrophages, which decreased in numbers after NM; and two infilt...