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AbstractAbstract Amphiregulin was expressed in alveolar macrophages after acute lung injury. Exogenous amphiregulin protected, whereas amphiregulin antibodies exacerbated lung injury. The results are consistent with the notion up-regulation of amphiregulin in activated alveolar macrophages can exert a protective effect on the lung tissue in a model of acute lung injury. Supplemental Digital Content is available in the text. Background Alveolar macrophages (AMs) activated into M1 phenotype are involved in the development of lipopolysaccharide-induced acute lung injury (ALI). However, whether AMs express amphiregulin and what roles amphiregulin plays in lipopolysaccharide-induced ALI remain poorly understood. Methods Acute lung injury was induced by intratracheal instillation of lipopolysaccharide in male C57BL/6 mice. Lung injury scores, level of protein, and level of neutrophils in bronchial alveolar lavage fluid of lipopolysaccharide-induced ALI mice were compared with those in mice challenged with recombinant exogenous amphiregulin and antiamphiregulin antibody. Amphiregulin expression in macrophages and neutrophils in bronchial alveolar lavage fluid of lipopolysaccharide-induced ALI mice was determined by using immunofluorescence technique and further detected in M0, M1, and M2 phenotypes of both peritoneal macrophages and AMs. The effect of amphiregulin on apoptosis of MLE12 cells and activation of epithelial growth factor receptor-AKT pathway were, respectively, examined by using flow cytometry and western blotting. Results Alveolar macrophages were found to highly express amphiregulin in ALI mice. Amphiregulin neutralization aggravated, whereas recombinant exogenous amphiregulin attenuated lipopolysaccharide-induced ALI in mice (n = 6). In cultured AMs and peritoneal macrophages, amphiregulin was mainly generated by M1, rather than M0 or M2 phenotype (n = 5). Apoptosis ratio of lipopolysaccharide-challenged MLE12 cells was significantly reduced by recombinant exogenous amphiregulin from 16.60 ± 1.82 to 9.47 ± 1.67% (n = 5) but significantly increased from 17.45 ± 1.13 to 21.67 ± 1.10% (n = 5) after stimulation with supernatant of M1-polarized AM media conditioned with amphiregulin-neutrolizing antibody. Western blotting revealed that amphiregulin activated epithelial growth factor receptor and AKT in the lung tissues and MLE12 cells (n = 5). Conclusions Different from the common notion that classically activated AMs have just a detrimental effect on the lung tissues, the results of this study showed that classically activated AMs also exerted a protective effect on the lung tissues by producing high-level amphiregulin in lipopolysaccharide-induced ALI.