Abstract
Excessive neutrophil infiltration to the lungs is a hallmark of acute lung injury (ALI). Milk fat globule epidermal growth factor-factor 8 (MFG-E8) was originally identified for phagocytosis of apoptotic cells. Subsequent studies revealed its diverse cellular functions. However, whether MFG-E8 can regulate neutrophil function to alleviate inflammation is unknown. We therefore aimed to reveal MFG-E8 roles in regulating lung neutrophil infiltration during ALI. To induce ALI, C57BL/6J wild-type (WT) and Mfge8(-/-) mice were intratracheally injected with LPS (5 mg/kg). Lung tissue damage was assessed by histology, and the neutrophils were counted by a hemacytometer. Apoptotic cells in lungs were determined by TUNEL, whereas caspase-3 and myeloperoxidase activities were assessed spectrophotometrically. CXCR2 and G protein-coupled receptor kinase 2 expressions in neutrophils were measured by flow cytometry. Following LPS challenge, Mfge8(-/-) mice exhibited extensive lung damage due to exaggerated infiltration of neutrophils and production of TNF-α, MIP-2, and myeloperoxidase. An increased number of apoptotic cells was trapped into the lungs of Mfge8(-/-) mice compared with WT mice, which may be due to insufficient phagocytosis of apoptotic cells or increased occurrence of apoptosis through the activation of caspase-3. In vitro studies using MIP-2-mediated chemotaxis revealed higher migration of neutrophils of Mfge8(-/-) mice than those of WT mice via increased surface exposures to CXCR2. Administration of recombinant murine MFG-E8 reduces neutrophil migration through upregulation of GRK2 and downregulation of surface CXCR2 expression. Conversely, these effects could be blocked by anti-α(v) integrin Abs. These studies clearly indicate the importance of MFG-E8 in ameliorating neutrophil infiltration and suggest MFG-E8 as a novel therapeutic potential for ALI.