Objective By using mouse orthotopic lung transplant model, we investigated the immune mechanisms of an-ti-CD3 induced lung allograft protection .Our study intends to further dissect the features of lung transplant immunology and to provide a novel therapeutic insight for the clinical application of anti-CD3 mAbs after lung transplantation.Methods Murine orthotopic allogeneic lung transplants were performed in C57BL/6 wild type(WT) mice using major histocompatibility complex (MHC) fully mismatched BALB/c donors.Syngeneic transplants were also performed in WT C57BL/6 mice using C57BL/6 donors.For immunosuppressive therapy, allograft recipients received 50g dose of anti-CD3 by intraperitoneal injection on days 2, 3, 4, 5, 6 and 9 post-operation(n=4).At day 10, histopathologic characteristics and rejection status of the pulmonary grafts were assessed.The severity of acute rejection was graded by the pathological score , and T cell and neutrophil infiltration in the pulmonary grafts was evaluated by immunohistochemical(IHC) staining for CD3 and myeloperoxidase(MPO) respective-ly.Real-time RT-PCR was performed for FoxP3, IL-17A and IFN-γexpression in the pulmonary grafts.The percentage of FoxP3+Treg in total CD4+T lymphocytes from the recipient spleens was analyzed by FACS.Results 10 days after transplan-tation, histopathologic examination demonstrated that there is no apparent acute rejection observed in the pulmonary isografts , whereas allografts from untreated recipients have marked inflammatory cell infiltration and pulmonary parenchyma lesion .IHC staining for CD3 and MPO showed that the allograft-infiltrating cells of perivascular layers are mainly T lymphocytes , and the cells around the small airways are mostly neutrophils .Anti-CD3 treatment significantly alleviated the acute rejection of pulmo-nary allografts, when compared with the untreated group.Real-time RT-PCR showed that the expression levels of IL-17A and IFN-γin allografts were markedly elevated compared to those in isografts, and anti-CD3 increased the expression of FoxP3, and reduced the expression of IL-17A and IFN-γin the pulmonary allografts.FACS analysis of splenocytes showed that the percent-age of Treg in total CD4+T lymphocytes increased significantly in the anti-CD3 treated allograft recipients, as compared with the isograft and untreated allograft recipients.Conclusion Anti-CD3 mAbs may alleviate acute rejection of the pulmonary al-lografts by promoting FoxP3 expression and Treg development.