AIM: To investigate the effect of IL-4 on the altered expression of complement activation regulators in pancreas and pancreatic necrosis during experimental severe acute pancreatitis (SAP).METHODS: SAP model of rats was established by retrograde injection of 5% sodium taurocholate (1 mL/kg)into the pancreatic duct. We immunohistochemically assayed the expression of three complement activation regulators: decay accelerating factor (DAF; CD55), 20ku homologous restriction factor (HRF20; CD59) and membrane cofactor protein (MCP; CD46), in the pancreatic acinar cells of rats at 0, 3, 6, 12, and 24 h after the induction of SAP model. Meanwhile the levels of amylase and lipase were determined, and morphological examination was performed. Then, 61 rats were randomly divided into three groups. Group A (n = 21) received notreatment after the SAP model was established; group B (n = 20) was given IL-4 (8 μg/animal) intraperitoneally 0.5 h before the SAP model was established; group C (n = 20) was given IL-4 (8 μg/animal) intraperitoneally 0.5 h after the SAP model was established. Plasma amylase and lipase, extent of pancreatic necrosis and expression of complement activation regulators were investigated 6 h after the induction of SAP model.RESULTS: Three complement activation regulators were all expressed in pancreatic acinar cells. MCP was not found on the basolateral surface as reported. Contrary to the gradually increasing plasma level of amylase and lipase, expression of complement activation regulators decreased after SAP model was set up. At the same time, the severity of pancreatic necrosis was enhanced.A strong negative correlation was found between the expression of MCP, DAF, CD59 in pancreatic acinar cells and the severity of pancreatic necrosis (r = -0.748, -0.827,-0.723; P＜0.01). In the second series of experiments,no matter when the treatment of IL-4 was given (before or after the induction of SAP model), the serum level of amylase or lipase Was decreased and the extent of pancreatic necrosis was ameliorated significantly. Compared to SAP control group, the expression of DAF and CD59in pancreas was reinforced when IL-4 was given before the induction of SAP model (P＜0.01, P＜0.05), but the expression of MCP was not influenced (P＞0.05). The expression of DAF was enhanced, when IL-4 was given after the induction of SAP model (P＜0.05), but the expression of CD59 and MCP did not change (P＞0.05).CONCLUSION: Complement activation regulators may participate in the pathogenesis of pancreatic inflammation. Downregulation of complement activation regulators expression may be one of the causes of pancreatic necrosis. IL-4 treatment may control SAP aggravation by enhancing expression of DAF and CD59 in pancreas and decreasingpancreaticnecrosis. Moreover, DAF and CD59may play an important role in the regulation of complement activation regulators during SAP.