AIM: To evaluate the expression of fibrinogenlike protein 2 (fgl2) and its correlation with disease progression in both mice and patients with severe viral hepatitis.METHODS: Balb/cJ or A/J mice were infected intraperitoneally (ip) with 100 PFU of murine hepatitis virus type 3 (MHV-3), liver and serum were harvested at 24, 48, and 72 h post infection for further use. Liver tissues were obtained from 23 patients with severe acute chronic (AOC) hepatitis B and 13 patients with mild chronic hepatitis B. Fourteen patients with mild chronic hepatitis B with cirrhosis and 4 liver donors served as normal controls. In addition, peripheral blood mononuclear cells (PBMC) were isolated from 30 patients (unpaired) with severe AOC hepatitis B and 10 healthy volunteers as controls. Procoagulant activity representing functional prothrombinaseactivity in PBMC and white blood cells was also assayed. A polyclonal antibody against fgl2 was used to detect the expression of both mouse and human fgl2 protein in liver samples as well as in PBMC by immunohistochemistry staining in a separate set of studies. Alanine aminotransferase (ALT) and total bilirubin (TBil) in serum were measured to assess the severity of liver injury.RESULTS: Histological changes were found in liver sections 12-24 h post MHV-3 infection in Balb/cJ mice.In association with changes in liver histology, marked elevations in serum ALT and TBil were observed. Mouse fgl2 (mfgl2) protein was detected in the endothelium of intrahepatic veins and hepatic sinusoids within the liver 24 h after MHV-3 infection. Liver tissues from the patients with severe AOC hepatitis B had classical pathological features of acute necroinflammation. Human fgl2 (hfgl2)was detected in 21 of 23 patients (91.30%)with severe AOC hepatitis B, while only 1 of 13 patients(7.69%) with mild chronic hepatitis B and cirrhosis had hfgl2 mRNA or protein expression. Twenty-eight of thirty patients (93.33%) with severe AOC hepatitis B and 1of 10 with mild chronic hepatitis B had detectable hfgl2expression in PBMC. No hfgl2 expression was found either in the liver tissue or in the PBMC from normal donors. There was a positive correlation between hfgl2expression and the severity of the liver disease as indicated by the levels of TBil. PCA significantly increased in PBMC in patients with severe AOC hepatitis B.CONCLUSION: The molecular and cellular results reported here in both mice and patients with severe viral hepatitis suggest that virus-induced hfgl2prothrombinase/fibroleukin expression and the coagulation activity associated with the encoded fgl2protein play a pivotal role in initiating severe hepatitis.The measurement of hfgl2/fibroleukin expression in PBMC may serve as a useful marker to monitor the severity of AOC hepatitis B and a target for therapeutic intervention.