Fgl2: link between hepatitis B and SARS?

A new clotting factor protein, Fgl2/fibroleukin prothrombinase, has been found to be important in viral infection, including hepatitis B [1], by researchers at the University of Toronto, Canada (http://www.utoronto.ca/).

A greater understanding

Chronic hepatitis B virus affects ∼300 million people worldwide 2., 3. and a greater understanding of the pathogenesis of viral-induced hepatocellular injury is required. The Fgl2/fibroleukin protein is also triggered by corona virus in mice, which suggests a possible link to the human corona virus that causes SARS.

Almost 80,000 people become infected with hepatitis B virus each year in the USA, despite the fact that it is a vaccine-preventable disease [4]. There are >1 million chronically infected individuals within the USA and these patients are at risk of developing chronic liver disease, cirrhosis and hepatocellular carcinoma. Chronic hepatitis B virus can often lead to severe complications and death after decades of infection.

Fgl2/fibroleukin

Fgl2/fibroleukin is an immune coagulant, which can directly cleave prothrombin into thrombin and has the characteristics of a serine protease. Expression is markedly upregulated by interferon-γ (IFN-γ), which is also important in viral-induced liver disease in humans and model systems. It was initially cloned from CD8+ cytotoxic T cells and shares homology of its carboxyl terminus with fibrinogen β and γ chains.

The viral pathogens that cause liver disease are not all directly cytopathic for the hepatocyte. For example, an immune response to the virus causes the hepatocellular injury that is associated with hepatitis B virus, rather than direct hepatocellular necrosis induced by the virus. Fibrin deposition and thrombosis within the microvasculature of the liver is also important in the immune response to viral infection of the liver. Importantly, the pathways by which vascular thromboses and fibrin generation occurs in viral hepatitis might be mechanistically distinct from the classical pathways of coagulation, which are induced by bacterial lipopolysaccharide or mechanical trauma.

Liver disease

The link between Fgl2/fibroleukin and liver disease was identified when a protein with unique clotting ability was isolated from the livers of mice infected with corona virus. That protein was Fgl2/fibroleukin, which, once produced, cleaves prothrombin to thrombin, resulting in a fibrin clot at the site of acute viral infection.

Therefore, this new research has generated Fgl2/fibroleukin-deficient mice to further study and define the role of this protein in the initiation and localization of fibrin deposition in viral hepatitis [1]. The effect of Fgl2/fibroleukin genotype on in vitro and in vivo responses to murine hepatitis virus type-3 (MHV3) was then assessed, together with Fgl2/fibroleukin expression in patients.

A model of MHV3 – a member of the Coronaviridae group of positive-stranded enveloped RNA viruses – was employed and infected mice developed markedly reduced liver necrosis and showed increased survival. Fibrin deposition was also reduced (Fig. 1 ). Thus, hepatocellular injury induced by MHV3 is dependent on the Fgl2/fibroleukin prothrombinase.

Fig. 1

Expression of fibrin in the liver of coronavirus-infected mice. As indicated by the brown staining, fibrin expression is minimal in Fgl2/fibroleukin-deficient mice (a) compared to wild-type susceptible mice (b). Fibrin expression correlates with hepatocellular injury. Figure courtesy of Philip Marsden, University of Toronto (http://www.utoronto.ca/).

The study also shows that Fgl2/fibroleukin mRNA expression and protein production varies markedly in patients with chronic viral hepatitis B versus those with minimal chronic viral hepatitis B, being highly correlated with fibrin expression. Philip Marsden, lead author of the study and Professor of Medicine at the University of Toronto, says, ‘Fibrin deposition in tissue is very important and Fgl2/fibroleukin is only now recognised as being important in liver disease’.

The promise of drug therapy?

Marsden believes this work paves the way for future therapies and provides a new approach to combating viral disease. ‘I am excited by this work and believe it will make a real difference to hepatitis patients’, says Marsden. The pharmacological blockade of Fgl2/fibroleukin could offer an important new treatment approach in hepatitis B virus-induced disease. Marsden comments, ‘Antibodies are now being generated to neutralize Fgl2 activity, which we hope will be useful in treating patients with viral hepatitis’.

Thomas Lane, Associate Professor, Department of Molecular Biology & Biochemistry, University of California (http://www.ucla.edu/), commented: ‘These results are novel and exciting in that they use a genetic approach to clearly show that Fgl2/fibroleukin expression is important in contributing to the pathology of experimental viral-induced hepatitis. While the evidence is compelling, additional work is necessary to determine if this is a safe and effective target. However, these observations lend significant support to arguments that targeting Fgl2/fibroleukin offers a novel method for intervention and treatment for patients with viral hepatitis.’

Applicability to SARS?

With the recent worldwide outbreak of SARS, the relevance of this work in chronic hepatitis B virus infection is timely. However, as Marsden is keen to stress, there is as yet no direct evidence to link Fgl2/fibroleukin and SARS. ‘We are now examining patients with SARS for Fgl2 activity and the possible use of neutralizing antibodies for their treatment,’ he says. As a logical target for molecular manipulation, the clotting protein offers hope for the development of newer treatment protocols for hepatitis patients. This research opens the way for initial thoughts of a hepatitis treatment by blocking a clotting protein and, if such inhibitors are generated that work in hepatitis patients, perhaps the same inhibitors have the potential to be used in the treatment of SARS.

‘[The researchers] found that the MHV3 coronavirus nucleocapsid protein is critical in the transcriptional activation of the gene,’ said K.Y. Yuen, a Professor in the Department of Microbiology, Queen Mary Hospital, University of Hong Kong (http://www.hku.hk/). ‘At this stage we still have no idea whether the SARS coronavirus infection in human can induce Fgl2/fibroleukin prothrombinase expression. However, postmortem examination in some HK patients dying from SARS revealed the presence of vascular thrombosis in the pulmonary vessels [5]. Taken together, these findings warrant further investigation of the role of Fgl2/fibroleukin prothrombinase in the pathogenesis of SARS.’