Hepatitis B is a worldwide health problem with effects that rangefrom silent infection to fatal liver diseases. Approximately 10 percentof adults and 90 percent of infants who are infected with hepatitis Bvirus (HBV) become chronic carriers of the disease.

Most chronic carriers have minor or no symptoms, however, lifelongHBV infection is associated with an almost 50 percent chance ofdeath from cirrhosis or cancer of the liver. In contrast to chronicallyinfected individuals, acute hepatitis sufferers usually overcome theirdisease with the elimination of detectable free virus and virus-infected cells. But sometimes these acute cases convert into chronicones.

Some groups of individuals, most prominently health care workersand drug users, have particularly high rates of infection. Somegeographic areas of the world have high levels of hepatitis infection.In Japan, more than 75 percent of the population shows evidence ofpast infection, compared to only 5 to 10 percent in most othercountries. In areas where the virus is endemic, the main route oftransmission is from mother to child.

Suitable experimental model systems for HBV have been hard todevelop. Since HBV has a narrow host range, direct infection ofanimals is not possible. Transgenic mice that have HBV introducedinto their germline cells are immunologically tolerant to HBVantigens since they do not recognize these antigens as being foreign.As a result, they do not show spontaneous hepatitis and are not agood model system. Because of these problems, the molecularmechanisms responsible for liver cell death and viral clearance inHBV infection are not well understood.

In the Feb. 27 Proceedings of the National Academy of Sciences,Hiroshi Takahashi and his colleagues from Harvard Medical Schooland Massachusetts General Hospital report on their development of arat model system for hepatitis B. In an article entitled, "Acutehepatitis in rats expressing human hepatitis B virus transgenes," theseresearchers describe the introduction of a modified HBV genome thatcan replicate into rat liver cells.

Transfection, Not Infection, Does The Trick

These scientists have overcome the previous difficulties of narrowhost range infectivity and immune system tolerance in transgenicanimals. As Hiroshi Takahashi, a molecular immunologist and authorof the article, told BioWorld Today, "The human virus cannot enterthe rat cells because there is no receptor. However, once HBV isinside the cell, it can replicate. By using a cationic lipid complexedwith the viral DNA, we are able to get the HBV inside the rat cellswhen the lipid complex fuses with liver cell membranes. Since we dothis in adult rats, the problem of immune system tolerance seen intransgenic rats is eliminated."

HBV was expressed in the liver of these transfected rats. HBVvirions and hepatitis B e antigen were seen in the blood betweenthree and seven days after transfection. Antibodies specific for the eantigen then appeared. After two to three weeks, liver cell deathbegan. This was indicated by elevated blood levels of glutamic-pyruvic transaminase, an enzyme released by liver cells into thebloodstream as they die. At this time, liver cell death and lymphocyteinfiltration were observed near the portal vein of the liver and HBVvirions had disappeared from the blood. In short, these scientists wereable to mimic the effects seen in acute HBV-induced infection inhumans.

In addition to studying normal rats, T cell-deficient rats were alsotransfected with HBV. No liver cell injury was seen, but there was aprolonged presence of HBV in the blood of these immune systemcompromised rats. Thus, these investigators were able to demonstratethat T cells play an essential role in liver cell injury and HBVclearance from the blood.

General Strategy For Studying Hepatitis

When BioWorld Today asked Takahashi what aspects of this work heand his colleagues were going to pursue, he said, "We now have ageneral strategy that can help us develop other disease modelsystems. Specifically, we are currently working on making a modelfor hepatitis C. With these systems in place, we hope to be able todevelop tests for anti-hepatitis reagents.

"Further out in the future," said Takahashi, "we see the ability tointroduce foreign genes into our hepatitis constructs. We may even beable to exploit hepatitis to make `infective DNA' constructs. Non-liver cells and tissues could be targeted by these constructs."

Takahashi said, "At present our options are wide open. We wouldlike to have a partnership with industry, but we have none as yet.However, because of the prevalence of hepatitis in Japan, scientiststhere have taken a great interest in our work." n

-- Chester Bisbee Special To BioWorld Today

(c) 1997 American Health Consultants. All rights reserved.