Key to hepatitis C virus persistence found
Scientists at two Texas universities have discovered how hepatitis C virus thwarts immune system efforts to eliminate it.
The finding, published online today in Science Express, could lead to more effective treatments for liver disease caused by hepatitis C virus, says author Michael Gale, Jr, PhD, of University of Texas Southwestern Medical Centre at Dallas. Dr Gale and coauthor Stanley Lemon, MD, of University of Texas Medical Branch at Galveston, are grantees of the National Institute of Allergy and Infectious Diseases (NIAID).
“Persistent hepatitis C virus (HCV) infection is a major cause of liver disease worldwide and is the leading reason for liver transplants in this country,” notes NIAID Director Anthony Fauci. “The most prevalent form of HCV in the United States is, unfortunately, the least responsive to available treatments. Moreover, African Americans are even less responsive to therapy than Caucasians,” he adds.
The immune system has many ways to detect and fight off invading microbes, and microbes have just as many ways to elude and disarm immune system components. Through a series of experiments on cells grown in the laboratory, Drs Gale and Lemon defined the strategy HCV uses to evade the host’s immune response. As HCV begins to replicate in its human host, it manufactures enzymes, called proteases, which it requires to transform viral proteins into their functional forms. The Texas investigators determined that one viral protease, NS3/4A, specifically inhibits a key immune system molecule, interferon regulatory factor-3 (IRF-3). IRF-3 orchestrates a range of antiviral responses. Without this master switch, antiviral responses never begin, and HCV can gain a foothold and persist in its host.
Next, the scientists searched for ways to reverse the IRF-3 blockade. They applied a protease inhibitor to human cells containing modified HCV. This prevented the virus from making functional NS3/4A and restored the cells’ IRF-3 pathway. Follow-up studies have shown that once restored, the immune response reduced viral levels to nearly undetectable levels within days, according to Dr Gale.
The identification of this viral protease-regulated control of IRF-3 opens new avenues in both clinical and basic research on hepatitis C, notes Dr Gale. Until now, scientists had not considered the possibility that inhibiting this protease did anything more than halt viral replication. “Now that we know NS3/4A inhibition essentially restores the host’s immune response to the virus, we can assess hepatitis drug candidates for this ability as well,” Dr Gale says.
NS3/4A will be a valuable tool in further dissecting the roles of viral proteases and their host cell targets, says Dr Gale. For example, the scientists plan to use NS3/4A to hunt for the still unknown host cell enzyme responsible for activating IRF-3. Conceivably, Dr Gale explains, future therapeutic approaches to viral disease could involve boosting the activity of any key host enzymes that are found.
“Understanding the tricks that the hepatitis C virus employs to impair the immune system represents an important advance with potential implications for successful cure of those suffering from liver disease,” says Leslye Johnson, PhD, chief of NIAID’s enteric and hepatic diseases branch.
NIAID is a component of the National Institutes of Health (NIH), which is an agency of the United States Department of Health and Human Services. NIAID supports basic and applied research to prevent, diagnose, and treat infectious and immune-mediated illnesses, including HIV/AIDS and other sexually transmitted diseases, illness from potential agents of bioterrorism, tuberculosis, malaria, autoimmune disorders, asthma and allergies.
News releases, fact sheets and other NIAID-related materials are available on the NIAID Web site at http://www.niaid.nih.gov/. (The paper will be available online at www.scienceexpress.org on April 17, 2003).
E Foy et al. Regulation of interferon regulatory factor-3 by the hepatitis C virus serine protease. Science, April 17, 2003. DOI 10.1126/science.1082604.
This is conceptually interesting data, but to date no there is no animal model and no real cell culture system in which HCV can replicate as a whole virus. Replicon systems with artificial virus are used so far.
Interactions of immune system and the virus are difficult to extrapolate from cell culture to humans. This may limit the implications of any results as from the paper above. A serin-protease inhibitor from Boehringer Ingelheim is in clinical development and has demonstrated potent viral inhibition during 48 hours of application. All patients experienced a rapid viral rebound after cessation of therapy but this indicates at least initial antiviral activity.