Scientists discover how virus evades immune system in acute and chronic infections; new vaccines may result
Researchers at Johns Hopkins have uncovered how a majority of the genetic changes in the hepatic-C virus, the most common cause of liver disease, allow it to evade the body's immune system during infection. Hepatitis C infection can lead to cirrhosis, cancer and even death. In a series of experiments that describe the virus' transition from an acute to chronic infection, the Hopkins team found that one-half of the virus' changes in its genome are in sites under attack by the body's immune system. As the virus evolves and these changes weaken the body's immune response, a second set of changes at other sites in the genome are reverting back to an "ancestral" set of amino acids.
"We think this piecemeal exchange is helping the virus evade the body's immune system," says study investigator and infectious disease specialist Stuart Ray, M.D., an associate professor at The Johns Hopkins University School of Medicine. "In a newly infected person, the virus may need to adopt new mutations to escape recognition by the immune system's T cells, which fight infection, but it may need to lose the mutations that had protected it in someone else. Despite pressure to change, the virus is always is restoring its shape."
The Hopkins findings, published in a pair of studies in the Journal of Experimental Medicine this week, are believed to be the first description of the precise genetic changes taking place in the virus during the acute phase of infection, when hepatitis C initially escapes the body's defenses and establishes itself in the body. As the infection moves into the chronic stage, the immune response becomes weak and less effective, but until now, no one could explain exactly why.
A second, related experiment produced similar findings when the Hopkins team partnered with researchers in Ireland to perform what is believed to be the first comparison of genetic changes across multiple genes in strains from chronically infected people to the original strain that infected them.
Ray, who served as senior investigator on the first study and led the second, believes the newly identified ancestral component of the viral genome, called a consensus sequence, could serve as the basis for development of a vaccine that is effective against both acute and chronic infections, thereby stemming the epidemic that currently afflicts more than 170 million people worldwide, including 3 million Americans.
"Hepatitis C is extremely difficult to treat if it becomes chronic," says infectious disease specialist Andrea Cox, M.D., Ph.D., an assistant professor at Hopkins who was lead author of the first study. "While approximately 30 percent of patients have a strong enough immune response to rid themselves of the virus during the acute phase, and current treatments are 90 percent effective at treating any remaining acute infections, these treatments are only 50 percent effective against chronic infections, which otherwise persist for life and can cause death."
According to Cox, the hepatitis C virus naturally mutates, or alters its genome, very rapidly. Its strains have two to three times more genetic variability, for example, than HIV, the virus that causes AIDS, and hepatitis C reproduces more than 100 billion times per day, 100 times faster than HIV. Compounding the problem, the infection is asymptomatic in the acute stage, making it less likely that diagnosis will be made early, when it is easiest to treat.
Conventional wisdom, the researchers say, was that the large numbers of mutations were simply random in the virus' ever-changing genome, but the new study suggests that Darwinian genetic selection is at play. That is, the virus' genome changes in ways that make it more reproductively "fit" in the face of each immune system it encounters, changing what is must to evade the immune system in one host, then restoring itself when the pressure is off.
What Ray's team found when the immune response weakens was that the virus naturally mutates toward a set of 3,000 common amino acids, what the researchers considered the virus' most preferred state. During the acute phase, Ray says, the virus is under severe pressure from the immune response and forced to drift away from the consensus sequence, using mutations to evade the immune response. However, the drift was reversible and, once the virus successfully evaded a particular immune cell, its amino acids reverted back to the consensus set.
To assess the genetic changes in the early stages of infection, the researchers decoded, or sequenced, the virus' genome, made up of RNA, which is very similar to the more widely known DNA that makes up the genome of most organisms. The RNA was gathered from eight newly infected patients in Baltimore, Md., all of whom were offered treatment and were participants in a larger study of infectious diseases in intravenous drug users. The sample group was unusual, allowing analyses before and during the early stages of infection. One patient self-recovered, while the rest proceeded to chronic infection.
Using advanced blood-sorting techniques, the Hopkins team extracted millions of immune system cells, including the systems' principal fighters, called T cells, from blood samples taken between 30 days and six months after infection, when the body's initial immune response kicks in and subsequently peaks.
Immune responses were mapped using a series of more than 500 overlapping synthetic peptides, or strings of amino acids whose code was already known. This allowed the researchers to compare changes observed in the RNA sequence to corresponding shifts in the body's immune response to the infection.
When specifically recognized by T cells, the peptides trigger production of interferon gamma, a protein that acts as a signal to many other immune cells to respond to a new infection. Reductions in the production of interferon gamma would indicate, the scientists say, that the immune system was weakening in its response to the virus' mutations.
After analyzing the genetic changes in the sites, called epitopes, where the T cells specifically bind to the virus, the researchers found no changes had occurred during the one year of follow-up in the one patient who self-recovered. However, in the remaining seven patients, there were changes in 69 percent of T-cell epitopes, showing that the virus had mutated at key locations necessary for chronic infection to proceed.
Additional analysis showed that changes in T-cell epitopes were 13 times more frequent than changes in the remaining genome of the virus. The researchers examined the binding ability of T cells obtained early in infection to recognize 10 viral peptides known to have changed during the first six months of infection. Eight showed severely reduced capacity to stimulate production of interferon gamma, offering confirmation that the virus was mutating to evade the immune system.
Analysis of the viral RNA in the blood of seven patients with chronic infections revealed that eight of 16 changes in genome matched to the consensus sequence, confirming the presence of selective evolutionary pressure toward restoration of an ancestral form of the virus.
In the second study, using blood samples collected in Cork, Ireland, the researchers compared the genetic makeup of the virus in 22 chronically infected women to the original strain that had infected them more than 20 years before. The women were among hundreds accidentally infected in 1977 by a blood product tainted with hepatitis C, providing the researchers with unique access to the source of the infection, which came from a single donor unaware of having the illness.
Using computer analysis techniques developed at Hopkins, the scientists mapped these changes against the genetic makeup of the women's immune response. The researchers found that when viral mutations were clustered in epitopes specific to each woman's immune system, the changes were directed away from the consensus sequence, suggesting immune escape. However, when mutations were clustered in epitopes that were not specific, the mutations were reversions back to the consensus sequence.
When the individual genome changes in each woman were mapped on a grid, each woman formed a unique cluster indicating individual, evolutionary selection. However, some of the changes were shared, suggesting convergence, which would not have occurred had the virus simply mutated at random.
"Our results raise the possibility that a hepatitis-C consensus sequence could be the best practical option for a vaccine," says infectious disease specialist David Thomas, M.D., a professor of medicine at Hopkins who served as senior author of the study of Irish women. "If we can focus vaccine development on the common genetic element in chronically infected patients, then we may be able to make a more effective vaccine."
Funding for these studies, which took place from January 2002 to January 2005, was provided by the National Institutes of Health, including the National Institute for Allergy and Infectious Disease, and the National Institute on Drug Abuse.
Other Hopkins investigators in this research were Timothy Mosbruger; Qing Mao, M.D., Ph.D.; Zhi Liu, M.D.; Xiao-Hong Wang, M.D.; Hung-Chih Yang; Xiao-Hang Wang, Ph.D., Dale Netski, Ph.D.; and Drew Pardoll, M.D. Co-investigators in the first study also included John Sidney, Ph.D., and Alessandro Sette, Ph.D., from the La Jolla Institute for Allergy and Immunology, in San Diego, Calif. Collaborators in the second study, conducted at Hopkins and at Cork University Hospital in Ireland, were Liam Fanning, Ph.D., and Kelizaeth Kenny-Walsh, M.D.
Hepatitis C is the leading cause of liver disease in the United States, causing an estimated 10,000 to 12,000 deaths each year. Hepatitis C is transmitted when blood and possibly other body fluids of an infected person enter another person, primarily through injection drug use, exposures in health care settings, from an infected mother to her baby during birth, and occasionally through sexual exposure. Symptoms of hepatitis C may not appear for years after infection, and diagnosis must be confirmed by blood tests. However, in addition to liver inflammation and tumors, earlier signs of infection are persistent flulike symptoms, including any combination of body aches, headaches, night sweats, loss of appetite, diarrhea, fatigue, rash, nausea and mild abdominal pain. Current treatments for hepatitis C involve weekly injections of pegylated interferon for one year, plus twice daily doses of oral ribavirin. While some patients recover on their own, with their immune system attacking the virus and clearing it from the body, most do not. Scientists have not yet determined why this happens in some patients and not in others.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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