April 22, 2005 — A newly developed monoclonal antibody can cure mice infected with the West Nile virus, scientists at Washington University School of Medicine in St. Louis report. If further studies confirm the effectiveness and safety of the antibody, it could become one of the first monoclonal antibodies used as a treatment for an infectious disease.
In a strain of mice that normally only has about a 10 percent survival rate after West Nile infection, scientists found that single doses of the antibodies given soon after infection could boost survival rates to 90 percent or higher.
"To our knowledge, these experiments are the first successful demonstration of the use of a humanized antibody as a post-exposure therapy against a viral disease," says senior investigator Michael Diamond, M.D., Ph.D., assistant professor of molecular microbiology, pathology and immunology and of medicine. "They also suggest antibody-based therapeutics may have a broader utility against other infectious diseases."
Diamond points out that Macrogenics Inc., of Rockville, Md., a company that contributed to the study and licensed the antibody from Washington University, must complete other preliminary studies before the antibody can be tested in humans. But he and his colleagues are excited both by the apparent potency of the antibody and its potential to help them explore new possibilities for treating related viruses that are more prolific causes of human disease and death.
"We could give a single dose of this antibody to mice as long as five days after infection, when West Nile virus had entered the brain, and it could still cure them," says Diamond. "It also completely protected against death from the disease."
Diamond and his colleagues will report their results in the May issue of Nature Medicine.
In 2004, West Nile virus reportedly caused 2,470 infections and 88 deaths in the United States. The mosquito-borne virus, first isolated in Africa in 1937, spread to the Middle East, Europe, and Asia before arriving in the United States in 1999. Most infections with the virus are mild or symptom-free, but infections in people with weakened immune systems and those over 50 sometimes lead to serious complications or death.
Scientists initially produced a panel of many West Nile virus antibodies from mouse cells. The human immune system would clear out these foreign antibodies quickly, so when they had identified a potent antibody, scientists at Macrogenics clipped out the genetic material that controls the antibody's targeting and cloned it into a human antibody. The "humanized" antibody should be less likely to induce an adverse human immune system response. A second round of tests in mice confirmed that the new antibodies retained their ability to stop West Nile virus.
Other monoclonal antibodies are currently in development or use as anti-cancer and anti-inflammatory treatments. An antibody against respiratory syncytial virus (RSV) is approved for use as a prophylactic treatment in children at risk of the disease in hospitals. Unlike the West Nile virus antibody, though, the RSV antibody has to be given prior to infection.
West Nile virus belongs to a family of viruses known as flaviviruses, several of which are spread by mosquito bites. Other flaviviruses include the virus that causes dengue fever, a potentially life-threatening infection prevalent in tropical cities. Centers for Disease Control and Prevention epidemiologists estimate that there are100 million cases of dengue worldwide every year.
"A lot of what we're learning from the West Nile virus antibody will be of consequence for the development of a pediatric dengue vaccine," says co-author Daved Fremont, Ph.D., associate professor of biochemistry and molecular biophysics and of pathology and immunology. "Currently there are no safe vaccines for dengue infections."
Important insights from the production and selection of the new antibody include a close fix on where the antibody binds to West Nile virus. Antibodies typically work by attaching to a piece of a foreign cell or substance, which causes immune system cells known as macrophages to pick up the substance and clear it from the body.
Binding to the invader is just the beginning of the battle, though. Some antibodies can bind to an invader but do so in a way that fails to slow the invader down or trigger a response from macrophages. In one rare case that involves the dengue fever virus, antibodies can adhere to the virus in a way that accelerates the infection.
From their initial pool of West Nile virus antibodies, researchers identified 46 that could bind to the West Nile virus' envelope (E) protein. Further testing showed that 12 could bind to the virus in a way that consistently neutralized it, shutting down infections in cell cultures and in mice.
To determine where these potently neutralizing antibodies were binding to the envelope protein, a task known as epitope mapping, researchers modified a yeast-based screening system. The system let them test individual antibodies for their ability to bind to many versions of the E protein, each with slight alterations. By analyzing the changes in the versions of the protein that antibodies had difficulty binding to, they isolated first a region of the E protein, known as domain III, and then a group of amino acids in that domain.
"The big surprise for us was that all of the potently neutralizing antibodies appear to recognize the same general region of this domain," says Fremont. "It was very consistent—all the neutralizing antibodies that bind this domain adhere to that area; all the non-neutralizing antibodies that bind this domain adhere to different areas."
Fremont notes that while the E proteins of various flaviviruses are generally very similar, domain III can vary significantly. He and others are working to detail the precise mechanisms that allow the new West Nile antibody to block viral infection.
Diamond and Fremont are looking for other areas of the West Nile virus E protein that antibodies can bind to and neutralize the virus. Diamond is also using the yeast screening system to epitope map the sites on the dengue fever virus where antibodies can bind and inadvertently enhance infection instead of fighting it.
"We don't really understand on a molecular level what's happening in these cases, which are called enhancing antibodies," Diamond explains. "Epitope mapping may help us better understand this potentially dangerous interaction."
Researchers at Macrogenics were coauthors on this paper, and Diamond now serves as a consultant for the company.
Oliphant T, Engle M, Nybakken GE, Doane C, Johnson S, Huang L, Gorlatov S, Mehlhop E, Marri A, Chung KM, Ebed GD, Kramer LD, Fremont DH, Diamond MS. Development of a humanized monoclonal antibody with therapeutic potential against the West Nile virus. Nature Medicine, May 2005.
This research was supported by funding from the National Institute of Allergy and Infectious Diseases and the Pediatric Dengue Vaccine Foundation.
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