January 26, 2007

Hunt for cold clues crosses disciplines

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David Wright, Ph.D., left, John Williams, M.D., and Sharon Tollefson are studying how metapneumovirus causes disease and how to stop it. (Photo by Susan Urmy)

Hunt for cold clues crosses disciplines

Sneeze, sniffle, wheeze. The culprit behind that head cold could be human metapneumovirus (MPV) — a relative newcomer to the world of known respiratory viruses.

MPV was first identified in 2001, but it's been around for at least 50 years, “and probably hundreds of years,” said John Williams, M.D., a pediatric infectious disease specialist studying how MPV causes disease and how to stop it.

Williams and Vanderbilt colleagues including chemist David Wright, Ph.D., reported in two separate studies this month in the Journal of Virology that an MPV protein responsible for helping the virus infect cells could be both a good target for anti-viral drugs and a candidate vaccine.

MPV is the second most common cause of serious respiratory illness in young children, Williams said. The virus is related to RSV (respiratory syncytial virus) — the No. 1 cause of respiratory disease and hospitalization in children under age 5.

Like RSV, MPV is most dangerous for very young infants and for children with heart and lung disease or compromised immune systems. The elderly too — particularly individuals with chronic lung disease — are vulnerable to the lower respiratory illnesses (bronchiolitis and pneumonia) caused by MPV.

“We all get reinfected with RSV and MPV over and over, and repeat infections tend to be mild — just a cold,” Williams said. “It's that first infection in infancy, especially for premature babies, and infections in other high-risk populations that we need to prevent with vaccines or treat with good anti-virals.”

Williams, Wright and colleagues focused on the MPV fusion protein — a viral protein that participates in getting the virus inside cells. By understanding how the fusion protein works, the investigators reasoned that they may discover ways to disable it and prevent infection.

The researchers found that peptides (small pieces of protein) corresponding to certain parts of the MPV fusion protein could block the virus from infecting cells in culture.

“These small synthetic proteins were very, very potent at inhibiting the virus, which opens the possibility that they could be used to treat people infected with virus,” Williams said.

A similar strategy is used against HIV — patients are treated with peptides that block viral fusion. The current studies suggest that the MPV fusion protein works in a way similar to the HIV fusion protein.

“The next step is to test these peptides in an animal model for MPV infection, by delivering them as an aerosol directly to the infected respiratory tract,” Williams said.

In other studies, the investigators developed what Williams called a “breakthrough technology” for making large quantities of pure MPV fusion protein in the laboratory.

“It's been extraordinarily difficult in the field of respiratory virus research to make enough of the viral proteins in the laboratory for studies,” he noted.

Gabriella Cseke, Ph.D., a former graduate student, spent about two years developing a technology for synthesizing MPV fusion protein so that the synthetic protein resembled the native virus protein. The group has applied the technology to other virus proteins as well.

The investigators tested the synthetic MPV fusion protein, along with DNA constructs coding for the protein, as vaccine candidates in cotton rats. Williams and colleagues had previously established that the cotton rat — a small rodent that resembles a gerbil — is a good animal model for MPV infection. Cotton rats are easily infected by the virus and have lung disease that looks like human disease.

The team vaccinated cotton rats with the fusion protein or DNA construct — two intramuscular injections two weeks apart — and then infected the animals with MPV. All of the vaccinated animals developed protective antibodies after immunization, and were “almost completely protected in the lungs against virus infection,” Williams said.

The researchers also examined the “type” of immune response induced by the protein to rule out the possibility that the animals were having an aberrant, allergic-type response. This was important, Williams explained, because RSV vaccine candidates tested in the 1960s induced an allergic response that led to worsened disease.

“To the extent that we can measure in this model, it looks like the cotton rats are having an appropriate natural immune response, and that that response is protecting them from MPV infection,” Williams said. “We're very excited.”

The investigators will pursue the fusion protein as a vaccine candidate in animal models, including primates. They are also using the purified protein as a reagent to identify MPV receptors and to understand the biology and immunity of MPV.

Williams credits the collegial atmosphere at Vanderbilt with fostering rapid discoveries in the MPV field.

“In five years, we've gone from discovery of a virus to vaccine and anti-viral candidates. The seamless collaboration at Vanderbilt across disciplines like Pediatrics and Chemistry has made this possible.”

Other authors of the two Journal of Virology studies include Scott Miller, Ph.D., Sharon Tollefson, Joyce Johnson, M.D., and James Crowe Jr., M.D. The research was supported by the National Institute of Allergy and Infectious Diseases, a Vanderbilt Department of Pediatrics Hazinski-Turner Award, and a Burroughs Wellcome Fund Clinical Scientist Award in Translational Research.