Cell-Grown Vaccine Protects Against Avian Flu Virus

University of Pittsburgh (www.upmc.com) researchers have genetically engineered an avian flu vaccine from the critical components of the H5N1 virus, and shown that the vaccine completely protected mice and chickens from infection.

Researchers grew the vaccine in cell cultures, developing the recombinant vector vaccine in 36 days. In contrast, conventional vaccines prepared in fertilized chicken eggs require a minimum of several months to develop.

According a university press release, the vaccine was constructed by genetically engineering adenovirus, a common cold virus, to express either all or parts of an avian influenza protein called hemagglutinin on its surface. Hemagglutinin allows the virus to attach to the cell that is being infected and is, therefore, vital to the influenza virus’s ability to cause illness and death.

The investigators constructed several adenovirus vectors, containing either the full genetic sequence of the HA protein or sequences for subunits of of the protein. Collaborating with investigators from the Centers for Disease Control and Prevention, the researchers tested the ability of their vaccines to protect mice from infection by wild-type H5N1 by comparing its performance with an adenovirus vector containing no H5N1 genes (i.e., an empty vector). The H5N1-exposed mice were studied for any signs of illness, and their blood was checked for antiviral antibodies and other markers of H5N1-specific immunity.

All of the mice immunized with the empty-vector vaccine experienced substantial weight-loss after exposure to wild-type H5N1, and all were dead within six to nine days of avian flu exposure. In contrast, most of the mice immunized with the adenovirus containing all or part of the hemagglutinin protein showed only mild and short-lived weight loss and survived H5N1 infection. Six days after infection, researchers could not detect any infectious H5N1 in the organs of mice immunized with the full-length HA vaccine. Moreover, when they looked at the cellular immune response to vaccination, they found that all of the animals immunized with full-length hemagglutinin or the subunit vaccines developed strong cellular immune responses, and the full-length hemagglutinin-immunized mice developed strong T-cell responses to both of the hemagglutinin subunits.

The fact that two types of immunity were produced (antibodies that block hemagglutinin and a strong T-cell response) could mean that that even if the H5N1 virus mutates, the vaccine may still be effective against it, said the researchers.

The vaccine was then tested in chickens, which have almost a 100% mortality rate after H5N1 exposure. Chickens were inoculated either intranasally or subcutaneously with either the hemagglutinin-containing vaccine or the empty-vector vaccine. The chickens were challenged with a dose of whole H5N1 virus 10,000 times greater than the dose given to the mice and significantly greater than the dose farm chickens are likely to be exposed to during a natural outbreak. All of the chickens that were immunized subcutaneously survived exposure to H5N1, developed strong HA-specific antibody responses, and showed no clinical signs of disease. Half of the chickens immunized intranasally died, and all of the chickens immunized with the empty vector died within two days of H5N1 exposure.

Researchers suggest their adenovirus-based vaccine could complement traditional inactivated influenza vaccines by immunizing chickens against H5N1. The widespread inoculation of susceptible poultry populations could help prevent the spread of the virus. Also, if there were a disruption in the traditional vaccine production pipeline, a recombinant vaccine could be an alternative for human immunization.