Special reverse genetics used for vaccine against H5N1

    Memphis, Tennessee, April 2, 2003

    New technique created at St. Jude allowed vaccine to be custom-made within weeks of emergence of virus

    Scientists at St. Jude Children’s Research Hospital announced today the development of a vaccine against H5N1, a new lethal influenza virus that triggered the World Health Organization (WHO) to declare a pandemic alert in February 2003.

    The virus appeared in birds in Hong Kong late last year and subsequently killed one of two infected people with rapidly progressive pneumonia in the past month.

    St. Jude developed the vaccine in only four weeks from the time it received the H5N1 sample from colleagues in Hong Kong.

    The announcement comes at a time when a second, as-yet-unidentified virus, has taken several lives around the world. The unknown virus, which causes severe acute respiratory syndrome (SARS), appears to have originated at the same time and in the same place as the new “flu.”

    The development of the initial (“seed”) batch of H5N1 vaccine is significant because humans do not have a natural immunity to the virus, according to Robert Webster, Ph.D., a member of the Department of Infectious Diseases at St. Jude. Rather, humans appear to become infected through contact with chickens and other birds. In the past the virus killed only the chickens it infected. But the new variant of H5N1 also killed many kinds of wild birds, which is unusual.

    If H5N1 acquires the ability to pass from human to human, there would be the potential for concern similar to that for SARS, according to Webster.

    “It’s likely there were two things that prevented the 1997 poultry influenza outbreak in Hong Kong from becoming more deadly—its inability to spread from human to human and the slaughter of more than 1.5 million chickens and other birds in the open-air markets of Hong Kong, which eliminated the source of the virus,” Webster said. “In fact, the sudden appearance of SARS in the same region of the world is just another warning that the large populations of people and poultry in this region are a potential source of viruses.”

    Webster is the director of the WHO’s U.S. Collaborating Center at St. Jude that studies animal influenza viruses. It is the only WHO laboratory that focuses on the transmission of animal viruses to humans.

    Webster’s laboratory has sent the seed H5N1 vaccine to the Centers for Disease Control in Atlanta and the World Influenza Center in London for further testing, in preparation for initial Phase I and Phase II trials in humans.

    “It’s important to move right along with these trials in case the virus begins spreading from person to person,” Webster says.

    Led by Richard Webby, Ph.D., and Daniel Perez, Ph.D., the St. Jude laboratory team successfully modified a technique called reverse genetics to permit them to develop the H5N1 vaccine so quickly. Using the samples of H5N1 obtained from Hong Kong, Webby mixed two genes from H5N1 with six genes from a second virus (A/PR8/34)[H1N1]). H1N1 is a rapidly growing “master” strain of virus commonly used to make vaccines.

    The genes from flu viruses produce proteins called HA and NA, which are on the surface of the virus, in full “view” of the immune system. Webby took the modified gene for HA and the NA from H5N1 and mixed them inside a cell with six genes from H1N1. The HA gene was modified to abolish its ability to cause disease and therefore made it safer to use in the vaccine.

    The genes mixed together, and the resulting vaccine virus produced in the cell thus carried HA and NA from H5N1. But because of the alterations to the HA, and the rest of the genes being derived from H1N1, the new virus vaccine cannot cause disease. Rather, it can only stimulate the immune system to respond to H5N1.

    “The St. Jude vaccine is like a gun without ammunition,” said Elaine Tuomanen, M.D., director of the St. Jude Department of Infectious Diseases. “The vaccine looks deadly enough for its HA and NA proteins to alert the immune system. But in reality, it’s carrying blanks that can’t cause disease.”