Focus on the Flu : Prevention and Vaccine, Part 4

by National Institute of Health

(Page 6 of 15)

Reverse Genetics: Building Flu Vaccines Piece by Piece

A new technology that's looking increasingly promising in the fight against the flu is plasmid-based reverse genetics, a procedure developed by Yoshihiro Kawaoka, Ph.D., of the University of Wisconsin, and further refined by Robert Webster, Ph.D., and Erich Hoffman, Ph.D., of St. Jude Children's Research Hospital in Memphis, TN. The technique could speed up the process by which a vaccine is created.

In the conventional egg method, two flu strains with the preferred features for a new vaccine are injected into an egg, and their genes reassort naturally. Researchers sift through the 250-plus possible combinations of viruses to find one that displays the desired hemagglutinin (HA) and neuraminidase (NA) antigens and is still able to grow inside an egg.

With reverse genetics, scientists can custom make a flu vaccine by assembling genes that code for the desired features. Two genes representing the HA and NA antigens are selected from the target virus, while the remaining six genes come from a virus that's time-tested for its ability to grow inside an egg. (Although the influenza virus actually uses RNA as its genetic material, the researchers make complementary pieces of DNA because DNA is easier to work with.) The DNA that codes for the viral genes is then inserted into animal cells, which make new copies of the virus. Researchers recover the resulting virus that will be used to manufacture the vaccine. Although the vaccine still needs to be grown inside eggs for large-scale production, animal cells could also be used as that technology advances.

One key benefit of the new technology is that if portions of a targeted virus, such as the H5 and H7 antigens, are too toxic to grow inside eggs, the segments of the genes coding for these antigens that make them so dangerous can be clipped and removed.

Because of this characteristic, Dr. Webster and collaborators are using reverse genetics to attempt to develop a vaccine for the deadly H5N1 virus, also known as the bird flu, that has appeared in Vietnam, Thailand, and other Asian countries in winter 2003-4.

With further research, scientists will try to determine if reverse genetics technology may prove a quick and effective way for developing a suitable vaccine in the event of a pandemic.

Scrambling for an Egg Alternative

Since the 1940s, influenza virus used in vaccines has been grown inside chicken eggs. Early each year, experts from the Centers for Disease Control and Prevention predict the three flu strains most likely to cause disease in the coming flu season. Each strain is grown in separate batches of fertilized chicken eggs. Next, vaccine manufacturers draw off virus-laden fluids from the eggs and combine them into a single vaccine product. The slow, laborious process requires some 270 million eggs to make the roughly 90 million doses of seasonal flu vaccine needed each year (90 million doses x 3 eggs per dose).

William J. Hillegas, Ph.D., of SoloHill Engineering, Inc., a biotechnology firm in Ann Arbor, MI, thinks that a cell culture-based approach may be better. He and a group of researchers are using tiny beads called microcarriers as the basis for a faster, more cost-effective way to grow virus strains used for influenza vaccines. SoloHill is working to improve the performance of its patented microcarrier beads by coating them with a synthetic peptide instead of an animal-based collagen; thus no animal-derived proteins will be used to manufacture the beads.

In the cell culture-microcarrier technique, flu virus is grown inside mammalian cells, such as the kidney cells of African green monkeys or dogs. The cells are anchored to the microcarrier beads, which are placed in large bioreactors and immersed in a liquid medium that encourages their growth.

Producing vaccines in bioreactors using cell culture and microcarrier beads has several advantages over the egg-based technique: The microcarrier cell culture technology requires much less manufacturing space to produce large quantities of virus. Microcarriers provide a large surface area on which the animal cells thrive. The growth media and microcarriers are totally free of animal-derived components, enhancing safety and simplifying subsequent manufacturing steps. Animal cells growing on beads reproduce rapidly, whereas hens rarely lay more than one egg a day. Some of the more dangerous flu strains (including avian influenza) are lethal to chicken embryos and cannot grown in eggs, whereas they could be grown in animal cells.

Another positive: people who cannot receive the standard, egg-based flu vaccine because they are allergic to eggs may safely tolerate a vaccine made with cell culture technology.

In 2005, Dr. Hillegas and his coworkers reported at a scientific meeting success in developing their animal-produce-free microcarrier beads. Says Dr. Hillegas, "We showed that our technique can produce virus in quantities comparable to the egg-based technique."

By speeding up the production process, allowing rapid turnaround when new viruses are discovered, and keeping costs down, the technique could significantly ratchet up our ability to keep ahead of the flu, says Dr. Hillegas.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.