http://news.bmn.com/news/story?day=030704&story=2

3 July 2003 12:49 GMT

by Paul D. Thacker, Drug Discovery Today

Researchers at the Wistar Institute in Philadelphia are currently devising a prototype mouse influenza vaccine that promises to last longer and provide increased protection against this rapidly evolving pathogen.

The vaccine has three novel features: a synthetic peptide as the vaccine, a nasal delivery system, and an immune response to unique protein on the virus.

The World Health Organization estimates that five million people are affected by influenza every year and, of those, a quarter to half a million patients die. Although a vaccine for the pathogen exists, the virus mutates rapidly, forcing researchers to constantly search and identify new strains and then ascertain which viruses to incorporate into the annual flu shot.

"Normally against influenza you use an inactivated virus," said Wistar Institute associate professor, Laszlo Otvosz. "But like many other groups, we are trying to use subunit vaccines which come from proteins or peptides."

Current vaccines

The influenza virus has two transmembrane proteins - hemagglutinin and neuraminidase - which have protein sections found on the outside of the virus. These ectodomains mutate rapidly in response to immune attack so new strains constantly evolve. These new strains are then identified by sequencing the extracellular portion of hemagglutinin.

To create a vaccine, virus is grown in chicken eggs and then purified. The virus is then chemically inactivated and the vaccine is standardized to a titer of the hemagglutinin. However, Otvosz and his team plan a different approach.

Creating a synthetic vaccine

The Wistar group is focusing on a third transmembrane protein, matrix protein 2 (M2). This protein is highly conserved, with an extremely short extracellular domain consisting of only 23 amino acids. Because it does not mutate and is immunogenic in mice, M2 has become the focus of several investigations trying to create an M2-based vaccine.

The Otvosz vaccine consists of a poly-(lysine-glycine) backbone for the four M2 ectodomains. Peptide vaccines are ideal because they can be made in large quantities with a high purity, but they are not generally immunogenic enough. To solve this problem, Otvosz also adds two protein fragments, which attract T-helper cells.

"When you want to elicit an antibody response you need T-helper cells involved," said Otvosz. "So we had to add these epitopes." They then vaccinated the mice twice intranasally. In nature, influenza infects through the nasal passages and Otvosz says they wanted to mimic this effect.

After five weeks, blood was drawn to see if the mice had generated antibodies. "We got a very good antibody response," says Otvosz. "Sometimes better than a natural infection." He adds that the antibody was protective when mice were later infected with a live a virus. "We saw a one log decrease in virus titer in mice that had been vaccinated versus the untreated animals."

Drawbacks and future testing

Although the vaccine shows great promise, Otvosz says there is much more to explore. "We know that this reduction in virus is scientifically significant, but we're not certain if it is clinically significant." The virus they used was not lethal and in the future he would like to try a lethal strain.

"A good human vaccine is always going to involve antibodies to hemagglutinin," said Jacqueline Katz, chief of the Immunology and Viral Pathogens Section at the US Centers for Disease Control and Prevention. "This approach using another protein is something I've always imagined as a supplement where you get increased immunity or broad immunity in a crisis situation if you don't have the vaccine." Further, she adds that vaccines based on M2 will more likely ameliorate disease rather than provide true immunity.

Otvosz adds that he would like to test other constructs using different T-helper epitopes. "Mice are inbred and we knew this strain would raise antibodies to this protein. So we would like to try different T-cell determinants with different mice strains," he said.

In the next round of studies, he is also focusing on different M2 mutants to see how mice respond. "Unlike mice, humans are outbred and have numerous T-cell determinants. So we need to see if this will work in everybody, probably making a more promiscuous T helper epitope."

This article was originally published in Drug Discovery Today.

© Elsevier Limited 2003