May 2, 2008 (CIDRAP News) – An H5N1 influenza vaccine based on a weakened adenovirus was tested successfully in mice and may offer advantages as a tool for combating a human flu pandemic, according to researchers from the Centers for Disease Control and Prevention (CDC) and Purdue University.
The vaccine is grown in mammalian cell cultures and therefore can be produced faster than egg-based vaccines, and it has the potential to provide broad protection against H5N1 viruses, say the researchers.
The scientists, led by Mary A. Hoelscher of the CDC as the first author, tested several versions of the vaccine in mice and found that they triggered both humoral (antibody) and cellular immune responses, suggesting that they could offer protection against a range of H5N1 strains. Their report was published in the Apr 15 issue of the Journal of Infectious Diseases.
"This approach may prevent severe illness and death or shorten the course of future infection with H5N1 virus strains that are antigenically distinct from currently circulating strains, and it may offer stockpiling advantages that overcome the limitations associated with storage of egg-derived vaccines," the report says.
The authors' method involves splicing the hemagglutinin (HA) gene from an avian flu virus into a human adenovirus that has been modified so it can't replicate in the body. Though it can't replicate, it can enter host cells, according to Hoelscher.
In a previous report, published last December, the scientists said a version of their vaccine that contained HA from a 1997 Hong Kong strain of H5N1 induced long-lasting immunity in mice and protected them from more recent H5N1 strains. The study was published in Clinical Pharmacology and Therapeutics.
Vietnam and Indonesia strains used
In the new study, the researchers made several versions of the adenovirus-based vaccine. One contained HA from a 2004 Vietnam strain of H5N1 virus (a clade 1 virus), while a second contained HA from a 2005 Indonesia H5N1 virus (clade 2). A third vaccine contained HA from the clade 2 strain plus the nucleoprotein gene from the clade 1 strain. Nucleoprotein is a virus component that varies little among different H5N1 strains—unlike HA, a surface protein that mutates often.
Still another version of the vaccine contained the nucleoprotein gene but no HA gene, and a control version contained the adenovirus without any flu virus genes.
Groups of 20 mice were injected with the vaccines (one vaccine per group), and another group was given both the clade 1 and clade 2 vaccines (in half doses). In addition, two more groups were vaccinated with one of two viruses generated through reverse genetics. These contained the HA and neuraminidase genes from either the clade 1 or the clade 2 H5N1 virus along with other genes from a human-adapted flu virus and were labeled C1RG and C2RG.
The mice received two doses of vaccine 4 weeks apart. Four weeks after the second dose, five mice from each group were challenged with a high intranasal dose of the C1RG or C2RG virus, both of which can replicate in mice but do not cause clinical disease. The lungs of these mice were analyzed for presence of the virus, while spleen cells from other mice were tested for evidence of cellular immune responses.
Serum tests showed that the mice given the clade 1 or clade 2 vaccine produced antibodies against wild-type virus of the same clade but not against the other clade. The mice that received both vaccines had antibodies against both clades, but the titer was cut in half.
Evidence of cellular immune response
In the mice that were challenged with the C1RG and C2RG viruses, the HA-containing vaccines led to clearance of virus from the lungs, as virus amounts were near or below the level of detection, the report says. The finding that mice vaccinated against only one H5N1 clade had lungs clear of virus from the other clade, despite a lack of antibodies against the latter, suggests that some mechanism other than neutralizing antibody response may have contributed to viral clearance, the authors write. They suggest that cell-mediated immune responses might have played a role.
Further, they report finding specific evidence of a cellular immune response in the mice that received the HA-containing vaccines: They had significantly higher levels of CD8+ T cells specific to a certain region (epitope) of the HA protein than did mice that received a vaccine with no HA.
"We demonstrated that the adenoviral vector–based strategy elicited cell-mediated CD8+ T cell immune responses as well as neutralizing antibodies against C1 and C2 strains, thereby broadening the vaccine's coverage," the report says.
CDC researcher Suryaprakesh Sambhara, senior author of the study, told CIDRAP News his team chose the adenovirus vector because it has already been used in a number of clinical trials. He said the approach used in the study has been licensed to PaxVax Inc., a San Diego company, which intends to launch a clinical trial later this year.
"In the event of a pandemic, we can make a vaccine in a relatively short period of time," Sambhara said. With cell-culture production, he estimates it would be possible to start producing vaccine in quantity within 1 to 2 months after a seed virus becomes available. By comparison, experts typically say it would take 4 to 6 months to provide a pandemic vaccine using the conventional egg-based technology.
Will it work in humans?
Gregory A. Poland, MD, a vaccine expert at the Mayo Clinic in Rochester, Minn., called the report promising but offered some caveats.
"The concept of the technology is a promising one," he said. "A lot of scientists in this area have hope that this will be one way we can rapidly produce a broadly protective vaccine."
However, he added, "This is a mouse study, and mice lie. It really is another issue altogether whether it'll work in humans." Poland is a professor of medicine in infectious diseases at the Mayo College of Medicine and director of the Mayo Vaccine Research Group and Program in Translational Immunovirology.
One potential obstacle, he said, is that laboratory mice have no previous exposure to the adenovirus serotype 5 used in the vaccine, but half or more of humans have been exposed to it and thus have neutralizing antibodies. If a vaccine based on the same adenovirus were used in humans, their existing antibodies might bind to and neutralize it before it could trigger the desired immune response, he said. He suggested that the next step should be to test the vaccine in mice that have already been exposed to the adenovirus.
Poland said the evidence of cellular immune responses in the study is of interest. There is some previous evidence of cell-mediated immune responses to flu in humans, but it's not widely known or accepted, he explained. "What the study showed is that this vaccine induced humoral and cell-mediated immunity; it caused viral clearance from the lung. So it's a very interesting study."
But it's not clear how important a contribution cell-mediated immunity might make, in Poland's view. "What I want is a vaccine that prevents infection, not one that clears it after it's established, because you might die before then," he said.
Hoelscher MA, Singh N, Garg S, et al. A broadly protective vaccine against globally dispersed clade 1 and clade 2 H5N1 influenza viruses. J Infect Dis 2008 Apr 15:1185-8 [Abstract]
Hoelscher, Jayashankar L, Garg S, et al. New pre-pandemic influenza vaccines: an egg- and adjuvant-independent human adenoviral vector strategy induces long-lasting protective immune responses in mice. Clin Pharmacol Therapeutics 2007 Dec;82(6):665-70 [Full text]
2007 CIDRAP News series "The pandemic vaccine puzzle"