Wilson, Anna Tretiakova, PhD, Senior Research Scientist, Maria P. Limberis, PhD, Research Assistant Professor, all from the Penn Gene Therapy Program, and colleagues published their findings online this week in Science Translational Medicine ahead of print. In addition to the Penn scientists, the international effort included colleagues from the Public Health Agency of Canada, Winnipeg; the University of Manitoba, Winnipeg; and the University of Pittsburgh. Tretiakova is also the director of translational research, and Limberis is the director of animal models core, both with the Gene Therapy Program
"The experiments described in our paper provide critical proof-of-concept in animals about a technology platform that can be deployed in the setting of virtually any pandemic or biological attack for which a neutralizing antibody exists or can be easily isolated," says Wilson. "Further development of this approach for pandemic flu has taken on more urgency in light of the spreading infection in China of the lethal bird strain of H7N9 virus in humans."
At the Ready
Influenza infections are the seventh leading cause of death in the United States and result in almost 500,000 deaths worldwide per year, according to the Centers for Disease Control. The emergence of a new influenza pandemic remains a threat that could result in a much loss of life and worldwide economic disruption.
There is also interest by the military in developing an off-the-shelf prophylactic vaccine should soldiers be exposed to weaponized strains of infectious agents in biologic warfare.
Human antibodies with broad neutralizing activity against various influenza strains exist but their direct use as a prophylactic treatment is impractical. Now, yearly flu vaccines are made by growing the flu virus in eggs. The viral envelope proteins on the exterior, namely hemagglutinin, are cleaved off and used as the vaccine, but vary from year to year, depending on what flu strains are prevalent. However, high mutation rates in the proteins result in the emergence of new viral types each year, which elude neutralization by preexisting antibodies in the body (specifically specific receptor binding sites on the virus that are the targets of neutralizing antibodies).
This approach has led to annual vaccinations against seasonal strains of flu viruses that are predicted to emerge during the upcoming season. Strains that arise outside of the human population, for example in domestic livestock, are distinct from those that normally circulate in humans, and can lead to deadly pandemics.
These strains are also not effectively controlled by vaccines developed to human strains, as with the 2009 H1N1 pandemic. The vaccine development time for that strain, and in general, was not fast enough to support vaccination in response to an emerging pandemic.
Knowing this, the Penn team proposed a novel approach that does not require the elicitation of an immune response, which does not provide sufficient breadth to be useful against any strain of flu other than the one for which it was designed, as with conventional approaches.
The Penn approach is to clone into a vector a gene that encodes an antibody that is effective against many strains of flu and to engineer cells that line the nasal passages to express this broadly neutralizing antibody, effectively establishing broad-based efficacy against a wide range of flu strains.
A Broad Approach
The rational for targeting nasal epithelial cells for antibody expression was to focus this expression to the site of the body where the virus usually enters the body and replicates which is the nasal and oral mucosa. Antibodies are normally expressed from B lymphocytes so one challenge was to design vectors that could deliver antibody genes to the non- lymphoid respiratory cells of the nasal and lung passages and could express functional antibody protein.
Targeting the respiratory cells was achieved through the use of a vector based on a primate virus -- AAV9 -- which was discovered in the Wilson laboratory and evaluated previously by Limberis for possibly treating patients with cystic fibrosis. The team constructed a genetic payload for AAV9 that expressed an antibody that was showed by other investigators to have broad activity against flu.
Efficacy of the treatment was tested in mice that were exposed to lethal quantities of three strains of H5N1 and two strains of H1N1, all of which have been associated with historic human pandemics (including the infamous H1N1 1918).
Flu virus rapidly replicated in untreated animals all of which needed to be euthanized. However, pretreatment with the AAV9 vector virtually shut down virus replication and provided complete protection against all strains of flu in the treated animals. The efficacy of this approach was also demonstrated in ferrets, which provide a more authentic model of human pandemic flu infection.
"The novelty of this approach is that we're using AAV and we're delivering the prophylactic vaccine to the nose in a non-invasive manner, not a shot like conventional vaccines that passively transfer antibodies to the general circulation," says Limberis.
"There's a long history of using antibodies for cancer and autoimmune disease, but only two have been approved for infectious diseases," notes Tretikova. "This novel technique has allowed for the development of a prophylactic passive vaccine that is cost effective, easily administered, and quickly manufactured."
The team is working with various stakeholders to accelerate the development of this product for pandemic flu and to explore the potential of AAV vectors as generic delivery vehicles for countermeasures of biological and chemical weapons.