In what is being heralded as a breakthrough in research to find a universal flu drug, scientists in the US have identified a small family of human monoclonal antibodies that can neutralize an unprecedented range of influenza A viruses, including the bird flu virus (H5N1), previous pandemic viruses (such as the 1918 Spanish flu that killed millions) and some seasonal flu viruses.
The scientists showed that the antibodies were effective at protecting mice from illness. They said that while more tests are needed, because large quantities of monoclonal antibodies don’t take long to make, this family of flu-fighting monoclonal antibodies could be combined with antiviral drugs to prevent or treat flu during an outbreak or pandemic.
The study was the work of researchers at the Dana-Farber Cancer Institute and Harvard Medical School, both in Boston, Massachusetts, the Burnham Institute for Medical Research, in La Jolla, California, and the US Centers for Disease Control and Prevention in Atlanta, Georgia, and is published in the 22 February online issue of Nature Structural & Molecular Biology.
The lead investigator was Dr Wayne Marasco, associate professor of medicine at the Dana-Farber Cancer Institute and Harvard Medical School. The National Institute of Allergy and Infectious Diseases (NIAID) of the National Institutes of Health and the Centers for Disease Control and Prevention sponsored the research.
NIAID director Dr Anthony S Fauci told the press that:
“This is an elegant research finding that holds considerable promise for further development into a medical tool to treat and prevent seasonal as well as pandemic influenza.”
“In the event of an influenza pandemic, human monoclonal antibodies could be an important adjunct to antiviral drugs to contain the outbreak until a vaccine becomes available,” he added.
Current estimates suggest initial doses of a new vaccine against a flu pandemic would take four to six weeks to produce.
The flu virus remains a serious health threat; it evades surveillance by the immune system because by the time the immune system has figured out what it is and how to fight it, the virus has mutated.
An important part of the work that Marasco and colleagues did on this study was to describe the detailed atomic structure of a section of the flu virus that the monoclonal antibodies they produced bound to. This is in a hidden part of the virus, located in the “neck” below the peanut-shaped “head” of the hemagglutinin (HA) protein. HA and neuraminidase are two main proteins found on the surface of the virus.
Another important discovery was that once the antibodies bound to this part of the virus, it couldn’t change shape, which prevented it from fusing and entering host cells and causing infection. This was the key to the neutralizing power of these antibodies.
There are 16 known subtypes of HA proteins and influenza A viruses can include any of them. The HA proteins fall into two groups, Group 1 and Group 2. Group 1 has 10 of the known HA subtypes and Group 2 has 6.
At first the researchers worked on avian flu viruses. They scanned tens of billions of monoclonal antibodies produced by viruses that infect bacteria (bacteriophages) and found 10 of them were active against the four main strains of H5N1 bird flu virus. When they tested them in cell cultures and mice they showed these were also able to neutralize other known influenza type A viruses.
The monoclonal antibodies that Marasco and colleagues found neutralized all testable viruses that contained the 10 Group 1 HA proteins, including the H1 virus that caused the 1918 Spanish flu and the H5 bird flu types. However, they didn’t work against any of the viruses containing the Group 2 HAs.
Co-author Dr Ruben Donis, chief of the Molecular Virology and Vaccines Branch at CDC said:
“Our human monoclonal antibody protected mice from the lethal H5N1 virus even when injected three days after infection.”
“This is good news, but many antibodies can do this. What surprised us is that the same antibody protected mice from a lethal infection with a very different virus such as the H1N1 subtype that causes seasonal human infections; this is really remarkable,” he explained.
The work on examining the atomic structure started with an investigation into one of the monoclonal antibodies bound to the H5N1 HA. The researchers found that an arm of the antibody reaches into a genetically stable pocket in the neck of the HA protein and it is this that stops the virus changing shape and being able to fuse with the membrane of a host cell and get inside it.
Marasco and colleagues then looked at more than 6,000 other genetic sequences of the 16 Group 1 and Group 2 HA subtypes and found that within a group the pockets were similar, but the groups were quite different to each other.
They speculated that the pockets are genetically stable because they are an “evolutionary constraint” that enable the virus to fuse with the cell (if it lost this ability it would lose its main survival advantage). This could also explain why they couldn’t find any “escape mutants”, these are viruses that gain an advantage and escape the antibodies by mutating into a form that they can’t bind with any more.
Marasco explained:
“One of the most remarkable findings of our work is that we identified a highly conserved region in the neck of the influenza hemagglutinin protein to which humans rarely make antibodies.”
“We believe this is because the head of the hemagglutinin protein acts as a decoy by constantly undergoing mutation and thereby attracting the immune system to produce antibodies against it rather than against the pocket in the neck of the protein.”
Co-author Dr Robert Liddington, professor and director, Infectious and Inflammatory Disease Center at Burnham, explained:
“The head portion of hemaglutinin is highly mutable, leading to the rise of forms of the virus that can evade neutralizing antibodies.”
“However, the stem [neck] region of hemaglutinin is highly conserved because it undergoes a dramatic conformational change to allow entry of viral RNA into the host cell. It’s very difficult to get a mutation that doesn’t destroy that function, which explains why we aren’t seeing escape mutants and why these antibodies neutralize such a variety of strains of influenza,” he added.
The researchers think these findings could also help scientists developing flu vaccines. Current flu vaccines attack the mutating head of the HA protein. Marasco said if the vaccine could be adapted so it attacked the genetically invariant neck of the HA protein it might then confer a more durable lifelong immunity.
Marasco said the monoclonal antibodies they identified are ready for “advanced preclinical testing”. He is arranging to test them in ferrets (the gold standard for animal testing for flu drugs) and then develop a clinical grade version of one of them to prepare for human clinical trials which could be as soon as 18 months later. If that trial is successful and the antibodies proven to be safe in humans, it could still take several years to develop a product that is approved.
“Structural and functional bases for broad-spectrum neutralization of avian and human influenza A viruses.”
Jianhua Sui, William C Hwang, Sandra Perez, Ge Wei, Daniel Aird, Li-mei Chen, Eugenio Santelli, Boguslaw Stec, Greg Cadwell, Maryam Ali, Hongquan Wan, Akikazu Murakami, Anuradha Yammanuru, Thomas Han, Nancy J Cox, Laurie A Bankston, Ruben O Donis, Robert C Liddington & Wayne A Marasco.
Nature Structural & Molecular Biology Published online: 22 February 2009.
doi:10.1038/nsmb.1566
Sources: Journal abstract, NIH/National Institute of Allergy and Infectious Diseases, Dana-Farber Cancer Institute.
Written by: Catharine Paddock, PhD