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Scientists have identified a small family of lab-made proteins that neutralize a broad range of influenza A viruses, including the H5N1 avian virus, the 1918 pandemic influenza virus and seasonal H1N1 flu viruses. These human monoclonal antibodies, identical infection-fighting proteins derived from the same cell lineage, also were found to protect mice from illness caused by H5N1 and other influenza A viruses. Because large quantities of monoclonal antibodies can be made relatively quickly, after more testing, these influenza-specific monoclonal antibodies potentially could be used in combination with antiviral drugs to prevent or treat the flu during an influenza outbreak or pandemic.
A report describing the research, supported by the National Institute of Allergy and Infectious Diseases (NIAID) of the National Institutes of Health as well as the Centers for Disease Control and Prevention, appears online today in Nature Structural & Molecular Biology. Wayne Marasco, MD, PhD, associate professor of medicine at the Dana-Farber Cancer Institute and Harvard Medical School in Boston led the research team, which included collaborators from the Burnham Institute for Medical Research in La Jolla, Calif., and the CDC in Atlanta.
"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," notes NIAID director Anthony S. Fauci, MD. "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."
Using standard methods of production, initial doses of a new influenza vaccine to fight pandemic influenza would be expected to take four to six months to produce.
Key to their research, Marasco and his colleagues discovered and described the atomic structure of an obscure but genetically stable region of the influenza virus to which their monoclonal antibodies bind. The hidden part of the influenza virus is in the neck below the peanut-shaped head of the hemagglutinin (HA) protein. HA and neuraminidase are the two main surface proteins on the influenza virus.
The scientists also identified a new mechanism of antibody action against influenza: Once the antibody binds, the virus cannot change its shape, a step required before it can fuse with and enter the cell it is attempting to infect.
Marasco, Jianhua Sui, MD, PhD, and other Dana-Farber colleagues began their study with avian flu viruses. They scanned tens of billions of monoclonal antibodies produced in bacterial viruses, or bacteriophages, and found 10 antibodies active against the four major strains of H5N1 avian influenza viruses. Encouraged by these findings, they collaborated with Ruben O. Donis, PhD, of the CDC Influenza Division, and found that three of these monoclonal antibodies had broader neutralization capabilities when tested in cell cultures and in mice against representative strains of other known influenza A viruses.
Influenza A viruses can include any one of the 16 known subtypes of HA proteins, which fall into two groups, Group 1 and Group 2. Their monoclonal antibodies neutralized all testable viruses containing the 10 Group 1 HAs--which include the seasonal H1 viruses, the H1 virus that caused the 1918 pandemic and the highly pathogenic avian H5 subtypes--but none of the viruses containing the six Group 2 HAs.
Simultaneously, Marasco's group teamed up with Robert C. Liddington, PhD, professor and chair of the Infectious and Inflammatory Disease Center at Burnham, to determine the atomic structure of one of their monoclonal antibodies bound to the H5N1 HA. Their detailed picture shows one arm of the antibody inserted into a genetically stable pocket in the neck of the HA protein, an interaction that blocks the shape change required for membrane fusion and virus entry into the cell.
When they surveyed more than 6,000 available HA genetic sequences of the 16 HA subtypes, they found the pockets to be very similar within each Group but to be significantly different between the two Groups. The genetically stable pockets, they note, may be a result of evolutionary constraints that enable virus-cell fusion. This could also explain why they did not detect so-called escape mutants, viruses that elude the monoclonal antibodies through genetic mutation.
"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," says Marasco. "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."
Their findings could also assist vaccine developers. Current influenza vaccines target the constantly mutating head of the HA protein and do not readily generate antibodies against the conserved region in the neck.
"An important goal is to redirect the immune response of vaccines to this invariable region of the hemagglutinin to try to obtain durable lifelong immunity," Marasco states.
The monoclonal antibodies identified in their paper are very well-characterized, Dr. Marasco notes, and he is optimistic about their further clinical development. "These are fully human monoclonal antibodies that are ready for advanced preclinical testing," he says. He currently is arranging to use NIAID research resources to take the next steps: first, testing the antibodies in ferrets, the gold standard animal model for influenza, and then developing a clinical grade version of one antibody that could enter human clinical trials as soon as 18 months from when the development program begins. Should the antibodies prove safe and effective in humans, it could take several years to develop a licensed product.
Despite the availability of influenza drugs and vaccines, seasonal influenza still kills more than 250,000 people worldwide each year. During seasonal flu outbreaks, monoclonal antibodies could be used to treat individuals with impaired immunity due to pre-existing medical conditions or advanced age. In the event of an influenza pandemic, these individuals plus others at risk--for example, first responders and medical personnel and exposed family members and coworkers--could also benefit from this type of therapy.
Reference: J Sui et al. Structural and functional bases for broad-spectrum neutralization of avian and human influenza A viruses. Nature Structural & Molecular Biology DOI: 10.1038/nsmb.1566 (2009).