Monoclonal antibody protects mice from lethal influenza A doses

Scientists have engineered a monoclonal antibody that can protect mice from a lethal dose of influenza A, a new study shows. The new molecule combines the specificity of a mature flu fighter with the broad binding capacity of a more general immune system defender.

The protective effect was enhanced by delivering the antibody in a nasal spray that disperses these molecules throughout the respiratory tract, where they stick to the slippery mucus lining to lie in wait for invading viral particles.

The engineered molecule is based on the IgM version of an immunoglobulin antibody, the generic first line of antibody defense against infection – but with an added structural feature that attaches to a very specific location on the influenza A surface to neutralize the virus.

It's a better platform with a better antibody. If we can prepare the respiratory environment with this enhanced engineered molecule, it can capture and intercept the virus in an early stage. This would be a therapy that not only can be used for prevention of seasonal flu – it potentially could also be used against pandemic strains in the future."

Kai Xu, co-lead author, assistant professor of veterinary biosciences, The Ohio State University

The study was published April 29 in Nature Communications.

Influenza A viruses are divided into subtypes based on characteristics of two major surface proteins, hemagglutinin (HA) and neuraminidase (NA) – as in, for example, the strain annually circulating in people, H1N1, and a less common H3N2 virus that co-circulates during flu season.

Previous work has shown that monoclonal IgG antibodies engineered to target the influenza A HA protein lose potency as the virus mutates its way out of their crosshairs.

These antibodies, mimicking mature natural flu-fighters of the adaptive immune system, attach to a precise section of the surface protein to disable the virus's ability to bind to a host cell receptor and enter the cell to make copies of itself. But once viruses figure out where they're being targeted by antibodies, they mutate those sites to escape recognition by the immune system.

Xu and colleagues theorized that building an anti-flu antibody based on the IgM isotype – an early, first-line immune responder known for its pentameric shape – featuring more structural sites capable of binding to the HA protein as well as an IgG-level tight and precise target, would increase the antibody's neutralization capacity.

The team constructed and evaluated 18 IgM antibodies based on existing IgG antibodies known to target different segments of the HA protein, landing on one IgM version that was superior in cell culture studies at neutralizing a panel of H1N1 and H3N2 flu viruses that have been circulating in people for at least 50 years.

"The advantage is that we utilized the multivalency feature of IgM combined with the potency of a mature broadly neutralizing antibody," Xu said. "Multivalent means that this molecule has multiple arms to capture the virus, so the virus has very little chance to escape.

"And because IgM is a large molecule, when it binds to the viral surface it effectively covers the surface, so it prevents receptor engagement."

Experiments showed that even if a mutation affected this IgM antibody's strong attraction to a specific HA binding site, the antibody's large size and multivalency could help it outpace viral escape attempts.

"By enhancing the capacity for it to recognize the mutated or differentiated viral surface regions, it can cross-protect against different influenza virus infections and can also work even after mutations occur," Xu said.

In mouse experiments, researchers delivered a single dose of the IgM antibody in a nasal spray, intending to place a layer of the engineered antibody across the mucosal surfaces of the nasal cavities and lungs.

"The primary purpose is to refuse viruses before they can get into a host cell," Xu said. "The mucosal immunity is more efficient than systemic immunity at intercepting, so the initial infection can be reduced or eliminated."

Researchers followed the preventive nasal spray with lethal doses of H1N1 and H3N2 flu viruses. Compared to control animals that did not receive preventive treatment, mice receiving the IgM antibody had better outcomes: No mice got sick from the H3N2 strain, and most survived the H1N1 infection and recovered from symptoms.

The molecules remained on the mouse mucosal surface for seven days, an indication that a single spray could last for weeks in humans.

"The adhesiveness of the molecule extends its protection window, and that's one advantage of this strategy," Xu said.

Though this work focused on historic and seasonal versions of influenza A, Xu said the team of structural biologists, immunologists and virologists see the findings as a proof of principle to help establish the IgM platform as a potential therapeutic technology for different viruses – including bird flu, an influenza A H5 virus – and even non-viral diseases like cancer.

Xu co-led the study with Xinli Liu of the University of Houston, Tong-Ming Fu of IGM Biosciences Inc. and Zhiqiang An of the University of Texas Health Science Center at Houston. Additional co-authors from those institutions and the National Institute of Allergy and Infectious Diseases also worked on the study.

This research was supported by grants from the National Institutes of Health, IGM Biosciences and the Welch Foundation.