The researchers developed a system that rapidly identifies regions within influenza viral proteins that undergo genetic mutations and subsequently infect the host, potentially contributing to flu success. The results were published today in the journal _Cell Host & Microbe_.
"Influenza virus evolves and changes very quickly," said senior author Sarah Fortune, PhD, a professor of Microbiology at Penn and a Howard Hughes Medical Institute Investigator. "We have known that the viral genome sequence changes, but we didn't know very well the relationship between the sequence changes and the ability of the virus to grow and transmit in humans.
"In this study, we were able to rapidly identify and map which parts of the viral genome were changing, and associate those changes with the ability to replicate and grow in nasal tissues. This allowed us to identify the areas of the virus we should watch to monitor how it changes and evolves over time, and better understand how certain strains may be more or less successful at becoming seasonal strains or pandemic viruses."
Influenza remains one of the world's most pressing infectious disease threats, causing seasonal epidemics that lead to significant morbidity and mortality globally. Seasonal flu viruses alone are responsible for an estimated 290,000 to 650,000 deaths each year, while pandemic influenza viruses have caused some of the deadliest pandemics in modern history.
The ability of influenza virus to cause disease in humans is largely dependent on viral proteins that interact directly with human host proteins. In particular, the success and transmission of specific strains or variants of influenza depend upon their abilities to bind to cellular receptors on the surfaces of respiratory cells and then replicate inside those cells. While it is well-known that influenza viruses are constantly evolving genetically, researchers still have a limited understanding of the specific molecular mechanisms by which influenza variants exploit the human host range and immune system.
To address this knowledge gap, Fortune's team developed a versatile molecular system to rapidly create thousands of genetically diverse influenza virus variants and then quantify how well each variant can replicate in human respiratory cells. They systematically introduced genetic mutations in two key viral proteins–the hemagglutinin (HA) and neuraminidase (NA) that help the virus enter and exit cells. Next, they screened these large viral mutant libraries for variants that better utilized mutations in host proteins.
"Because influenza virus replicates very fast and grows to high titers, we can do experiments to understand the evolutionary and functional consequences of individual mutations very rapidly, compared to other viruses that may have long generation times or complex growth requirements," said co-senior author Christopher Lazear, PhD, a professor in the Department of Bioinformatics and Biostatistics at CHOP. "We use this as an advantage in our studies, allowing us to perform deep and systematic studies to understand the evolution of the virus."
The study revealed that influenza viruses can effectively exploit naturally occurring variations in human proteins to acquire new functions that enhance their ability to infect nasal cells. Mutations within HA and NA proteins on the surface of the virus were specifically linked to how efficiently the virus was able to enter human nasal cells and replicate therein, both necessary steps in the flu's ability to spread and cause disease.
"These results provide a framework to rapidly dissect the molecular mechanisms that underpin the success and transmission of influenza and, more broadly, of any respiratory pathogen," Fortune said. "Further, our system can uncover host determinants of influenza susceptibility, which could provide novel therapeutic avenues to broadly prevent influenza virus infection."
Other co-authors on the study include: Penn's Katherine Brown, Elizabeth B. Creech, Hannah M. Bartsch, and Scott Hensley; and CHOP's James V. Seeley, Andrew L. Vaughan, and Emily S. Crawford.
The research was supported by National Institute of Allergy and Infectious Diseases (grants NIAID-U19AI118610, NIAID-R01AI120994, NIAID-R21AI141445), the Pew Charitable Trusts, and a Burroughs Wellcome Fund Career Award for Medical Scientists.
