Most influenza viruses enter human or animal cells via specialised routes on their surfaces. Researchers at the University of Zurich have revealed that certain human and avian influenza viruses may infect cells via a second entrance mechanism, an immune system protein complex.This ability helps the viruses infect different species – and potentially jump between animals and humans.
The majority of type A influenza viruses found in birds and pigs do not pose a health concern to people. However, the viruses might represent a problem if there is an outbreak, such as the one presently occurring in dairy cattle in the United States, or during seasonal epidemics. In rare situations, a virus can spread from animals to people, potentially causing a worldwide pandemic.
Most influenza viruses enter host cells by using their envelope proteins, which stand up from the surface like spikes. The so-called hemagglutinin binds to sialic acid, a chemical group on the surface of human cells and the cells of various animal species. An international research team led by Professor Silke Stertz from the Institute of Medical Virology at the University of Zurich (UZH) has now shown that flu viruses also have a second method to infect host cells. “Human influenza A viruses of subtype H2N2 and related H2N2 avian influenza viruses can enter cells through a second receptor. They use an alternative entry pathway,” says Stertz.
The researchers found that hemagglutinin also binds to MHC class II protein complexes. These complexes on the surface of certain immune and respiratory cells are responsible for differentiating between the body’s own cells and foreign cells. “We found that MHC class II complexes in humans, pigs, ducks, swans and chickens allow viruses to enter cells, but not those in bats,” says Stertz.