Viruses entrust their most fundamental function - reproduction -
to the host cells they infect. But it turns out this highly
economical approach also creates vulnerability.
Researchers at Rockefeller University, with input from a team at
the Medical Research Council's National Institute for Research
(NIMR; now part of the Francis Crick Institute), have found an
unexpected way the immune system exploits the flu virus's
dependence on its host's machinery to create new viruses capable of
spreading infection. This discovery suggests a new approach to
combating flu.
"Influenza A, the virus we studied, relies on a host's
protein-cutting machinery to put the final touches on new viral
particles. Our research has shown that the host immune system
fights back by turning off this machinery," said Professor Charles
Rice of Rockefeller University. "This concept, that a host would
inhibit its own protein-cutting enzymes in order to fight off a
virus, is entirely new."
The research reveals a new function for the well-studied protein
known as PAI-1 (plasminogen activator inhibitor 1) as the key to
this defensive strategy. PAI-1 shuts down proteases, which are
enzymes that break the chemical bonds within protein molecules.
PAI-1 is best known for inhibiting proteases involved in the break
down of blood clots. After seeing evidence of a new role for PAI-1,
the researchers found that human and mouse cells unable to properly
produce it appeared more vulnerable to infection by influenza A. In
experiments, they used the subtype H1N1, a derivative of the 1918
pandemic flu and a member of a large family of flu viruses that
include seasonal flu.
A cell infected by a virus releases chemical signals known as
interferons, which turn up the volume on a legion of defensive
genes. "The hundreds of host proteins produced by these
interferon-stimulated genes are like an army. We know that,
together, they can effectively defend against a viral infection,
but we don't know how the individual soldiers fight back,
particularly those that interfere with later stages of viral
replication, when the virus exits the cell and spreads the
infection," said Dr Meike Dittmann, also of Rockefeller
University.
They started out by testing a large suite of genes activated by
interferon and introduced these individual genes into cells, then
infected the cells with the flu. They then watched to see which
genes blocked the ability of influenza to spread. As expected,
numerous genes inhibited late stages of infection, but one stood
out: SERPINE1, the gene that codes for PAI-1.
Given what was already known about PAI-1, Dr Dittmann suspected
how it might help cells fight flu.
"A virus attacks a cell using fusion proteins, and if these
don't work properly, new virus particles get out of an infected
cell just fine, but they cannot spread the infection to other
cells. Proteases activate fusion proteins by clipping them, but on
its own influenza A doesn't have the gene for the protease it
needs. As a result, the virus relies on the host proteases to do
the job," Dr Dittmann said.
Subsequent experiments confirmed PAI-1 did indeed prevent the
cutting of the fusion protein, known as hemagglutinin, and that
high levels of PAI-1 prevented the virus from producing particles
capable of spreading the infection. Furthermore, mice that lacked
the gene for PAI-1 generally fared worse than their peers when
infected with the influenza A virus. Experiments led by Andreas
Wack of NIMR used tissues cultured from mice's tracheae to confirm
PAI-1's role in fighting off the infection.
Human cells were the final step. Cells derived from patients
with mutations in SERPINE1 were infected and accumulated higher
loads of the virus than cells derived from people without mutations
in the gene.
Dr Andreas Wack of NIMR said: "This was a great collaboration as
we were able to build a bridge between Dittmann's findings on cell
lines in the lab and what was seen in PAI-1 deficient animals,
thereby contributing to the discovery of a novel way to fight virus
infection. PAI-1 has been studied for a long time but was never
linked to influenza. Since PAI-1 is a member of a larger family of
protease inhibitors and many viruses rely on host proteases during
their life cycle, this will become an interesting field for future
antiviral strategies."
The paper, A
Serpin Shapes the Extracellular Environment to Prevent Influenza A
Virus Maturation, is published inCell.