Our cells have many defenses against viruses. When cells are infected, they build enzymes
that slow protein synthesis, and thus also slow down viral growth, and they build enzymes to
chop up double-stranded RNA, which is made primarily by viruses. Infected cells also alert
the immune system by displaying pieces of the virus on their surfaces. In the worst cases,
infected cells make the ultimate sacrifice and destroy themselves by apoptosis. Of course,
our cells can't normally be doing all these things--these draconian measures need to be
initiated only when a cell is in trouble. Interferon is the way that cells signal that it's
time to shift into virus-fighting mode.
Interferons are secreted by infected cells to warn their neighbors, and once stimulated,
cells of the immune system secrete interferons as part of their viral surveillance.
Interferons are small proteins that bind to receptors on the cell surface. This signal is
transmitted into the cells and leads to production of hundreds of proteins involved in viral
defense. Several types of interferons are made by our cells. Interferon-alpha and
interferon-beta, shown here from PDB entries
are the most common types,
and are made by most types of cells, especially cells of the immune system. They send a
basic signal to stop growing and focus on defense. Interferon-gamma, shown here from PDB entry
is secreted primarily by T-cells, and sends signals that tune the response
of the immune system.
When interferon was first discovered, it seemed to be the perfect treatment for fighting
viral infection. Since it also slows down the growth of cells, it also seemed like a perfect
treatment for stopping the uncontrolled growth of cancer cells. Interferons are very
specific, though, and only interferon from human cells or other primates is effective for
treatment. So widespread use and testing needed to wait until the 1980s, when methods
for genetic engineering progressed enough to allow production of recombinant interferon.
Today, recombinant interferon is used to treat hepatitis and other
viruses, multiple sclerosis, and a few types of cancer. However, since it has such an extreme effect on cells, it
causes significant side effects, and it is currently only used in specialized cases.
Viruses Fight Back
Viruses are tricky, and as you might expect, they have evolved many ways to fight the
protection that is elicited by interferons. Different viruses block different steps in
interferon action, all the way from the binding of interferon to its receptor to the cascade
of signals that ultimately reach the nucleus. For instance, the protein shown here (PDB entry
is from the virus that causes a smallpox-like disease in mice. It traps
interferon (shown in red) and prevents it from binding to its receptor.