The protein, known as interleukin-2 or IL-2, is a master regulator of the
immune system. It acts as a growth factor for many different kinds of immune
cells, including an all-important class called T cells. These cells can both
recognize and organize attacks against pathogens or tumors.
IL-2 stimulates T cells' proliferation in response to these threats. That
makes it a potent anti-cancer drug. When injected into a patient, it spurs
fierce anti-tumor activity.
"In a substantial subset -- about 7 percent -- of patients with advanced
metastatic melanomas or kidney cancers, IL-2 treatment actually cures the
disease," said Christopher Garcia, professor of molecular and cellular
physiology and of structural biology and the study's senior author. That's an
impressive result, considering the failure of most treatments at such a late
stage of cancer.
IL-2 is also used off-label for various other cancers and a wide range of
other indications including HIV. But its use is restricted because it can cause
severe toxic side effects such as difficulty in breathing due to pulmonary
edema, or swelling of the lung, caused by the buildup of fluid in that organ.
This in turn is the result of leakage from the copious capillaries that permeate
lung tissue, the better to carry away oxygenated blood to distant tissues.
"The cells that cause these toxic effects appear to express different levels
and types of IL-2 receptors than do the cells that produce the therapeutic
effects," Garcia said. The various classes of immune cells activated by IL-2
have their own characteristic receptor complexes for the protein. Accordingly,
each different cell type requires a different concentration of IL-2 for its
activation, and each responds in its own way.
In 2005, Garcia and his colleagues determined the structure of IL-2, making
it possible to visualize its internal features. "We thought we might be able to
tilt the balance of therapeutic-to-toxic effects by modifying this protein in a
way that preferentially trips off activation of a desired immune-cell type,
while minimizing the activation of an unwanted cell type," he said.
For this study, Garcia's group produced a vast variety of mutated versions of
the protein, and then, in a test-tube competition, compared the strength of
these mutant proteins' binding to a particular cell-surface receptor, a process
that is crucial to the T-cell activation needed to treat cancer. The researchers
eventually obtained a mutant that Garcia dubbed "Super-2," which had more than
300 times the receptor-binding strength of natural IL-2. In subsequent tests
designed to assess Super-2's ability to impede tumor growth, the new molecule
outperformed natural IL-2 by a significant margin.
The researchers also tested Super-2 to determine the extent of the side
effects it would cause. To do this, they collaborated with a co-author of the
paper, Onur Boyman, MD, of University Hospital Zurich in Switzerland, who had
previously found that the type of cells in the lung that are responsible for
capillary leakage have receptors for IL-2. Boyman developed an assay for IL-2's
most dose-limiting side effect, pulmonary edema. This assay compares the weight
of lungs from mice treated with a test compound versus lungs that are not thusly
treated. The greater the weight difference, the more fluid buildup has
occurred.
Boyman and a member of his group, Carsten Krieg, PhD, who is one of four
investigators sharing first authorship of the
Nature paper, carried out
all the animal research used for the study. By this assay, pulmonary edema
caused by Super-2 was significantly and substantially less than by natural
IL-2.
Others sharing first authorship of the study were Aron Levin, PhD, (now at
Technion University in Israel) and Darren Bates, PhD, (now a scientist at Amgen)
who were formerly in Garcia's lab, and his MD/PhD student Aaron Ring.
What makes Super-2 so effective, said Garcia, is its altered shape. A
T-cell's IL-2 receptor complex consists of three separate protein components
sitting on the cell's surface. These receptors, sometimes referred to as alpha,
beta and gamma, act in concert: IL-2 must first touch bases with alpha before it
can assume the right shape to bind to beta. Typically, T cells that have never
been activated in the past have vanishingly small amounts of alpha on their
surfaces, and so require high concentrations of IL-2 to get the process
started.
But the mutations Garcia's team induced lock Super-2 into a configuration
whose optimized shape lets it bind directly to beta, bypassing alpha. The
three-dimensional structure of Super-2, together with computer simulations from
the laboratory of associate professor of chemistry Vijay Pande, PhD, suggested
this was because the mutant form of IL-2 was less "floppy" than the natural
form, so that it presented a "tighter" binding surface to the beta receptor.
This souped-up form of the protein was several times as potent as the naturally
occurring form of IL-2 at slowing tumor growth, as measured by assays employing
three different tumor types in culture.
