August 27, 2004
Streptococcus Infects Humans by Thwarting Blood Clotting
Streptococcal bacteria may infect humans by using a bacterial enzyme
to “hijack” the blood-clotting system, according to new
research by Howard Hughes Medical Institute scientists.
In studies published in the August 27, 2004, issue of the journal
Science, the researchers establish that the enzyme streptokinase
is responsible for the pathogen's ability to infect humans while
exhibiting little activity against other mammals.
“I didn’t really think that this would work, because it seemed unlikely that, since pathogenicity seemed to be such a complex process, one factor could have such a dramatic effect by itself.”
David Ginsburg
The scientists genetically altered strains of mice to make the
animals susceptible to infection by streptococcus. They say their
strategy outlines a new path for developing animal models for
human-specific microbes. The research is also likely to open the way to
new understanding of the factors that enable bacteria to evolve host
specificity, the researchers said.
Howard Hughes Medical Institute investigator David Ginsburg led the
research team, which included lead author Hongmin Sun and colleagues at
the University of Michigan and Lund University in Sweden.
“Understanding why bacteria in general are so species-specific
has been a major problem for a long time,” said Ginsburg.
“And this species-specificity had greatly hindered our ability to
develop an animal model for human-specific bacteria such as Group A
streptococci, which are an important human pathogen.”
Ginsburg said that Hongmin Sun's achievement of constructing a
transgenic mouse susceptible to streptococcus infection represents a
major step not only in understanding infection by that bacterium, but
in opening the way to similar studies of other bacteria.
In infecting its human host, the group A streptococcus secretes its
own streptokinase, which activates the human form of the enzyme,
plasminogen. Plasminogen, in turn, dissolves blood clots by degrading
the protein, fibrin. A major question was what role streptokinase
played in the bacterium's overall pathogenicity, said Ginsburg.
To develop the “humanized” mouse that would be
vulnerable to bacterial streptokinase, Sun attached the gene for human
plasminogen to a regulatory DNA sequence that normally activates the
gene for a mouse blood protein, albumin. This protein is produced in
large amounts in the animal. The result was a transgenic mouse that
made significant amounts of human plasminogen.
To show that the human plasminogen was functional in the mice, Sun
crossed the transgenic mice with another strain in which their own
plasminogen genes had been deleted. This cross essentially restored
plasminogen function in the resulting mice. In test-tube experiments,
Sun also demonstrated that streptokinase acted on the human plasminogen
from the transgenic mice to dissolve blood clots just as if it were
acting on a human clot.
“The critical experiment, though, was when Hongmin infected
the skin of these transgenic mice with the group A streptococcus
bacteria,” said Ginsburg. “She found that the bacteria were
much, much more toxic to these mice than the normal mice. This fit with
the idea that streptokinase was an important component of the
pathogenicity of strep.
“I didn't really think that this would work, because it seemed
unlikely that, since pathogenicity seemed to be such a complex process,
one factor could have such a dramatic effect by itself,” he
said.
In further experiments, the researchers found that when they removed
the streptokinase gene from group A streptococci bacteria, there was
little difference in their infectivity between normal and the
transgenic mice.
Such studies have led Ginsburg and his colleagues to theorize that
streptokinase “hijacks” the human clot-forming system for
the bacteria's own infective ends. “The theory is that the
bacteria cause a local infection and begin to grow. Many of the
bacterial products, as well as our immune cells, trigger the human
clotting system, which evolved in part as a defense against such
infection,” said Ginsburg. “This system produces clots in
the blood vessels around the infection, closing the highways that the
bacteria would use to spread. However, the bacterial streptokinase
bypasses this system causing the blood clot to dissolve so the bacteria
can spread.”
Sure enough, when the researchers bypassed the clotting defense by
injecting the streptococcus directly into the bloodstream of both
normal and transgenic mice, they both showed similar susceptibility to
infection. In another experiment to demonstrate the defensive
importance of the clotting system, the researchers administered a
substance derived from snake venom that degrades another clotting
protein, fibrinogen, discovering that the treatment greatly increased
the mice's mortality from this streptococcus infection.
Streptokinase's importance to group A streptococci may generalize to
many other human-specific bacteria that have evolved their own
distinctive plasminogen-activating enzymes, said Ginsburg. Also, he
said, the findings highlight the evolutionary arms race between
bacteria and humans.
“Clearly, if we could mutate our plasminogen so it still
worked, yet was resistant to a bacterial streptokinase, it would give
us an advantage,” said Ginsburg. “But then the bacteria
could mutate their streptokinase to keep up. So, you can see how one
bacterial species and one host get locked in this evolutionary dance
and would evolve apart from other host-bacterial pairs — ending up
with a multitude of variants of streptococci, one for each host.
“This evolutionary mechanism probably functions for many other
pathogenicity factors, not just streptokinase, and probably underlies
the species-specificity of all kinds of infectious organisms,”
said Ginsburg.
Such findings also hint that subtle variations in plasminogen genes
among humans could partially explain differences in susceptibility to
certain infection in different people. Thus, he said, his laboratory is
exploring the genetic variations in the blood-clotting system that
might affect risk factors for infection. “Although this is
speculation at this point, it might ultimately be possible to tailor
treatment of infections to the pattern of genetic variability in
clotting genes or other pathogenicity factors,” said
Ginsburg.
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