A new study found that an extremely infectious pneumonia-like disease in
humans slips through the immune system's usual defense mechanisms.
The bacterium at fault, Francisella tularensis, causes the disease tularemia.
Also known as rabbit fever, tularemia is fatal in less than 1 percent of treated
cases and in about 5 percent of untreated cases. It is a rare disease with only
about 300 cases per year occurring in the United States .
But the disease can make many people very ill very fast, said Mark Wewers,
the study's lead author and an assistant director of the Davis Heart and Lung
Research Institute at Ohio State
University. He and his colleagues report their findings in this week's
online edition of the Proceedings of the National Academy of Sciences.
In this study the researchers found that, unlike other kinds of bacteria,
Francisella is fully detected by the immune system only after it gets inside a
monocyte, an immune cell whose job is to detect pathogens when they enter the
body. Most pathogens are detected by sensors on the surface of monocytes, and
these cells immediately respond by launching an attack.
However, monocytes don't immediately recognize Francisella as a threat
because the bacteria can bypass those sensors. They cause a reaction only once
they are inside the monocyte.
The fact that Francisella can spread so readily makes it an excellent
possible weapon for bioterrorism, according to some experts.
"We estimate that if a terrorist dropped Francisella on a city it could make
tens of thousands of people seriously ill," said Wewers, who is also a professor
of molecular virology, immunology and medical genetics. "A widespread infection
would put a lot of people out of commission for a long time."
During the Cold War, both the United States and the former U.S.S.R.
stockpiled highly infectious strains of Francisella, Wewers said.
In North America , tularemia most commonly affects hunters and other outdoor
enthusiasts. The bacterium is usually transmitted by ticks or by contact with an
infected animal. Symptoms of tularemia can include high fever, swollen glands,
throat infection, diarrhea and vomiting and large, reddish ulcers on the skin.
Understanding how the human immune system reacts to F. tularensis may help
scientists to better comprehend how the body reacts to other infectious
diseases, such as tuberculosis and the plague. This ultimately could lead to
better treatments.
In laboratory experiments, the researchers infected human blood cells called
monocytes with live cultures of Francisella novicida, a less-infectious form of
Francisella. Monocytes are the "soldiers" at the immune system's frontline –
these cells are the first to react when a pathogen enters the body.
The surface of a monocyte is dotted with structures called Toll-like
receptors. These receptors function like barcode readers in that they scan for
and determine the type of pathogen that has entered the body. The feedback that
these receptors give the monocyte tells the cell how to react to the intruder.
Also on the surface of every monocyte is a tiny opening for a "sack" called a
phagosome. Resembling a garbage bag, in a healthy immune system, the phagosome
consumes disease-causing pathogens, eliminating them.