Mar. 31, 1999 Laboratory researchers have developed a new way to cripple a virulent class of microbes that have become increasingly resistant to antibiotics and continue to kill hundreds of thousands of hospital patients every year.
The discovery, based on experiments with laboratory mice, provides immunization against the Pseudomonas aeruginosa bacterium, suggesting a fresh alternative to antibiotics in the fight against the microbes responsible for most hospital-acquired pneumonia deaths. The bacteria kill half the hospital patients they infect.
The new finding led by scientists at the University of California San Francisco and the Medical College of Wisconsin is reported in the April issue of Nature Medicine. A commentary on the research and its significance appears in the same issue.
Nationally, more than two billion dollars a year are spent combating hospital pneumonia, a condition most often contracted by patients already under intensive care.
The Pseudomonas species targeted in the research is also responsible for the untreatable lung inflammations which devastate the lives of half a million people with cystic fibrosis. In addition, it belongs to the same group of gram-negative bacteria as E. coli, Salmonella, and the microbe responsible for plague, all of which may be candidates for the same immunization strategy developed by the scientific team.
"All these species utilize the same combination of proteins to deliver deadly toxins to host cells," says Jeanine Wiener-Kronish, MD, professor of anesthesiology and medicine UC San Francisco and one of the senior authors on the Nature Medicine paper. "We have developed an antibody against one of these proteins and shown that it blocks Pseudomonas from injecting toxins into lung cells. This antibody may well provide immunization against these other bacteria species too."
Senior author on the Nature Medicine paper is Dara W. Frank, PhD, professor of microbiology and molecular genetics at the Medical College of Wisconsin. The collaboration involved seven scientists from the two institutions, combining basic research in critical patient care and bacterial genetics to develop the antibody and demonstrate its ability to provide immunity against the Pseudomonas bacterium. First author on the paper is Teiji Sawa, MD, PhD, adjunct assistant professor of anesthesiology at UCSF.
The bacterium employs what is known as a type III secretory system to infect lung cells. A combination of at least 25 bacterial proteins work in concert to allow the microbe access to penetrate the outer surface of the host cell and inject toxins into it. The toxins are particularly difficult for the cell to defend against, Wiener-Kronish explained, because they wipe out the primary line of defense -- the macrophages which would normally engulf such invaders.
The research team developed antibodies to five of the proteins in the bacterium's toxin delivery system and tested the ability of each to block the deadly process. One of the five, an antibody to the bacterium's PcrV antigen, prevented the invading microbe from destroying the lung cell macrophages. As a result, the cells could mount a defense against the bacteria and prevent delivery of the toxin. Most importantly, lung cells were protected, the researchers report.
The scientists do not yet know the precise role of the bacterial protein they have blocked. They know it is needed for successful delivery -- or translocation -- of the toxin from the bacterium to the lung tissue, and that it itself is not a toxin, they report.
Immunization could be useful not only against the Pseudomonas-induced pneumonia, but also against the devastating lung inflammations suffered by people with cystic fibrosis, says Wiener-Kronish. These same virulent bacteria cause the chronic inflammations of cystic fibrosis, and sufferers are never able to clear their lungs of the microbes. A systemic approach such as immunization may hold more promise than efforts at control through antibiotics, she says.
The bacteria also cause sepsis, or infection of the blood, by passing from the lungs into the bloodstream, and the scientists hope that sepsis too may be controlled through antibody therapy.
The research group is now working with InterMune Pharmaceutical, Inc. to develop an antibody therapy and a vaccine for use in clinical trials with patients. The scientists are also seeking to better understand how the bacterial type III secretory system works, expecting that further refinements in the immunization strategy might be possible.
"We're still in the development stages, but we think that studying PcrV may well reveal significant information about the process of intoxication by type III systems," said Dara Frank of the Medical College of Wisconsin.
Collaborating in the research and on the paper with Sawa, Frank and Wiener-Kronish were Maria Ohara, MD, and Kiyoyasu Kurahashi MD, both Fellows in anesthesiology and Michael Gropper, MD, PhD, assistant professor of anesthesiology, all at UCSF, and Timothy L. Yahr, PhD, a postdoctoral scientist at Dartmouth Medical School. The research was funded by the National Institutes of Health and the Cystic Fibrosis Foundation.
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