DURHAM, N.C. -- An international team led by Duke University Medical Center researchers has discovered that a genetic variant of an immune system receptor appears to simultaneously dampen the body's immune response to bacteria and other microbial toxins and to provide some protection against atherosclerosis, or clogging of the arteries. The scientists believe their discovery suggests a possible new approach to anti-atherosclerosis drugs.
The genetic variant, or polymorphism, occurs in one of a family of 10 receptors known collectively as "Toll-like receptors" (TLRs), reflecting the fact that the human receptors resemble Toll receptors first discovered in flies. These TLRs -- which are located on the surface of immune cells, heart muscle cells, airway epithelial cells and cells lining blood vessels -- are crucial to immune responses to bacteria. They recognize a specific lipid on the surface of bacteria and provide the first warning to the immune system that an invader is present. The researchers estimate that the polymorphism of the Toll-like receptor 4 (TLR4) they studied occurs in about 10 percent of the population.
"This particular polymorphism appears to attenuate the receptor signaling ability and to diminish the inflammatory response. Our results suggest that the diminished inflammatory response is responsible for a decreased risk of atherosclerosis," said Duke's David Schwartz, M.D., principal investigator for the study, the results of which were published today (July 18, 2002) in the New England Journal of Medicine. "We were quite encouraged that the data were so compelling in the degree to which this polymorphism is so protective.
"This represents a completely new mechanism involved in atherosclerosis and cardiovascular disease, and it provides a potential novel target for decreasing one's chances of developing heart disease," Schwartz continued. "In addition to using traditional means to reduce the risk of heart disease, such as exercise, smoking cessation and lowering cholesterol, in the future, it may also be possible to reduce risk by influencing the activity of this receptor."
Theoretically, an agent or drug could be given to modulate the activity of these receptors, Schwartz said, adding that they will attempt to initiate clinical studies using TLR4 inhibitors on humans at high-risk for developing atherosclerosis in the near future.
Toll-like receptors, which were only discovered two years ago, appear to be the critical link in what is known as the innate immune system, or the body's "first-responders." When activated by a pathogen or bacteria, these receptors stimulate the release of anti-microbial proteins, inflammatory cells and other immune molecules. While this response is effective in battling infections, Schwartz' findings suggest that these agents also appear to play a role in the development of plaques within blood vessels, the hallmark of atherosclerosis.
When the Toll-like receptors were discovered, Schwartz wanted to know if there was a relationship between this innate immune response and the process of atherosclerosis. To determine if such a relationship existed, the team consulted with an investigator in Austria and used data collected during the Bruneck Study, which followed 810 patients in Italy from 1990 to 1995, with another five-year follow-up.
The Bruneck Study was a prospective survey of the natural history of atherosclerosis, and what made the study population interesting to the Duke researchers was that it included detailed information of patients' immunological status, as well as ultrasound studies of the major arteries in the neck.
Of the 810 patients, the Duke researchers found 55 to have the TLR4 polymorphism.
"When compared to the other patients, those with the polymorphism had lower levels of inflammatory immune proteins known as cytokines, as well as other adhesion and inflammatory molecules," Schwartz explained. "These patients also had a substantially higher frequency of bacterial infections."
The researchers then examined the results of three different ultrasound tests used to determine the degree to which the carotid artery -- the main artery supplying oxygen-rich blood to the brain -- was clogged in the affected patients. The degree of clogging, or intimal thickening, is a commonly used indicator for atherosclerosis.
"Whichever measure of atherosclerosis we used, the specific polymorphism emerged as significant protective factor," Schwartz said. "Furthermore, all of these results were virtually unchanged when we statistically adjusted for other common cardio-vascular risk factors."
TLR4 is activated when it encounters lipopolysaccharides -- molecules that include both lipid and sugar components -- on the surface of common bacteria, including Chlamydia pneumonias and Helicobacter pylori, the most common bacteria implicated in atherosclerosis. Lipopolysaccharides, the major component of the cell walls of many common bacteria, are found circulating in variable levels in virtually all healthy humans.
"The diminished levels of inflammation within blood vessels and the enhanced risk of severe bacterial infections found in the patients with the TLR4 polymorphism are clear indications of a dampened innate immune system," Schwartz said. "Conversely, people without the polymorphism would have fewer bacterial infections, but would be at higher risk for atherosclerosis."
The research was supported by the National Institutes of Health (NIH), the Department of Veterans Affairs, the Pustertaler Verein zur Pravention von Herz und HirngefaBerkrankungen, the Sanitatacinhelt Ost, and the Assesaorat fir Gesundheit.
Other members of the team included: from University Clinic, Innsbruck, Austria, Stefan Kiechl, M.D., Markus Reindl, Ph.D., Johann Willeit, M.D., and Christian Wiedermann, M.D.; Eva Lorenz, Ph.D., Wake Forest University Medical School; Friedrich Oberhollenzer, Bruneck Hospital, Italy; and Enzo Bonora, M.D., University of Verona, Italy.
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