Chlamydia protein mimics heart muscle.

An attempt to unravel how certain viral infections harm the heart may have produced an explanation for the tantalizing link between some bacteria and the development of heart disease. What researchers have stumbled upon is in essence a dangerous case of mistaken identity.

Josef M. Penninger of the Amgen Institute in Toronto and his colleagues have been studying how infections by members of the coxsackie virus family stimulate an animal's immune system to attack its heart. The researchers found that injections of a small fragment of the heart-muscle protein myosin generated heart damage nearly identical to that caused by the viruses. They therefore wondered whether the viruses have proteins structurally similar to the myosin fragment. Such molecular mimicry could explain why the immune system responds to the microbes by attacking the heart.

Yet when Penninger's team searched a database of viral and bacterial proteins, the only match to the myosin fragment was part of a surface molecule made by the bacterium Chlamydia trachomatis. That match intrigued Penninger because another member of the chlamydia family, Chlamydia pneumoniae, has been associated with heart disease (SN: 6/14/97, p. 375). A recent study even suggested that antibiotics might prevent the development of heart attacks (SN: 2/6/99, p. 86).

Penninger found that C. pneumoniae has a surface molecule identical to the one in C. trachomatis that mimics myosin. He and his colleagues even showed that injections of this bacterial protein have a dramatic effect. "We can take a piece of the bacteria, put it into [mice], and give them heart disease," says Penninger. He argues that his group has offered the first proof of a mechanism by which chlamydia bacteria may trigger heart problems.

Epidemiology studies, however, have linked the bacteria to atherosclerosis, a thickening of blood vessel walls, not to a direct immune attack on the heart, comments J. Thomas Grayston of the University of Washington in Seattle. Grayston, who was one of the first scientists to connect C. pneumoniae to heart disease, notes that at least two other theories have been put forth to explain how the bacteria induce heart disease. "There has been lots of speculation about what the mechanism might be," he says.