By David N. Leff
Beavers (Castor canadensis) build their dams across streams with twigs, branches and logs (from which they've eaten the bark), sealed by gravel and mud. The water behind these barriers backs up to form lakes or ponds.
Humans (Homo sapiens) courting coronary artery disease (CAD) build their dams — atherosclerotic plaques — across arterial blood vessels. The bloodstream behind these coronary artery barriers backs up over time to starve the heart of the blood-borne oxygen and nutrients it needs nonstop to keep pumping.
That stoppage constitutes the greatest single serial killer in the U.S. and the Western world generally. Heart attacks take twice as many lives as cancer, 10 times the toll of accidents.
Often CAD ends in sudden death, but this seeming bolt-from-the-blue event is usually heralded by subtle warnings — CAD risk factors — especially:
* cigarette smoking;
* diabetes mellitus;
* high LDL (low-density lipoprotein) in the blood.
When any of these CAD promoters has sufficiently abused a once-healthy, heart-nourishing coronary artery, the endothelial cells making up that vessel's inner wall signal the immune system for help.
"One of the first steps in human plaque formation," explained clinical cardiologist and vascular biologist Peter Libby, "is the recruitment of leukocytes — macrophages and T cells — to lesioned areas of the artery. Then follows the second phase, lesion progression, when leukocytes and probably other cells release growth factors that stimulate extracellular matrix synthesis, which leads to the growth of the plaque.
"Progression involves formation of the fibrous cap of the plaque and a lipid-rich core. The cap consists of smooth-muscle cells shed from the artery's wall and extracellular matrix connective tissue — collagen and elastin."
Eventually the hidden, growing plaque begins to make its presence known clinically.
"Third comes a phase of complication of atherosclerosis," Libby went on, "when the previously silent lesions will manifest themselves. All too often this takes the form of acute cardiac complication, such as unstable angina pectoris or myocardial infarction.
"There are two ways atherosclerosis can manifest itself clinically," Libby narrated.
"One is usually chronic: the plaque causes blockage of the blood vessel, constricting blood flow. Particularly when exertion puts increased metabolic demand on the heart pump, this can cause myocardial ischemia in the heart. Not enough blood getting to the cardiac muscle, and that causes the intense discomfort known as angina pectoris.
CAD Kills More Ways Than One
"The other way in which atherosclerotic disease can manifest itself is thrombotic complications when a blood clot forms. That doesn't necessarily require a narrow blockage of the blood vessel. In fully a third of patients with CAD," he pointed out, "its first manifestation is sudden death — without the warning of developing angina."
Libby, who is chief of cardiovascular medicine at Harvard University-affiliated Brigham and Women's Hospital, in Boston, is senior author of a research paper in today's Nature, dated July 9, 1998. Its title is "Reduction of atherosclerosis in mice by inhibition of CD40 signaling."
He explained: "The CD40-CD40-ligand signaling system can be most simply thought of as a cytokine, similar to tumor necrosis factor (TNF).
"CD40, which is the receptor, is homologous to the TNF receptor; CD40L [for ligand] is homologous to TNF's ligand. CD40-CD40L occurs in many places. We were able to localize both CD40 and CD40L within advanced human atherosclerotic plaques. Which is one of the reasons," Libby pointed out, "why we undertook the rather arduous set of animal experiments reported in Nature.
"But first," he recounted, "we embarked on a series of in vitro experiments, which showed that CD40 ligation was uniquely capable of activating certain functions of atheroma-associated cells that we believe are important in atherosclerosis. For example, CD40 ligation, but not the usual soluble cytokines, can induce tissue factor, a potent procoagulant. So we believe now that tissue factor is one of the major reasons that clots form in coronary arteries."
Libby's in vivo stand-in for human atherosclerosis is a knockout mouse that builds up high levels of the LDL risk factor molecule, for which it lacks the receptor. He and his co-authors fed these animals a diet high in cholesterol and lipid for 12 weeks, which duly produced plaques in their arteries. These lesions were largely brought on by CD40L-CD40 cell signaling.
Success Doesn't Signal New Drug Candidate
Then, to "cure" the animals, they treated them with an antibody that abolished the CD40L-CD40 activity.
Instead of the minuscule murine coronary arteries, they examined the more wieldy aorta, also assailed with atherosclerotic plaque. Result: The antibody reduced the size of those arterial lesions by 59 percent and their lipid content by 79 percent.
"These data," the Nature paper observed, "support the involvement of inflammatory pathways in atherosclerosis and indicate a role for CD40 signaling during atherogenesis in hyperlipidemic mice."
Does this point toward a CD40-based therapeutic approach to CAD in people?
"I do not believe, I am not advocating," Libby stated, "that we translate interruption of CD40 signaling to a treatment for patients, at this stage.
"We actually have ways of interrupting inflammatory signaling in patients," he pointed out, "that are proven to save lives and reduce heart attacks, but which are underutilized. So from a clinical perspective, as a cardiologist, I'd like to see application of our current knowledge become more widespread, rather than climb on the bandwagon for novel and unproven therapies."
Meanwhile, he and his co-authors have moved on from interrupting CD40 signaling by treatment with a neutralizing antibody to "another important strategy," Libby continued, "which provides further evidence favoring a role for CD40 signaling in this model of atherosclerosis. It involves us in making a compound mutant of an animal that is deficient in the LDL receptor, thus prone to develop atherosclerosis, and of a mouse that is deficient in CD40L.
"Here we have two genetically altered animals. We are now mating these compound knockouts, to ask the same kinds of questions — the same hypothesis as in the Nature paper, namely, an independent method for interfering with CD40 signaling."
Even as he speaks, "the mice are busy at work reproducing. We don't at this point have answers to the experiment.
"No one experiment can change medical practice," Libby observed, "and no one experiment can definitively establish the role of the particular signaling system. We feel," he concluded, "that taking this independent approach to test the same hypothesis is an important step." *