"For a medical disorder affecting more than a quarter-million Americans each year," said vascular surgeon Thomas Wakefield, "we researchers don't know much more today about what causes blood clots in veins than our predecessors did over 100 years ago." But, he added, "deep-vein thromboses [DVTs] are a serious health problem, especially in the elderly. When blood clots form in deep leg veins, they can permanently damage the venous system, or even be fatal if a clot travels to the lungs."

Wakefield, who practices vascular surgery at the University of Michigan (Ann Arbor, Michigan), was senior author of an article in the November issue of The Journal of Vascular Surgery.

"Selectins are the first up-regulated glycoprotein that gets expressed when we have something that causes inflammation in the body," he said. "It's expressed on endothelial cells, which line the interior blood vessels. Selectin also is expressed on platelets, which are the elements in the blood that helps it cause clot formation."

Until recently, Wakefield said, DVT was thought to be solely a blood or vascular disorder. "Now we have discovered intriguing new evidence to support the idea that the development of blood clots in veins just like blocked arteries in atherosclerosis is an inflammatory process." He added that when a blood clot develops in superficial veins of the leg, the condition called phlebitis, "the redness and swelling of inflammation are visible. When a clot forms deep inside a leg, these signs are hidden, so physicians have rarely connected DVTs with inflammation."

Wakefield said that Denisa Wagner, a pathologist at Harvard Medical School (Boston, Massachusetts) and a co-author of the paper, altered mice genetically to have higher circulating levels of P-selectin. "We were the first group to actually take these mice and do some intervention to them," he noted. "We discovered that the animals with elevated levels of P-selectin made much bigger blood clots. This was a pretty remarkable find. We were seeing 50% to 60% increases in clot size."

That was the first of four types of transgenic mice the researchers used in their study. A second group of animals was altered to lack the gene required to produce P-selectin. A third cohort was rendered unable to express either P-selectin or a related molecule called E-selectin (for endothelial cells). The fourth set of mice consisted of normal controls.

"Mice were surgically treated to induce thrombosis in the inferior vena cava [IVC], the major vein carrying blood from their lower body back up to the heart," Wakefield said. On the second and sixth day after surgery, the researchers measured the size and weight of blood clots in the IVC, examined the vein walls and took blood from mice in all four groups.

Blood from mice with high P-selectin levels also contained microparticles small fragments of cell membrane from degraded cells. "Most of these microparticles came from immune-system leukocytes [white blood cells], but some were delivered from blood platelets, which are responsible for initiating clots," he said. Some platelets may have come from endothelial cells lining the vein walls.

"These hidden clots can do one of two very bad things," Wakefield said. "One is they break off and embolize, go into the lungs and cause a pulmonary embolism, which can kill people." Or else they will eventually destroy the inside surfaces of the veins and their valves. "In this case," he added, "patients will develop typical pains and swellings. As time goes on, these symptoms can lead to discoloration and fibrosis of the skin, even ulcers. So this is a syndrome called 'chronic venous insufficiency.'"

Wakefield said that two very topical areas give rise to venous thrombosis. Prolonged immobilization is associated with development of blood clots. "In the last few years," he said, "there's been a lot of debate about 'coach-class syndrome.' This is where patients develop blood clots after they've made transatlantic flights. We know that if you're immobile for hours at a time, that puts you at risk of the thrombosis."

Wakefield said another pronounced risk factor is advanced age. "The older you get, the more chance you have of developing a blood clot, so age is a significant factor." There are many other factors, such as use of oral contraceptives. "Some people have what we call hypercoagulable state, which means they have a tendency for blood to clot more than others," he said. "Cancer is a very big associated factor for developing blood clots."

He said his goal is to at some point have a way of interfering with the clot formation in the venous system without putting a patient at risk or jeopardy of bleeding. "[That's] the bottom line we're trying to reach." As for human clinical trials, Wakefield said, "at some point we would like to try a clinical compound that inhibits P-selectin." He said that such a compound is "not imminent; perhaps in the next year or two."

He said he was a few weeks away from "knowing whether we can measure these microparticles in patients. "I don't know for sure if it will translate, but I think it will," Wakefield said.