( Each year 200000 Americans find out t...)Why do aneurysms form? New studies suggest leading role for white blood cells

But despite the deadly toll of this condition, called abdominal aortic aneurysm or AAA ("triple A"), experts know very little about why it happens. Now, a pair of papers from researchers at the University of Michigan Cardiovascular Center is shedding new light on the mystery.

In the July 12 issue of the journal Circulation, U-M researchers report that a certain type of white blood cell called a neutrophil appears to be crucial to AAA formation.

Neutrophils are the "first responders" of the body's immune system, appearing rapidly on the scene of an infection or injury to an area of tissue, and sounding the alarm to summon other immune cells to join the fight. They are key to the process of inflammation by which the body fights off insults from bacteria, viruses and injuries.

The two new papers show for the first time that neutrophils are important in the very early stages of AAA formation, when the aorta wall begins to weaken and bulge. The researchers think the cells may somehow act in combination with other AAA risk factors such as smoking, high blood pressure and inherited genetic vulnerability.

The U-M researchers made the findings in laboratory experiments on mice that had been treated to wipe out their neutrophils temporarily, and in rats that lacked the protein that lets neutrophils attach to tissue and start the "first aid" process. The researchers exposed a small part of the rodent aortas to a chemical that breaks down blood vessel tissue, then studied what happened in the rodents that had altered neutrophil systems and in comparison rodents that were normal.

In both cases, says senior author and U-M vascular surgeon Gilbert Upchurch, M.D., th
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Source:University of Michigan Health System