Scientists have solved a long-running conundrum connecting high levels of air pollution with an increased risk of heart attacks and strokes.
Writing in the Journal of Clinical Investigation, Gokhan Mutlu and colleagues from Northwestern University in Illinois found that mice exposed to airbourne particulate matter formed blood clots much more quickly than normal. Tests on the animals' blood showed higher levels of the clotting chemical fibrinogen and elevated coagulation factors II, VIII and X. To find out what was triggering this effect the team tested mice lacking the gene for an immune signalling hormone called IL6. Surprisingly these animals were insensitive to the effects of pollution.
Next the team gave normal mice a drug called clodronate to remove a population of cells known as macrophages from theirlungs. These cells are phagocytes, meaning that they can ingest foreign inhaled material and when they do so they activate and pump out chemical signals including IL6. The clodronate-treated animals also showed little response to pollution exposure, like their IL6-lacking counterparts. This suggests that when particulate matter in polluted air enters the lungs it is picked up by macrophages, causing them to activate. The macrophages then pump out IL6, which provokes increases in blood coagulation factors and makes blood much stickier and more likely to clot, which in turn increases the chances of heart attacks and strokes.