Termed neutrophil extracellular traps (NETs), these web-like structures are primarily released to capture and kill certain pathogenic microorganisms. However, the authors now show that NETs also form in atherosclerotic lesions and lead to the production proinflammatory cytokines and the accumulation of pathogenic immune cells in the lesion.
The findings shed light on the processes that promote artherosclerosis, a disease in which plaque builds up inside a patient's arteries. Atherosclerosis is among the most widespread of chronic inflammatory diseases and is a major cause of death in Western countries.
Dr Veni Papayannopoulos of the Crick (currently based at Mill Hill) explained: "Like many chronic inflammatory diseases, atherosclerosis is driven by molecules called proinflammatory cytokines. In atherosclerosis, these proteins orchestrate the continuous recruitment of immune cells to the arterial walls, leading to the build up of cholesterol-containing lesions that can obstruct blood flow."
"One of the key challenges in understanding inflammatory diseases that are not caused by infections is to discover what triggers the production of these inflammatory cytokines."
Previous work had showed that when the concentration of certain lipids increases in our blood, cholesterol crystals are produced. These crystals activate a process in immune cells called macrophages that is required to process proinflammatory cytokines into their mature and active form. However, it remained unclear how these molecules are produced in the first place.
The team used a variety of methods to shed light on the topic. First, they isolated human neutrophils from healthy individuals and showed that cholesterol crystals, trigger neutrophils to release NETs.
They then placed mice on a high fat western diet to promote artherosclerosis and isolated their aortas (the main artery in their hearts) to measure the size of the artherosclerotic plaques and to look for the presence of NETs. Lastly, the team measured the concentrations of proinflammatory cytokines and inflammatory immune cells in these plaques and in the blood of the mice.
The research showed that exposure of neutrophils to cholesterol crystals promotes the release of NETs, which activate macrophages to produce proinflammatory cytokines and recruit additional disease-causing immune cells to the lesion.
However, mice with a genetic mutation that hampered the ability to release NETs in response to cholesterol crystals, and mice receiving treatments that degrade NETs, had substantially lower concentrations of proinflammatory cytokines and smaller lesions covering their aortas.
Dr Annika Warnatsch of the Crick (also at Mill Hill) said: "These findings explain how cholesterol crystals and other internal signals are sufficient to promote inflammation in the absence of infection.
"Our work sheds light on the mechanisms that drive atherosclerosis and potentially and other chronic inflammatory diseases and suggests new strategies for their treatment."