Neutrophils exacerbate tissue damage and inflammation in advanced atherosclerosis by triggering smooth muscle cell (SMC) lysis and death, according to a study published in Nature. Oliver Soehnlein and colleagues show that lesional neutrophils release neutrophil extracellular traps (NETs) that contain histone H4, which interacts with SMCs to induce cell lysis, leading to the destabilization of atherosclerotic plaques.

Credit: Lara Crow/Springer Nature Limited

To study the role of neutrophils during the transition of a stable to an unstable lesion, the researchers used a mouse model of advanced atherosclerosis. “Much to our surprise, we observed striking phenotypes on SMCs rather than macrophages when we manipulated neutrophil counts,” explains Soehnlein. The number of neutrophils in atherosclerotic lesions inversely correlated with SMC counts and fibrous cap thickness, and positively correlated with necrotic core area, lesion size and plaque vulnerability. Decreasing the number of circulating neutrophils in these mice led to an increase in SMC content and fibrous cap thickness in atherosclerotic lesions, resulting in plaque stabilization. By contrast, increasing neutrophil numbers destabilized the plaques. The changes in SMC numbers were due to increased SMC death rather than proliferation. “Therefore, we shifted our focus from a neutrophil–macrophage-centred perspective to the question of how neutrophils regulate SMC survival,” adds Soehnlein.

Using in vivo and in vitro assays, Soehnlein and colleagues showed that activated SMCs in atherosclerotic plaques release chemotactic factors that attract neutrophils and trigger the release of NETs containing histone H4, which has cytotoxic effects on SMCs. Extracellular histone H4 induced a type of non-programmed, receptor-independent death in SMCs via the formation of pores in the plasma membrane, which reduced membrane integrity and resulted in cell lysis. Antibody-mediated neutralization of histone H4 in mice with established atherosclerotic plaques increased SMC content and reduced lesion instability. Finally, the researchers explored therapeutic strategies to target histone H4-mediated cell lysis, and showed that disruption of the histone H4–plasma membrane interaction with a tailored peptide that binds to the N terminus of histone H4 reduced SMC death and increased plaque stability in mice with established atherosclerotic lesions.

“Atherosclerosis is the main focus of our study, but certainly the mechanisms identified here should be applicable to other forms of inflammation in which neutrophil infiltration associates with cell death, such as arthritis, inflammatory bowel disease or dementia,” says Soehnlein. The therapeutic strategy for these chronic inflammatory conditions would involve preventing histone H4-induced cell death, thus reducing tissue damage and inflammation. “In addition, the mechanism of histone H4-induced cytotoxicity could also be explored in a setting in which cell death is warranted, for example, the local delivery of histone peptides in tumours,” he concludes.