We investigate how cell death affects the local tissue environment and in particular how necrosis drives sterile inflammation. We have a special interest in the pathogenesis of atherosclerotic plaques, which are characterized by chronic inflammation and an accumulation of apoptotic and necrotic cells. Much of our work focuses on the key inflammatory cytokine IL-1α, which acts as a powerful danger signal upon release from dead cells. However, our recent work has shown that an elaborate molecular system controls IL-1α in a conditional and cell type-dependent manner to prevent inappropriate activation.
Cell death in Atherosclerosis
Atherosclerosis is a chronic inflammatory disease characterised by lipid accumulation in the vessel wall, inflammation and death of multiple cell types, including macrophages, vascular smooth muscle cells (VSMC) and lymphocytes. Both apoptosis and necrosis occur in advanced plaques at higher levels than in normal vessels, and levels are increased in patients with unstable angina. However it is unclear whether dying cells drive atherosclerosis, or simply occur as a response to it.
Therefore, a major focus of our work is to elucidate the role of cell death, particularly VSMC death, in both initiating and progressing atherosclerosis. The loss of VSMCs from plaques is particularly bad because they maintain the structural extracellular matrix that stabilises plaques. Furthermore, we also believe that the cellular interactions and reactions that occur between adjacent normal VSMCs and dying VSMCs may provide key signals that perpetuate the inflammation associated with atherosclerosis. Specifically, the release of IL-1α from necrotic VSMCs is able to induce local and systemic vessel inflammation.
IL-1α and IL-1R2
Necrosis can induce profound inflammation or be clinically silent. However the mechanisms underlying such tissue specificity are unknown. IL-1α is a key danger signal released upon necrosis that exerts effects on both innate and adaptive immunity, and is considered to be constitutively active. In contrast, we have shown that necrosis-induced IL-1α activity is tightly controlled in a cell type-specific manner. Most cell-types examined expressed a cytosolic IL-1 receptor 2 (IL-1R2) whose binding to pro-IL-1α inhibited its cytokine activity. In cell-types exhibiting a silent necrotic phenotype IL-1R2 remained associated with pro-IL-1α. Cell-types possessing inflammatory necrotic phenotypes either lacked IL-1R2 or had activated caspase-1 before necrosis, which degraded and dissociated IL-1R2 from pro-IL-1α. Full IL-1α activity required cleavage by calpain following necrosis, which increased its affinity for IL-1 receptor 1. These cell type-dependent processes fundamentally govern IL-1α-activity post-necrosis. We believe changes in the level of intracellular IL-1R2 may underlie many chronic inflammatory diseases, including atherosclerosis.
Phagocytosis of dead cells
The final stage of cell death is the removal of the remnant cell debris by cells capable of phagocytosis (the specific internalisation and digestion of dead cells). This process removes over 50 billion cells daily and is essential in maintaining tissue homeostasis. Indeed, failure to phagocytose dead cells is a hallmark of many auto-inflammatory and auto-immune diseases. Atherosclerotic plaques show evidence of failed phagocytosis, and we believe this is an important factor in driving inflammation. Failed phagocytosis leads to secondary necrosis and the leakage of intracellular contents, in particular IL-1α, which can provide powerful activation signals that induce inflammation and plaque growth.