Atherosclerosis is instigated, driven and worsened by inflammation. However, the processes that drive this and cause chronic inflammation are not well understood, but likely involve interplay between macrophages, endothelial cells (ECs) and vascular smooth muscle (VSMCs). Macrophages, ECs and VSMCs all undergo senescence in advanced plaques, which results in the establishment of the senescence-associated secretory phenotype (SASP). The SASP is driven by membrane-bound IL-1α that drives the perpetual secretion of IL-6, IL-8, MCP-1 and numerous other inflammatory cytokines and growth factors that likely potentiate plaque growth. However, how the cytokine IL-1α, which lacks a signal peptide or a transmembrane domain, is tethered to the cell surface is unknown.
Using flow cytometry we show that senescent fibroblasts bear IL-1α on the cell surface. Similarly, resting human and murine macrophages express IL-1α on the cell surface, with expression significantly upregulated upon LPS treatment. However, significant false-positive staining can occur without the use of a robust dead cell gate. Furthermore, the level of membrane IL-1α is significantly lower in IL-1R1-/- or IL-1R2-/- macrophages compared to wild-type, indicating that cell surface IL-1α may in fact be tethered to its cognate receptors IL-1R1 and IL-1R2. In addition, we find human and mouse macrophages do not respond to IL-1α or IL-1β treatment, despite expression of IL-1R1 and response to LPS that shares the same signalling pathway.
We believe the SASP could be a key driver of the chronic inflammation witnessed in vascular disease. Therefore, the elucidation of mechanisms that control SASPs are important to help identify new targets that could specifically target vascular inflammation whilst sparing normal host defence
