Atypical mechanisms of synthesis, activation and release of interleukin-1β from vascular smooth muscle cells

Atherosclerosis is a widespread disease that often leads to serious conditions such as myocardial infarction and stroke. Chronic inflammation plays an overarching role in its pathophysiology, driving initiation and progression of atherosclerosis in humans and animal models. IL-1β is a potent inflammatory cytokine implicated in atherogenesis and other vascular diseases. However, control of its activation and release has been predominantly studied in macrophages, with scant data on whether vascular smooth muscle cells (VSMCs) can release IL-1β. I investigated the expression, activation and release of IL-1β from primary human VSMCs. VSMCs expressed caspase-1, -4, and -8 under basal conditions, but not caspase-5. Stimulation of VSMCs with IL-1α and/or LPS caused rapid processing of IL-1β to the mature form, which could be detected by Western and with a mature IL-1β-specific ELISA. Although IL-1β production and cleavage was dependent on NF-κβ activation, neither caspase or NLRP3 inflammasome inhibitors prevented generation of mature IL-1β, suggesting caspase- and NLRP3-independent processing. Instead, production of mature IL-1β was blocked by the serin protease inhibitor dichloroisocoumarin, with cathepsin G the most likely enzyme responsible for pro-IL-1β activation. Interestingly, mature IL-1β was never detected in the conditioned media and instead accumulated to high levels inside VSMCs. VSMCs expressed the pore-forming protein Gasdermin D, and the active form of Gasdermin D permeabilised VSMCs, yet mature IL-1β was still not released. Finally, mature IL-1β extracted from VSMCs has 13-fold lower activity than equal amounts of mature IL-1β from Thp1 cells, however the existence of a binding partner or inhibitor of IL-1β is unlikely the cause of this phenomenon. In conclusion, the work presented in this thesis suggests that VSMCs have a novel mechanism that specifically prevents the release of active IL-1β, and that perturbation of this could potentially contribute to vascular inflammation and atherogenesis.