Inflammasomes – mediators of renal inflammation in hypertension

Hypertension is the most important known risk factor for the two leading causes of death worldwide, namely myocardial infarction and stroke. Despite widespread prescribing and use of anti-hypertensive therapies across society, the prevalence of hypertension continues to rise. This is largely because in over 90% of cases the cause of hypertension is unknown, and our current non-targeted treatment approaches are ineffective in up to 30% of hypertensive patients. Therefore, it is imperative to ascertain the underlying mechanisms involved in the pathogenesis of hypertension to identify new drug targets that will allow us to treat the condition more effectively. Hypertension is now widely accepted to be a chronic low-grade inflammatory disease, with the kidneys representing a primary site of elevated inflammatory molecules and immune cell accumulation. Inflammasomes are multimeric complexes that act as caspase-1 activating platforms to drive the maturation and production of the classic pro-inflammatory interleukin (IL)-1 family of cytokines, IL-1β and IL-18. The release of these cytokines into the extracellular space allows them to act on their respective receptors present on neighbouring effector cells such as other macrophages, T-cells, endothelial cells, epithelial cells, etc. This induces further production of other pro-inflammatory cytokines such as IL-6, IL-17 and IFN-γ, which in turn act to propagate and amplify the inflammatory response. Previously, it has been shown that patients with hypertension have increased circulating levels of IL-1β and IL-18. However, whether this occurs downstream of inflammasome activity and whether IL-1β and/or IL-18 play a causative role in the development of elevated blood pressure (BP) is unknown. This thesis investigates the roles of inflammasomes and signalling by IL-1 family cytokines in the development of hypertension and its associated renal pathologies. In Chapter 3, we utilized a model of one-kidney (1K)/deoxycorticosterone acetate (DOCA)/salt in mice to show that hypertension is associated with an increase in mRNA expression of the various subunits of the NLRP3 inflammasome (NLRP3, ASC and pro-caspase-1) along with pro-IL-1β, indicative of inflammasome priming. We also demonstrated increased protein expression of the cleaved (active) caspase-1 p10 fragment and the mature IL-1β p17 subunit, suggesting that inflammasome activity is also higher in the kidneys of hypertensive mice. Critically, mice genetically deficient in the inflammasome adaptor protein, ASC, were protected not only from the chronic pressor, renal inflammatory and renal-fibrotic effects of 1K/DOCA/salt, but also from the hypertensive effects of an alternative stimulus, namely angiotensin II. Hence, data from this study provided proof-of-concept that inflammasome activity is crucial for the development of hypertension and a rationale for our subsequent pre-clinical evaluations of the potential anti-hypertensive effect of pharmacotherapies known to inhibit inflammasome/IL-1β signalling. In Chapters 4 and 5, we investigated the effects of intervention therapies with two inhibitors of the inflammasome/IL-1 signalling pathway in mice with established hypertension. First we used anakinra, a clinically-relevant human IL-1 receptor antagonist used for the treatment of classical inflammatory conditions such as rheumatoid arthritis and gout. Although anakinra was found to reduce systolic BP and collagen deposition in the kidneys of hypertensive mice; it had only modest effects in reducing renal inflammation and may even have exacerbated renal hypertrophy. By contrast, a novel small-molecule, MCC950, that directly inhibits NLRP3 assembly, thereby abrogating signalling downstream of both IL-1β and IL-18, was highly effective at inhibiting renal inflammation and leukocyte influx in hypertensive mice, along with reducing BP. MCC950 also appeared to partially reverse the impairment in kidney function that normally accompanies the development of hypertension in this model. These findings, along with work published by other groups during the course of my investigations, have provided strong evidence that the inflammasome/IL-1 axis is a crucial mediator of chronic inflammation and elevated BP in hypertension and a promising candidate as a target for future anti-hypertensive therapies.