Old Drugs for an Old Pathology? Drug Repurposing for Calcific Aortic Valve Disease

Immunotherapy in the Context of Aortic Valve Diseases

PURPOSE

Aortic valve disease (AVD) affects millions of people around the world, with no pharmacological intervention available. Widely considered a multi-faceted disease comprising both regurgitative pathogenesis, in which retrograde blood flows back through to the left ventricle, and aortic valve stenosis, which is characterized by the thickening, fibrosis, and subsequent mineralization of the aortic valve leaflets, limiting the anterograde flow through the valve, surgical intervention is still the main treatment, which incurs considerable risk to the patient.

RESULTS

Though originally thought of as a passive degeneration of the valve or a congenital malformation that has occurred before birth, the paradigm of AVD is shifting, and research into the inflammatory drivers of valve disease as a potential mechanism to modulate the pathobiology of this life-limiting pathology is taking center stage. Following limited success in mainstay therapeutics such as statins and mineralisation inhibitors, immunomodulatory strategies are being developed. Immune cell therapy has begun to be adopted in the cancer field, in which T cells (chimeric antigen receptor (CAR) T cells) are isolated from the patient, programmed to attack the cancer, and then re-administered to the patient. Within cardiac research, a novel T cell-based therapeutic approach has been developed to target lipid nanoparticles responsible for increasing cardiac fibrosis in a failing heart. With clonally expanded T-cell populations recently identified within the diseased valve, their unique epitope presentation may serve to identify novel targets for the treatment of valve disease.

CONCLUSION

Taken together, targeted T-cell therapy may hold promise as a therapeutic platform to target a multitude of diseases with an autoimmune aspect, and this review aims to frame this in the context of cardiovascular disease, delineating what is currently known in the field, both clinically and translationally.

Uncoupling the Vicious Cycle of Mechanical Stress and Inflammation in Calcific Aortic Valve Disease

Calcific aortic valve disease (CAVD) is a common acquired valvulopathy, which carries a high burden of mortality. Chronic inflammation has been postulated as the predominant pathophysiological process underlying CAVD. So far, no effective medical therapies exist to halt the progression of CAVD. This review aims to outline the known pathways of inflammation and calcification in CAVD, focussing on the critical roles of mechanical stress and mechanosensing in the perpetuation of valvular inflammation. Following initiation of valvular inflammation, dysregulation of proinflammatory and osteoregulatory signalling pathways stimulates endothelial-mesenchymal transition of valvular endothelial cells (VECs) and differentiation of valvular interstitial cells (VICs) into active myofibroblastic and osteoblastic phenotypes, which in turn mediate valvular extracellular matrix remodelling and calcification. Mechanosensitive signalling pathways convert mechanical forces experienced by valve leaflets and circulating cells into biochemical signals and may provide the positive feedback loop that promotes acceleration of disease progression in the advanced stages of CAVD. Mechanosensing is implicated in multiple aspects of CAVD pathophysiology. The mechanosensitive RhoA/ROCK and YAP/TAZ systems are implicated in aortic valve leaflet mineralisation in response to increased substrate stiffness. Exposure of aortic valve leaflets, endothelial cells and platelets to high shear stress results in increased expression of mediators of VIC differentiation. Upregulation of the Piezo1 mechanoreceptor has been demonstrated to promote inflammation in CAVD, which normalises following transcatheter valve replacement. Genetic variants and inhibition of Notch signalling accentuate VIC responses to altered mechanical stresses. The study of mechanosensing pathways has revealed promising insights into the mechanisms that perpetuate inflammation and calcification in CAVD. Mechanotransduction of altered mechanical stresses may provide the sought-after coupling link that drives a vicious cycle of chronic inflammation in CAVD. Mechanosensing pathways may yield promising targets for therapeutic interventions and prognostic biomarkers with the potential to improve the management of CAVD.

Innate and adaptive immunity: the understudied driving force of heart valve disease

Calcific aortic valve disease (CAVD), and its clinical manifestation that is calcific aortic valve stenosis, is the leading cause for valve disease within the developed world, with no current pharmacological treatment available to delay or halt its progression. Characterized by progressive fibrotic remodelling and subsequent pathogenic mineralization of the valve leaflets, valve disease affects 2.5% of the western population, thus highlighting the need for urgent intervention. Whilst the pathobiology of valve disease is complex, involving genetic factors, lipid infiltration, and oxidative damage, the immune system is now being accepted to play a crucial role in pathogenesis and disease continuation. No longer considered a passive degenerative disease, CAVD is understood to be an active inflammatory process, involving a multitude of pro-inflammatory mechanisms, with both the adaptive and the innate immune system underpinning these complex mechanisms. Within the valve, 15% of cells evolve from haemopoietic origin, and this number greatly expands following inflammation, as macrophages, T lymphocytes, B lymphocytes, and innate immune cells infiltrate the valve, promoting further inflammation. Whether chronic immune infiltration or pathogenic clonal expansion of immune cells within the valve or a combination of the two is responsible for disease progression, it is clear that greater understanding of the immune systems role in valve disease is required to inform future treatment strategies for control of CAVD development.