Therapeutic modulation of calcium- and calciprotein particle-induced NLRP3 inflammasome activation in rheumatoid arthritis

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Activation of the NLRP3 inflammasome and subsequent IL-1β release is the most powerful pro-inflammatory mechanism of the innate immune system and has been reported to drive inflammation in numerous pathological scenarios. The calcium sensing receptor is monitoring systemic calcium homeostasis in parathyroid gland and kidney, but it is also strongly expressed in immune cells and has been found to be an important pro-inflammatory mediator in inflammatory diseases. We have reported previously, that calcium-induced signaling via the calcium sensing receptor triggers NLRP3 inflammasome activation in monocytes resulting in the release of high amounts of IL-1β and other Caspase1-dependent cytokines, but also of TNF and IL-6. Recently, we could further elucidate the underlying mechanism by showing, that in the presence of sufficient calcium concentration, calciprotein particles containing phosphate and fetuin A are formed und taken up by monocyte macropinocytosis in a CaSR signaling dependent fashion. Rheumatoid arthritis is one of the most common autoimmune diseases and characterized by a destructive and often debilitating inflammation of small and medium sized joints, and in some patients also of internal organs. Long-term consequences of the disease, in particular in patients with poorly controlled disease activity, include a significantly decreased life expectancy primarily due to an increased frequency of cardiovascular events. In a clinical study, our group could show that ionized calcium induces increased IL-1β responses in monocytes from patients with rheumatoid arthritis, and that RA monocytes take up calciprotein particles at a much higher rate than control monocytes. In this project, we will test various approaches to interfere therapeutically with this pathway, in order to inhibit calcium induced inflammation in RA. First, we will use pharmacologic and genetic methods to inhibit CaSR signaling and investigate the course of autoimmune arthritis under those conditions. In the second approach, arthritic mice will be treated with inhibitors of macropinocytosis in order to modulate the course of arthritis. And thirdly, Cathepsin inhibitors and cathepsin B-deficient mice will be used to investigate lysosomal breakdown of calciprotein particles in arthritis. In addition, we will further investigate the intracellular pathway leading from CPP degradation to NLRP3 activation and IL-1β release. The relevance of each step of this pathway for the initiation and perpetuation of inflammation will be tested using genetic mouse model of arthritis as well as collagen- and collagen-antibody-induced arthritis. At the conclusion of the project, we will have identified the most promising therapeutic approach to the inhibition of calcium induced NLRP3 inflammasome activation in human rheumatoid arthritis.