Inflammasomes are intracellular protein complexes and important components of the innate immune system. The best described inflammasome is NLRP3, which contains three major components – nucleotide-binding and oligomerization domain-like receptor, adapter protein apoptosis-associated speck-like protein (ASC) and procaspase-1 [1]. NLRP3 inflammasome activation results in cleavage and activation of inflammatory cytokines, like IL-1β, and inducing inflammatory cell death - pyroptosis [2]. Activation of NLRP3 inflammasome is associated with various diseases, including asbestosis, gout, Alzheimer's disease and auto-inflammatory diseases. In our previous research we showed that NLRP3 inflammasome is activated by soluble amyloid-beta protein oligomers and protofibrils [3]. The aim of this study was to extend the latter research and determine whether oligomeric proteins of different structure activate NLRP3 inflammasome in macrophages.
Human monocytic cell line THP-1, differentiated to macrophages were selected as cell culture model for this study. Cells were treated with various viral oligomeric proteins: filaments forming measles and mumps viral proteins [4, 5]; spherical viral-like particles (VLPs) of these polyomaviruses – Karolinska Institutet polyomavirus (PyV), Merkel cell PyV [6], John Cunningham PyV [7]. MCC950 was used to inhibit NLRP3 inflammasome activation. NLRP3 activation was studied by evaluating cell viability by LDH detection assay, IL-1β and TNF-α cytokine release by ELISA, active caspase-1 detection using fluorescent probe FLICA, which binds to the activated caspase-1, and ASC speck detection using THP-1 macrophages expressing ASC protein fused to green fluorescent protein.
It was found that filament-like nucleocapsid proteins of measles and mumps viruses did not cause any inflammatory response in macrophages. In contrast spherical VLPs of polyomaviruses triggered cell death, induced IL-1β secretion and ASC speck formation in human cell model indicating NLRP3 inflammasome activation. In addition, it was demonstrated that cathepsins are involved in NLRP3 activation induced by polyomavirus VLPs. To conclude, our results demonstrate that viral proteins can activate inflammasome depending on their structural properties.