Podocytes were blocked with 3% BSA in PBS. the tubular c-Fms-IN-1 epithelial compartment. NLRP3 mRNA and protein expression were transiently induced in HPTC by TGF-1 but subsequently diminished over time as cells lost their epithelial phenotype in a process regulated by transcription and ubiquitin-mediated degradation. Consistent with the data, low NLRP3 mRNA expression in kidney biopsies was associated with a linear trend of higher risk of composite endpoint of doubling serum creatinine and c-Fms-IN-1 end stage renal disease in patients with IgAN. Taken together, these data show that NLRP3 is primarily a kidney tubule-expressed protein that decreases in abundance in progressive IgAN. NLRP3 (NOD-like receptor, pyrin domain-containing 3) is a member of the NOD-like receptor (NLR) of innate immune genes. NLRP3 is best known for its role as a component of the inflammasome, which is a multi-protein caspase activating platform that regulates a variety of host defense pathways in response to pathogen or damage-associated molecular patterns (PAMPs or DAMPs, respectively). The NLRP3 inflammasome regulates caspase-1 activation which in turn regulates the maturation and secretion of pro-inflammatory cytokines such as IL-1 and IL-181,2. There is growing evidence for inflammasome-independent or non-canonical roles for NLRP3 in the kidney tubular epithelial injury and fibrosis3,4,5,6,7. Given these properties, NLRP3 not surprisingly has been implicated in the pathogenesis of numerous kidney diseases at the experimental level including ischemia/reperfusion injury, unilateral ureteric obstruction, diabetic nephropathy, calcium oxalate-induced renal injury, diet-induced nephropathy and hyperhomocysteinemia3,6,8,9,10,11,12,13. However, despite the increasing number of reports describing a role for NLRP3 in animal renal injury models, the characterization of NLRP3 in the context of human kidney diseases remains largely unexplored. IgA nephropathy (IgAN) is the most common type of primary glomerulonephritis in the world14. Although previously thought to have a relatively benign course, it is now known that 20C40% of patients with IgAN progress to end stage renal disease (ESRD) within 20 years15,16. IgAN is characterized by galactose-deficient IgA1 immune complex deposition in the glomerular mesangium that leads to activation of the complement cascade and other immunologic processes stimulating cell proliferation and secretion of growth factors, proinflammatory and profibrotic cytokines17,18. Glomerular inflammation leads to injury of podocytes and proximal tubular epithelial cells causing a sequelae of glomerulosclerosis, tubular injury/atrophy, and interstitial fibrosis. The mechanisms that initiate renal tubular injury downstream of glomerular inflammation remain poorly understood but tubular injury and interstitial fibrosis are critical to IgAN progression and remain the strongest pathologic predictors of disease outcome19. Given the current understanding of c-Fms-IN-1 NLRP3 in the regulation of inflammation, tubular epithelial cell injury and fibrosis, the possibility exists that NLRP3 may be associated with the progressive chronic kidney disease induced by IgAN. While NLRP3 is well known to be expressed in macrophages, its biology in kidney disease c-Fms-IN-1 is believed to be largely non-canonical and dependent on non-hematopoietic cellular compartments5,6. Studies have demonstrated NLRP3 expression in tubular epithelial cells as well as podocytes that plays an important role in experimental disease pathogenesis3,7,10,20,21. Collectively, these data suggest that the biology of the NLRP3 in the kidney may differ from the canonical inflammasome pathway described in macrophages and other non-renal disease CD80 models that rely primarily caspase-1 activation and cytokine maturation22. Despite these observations, the cellular localization and characterization of NLRP3 in the human kidney or a temporal relationship to human kidney disease has yet to be confirmed. We previously demonstrated increased NLRP3 mRNA in kidney biopsies from a variety of nondiabetic kidney diseases including IgAN6. In this extension of our prior work, we employed human nephrectomy samples, kidney disease biopsies and primary tubular epithelial cells to characterize NLRP3 in the context of the human kidney and IgAN, a common chronic human kidney disease. Results NLRP3 c-Fms-IN-1 localizes primarily to the tubular epithelium in the human kidney NLRP3 localization in the human kidney has been inconclusive with reported localization at podocytes and tubular epithelial cells7,10,20,21. To clarify NLRP3 localization in the human kidney, cryosections and paraffin embedded sections of histologically normal tissue obtained from human kidney nephrectomies were stained with immunoperoxidase or processed for indirect immunofluorescence (IF) and confocal microscopy. In normal kidney tissue stained with immunoperoxidase, NLRP3 localized primarily to tubules whereas platelet derived.