The NLRP3 Inflammasome as a Driver of the MDS Phenotype

Abstract

Myelodysplastic syndromes (MDS) share features of cytological dysplasia and ineffective hematopoiesis while demonstrating profound molecular and genetic heterogeneity. Our investigations show that many of the hallmark features of MDS arise from activation of the NLRP3 inflammasome that directs clonal expansion and pyroptotic cell death, a caspase-1–dependent pro-inflammatory, lytic form of cell death. Independent of genotype, MDS hematopoietic stem and progenitor cells (HSPC) overexpress inflammasome proteins and manifest activated NLRP3 complexes that direct the activation of caspase-1, IL-1β and IL-18 generation, and pyroptosis. NLRP3 is a redox-sensitive, cytosolic sensors of danger signals, which upon stimulation, undergoes a conformational change fostering homotypic pyrin domain interaction with the adaptor protein ASC [apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (CARD)] and ASC polymerization to create large, cytoplasmic filaments. The ASC speck formed by ASC filament clusters creates abundant caspase-1 activation sites, thereby acting as an inflammasome signal amplification complex. Mechanistically, pyroptosis is triggered by the alarmin S100A9 found in excess in MDS HSPC, myeloid-derived suppressor cells (MDSC) and bone marrow plasma, which like founder gene mutations, induces reactive oxygen species (ROS) to initiate cation influx, cell swelling and β-catenin activation. Accordingly, knockdown of caspase-1, neutralization of S100A9 in BM plasma, and pharmacologic inhibition of the NLRP3 inflammasome or NADPH oxidase suppresses pyroptosis, ROS generation and nuclear β-catenin in MDS HSPC and restores effective hematopoiesis. NLRP3 inflammasome activation appears specific to MDS compared to other bone marrow malignancies, with ASC specks detected and quantified in the peripheral blood providing an index of ineffective hematopoiesis and pyroptotic cell death in MDS patients These data indicate that DAMP signals and oncogenic mutations in MDS HSPC license a common redox-sensitive inflammasome platform, and that targeted disruption of the inflammasome signal can restore effective hematopoiesis.