Activation of the NLRP1 Inflammasome Induces the Pyroptotic Death of Hematopoietic Progenitor Cells

Abstract 1213

The inflammasome is an innate immune complex that recognizes a wide range of pathogen, cellular stress and damage signals. It triggers the Caspase-1-dependent production of inflammatory cytokines including IL-1β and IL-18. Sensors for the inflammasome include the nucleotide-binding oligomerisation domain, leucine-rich repeat (NLR) proteins. The best-characterised of these is NLRP3, which mediates antibacterial, viral, fungal and parasitic immune responses, and is mutated in a spectrum of autoinflammatory diseases. Activation of mammalian NLRs can also result in a Caspase-1-dependent form of cell death termed pyroptosis, the importance of which in disease states remains unclear. For example, it is known that NLRP3 activating mutations in humans can be effectively treated by neutralising IL-1β, suggesting that cell death induced by NLRP3 activation does not play a significant role in pathology. Similarly, the NLRP1b inflammasome is activated by anthrax lethal toxin to cause macrophage pyroptosis, but this does not play a role in anthrax sensitivity in vivo.

Here we define a role for NLRP1 in autoinflammatory disease and the pyroptotic death of hematopoietic progenitor cells. We began by generating two mouse models, one carrying a point mutation in NLRP1a that results in constitutive activation of the protein, and another harbouring a deletion of the entire NLRP1 locus. Animals homozygous for the NRLP1a activating mutation developed a multi-organ neutrophilic inflammatory disease characterised by meningitis, hepatitis, pneumonitis, pancreatitis, pulmonary peri-arteritis, myocarditis and inflammatory bowel disease. Mean survival was approximately 3 months of age. Genetic crosses established that this inflammatory disease was driven by Caspase-1 and IL-1β, but was independent of ASC and Caspase-11, and ameliorated by IL-18. Surprisingly, in the absence of IL-1β-driven inflammation, constitutively active NLRP1a triggered the Caspase-1-dependent death of hematopoietic progenitor cells resulting in leukopenia at steady state.

To evaluate the effect of activated NLRP1a during hematopoietic stress, IL-1 receptor-deficient mice homozygous for the NRLP1a activating mutation were challenged with 5-fluorouracil. Strikingly, these animals succumbed shortly after the nadir of leukopenia at 12 days post-injection. They exhibited hypoplastic bone marrow, lymphopenia, monocytopenia and a deficit of reticulocytes consistent with a functional deficiency in hematopoietic progenitor cells. Conversely, in mice lacking NLRP1, we observed improved recovery of the hematopoietic compartment following hemoablative chemotherapy, with increases in the numbers of platelets, lymphocytes and monocytes relative to littermate controls.

Severe infections are commonly associated with a range of cytopenias including anemia, lymphopenia, neutropenia and thrombocytopenia, however, the etiological triggers for these conditions have not been elucidated. We therefore infected IL-1 receptor-deficient mice homozygous for the NRLP1a activating mutation with lymphocytic choriomeningitis virus (LCMV). Relative to controls, more severe cytopenia and bone marrow hypoplasia was observed. In contrast, NLRP1-deficient animals showed enhanced recovery from LCMV. Our results suggest that activation of the NLRP1 inflammasome in hematopoietic progenitors may contribute to cytopenias induced by hematopoietic stresses such as chemotherapy or infection.