Summary Bone marrow inflammation induced by the dysregulated innate immune system and inflammasome activation is increasingly indicated to be critical in the pathogenesis and progression of myelodysplastic syndromes (MDS). Despite the significant progress in this field in the past few years, one of the key unresolved questions is: what are the major regulators that mediate the development of bone marrow inflammation. Resolving this question will be critical for the development of novel immunotherapies to treat MDS and prevent its progression, which remains to be a critical unmet medical need. Using a mouse model of double knockout (DKO) of Diap1 and miR146a, both are involved in the innate immune signaling and significantly downregulated in patients with MDS, we revealed that the old DKO mice developed MDS due to the upregulation of pro-inflammatory cytokines in the bone marrow. One of the unique features of the DKO model is that these mice are phenotypically normal at younger age but develop MDS that eventually progress to leukemia with aging, which closely phenocopies the development and progression of MDS in patients. More recently, we showed that genetic depletion of IL-6 in the DKO mice significantly delayed MDS progression. However, IL-6 is one of the highly upregulated cytokines in DKO mice and MDS patients. Loss of IL-6 failed to revert the inflammatory bone marrow microenvironment and many MDS phenotypes in the DKO model. It is essential to reveal the common upstream regulator(s) that mediate the release of inflammatory cytokines. In this respect, IL-6 is a well-known downstream target of IL-1, which is released by inflammation induced pyroptotic cells. Pyroptosis is activated upon inflammasome assembly including NLRP3, which is a downstream target of NF-b and upregulated by the activation of the innate immune signaling. Activation of NLRP3 is a hall mark of MDS independent of mutation status. Upregulated and activated NLRP3 inflammasome activates caspase 1, which in turn cleaves gasdermin D. The cleaved gasdermin D results in increased cell permeability, pyroptosis, and the release of cytokines including IL-1 family. Given its central role in pyroptosis, gasdermin D could also be involved in the pathogenesis and progression of MDS. We therefore crossed DKO mice with gasdermin D knockout mice. Indeed, loss of gasdermin D markedly reverted MDS phenotypes in the DKO mice. Gasdermin D is also highly upregulated in patients with MDS. Our preliminary studies indicate that gasdermin D also mediates inflammation in bone marrow stromal cells. Based on these data, we hypothesize that gasdermin D mediated inflammatory cues and dysregulation of innate immune signaling in hematopoietic cells and bone marrow stromal cells are pivotal in facilitating MDS development and progression. To test this hypothesis, we will perform experiments in the following three aims. In aim 1, we will investigate the hematopoietic cell intrinsic mechanisms of gasdermin ...