Retention of the NLRP3 Inflammasome-primed Neutrophils in the Bone Marrow is Essential for Myocardial Infarction-induced Granulopoiesis.

Background: Acute myocardial infarction (MI) results in overzealous production and infiltration of neutrophils to the ischemic heart. This is mediated in-part by granulopoiesis induced by the S100A8/A9-NLRP3-IL-1β signaling axis in injury-exposed neutrophils. Despite the transcriptional upregulation of the NLRP3 inflammasome and associated signaling components in neutrophils, the serum levels of IL-1β, the effector molecule in granulopoiesis was not impacted by MI suggesting that IL-1β is not released systemically. We hypothesize that IL-1β is released locally within the bone marrow (BM) by inflammasome-primed and reverse-migrating neutrophils. Methods: Using a combination of time-dependent parabiosis and flow cytometry techniques, we first characterized the migration patterns of different blood cell types across the parabiotic barrier. We next induced MI in parabiotic mice by permanent ligation of the LAD artery, and examined the ability of injury-exposed neutrophils to permeate the parabiotic barrier and induce granulopoiesis in non-infarcted parabionts. Finally, utilizing multiple neutrophil adoptive and BM transplant studies, we studied the molecular mechanisms that govern reverse migration and retention of the primed neutrophils, IL-1β secretion and granulopoiesis. Cardiac function was assessed by echocardiography. Results: MI promoted greater accumulation of the inflammasome-primed neutrophils in the BM. Introducing a time-dependent parabiotic barrier to the free movement of neutrophils inhibited their ability to stimulate granulopoiesis in the non-infarcted parabionts. Prior priming of the NLRP3 inflammasome is not a prerequisite, but the presence of a functional CXCR4 (C-X-C-motif chemokine receptor 4) on the primed neutrophils and elevated serum S100A8/A9 levels are necessary for homing and retention of the reverse-migrating neutrophils. In the BM, the primed neutrophils secrete IL-1β through formation of gasdermin D pores and, promote granulopoiesis. Pharmacological and/ or genetic strategies aimed at inhibition of neutrophil homing or release of IL-1β in the BM markedly suppressed MI-induced granulopoiesis and, improved cardiac function. Conclusions: Our data reveal a new paradigm of how circulatory cells establish a direct communication between organs by delivering signaling molecules (e.g., IL-1β) directly at the sites of action rather through systemic release. We suggest that this pathway may exist to limit the off-target effects of systemic IL-1β release.