Abstract 13996: Retention of the NLRP3 Inflammasome-Primed Neutrophils in the Bone Marrow is Essential for Myocardial Infarction-Induced Granulopoiesis

Abstract

Background: Excessive infiltration of neutrophils to the ischemic heart after an acute myocardial infarction (MI) is mediated in-part by the S100A8/A9-NLRP3-IL-1β signaling axis in neutrophils. Despite upregulation of NLRP3 inflammasome signaling components in neutrophils, the serum levels of IL-1β, 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 parabiosis techniques, we first characterized the migration patterns of different blood cell types across the parabiotic barrier. We next induced MI in one of the parabiotic mice and examined the ability of injury-exposed neutrophils to permeate the parabiotic barrier and induce granulopoiesis in non-infarcted parabionts. Finally, we studied the molecular mechanisms that govern reverse migration and retention of primed neutrophils, IL-1β secretion and granulopoiesis. Cardiac function was assessed by echocardiography. Results: MI promotes granulopoiesis in the BM and introducing a time-dependent parabiotic barrier to the free movement of neutrophils inhibited granulopoiesis in the non-infarcted parabionts. MI also promotes greater accumulation/ retention of inflammasome-primed neutrophils in the BM in a neutrophil CXCR4 (C-X-C-motif chemokine receptor 4) and serum S100A8/A9 dependent manner. In the BM, the primed neutrophils secrete IL-1β through formation of gasdermin D pores and, promote granulopoiesis. 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 reveals a new paradigm of how circulatory cells can deliver signaling molecules (e.g., IL-1β) directly to the site of action rather than through systemic release. The absolute requirement of signal-2 in the BM for IL-1β release is consistent with our model where neutrophils undergo programming in the infarcted heart and inertly return to the sites of production (i.e., BM) with cytokine cargo (i.e., IL-1β) to regulate granulopoiesis. We suggest that this pathway may exist to limit the off-target effects of systemic IL-1β release.