β2 adrenergic receptors orchestrate neutrophil demargination and recruitment to the ischemic heart following myocardial infarction.

Neutrophils play a crucial role in instigating inflammation as well as its resolution post-myocardial infarction (MI). Although granulopoiesis in the bone marrow (BM) is the major source of cardiac neutrophils post-MI, infiltration of neutrophils to the heart occurs much quicker than peak granulopoiesis. These observations suggest that sources other than granulopoiesis may supply neutrophils to the heart during the early hours post-MI. Using a combination of flow cytometry, BM ablation of hematopoietic stem cells, confocal microscopy and multiple proteomics analysis, we found that the first wave of neutrophils recruited to the ischemic heart is exclusively sourced from vasculature and not from granulopoiesis in the BM/ spleen. The MI-evoked neutrophilia during the early hours bore all hallmarks of demargination induced by classical demarginating agents such as dexamethasone/ norepinephrine (NE). Various pharmacological and genetic strategies aimed at suppressing NE synthesis or disruption of {beta}-AR signaling reduced both neutrophil demargination as well as recruitment to the heart. Interestingly, however, despite a marked reduction in cardiac neutrophil burden only short-term inhibition of {beta}-ARs improved cardiac remodeling and function. Our findings support a pharmacological strategy to contain the initial onslaught of neutrophils on the ischemic heart using {beta}2-AR blockers to regulate the otherwise runaway inflammatory response.