SiglecF-expressing neutrophils differentiate from mature neutrophils locally in the heart after myocardial infarction.

Myocardial infarction (MI) triggers an immune response marked by rapid infiltration of neutrophils. Although once considered a uniform population, neutrophils display remarkable heterogeneity post-MI, including a subset expressing SiglecF, a marker typically associated with eosinophils. In this study, we investigated the origin of SiglecF+ neutrophils following MI. We found that these cells appear early in the heart after MI, coinciding with the influx of CD101- immature neutrophils. However, SiglecF+ neutrophils were absent in hematopoietic organs, suggesting their local differentiation. Interestingly, SiglecF+ neutrophils coexpressed CD101, indicating their maturity. Neutrophil stimulation in vitro with transforming growth factor β (TGF-β) and granulocyte-macrophage colony-stimulating factor (GM-CSF) induced coexpression of SiglecF and CD101. When sorted, CD101- and CD101+ neutrophils were exposed to the same stimuli; only the mature (CD101+) subset could acquire SiglecF expression, indicating maturation as a prerequisite for SiglecF induction. Using 5-ethynyl-2'-deoxyuridine (EdU) labeling before MI to track neutrophil origins in vivo, we found that most SiglecF+ neutrophils in infarcted hearts were derived from mature circulating neutrophils existing before MI. SiglecF+ neutrophils emerge during heightened inflammation, which drives emergency granulopoiesis in the bone marrow. Neutrophils from mice treated with granulocyte colony-stimulating factor (G-CSF) to stimulate emergency granulopoiesis exhibited enhanced capacity for SiglecF expression. Exposing neutrophils to G-CSF in vitro increased SiglecF induction by TGF-β, indicating that G-CSF primes neutrophils for SiglecF expression in vivo and ex vivo. These findings support a model in which SiglecF+ neutrophils originate from mature CD101+ precursors following MI, and emergency granulopoiesis enhances their capacity to acquire this phenotype, contributing to neutrophil heterogeneity in MI.