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. Though 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 Pos neutrophils following MI. We found that these cells appear early in the heart after MI, coinciding with the influx of CD101 Neg immature neutrophils. However, SiglecF Pos neutrophils were absent in hematopoietic organs, suggesting their local differentiation. Interestingly, SiglecF Pos neutrophils co-expressed CD101, indicating their maturity. Neutrophil stimulation in vitro with TGF-β and GM-CSF induced co-expression of SiglecF and CD101. When sorted CD101 Neg and CD101 Pos neutrophils were exposed to the same stimuli, only the mature (CD101 Pos ) subset could acquire SiglecF expression, indicating maturation as a prerequisite for SiglecF induction. Using EdU labeling prior to MI to track neutrophil origins in vivo , we found that most SiglecF Pos neutrophils in infarcted hearts were derived from mature circulating neutrophils existing prior to MI. SiglecF Pos neutrophils emerge during heightened inflammation, which drives emergency granulopoiesis in the bone marrow. Neutrophils from mice treated with G-CSF to stimulate emergency granulopoiesis exhibited enhanced capacity for SiglecF expression. Neutrophils exposed to G-CSF in vitro exhibited enhanced 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 Pos neutrophils originate from mature CD101 Pos precursors following MI, and emergency granulopoiesis enhancing their capacity to acquire this phenotype and contributing to neutrophil heterogeneity in cardiac injury. Teaser Abstract: Myocardial infarction provokes neutrophil infiltration and the emergence of a SiglecF Pos subset. Using in vivo EdU labeling and in vitro assays, we demonstrate that SiglecF is acquired by circulating mature neutrophils within the infarcted myocardium, rather than by local maturation of immature precursors. G-CSF and emergency granulopoiesis prime neutrophils with an enhanced capacity to upregulate SiglecF, highlighting a stress-induced program that reshapes neutrophil fate and function after cardiac injury.