Identification of a phenotypically and functionally distinct population of long-lived neutrophils in a model of reverse endothelial migration

Recent studies have demonstrated that neutrophils are not a homogenous population of cells. Here, we have identified a subset of human neutrophils with a distinct profile of cell-surface receptors [CD54^high, CXC chemokine receptor 1^low (CXCR1^low)], which represent cells that have migrated through an endothelial monolayer and then re-emerged by reverse transmigration (RT). RT neutrophils, when in contact with endothelium, were rescued from apoptosis, demonstrate functional priming, and were rheologically distinct from neutrophils that had not undergone transendothelial migration. In vivo, 1-2% of peripheral blood neutrophils in patients with systemic inflammation exhibit a RT phenotype. A smaller population existed in healthy donors (≈0.25%). RT neutrophils were distinct from naïve circulatory neutrophils (CD54^low, CXCR1^high) and naïve cells after activation with formyl-Met-Leu-Phe (CD54^low, CXCR1^low). It is important that the RT phenotype (CD54 ^high, CXCR1^low) is also distinct from tissue-resident neutrophils (CD54^low, CXCR1^low). Our results demonstrate that neutrophils can migrate in a retrograde direction across endothelial cells and suggest that a population of tissue-experienced neutrophils with a distinct phenotype and function are present in the peripheral circulation in humans in vivo.