Abstract 17039: Neutrophil Subset in Blood Contributes to Acute Oxidant Signals in the Bone Marrow, Which Regulate Innate Immune Responses After Ischemic Tissue Damage

Abstract

Blood neutrophils are short-lived and have distinct subsets. Aged (senescent) neutrophils are cleared by macrophages in multiple organs including bone marrow (BM), at least, in the steady state. It is unknown, after ischemic tissue damage, how these aged-type neutrophils regulate bone marrow microenvironment and impact on downstream bone marrow responses. In a mouse model of critical limb ischemia, we found that CD62L lo aged-type neutrophils that return to the BM, measured by in vivo circulating cell-labeling, are increased (2.1-fold) after acute limb ischemia, which is corresponded with acute increase in CXCR4, a major chemokine receptor promoting BM returning of blood neutrophils (31%). Interestingly, neutrophil uptake by macrophages is reduced (by 76%) in the BM after limb ischemia, suggesting increased retention of blood-derived CD62L lo neutrophils in BM microenvironment after ischemia. We also found, in the BM, that blood-derived neutrophils have higher reactive oxygen species (ROS) than other neutrophils (61% in MFI) and that limb ischemia indeed increases ROS (34%) and oxidized lipid microparticles (61%) in BM plasma in a Nox2 NADPH oxidase-dependent manner. These suggest that increased returning and retention of blood-derived CD62L lo neutrophils in the BM may contribute to oxidant signals in the BM after limb ischemia. Indeed, using aged neutrophils generated ex vivo and adoptive transfer of these cells, we found that aged neutrophils modulate BM innate immune responses after limb ischemia through ROS—reduced activation of hematopoietic stem progenitor cells (50%); increased acute activation of myeloid progenitor cells (4%); and increased acute but inhibited later Ly6C hi inflammatory monocytes/macrophages in the ischemic tissue (72% and 45%, respectively)—resulting in improved tissue recovery from ischemic damage (2.3-fold). In summary, an aged-type neutrophil subset in blood contributes to acute oxidant signals in the BM, which coordinate innate immune responses to promote tissue recovery from ischemic damage.