Aged Neutrophils in Sickle Cell Disease Impede Ischemic Repair

INTRODUCTION: Repeated bout of tissue ischemia is a hallmark of sickle cell disease (SCD). Neutrophils are critical first responder cells that initiate host response to sterile injury, including ischemia. Of the numerous neutrophils in circulation, a small proportion undergo physiologic changes (termed aging) which directs their homing to the bone marrow for clearance. How neutrophil homeostasis is regulated in sickle cell disease (SCD) to influence ischemic repair is unknown.

HYPOTHESIS: We hypothesized that SCD may alter the homeostasis and function of aged neutrophil.

METHODS: Aged neutrophils from the Townes humanized sickle cell (SS) mice were compared to aged neutrophils from MerTK deficient mice (MerKO), which age in the circulation owing to defective efferocytosis. Hind limb ischemia (HLI) was used as model of vascular injury. LASER Doppler perfusion imaging (LDPI) measured perfusion recovery after HLI.

RESULTS: Compared to wild-type mice (7 ± 1.2%), the proportion of CXCR4^HiCD62L^lo aged neutrophils in circulation was markedly elevated in the SS (34 ± 4%) and MerKO mice (38 ± 4%). However, in contrast with aged neutrophils from MerKO mice, those from SS mice showed impaired phagocytosis, increased capacity to produce reactive oxygen species, and pronounced release of extracellular traps (NETs). In response to HLI, perfusion recovery in the MerKO and wild type mice were similar, but this was dramatically impaired in SS mice. Importantly, the phenotype of SS mice was reversed by treatment with the anti-oxidant N-acetyl cysteine (NAC), but NAC therapy had no impact on MerKO mice.

CONCLUSIONS: Although SS and MerKO mice demonstrated elevated numbers of aged neutrophils, our data suggest that oxidant stress causes aged CXCR4^HiCD62L^lo neutrophils in SS to impede vascular repair after ischemia. This specialized subset of neutrophils may be targeted to improve vascular health in SCD.