Prolonged exposure to hypoxia induces an autophagy-like cell survival program in human neutrophils

Abstract

Neutrophils contribute to low oxygen availability at inflammatory sites through the generation of reactive oxidants. They are also functionally affected by hypoxia, which delays neutrophil apoptosis. However, the eventual fate of neutrophils in hypoxic conditions is unknown and this is important for their effective clearance and the resolution of inflammation. We have monitored the survival and function of normal human neutrophils exposed to hypoxia over a 48 h period. Apoptosis was delayed, and the cells remained intact even at 48 h. However, hypoxia promoted significant changes in neutrophil morphology with the appearance of many new cytoplasmic vesicles, often containing cell material, within 5 hours of exposure to low O 2 . This coincided with an increase in LC3B-II expression, indicative of autophagosome formation and an autophagy-like process. In hypoxic conditions, neutrophils preferentially lost myeloperoxidase, a marker of azurophil granules. Short-term (2 h) hypoxic exposure resulted in sustained potential to generate superoxide when O 2 was restored, but the capacity for oxidant production was lost with longer periods of hypoxia. Phagocytic ability was unchanged by hypoxia, and bacterial killing by neutrophils in both normoxic and hypoxic conditions was substantially diminished after 24 hours. However, pre-exposure to hypoxia resulted in an enhanced ability to kill bacteria by oxidant-independent mechanisms. Our data provide the first evidence for hypoxia as a driver of neutrophil autophagy that can influence the function and ultimate fate of these cells, including their eventual clearance and the resolution of inflammation.

Circulating activated neutrophils in COVID-19: An independent predictor for mechanical ventilation and death

“…However, there may also be untoward consequences from the large influx of neutrophils into a hypoxic environment. Hypoxia has been shown to inhibit spontaneous neutrophil apoptosis (Talla et al, 2019) and is also thought to induce degradation of azurophilic granules with the potential for exacerbation of tissue injury from spillage of granule proteins at hypoxic injury sites. While extended neutrophil survival could be beneficial for pathogen clearance, it is unclear what effects their continuing accumulation could have on an already damaged tissue environment.…”

The Impact of Hypoxia on Neutrophil Degranulation and Consequences for the Host

“…Autophagy, the process by which damaged intracellular components are cleared by a homeostatic degradation pathway, promotes neutrophil degranulation-autophagy-deficient murine neutrophils exhibited reduced secretion of all three major granule subtypes (assessing MPO, lactoferrin, and MMP-9 release) [88]. Hypoxia has been shown to increase expression of LC3B-II, a protein indicative of autophagosome formation, in neutrophils, [89] and, hence, hypoxia-driven autophagy may contribute to increased degranulation. Furthermore, pan-PI3K inhibition reduced the formation of autophagosomes in hypoxic but not normoxic neutrophils, although this effect was only seen at 5 h (81).…”

“…Both phagocytosis-dependent and -independent induction of autophagy have been shown, with uniform upregulation of LC3B and both processes reduced by PI3K inhibition; however, the reliance of phagocytosis-independent autophagy on ROS is less strict [94]. Of note, the aforementioned studies addressing neutrophil degranulation or hypoxia in association with autophagy demonstrated no differences in phagocytic capacity [88,89]. Overall, autophagy, which appears to increase degranulation, can be enhanced by both hypoxia and PI3K signalling, but the balance of interaction between autophagy and ROS in hypoxic situations, and the potential effect this has on degranulation, is complex and requires further clarification.…”

hematological changes in COVID-19 patients; its relations to outcome in a retrospective study in Makkah city

“…Or inhibition of margination [10] or hypoxia. Hypoxia inhibits neutrophil apoptosis and subsequently increases neutrophil numbers [11]. In this work, 61.7%, 58.2%, 59.3%,25%,52.9% of leukocytosis, neutrophilia, monocytosis, eosinophilia, and basophilia were present in the critical cases at presentation.…”