Abstract
During skin injury, immune response and repair mechanisms have to be coordinated for rapid skin regeneration and the prevention of microbial infections. Natural Killer (NK) cells infiltrate hypoxic skin lesions and Hypoxia-inducible transcription factors (HIFs) mediate adaptation to low oxygen. We demonstrate that mice lacking the Hypoxia-inducible factor (HIF)-1α isoform in NK cells show impaired release of the cytokines Interferon (IFN)-γ and Granulocyte Macrophage – Colony Stimulating Factor (GM-CSF) as part of a blunted immune response. This accelerates skin angiogenesis and wound healing. Despite rapid wound closure, bactericidal activity and the ability to restrict systemic bacterial infection are impaired. Conversely, forced activation of the HIF pathway supports cytokine release and NK cell-mediated antibacterial defence including direct killing of bacteria by NK cells despite delayed wound closure. Our results identify, HIF-1α in NK cells as a nexus that balances antimicrobial defence versus global repair in the skin.
Introduction
After tissue injury, an adequate immune and repair response are prerequisites for rapid wound closure and the prevention of microbial infections1. Skin injury triggers key components of the skin repair and defence machinery including inflammation and regenerative angiogenesis, that have to be tightly coordinated2. Innate lymphoid cells (ILCs) are an early source of cytokines and particularly, NKp46+ group 1 ILCs (ILC1s), that comprise Natural Killer (NK) cells, guide inflammation and tailor the immune response to the type of the encountered insult3. Whereas the role of skin-resident ILC1s for steady-state skin homoeostasis is increasingly recognised, the significance of infiltrating NK cells for skin repair and antimicrobial defence remains unknown4,5. NK cells directly kill tumour cells and microbes6,7 and secrete cytokines including Interferon y (IFN-γ), Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) and Tumour Necrosis Factor (TNF) to instruct immune responses as well as repair processes3,6,8,9. IFN-γ and GM-CSF drive macrophage activation and favour proinflammatory M1 over a pro-regenerative M2 polarisation1,2. In addition, IFN-γ has been shown to exert anti-angiogenic effects10. Hypoxia is a characteristic feature of the tissue microenvironment during skin repair and bacterial infections, with tissue oxygen tensions lower than 10 mmHg in wounds and necrotic tissue foci11,12. Wound-infiltrating NK cells need to function under such conditions and adapt to low oxygen, which is mediated by Hypoxia-inducible transcription factors (HIFs), with HIF-1 and HIF-2 being the most extensively studied isoforms13,14. HIFs are basic-helix-loop-helix transcription factors that consist of a constitutively expressed β-subunit and an oxygen-regulated α-subunit that is hydroxylated by prolyl hydroxylases in the presence of oxygen and subsequently degraded through the ubiquitin proteasome pathway via interaction with its negative regulator von Hippel-Lindau (VHL) protein15,16. The role of HIF in tumour-infiltrating NK cells is controversial but in general evidence dominates that HIFs are important for NK cell performance in low oxygen environments17,18. Yet, the impact of the hypoxic response in NK cells upon skin injury and infection remains enigmatic. We here sought to elucidate if and how the HIF pathway in NKp46-expressing NK cells modulates wound healing and antimicrobial defence.
Here we show that hypoxic NK cells are a key element to balance antimicrobial defence versus repair mechanisms in the skin. Loss of HIF-1α in NK cells impairs release of IFN-γ and GM-CSF and the mounting of an adequate myeloid cell response. This results in accelerated wound healing but reduces the defence against bacterial skin infections. In conclusion, we propose that adequate antimicrobial defence in the skin is achieved at the expense of a limited repair capacity, whereas acceleration of physiological wound healing comes with a lower response against infections.
