Does hypoxia drive alveolar macrophages to potentiate fibrosis in idiopathic pulmonary fibrosis?

Abstract

<b>Background:</b> Tissue hypoxia is implicated as a contributing factor in the progressive fibrosis seen in idiopathic pulmonary fibrosis (IPF). IPF lung tissue contains hypoxia markers and patients experience hypoxaemia. Although hypoxia drives fibroproliferation in epithelial cells and fibroblasts, the effect on the homeostatic function of macrophages is unknown. Hypothesis: Hypoxia impairs alveolar macrophage (AM) function and promotes the profibrotic phenotype seen in IPF AMs. <b>Aim:</b> To model the effects of hypoxia on AM function. <b>Methods:</b> Human AMs isolated by lavage of normal lung tissue (n=4) from non-smokers undergoing cancer resection, were cultured for 48 hours in hypoxia (1% O₂, 5% CO₂) or normoxia (21% O₂, 5% CO₂). AM viability and phagocytosis were measured by flow cytometry using Annexin V/Propidium Iodide staining and heat-killed Streptococcus pneumoniae respectively. mRNA was measured by reverse transcriptase qPCR. <b>Results:</b> Hypoxia impaired AM phagocytosis by 46%, whilst viability was unaffected. mRNA expression of vascular endothelial growth factor A (VEGF-A) was increased in hypoxia. However, matrix metalloproteinase (MMP)-9 and MMP-7; pro-resolution factor, transforming growth factor β; pro-inflammatory cytokines, tumour necrosis factor and interleukin 1β, were unchanged in hypoxic conditions. <b>Conclusion:</b> These results show that hypoxia impairs AM phagocytosis. Consequently, in hypoxic IPF lung, bacterial clearance may be reduced, increasing acute exacerbation risk. Upregulated VEGFA expression by hypoxic macrophages may also be a pro-fibrotic driver.&nbsp;Further work is needed to explore the mechanism of these changes, and ascertain effects on the lung environment.