Background In rheumatoid arthritis (RA), the influx of inflammatory cells as well as the aggressive proliferation of fibroblast-like synovial cells (FLS) outstrips the oxygen supply from blood vessels leading to joint hypoxia. Macrophages accumulate in hypoxic disease sites including RA joints where they possess broad pro-inflammatory, destructive and remodelling potential leading to inflammation and joint destruction. Macrophages respond to hypoxia by up regulating the hypoxia inducible transcription factors– HIF-1 and -2, which leads to the up-regulation of genes involved in proliferation, angiogenesis, and glucose metabolism.
Objectives The objective of our study was to characterise the HIF-1 and HIF-2 expressing macrophage populations in response to joint hypoxia, and in particular to dissect out the effects of HIF-2 in a murine model of arthritis.
Methods Arthroscopy sections (5 microns) from RA patients with mild (~40mmHg), moderate (~15mmHg) & severe (~3mmHg) joint hypoxia were immunostained with anti-HIF 1 and 2 and co-localised with the pan-macrophage marker CD68 as well as other macrophage markers. Murine arthritis using K/BxN serum transfer was induced in 10 – 12 week old mice bearing a targeted deletion of HIF-2 in myeloid cells (HIF2fl/fl; LysM-Cre+’ mice) or control mice (HIF2fl/fl; LysM-Cre-). Mice were given 100ul injections of serum on day 0 and 2 and inflammation was monitored daily.
Results In patients with mildly hypoxic joints, macrophages expressed HIF-1+ and CD147 and were found in small clusters localised to the lining layer. Macrophages in patients with severely hypoxic joints predominately expressed HIF-2+ and the VEGF receptor Flt-1. These macrophages also expressed the angiopoietin receptor ‘Tie2’ and the mannose receptor ‘CD206’ both associated with M2-like macrophages. The significance of HIF-2 was evaluated in a murine arthritis model where macrophages having a genetic deletion of HIF-2 developed less severe arthritis compared to control mice.
Conclusions HIF-2+ macrophages expressing M2-like markers were predominantly expressed in patients with severely hypoxic joints and loss of HIF-2 expression by macrophages in a mouse model of arthritis prevented disease development. Taken together, these findings suggest that HIF-2 is the major hypoxia-inducible transcription factor in synovial macrophages and targeting HIF-2 is likely to be an effective therapeutic strategy.
Disclosure of Interest None Declared