TIE2-expressing monocytes regulate revascularisation of the ischaemic limb

BackgroundMonocytes (CD14+ cells) expressing the receptor TIE2 are a highly angiogenic subset that are pivotal to neovascularisation in the tumour environment. We hypothesised that TIE2-expressing monocytes (TEMs) are also important in neovascularisation of ischaemic tissues.MethodsFlow cytometry was used to quantify circulating TEMs in 40 patients with critical limb ischaemia (20 age-matched and 20 healthy controls). RT-PCR was used to confirm TIE2 expression in FACS-sorted TEMs. ELISA was used to measure circulating levels of the TIE2 ligand angiopoietin 2 (ANG2). Mice were subjected to hindlimb ischaemia and TEMs quantified. In an additional study, haemopoietic stem/progenitor cells, isolated from Pgk-rtTA-miR-126T transgenic mice, were transduced ex vivo with a TRE-miR-Tie2-OFP lentiviral vector and used to reconstitute lethally irradiated mice. These mice were treated with alternate daily doses of doxycycline to silence Tie2 expression on TEMs, and hindlimb ischaemia (HLI) was induced. Conversely, bone-marrow-derived macrophages (BMDMs) were enforced to express Tie2 using a Pgk-Tie2 lentivirus and delivered into the ischaemic hindlimb. Recovery of ischaemia was measured with laser Doppler.FindingsFlow cytometry revealed a ten-fold higher number of circulating TEMs in patients with critical limb ischaemia than in matched controls (mean 3·52% [SE 0·28] vs 0·39 [0·09], p