The role of immune-related myeloid cells in angiogenesis

Macrophage function is not restricted to the innate and adaptive immune responses, but also includes host defence, wound healing, angiogenesis and homeostatic processes. Within the spectrum of macrophage activation there are two extremes: M1 classically activated macrophages which have a pro-inflammatory phenotype, and M2 alternatively activated macrophages which are pro-angiogenic and anti-inflammatory. An important property of macrophages is their plasticity to switch from one phenotype to the other and they can be defined in their polarisation state at any point between the two extremes. In order to determine what stage of activation macrophages are in, it is essential to profile various phenotypic markers for their identification. This review describes the angiogenic role for myeloid cells: circulating monocytes, Tie-2 expressing monocytes (TEMs), myeloid-derived suppressor cells (MDSCs), tumour associated macrophages (TAMs), and neutrophils. Each cell type is discussed by phenotype, roles within angiogenesis and possible targets as a cell therapy. In addition, we also refer to our own research on myeloid angiogenic cells (MACs), outlining their ability to induce angiogenesis and their similarities to alternatively activated M2 macrophages. MACs significantly contribute to vascular repair through paracrine mechanisms as they lack the capacity to differentiate into endothelial cells. Since MACs also retain plasticity, phenotypic changes can occur according to disease states and the surrounding microenvironment. This pro-angiogenic potential of MACs could be harnessed as a novel cellular therapy for the treatment of ischaemic diseases, such as diabetic retinopathy, hind limb ischaemia and myocardial infarction; however, caution needs to be taken when MACs are delivered into an inflammatory milieu.