The diverse identity of angiogenic monocytes

The role of bone marrow-derived cells in stimulating angiogenesis, vascular repair or remodelling has been well established, but the nature of the circulating angiogenic cells is still controversial.

The existing literature on different cell types that contribute to angiogenesis in multiple pathologies, most notably ischaemic and tumour angiogenesis, is reviewed, with a focus on subtypes of angiogenic mononuclear cells and their local recruitment and activation.

A large number of different cells of myeloid origin support angiogenesis without incorporating permanently into the newly formed vessel, which distinguishes these circulating angiogenic cells (CAC) from endothelial progenitor cells (EPC). Although CAC frequently express individual endothelial markers, they all share multiple characteristics of monocytes and only express a limited set of discriminative surface markers in the circulation. When cultured ex vivo, or surrounding the angiogenic vessel in vivo, however, many of them acquire similar additional markers, making their discrimination in situ difficult.

Different subsets of monocytes show angiogenic properties, but the distinct microenvironment, in vitro or in vivo, is needed for the development of their pro-angiogenic function.