Endothelial progenitor cells (EPCs) promote angiogenesis and facilitate vascular repair. Recent clinical trials have demonstrated the feasibility, effectiveness and safety of EPCs for treatment of various ischaemic diseases. However, results from such trials have been inconsistent. This is due to the heterogeneous mix of cells used, as the term EPC has been applied to a broad range of blood derived cells that display endothelial markers and enhance angiogenesis. We have previously characterised two EPC subsets in vitro, Myeloid angiogenic cells (MACs) and Outgrowth endothelial cells (OECs). Although OECs are the EPC subset with progenitor and endothelial characteristics, MACs may still play an important role in angiogenesis indirectly. We have recently shown that cell therapy using MACs significantly enhanced vascular repair in a murine model of retinal ischaemia. This MAC response is mediated by the paracrine release of IL8 since it can be blocked using CXCR receptor inhibitors or a neutralising IL8 antibody. This pro-angiogenic potential of MACs could be harnessed as a novel cellular therapy for the treatment of ischaemic retinopathy but further optimisation of MACs as a cell therapy, including optimisation of the density of cells is required. A co-culture system of MACs with retinal microvascular endothelial cells (RMECs) was used as an in vitro angiogenesis model to determine MACs effects at various densities. An angiogenesis protein array was then used to identify conditioned media components. An in vitro angiogenesis assay demonstrated that MACs significantly increased RMEC tube formation (p<0.01) when used at a ratio of 4/1(RMECs/MACs). However, RMECs co-cultured with a higher density of MACs significantly decreased RMEC tube formation (p<0.01). Analysis of conditioned media by an angiogenesis protein array highlighted upregulation of specific anti-angiogenic factors (PTX3, PAI1, CXCL16) in the high density group, whilst showing a significant decrease in IL8 levels and an imbalance in the MMP9/TIMP1 ratio. Exposure of RMECs to recombinant PTX3 confirmed the anti-angiogenic role of this protein by showing a significant decrease in tube formation (p<0.05) and migration (P<0.01). Furthermore, the anti-angiogenic effect of high density MACs was partly rescued by using an antibody targeted against PTX3 (p<0.01). We have shown that MACs are capable of inducing angiogenesis and act as M2 macrophages. However, at high density, MACs significantly inhibit angiogenesis. This effect is due to secretome changes, and release of negative regulators of angiogenesis, such as PTX3. Therefore, MACs density is an important consideration when using them as a cellular therapy for ischaemic retinopathy.