Macrophages Are Essential for the Early Wound Healing Response and the Formation of a Fibrovascular Scar

After wounding, multiple cell types interact to form a fibrovascular scar; the formation and cellular origins of these scars are incompletely understood. We used a laser-injury wound model of choroidal neovascularization in the eye to determine the spatiotemporal cellular events that lead to formation of a fibrovascular scar. After laser injury, F4/80+ myeloid cells infiltrate the wound site and induce smooth muscle actin (SMA) expression in adjacent retinal pigment epithelial cells, with subsequent formation of a SMA+NG2+ myofibroblastic scaffold, into which endothelial cells then infiltrate to form a fibrovascular lesion. Cells of the fibrovascular scaffold express the proangiogenic factor IL-1β strongly, whereas retinal pigment epithelial cells are the main source of VEGF-A. Subsequent choroidal neovascularization is limited to the area demarcated by this myofibroblastic scaffold and occurs independently of epithelial- or myeloid-derived VEGF-A. The SMA+NG2+ myofibroblastic cells, F4/80+ macrophages, and adjacent epithelial cells actively proliferate in the early phase of the wound healing response. Cell-lineage tracing experiments suggest that the SMA+NG2+ myofibroblastic scaffold originates from choroidal pericyte-like cells. Targeted ablation of macrophages inhibits the formation of this fibrovascular scaffold, and expression analysis reveals that these macrophages are Arg1+YM1+F4/80+ alternatively activated M2-like macrophages, which do not require IL-4/STAT6 or IL-10 signaling for their activation. Thus, macrophages are essential for the early wound healing response and the formation of a fibrovascular scar.