A Linkage between Angiogenesis and Inflammation in Neovascular Age-Related Macular Degeneration

Abstract

Age-related macular degeneration (AMD) is the leading cause of visual impairment in the aging population with a limited understanding of its pathogenesis and the number of patients are all the time increasing. AMD is classified into two main forms: dry and neovascular AMD (nAMD). Dry AMD is the most prevalent form (80-90%) of AMD cases. Neovascular AMD (10-20% of AMD cases) is treated with monthly or more sparsely given intravitreal anti-vascular endothelial growth factor inhibitors, but unfortunately, not all patients respond to the current treatments. A clinical hallmark of nAMD is choroidal neovascularization. The progression of AMD is initially characterized by atrophic alterations in the retinal pigment epithelium, as well as the formation of lysosomal lipofuscin and extracellular drusen deposits. Cellular damage caused by chronic oxidative stress, protein aggregation and inflammatory processes may lead to advanced geographic atrophy and/or choroidal neovascularization and fibrosis. Currently, it is not fully known why different AMD phenotypes develop. In this review, we connect angiogenesis and inflammatory regulators in the development of nAMD and discuss therapy challenges and hopes.

Keywords: aggregation; aging; angiogenesis; degeneration; inflammation; macula.

Figure 1

Fundus photographs (upper panel) and optical coherent tomographs (lower panel) from healthy individual, early AMD with pigment mottling (black arrows), intermediate dAMD with drusen (white arrows), GA (blue arrows) and nAMD with choidal neovascularization and hemorrhage (red arrows) that coincide with intra- (yellow arrow) and subretinal (green arrow) fluids. Black column indicates neural retina and asterisk the RPE layer that both are atrophic or missing in GA. Abbreviations: AMD, age-related macular degeneration; BM, Bruch’s membrane; Ch, choriocapillaris; d, dry; GA, geographic atrophy; n, neovascular and RPE, retinal pigment epithelium. Note that BM (red line) is not optically recognized right when pathology increases and thus it starts to wave.

Figure 2

Mechanism of action of anti-VEGF treatments currently used to treat neovascular age related macular edema. Activation of VEGFR2 by VEGF-A plays a major role during angiogenesis, the role of VEGFR1 and VEGFR3 during neovascularization is less pronounced. Binding of ANG2 to TIE2 and Integrin α5β1, αvβ5, α3β1 and αvβ3 will further. All the antiangiogenic agents used to treat age related macular degeneration, binding to VEGF-A and prevent its signaling. Aflibercept, which is a fusion protein, will also bind to PLGF and VEGF-B. Faricimab instead is a bi-specific antibody and will bind to both ANG2 and VEGF-A. Whether faricimab will inhibit binding of ANG2 to integrins is less well characterized than its capability to intervene with ANG2-TIE2 signaling. Both ranibizumab and brolucizumab antibody fragments that inhibit VEGF-A signaling. Brolucizumab is a humanized monoclonal single-chain Fv (scFv) antibody fragment and ranibizumab is a humanized monoclonal antibody fragment. VEGF-C and VEGF-D are ligands for both VEGFR-3 and VEGFR-2 and may mediate pro-angiogenic signals even in the presence of previously mentioned anti-VEGF treatments. Abbreviations: PLGF, placental growth factor; VEGF, vascular endothelial growth factor; ANG, angiopoietin; VEGFR, vascular endothelial growth factor receptor, TIE, tyrosine kinase with Ig and EGF homology domains.

Figure 3

Mechanisms behind age related macular degeneration. (A) Age, light exposure, smoking, high-fat diet and unknown factors contribute to elevated oxidative stress, mitochondrial dysfunctions as well as decreased phagocytosis, autophagy and toxin clearance mechanisms in RPE cells. This leads to accumulation of intra- and extra-cellular waste products such as intracellular light absorbing lipofuscin and extracellular drusens. Affected retina may eventually face structural changes such as, RPE degeneration, photoreceptor loss, BrM thickening, choriocapillary degeneration, low-level inflammation and fibrosis. Activated microglia migrate to the outer nuclear layer to remove rod cell debris and, at the same time, may kill adjacent photoreceptors. (B) During an angiogenic switch, inflammatory cell recruitment is initiated, and as newly formed vessels are leaky, they contribute to retinal edema, hemorrhage and therefore further potentiate the pathological wound healing response. In nAMD, there exist many mechanisms how neovascularization and low-level inflammation can be stimulated. For example, damaged RPE produces excessive amounts of growth factors, which stimulate vascular growth. In addition, BrM thickening and drusens increase distance of RPE cells from choroidal vasculature and thus increases hypoxia, which in turn will upregulate pro-angiogenic and pro-inflammatory growth factors. Interestingly, drusens also consist of pro-inflammatory components, such as amyloid structures and C3a. Furthermore, accumulating immune cells may secrete inflammatory cytokines and angiogenic growth factors and therefore support the disease progression. (C) In nAMD, fibrosis is the product of defective and excessive wound healing response. It is believed that pro-inflammatory cytokines can promote differentiation and activation of matrix-producing myofibroblasts (e.g., EndMT and EMT). Multiple cell types, including fibroblast, fibrocytes, macrophages, RPE cells and endothelial cells, may potentially participate in this process. Mesenchymal cells in subretinal fibrotic lesions can for example, originate from the retinal pigment epithelium and/or choroidal endothelial cells through EMT and EndMT. In addition, macrophages are able to transdifferentiate to myofibroblasts through MMT. RPE cells and infiltrating macrophages are believed to be a major source of these cytokines. Moreover, hypoxia may promote EMT in RPE cells. Abbreviations: RPE, retinal pigment epithelium; EMT, epithelial to mesenchymal transition; C3a, complement component 3a; C5a, complement component 5a; CFB, complement factor B; MAC, membrane attack complex; VEGF-A, vascular endothelial growth factor A; EMT, epithelial–mesenchymal transition; EndMT, endothelial–mesenchymal transition; MMT, macrophage-mesenchyman-transition.

Figure 4

Illustration of a blood vessel in quiescent and angiogenic state. Cross-section of a capillary, fenestrated capillary and blood vessel during neovascularization. During vascular homeostasis, endothelial cells are connected by tight junctions and covered by both basement membrane and pericytes. Growth factors such as VEGF-A and ANG2 induce angiogenic switch that will lead to weakening of endothelial tight junctions, pericyte dropout and initiation of angiogenesis. Endothelial destabilization will further lead to vascular leakage and leukocyte transmigration. Abbreviations: VEGF, vascular endothelial growth factor; ANG, angiopoietin.