Introduction
Diabetic retinopathy (DR), a retinal neurodegenerative and microvascular disease, is one of the leading causes of severe but preventable vision loss in the working-age population.1 Approximately 285 million people worldwide have diabetes, and this number is expected to reach 300 million by 2025.2 Retinal neurodegeneration and subsequent microvascular deterioration are the basic pathogenic changes in DR, but their mutual relationship remains elusive.3 About one-third of patients with diabetes show signs of DR, and one-third of these patients can be categorized as vision-threatened due to severe non-proliferative diabetic retinopathy (PDR), PDR, or diabetic macular edema (DME).4 PDR and DME, which are the major complications of DR, can both lead to severe visual impairment.5 For several decades, the gold standard for the management of advanced DR has been laser photocoagulation; however, recent reports have shown that intravitreal injection of antivascular endothelial growth factor (VEGF) agents is highly effective in ameliorating DME6 and is hence being widely used currently.
The worldwide increase in the use of anti-VEGF agents against DR has ushered in the so-called anti-VEGF era. Among the anti-VEGF agents, the fusion protein aflibercept stands out due to its higher binding affinity for VEGF.7 Aflibercept is a soluble VEGF decoy receptor generated with the trap technology and binds several members of the VEGF family: VEGF-A, VEGF-B and placental growth factor (PlGF), which activate the VEGF receptor-1 (VEGFR-1). Therefore, aflibercept can further inhibit VEGFR-1 signaling by trapping the PlGF ligand. The signaling pathway of VEGFR-1 modulates vascular development, angiogenesis, cell survival, inflammation, and chemotaxis of inflammatory cells. However, current clinical indications for aflibercept are mainly limited to advanced-stage diseases such as severe DME, which is characterized by leakage from the capillary endothelium. Prior to the development of vascular proliferation or widespread non-perfusion, blood glucose control was considered the only measure for delaying the progression of DR to advanced stages. However, several recent clinical trials have shown that anti-VEGF agents, including aflibercept, are highly effective at improving not only DME but also the Diabetic Retinopathy Severity Scale (DRSS) scores or retinal non-perfusion, a condition mainly involving the microvasculature,6 8–11 thus suggesting the possibility that aflibercept may exert beneficial effects on microvascular structures such as retinal endothelial cells or pericytes. Recently, it has been demonstrated that aflibercept regulates retinal inflammation elicited by high glucose via the PlGF/extracellular signal-regulated kinase pathway, and the effects of anti-VEGF agents such as aflibercept are related to molecular interactions and stabilizing energy.12 13 If the DR pathogenic cascade could be impeded earlier before the degree of vascular destruction becomes irreversible, more viable vessels can be salvaged and can halt the progression to advanced stages.
Recent pieces of evidence suggested that the primary diabetic retinal manifestation is neurodegeneration.3 14 Then, the early stages of vasculopathic DR likely start with a loss of retinal pericytes and consequent disruption of the blood–retinal barrier (BRB), which then gradually leads to retinal capillary hyperpermeability and occlusion.15 16 Since pericytes are located adjacent to blood vessels and regulate endothelial cell properties and contribute to the stability and maintenance of blood vessels, the loss of retinal pericytes is considered to play a critical role in the early pathogenesis of DR.17–19 Pericyte loss has also been experimentally observed as an initial structural change in DR using streptozotocin (STZ)-induced murine model of diabetes.20 21
Together with pericyte loss, retinal inflammation has also been highlighted as a contributing factor to the pathology of DR. Inflammatory changes associated with DR include increased inflammatory cytokine expression, macrophage infiltration, leukocyte adhesion and infiltration, complement activation, and acute phase reactant expression, which collectively lead to the breakdown of the BRB and compromised perfusion.22 These observations strongly suggest that DR may be an inflammatory or a parainflammatory disease,23 and it has been shown that such inflammatory cascades are accelerated by the upregulation of VEGF.24
Here, using an STZ-induced murine model of diabetes, we examined the protective effects of aflibercept against microvascular structural changes that occur in the early stage of DR, such as pericyte loss, hyperpermeability, perfusion blockage, and inflammation.