Neutrophils promote inflammatory angiogenesis via release of preformed VEGF in an in vivo corneal model
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We investigated the role of neutrophilic cells (neutrophils) in inflammatory angiogenesis and explored the possible mechanisms involved. Corneal angiogenesis was induced in vivo with a 75% silver nitrate applicator. Depletion of neutrophils was accomplished by the intraperitoneal administration of RB6-8C5, a neutrophil-depleting antibody. Angiogenesis, neutrophil infiltration, and the localization of vascular endothelial growth factor (VEGF) were evaluated by biomicroscopic observations, histology, and immunohistochemistry in control and RB6-8C5 treatment groups. Protein levels of VEGF, macrophage inflammatory protein-1alpha (MIP-1α), macrophage inflammatory protein-2 (MIP-2), and tumor necrosis factor alpha in the cornea were determined by enzyme-linked immunosorbent assay. An in vitro model of neutrophil activation was also used to examine the ability of neutrophils to produce and release VEGF, MIP-1α, and MIP-2. At day 1 after injury, neutrophil infiltration in the cornea was highest, and VEGF was expressed in the infiltrating neutrophils. The enhanced protein levels of VEGF, MIP-1α, and MIP-2 correlated with the degree of neutrophil infiltration. Neutrophil depletion significantly inhibited corneal angiogenesis and reduced the protein levels of VEGF, MIP-1α, and MIP-2 in the cornea. Upon stimulation, isolated neutrophils released VEGF from preformed stores and MIP-1α and MIP-2 by de novo synthesis. Neutrophil depletion thus significantly impaired inflammatory angiogenesis, identifying neutrophils as an important player in inflammatory angiogenesis. Neutrophils may exercise their angiogenic function by releasing proangiogenic factors such as VEGF. Intervention measures targeting neutrophils may therefore help to deal with abnormal angiogenesis involved in chronic inflammatory diseases.