Role of Inflammatory Leukocytes in Ischemic Retinopathy

Abstract

Purpose: : Ischemic retinopathies (such as retinopathy of prematurity and diabetic retinopathy) are major causes of blindness across all age groups. Several reports have described an inflammatory contribution to the pathogenesis of the disease. For instance, chemokines such as MCP-1 or MIP-1a have been demonstrated to be implicated in the progression of pathological neovascularization. Although neutrophils are the most abundant leukocytes in the human blood, their contribution to retinopathy remains elusive. Here we investigated the role of neutrophils in a mouse model of oxygen-induced retinopathy (OIR), with particular emphasis on the proliferative phase of the disease. We also looked at the implication of netrin-1, an axonal guidance cue recently demonstrated to be implicated in the control of inflammation and has interestingly been proven to inhibit chemotaxis of the neutrophil.

Methods: : Mice were subjected to 75% O2 from postnatal day 7 (P7) to P12. Depletion of neutrophils was accomplished by systemic injections at P10 and P12 of the Gr-1 neutrophil-neutralizing antibody. Mouse retinas were analyzed for extent of vascular regeneration at P17. The efficacy of neutrophil depletion was determined on bloodsmears at P14. The presence of neutrophils was determined using a myeloperoxydase (MPO) enzymatic assay. Netrin-1 was injected intravitreally at P14 and retinas were analyzed at P17 for revascularization using flatmounts and expression of angiogenic and inflammatory genes using real-time PCR.

Results: : We observed an increase in MPO activity in OIR retinas at P14 and a peak at P17 indicative of a neutrophil infiltration. Depletion of neutrophils with the Gr-1 antibody to less than 10% of total leukocytes (non-treated : 25%) effectively enhanced vascular regeneration by ~5%. Netrin-1 addition was also demonstrated to be a promoter of retinal revascularization by enhancement of expression of angiogenic markers (i.e.VEGF) and downregulation of pro-inflammatory genes.

Conclusions: : These results demonstrate that neutrophils partake actively in delaying physiological revascularization in OIR. Future studies will determine the merit of interfering with this population of leukocytes as a therapeutic strategy for ischemic retinopathies.

Keywords: retinopathy of prematurity • inflammation • hypoxia