Distribution and Quantification of Choroidal Macrophages in Human Eyes With Age-Related Macular Degeneration.

Age-related macular degeneration (AMD) is the leading cause of severe vision loss in patients over 50 years of age in industrialized countries.1 Despite its high prevalence, the cause of AMD remains largely unknown. Clinically and histologically, AMD is generally classified as nonexudative or dry AMD, of which geographic atrophy (GA) is the severe form, and exudative or neovascular AMD. Dry AMD progresses more slowly and is characterized by drusen, geographic or focal atrophy of the retinal pigment epithelium (RPE), and photoreceptor dysfunction and degeneration. The advanced exudative form, neovascular AMD, can develop after early and intermediate dry AMD.2 The key feature of the advanced subtype is choroidal neovascularization (CNV), the growth of new blood vessels from the choroid into the region underlying the RPE or extending into the subretinal space. Emerging evidence supports the association between chronic inflammation and AMD. Penfold et al.3 described inflammatory cells (macrophages, lymphocytes, and mast cells) in human AMD choroids. More recent findings suggest a role for immunologic processes in AMD pathogenesis, including recruitment of macrophages,4 involvement of systemic inflammatory processes,5–7 complement activation,8–12 and microglial activation and accumulation.13 Historically, macrophages have been observed at sites of CNV in patients with AMD,14,15 and CD68+ macrophages were detected in rapidly progressive fibrovascular AMD membranes.16 Grossniklaus et al.17 evaluated human CNV and observed that macrophages express proangiogenic vascular endothelial growth factor (VEGF) and suggested that they directly promote CNV.18 Macrophage populations are simplistically divided into phenotypes M1 and M2. Phenotype M1 is proinflammatory and causes tissue injury, whereas M2 phenotype is pro-angiogenic and promotes tissue repair.19 In the laser-induced mouse CNV model with scar, M2 macrophages were recruited to the site of laser injury20,21; however, M1 macrophages were identified in the subretinal space when mice were immunized with carboxyethylpyrrole), an immune response-initiated model of AMD.22,23 Determination of M1 and M2 macrophages in a human is not as straightforward as it is in mouse. In the present study, we chose to localize all macrophages with the monocyte marker ionized calcium-binding adapter molecule 1 (IBA1) and determine if they were activated using an antibody to human leukocyte antigen-antigen D-related (HLA-DR), a subunit of major histocompatibility complex (MHC) class II. The present study aimed to quantify the number, distribution, and activation of choroidal macrophages in choroidal whole mounts from aged control eyes and in eyes from patients with clinically documented early and intermediate AMD, as well as two advanced subtypes, neovascular AMD and GA. The presence of macrophages was related to changes in choriocapillaris (CC) and CNV.