In our model, increased microglial activation and monocyte infiltration result in a significant proinflammatory environment. Cytokine and chemokine profiling in the retina identified a number of changes in OHT and in contralateral NT eyes. CNTF, fetuin A, FGF-1, galectin-3, ICAM-1 (CD54), and lipocalin-2 were all differentially elevated. CNTF, which can be released from microglia as a response to central nervous system injury, has demonstrated neurotrophic properties and increases RGC survival in glaucoma models.
73,74 CNTF also induces gliosis in Müller cells and leads to the upregulation of proinflammatory cytokines and chemokines,
75–77 and as such its action is likely highly context dependent. Fetuin-A, a serum carrier protein, has been proposed as a biomarker of systemic inflammation, vascular disease, and neurodegenerative disease.
78–80 A decreased expression of fetuin-A correlates with the severity of cognitive impairment in Alzheimer's disease and fetuin-A–deficient mice demonstrate protection in experimental models of multiple sclerosis.
81,82 Galectin-3 mediates cell migration and adhesion, inflammatory cytokine release, and apoptosis and has been shown to activate microglia, astrocytes, monocytes, and other immune cells.
83 The plasma levels of galectin-3 correlate with Huntington's disease severity in patients and animals, and are associated with adverse outcomes in stroke and cerebral infarction.
84,85 The expression of galectin-3 was increased in Huntington's disease and genetic ablation of
Lgals3 is protective in animal models.
84 Microglial released galectin-3 can act as a ligand for Toll-like receptor 4, sustaining inflammatory responses.
86 Increased expression of galectin-3 has been detected in human glaucomatous donor tissue in the trabecular meshwork and optic nerve head in association with increased fibrosis.
87 ICAM-1 is a glycoprotein that is expressed on leukocytes, macrophages and vascular endothelium. Its expression is upregulated in response to inflammatory stimuli (IL-1, tumor necrosis factor-α), where it functions as a ligand for integrins to facilitate endothelial binding and the transmigration of leukocytes.
88 ICAM1 expression is increased in glaucoma patient leukocytes, but not blood plasma.
89,90 The upregulation of
ICAM1 is early and persistent in the optic nerve head and retina of DBA2/J mice, and is decreased by radiation, treatment which is highly protective for RGCs.
41 Lipocalin-2 is an antibacterial protein (acting through iron sequestration) that elicits proinflammatory responses and is implicated in a number of inflammatory and neurodegenerative diseases, including Stargardt disease and age-related macular degeneration in the retina.
91,92 Lcn2 expression is also highly upregulated in DBA2/J glaucoma and in the rat hypertonic saline glaucoma model.
41,93 A recent Gene Expression Omnibus screen identified
LCN2 involvement in glaucoma pathogenesis.
94 Increased FGF-1 expression was identified in NT/OHT eyes. FGF-1 has demonstrated neuroprotective responses in neurodegenerative disease, but also activates astrocytes in the spinal cord, leading to proinflammatory responses.
95–98 Whether this response is neuroprotective or inflammatory and related to microglial activation warrants further exploration. Based on our findings, the earlier day 3 time point in which IOP peaks but neurodegeneration is not detected may offer an ideal time point to conduct additional cytokine profiling to support these findings at day 14, as well as additional microglia morphologic assessments to capture earlier responses. These proinflammatory events represent ideal candidates for further exploration using targeted knockouts, gene therapies, or antibody therapies in glaucoma.