The Role of Microglia and Peripheral Monocytes in Retinal Damage after Corneal Chemical Injury

Abstract

Eyes that have experienced alkali burn to the surface are excessively susceptible to subsequent severe glaucoma and retinal ganglion cell loss, despite maximal efforts to prevent or slow down the disease. Recently, we have shown, in mice and rabbits, that such retinal damage is neither mediated by the alkali itself reaching the retina nor by intraocular pressure elevation. Rather, it is caused by the up-regulation of tumor necrosis factor-α (TNF-α), which rapidly diffuses posteriorly, causing retinal ganglion cell apoptosis and CD45 cell activation. Herein, we investigated the involvement of peripheral blood monocytes and microglia in retinal damage. Using CX3CR1::CCR2 reporter mice and bone marrow chimeras, we show that peripheral CX3CR1CD45CD11bMHC-II monocytes infiltrate into the retina from the optic nerve at 24 hours after the burn and release further TNF-α. A secondary source of peripheral monocyte response originates from a rare population of patrolling myeloid CCR2 cells of the retina that differentiate into CX3CR1 macrophages within hours after the injury. As a result, CX3CR1CD45CD11b microglia become reactive at 7 days, causing further TNF-α release. Prompt TNF-α inhibition after corneal burn suppresses monocyte infiltration and microglia activation, and protects the retina. This study may prove relevant to other injuries of the central nervous system.

Microglia Regulate Neuroglia Remodeling in Various Ocular and Retinal Injuries

“…We previously showed that corneal injury led to prompt infiltration of peripheral CX3CR1 + CCR2 + monocytes into the retina and subsequent inflammation that resulted in retinal cell loss (28,32,33). In order to assess the role of microglia in this neurodegenerative process we assessed the damage to the retina the setting of microglia depletion by CSF1R inhibitor.…”

“…Reactive microglia have been identified in almost all neuroretinal diseases and injuries (12), however, their exact role is not yet fully elucidated. Upon activation, microglia have been shown to secrete neurotrophic factors that promote neuronal survival and regeneration (4,(13)(14)(15) or release inflammatory cytokines that cause cell death (16)(17)(18)(19). This functional duality has been confirmed by single-cell RNAseq techniques where microglia were shown to have a heterogeneous transcriptional profile, with signatures ranging from inflammatory to neuroprotective (10,20).…”

“…This process of peripheral monocyte engraftment is regulated by yolk sac-derived microglia, and microglia depletion leads to subsequent repopulation by peripheral CX3CR1 + monocytes (24). Considering that microglia and macrophages express common cell markers, distinguishing between the two cell populations is not easy, and often requires novel fate mapping techniques (5,19,(25)(26)(27). Instead, by using busulfan myelodepletion and bone marrow CX3CR1 +/EGFP ::CCR2 +/RFP chimeras, we showed that peripherally engrafted monocytes remain transcriptionally different compared to microglia (24).…”

“…We recently showed that microglia may not be the direct mediators of retinal inflammation after acute ocular surface injury. Instead, peripheral monocytes infiltrate into the retina within hours, initiating neuroretinal inflammation and microglia activation (19). Interestingly, peripheral monocytes that infiltrate into the retina after injury do not vanish from the tissue over time.…”

CSF1R inhibition by a small molecule inhibitor affects hematopoiesis and the function of macrophages

“…However, this argument has been solely based on cell count measurements of blood monocytes and evaluation of blood brain barrier rather than direct assessment of cellular subtypes and their function. Given that peripheral monocytes are known to participate in CNS disease (6)(7)(8)(9)(10)(11)(12) and in the repopulation of microglia following their depletion, (10,12) it is important to definitively determine if CSF1R inhibition affects the function of peripheral immune cells (3,5,6,(12)(13)(14)(15).…”