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Ahad M. Siddiqui, Thomas F. Sabljic, Alexander K. Ball; Migration of HAPI Microglial Cells to the Retina and Optic Nerve After Injury. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1005.
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Transection of the optic nerve (ON) results in the gradual death of retinal ganglion cells by apoptosis (RGCs) over a 2 wk period. During this time microglia become reactive and proliferate. Whether retinal microglial cells acquire a neuroprotective or proinflammatory phenotype appears to depend on the nature and time-course of the injury. HAPI cells are immortalized microglial cells whose phenotype can be manipulated in vitro. The purpose of this study was to determine if cultured HAPI microglial cells injected into the eye, or systemically, would migrate to the retina and optic nerve after ON injury.
HAPI microglial cells (gift of Dr. J. Connor) were labelled with Wheat Germ Agglutinin conjugated to Oregon Green (OG)(0.05mg/ml; 10min) in vitro, and resuspended in 100 µL of PBS. HAPI-OG cells remained labelled for up to 21 days in vitro. Rats received ON transaction or crush 4 days before the HAPI cells were injected. Approximately 1000 HAPI-OG cells were injected into the vitreous (intravitreal=IV) of unlesioned or transected rats and 20,000 HAPI-OG cells injected into the lateral tail vein (Systemic Tail Vein=TV) of normal, ON crushed, or ON transected rats. Four days later, retinas and ONs were removed, fixed in phosphate buffered formaldehyde, cryostat sectioned, and examined by epifluorescence microscopy.
When HAPI-OG cells were injected into the eye (IV) or the tail vein (TV) without injury, HAPI-OG cells were not visible in either the retina or ON. Eight days after ON injury and 4 days after HAPI-OG IV injection, HAPI-OG cells were located on the vitreal surface, however there was severe disruption of the retinal layers typical of an inflammatory response. Eight days after ON injury and 4 days after HAPI-OG TV injection there were no HAPI-OG cells present in the eye, however HAPI-OG cells were aggregated along the ON crush site. The retinas from these rats were highly inflamed, in contrast to retinas from rats that had an ON injury but no HAPI-OG TV injection.
When HAPI-OG cells were injected into the eye or tail vein they migrated to the retina or optic nerve (respectively) if there was an ON injury signal present. These results suggest that ON injury signals attract extrinsic microglial cells to the retina, but results in a destructive, pro-inflammatory response.
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