Purpose: Glaucoma is widely accepted as a neurodegenerative disease in which retinal ganglion cell (RGC) loss is initiated by a primary insult to the optic nerve head, caused, for example, by increased intraocular pressure (IOP). In some cases, the surviving RGCs, despite adequate IOP control, may continue to degenerate as a result of their heightened susceptibility to self-destructive processes evoked by the initial damage. In animal models of mechanical or biochemical injury to the optic nerve or retina, a T-cell-mediated immune response evoked by the insult helps to reduce this ongoing loss. The current study was conducted to find out whether the ability to resist the IOP-induced loss of RGCs in a rat model is affected by the immune system.
Methods: The ocular veins and limbal plexus of rats of two strains differing in their resistance to experimental autoimmune encephalomyelitis (EAE) and in their ability to manifest a beneficial autoimmune response were laser irradiated twice to induce an increase in IOP. The pressure was measured weekly, and RGC losses were assessed 3 and 6 weeks after the first irradiation. To verify the existence of a relationship between the immune system and RGC survival, we assessed neuronal survival in Sprague-Dawley (SPD) rats devoid of mature T cells as well as after transferring splenocytes from Fisher rats, an EAE-resistant rat strain capable of manifesting T-cell-mediated neuroprotection, to rats of a major histocompatibility complex (MHC)-matched EAE-susceptible strain (Lewis), in which the ability to manifest such protective immunity is limited.
Results: Both 3 and 6 weeks after the increase in IOP was initiated, the number of surviving RGCs in SPD rats, a strain in which a beneficial autoimmune response can be evoked spontaneously, was significantly higher than in Lewis rats. Moreover, in SPD rats that were thymectomized at birth, the number of surviving RGCs after an increase in IOP as adults was significantly diminished. Passive transfer of splenocytes from Fisher rats to Lewis rats significantly reduced the IOP-induced loss of RGCs in the latter.
Conclusions: In rats of different strains, a similar increase in IOP results in differing amounts of RGC loss. This disparity was found to correlate with immune potency. These findings may explain why patients with glaucoma experience different degrees of visual loss after pressure reduction, even when the severity of the disease at the time of diagnosis is similar. The results have far-reaching prognostic and therapeutic implications.