The influence of penetrating keratoplasty and cyclosporin A therapy on MHC class II (Ia)-positive cells in the rat iris and choroid

Resident and infiltrating cells in the rat iris during the early stages of experimental melanin protein-induced uveitis (EMIU)

Experimental melanin protein-induced uveitis (EMIU) is reported to be a model of anterior uveitis and choroiditis in which the retina is spared. In this study, we chose to compare EMIU with experimental autoimmune uveoretinitis (EAU), a well-recognised model of endogenous posterior uveitis, with regard to the nature and dynamics of the cellular infiltrate in the iris. Female Lewis albino rats were immunised with mixtures of crude retinal extract/complete Freund's adjuvant (CFA) (EAU), phosphate-buffered saline/CFA (controls), or iris melanin/CFA (EMIU) using established protocols. Animals were sacrificed on days 10 and 13 (around disease onset). Following whole body perfusion fixation, irides were dissected from the remainder of the globe. Iris wholemount preparations were then subjected to immunohistochemical analysis in order to investigate both the dynamics of infiltrating leukocytes and the effects of the inflammatory changes on resident immune cells in the iris. The nature of the cellular infiltrate in both EMIU and EAU models was essentially similar, namely there was a rich infiltrate of EDI+ mononuclear cells, Ox42+ neutrophils and T cells. Resident tissue macrophages (ED2+) were slightly below normal densities in the iris of EAU animals and marginally elevated in EMIU animals, MHC class II (Ia) staining, associated in the normal eye with dendritic cells (DC), was considerably elevated in EMIU. It is likely that this was due to both increased DC numbers and an influx of Ia+ exudate macrophages. No striking difference was found in the nature and phenotype of the cellular infiltrate in the iris at the onset of the disease in these two models of uveitis (EAU and EMIU). This suggests that the anterior segment inflammation in both models represents non-specific changes secondary to cytokine release associated with interaction of activated antigen-specific T cells and target antigens, namely retinal photoreceptors in EAU and uveal tract melanin-containing cells in EMIU. Alternatively, it may suggest that antigen-presenting cells resident in the iris and ciliary body in normal eyes have access to ocular antigens on both sides of the blood-ocular barrier and are capable of activating circulating antigen-specific T cells in these models.

The behaviour of ED1- and ED2-positive cells in the rat iris and choroid following penetrating keratoplasty and cyclosporin A therapy

BACKGROUND

The presence, morphology and distribution of ED1 and ED2+ cells have been recently reported in detail in the uveal tissues of the rat. These cells, particularly those of dendritic morphology, are possibly capable of antigen presentation and, therefore, may play an important role in immune processes of uveal and other ocular tissues. Using the whole-mount technique, the distribution of ED1+ and ED2- cells in the rat iris and choroid was investigated following penetrating keratoplasty (PKP) and following treatment with cyclosporin A (CsA).

METHODS

Lewis (LW) rats received corneal buttons from Lewis-Brown Norway (LW-BN) donors and were randomly assigned to one of two groups: (I) operated, untreated (n = 24); (II) operated, CsA treated (10 mg/kg i.m.: n = 22). Four groups served as controls: normal LW rats (n = 13); (IV) unoperated, CsA treated (16 days' treatment; n = 8); (V) eight corneal sutures only, representing a simulated or "sham" operation (n = 4); (VI) syngeneic operated (LW to LW: n = 4). Animals of groups I and II were killed on the 5th, 9th and 13th postoperative days and on appearance of the corneal rejection (group I, day 13; Group II, day 16). Both eyes were enucleated, immediately fixed, and iris-choroid flat mounts were examined for ED1+ and ED2+ cells using APAAP immunohistochemistry.

RESULTS

In the normal LW rat iris, ED1+ and ED2+ cells of both non-dendritic and dendritic morphology were observed. The placement of sutures in the cornea and PKP with or without treatment resulted in a reasonably regular response in both the iris and the choroid in the operated and partner eye. These included: (a) an increase in round iridal ED1+ and ED2+ cells in the operated eye and in round ED1+ cells in the partner eye; (b) a decrease in dendritiform ED2+ cells in the iris of the operated eyes as well as in the partner eye; and (c) a decrease in the dendritiform ED2+ cells in the choroid of the operated and partner eye. CsA treatment alone in unoperated animals resulted in significant decreases in the number of dendritiform ED1+ cells in the iris and in the dendritiform ED2+ cells in the choroid.

CONCLUSION

Corneal transplantation in the Lewis rat results in responses in ED1+ and ED2+ cells in uveal tissues in both the operated eye as well as in the partner eye. The differences in cell behaviour supports the idea that distinct immune-competent cell populations are present within uveal tissues and that they may have differing roles in the pathological eye.