Ultrastructural pathology of experimental autoimmune uveitis: A review

Microglial mediators in autoimmune Uveitis: Bridging neuroprotection and neurotoxicity

Autoimmune uveitis, a severe inflammatory condition of the eye, poses significant challenges due to its complex pathophysiology and the critical balance between protective and detrimental immune responses. Central to this balance are microglia, the resident immune cells of the central nervous system, whose roles in autoimmune uveitis are multifaceted and dynamic. This review article delves into the dual nature of microglial functions, oscillating between neuroprotective and neurotoxic outcomes in the context of autoimmune uveitis. Initially, we explore the fundamental aspects of microglia, including their activation states and basic functions, setting the stage for a deeper understanding of their involvement in autoimmune uveitis. The review then navigates through the intricate mechanisms by which microglia contribute to disease onset and progression, highlighting both their protective actions in immune regulation and tissue repair, and their shift towards a pro-inflammatory, neurotoxic profile. Special emphasis is placed on the detailed pathways and cellular interactions underpinning these dual roles. Additionally, the review examines the potential of microglial markers as diagnostic and prognostic indicators, offering insights into their clinical relevance. The article culminates in discussing future research directions, and the ongoing challenges in translating these findings into effective clinical applications. By providing a comprehensive overview of microglial mechanisms in autoimmune uveitis, this review underscores the critical balance of microglial activities and its implications for disease management and therapy development.

Involvement of CCR5 in the passage of Th1-type cells across the blood-retina barrier in experimental autoimmune uveitis

Although the recruitment of T helper cell type 1 (Th1)/Th2 cells into peripheral tissues is essential for inflammation and the host response to infection, the traffic signals that enable the distinct positioning of Th1/Th2 cells are unclear. We have determined the role of CC chemokine receptor 5 (CCR5) in this using experimental autoimmune uveitis (EAU) as a model system. In EAU, Th1-like cells are preferentially recruited into the retina across the blood-retina barrier, partly as a result of expression of the adhesion molecules P-selectin glycoprotein ligand 1 and lymphocyte function-associated antigen-1 on these cells. CD3+ T cells, infiltrating the retina, also expressed the chemokine receptor CCR5, and CCR5 ligands, macrophage-inflammatory protein-1alpha (MIP-1alpha), MIP-1beta, and regulated on activation, normal T expressed and secreted (RANTES), were strongly expressed in the retina at peak EAU. Th1-like cells, polarized in vitro, expressed high levels of CCR5. The trafficking of these CCR5+ cells was examined by tracking them after adoptive transfer in real time in vivo at an early disease stage using scanning laser ophthalmoscopy. Treatment of the cells with antibody against CCR5 prior to transfer resulted in a reduction in their infiltration into the retina. However, rolling velocity, rolling efficiency, and adherence of the cells to retinal endothelium were not reduced. CCR5 is clearly important for Th1 cell recruitment, and this study demonstrates for the first time in vivo that CCR5 may act at the level of transendothelial migration rather than at the earlier stage of rolling on the endothelium.

Regulation of granulocyte-macrophage colony-stimulating factor in human retinal pigment epithelial cells by IL-1beta and IFN-gamma

GM-CSF production by RPE cells, which form part of the blood-retina barrier, is upregulated by IL-1beta and this increase can be reversed by IFN-gamma. IL-1beta up-regulation is not dependent on PKC but the PKC activator PMA induces low levels of GM-CSF production and acts synergistically with IL-1beta to further increase GM-CSF. Although A23187 and ionomycin stimulated low levels of GM-CSF production, the IL-1beta pathway was cyclosporin A insensitive and did not interact with the calcium pathway. IL-1beta-stimulated GM-CSF mRNA expression and production was strongly dependent on NF-kappaB. IFN-gamma inhibition of the GM-CSF response to IL-1beta acted via NF-kappaB, reducing the translocation of NF-kappaB to the nuclei of RPE cells treated with IL-1beta and IFN-gamma. The results show that IFN-gamma down-regulation acts either directly on NF-kappaB or its activation or by blockade of a pathway upstream of NF-kappaB. However, any such blockade does not involve PKC or intracellular calcium.

Control of chemokine production at the blood-retina barrier

Chemokine production at the blood-retina barrier probably plays a critical role in determining the influx of tissue-damaging cells from the circulation into the retina during inflammation. The blood-retina barrier comprises the retinal microvascular endothelium and the retinal pigment epithelium. Chemokine expression and production by human retinal microvascular endothelial cells (REC) have never been reported previously, so we examined the in vitro expression and production of monocyte chemoattractant protein-1 (MCP-1), regulated on activation of normal T-cell expressed and secreted (RANTES), macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, interleukin (IL)-8, epithelial cell-derived neutrophil activating protein-78 (ENA-78) and growth related oncogene alpha (GROalpha) in these cells, both unstimulated and stimulated by cytokines likely to be present during the evolution of an inflammatory response. We compared this to expression and production of these chemokines in vitro in human retinal pigment epithelial cells (RPE). MCP-1 was expressed and produced constitutively by REC but all the chemokines were produced in greater amounts upon stimulation with the proinflammatory cytokines IL-1beta and tumour necrosis factor-alpha (TNF-alpha). MCP-1 and IL-8 were produced at much higher levels than the other chemokines tested. MIP-1alpha and MIP-1beta were present only at low levels, even after stimulation with IL-1beta and TNF-alpha. Cytokines with greater anti-inflammatory activity, such as IL-4, IL-10, IL-13, transforming growth factor-beta (TGF-beta) and IL-6, had little effect on chemokine production either by REC alone or after stimulation with IL-1beta and TNF-alpha. RPE, although a very different cell type, showed a similar pattern of expression and production of chemokines, indicating the site-specific nature of chemokine production. Chemokine production by REC and RPE is probably significant in selective leucocyte recruitment during the development of inflammation in the retina.

Cytokine regulation of granulocyte-macrophage colony-stimulating factor (GM-CSF) production by human retinal pigment epithelial cells

GM-CSF is an important regulator of macrophage, granulocyte and dendritic cell behaviour and function. These cell types have been implicated in the retinal damage characteristic of endogenous posterior uveitis. Dendritic cells in the choroid have access to retinal antigens processed by the retinal pigment epithelial (RPE) cells of the blood-retinal barrier and are thought to be candidates for the presentation of antigen in uveoretinitis. We therefore investigated the production of GM-CSF and its regulation in human RPE cells. IL-1beta, tumour necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) all stimulated GM-CSF production by RPE cells and a combination of these cytokines increased GM-CSF production over five-fold compared with that with the individual cytokines alone. Interferon-gamma (IFN-gamma) rapidly down-regulated these responses. IFN-gamma did not appear to be acting directly on IL-1beta or via the synthesis of another protein. GM-CSF mRNA expression showed the same pattern of response to these cytokines, indicating transcriptional or pre-transcriptional regulation, and there was no evidence that IFN-gamma was acting by destabilizing GM-CSF mRNA. These results are generally important in understanding the ways in which cytokine regulation differs between different cell types and also more specifically for determining ways in which a cytokine with a significant role in the development of autoimmune uveoretinitis may be manipulated.

Increased vascular endothelial growth factor (VEGF) and transforming growth factor beta (TGFbeta) in experimental autoimmune uveoretinitis: upregulation of VEGF without neovascularization

Experimental autoimmune uveoretinitis (EAU) was induced in Lewis rats and B10.A mice by immunization with S-antigen (S-Ag) to study the potential roles of vascular endothelial growth factor (VEGF) and the beta1 and beta2 isoforms of transforming growth factor (TGFbeta1 and TGFbeta2) during the progression of the disease. VEGF has been implicated as an angiogenic factor in ischemic retinopathies; however, Lewis rats developing EAU have high levels of VEGF in the retina, but no neovascularization. In the present study, immunohistochemical staining for VEGF, TGFbeta1 and TGFbeta2 was performed on the retinas of Lewis rats developing EAU or with oxygen-induced ischemic retinopathy. In rats immunized with S-antigen, a marked upregulation of VEGF was immunohistochemically visualized from the inner nuclear layer to the inner limiting membrane prior to blood-retinal barrier (BRB) failure and lymphocytic infiltration. VEGF is normally induced by hypoxia and its induction leads to neovascularization. Coincident with the increase in VEGF, there was increased immunoreactivity for TGFbeta1 and TGFbeta2 within the same layers of the retina. In contrast, rats with ischemic retinopathy and retinal neovascularization showed only a modest increase in VEGF immunoreactivity, which is largely confined to retinal ganglion cells and inner retinal vessels, and little or no increase in TGFbeta1 or TGFbeta2. In addition, in mice developing EAU, which does not have an abrupt onset as it does in rats and may involve neovascularization, a comparable upregulation of VEGF in the inner retina to that seen in rats developing EAU occurs with no increase in TGFbeta1 or TGFbeta2. Since TGFbeta can inhibit endothelial cell proliferation, it is likely that an increase in TGFbeta may prevent VEGF from exerting its endothelial growth activity in the rat EAU model, but VEGF may be operative in inducing BRB failure. These data suggest that there is a complex interaction among growth factors in the retina and that retinal neovascularization may require an imbalance between stimulatory and inhibitory factors.

Cytokine regulation of RANTES production by human retinal pigment epithelial cells

The mechanism whereby inflammatory cells gain access to the retina in posterior intraocular inflammatory disease remains unclear. The chemokine RANTES has the potential to influence the migration of memory T cells and monocytes across the blood-retinal barrier during inflammatory eye disease. We have therefore examined the production of RANTES by cultured human retinal pigment epithelial cells (RPE), which form a part of the blood-retinal barrier, in response to cytokines likely to be present in the microenvironment. IL-1 beta and TNF alpha stimulated RANTES production by these cells. IFN-gamma acted synergistically with TNF alpha to increase RANTES production. In contrast, IL-4 downregulated RANTES production stimulated by TNF alpha. RT-PCR studies showed that RANTES mRNA from RPE followed the same pattern of expression in response to cytokines as did RANTES production indicating that RANTES production was controlled at, or prior to, transcription. RANTES is produced in vitro by RPE in response to the proinflammatory cytokines IL-1 beta, TNF alpha, and IFN-gamma and is therefore likely to play a role in the development of the inflammatory eye disease endogenous posterior uveitis.

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.

Uveitogenic 28/30 kD and 43 kD polypeptides in pigment epithelial membranes of the retina

The purpose of this study was the search for new intrinsic polypeptides of the retinal pigment epithelium (RPE) capable of evoking experimental uveitis. A membrane fraction was prepared from purified bovine RPE cells. The Triton X-100 soluble protein fraction was separated into polypeptide fractions by preparative gel electrophoresis, and the pathogenicity of the main isolated polypeptides was investigated in Lewis rats. After immunization, two hitherto unknown pigment epithelial polypeptides with M(r) 28/30 kD (doublet) and 43 kD (PEP-28/30 and PEP-43, respectively) elicited progressive pigment epitheliitis and choroiditis accompanied by extending plaque-shaped macrophage accumulations along the RPE-Bruch's membrane layer. No inflammatory foci were found within the neural retina. Polypeptide fractions with M(r) 14-17, 25 and 32/34 kD (doublet) (PEP-14/17, PEP-25 and PEP-32/34, respectively) appeared to be non-uveitogenic at the tested dose. PEP-28/30 and PEP-43 exhibited a partial antigenic relationship to PEP-65. PEP-28/30 exhibited marked reactivity to a monoclonal antibody previously raised to a 32 kD RPE-specific protein.

IN CONCLUSION

in addition to the previously described main RPE-specific membrane polypeptide PEP-65, the RPE appears to contain two more uveitogenic components, the intrinsic pigment epithelial membrane polypeptides PEP-28/30 and PEP-43. Like PEP-65, these antigens are able to evoke experimental autoimmune pigment epithelial protein-induced uveitis (EAPU) in rats.