Macrophage subpopulations and RPE elimination in the pathogenesis of experimental autoimmune pigment epithelial protein-induced uveitis (EAPU)

Sialoadhesin - a macrophage-restricted marker of immunoregulation and inflammation

Sialoadhesin (Sn, also known as Siglec-1 and CD169) is a macrophage-restricted cell surface receptor that is conserved across mammals. Sn is a member of the sialic acid-binding IgG-like lectin (Siglec) family of proteins characterized by affinity to specifically sialylated ligands, and under normal conditions is expressed on subsets of macrophages in secondary lymphoid tissues, such as lymph node and spleen. However, Sn-positive macrophages can also be found in a variety of pathological conditions, including (autoimmune) inflammatory infiltrates and tumours. Sn has been shown to contribute to sialylated pathogen uptake, antigen presentation and lymphocyte proliferation, and to influence both immunity and tolerance. This review presents Sn as a macrophage-specific marker of inflammation and immunoregulation with the potential to becoming an important biomarker for immunologically active macrophages and a target for therapy.

Free radical mediated photoreceptor damage in uveitis

Uveitis is a major cause of blindness, with the visual loss that occurs being due primarily to retinal tissue damage. The tissue damage is mediated mainly by phagocytic inflammatory cells, such as macrophages, by the release of various proteolytic enzymes, arachidonic acid metabolites, cytokines and free radicals. The latter are found to be potent cytotoxic agents that readily cause tissue damage by peroxidation of lipid cell membranes. Recent studies of experimental uveitis indicate that other potent oxidants are generated in uveitis by macrophages. One of these is ONOO-, which is formed from *NO and O(-)2. The macrophages generate *NO preferentially in the outer retina following iNOS expression. In these phagocytes, outer retinal proteins, especially arrestin, are found to be potent iNOS inducers. Current studies of RPE show that these cells protect the retina from ONOO- mediated damage in uveitis by releasing a novel protein called retinal pigment epithelial protective protein. This protein is found to suppress O(-)2 and *NO generation by the phagocytes, in both in vitro and in vivo uveitis models. The protective protein expression is restricted to RPE, its suppressive effect is a result of the inhibition of the phosphorylation of cytosolic proteins, p47-phox, required for the assembly of NADPH and activation of NFkappaB, which are required for generation of 0(-)2 and expression of iNOS respectively. Either pharmacologically or chemically, up-regulation of RPP generation could help in preventing retinal degeneration in uveitis or other degenerative dis

Differential effect of macrophage depletion on two forms of experimental uveitis evoked by pigment epithelial membrane protein (EAPU), and by melanin-protein (EMIU)

The purpose of the present study was to clinically and histologically investigate the influence of macrophage depletion on the development of experimental autoimmune pigment epithelial membrane protein-induced uveitis (EAPU), and experimental melanin-protein induced uveitis (EMIU) in the Lewis rat. EAPU is mainly characterized by pigment epitheliitis. Posterior mononuclear cell accumulations enclose and destroy the retinal pigment epithelium (RPE). In EMIU the inflammation is specifically localized in the uvea. EAPU was induced by immunization with RPE membrane protein, and EMIU was evoked by immunization with purified choroidal melanin. Systemic treatment with dichloromethylene diphosphonate (Cl2MDP)-containing liposomes just before the expected beginning of the clinical signs of EAPU (at day 7 and 9 after immunization) resulted in a considerable delay of the uveitis process. In the treated animals the typical plaque shaped cell accumulations (containing many macrophages) along the RPE were lacking. Two weeks after the treatment, severe rebound EAPU developed. Local treatment by subconjunctival liposome injections did not exert any effect on EAPU. In EMIU, macrophage depletion by systemic treatment did not noticeably influence the clinical and histological development of the inflammation. Systemic treatment at the peak stage of EAPU (at day 12 and 14 after immunization) resulted in the rapid disappearance of the clinical signs of uveitis. Vitreous and anterior chamber cells were virtually absent two days later. This situation remained unchanged until the experiment was terminated two weeks later. Already deposited cell accumulations along the RPE did not regress but stopped their progression. Hematogenous macrophages thus appear to play a crucial role in the development of EAPU but the effect of early macrophage depletion on EAPU appeared to be temporary due to blood repopulation. A possible explanation for the differential influence of macrophage depletion on EAPU and EMIU is discussed, and is based on differences in immunopathogenesis.

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.