Ординатура / Офтальмология / Английские материалы / Antigen Presenting Cells and the Eye_Zierhut, Rammensee, Streilein_2007

Figure 2 Allergen stimulation of conjunctival mast cells.

Therefore, aeroallergens such as ragweed and grass pollen or house dust mite allergens can invade into the eye.

These allergens acitvate mast cells, which are the earliest responder to allergen challenge, leading to the degranulation of pre-formed mediators such as histamine and tryptase and newly synthesized mediators such as arachidonic acid and leukotrienes. Mast cells also show an enhanced production of inflammatory mediators such as cytokines and chemokines.

This leads to a defect of conjunctival epithelial cells, which release the apoptotic factors Fas-L, upregulate ICAM-1 expression, and produce proinflammatory mediators such as IL-8, TNF-α, and GM-CSF (Fig. 3) (8–10).

In addition, an enhanced number of CD1a+ DCs, which bear high amounts of IgE molecules on their cellular surface, can be detected after ocular allergen challenge in the conjunctiva of these patients.

CONCLUSION

Although there are numerous similarities between skin allergy and ocular allergy, several aspects such as the existence of inflammatory DC subtypes and the role of autoallergens or superantigens in allergic ocular diseases remain to be verified. Unraveling similarities and differences between skin allergy and ocular allergy would form the basis for related therapeutic strategies that might be useful for the treatment of both skin allergy and ocular allergy in the future.

Figure 3 Damage of conjuctival epithelial cells.

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T cells rather than activate them (2–5). Despite these multiple mechanisms, ocular inflammation involving the retina and uveal tract is not uncommon (5,6), indicating that the dynamic interactions between the cells and tissues of the eye and the immune system maintaining privilege can break down in response to infection or immune dysfunction. Posterior uveitis, the mononuclear cell inflammation of the retina and uveal tract that can result, is thought to represent an autoimmune response to retinal antigens (5,7,8), and in this review we examine some data supporting the notion that endogenous posterior uveitis (EPU) represents an autoimmune response. In addition, we will briefly consider the evidence for local priming of T cells to retinal autoantigens and evidence for local regulation of the response.

RETINAL AUTOANTIGENS

The retina is known to contain several potential autoantigens that have been implicated in the pathogenesis of EPU (9). The most abundant of these are S-antigen and interphotoreceptor retinoid binding protein (IRBP) (10). Both antigens are highly conserved between species, reflecting their importance in the process of vision, but both antigens contain epitopes that are also potent immunogens, inducing clinically and histologically similar CD4+ T-cell-dependent autoimmune uveitis in a range of animal models (10–12).

Recent strategies for the treatment of CD4+ T-cell-mediated autoimmune disease have focused on the possibility of altering the immune response by peptide therapy. This process involves blocking or modulation of the major histocompatibility complex (MHC)-peptide–T-cell receptor (TCR) recognition event that controls the immune response, either directly, or through manipulation of the immune system to re-establish a state of immunological tolerance to antigen. Both S-antigen and IRBP have been extensively studied to obtain information on immuno-dominant pathogenic or modulatory epitopes and the mechanisms by which they are recognized by the immune system (13,14). For this approach to succeed in vivo it is necessary to consider the effect of antigen processing on therapeutically administered antigen or peptide by host antigen-presenting cells (APCs). We have studied the proteases involved in antigen processing to predict which epitopes would be generated by APCs during normal processing. In vitro experiments had shown that S-antigen processing is independent of the ubiquitous endosomal protease cathepsin B, suggesting that S-antigen processing and peptide loading occurs outside the normal endo-lysosomal pathway (15). Further analysis of the role of cathepsins and acidic proteolysis in retinal antigen processing confirmed that processing of S-antigen is also independent of other cysteine proteinases and lysosomal enzymes in vivo. Different antigen processing pathways were found to be involved in the induction of experimental autoimmune uveoretinitis (EAU) by S-antigen and IRBP. IRBP was processed via conventional pathways requiring extensive proteolytic cleavage by cathepsins D and B. In contrast, our data suggested that an alternative pathway of antigen processing exists for S-antigen,

which is independent of processing within the normal endo-lysosomal pathway and that uveitogenic peptides of naturally processed S-antigen bind to MHC class II antigens either at the cell surface or within very early endosomes where cathepsin B is inactive (16). This diversity in MHC class II presentation, although highly unusual, is not unique. Other examples of aberrant processing have been detected in various disease states and it has been shown that epitopes bound to MHC class II in alternative sites are often associated with pathogen evasion of the immune system or induction of autoimmunity (17).

Why should soluble S-antigen taken up by APCs be processed via the alternative pathway, while soluble IRBP is processed through the conventional acidic proteolytic pathway? To answer this question, it was necessary to examine dendritic cell (DC) processing and antigen presentation function in induction of retinal autoimmunity.

The unique ability of DCs to activate naïve T cells and prime the immune response has been shown to depend on the differentiation status of the cell (18). DCs also control the nature of immune responses through the shaping of tolerance induction. The default role of tissue DCs is thought to be tolerance, thus tissue antigens acquired in normal tissues through uptake of apoptotic cells maintain peripheral tolerance, and inert particles in the gut and airway are “ignored” by DCs conditioned by the mucosal microenvironment (19). The key to induction of immunity is the triggering of specific pattern recognition receptors on DCs and other cells of the innate immune system by pathogens or by pro-inflammatory cytokines (20). Induction of autoimmunity to tissue antigens presented during a response to injury or infection should normally be prevented by central and peripheral tolerance mechanisms, but as retinal antigens are sequestered from normal immune surveillance, tolerance to them must be compromised. Furthermore, uveitogenic epitopes of S-antigen are known to contain microbial homologies (21,22) and a TNF-α homologous sequence (23). Crucially, the crystal structure of S-antigen shows that this epitope, GVXLXD, is located close to the amino terminal of the molecule well away from the rhodopsin-binding site, is exposed on the surface of the molecule, and is therefore available for recognition by ligands on other cells (24). We have also shown that the epitope is functional, and can elicit TNF-α-dependent responses in other cells (25). S-antigen was shown to drive maturation and differentiation of cultured DCs or choroidal DCs in tissue explants via the p55 TNF-α receptor. Choroidal DCs were stimulated to migrate and cultured DCs to accumulate surface MHC class II with a corresponding loss of acidic intracellular vesicles. DCs were also stimulated to express maturation dependent mRNA for the cytokines IL-1β and IL-12. S-antigen-pulsed DCs were also able to induce an immune response in vivo and to initiate Ig class switching. In contrast, IRBPpulsed DCs appeared inert and had no priming effect in vivo (25). This data confirms that retinal antigens can have different effects upon the immune system and supports the hypothesis that exposure of the immune system to S-antigen in vivo, even in the absence of infection, could over-ride peripheral and ocular specific tolerance mechanisms and induce autoimmunity.

IMMUNE PROCESSES IN EYES OF UVEITIS PATIENTS

What evidence is there for an autoimmune response to retinal autoantigens in uveitis patients? Several studies have indicated that T-cell–mediated autoimmune processes play a major role (7,26), and that responses to S-antigen epitopes in particular may be involved (27–29). In addition, histopathological studies show that mononuclear cells pre-dominate in the inflammation, with granuloma formation consistent with a delayed type hypersensitivity response to antigens within the retina or uvea (30–32). Helper T cells and Th1 cytokines have been demonstrated in ocular fluids (8,32,33), but their relationship to the disease process is not well defined, as Th2 responses can also be damaging (34). Our studies have shown that vitreous cells are predominantly CD4 and CD8 T cells, and present in approximately equal numbers. These make up to 60% of the total infiltrate together with approximately 20% macrophages and 10% B cells (Fig. 1). Immunocytochemical analysis of cells showed that both T cells and macrophages also exhibited an activated phenotype. Large mononuclear cells strongly expressing MHC class II antigens were often associated with smaller lymphocytes, suggesting that cognate interactions between potential APCs and T cells were occurring within the vitreous and retina during uveitis.

Auto-aggressive T cells may use a restricted range of T-cell receptor (TCR) genes, so we compared the TCRβ chain repertoire of the activated (CD25+) subset with total lymphocytes isolated from the peripheral blood of patients with clinically active disease, using data from healthy individuals as controls. Significantly elevated TCR Vβ1 usage within the CD25+ T-cell population of patients was detected (35). To confirm peripheral bias was relevant to events in the disease site, vitrectomy samples were also examined. To detect local T-cell proliferation within the eye, T-cell clones present in the posterior chamber were examined using CDR3 spectra-typing. This method allows the polyclonality of an expressed TCR V gene family to be quantitatively determined on the basis of variable (V), diversity (D), and joining (J) (VDJ) gene segment lengths. Oligoclonality was observed in 3 out of 5 patients, but no evidence of TCR Vβ1 bias was detected in these samples. Figure 2 shows the results of one CDR3 spectra-typing experiment carried out to detect oligoclonality within each Vβ family within the peripheral blood, the vitreous, and from the sub-retinal space of a uveitis patient undergoing surgery. Polyclonality of each Vβ family within the circulation can be seen, with oligoclonality or even monoclonality evident within the vitreous and sub-retinal space for some families. In particular dominant monoclonality of Vβ6, Vβ12, Vβ15, Vβ17, and Vβ22 families in the sub-retinal space is evident, indicating epitope-specific proliferation is occurring within the retina. The number of Vβ families involved also suggests that the immune response is to more than one epitope in this region, and the oligoclonality of response in the vitreous indicates T-cell epitope spreading to other ocular antigens within the eye and that certain T-cell populations may be retained within the ocular space. This evidence of multiple antigen specificity and epitope spreading within the eye of a single patient will certainly pose difficulties when devising rational peptide therapy for uveitis.

Figure 2 Evidence for multiple antigen specificity and epitope spreading in the response of a uveitis patient. CDR3 spectra-typing data of 22 TCR Vβ regions to show quantitative analysis of T-cell clones present within the peripheral blood (a), the vitreous (b), and the sub-retinal space (c). Monoclonality of response was evident in the sub-retina for Vβ6, Vβ12, Vβ15, Vβ17, and Vβ22, where only a single band was evident.

proliferation of activated T cells. These include Fas-Fas ligand induced apoptosis (36,37), and immunosuppressive cytokines such as TGFβ (1,2). Analysis of vitreous humor from uveitis patients and controls undergoing surgery (diabetes and cataract), showed that vitreous from uveitis patients contained high levels of

sFas-ligand (150–1500 pg/ml), whereas control levels were undetectable (cataract) or <30pg/ml (diabetes). This finding, together with data from an anterior

uveitis study that showed evidence for Fas-ligand–mediated apoptosis contributing to local immune regulation of ocular inflammation (38), provides a mechanism to explain the refractoriness of vitreous T cells to cloning and may account for the self-limiting clinical course of acute anterior uveitis. This hypothesis is also supported by our observation in EAU where FAS+ T cells in the vitreous were observed to undergo activation-induced cell death (39).

Apoptotic T cells also produce IL-10, and APCs taking up such apoptotic cells can induce tolerance rather than immunity (37). So why does chronic

inflammation persist in some uveitis patients? Genetically controlled immune dysfunction is thought to underlie persistent inflammation, and an imbalance in the production of other cytokines by ocular resident and infiltrating cells may continue to drive the immune response. Retinal pigment epithelial cells produce a wide range of both pro-and anti-inflammatory cytokines and other mediators in response to cytokine stimulation or contact with inflammatory cells (2), including TNF-α,

IL-6, and IL-8 (40–42), as well as PGE2, nitric oxide (NO), IL-15, and TGF-β (43,44). The outcome of any inflammatory response will therefore depend upon

the balance of mediators present. Interleukin 15, IL-8, and PGE2 all inhibit apoptosis by both stress-induced and Fas-ligand-induced pathways, potentially rescuing T cells from the effects of Fas ligation. Equally, uncontrolled production of excessive TNF-α and IL-1 will continue to drive damaging inflammation (45).

SUMMARY

This review highlighted data supporting the hypothesis that peripheral tolerance mechanisms for retinal S-antigen may be compromised. S-antigen can induce tissue DC maturation and migration, resulting in in vivo priming of immune responses in EAU. Although evidence for S-antigen peptide responsiveness in uveitis patients is accumulating, peptide therapy based on knowledge of pathogenic and regulatory epitopes on retinal antigens may not be practical, given the oligoclonal expansion and epitope spreading evident in patients. Therapeutic advances may lie in manipulation of APCs to induce tolerance. This could be directly through IL-10, or indirectly through administration of anti-pro-inflammatory cytokine reagents, such as TNF-α fusion proteins, allowing endogenous IL-10 or other suppressive factors to exert their effects, and allowing immunological homeostasis to be restored through generation of tolerance and regulatory cell populations (46–48).

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