CD8+ T Cells Play Disparate Roles in the Induction and the Effector Phases of Murine Experimental Allergic Conjunctivitis

Of mice and not humans: how reliable are animal models for evaluation of herpes CD8(+)-T cell-epitopes-based immunotherapeutic vaccine candidates?

Herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2)-specific CD8(+) T cells that reside in sensory ganglia, appear to control recurrent herpetic disease by aborting or reducing spontaneous and sporadic reactivations of latent virus. A reliable animal model is the ultimate key factor to test the efficacy of therapeutic vaccines that boost the level and the quality of sensory ganglia-resident CD8(+) T cells against spontaneous herpes reactivation from sensory neurons, yet its relevance has been often overlooked. Herpes vaccinologists are hesitant about using mouse as a model in pre-clinical development of therapeutic vaccines because they do not adequately mimic spontaneous viral shedding or recurrent symptomatic diseases, as occurs in human. Alternatives to mouse models are rabbits and guinea pigs in which reactivation arise spontaneously with clinical herpetic features relevant to human disease. However, while rabbits and guinea pigs develop spontaneous HSV reactivation and recurrent ocular and genital disease none of them can mount CD8(+) T cell responses specific to Human Leukocyte Antigen- (HLA-)restricted epitopes. In this review, we discuss the advantages and limitations of these animal models and describe a novel "humanized" HLA transgenic rabbit, which shows spontaneous HSV-1 reactivation, recurrent ocular disease and mounts CD8(+) T cell responses to HLA-restricted epitopes. Adequate investments are needed to develop reliable preclinical animal models, such as HLA class I and class II double transgenic rabbits and guinea pigs to balance the ethical and financial concerns associated with the rising number of unsuccessful clinical trials for therapeutic vaccine formulations tested in unreliable mouse models.

Immunopathogenesis of ocular allergy: a schematic approach to different clinical entities

PURPOSE OF REVIEW

The immunopathogenesis of ocular allergic disorders is generally related to the specific immunoglobulin E-mediated mast cell activation and the following cascade of inflammatory mediators. Seasonal and perennial allergic conjunctivitis, however, are the only ocular diseases to involve solely type I hypersensitivity. The other main forms, vernal and atopic keratoconjunctivitis, have a more complex immunological basis and a chronic inflammatory component. Involvement of inflammatory cells, particularly eosinophils and T cells, cytokines and proteases can lead to more serious corneal damage with vision-threatening potential.

RECENT FINDINGS

Experimental allergic conjunctival models and clinical research studies have shown that T helper type 2-related mechanisms are definitely involved in the sensitization phase of ocular allergy, however, both T helper type 1 and type 2 cytokines are overexpressed in the active disease, contributing to the development of ocular inflammation.

SUMMARY

A review of the recent literature allows us to better understand the mechanisms involved in the development of ocular allergy and to guide us toward a more schematic approach, which could possibly be useful in forming a new classification, standardizing clinical phases and individuating new treatment targets.

Classification of ocular allergy

PURPOSE OF REVIEW

The purpose of this article is to summarize the clinical presentations associated with the classification of ocular allergy. This article also serves to summarize recent findings of pathophysiological mechanisms associated with ocular allergy and to highlight recently improved diagnostic methods for ocular allergic inflammation.

RECENT FINDINGS

The term allergic conjunctivitis may not sufficiently describe all forms of allergic eye disease, thus a new classification system is desirable, preferably derived from the varied pathophysiological mechanisms operating in the different forms of ocular allergy. Recent published material has further characterized the roles that inflammatory and structural cells have in ocular allergic inflammation. Improved diagnostic methods have also been developed to assess the underlying causes of ocular allergy.

SUMMARY

The underlying immune responses of ocular allergies are complex, indicating the critical need to understand the pathophysiology behind these diseases. Extensive research over the past several years has provided valuable insight into understanding the pathophysiology associated with the different forms of allergic conjunctivitis. Further clarification of the mechanisms associated with different forms of ocular allergy is essential for improved methods of classification, diagnosis, and treatment.

Roles of T-Cells in the Development of Allergic Conjunctival Diseases

Massive infiltration of eosinophils is the typical histopathologic feature of severe forms of allergic conjunctival diseases (ACDs) such as vernal keratoconjunctivitis. Although ACD is an antigen (Ag)-specific disease, eosinophils lack Ag-specific receptors. Therefore, my group studied the likely roles of immunocompetent cells bearing Ag-specific receptors in inducing conjunctival eosinophil infiltration in ACDs. To induce experimental conjunctivitis (EC), Brown Norway rats and C57BL/6 and Balb/c mice were passively immunized by the transfer of Ag-specific or primed IgE, splenocytes, or purified T-cells and then challenged with Ag in eyedrops. The transfer of Ag-primed splenocytes or T-cells, but not of Ag-specific IgE, induced conjunctival eosinophil infiltration. We also showed that soon after EC induction by T-cell transfer, activated Ag-specific T-cells infiltrated the conjunctiva. Moreover, when EC-developing animals were topically treated with the T-cell-specific immunosuppressants cyclosporin A and FK506, conjunctival eosinophil infiltration was abrogated. In addition, only the transfer of Ag-specific type 2 helper T-cells (TH2 cells), but not of type 1 helper T-cells or CD8 T-cells, could induce conjunctival eosinophil infiltration. Our data show that Ag-specific TH2 cells play a crucial role in inducing conjunctival eosinophil infiltration during the development of EC. It is necessary to elucidate the roles that conjunctival residential cells and immunocompetent cells other than T-cells play in the conjunctival eosinophil infiltration of EC.

Endogenous interleukin-10 produced by antigen-irrelevant cells promotes the development of experimental murine allergic conjunctivitis

Interleukin (IL)-10 is known to act as an immunoregulatory cytokine in both T helper cell 1 (Th1)- and Th2-mediated immune responses. Here, we ask whether IL-10 regulates the development of experimental allergic conjunctivitis (EC), a Th2-mediated inflammatory disease. Wild-type (WT) and IL-10 knockout (IL-10 KO) mice were immunized with ragweed (RW) and then repeatedly challenged with RW in eye drops. Twenty-four hours after the final challenge, conjunctivas were harvested for histological analysis, while the blood and spleens were used to determine the RW-specific immunoglobulin levels in serum and proliferation or cytokine responses and splenocyte transfer, respectively. The IL-10 KO mice had significantly less severe EC (as determined by conjunctival eosinophil infiltration) than the WT mice and evinced greater RW-specific splenocyte proliferation and cytokine production. However, the RW-specific immunoglobulin levels of the two strains did not differ. When the splenocytes from RW-primed WT mice were transferred into IL-10 KO or WT mice, the IL-10 KO mice showed significantly less conjunctival eosinophil infiltration. In contrast, when the splenocytes from RW-primed IL-10 KO or WT mice were transferred into WT mice, both splenocyte populations generated equivalent severe EC. These data indicate that IL-10 does not serve as an immunoregulatory cytokine in the development of EC. Instead, it appears that IL-10 produced by antigen-irrelevant cells acts in the effector phase to promote the development of EC.

Functional Foxp3+ CD4+ CD25(Bright+) "natural" regulatory T cells are abundant in rabbit conjunctiva and suppress virus-specific CD4+ and CD8+ effector T cells during ocular herpes infection

We studied the phenotype and distribution of "naturally" occurring CD4(+) CD25(+) T regulatory cells (CD4(+) CD25(+) nT(reg) cells) resident in rabbit conjunctiva, the main T-cell inductive site of the ocular mucosal immune system, and we investigated their suppressive capacities using herpes simplex virus type 1 (HSV-1)-specific effector T (T(eff)) cells induced during ocular infection. The expression of CD4, CD25, CTLA4, GITR, and Foxp3 was examined by reverse transcription-PCR, Western blotting, and fluorescence-activated cell sorter analysis in CD45(+) pan-leukocytes isolated from conjunctiva, spleen, and peripheral blood monocyte cells (PBMC) of HSV-1-infected and uninfected rabbits. Normal conjunctiva showed a higher frequency of CD4(+) CD25((Bright+)) T cells than did spleen and PBMC. These cells expressed high levels of Foxp3, GITR, and CTLA4 molecules. CD4(+) CD25((Bright+)) T cells were localized continuously along the upper and lower palpebral and bulbar conjunctiva, throughout the epithelium and substantia propria. Conjunctiva-derived CD4(+) CD25((Bright+)) T cells, but not CD4(+) CD25((low)) T cells, efficiently suppressed HSV-specific CD4(+) and CD8(+) T(eff) cells. The CD4(+) CD25((Bright+)) T-cell-mediated suppression was effective on both peripheral blood and conjunctiva infiltrating T(eff) cells and was cell-cell contact dependent but independent of interleukin-10 and transforming growth factor beta. Interestingly, during an ocular herpes infection, there was a selective increase in the frequency and suppressive capacity of Foxp3(+) CD4(+) CD25((Bright+)) T cells in conjunctiva but not in the spleen or in peripheral blood. Altogether, these results provide the first evidence that functional Foxp3(+) CD4(+) CD25((Bright+)) T(reg) cells accumulate in the conjunctiva. It remains to be determined whether conjunctiva CD4(+) CD25(+) nT(reg) cells affect the topical/mucosal delivery of subunit vaccines that stimulate the ocular mucosal immune system.