Phenotypic and immunoregulatory characteristics of monocytic iris cells

Pathophysiological Aspects of Ocular Toxoplasmosis: Host-parasite Interactions

Purpose: This review aims to present the state of the art to understand the pathophysiology of ocular toxoplasmosis (OT), providing further foundations that would help to improve the future treatment and prognosis of this potentially blinding disease.Methods: A thorough literature search was performed in PubMed database. An additional search was made in Google Scholar to complete the collected items.Results: Toxoplasma gondii ocular infection is one of the most frequent causes of posterior uveitis. Despite the ocular barriers, the parasite reaches the eye through different mechanisms. Once inside, it remains encysted livelong within the retina, and recurrences cannot be completely avoided. The complexity of host-parasite interactions, leading to the success of this parasite, encompasses host factors such as genetic predisposition, immune status, and age; and parasite factors such as strain diversity, virulence, phylogenetic origin, and geographical distribution. These factors influence the clinical presentation, course, and progression of the disease. Additional elements, such as pregnancy, eating behavior, and environmental, social, and cultural factors may also contribute to this complex balance.Conclusions: The host-parasite interaction in OT is a complex and multifactorial relationship, with the parasite always on the driving edge of the game. There are still multiple incompletely understood fields to be investigated. Future research would permit further insight into the immune-biology of the parasite and recognition of the host-parasite interplay to improve the diagnostic and management performance.

Eye-mediated induction of specific immune tolerance to encephalitogenic antigens

AIMS

Administration of antigens into the anterior chamber (AC) of the eye induces a form of antigen-specific immune tolerance termed anterior chamber-associated immune deviation (ACAID). This immune tolerance effectively impairs host delayed-type hypersensitivity (DTH) responses. We hypothesized that ACAID could be generated in BALB/c mice following AC inoculation of the encephalitogenic antigens myelin oligodendrocyte glycoprotein (MOG) and myelin basic protein (MBP).

METHODS

We used DTH assays and local adoptive transfer (LAT) assays to test whether MOG/MBP-induced ACAID following their administration into the AC, whether they elicited this immune tolerance via CD8(+) T cells, and whether their AC coadministration (MOG/MBP) induced specific immune tolerance to one or both antigens.

RESULTS

We showed that MOG/MBP-induced AC-mediated specific immune tolerance, as evident from impaired DTH responses. This antigen-driven DTH suppression was solely mediated via splenic CD8(+) T cells as confirmed by LAT assays. Finally, a single AC injection with both antigens was sufficient to induce specific immune tolerance to these antigens, as evident from DTH and LAT assays.

CONCLUSION

ACAID T-cell regulation could be used as a therapeutic tool in the treatment of complicated autoimmune diseases that involve multiple antigens such as multiple sclerosis.

Immune amplification of murine CD8 suppressor T cells induced via an immune-privileged site: quantifying suppressor T cells functionally

Background

CD8⁺ suppressor T cells exert antigen-specific suppression of the expression of hypersensitivity by activated T cells. Therefore, CD8⁺ suppressor T cells serve a major regulatory role for the control of active immunity. Accordingly, the number and/or activity of CD8⁺ suppressor T cells should be influenced by an immune response to the antigen. To test this hypothesis we used an adoptive transfer assay that measures the suppression of the expression of delayed-type hypersensitivity (DTH) by CD8⁺ suppressor T cells to quantify the antigen-specific suppression of DTH by these suppressor T cells.

Methods

Suppressor T cells were induced in the spleens of mice by the injection of antigen into the anterior chamber of an eye. Following this injection, the mice were immunized by the same antigen injected into the anterior chamber. Spleen cells recovered from these mice (AC-SPL cells) were titrated in an adoptive transfer assay to determine the number of AC-SPL cells required to effect a 50% reduction of antigen-induced swelling (Sw50) in the footpad of immunized mice challenged by antigen.

Results

Suppression of the expression of DTH is proportional to the number of AC-SPL cells injected into the site challenged by antigen. The number of AC-SPL cells required for a 50% reduction in DTH-induced swelling is reduced by injecting a cell population enriched for CD8⁺ AC-SPL cells. Immunizing the mice receiving intracameral antigen to the same antigen decreases the RSw50 of AC-SPL cells required to inhibit the expression of DTH.

Conclusions

The results provide the first quantitative demonstration that the numbers of antigen-specific splenic CD8⁺ suppressor T cells are specifically amplified by antigen during an immune response.

Antigen-specific splenic CD4+ and CD8+ regulatory T cells generated via the eye, suppress experimental autoimmune encephalomyelitis either at the priming or at the effector phase

The injection of antigen into the ocular anterior chamber (AC) induces the generation of splenic CD4(+) and CD8(+) regulatory T (Treg) cells, specific for the antigen injected into the AC. These Treg cells inhibit the induction (CD4(+)) and also the expression (CD8(+)) of a delayed-type hypersensitivity response. The ability of AC-induced self-antigen-specific Treg cells in modulating autoimmunity is not well defined. Here we show that an injection of encephalitogenic myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide into the anterior chamber of the eye (AC-MOG), before the induction of or during established experimental autoimmune encephalomyelitis (EAE) induced by MOG(35-55), suppresses the induction or progression of EAE, respectively. CD4(+) or CD8(+) splenic Treg cells induced by an injection of AC-MOG prevent EAE either at the inductive (priming) or at the progressive (effector) phase, respectively. This suppression of EAE by an AC-MOG injection or by intravenous transfer of splenic regulatory cells induced by an AC-MOG injection is specific for the antigen injected into the AC. Additionally, our data suggest that splenic CD8(+) Treg cells that suppress active EAE may use a transforming growth factor (TGF)-β-dependent suppression mechanism while the suppression of the induction of EAE by the AC-induced CD4(+) Treg cells is independent of TGF-β. Thus, we show for the first time that regulation of EAE at the priming or the chronic phase requires different phenotypes of Treg cells. Hence, it is important to consider the phenotype of Treg cells while designing effective cell-based therapies against autoimmune disorders.

The Induction of Circulating, ACAID-Inducing Monocytes Requires CCR2/CCL2-Dependent Migration of Circulating F4/80(+) Cells into the Anterior Chamber

To determine the origin of peripheral blood mononulclear cells (PBMC) that activate regulatory T cells in anterior chamber-associated immune deviation (ACAID), fluorescein-labeled PBMC were intravenously injected into mice before the mice received an intracameral injection of antigen. Six-24 hr after intracameral injection, fluorescein-labeled PBMC increased in the iris. Twenty-four-48 hr labeled cells decreased in the iris and increased in the thymus and spleen. The entry of the labeled PBMC into the anterior chamber and subsequent production of PBMC that transfer ACAID required the expression of CCR2 by the PBMC and the production of the chemokine CCL2 by the recipient of the PBMC. The results suggest that the intracameral injection of antigen induces i) the infiltration of F4/80⁺ PBMC into the AC, ii) where these PBMC are converted to a regulatory phenotype, and iii) recirculate to activate T cells that suppress cell-mediated immunity.

Anterior Chamber-Associated Immune Deviation (ACAID): An Acute Response to Ocular Insult Protects from Future Immune-Mediated Damage?

The “immune privilege” that inhibits immune defense mechanisms that could lead to damage to sensitive ocular tissue is based on the expression of immunosuppressive factors on ocular tissue and in ocular fluids. In addition to this environmental protection, the injection of antigen into the anterior chamber or infection in the anterior chamber induces a systemic suppression of potentially damaging cell-mediated and humoral responses to the antigen. Here we discuss evidence that suggests that Anterior Chamber-Associated Immune Deviation (ACAID)^a is initiated by an ocular response to moderate inflammation that leads to a systemic immunoregulatory response. Injection into the anterior chamber induces a rise in TNF-α and MCP-1 in aqueous humor and an infiltration of circulating F4/80⁺ monocytes that home to the iris. The induction of ACAID is dependent on this infiltration of circulating monocytes that eventually emigrate to the thymus and spleen where they induce regulatory T cells that inhibit the inductive or effector phases of a cell-mediated immune response. ACAID therefore protects the eye from the collateral damage of an immune response to infection by suppressing a future potentially damaging response to infection.

Immune regulation and the eye

The eye is an immune privileged site that is styled to maintain the visual pathway while at the same time provide defense against invading organisms. The eye does this by selecting immune responses that function in the absence of inflammation. Immune regulation by the eye takes the form of several active processes including a local immunosuppressive environment, the contribution of soluble factors, Fas-FasL-induced apoptosis and unique suppressive mechanisms used by pigment epithelial cells in the eye. These processes are so effective that antigens encountered in the eye result in specific systemic tolerization; a phenomenon akin to gut-induced oral tolerance. This review discusses the cellular and molecular basis of tolerance induction by the eye and notes the parallels to gut-induced peripheral tolerance.

The suppression of hypersensitivity by ocular-induced CD8(+) T cells requires compatibility in the Qa-1 haplotype

The injection of antigen into the anterior chamber (AC, intracameral injection)2 of a murine eye induces the generation of splenic CD8⁺ regulatory T cells (AC-SPL cells) that effect the antigen-specific suppression of a Delayed-Type Hypersensitivity (DTH) reaction. Here we show (i) for the first time that the local antigen-specific suppression of DTH-induced swelling in immunized mice by either an intracameral injection of antigen or by the direct injection of CD8⁺ AC-SPL cells into an antigen-challenged site is associated with an absence of infiltrated mononuclear cells, (ii) the local antigen-specific suppression of the DTH reaction by CD8⁺ AC-SPL cells requires compatibility between the Qa-1 but not H2 antigen haplotype of the immunized recipient and the injected AC-SPL regulatory T cells, (iii) The suppression of the DTH reaction by CD8⁺ AC-SPL cells requires the expression of Qa-1 but not H2 antigens and is not due to bystander suppression.

Implication for the CD94/NKG2A-Qa-1 system in the generation and function of ocular-induced splenic CD8+ regulatory T cells

The injection of antigen into the anterior chamber (AC) induces the production of antigen-specific splenic CD8+ regulatory T cells (Tregs) /suppressor T cells that perform the local suppression of delayed-type hypersensitivity (DTH) responses. Because CD94/NKG2A-Qa-1-dependent interactions have been implicated in CD8+ Treg-mediated immune suppression and DBA/2J mice are deficient in CD94/NKG2R, we have utilized these mice to test the hypothesis that the CD94/NKG2A-Qa-1 system is essential to the induction and immunosuppressive function of CD8+ Tregs in anterior chamber-associated immune deviation (ACAID). We show that: (i) neither ACAID-mediated suppression of DTH to ovalbumin nor splenic Tregs/suppressor T cells was induced in DBA/2J mice that received an injection of antigen into the AC; (ii) splenic CD8+ Tregs from ACAID-induced DBA/2NCr mice suppressed the initiation of DTH when transferred to DBA/2J mice; (iii) following injection of antigen into the AC, intravenous administration of splenocytes or Peripheral Blood Mononuclear Cells (PBMC) isolated from DBA/2NCr but not from DBA/2J mice transferred suppression of DTH to DBA/2NCr mice; (iv) antibodies to CD94/NKG2A reduced the ACAID CD8+ T cell-mediated suppression of DTH and (v) The deficiency of such immune regulation in DBA/2J mice also correlated with a decreased number of Qa-1(b+) B cells, F4/80+ cells, a deficient number of CD94/NKG2AR and Qa-1 tetramer binding by CD8+ T cells. These results demonstrate that defective ACAID in DBA/2J mice involves multiple regulatory lesions resulting in a lack of induction of a CD8+ Treg response and possibly defective CD94/NKG2A-dependent suppression of peripheral cell-mediated immunity.

Toll-like receptors and immune regulation: their direct and indirect modulation on regulatory CD4+ CD25+ T cells

Regulatory CD4(+) CD25(+) T (Treg) cells with the ability to suppress host immune responses against self- or non-self antigens play important roles in the processes of autoimmunity, transplant rejection, infectious diseases and cancers. The proper regulation of CD4(+) CD25(+) Treg cells is thus critical for optimal immune responses. Toll-like receptor (TLR)-mediated recognition of specific structures of invading pathogens initiates innate as well as adaptive immune responses via antigen-presenting cells (APCs). Interestingly, new evidence suggests that TLR signalling may directly or indirectly regulate the immunosuppressive function of CD4(+) CD25(+) Treg cells in immune responses. TLR signalling may shift the balance between CD4(+) T-helper cells and Treg cells, and subsequently influence the outcome of the immune response. This immunomodulation pathway may therefore have potential applications in the treatment of graft rejection, autoimmune diseases, infection diseases and cancers.

The suppression of delayed-type hypersensitivity by CD8+ regulatory T cells requires interferon-gamma

CD8(+) regulatory (suppressor) T cells are induced by complex cellular pathways in the spleens of mice that have received an injection of antigen into the anterior chamber (AC) of an eye, an immune-privileged site. Although these CD8(+) regulatory T cells perform an antigen-specific regulatory function for an immune response to self and non-self antigens, the mechanisms of the activation or function of these regulatory cells are not clear. Here, we describe a novel mechanism for the activation of splenic CD8(+) regulatory T cells induced by injection of antigen into the AC. Immunization of mice with trinitrophenyl and bovine serum albumin (TNP-BSA) amplified AC-induced splenic CD8(+) regulatory T cells that suppressed the initiation of contact sensitivity when transferred to immunized, challenged mice. These CD8(+) regulatory T cells were produced independently of perforin, indicating that they are not canonical cytotoxic T cells. Fas ligand (FasL)-deficient CD8(+) regulatory T-cell function was rescued by inclusion of exogenous interferon-gamma (IFN-gamma), demonstrating that the expression of FasL by CD8(+) regulatory T cells was dispensable, but IFN-gamma was not. Ultimately, we demonstrated that the generation of these CD8(+) regulatory T cells occurred independently of IFN-gamma, but their suppressor function required IFN-gamma receptor stimulation.