Expression of a nonclassical MHC class Ib molecule in the eye

The anterior chamber of the eye technology and its anatomical, optical, and immunological bases

The anterior chamber of the eye (ACE) is distinct in its anatomy, optics, and immunology. This guarantees that the eye perceives visual information in the context of physiology even when encountering adverse incidents like inflammation. In addition, this endows the ACE with the special nursery bed iris enriched in vasculatures and nerves. The ACE constitutes a confined space enclosing an oxygen/nutrient-rich, immune-privileged, and less stressful milieu as well as an optically transparent medium. Therefore, aside from visual perception, the ACE unexpectedly serves as an excellent transplantation site for different body parts and a unique platform for noninvasive, longitudinal, and intravital microimaging of different grafts. On the basis of these merits, the ACE technology has evolved from the prototypical through the conventional to the advanced version. Studies using this technology as a versatile biomedical research platform have led to a diverse range of basic knowledge and in-depth understanding of a variety of cells, tissues, and organs as well as artificial biomaterials, pharmaceuticals, and abiotic substances. Remarkably, the technology turns in vivo dynamic imaging of the morphological characteristics, organotypic features, developmental fates, and specific functions of intracameral grafts into reality under physiological and pathological conditions. Here we review the anatomical, optical, and immunological bases as well as technical details of the ACE technology. Moreover, we discuss major achievements obtained and potential prospective avenues for this technology.

Ocular immunosuppressive microenvironment and novel drug delivery for control of uveitis

Ocular immune privilege is a phenomenon described by Peter Medawar in relation to the indefinite survival of the placement of foreign tissue grafts into the eye. Several mechanisms have been described that contribute to ocular immune privilege, such as a blood-ocular barrier and lack of lymphatics in the eye, the production of immune-suppressing molecules inside the ocular microenvironment, and the induction of systemic regulatory immunity against antigens found in the eye. Because ocular immune privilege is not absolute, failure of it can result in uveitis. Uveitis is a group of inflammatory disorders that can lead to vision loss if not treated properly. The current uveitis treatments involve the use of immunosuppressive and anti-inflammatory medications. Researching mechanisms of ocular immune privilege and the development of novel treatments for uveitis is ongoing. This review discusses mechanisms of ocular immune privilege, followed by an overview of uveitis treatments and ongoing clinical trials.

A Fresh Glimpse into Cartilage Immune Privilege

The increasing prevalence of degenerative cartilage disorders in young patients is a growing public concern worldwide. Cartilage's poor innate regenerative capacity has inspired the exploration and development of cartilage replacement treatments such as tissue-engineered cartilages and osteochondral implants as potential solutions to cartilage loss. The clinical application of tissue-engineered implants is hindered by the lack of long-term follow-up demonstrating efficacy, biocompatibility, and bio-integration. The historically reported immunological privilege of cartilage tissue was based on histomorphological observations pointing out the lack of vascularity and the presence of a tight extracellular matrix. However, clinical studies in humans and animals do not unequivocally support the immune-privilege theory. More in-depth studies on cartilage immunology are needed to make clinical advances such as tissue engineering more applicable. This review analyzes the literature that supports and opposes the concept that cartilage is an immune-privileged tissue and provides insight into mechanisms conferring various degrees of immune privilege to other, more in-depth studied tissues such as testis, eyes, brain, and cancer.

Role of Natural Killer Cells in Uveal Melanoma

Simple Summary

Metastatic Uveal Melanoma (MUM) is a lethal malignancy with no durable treatment available to date. A vast majority of patients with MUM present with liver metastasis. The liver harbors metastatic disease with an apparent lack of a cytotoxic T cell response. It is becoming evident that MUM is not an immunologically silent malignancy and the investigation of non-T cell anti-tumor immunity is warranted. In this review, we highlight the relevance of Natural Killer (NK) cells in the biology and treatment of MUM. Potent anti-NK cell immunosuppression employed by uveal melanoma alludes to its vulnerability to NK cell cytotoxicity. On the contrary, micro-metastasis in the liver survive for several years within close vicinity of a plethora of circulating and liver-resident NK cells. This review provides unique perspectives into the potential role of NK cells in control or progression of uveal melanoma.

Abstract

Uveal melanoma has a high mortality rate following metastasis to the liver. Despite advances in systemic immune therapy, treatment of metastatic uveal melanoma (MUM) has failed to achieve long term durable responses. Barriers to success with immune therapy include the immune regulatory nature of uveal melanoma as well as the immune tolerant environment of the liver. To adequately harness the anti-tumor potential of the immune system, non-T cell-based approaches need to be explored. Natural Killer (NK) cells possess potent ability to target tumor cells via innate and adaptive responses. In this review, we discuss evidence that highlights the role of NK cell surveillance and targeting of uveal melanoma. We also discuss the repertoire of intra-hepatic NK cells. The human liver has a vast and diverse lymphoid population and NK cells comprise 50% of the hepatic lymphocytes. Hepatic NK cells share a common niche with uveal melanoma micro-metastasis within the liver sinusoids. It is, therefore, crucial to understand and investigate the role of intra-hepatic NK cells in the control or progression of MUM.

Immune responses to retinal gene therapy using adeno-associated viral vectors - Implications for treatment success and safety

Recombinant adeno-associated virus (AAV) is the leading vector for gene therapy in the retina. As non-pathogenic, non-integrating, replication deficient vector, the recombinant virus efficiently transduces all key retinal cell populations. Successful testing of AAV vectors in clinical trials of inherited retinal diseases led to the recent approval of voretigene neparvovec (Luxturna) for the treatment of RPE65 mutation-associated retinal dystrophies. However, studies applying AAV-mediated retinal gene therapy independently reported intraocular inflammation and/or loss of efficacy after initial functional improvements. Both observations might be explained by targeted removal of transduced cells via anti-viral defence mechanisms. AAV has been shown to activate innate pattern recognition receptors (PRRs) such as toll-like receptor (TLR)-2 and TLR-9 resulting in the release of inflammatory cytokines and type I interferons. The vector can also induce capsid-specific and transgene-specific T cell responses and neutralizing anti-AAV antibodies which both limit the therapeutic effect. However, the target organ of retinal gene therapy, the eye, is known as an immune-privileged site. It is characterized by suppression of inflammation and promotion of immune tolerance which might prevent AAV-induced immune responses. This review evaluates AAV-related immune responses, toxicity and inflammation in studies of retinal gene therapy, identifies influencing variables of these responses and discusses potential strategies to modulate immune reactions to AAV vectors to increase the safety and efficacy of ocular gene therapy.

Humic-acid-driven escape from eye parasites revealed by RNA-seq and target-specific metabarcoding

Background

Next generation sequencing (NGS) technologies are extensively used to dissect the molecular mechanisms of host-parasite interactions in human pathogens. However, ecological studies have yet to fully exploit the power of NGS as a rich source for formulating and testing new hypotheses.

Methods

We studied Eurasian perch (Perca fluviatilis) and its eye parasite (Trematoda, Diplostomidae) communities in 14 lakes that differed in humic content in order to explore host-parasite-environment interactions. We hypothesised that high humic content along with low pH would decrease the abundance of the intermediate hosts (gastropods), thus limiting the occurrence of diplostomid parasites in humic lakes. This hypothesis was initially invoked by whole eye RNA-seq data analysis and subsequently tested using PCR-based detection and a novel targeted metabarcoding approach.

Results

Whole eye transcriptome results revealed overexpression of immune-related genes and the presence of eye parasite sequences in RNA-seq data obtained from perch living in clear-water lakes. Both PCR-based and targeted-metabarcoding approach showed that perch from humic lakes were completely free from diplostomid parasites, while the prevalence of eye flukes in clear-water lakes that contain low amounts of humic substances was close to 100%, with the majority of NGS reads assigned to Tylodelphys clavata.

Conclusions

High intraspecific diversity of T. clavata indicates that massively parallel sequencing of naturally pooled samples represents an efficient and powerful strategy for shedding light on cryptic diversity of eye parasites. Our results demonstrate that perch populations in clear-water lakes experience contrasting eye parasite pressure compared to those from humic lakes, which is reflected by prevalent differences in the expression of immune-related genes in the eye. This study highlights the utility of NGS to discover novel host-parasite-environment interactions and provide unprecedented power to characterize the molecular diversity of cryptic parasites.

Mammalian non-classical major histocompatibility complex I and its receptors: Important contexts of gene, evolution, and immunity

The evolutionary conserved, less-polymorphic, nonclassical major histocompatibility complex (MHC) class I molecules: Qa-1 and its human homologue human leukocyte antigen-E (HLA-E) along with HLA-F, G and H cross-talk with the T-cell receptors and also interact with natural killer T-cells and other lymphocytes. Moreover, these nonclassical MHC molecules are known to interact with CD94/NKG2 heterodimeric receptors to induce immune responses and immune regulations. This dual role of Qa-1/HLA-E in terms of innate and adaptive immunity makes them more interesting. This review highlights the new updates of the mammalian nonclassical MHC-I molecules in terms of their gene organization, evolutionary perspective and their role in immunity.

Corneal lymphangiogenesis in herpetic stromal keratitis

Corneal lymphangiogenesis is the extension of lymphatic vessels into the normally alymphatic cornea, a process that compromises the cornea's immune-privileged state and facilitates herpetic stromal keratitis (HSK). HSK results most commonly from infection by herpes simplex virus-1 (HSV-1) and is characterized by immune- and inflammation-mediated damage to the deep layers of the cornea. Current research demonstrates the potential of anti-lymphangiogenic therapy to decrease and prevent herpes-induced lymphangiogenesis.

Role of NKT cells in anterior chamber-associated immune deviation

Cells in the eye have a limited capacity for regeneration and, as such, immune-mediated inflammation can lead to blindness. The eye is designed to quench immune-mediated inflammation - a condition known as immune privilege. An important component of immune privilege is the dynamic immunoregulatory process termed anterior chamber-associated immune deviation (ACAID), which is initiated when antigens enter the eye. ACAID suppresses the initiation of antigen-specific inflammation in the eye and the effector stages of immune reactions. Four organ systems are crucial for the induction of ACAID: the eye, thymus, spleen and sympathetic nervous system. Multiple cell populations contribute to ACAID, with natural killer T cells playing a crucial role in the thymic and splenic phases of ACAID. Interactions between natural killer T cells and multiple cell populations in the spleen culminate in the tight regulation of immune-mediated inflammation in the eye and the preservation of vision.

Ocular immune privilege and ocular melanoma: parallel universes or immunological plagiarism?

Evidence of immune privilege in the eye was recorded almost 140 years ago, yet interest in immune privilege languished for almost a century. However, the past 35 years have witnessed a plethora of research and a rekindled interest in the mechanisms responsible for immune privilege in the anterior chamber of the eye. This research has demonstrated that multiple anatomical, structural, physiological, and immunoregulatory processes contribute to immune privilege and remind us of the enormous complexity of this phenomenon. It is widely accepted that immune privilege is an adaptation for reducing the risk of immune-mediated inflammation in organs such as the eye and brain whose tissues have a limited capacity to regenerate. Recent findings suggest that immune privilege also occurs in sites where stem cells reside and raise the possibility that immune privilege is also designed to prevent the unwitting elimination of stem cells by immune-mediated inflammation at these sites. Uveal melanoma arises within the eye and as such, benefits from ocular immune privilege. A significant body of research reveals an intriguing parallel between the mechanisms that contribute to immune privilege in the eye and those strategies used by uveal melanoma cells to evade immune elimination once they have disseminated from the eye and establish metastatic foci in the liver. Uveal melanoma metastases seem to have “plagiarized” the blueprints used for ocular immune privilege to create “ad hoc immune privileged sites” in the liver.

Arginine depletion as a mechanism for the immune privilege of corneal allografts

The cornea is an immune privileged tissue. Since arginase has been found to modulate T-cell function by depleting arginine, we investigated the expression of arginase in the cornea and its possible role in immune privilege using a murine transplant model. We found that both the endothelium and epithelium of murine corneas express functional arginase I, capable of down-regulating T-cell proliferation in an in vitro culture system. The administration of the specific arginase inhibitor N-hydroxy-nor-L-Arg to recipient mice resulted in an accelerated rejection of allogeneic C57BL/6 (B6) corneal grafts. In contrast, in vivo blockade of arginase activity had no effect in altering the course of rejection of primary skin grafts that express little, if any, arginase. In addition, the inhibition of arginase did not alter systemic T-cell proliferation. These data show that arginase is functional in the cornea and contributes to the immune privilege of the eye, and that modulation of arginase contributes to graft survival.

Abrogating TNF-α expression prevents bystander destruction of normal tissues during iNOS-mediated elimination of intraocular tumors

Although intraocular tumors reside in an immune privileged site, some tumors are rejected nonetheless. For example, intraocular adenovirus-induced (Ad5E1; adenovirus type 5 early region 1) tumors are rejected in syngeneic C57BL/6 mice by one of two pathways. One pathway leads to extensive necrosis of innocent bystander cells and culminates in destruction of the eye, a condition called phthisis. The second pathway is characterized by piecemeal tumor cell death that rids the eye of the tumor while preserving the architecture and function of the eye. To study the mechanisms of phthisical tumor rejection, we isolated a cell clone-designated clone 2.1 that consistently undergoes rejection in a phthisical manner. CD4(+) T cells and macrophages were required for phthisical rejection of intraocular clone 2.1 tumors and M1 macrophages were involved in mediating tumor rejection. In vitro and in vivo inhibition of iNOS (inducible nitric oxide synthase) abolished macrophage-mediated killing of tumor cells and rejection of intraocular tumors. A role for M1 macrophages was further supported by investigations showing that intraocular tumors grew progressively in IFN-γ KO (knockout) mice. Studies in mice deficient in TNF-α, TNF receptor-1, or TNF receptor-2 revealed that although TNF-α was not needed for tumor rejection, it was required for the development of necrotizing inflammation and phthisis of tumor-bearing eyes. Together, our findings suggest new strategies to successfully eliminate ocular tumors while preserving the integrity of the eye.

Role of the Fas pathway in Pseudomonas aeruginosa keratitis

PURPOSE

The role of the Fas pathway was tested in Pseudomonas aeruginosa-infected mouse cornea by contrasting the responses of FasL(-/-) and wild-type (WT) mice.

METHODS

TUNEL staining, real-time RT-PCR, immunostaining, and ELISA assay were used.

RESULTS

Compared with WT (resistant) mice, BALB/c FasL(-/-) exhibited significantly elevated bacterial counts and polymorphonuclear leukocyte numbers at 1 and 3 days postinfection (p.i.) and worse outcomes from disease. Similar bacterial challenges in C57BL/6 FasL(-/-) compared with WT mice also led to worsened disease as evidenced by earlier corneal perforation in the susceptible mouse strain. Intense TUNEL staining of apoptotic cells was seen earlier (1 day vs. 3 days) p.i. in BALB/c WT than in knockout mice, This earlier apoptotic pattern correlated with increased expression of caspases 3, 8, and 9 and BAX and with decreased expression of the antiapoptotic molecule Bcl-2. Furthermore, expression levels of the proinflammatory molecule TNF-alpha and its receptor, MIP-2, inducible nitric oxide synthase (iNOS), and nitrite also were significantly elevated in the infected cornea of BALB/c FasL(-/-) compared with WT mice. In vitro, LPS-stimulated Mphi from BALB/c FasL(-/-) mice expressed significantly less caspase 3 and 9, BAX, and IL-10 and more TNF-alpha, MIP-2, and IL-1beta than did cells from WT mice.

CONCLUSIONS

Fas-FasL interaction in the cornea balances the host innate immune response to improve disease outcome by promoting earlier apoptosis and regulating proinflammatory cytokines/chemokines and nitric oxide (nitrite) production. Dysregulation of this interaction contributes to bystander tissue damage, enhancing nutrients for bacterial growth and worsened disease outcome after P. aeruginosa infection.

Immune escape mechanisms of intraocular tumors

The notion that the immune system might control the growth of tumors was suggested over 100 years ago by the eminent microbiologist Paul Ehrlich. This concept was refined and expanded by Burnet and Thomas 50 years later with their articulation of the "immune surveillance" hypothesis. In its simplest form, the immune surveillance hypothesis suggests that neoplasms arise spontaneously and express novel antigens that are recognized by the immune system, which either eliminates the tumors or restrains their growth. Within the eye, immune responses are controlled and sometimes profoundly inhibited - a condition known as immune privilege. Immune privilege in the eye is the result of a complex array of anatomical, physiological, and immunoregulatory mechanisms that prevent the induction and expression of many immune responses. Tumors arising in the eye would seem to have an advantage in evading immune surveillance due to ocular immune privilege. Uveal melanoma, the most common and malignant intraocular tumor in adults, not only benefits from the immune privilege of the eye but also has adopted many of the mechanisms that contribute to ocular immune privilege as a strategy for protecting uveal melanoma cells once they leave the sanctuary of the eye and are disseminated systemically in the form of metastases. Although the immune system possesses a battery of effector mechanisms designed to rid the body of neoplasms, tumors are capable of rapidly evolving and countering even the most sophisticated immunological effector mechanisms. To date, tumors seem to be winning this arms race, but an increased understanding of these mechanisms should provide insights for designing immunotherapy that was envisioned over half a century ago, but has failed to materialize to date.

Qa-1, a Nonclassical MHC Molecule with Immunomodulatory Functions, Is Ubiquitously Expressed in the Immune-Privileged Anterior Chamber of the Eye

PURPOSE

To determine whether the MHC class Ib gene, Qa-1, is expressed in the tissues that surround the immune-privileged anterior chamber (AC) of the murine eye.

METHODS

Transcription of Qa-1 mRNA in BALB/c ocular tissues was analyzed by reverse transcription-polymerase chain reaction. Expression of Qa-1 protein was assessed on ocular frozen tissue sections by immunohistochemistry, and within aqueous humor by western blotting.

RESULTS

Transcription of Qa-1 was found in all tissues surrounding the AC of the eye. Immunohistological staining revealed Qa-1 expression on corneal endothelium, corneal epithelium, and lens epithelium. No soluble Qa-1 was detected in aqueous humor.

CONCLUSIONS

Qa-1, unlike other MHC class I molecules, is ubiquitously expressed in tissues surrounding the AC of the eye, raising the possibility that Qa-1 plays a role in creating and maintaining ocular immune privilege.

Evidence that the bulge region is a site of relative immune privilege in human hair follicles

BACKGROUND

Recent gene profiling data suggest that, besides the anagen hair bulb, the epithelial stem cell region in the outer root sheath of hair follicles (HFs), termed the bulge, may also represent an area of relative immune privilege (IP).

OBJECTIVES

To investigate whether the human HF bulge is a site of relative IP within anagen VI HFs.

METHODS

Anagen VI HFs from normal human scalp skin were analysed using immunohistological staining techniques, quantitative histomorphometry and statistical analysis. For functional evidence we performed full-thickness human scalp skin organ cultures to investigate whether interferon (IFN)-gamma, a key inducer of IP collapse in hair bulbs, has a similar effect on the putative bulge IP.

RESULTS

Major histocompatibility complex (MHC) class Ia, beta(2)-microglobulin and MHC class II immunoreactivity are downregulated in the human bulge. The immunosuppressants alpha-melanocyte stimulating hormone, transforming growth factor-beta2, macrophage migration inhibitory factor and indoleamine-2,3-dioxygenase (IDO) are upregulated in the CD200+, stem cell-rich bulge region. These CD200+ cells also co-express HLA-E. Furthermore, IFN-gamma induces significant ectopic MHC class Ia expression in bulge cells of organ-cultured human scalp skin.

CONCLUSIONS

These data suggest that the bulge of human anagen HFs represents a hitherto unrecognized site of relative IP in human skin. Simultaneously, we present the first evidence of IDO and HLA-E protein expression in normal human HFs. Bulge IP presumably protects the HF epithelial stem cell reservoir from autoaggressive immune attack whereas a loss of bulge IP may play a central role in the pathogenesis of cicatricial alopecias.

The role of tapasin in MHC class I protein trafficking in embryos and T cells

Preimplantation mouse embryos express both classical (class Ia) and nonclassical (class Ib) MHC class I proteins, and yet are not rejected by the maternal immune system. Although the function of the embryonic MHC class Ia proteins is unknown, one MHC class Ib protein, Qa-2, the product of the preimplantation embryo development (Ped) gene, actually enhances reproductive success. Similar in structure to MHC class Ia proteins, Qa-2 protein is a trimer of the alpha (heavy) chain, beta(2) microglobulin and a bound peptide. Studies on the folding, assembly and trafficking of MHC class Ia molecules to the cell surface have revealed this process to be dependent on multiple protein chaperone molecules, but information on the role of chaperone molecules in Qa-2 expression is incomplete. Here, we report the detection of mRNA for four chaperone molecules (TAP1, TAP2, calnexin and tapasin) in preimplantation embryos. We then focused on the role of the MHC-dedicated chaperone, tapasin, on Qa-2 protein expression. First, we demonstrated that tapasin protein is expressed by preimplantation embryos. Then, we used tapasin knockout mice to evaluate the role of tapasin in Qa-2 protein expression on both T cells and preimplantation embryos. We report here that optimal cell surface expression of Qa-2 is dependent on tapasin in both T cells and preimplantation embryos. Identification of the molecules involved in regulation of MHC class I protein expression in early embryos is an important first step in gaining insight into mechanisms of escape of embryos from destruction by the maternal immune system.

B7-H1-induced apoptosis as a mechanism of immune privilege of corneal allografts

The programmed death-1 (PD-1) costimulatory pathway has been demonstrated to play a role in the regulation of immune responses and peripheral tolerance. We investigated the role of this pathway in establishing an immune privilege status of corneal allografts in mice. B7-H1, but not B7-DC or PD-1, was expressed constitutively in the eye, i.e., cornea, iris-ciliary body, and retina. After corneal allografting, PD-1(+)CD4(+) T cells infiltrated and adhered with B7-H1(+) corneal endothelium. Blockade of PD-1 or B7-H1, but not B7-DC, led to accelerated corneal allograft rejection. In B7-H1-expressing corneal allografts, apoptosis of the infiltrating PD-1(+)CD4(+) or CD8(+) T cells was observed, after which there was allograft acceptance. In contrast, B7-H1 blockade suppressed apoptosis of infiltrating PD-1(+) T cells, which led to allograft rejection. In vitro, destruction of corneal endothelial cells by alloreactive T cells was enhanced when the cornea was pretreated with anti-B7-H1 Ab. This is the first demonstration that the constitutive expression of B7-H1 plays a critical role in corneal allograft survival. B7-H1 expressed on corneal endothelial cells maintains long-term acceptance of the corneal allografts by inducing apoptosis of effector T cells within the cornea.

A structural perspective on MHC class Ib molecules in adaptive immunity

The highly polymorphic MHC class Ia molecules have a central role in adaptive immunity. By contrast, the closely related MHC class Ib molecules, which show limited polymorphism, are best known for regulating innate immune responses. Nevertheless, a recent area of interest is the emerging role of class Ib molecules in adaptive immunity, particularly in response to tumours and pathogens such as Mycobacteria, Listeria and Salmonella. Here, we review recent findings in this area, highlighting the structure of a T-cell receptor complexed with a cytomegalovirus peptide bound to the class Ib molecule, HLA-E. Collectively, these findings have implications for immunity, transplantation and autoimmunity, and our understanding of the evolution and plasticity of the molecular interactions mediating adaptive immunity.

See no evil, hear no evil, do no evil: the lessons of immune privilege

Immune-mediated inflammation and allograft rejection are greatly reduced in certain organs, a phenomenon called 'immune privilege'. Immune privilege is well developed in three regions of the body: the eye, the brain and the pregnant uterus. Immune-mediated inflammation has devastating consequences in the eye and brain, which have limited capacity for regeneration. Likewise, loss of immune privilege at the maternal-fetal interface culminates in abortion in rodents. However, all three regions share many adaptations that restrict the induction and expression of immune-mediated inflammation. A growing body of evidence from rodent studies suggests that a breakdown in immune privilege contributes to multiple sclerosis, uveitis, corneal allograft rejection and possibly even immune abortion.

Indoleamine 2,3-dioxygenase protects corneal endothelial cells from UV mediated damage

Indoleamine-2,3-dioxygenase (IDO) is an intracellular enzyme present in dendritic cells and macrophages. It is a known modulator of T-cell response and contributes to the UV protection of the lens. There yet is no information on IDO activity in the corneal endothelium, protecting the endothelial cells from light mediated damage. We exposed murine corneal endothelial cells (MCEC) with different doses of UV-B light 280-320 nm, probed for IDO mRNA (real-time PCR) and assessed apoptosis rate (flow cytometry) and caspase-3-activity in the cells. The metabolites of the IDO catalysed reaction, l-kynurenine, was also measured. Malondialdehyde was detected for quantification of UV-B-induced oxidative stress. To investigate specificity, IDO effects were blocked by 1-methyl-tryptophan. The effects of IDO overexpression in the MCEC were assessed by transfection of an expression vector. MCEC consistently express IDO at low levels. Exposure to UV-B light led to a dose-responding upregulation of IDO; IDO was found competent converting l-tryptophan into l-kynurenine. Irradiation led to increased apoptosis and caspase-3-activity of MCEC. Supplementation of l-kynurenine or overexpression of IDO in the MCEC could reduce apoptosis significantly following UV-B irradiation. Inhibition of IDO by 1-MT was potent to reverse this effect. IDO and its metabolite l-kynurenine can protect corneal endothelial cells from UV-B-induced oxidative stress and apoptosis. It may be an active protection mechanism against corneal endothelial damage.

Immune Privilege of the Eye and Fetus: Parallel Universes?

The eye is an extension of the brain and thus many of its tissues are incapable of regeneration. Ocular inflammation can produce extensive damage to innocent bystander cells leading to blindness. However, the eye possesses multiple strategies to control immune-mediated inflammation-a phenomenon known as immune privilege. The fetus of outbred mammals expresses paternal histocompatibility antigens and represents an allograft. However, the success of placental animals is a testament to the immune privilege of the allogeneic fetus. Extensive evidence suggests that the eye and the fetus employ similar strategies for establishing immune privilege for preserving vision and the unborn respectively.

Vitamins inhibit oxidant-induced apoptosis of corneal endothelial cells

PURPOSE

To determine the effects of vitamins A, C, and E supplementation on lipid peroxidation and apoptosis in corneal endothelial cells.

METHODS

Murine corneal endothelial cells were maintained in tissue culture medium supplemented with free iron ions, known to lead to increased lipid peroxidation. The concentration of antioxidative vitamins (ascorbic acid, tocopherol, and retinoic acid) in the cells and supernatant was determined using reversed-phase high-performance liquid chromatography. Apoptosis was assessed by quantification of caspase-3-like activity, using annexin-V/propidium iodide stains for flow cytometry. Lipid peroxidation was assessed using the malondialdehyde method. Supplementation of antioxidative vitamins was tested in the setting of apoptosis.

RESULTS

Increasing levels of free iron led to a rapid loss of antioxidative vitamins in the supernatant and corneal endothelial cells. This was correlated with rising levels of malondialdehyde and increased apoptosis. Supplementation with ascorbic acid or alpha-tocopherol alone was not sufficient to prevent lipid peroxidation in the cells, whereas a combination of vitamins C and E was able to do so. In contrast, supplementation with vitamin A alone significantly reduced oxidative stress and apoptosis.

CONCLUSIONS

We present an in vitro model to test the direct influence of vitamin supplementation on corneal endothelial cells with regard to lipid peroxidation and apoptosis. We show that supplementation with antioxidative vitamins of corneal endothelial cells significantly prevents the generation of free-radical injury, lipid peroxidation, and consequent apoptosis.

MHC class I and II antigens as targets of rejection in penetrating keratoplasty in low- and high-risk mouse eyes

PURPOSE

To determine the extent to which expression of major histocompatibility complex (MHC) class I and II molecules contributes to rejection of orthotopic corneal transplants in mice.

METHODS

Full-thickness corneas, prepared from eyes of normal C57BL/6 (B6) and BALB/c mice and from B6 mice in which the class II gene I-A or the beta2-microglobulin (beta2mu) gene was disrupted, were placed orthotopically in low- or high-risk eyes of BALB/c (fully incompatible), BALB.B [minor histoincompatible (H) only], and bm12 (class II only disparate) recipients. BALB/c grafts were placed in low-risk eyes of normal B6 and B6 mice with disrupted H-2 DMalpha genes. Graft survival was judged by clinical examination.

RESULTS

Recipient-identical class II, but not class I, molecules on fully allogeneic corneas grafted to low-risk beds promoted graft rejection. Allogeneic class II molecules on fully allogeneic corneas placed in high-risk beds promoted graft rejection more strongly than did allogeneic class I molecules. Neither allogeneic class I molecules nor recipient-identical class II molecules on grafts placed in high-risk beds contributed to graft outcome. Mice deficient in H-2 DMalpha failed to reject fully incompatible cornea grafts.

CONCLUSIONS

On corneal allografts, where minor H antigens are the major barriers to acceptance, allogeneic class II molecules promote rejection if the graft is placed in high-risk eyes, whereas recipient-type class II molecules promote rejection if the graft is placed in low-risk eyes. Allogeneic class I molecules make a minor contribution to rejection only if the grafts are placed in high-risk eyes.

Anti-oxidative vitamins prevent lipid-peroxidation and apoptosis in corneal endothelial cells

To determine the effects of vitamin supplementation on the lipid-peroxidation-mediated toxicity of iron-ions on corneal endothelial cells (CECs) leading to apoptosis, murine CECs were maintained in tissue culture medium supplemented with increasing concentrations of free iron-ions, a treatment known to lead to increased lipid-peroxidation. The concentration of anti-oxidative vitamins (ascorbic acid, tocopherol and retinoic acid) in the cell supernatant and in the cells was determined by high-pressure liquid chromatography. Apoptosis was assessed by quantification of caspase-3-like activity and by using annexin-V/propidium iodide stains for flow cytometry. Lipid-peroxidation was measured by the malondialdehyde method. Supplementation with anti-oxidative vitamins was tested for the ability to counteract the induction of apoptosis. The production of nitric oxide was assessed spectrophotometrically and the expression levels of inducible and endothelial nitric oxide synthase were determined by Western blot. Increasing levels of free iron led to a rapid loss of anti-oxidative vitamins in the supernatant and in the CECs. This was correlated with rising levels of malondialdehyde and increased apoptosis. Supplementation with ascorbic acid or alpha-tocopherol alone did not prevent lipid-peroxidation in the cells. A combination of vitamins C and E (ascorbic acid, tocopherol) or solitary supplementation with vitamin A (retinoic acid) prevented lipid-peroxidation. We thus present a novel in vitro model for testing the direct influence of pro-oxidative species on CECs. We also show that supplementation with anti-oxidative vitamins to CECs significantly prevents the generation of free-radical-induced oxidative injury and apoptosis. These findings may have important implications for the storage of human corneae prior to transplantation and for the prolongation of corneal graft survival.

Mechanisms of immune privilege in the eye and hair follicle

It has been recognized for over a century that the eye is endowed with remarkable properties that permit the long-term survival of foreign tumor and tissue grafts that are normally rejected at extraocular sites. This ocular immune privilege was originally attributed to a putative sequestration of antigens in the eye as a result of the conspicuous absence of intraocular lymphatic drainage channels. In the last 30 years, a sizeable body of information indicates that ocular immune privilege is a product of multiple anatomical, physiological, and immunoregulatory processes. Ocular tissues and fluids express a wide variety of anti-inflammatory and immunosuppressive molecules, including CD95L (FasL), transforming growth factor-beta, macrophage migration inhibitory factor, alpha-melanocyte-stimulating hormone, calcitonin gene-related peptide, somatostatin, and complement regulatory proteins. Moreover, antigens entering the anterior chamber of the eye evoke a unique form of immune deviation that culminates in the antigen-specific suppression of TH1 immune responses. Finally, the intraocular milieu contains both cell membrane and soluble factors that inhibit both the adaptive and innate immune systems. The hair follicle is also recognized for its immune privilege. Like the anterior chamber of the eye, it produces anti-inflammatory and immunosuppressive cytokines, such as transforming growth factor-beta and adrenocorticotrophic hormone. The cells of the hair follicle display limited expression of class Ia MHC molecules and, like cells that line the anterior chamber of the eye, are protected against CD8+ cytotoxic T lymphocyte attack. Gaining a better understanding of the immune privilege of the hair follicle may provide insights into the regulation and pathogenesis of immune-mediated diseases of the skin.

Expression of nonclassical MHC class Ib genes: comparison of regulatory elements

Peptide binding proteins of the major histocompatibility complex consist of the "classical" class Ia and "nonclassical" class Ib genes. The gene organization and structure/function relationship of the various exons comprising class I proteins are very similar among the class Ia and class Ib genes. Although the tissue-specific patterns of expression of these two gene families are overlapping, many class Ib genes are distinguished by relative low abundance and/or limited tissue distribution. Further, many of the class Ib genes serve specialized roles in immune responses. Given that the coding sequences of the class Ia and class Ib genes are highly homologous we sought to examine the promoter regions of the various class Ib genes by comparison to the well characterized promoter elements regulating expression of the class Ia genes. This analysis revealed a surprising complexity of promoter structures among all class I genes and few instances of conservation of class Ia promoter regulatory elements among the class Ib genes.

Role of TRAIL and IFN-gamma in CD4+ T cell-dependent tumor rejection in the anterior chamber of the eye

Although the anterior chamber of the eye expresses immune privilege, some ocular tumors succumb to immune rejection. Previous studies demonstrated that adenovirus-induced tumors, adenovirus type 5 early region 1 (Ad5E1), underwent immune rejection following transplantation into the anterior chamber of syngeneic mice. Intraocular tumor rejection required CD4(+) T cells, but did not require the following: 1) CD8(+) T cells, 2) B cells, 3) TNF, 4) perforin, 5) Fas ligand, or 6) NK cells. This study demonstrates that CD4(+) T cell-dependent tumor rejection does not occur in IFN-gamma-deficient mice. Ad5E1 tumor cells expressed DR5 receptor for TRAIL and were susceptible to TRAIL-induced apoptosis. Although IFN-gamma did not directly induce apoptosis of the tumor cells, it rendered them 3-fold more susceptible to TRAIL-induced apoptosis. Both CD4(+) T cells and corneal endothelial cells expressed TRAIL and induced apoptosis of Ad5E1 tumor cells. The results suggest that Ad5E1 tumor rejection occurs via TRAIL-induced apoptosis as follows: 1) tumor cells express TRAIL-R2 and are susceptible to TRAIL-induced apoptosis, 2) IFN-gamma enhances TRAIL expression on CD4(+) T cells and ocular cells, 3) IFN-gamma enhances tumor cell susceptibility to TRAIL-induced apoptosis, 4) apoptotic tumor cells are found in the eyes of rejector mice, but not in the eyes of IFN-gamma knockout mice that fail to reject intraocular tumors, 5) CD4(+) T cells and corneal endothelial cells express TRAIL and induce apoptosis of tumor cells, and 6) apoptosis induced by either CD4(+) T cells or corneal cells can be blocked with anti-TRAIL Ab.

The potential of antibody-based immunosuppressive agents for corneal transplantation

Corneal transplantation is a sight-restorative procedure but its success is limited by irreversible graft rejection, which accounts for up to 50 per cent of failures. The normal eye is an immune-privileged site. Multiple mechanisms maintain ocular privilege, including the blood-eye barrier, the lack of blood vessels and lymphatics in the normal cornea, the relative paucity of mature antigen-presenting cells in the central cornea, the presence of immunomodulatory factors in ocular fluids, and the constitutive expressive of CD95L (Fas ligand) within the eye. However, privilege can be eroded by the sequelae of inflammation and neovascularization. Corneal graft rejection in humans is currently suppressed with topical glucocorticosteroids, which are moderately effective. Systemically administered immunosuppressive therapy is of limited efficacy and may be accompanied by unacceptable morbidity. Alternative therapies are needed to improve outcomes. Corneal graft rejection is primarily a cell-mediated response controlled by the CD4+ T cell, and thus CD4 and costimulatory molecule blockade are appealing targets for new therapeutic interventions. A number of monoclonal antibodies have shown promise as immunosuppressants to prolong corneal graft survival in experimental animal models, and may eventually prove to be useful adjuncts to corticosteroids.

The nonclassical major histocompatibility complex molecule Qa-2 protects tumor cells from NK cell- and lymphokine-activated killer cell-mediated cytolysis

The cytotoxic activity of NK cells is regulated by class I MHC proteins. Although much has been learned about NK recognition of class I autologous targets, the mechanisms of NK self-tolerance are poorly understood. To examine the role of a nonpolymorphic, ubiquitously expressed class Ib Ag, Q9, we expressed it on class I-deficient and NK-sensitive B78H1 melanoma. Presence of this Qa-2 family member on tumor cells partially protected targets from lysis by bulk lymphokine-activated killer (LAK) cells. H-2K(b)-expressing B78H1 targets also reduced LAK cell activity, while H-2D(b) offered no protection. Importantly, blocking with F(ab')(2) specific for Q9 or removal of this GPI-attached molecule by phospholipase C cleavage restored killing to the level of vector-transfected cells. Experiments with LAK cells derived from H2(b) SCID and B6 mice established that NK1.1(+)TCR(-) NK and NK1.1(+)TCR(+) LAK cells were the prevalent cytolytic populations inhibitable by Q9. Treatment of mice with poly(I:C) also resulted in generation of Q9-regulated splenic cytotoxicity. LAK cells from different mouse strains responded to Q9, suggesting that the protective effect of this molecule is not detectably influenced by Ly49 polymorphisms or the presence/absence of Q9 in NK-harboring hosts. We propose that Q9 expressed on melanoma cells serves as a ligand for yet unidentified NK inhibitory receptor(s) expressed on NK1.1(+) NK/T cells.

Promiscuous antigen presentation by the nonclassical MHC Ib Qa-2 is enabled by a shallow, hydrophobic groove and self-stabilized peptide conformation

BACKGROUND

Qa-2 is a nonclassical MHC Ib antigen, which has been implicated in both innate and adaptive immune responses, as well as embryonic development. Qa-2 has an unusual peptide binding specificity in that it requires two dominant C-terminal anchor residues and is capable of associating with a substantially more diverse array of peptide sequences than other nonclassical MHC.

RESULTS

We have determined the crystal structure, to 2.3 A, of the Q9 gene of murine Qa-2 complexed with a self-peptide derived from the L19 ribosomal protein, which is abundant in the pool of peptides eluted from the Q9 groove. The 9 amino acid peptide is bound high in a shallow, hydrophobic binding groove of Q9, which is missing a C pocket. The peptide makes few specific contacts and exhibits extremely poor shape complementarity to the MHC groove, which facilitates the presentation of a degenerate array of sequences. The L19 peptide is in a centrally bulged conformation that is stabilized by intramolecular interactions from the invariant P7 histidine anchor residue and by a hydrophobic core of preferred secondary anchor residues that have minimal interaction with the MHC.

CONCLUSIONS

Unexpectedly, the preferred secondary peptide residues that exhibit tenuous contact with Q9 contribute significantly to the overall stability of the peptide-MHC complex. The structure of this complex implies a "conformational" selection by Q9 for peptide residues that optimally stabilize the large bulge rather than making intimate contact with the MHC pockets.

Gamma delta T cells are needed for ocular immune privilege and corneal graft survival

It has been recognized for over a century that the anterior chamber of the eye is endowed with a remarkable immune privilege. One contributing component is the Ag-specific down-regulation of systemic delayed-type hypersensitivity (DTH) that is induced when Ags are introduced into the anterior chamber. This phenomenon, termed anterior chamber-associated immune deviation (ACAID), culminates in the generation of regulatory cells that inhibit the induction (afferent suppression) and expression (efferent suppression) of DTH. Since gamma delta T cells play a major role in other forms of immune regulation, we suspected they might contribute to the induction and expression of ACAID. Mice treated with anti-gamma delta Ab failed to develop ACAID following anterior chamber injection of either soluble Ag (OVA) or alloantigens (spleen cells). Additional experiments with knockout mice confirmed that mice lacking functional gamma delta T cells also fail to develop ACAID. Using a local adoptive transfer of DTH assay, we found that gamma delta T cells were required for the generation of regulatory T cells, but did not function as the efferent regulatory cells of ACAID. The importance of gamma delta T cells in corneal allograft survival was confirmed by blocking gamma delta T cells with GL3 Ab before corneal transplantation. While in vivo treatment with normal hamster serum had no effect on corneal graft survival, infusion of anti-gamma delta Ab resulted in a profound increase in corneal allograft rejection. Thus, gamma delta T cells are needed for sustaining at least one aspect of ocular immune privilege and for promoting corneal allograft survival.

Qa-2-dependent selection of CD8alpha/alpha T cell receptor alpha/beta(+) cells in murine intestinal intraepithelial lymphocytes

Murine intestinal intraepithelial lymphocytes (iIELs) are made up of a heterogeneous mix of T cells with unique phenotypes. Whereas CD8(+) T cells in peripheral lymphoid organs use CD8alpha/beta and are selected on MHC class Ia molecules, a majority of iIELs use CD8alpha/alpha. Here, we report that the presence of CD8alpha/alpha TCR-alpha/beta cells in iIELs is independent of classical MHC class I molecules K(b) and D(b), as illustrated by their presence in K(b)/D(b) double-knockout mice and in mice lacking a nonclassical MHC class I molecule, CD1d. Most strikingly, their presence is decreased by approximately 70% in mice lacking transporter associated with antigen processing (TAP). The TAP-dependent nonclassical MHC class I molecule Qa-2 is strongly implicated in the presence of these cells, as inferred from the low numbers of CD8alpha/alpha TCR-alpha/beta T cells in mice deficient in Qa-2 genes. Second, a Qa-2-transgenic mouse made in a Qa-2(-) strain showed an increase in the numbers of CD8alpha/alpha cells among its iIELs. Thus, the presence of CD8alpha/alpha TCR-alpha/beta cells in iIELs is mainly dependent on the nonclassical MHC class I molecule Qa-2.