Ординатура / Офтальмология / Английские материалы / Ocular Periphery and Disorders_Dartt, Bex, Amore_2011

Lipid layer

Aqueous

layer

Mucin layer

Epithelium

Peter Mallen

shown to behave as antigen-presenting cells that phagocytose pathogens, produce cytokines, and present pathogenderived peptides to sensitized T cells. Therefore, while RPE acts as a physical barrier to the posterior segment, it is also important in shaping and regulating the adaptive immune response once this barrier has been breached.

Chemical Barriers

In addition to the anatomic and physical barriers designed to block invasive pathogens, there are also a number of soluble factors that inhibit bacterial growth, adherence, and survival. Several of these factors are constitutively expressed at low levels, providing a baseline of protection from foreign pathogens. However, many of these factors can also be upregulated in response to pathogens and inflammation; thus, these become an important product of the active arm of innate immunity. Some of the more important factors are discussed in more detail below.

Lysozyme

Lysozyme is bacteriacidal and makes up 20–40% of the total tear protein. Lysozyme kills bacteria by (1) binding to and creating pores in the bacterial cell wall, or (2) dissolving bacterial membranes by enzymatic digestion.

Secretory phospholipase A2

Secretory phospholipase A2 exhibits potent antibacterial activity against Gram-positive bacteria. Similar to lysozyme, secretory phospholipase A2 dissolves bacterial membranes by hydrolyzing the principal phospholipid, phosphatidylglycerol.

Cathelicidin (LL-37)

LL-37 is a small cationic peptide with potent antimicrobial activity against Gram-positive and Gram-negative bacteria as well some viruses. The precise mechanism of action is incompletely understood, but it is widely believed that the antimicrobial activity is due to disruption of the microbial membrane or viral envelope.

Defensins

Beta defensins are expressed in epithelial cells that line mucosal surfaces such as the cornea. Similar to LL-37, defensins have a broad spectrum of antimicrobial activity and are effective against: Gram-positive and Gramnegative bacteria, fungi, and enveloped viruses. In addition to their antimicrobial activities, defensins modulate a variety of cellular activities including immune cell chemotaxis, epithelial proliferation, cytokine secretion, and stimulation of histamine release from mast cells. Human

corneal epithelial cells constitutively express human beta defensin-1 (hBD-1) and hBD-3. By contrast, hBD-2 is induced in response to corneal injury, infection, or inflammation, and is approximately 10-fold more potent than hBD-1 with an even wider antibacterial spectrum. Therefore, while hBD1 and hBD-3 provide a baseline defense to protect the cornea from infection, upon injury or microbial invasion, hBD-2 is upregulated and displays increased antimicrobial activity.

Lactoferrin

Lactoferrin is bacteriostatic and binds to and depletes iron from the tear film, which is required for microbial metabolism and growth. Lactoferrin is also bactericidal and permeabilizes membranes of Gram-positive and Gramnegative bacteria. Additional functions of lactoferrin have also been described: (1) inhibits biofilm development,

(2) inhibits bacterial adhesion to host cells, (3) inhibits intracellular invasion, (4) amplifies apoptotic signals in infected cells, and (5) enhances bactericidal activity of neutrophils.

Lipocalin-A

Lipocalin A prevents bacteria from obtaining iron, an essential nutrient for microorganism survival. However, unlike lactoferrin, lipocalin-A does not bind iron directly. Lipocalin-A inhibits the iron acquisition system of microbes by binding to and blocking microbial sidephores used to transport iron into bacteria.

Secretory IgA

sIgA protects the ocular surface against colonization and possible invasion by pathogenic microorganisms by binding to bacteria and facilitating clearance. In addition, sIgA can opsonize bacteria for phagocytosis.

Complement

Complement components (such as C3 and C4) are constitutively expressed in the tear film and are involved in phagocytic chemotaxis, opsonization, and lysis of bacteria. The eye is unusual in that there is a constitutive low level of activated complement that is present even in uninfected normal eyes. It is believed that this low level of activated complement provides innate immune surveillance of microbes and allows for a rapid activation of the full complement cascade upon pathogen invasion.

Active Innate Defense System

Pattern Recognition Receptors

Innate immunity develops rapidly and is described historically as nonspecific, while adaptive immunity develops slowly and is antigen-specific. However, the discovery of pattern recognition receptors (PRRs) that detect unique

pathogen-associated molecular patterns revealed a level of specificity in innate immunity that not only allows discrimination between self and non-self, but also allows the development of innate immunity tailored to specific pathogens, such as bacteria, viruses, and fungi.

There are two classes of PRRs: (1) TLRs and (2) nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) (Table 1). TLRs are expressed on the cell surface and detect pathogens at the cell membrane. TLRs are also expressed within endosomes and detect pathogens that have been endocytosed. By contrast, NLRs are expressed within in the cytoplasm and detect the presence of microbial molecules inside the host cell. These two classes of PRRs are discussed in detail below.

Toll-like receptors

TLRs are type 1 transmembrane proteins with an extracellular domain for ligand binding composed of leucine rich repeats and a cytoplasmic domain for intracellular signaling which is known as the Toll/IL-1 receptor (TIR) domain. TLRs recognize bacteria, viruses, fungi, protozoa, and endogenous ligands, such as heat shock proteins and fibrinogen. Triggering of the TLR leads to activation of the transcription factor, nuclear factor-kappa B (NF-kB), and the expression of pro-inflammatory molecules, such as TNF-a, IL-1, and IL-2. To date, 10 human TLRs have been identified and each TLR has a unique ligand specificity (Table 1). TLRs were first identified on innate immune cells: neutrophils, macrophages, monocytes, and dendritic cells. More recently, TLRs were also identified on epithelial cells that lie at the host/environment interface: skin, gastrointestinal tract, respiratory tract, and urogenital tract. Furthermore, several TLRs have been identified on ocular tissue in both the anterior and posterior segment of the eye, including cornea, iris, ciliary body, choroid, and RPE.

Similar to other barrier epithelium, several TLRs have been identified on the RPE and corneal epithelial cells and are vital for sensing microbes and triggering a rapid response to eliminate the pathogen. The study of TLRs in ocular immunity is still relatively new and debate remains over which TLRs are expressed in the eye and whether TLRs are expressed on the cell surface or within endosomes. However, it is clear that TLRs are required for initiation of innate immunity in the eye. This is supported by studies using TLR knockout mice that demonstrate a significant decrease in inflammation characterized by decreased neutrophil infiltration and increased susceptibility to infection in the absence of TLRs.

NOD-like receptors

The NLRs comprise a large family of cytoplasmic PRRs that recognize bacteria and endogenous danger signals such as uric acid (Table 1). All members of the NLR family share a conserved NOD. However, the NLR family

can be subdivided into three groups based upon their N-terminal domains: caspase recruitment domain (NODs), pyrin domain (NALPs), or baculovirus inhibitor repeat (neuronal apoptosis inhibitor proteins, NAIPs). At present, the human NLR family comprises of 23 proteins, while at least 34 NLR genes have been identified in mice. NALPs are unique in that upon binding to their ligand, NALPs form a complex termed the inflammasome resulting in caspase-1 activation and the release of active IL-1b. Recently, NALP1, 3, and 10 were identified in corneal epithelial cells, but their potential function in regulating ocular innate immunity has not been determined. NAIPs inhibit caspase effectors and are mainly expressed in neurons where their primary role is to protect against apoptosis. NAIPs have not yet been found in ocular tissues. NODs are the most extensively studied members of the NLR family and are expressed in ocular tissue.

Both NOD1 and NOD2 are constitutively expressed within the eye; NOD1 in the anterior and posterior segments of the eye and NOD2 only in the anterior segment. However, which specific tissues express NODs is unknown. NOD1 and NOD2 recognize specific subcomponents of bacterial peptidoglycan. NOD1 recognizes D-g-glutamyl-meso-diaminopimelic acid, while NOD2 recognizes muramyl dipeptide. Recently, a mutation in NOD2 was linked to the development of uveitis in patients with Blau syndrome, suggesting that cytoplasmic

NODs may be important in ocular inflammation. While the importance of NOD receptors in innate immunity and the pathogenesis of inflammatory disease are recognized outside the eye, the study of NODs within the eye is at an early stage. A complete understanding of ocular host defense will require a better understanding of where NODs and other NLRs are expressed and how they regulate innate immunity within the eye.

Complement

Similar to PRRs, the complement system acts as an innate immune surveillance system, detecting the first signs of pathogen invasion. The complement system was first identified as a biochemical cascade of serum proteins that help or complement antibodies to clear pathogens and mark them for opsinization by phagocytes. It is now known, however, that the complement system can also be activated directly by microbial products via the alternative pathway. Several studies demonstrate that complement is constitutively active at low levels in the eye. This is thought to be a primary defense mechanism of the eye against pathogenic infection. Upon pathogen invasion the complement system is further activated to clear the infection through (1) generation of inflammatory factors (C3 and C5a), (2) chemotaxis of phagocytes (C3 and C5a),

(3) opsonization of Ab-coated cells (C3b), and (4) lysis of bacteria and virus-infected cells (C8 and C9).

Cytokines, Chemokines, and Effector Cells

Once pathogens breach the passive barriers of the eye and trigger the PRRs or the complement system, the primary function of innate immunity is to eliminate the invading pathogen as quickly as possible. To achieve this innate immunity must: (1) trigger an immediate immune response,

(2) amplify the response, (3) clear the pathogens, and (4) activate the adaptive system if the pathogen cannot be cleared quickly. Cytokines, chemokines, and adhesion molecules play critical roles in regulating each of these stages.

Initiation and amplification

PRRs (TLRs and NLRs) recognize microbial products in the earliest phase of host defense and activate many immune and inflammatory genes, the products of which are important in initiating and amplifying antimicrobial immunity. Triggering TLRs on either the corneal epithelium or the RPE leads to activation of NF-kB via the (1) MyD88-dependent or (2) MyD88-independent pathway. The MyD88-dependent pathway is utilized via most TLRs (TLR1, 2, 4, 5, 6, 7, 8, and 9) and leads to the production of proinflammatory cytokines and chemokines (IL-6, IL-8, IL-18, MIP-1, and TNF-a). By contrast, only TLR3 and TLR4 utilize the MyD88-independent pathway and leads to the production of IFN-a and IFN-b. Recruitment of neutrophils into inflamed tissues is controlled predominantly by two chemokines (MIP-2 (= IL-8 in humans) and KC). In Pseudomonas aeruginosa-induced corneal keratitis, elevated MIP-2 and KC correspond with increased infiltration of neutrophils. In the posterior segment, elevated TNF-a, IL-1b, and CINC (rat homolog of IL-8) contribute to the breakdown of the blood–retinal barrier and the recruitment of neutrophils in response to Staphylococcus aureus. The adhesion molecules ICAM-1 and E-selectin are also upregulated early in iris, ciliary body, and retinal vessels, serving to enhance the infiltration of neutrophils to the site of infection.

Clearing the pathogen

In response to inflammatory cytokines and chemokines, neutrophils and macrophages are recruited to the site of infection. Bacterial clearance by neutrophils is accomplished by (1) phagocytosis, (2) generation of reactive oxygen species, and (3) the release of granule-associated enzymes: cathepsin G, myeloperoxidase, lactoferrin, and elastase. While the recruitment of neutrophils to the site of infection is essential for clearance of the pathogen, the persistence of neutrophils and the prolonged release of inflammatory mediators is also associated with nonspecific host tissue damage. Similar to neutrophils, macrophages also phagocytose and directly kill microbes as part of innate immunity. However, macrophages are also antigenpresenting cells, and as such participate in the development of the adaptive immune response. The natural killer (NK) cell is another important innate effector cell in host defense

against viral infections of the cornea such as herpes simplex virus type-1. NK cells respond in an antigen-independent manner and kill virus-infected host cells through the release of perforin and granzymes or through binding of the death receptors Fas and TRAIL-R on the target cell. If innate effector cells fail to clear the infection, adaptive immunity will take over and finish eradicating the pathogen. However, if the infection is successfully cleared, the final and most important step of innate immunity is preventing nonspecific host tissue damage. This is accomplished in the eye through several mechanisms that together make up innate immune privilege.

Innate Immune Privilege

The primary role of innate immunity is to rapidly eradicate invading pathogens through the induction of inflammation. As a general rule, the level of inflammation is proportional to the size and virulence of the infection. Small, nonvirulent infections are cleared by mild inflammation, while larger, more virulent infections induce intense inflammation. The potential danger of inflammation occurs when intense and/or prolonged inflammation threatens the surrounding normal tissue, resulting in nonspecific tissue damage and scarring. The potential danger of inflammation in the eye is magnified by the presence of irreplaceable and highly sensitive ocular tissues. Therefore, it is not surprising that immune privilege in the eye has multiple mechanisms to control innate immunity and limit nonspecific tissue damage. The aqueous humor contains multiple factors that directly inhibit innate immunity including: (1) TGF-b, soluble Fas ligand, and alpha-melanocyte stimulating hormone (inhibit neutrophil activation); (2) macrophage migration inhibitory factor (inhibits NK cell-dependent lysis of target cells);

(3) calcitonin gene-related peptide (inhibits nitric oxide release from activated macrophages); and (4) complement regulatory factors: CD46, CD55, CD59, and Crry (inhibit complement activation). These factors work together to limit the damaging consequences of inflammation and to preserve the visual axis. Unfortunately, while these mechanisms evolved to limit local tissue destruction and preserve the visual axis, they may leave the eye more vulnerable to organisms whose virulence often requires a robust inflammatory response for eradication. Therefore, a delicate balance must be made between the amount of inflammation needed for eradicating the pathogen and the amount of nonspecific tissue damage.

Link between Innate and Adaptive Immunity

Innate and adaptive immunity have long been discussed as separate arms of the immune system. However, it is increasingly clear that they are indeed not separate but highly integrated. Several studies, both outside and inside

the eye, identified the dendritic cell as a central link between innate and adaptive immunity. Immature dendritic cells reside in peripheral tissues and through triggering of TLRs, participate in the primary immune response against microbial infections. Immature dendritic cells encounter invading pathogens, capture bacterial antigens, and migrate to the draining lymph node. Once in the lymph node, only mature dendritic cells can efficiently prime naı¨ve T cells and initiate adaptive immunity. Studies using TLR and MYD88 deficient mice, reveal that TLR signaling is required for bacteria-induced maturation of dendritic cells and induction of adaptive immunity. Therefore, TLRs on dendritic cells are essential for (1) sensing the microbe and initiating the immediate innate immune response, as well as (2) inducing the development of an adaptive immune response. While dendritic cells have been identified in the cornea and retina, additional studies are needed to completely understand their function in linking innate and adaptive immunity within the eye.

Conclusion

Innate immunity is a critical first line of defense against ocular infections. However, the regulation of innate immunity within the eye is just beginning to be unraveled. The identification of TLRs and NLRs has provided new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases. A better understanding of how microbial agents and endogenous host factors interact with TLRs and NLRs in the eye will be critical in advancing our knowledge of the pathogenesis of infectious and noninfectious eye diseases. Moreover, a better understanding of these mechanisms will lead to the identification of new therapeutic targets for treating and preventing sight-threatening infections.

See also: Corneal Epithelium: Wound Healing Junctions, Attachment to Stroma Receptors, Matrix Metalloproteinases, Intracellular Communications; Defense Mechanisms

of Tears and Ocular Surface; Immunosuppressive and AntiInflammatory Molecules that Maintain Immune Privilege of the Eye; Ocular Mucins.

Further Reading

Banchereau, J. and Steinman, R. M. (1998). Dendritic cells and the control of immunity. Nature 392: 245–252.

Creagh, E. M. and O’Neill, A. J. (2006). TLRs, NLRs and RLRs: A trinity of pathogen sensors that co-operate in innate immunity. Trends in Immunology 27: 352–357.

Gregory, M., Callegan, M. C., and Gilmore, M. S. (2007). Role of bacterial and host factors in infectious endophthalmitis. Chemical Immunology and Allergy 92: 266–275.

Haynes, R. J., McElveen, J. E., Dua, H. S., Tighe, P. J., and Liversidge, J. (2000). Expression of human beta-defensins in intraocular tissues.

Journal of Investigative Ophthalmology and Visual Science 41: 3026–3031.

Holtkamp, G. M., Kijlstra, A., Peek, R., and de Vos, A. F. (2001). Retinal pigment epithelium–immune system interactions: Cytokine production and cytokine-induced changes. Progress in Retinal and Eye Research 20: 29–48.

Kaplan, H. J. and Niederkorn, J. Y. (2007). Regional immunity and immune privilege. Chemical Immunology and Allergy 92: 11–26.

Kawai, T. and Akira, S. (2007). TLR signaling. Seminars in Immunology 19: 24–32.

Kolls, J. K., McCray, P. B., and Chan, Y. R. (2008). Cytokine-mediated regulation of antimicrobial proteins. Nature Reviews Immunology

8: 829–835.

Pearlman, E., Johnson, A., Adhikary, G., et al. (2008). Toll-like receptors at the ocular surface. Ocular Surface 6: 108–116.

Rodriguez-Martinez, S., Cancion-Diaz, M. E., Jimenez-Zamudio, L., et al. (2005). TLRs and NODs mRNA expression pattern in healthy mouse eye. British Journal of Ophthalmology 89: 904–910.

Sack, R. A., Nunes, I., Beaton, A., and Morris, C. (2001). Host-defense mechanism of the ocular surfaces. Bioscience Reports 21: 463–480.

Sohn, J. H., Bora, P. S., Jha, P., et al. (2007). Complement, innate immunity and ocular disease. Chemical Immunology and Allergy 92: 105–114.

Streilein, W. J. and Stein-Streilein, J. (2000). Does innate immune privilege exist? Journal of Leukocyte Biology 67: 479–487.

Tosi, M. F. (2005). Innate immune responses to infection. Journal Allergy and Clinical Immunology 116: 241–249.

Van Vilet, S. J., den Dunnen, J., Gringhuis, S. I., Geijtenbeek, T. B., and Van Kooyk, Y. (2007). Innate signaling and regulation of dendritic cell immunity. Current Opinion in Immunology 19: 435–440.

Glossary

ACAID (anterior chamber-associated immune deviation) – A dynamic antigen-specific downregulation of T-cell-mediated immunity that is elicited when antigens are introduced into the anterior chamber of the eye. ACAID is believed to be a mechanism to maintain homeostasis in the eye and reduce the likelihood of T-cell-dependent inflammation in response to noninfectious, nominal antigens that enter the eye. ACAID is also believed to be induced by orthotopic corneal allografts and promotes their survival.

Alloantigens – Histocompatibility antigens that provoke immune rejection of organ transplants. These include both major histocompatibility complex antigens and antigens encoded by a wide diversity of minor histocompatibility genes.

Delayed-type hypersensitivity – The T-cell- and immune-mediated inflammation that contributes to resistance to intracellular pathogens but also carries a heavy burden of collateral damage to innocent bystander tissues.

TGF-b (transforming growth factor-b) –

A cytokine that is produced by many cells and has pleiotropic effects on the immune system. TGF-b is present in the anterior chamber of the eye and is crucial for altering the behavior of ocular antigenpresenting cells within the eye such that they induce ACAID.

Tregs (T regulatory cells) – These are

T lymphocytes that can be induced in multiple ways and suppress T-cell-dependent immune processes. Introducing antigens into the anterior chamber or the vitreous cavity of the eye elicits the generation of

T regulatory cells.

VCAID (vitreous cavity-associated immune deviation) – Antigens introduced into the vitreous cavity or into the subretinal space elicit an immune deviation that is indistinguishable from ACAID and is characterized by antigen-specific suppression of T-cell-dependent inflammation.

Introduction

The immune privilege of the eye was recognized over a century ago by the Dutch ophthalmologist van Dooremaal, who introduced a variety of foreign bodies and tissues into the eyes of animals as a means of studying cataractogenesis, and, in the process, unwittingly discovered the prolonged survival of mouse skin grafts placed in the anterior chamber (AC) of the dog eye. Evidence suggesting that tissue grafts might also enjoy prolonged survival in the eyes of humans surfaced when the first successful corneal transplant in a human subject was reported in 1905, a landmark event that occurred over 60 years before antirejection drugs were used in the first human heart transplant. In the 1940s, experimental pathologists transplanted human tumor cells into the AC of the rabbit eye as a bioassay for determining the malignancy of biopsy specimens. We now know that the survival of such tumor implants in the AC of the rabbit eye was not a property of malignant cells per se, but like the survival of corneal transplants, was a manifestation of ocular immune privilege. It was not until the early 1950s that the preeminent immunologist and Nobel Laureate, Sir Peter Medawar, recognized the significance of the prolonged survival of skin grafts placed into the eye and the brain and coined the term ‘‘immune privilege.’’ Although the concept of ocular immune privilege is widely recognized, it is frequently misunderstood or over-simplified. There are two common misconceptions of ocular immune privilege. The first misconception is that corneal transplants are universally exempt from immune rejection. While it is true that corneal allografts benefit from immune privilege and enjoy a high success rate compared to other categories of transplants, they can undergo immune rejection and, in some cases, the rejection cannot be prevented, even with potent immunosuppressive drugs. The second misconception is that ocular immune privilege is due to the absence of lymph vessels draining the interior of the eye, which would sequester antigens in the eye and deny them access to regional lymph nodes where they would elicit an immune response. Although there are no anatomically detectable patent lymphatics draining the interior of the eye, antigens and cells placed into the AC do in fact reach

the cervical lymph nodes in rodents. Thus, other explanations are needed to account for ocular immune privilege.

Important clues as to the possible underlying mechanisms for ocular immune privilege surfaced in the mid-1970s in seminal studies from J. Wayne Streilein and colleagues, who demonstrated that introducing antigens into the AC elicited a deviation in the systemic immune responses resulting in the suppression of T-cell-mediated immunity. This anterior chamber-associated immune deviation (ACAID) is characterized by the antigen-specific suppression of Th1 immune responses, such as delayed-type hypersensitivity (DTH), and Th2-based inflammation, such as experimental allergic asthma in mice. Although antigens introduced into the AC suppress Th1and Th2based immune inflammation, other immune elements are preserved. Noncomplement fixing antibodies are generated, while complement-fixing antibody production is silenced. Initial studies demonstrated that the induction of ACAID was associated with the production of IL-10, which at the time was considered a Th2 cytokine, and the suppression of the Th1 cytokine, interferon-g (IFN-g). This led some to conclude that ACAID was simply the preferential cross regulation of Th1 immune response by Th2 cytokines. However, further analysis revealed that ACAID also suppressed Th2-based inflammatory responses and that ACAID did not require the participation of a key Th2 cytokine, IL-4.

Not only does ACAID suppress classical Th1 immune responses, but it also mitigates Th2-mediated allergic inflammatory lung disease. ACAID also deviates antibody responses by preserving the production of noncomple- ment-fixing antibodies while blocking the generation of complement-fixing antibodies. This reduces the likelihood of immune inflammation, which is precipitated by activation of the complement cascade. Once activated by complement-fixing antibodies, the complement cascade spins off pro-inflammatory molecules that stimulate granulocytic inflammation. Thus, ACAID suppresses both Th1and Th2-based inflammation and the generation of complement-fixing antibodies and thereby reduces the likelihood that antigens entering the eye will elicit immune inflammation that could injure tissues in the eye that possess little or no regenerative properties.

Induction of ACAID

Studies conducted over the past 30 years have shed light on the molecular and cellular basis of ACAID and have revealed the remarkable complexity of this systemic immunoregulatory phenomenon. The eye, thymus, spleen, and sympathetic nervous system are required for the induction of ACAID. Chemical sympathectomy prior to AC injection of antigen or removal of any of these organs within 3 days of AC injection prevents the induction of ACAID, and in

many cases, allows the development of robust Th1 immune responses, such as DTH.

Ocular Phase of ACAID

The induction of ACAID begins when antigens enter the AC. The eye is an active participant in the induction of ACAID, as enucleation of the eye within 3 days of AC injection not only prevents the induction of ACAID, but also results in active immunization and development of DTH to the antigens that were introduced into the AC. It is widely believed that within the eye, antigen is captured by F4/80+ macrophages, which under the influence of TGF-b (which is present in the aqueous humor), are imprinted to produce IL-10 while simultaneously extinguishing IL-12 production. The preferential production of IL-10 by ocular macrophages is crucial for the induction of ACAID, as macrophages from IL-10 knockout mice are incapable of inducing ACAID. TGF-b also stimulates ocular macrophages to produce macrophage inflammatory protein-2 (MIP-2), which is a potent chemokine that is pivotal in the splenic phase of ACAID (see below).

Thymic Phase of ACAID

Within 72 h of antigen entering the eye, F4/80+ ocular macrophages capture antigen and migrate to the thymus. Within the thymus, they evoke the generation of CD4 , CD8 , and NK1.1+ T cells (NKT cells), which then emerge from the thymus and enter the bloodstream where they migrate to the spleen. Other F4/80+ ocular macrophages are believed to migrate directly from the eye to the spleen. Both populations of F4/80+ ocular macrophages contribute to the generation of CD4+ and CD8+ T regulatory cells (Tregs) within the spleen.

Splenic Phase of ACAID

After entering the spleen, F4/80+ ocular macrophages secrete MIP-2, which attracts CD4+ NKT cells. The NKT cells in turn interact with the ocular macrophages and secrete the chemokine, RANTES, which recruits other cells into the marginal zone of the spleen. Within the marginal zone, F4/80+ ocular macrophages, NKT cells, B cells, and CD4+ T cells, in the presence of the third component of complement, collaborate to generate CD8+ Tregs. The CD8+ Tregs are the end-stage effector cells of ACAID that inhibit Th1and Th2-based immune-mediated inflammation and promote corneal allograft survival.

Sympathetic Nervous System and ACAID

All three organs involved in the induction of ACAID – eye, spleen, and thymus – possess dense sympathetic innervations. Many immune responses are influenced by

the sympathetic nervous system, including ACAID. Animals subjected to chemical sympathectomy prior to AC injection of antigen fail to develop ACAID. It is not clear at what level the sympathetic nervous system exerts its effect, but it appears that it is not at the ocular phase, as chemical sympathectomy does not affect the generation of F4/80+ ocular macrophages. Thus, the induction and expression of ACAID are remarkably complex and require the active participation of multiple organs and organ systems including the circulatory system, sympathetic nervous system, thymus, spleen, and eye (Figure 1).

ACAID T Regulatory Cells

Two phenotypically and functionally distinct populations

of Tregs are induced in ACAID. CD4+ Tregs inhibit the induction of T-cell-mediated immune responses, but do

not suppress T-cell effector responses, such as DTH.

Thus, CD4+ ACAID Tregs act at the afferent arm of the immune response and prevent the initiation of immune

responses, but have no effect if an immune response has already been initiated. CD4+ afferent Tregs are also needed for the generation of CD8+ Tregs that suppress immune responses at the effector stage. CD8+ Tregs are the end-stage regulatory cells of ACAID that block effector immune responses, such as DTH, even if the host has been previously immunized and is capable of mounting a robust DTH response. Both the CD4+ afferent Tregs and the CD8+ efferent Tregs are antigen-specific. It is not clear how the CD8+ ACAID Tregs suppress DTH and other T-cell immune effector responses, but evidence to date indicates that these Tregs do not lyse immune cells via a perforin-dependent mechanism or induce apoptosis by Fas/FasL interactions. Interestingly, CD8+ efferent Tregs express IFN-g receptors and require IFN-g to exert their suppressive properties. However, the precise molecular mechanisms whereby CD8+ efferent Tregs produce their effects remain to be elucidated. Although two distinct Treg populations have been detected in ACAID (i.e., CD4+ and CD8+), it is possible that other Tregs might also participate

Trabecular

meshwork

Blood

Blood

Spleen

in the induction and expression of ACAID. Although, most, if not all, of the studies performed to date have examined Tregs in the spleens of mice with ACAID, it is entirely possible that Tregs may also be present in lymph nodes, such as the cervical and submandibular lymph nodes.

What is the Relevance of ACAID?

One might argue that ACAID and immune privilege create an immunological blind spot that renders the eye potentially vulnerable to infectious agents. However, the eye and the brain are part of the central nervous system and possess very limited regenerative properties. Thus, limiting the immunological options to viral or bacterial infections is desirable for the preservation of vision. DTH is crucial for controlling intracellular pathogens, while activation of the complement cascade is an important mechanism for clearing bacterial infections. However, DTH and complement elicit granulocytic inflammation. Granulocytes produce a potpourri of reactive oxygen species and proteases that are notorious for producing extensive collateral damage to innocent bystander tissues. If unrestrained, such inflammation would produce extensive necrosis of ocular tissues leading to blindness. Thus, suppressing DTH and the complement cascade has a clear benefit for preserving the functional integrity of the visual apparatus.

The eye is also at risk from other T-cell-mediated immune responses, such as cytotoxic T lymphocyte (CTL)-mediated immunity. During virus infections, viral antigens are displayed on major histocompatibility complex (MHC) class I molecules of the virus-infected cells. The MHC class I/viral antigen complex serves as a docking station for CTL, which kill the virus-infected cells, and thus, eliminate the viral invaders. Although this is a highly efficient mechanism for controlling virus infections in most parts of the body, it would be devastating in the eye. However, CTL responses are suppressed as a consequence of ACAID. Interestingly, corneal endothelial cells and cells of the retina do not express the critical MHC class I molecules that are necessary for recognition and elimination of virus-infected cells by CTL. Thus, the active downregulation of CTL responses to ocular antigens combined with the absence of MHC class I molecules shield ocular cells from CTL-mediated injury. The preferential production of noncomplement fixing antiviral antibodies provides a level of protection by neutralizing viral particles without activating the complement cascade and provoking granulocytic inflammation.

Although ACAID seems to be an adaptation to prevent unwitting immune-mediated injury to ocular tissues, it may also benefit the corneal transplant recipient. Animal studies have shown that hosts bearing successful long-term orthotopic corneal allografts display an immune deviation

that is reminiscent of ACAID. Moreover, AC injection of donor cells prior to transplantation results in a dramatic enhancement of corneal allograft survival in rodent models of penetrating keratoplasty.

Vitreous Cavity-Associated

Immune Deviation

Immune privilege is not restricted to the AC, but appears to be equally expressed in the vitreous cavity and in the subretinal space. Although less is known about the immune privilege in the poster segment of the eye, it is clear that antigens introduced into the vitreous cavity elicit an immune deviation that appears to be identical to ACAID and has been termed vitreous cavity-associated immune deviation (VCAID).

Ocular Regulatory Cells Induced In Situ

The aqueous humor contains a myriad of anti-inflamma- tory and immunosuppressive molecules. Iris and ciliary body (I/CB) cells line a major portion of the AC and secrete constituents of the aqueous humor. I/CB cells not only secrete immunosuppressive and anti-inflamma- tory molecules such as TGF-b, but they also have the capacity to suppress T-cell proliferation and the secretion of pro-inflammatory cytokines, such as IFN-g, by cell contact-dependent mechanisms, which are independent of soluble anti-inflammatory molecules such as TGF-b, IL-10, and tumor necrosis factor-a (TNF-a ). I/CB cells are strategically located at sites where T cells can enter the eye and, thus, are positioned to exert their influence by inhibiting T-cell proliferation and production of IFN-g shortly after T cells enter the eye.

Alpha-melanocyte-stimulating hormone (a-MSH) is one of the numerous immunosuppressive constituents of the aqueous humor. In addition to suppressing the production of pro-inflammatory cytokines, a-MSH stimulates T cells to produce anti-inflammatory cytokines such as TGF-b. Moreover, a-MSH converts Th1 cells into CD4+, CD25+ Tregs, which suppress DTH and mitigate experimental autoimmune uveitis (EAU).

The induction of ACAID requires penetration of the eye (i.e., antigen injection via a needle) in order to introduce antigens into the AC. AC injections, even if performed in the least possible traumatic manner, elicit the local production of pro-inflammatory cytokines including TNF-a. Thus, ACAID can be envisioned as a form of immune tolerance induced by antigens entering the AC as a consequence of perforating injuries to the eye. However, the eye may need to regulate immune responses to endogenous ocular antigens. During ontogeny, some structures in the eye are isolated from the immune system and

express tissue-specific antigens that, under some circumstances, can initiate an immune response. This has led some to suspect that ACAID and VCAID might be failsafe mechanisms that generate Tregs, which prevent immune responses to tissue-specific ocular antigens (e.g., lens crystallins and retinal antigens). Studies using transgenic mice have demonstrated that novel neoantigens engineered to be exclusively expressed in the retina induce the development of Tregs that suppress DTH responses, yet are significantly different from the Tregs induced by AC injection of antigens. Thus, endogenous ocular antigens arising in situ in an intact eye can elicit the generation of Tregs, which presumably maintain immune homeostasis in the eye and serve as buffers against autoimmune inflammation, although through mechanisms that are likely distinct from ACAID and VCAID.

Conclusions

Ocular immune privilege is the product of multiple anatomical and physiological properties of the eye. The blood–eye barrier restricts entry of inflammatory and immune cells into the eye. Moreover, once in the eye, cells of the immune system encounter a milieu that is rich with soluble immunosuppressive and anti-inflammatory molecules. Cells lining the interior of the eye are decorated with membrane-bound molecules such as FasL, programmed death ligand-1 (PD-L1), and tumor necrosis factor-related apoptosis-inducing ligand, each of which can induce apoptosis of activated T cells. In addition, the limited expression of MHC complex molecules on the corneal endothelium and the retina render these cells invisible to CTL.

Immune privilege is also maintained by dynamic immunoregulatory processes that are initiated when antigens are introduced into the eye by injection, corneal transplantation, or endogenous ocular antigens. Each of these dynamic immunoregulatory processes relies on

Tregs that act to either prevent the induction or expression of immune processes that inflict injury on tissues with little or no regenerative properties, while preserving immune effector mechanisms that provide protection against pathogens without damaging innocent bystander cells in the eye (Table 1). As stated over two decades ago by the preeminent ocular immunologist J. Wayne Streilein, the eye and the immune system are engaged in ‘‘a dangerous compromise.’’ This compromise protects the eye from unwitting immune-mediated injury at the risk of being vulnerable to ocular infections and perhaps blindness. The reader’s capacity to read through the articles in this encyclopedia is a testament to the success of this compromise.

See also: Adaptive Immune System and the Eye: T CellMediated Immunity; Antigen-Presenting Cells in the Eye and Ocular Surface; Immunosuppressive and AntiInflammatory Molecules that Maintain Immune Privilege of the Eye.

Further Reading

Ashour, H. M. and Niederkorn, J. Y. (2006). Peripheral tolerance via the anterior chamber of the eye: Role of B cells in MHC class I and II antigen presentation. Journal of Immunology 176: 5950–5956.

Caspi, R. R. (2006). Ocular autoimmunity: The price of privilege?

Immunological Reviews 213: 23–35.

Cone, R. E., Li, X., Sharafieh, R., O’Rourke, J., and Vella, A. T. (2006). The suppression of delayed-type hypersensitivity by CD8+ regulatory T cells requires IFN-g. Immunology 120: 112–119.

Faunce, D. E., Sonoda, K. H., and Stein-Streilein, J. (2001). MIP-2 recruits NKT cells to the spleen during tolerance induction. Journal of Immunology 166: 313–321.

Faunce, D. E. and Stein-Streilein, J. (2002). NKT cell-derived RANTES recruits APCs and CD8+ T cells to the spleen during the generation of regulatory T cells in tolerance. Journal of Immunology 169: 31–38.

Li, X., Taylor, S., Zegarelli, B., et al. (2004). The induction of splenic suppressor T cells through an immune-privileged site requires an intact sympathetic nervous system. Journal of Neuroimmunology 153: 40–49.

McKenna, K. C. and Kapp, J. A. (2004). Ocular immune privilege and CTL tolerance. Immunologic Research 29: 103–112.