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Chapter 159 – Mechanisms of Uveitis

Chapter 159 – Mechanisms of Uveitis

 

GEETA PARARAJASEGARAM

 

 

 

 

 

DEFINITION

• Uveitis is a complex intraocular inflammatory process that primarily involves the uveal tract.

 

KEY FEATURES

• The disease is characterized by the classical signs of inflammation, including inflammatory exudates and cells in the anterior chamber and/or vitreous.

 

ASSOCIATED FEATURES

• In most instances the cause is unknown.

• When identified, the causative agents and/or mechanisms include infectious agents, trauma, and autoimmunity.

 

 

 

INTRODUCTION

Uveitis is an inflammatory condition of the uvea and adjacent structures that affects mainly children and young adults. In the majority of patients, the cause of uveitis remains obscure even after extensive investigation. The inflammation might be induced by infectious agents or trauma, but in most cases the underlying mechanisms appear to be autoimmune in nature. Many excellent publications address the basic elements involved in the circuitry of the immune system and the induction of autoimmunity.[1] [2] The following includes an overview only of the main elements involved in immunity-autoimmunity and emphasizes the factors that have been shown to be involved in uveitis.

The immune response is an intricate event that is regulated by a number of different types of cells and the soluble factors secreted by some of these cells. The cells fall into two main categories, leukocytes and tissue cells. The former subclass comprises lymphocytes (T and B), phagocytes, and auxiliary cells (basophils, mast cells, and platelets). Tissue cells include in situ antigen-presenting cells (APCs), some of the functions of which have been elucidated in vivo; others are putative APCs, based on in vitro studies.

CELLS OF THE IMMUNE SYSTEM

B and T Lymphocytes

Both B and T lymphocytes are capable of participating in specific responses to a particular antigen and are derived from stem cells in the bone marrow. In mammals, B cells mature in the bone marrow, whereas T cells are “educated” through a sojourn in the thymus. B cells play a role in the immune system by producing specific antibodies to an antigen that is encountered. The antibodies belong to a group of extensively studied proteins called immunoglobulins (Igs), of which five different types occur—IgD, IgM, IgG, IgA, and IgE. When the antibody response requires the aid of T cells, the antigen against which this type of antibody is formed is termed a T cell–dependent antigen. The antibody produced by the differentiated B cell or plasma cell is inserted into the surface of the same cell, where it acts as a specific antigen receptor. Whereas B cells have the ability to recognize native antigen that is not processed or presented by other cells, T cells require the presence of a compatible APC for this recognition phase.

T cells express various surface molecules, which play a significant role in antigen recognition. These include the T-cell antigen receptor (TCR), surface molecules (CD4 and CD8), and others. The TCR is a disulfide-linked glycoprotein that allows T cells to recognize a wide range of antigens.[3] Two forms of TCRs (aß TCR and ?d TCR) exist, based on the structure of the heterodimeric glycoproteins, and the two types have distinct anatomical locations. The aß TCR is present on more than 90% of peripheral T cells and on the majority of thymocytes that express TCRs. The preponderance of T cells present in the epithelium and substantia propria of the normal conjunctiva express the aß TCR.[4] The T cells that express the ?d receptor occur in abundance in the epithelia of the intestine, uterus, and tongue. TCR ?d T cells have also been detected in inflamed conjunctiva and isolated and cultured from inflamed vitreous.[4] [5]

Role of T Cells

Different groups of T cells occur, determined by the role played by each type. These functions are determined by surface receptors on the various subsets. Helper T cells (TH cells), of which further subsets exist, act in conjunction with other immune cells to produce a response. The TH cells in one subset activate B cells to produce antibodies, and those in another subset interact with phagocytes in the destruction of pathogens. Cytotoxic T cells (TC cells) attack foreign cells and host cells infected with viruses. TH cells are identified most commonly by their surface receptor CD4, and the CD8 marker distinguishes TC cells. TH cells can be divided into two subsets, TH1 and TH2, on the basis of the pattern of production of various soluble mediators called cytokines.[6] TH1 cells produce cytokines such as interleukin-2 (IL-2), interferon (IFN)-? and tumor necrosis factor (TNF)-a and TNF-ß. They are the major effectors in delayed-type hypersensitivity, cytotoxicity, and macrophage activation and are also implicated in autoimmune diseases. TH2 cells preferentially produce IL-4, IL-5, IL-6, IL-10, and IL-13 and are responsible for the immune response against extracellular pathogens, allergens, and parasites.

Other Cells

Although the role of T and B lymphocytes in immune responses has been discussed, it must be stressed that these cells alone are not capable of achieving the wide range of responses involved in the initiation, perpetuation, and termination of an inflammatory reaction. In addition to B and T cells, a third population of lymphocytes exists, referred to as natural killer (NK) cells. These cells lack both immunoglobulins, which are characteristic of B cells, and TCRs, which are the hallmark of T cells, and constitute about 15% of blood lymphocytes. The role of these cells, although not fully understood, appears to lie in their ability to recognize and kill certain tumor cells and virus-infected cells.

 

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Mononuclear phagocytes, to which group belong the APCs, are derived from the bone marrow. Macrophages (mononuclear phagocytes in tissues) are found in many organs and participate predominantly in the removal of particulate antigens. These cells bind microorganisms through specialized receptors on their surface and ingest the invading organism. Neutrophils, also termed polymorphonuclear neutrophils (PMNs), are a hallmark of an acute inflammatory response. Their ability to ingest and kill invading organisms is based largely on the presence of two types of granules within the cell, the primary or azurophilic granules and the secondary or specific granules. Eosinophils also appear to be capable of phagocytosing and killing ingested microbes, although they play a more specialized role in the immunity to parasitic worms through the extracellular release of a toxin referred to as major basic protein. Eosinophils are also important participants in the termination or damping of an immune response. This is effected through the secretion of molecules such as histaminase and aryl sulfatase, which inactivate soluble products of mast cells.

Mast cells, functionally similar to basophils, play a role in the body’s response to an allergen by the release of mediators such as histamine. This reaction occurs through an interaction of the allergen with IgE molecules present on the surface of the mast cells and basophils. Mast cells, which have been demonstrated in the conjunctiva, play a pivotal role in allergic conjunctivitis.[7]

Processing of Antigens

As stated before, T cells require processing and presentation of antigen by APCs prior to being activated. Antigen recognition by T cells is dependent on two steps. One occurs through a TCR,[3] and the second involves binding of accessory molecules present on the APC to appropriate receptors on the T cell ( Fig. 159-1 ). In the absence of antigen, a T cell is not reactive. However, antigen alone cannot stimulate the cell. In fact, this first signal received alone will “turn off” the T cell, resulting in one mechanism

 

 

Figure 159-1 Diagrammatic representation of antigen presentation to CD4 marker positive T cells. MHC, Major histocompatibility complex; TCR, T-cell antigen receptor.

in which a response to autoantigens is prevented. T-cell proliferation or stimulation requires signaling between costimulatory molecules on the APC and cell surface molecules on the T cell. The best studied costimulatory molecules for T-cell activation are B7-1 (CD80), B7-2 (CD86), and CD40 and their respective receptors on the T cell, namely CD28, CTLA4, and CD40L (CD40 ligand).[8] Compatibility of the APC and T cell is a pivotal step in antigen presentation. This recognition, also termed “restriction,” is determined by the proteins of the major histocompatibility complex (MHC).

The human MHC system, known as the human leukocyte antigen (HLA) system, consists of three classes of molecules, class I, class II, and class III. Class I molecules in the human system are further subdivided into A, B, C, E, F, and G. These molecules, which are present on the surface of almost all nucleated cells in the body, are required for activation of CD8+ cells. Class II molecules, which are derived from the genes located in the D region of the human HLA complex, are also referred to as the HLA-DP, DQ, and DR molecules. CD4+ T cells respond to peptides (antigens) when presented with MHC class II or HLA-D region molecules. The latter are constitutively present only on cells involved in immune responses, such as APCs, which include dendritic cells, macrophages, and B cells. Their expression can, however, be induced in a variety of cell types following stimulation by cytokines. Studies have demonstrated MHC class II molecule expression on activated CD4+ and CD8+ cells, which allows them to act as APCs to themselves or to other autologous T cells.[9] Class III molecules are encoded by over 20 genes, some of which encode complement system products whereas others are responsible for the production of molecules involved in antigen processing.

Antigen is processed by APCs via a complex intracellular system, which includes proteolytic enzymes, peptide transporters, and molecular chaperones. [2] The processed antigen is then bound to MHC molecules on the surface of the APC. For antigen recognition by T lymphocytes, the MHC molecule must present the antigenic peptide to the TCR, which is associated with a molecule termed CD3 on the T cell. Various types of T cells require different MHC molecules for antigen presentation. Once the initial antigen presentation has occurred, the T cells respond by either acting directly with other cells or generating soluble factors that act on other cells. As more information becomes available, it appears that these soluble factors or cytokines are pivotal in the maintenance of balance of the immune system.

Importance of Antigen Presentation

The importance of antigen presentation in immunity cannot be overstated. This presumed first step in the induction of an immune

 

 

Figure 159-2 Fluorescent antibody–labeled microglial cells in a rat retina. A whole mount of rat retina was stained with antibody to Ox42, a marker for microglia and/or monocytes. The microglial cells are observed both perivascularly and in locations distant from the vessel. Microglial cells are thought to be the local antigen-presenting cells (APCs) in the retina. V, Vessel.

 

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reaction is responsible for the protection afforded against microbes, parasites, and other infectious agents. The established APCs in the eye include Langerhans’ cells (which are dendritic cells) and macrophages in the peripheral cornea and bone marrow–derived cells in the uvea[10] and retina ( Fig. 159-2 ). [11]

TOLERANCE AND AUTOIMMUNITY

The delicate balance afforded by the interaction of the various types of cells and their soluble products results in an almost perfect immune system. However, even slight aberrations of this self-regulating balance can lead to disastrous consequences. A reaction that could certainly lead to deleterious effects for the host concerned is the mounting of an immune response to autoantigens. This phenomenon is referred to as autoimmunity. However, nature, to a large extent, has prevented this from occurring through a mechanism referred to as tolerance. Immunological tolerance to autoantigens is a basic property of the immune system and is a feature by which the immune system can differentiate between self and nonself. Antigens that induce tolerance are termed tolerogens; they are distinct from immunogens, which are antigens that produce an immune response. Under normal conditions, all autoantigens act as tolerogens.

TOLERANCE

Tolerance to self is an active process, and there are two well-established mechanisms by which it is achieved ( Box 159-1 ). These are termed central and peripheral tolerance.[12] Central tolerance occurs during T-cell development, when immature T cells migrate from the bone marrow to the thymus. In the thymus, these T cells encounter autoantigens presented by APCs in conjunction with the MHC. The T cells with high affinity for autoantigens

 

 

Mechanisms of Tolerance Induction

 

CENTRAL TOLERANCE

• Usually occurs in the thymus during T-cell development

• Death of immature T cells that express a TCR that recognizes autoantigens

• Process referred to as negative selection

• Autoreactive T cells undergo clonal deletion by apoptosis

 

PERIPHERAL TOLERANCE

Mechanisms by which mature autoreactive T cells are kept in check include anergy, deletion, suppression, and ignorance

 

 

Anergy

• Occurs when antigen presentation to T cell occurs in absence of costimulatory molecules

• Mediated by block in IL-2 transcription

 

Deletion

• Autoreactive cells may be deleted by apoptosis

• Also referred to as programmed cell death

• Mediated via Fas and FasL

 

Suppression

• Mediated by antigen-specific T cells, natural suppressor (NS) cells, and veto cells

• Antigen-specific T cells suppress the response of autoreactive T cells

• NS cell–mediated suppression is antigen nonspecific

• Veto cells present negative signals to CD4+ cells, causing inactivation of effector T cells

• May be due to TH subset that produces transforming growth factor-ß

 

Ignorance

• Nonrecognition of antigen by autoreactive T cells

• May be due to presentation of autoantigen by cells that do not possess co-stimulatory molecules or autoantigens that are anatomically sequestered

• Autoreactive cells recognize only “cryptic” determinants on the autoantigen

 

 

 

are programmed to die through a process termed apoptosis. This mechanism of clonal deletion occurs through a process referred to as negative selection and ensures that a large number of autoreactive T cells are deleted. Elimination of autoreactive B-cell clones also occurs through clonal deletion of immature B cells and takes place in the bone marrow. Not all the self-peptides that T and B cells might encounter during their lifetime are present in the thymus and bone marrow; hence, it is possible that occasional autoreactive cells may escape the process of negative selection. Peripheral tolerance comes into play, to ensure that regulatory mechanisms exist to curb the activity of these cells that have the potential to cause autoimmune disease. Mechanisms that prevent mature autoreactive cells in the periphery from causing autoimmune disease include anergy, deletion, suppression, and ignorance.

ANERGY.

Anergy, which is defined as the functional inactivation of lymphocytes without their elimination, is induced when an autoreactive T cell is presented antigen in the absence of a costimulatory molecule or “second” signal, thus preventing further activation of the T cell. Thus, although autoreactive cells are present, they are incapable of responding to antigen. The process is thought eventually to be due to a block in the transcription of the gene for IL-2.[13]

DELETION.

Peripheral autoreactive T cells may also be deleted through apoptosis. It is mediated by a cell surface protein termed Fas (CD95), which is expressed on many cell types, including hemopoietic and epithelial cells.[14] Following antigen receptor-mediated activation, the expression of Fas on T and B cells has been shown to increase. The distribution of the Fas ligand FasL is much more restricted; it is induced on CD4+ and CD8+ lymphocytes following activation but not expressed on any other hemopoietic cells. Interactions between FasL and Fas play an obligatory role in apoptosis of T cells. Studies that demonstrate the constitutive expression of FasL in the eye have led to the theory that the interaction of Fas and FasL could be significant in the immune privilege of the eye.[15] In other studies, it has been shown that memory T cells in the aqueous humor of patients with uveitis preferentially express Fas antigen, which suggests that this increase in the number of Fas+ T cells may be involved in the pathogenesis of uveitis.[16]

SUPPRESSION.

Another potential mechanism of peripheral tolerance is suppression, in which a reactive T cell is actively kept from carrying out its function by another cell. T cells that belong to the TH2 subgroup appear to be one of the principal regulatory cell types in this mechanism, based on their ability to produce cytokines that suppress immune responses associated with autoimmune diseases. Whereas antigen-specific T suppressor cells inhibit the responsiveness of autoreactive effector T cells, another population of suppressor cells termed natural suppressor cells mediate suppression in a manner that is not antigen specific and does not require MHC restriction. Yet another subset of T cells, termed veto cells, has also been identified. Veto cells are T cells that have an APC function, present antigen to CD4+ T cells, and deliver negative signals to produce inactivation of the effector T cell. This form of anergy seems to be unrelated to the absence of costimulatory molecules. Studies have implicated a separate population of TH cells, which elaborate large amounts of the cytokine transforming growth factor-ß, an effective inhibitor of lymphocyte proliferation. As a result of the difficulty encountered in cloning suppressor cells and the lack of a suppressor-specific surface marker, the role of suppressor cells in tolerance is still largely unknown.

IGNORANCE.

Autoreactive T cells do not initiate disease if they ignore or are protected from autoantigens. In this form of peripheral tolerance, autoreactive cells may ignore autoantigens either because the antigen is present on cells that do not possess costimulatory molecules or because the antigen is anatomically sequestered. In addition, the amount of antigen available may be below the threshold necessary for activation of the T cell. Only a small portion of any antigen, known as the dominant determinant,

 

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is usually presented to T cells. If an autoreactive T cell recognizes only a region of an autoantigen that is not normally accessible during presentation (a cryptic determinant), the antigen may be ignored by peripheral autoreactive cells. The mechanism of tolerance by ignorance is considered a passive one because it leads to neither deletion nor anergy of autoreactive cells.

Breakdown of Tolerance and Autoimmunity

Even as there are several ways in which tolerance to autoantigens is established and maintained, there are also multiple mechanisms through which this tolerance can break down ( Box 159-2 ). The selection of nonautoreactive clones in the thymus could be flawed in individuals who have certain MHC genotypes, which leads to either the presence of autoreactive T cells or the absence of immunoregulatory T cells that are necessary to inhibit the former. Any factor that causes the release of previously sequestered autoantigens could potentially lead to a breakdown of tolerance to those antigens. Mechanisms that overcome the maintenance of peripheral tolerance could also lead to autoimmunity. For example, the absence of costimulatory molecules on APCs in antigen recognition by T cells leads to tolerance. However, if conditions exist that lead to the expression of these molecules by APCs, loss of T-cell tolerance results. Clonal anergy is ultimately thought to be due to a block in transcription of IL-2. Certain studies indicate that the responsiveness of these anergic T cells can be restored by culture with IL-2; this leads to the suggestion that local production of IL-2 during T-cell responses to nonautoantigens may overcome the peripheral tolerance of autoreactive T cells present at the site and so lead to autoimmunity.[13]

Autoimmunity may also result from a stimulation of autoreactive lymphocytes not deleted during development. This stimulation by polyclonal activators, which stimulate many T- and B-cell clones, occurs irrespective of their antigenic specificity. Bacterial lipopolysaccharide is a well-known polyclonal B-cell activator, and polyclonal T-cell stimulation by bacterial “superantigens” has been proposed as another mechanism of autoimmunity. T cells that express the ?d TCR are thought to be involved in this response to superantigens. The polyclonal activation of B cells may be induced by bacterial products that act like lipopolysaccharide and may give some insight into the possible link between infection and autoimmunity. In some cases, a response to a foreign antigen that has determinants in common with autoantigens could potentially lead to an autoimmune reaction.

 

 

 

Possible Mechanisms of Tolerance Breakdown and Autoimmunity

Presence of certain MHC genotypes that could lead to:

• Presence of autoreactive T cells

• Absence of immunoregulatory T cells

Release of normally sequestered autoantigens due to:

• Trauma

• Infection

Alteration of autoantigen structure resulting from:

• Tissue injury

• Inflammation

Expression of costimulators on APCs can overcome peripheral tolerance

Polyclonal stimulation of self-reactive lymphocytes by:

• LPS (for B cells)

• Bacterial ‘superantigens’ (for T cells)

Both are characterized by antigen-independent stimulation

Molecular mimicry due to homology between:

• Pathogens and host tissue antigens

• Microbial proteins and certain HLA antigens

 

 

 

CAUSES OF UVEITIS

In the various types of uveitis encountered in humans in which the mechanism is known, three main underlying causes occur:

• Reaction to trauma

• Autoimmune reaction

• Response to an infectious agent

Uveitis related to trauma could follow a penetrating injury (see Chapter 179 ). In the case of the autoimmune component, this could be either a direct response to autoantigens as a consequence of tolerance breakdown, as discussed earlier, or a secondary reaction to autoantigens that results from damage to ocular structures from other causes. In the matter of infectious uveitis, Toxoplasma and other infectious agents have been implicated in many cases. The inflammatory response observed in infectious uveitis could result from two scenarios—first, a reaction to noxious agents such as toxins produced by pathogens, and second, an immune response to the pathogen itself. In some cases, a response to a particular pathogen could lead to a reaction against autoantigens because of similarity or cross-reaction in antigenic structure between the two. This cross-reaction or homology is termed molecular mimicry and is postulated to be one mechanism by which an autoimmune response could be initiated. Although many examples of molecular mimicry are found in viruses, bacteria, protozoa, and helminths, its biological relevance to human disease is yet to be confirmed. In experimental studies, microbial (nonself) proteins that have sequence homology to certain uveitogenic self-peptides have been shown to induce autoimmune uveitis in rats and subhuman primates. These nonself proteins include sequences from hepatitis B virus, Moloney murine leukemia virus, baker’s yeast, and Escherichia coli, among others. [17] [18] [19]

There has been some speculation about molecular mimicry playing a role in certain diseases with a genetic predisposition. Correlations between the expression of various HLAs and certain autoimmune diseases have been noted for antigens of HLA class I and II ( Table 159-1 ). For example, anterior uveitis is strongly associated with the presence of HLA-B27. Uveitis in Behçet’s disease has an association with HLA-B51 and birdshot choroidopathy with HLA-A29.[20] Because MHC class II molecules are involved in the selection and activation of CD4+ T cells and these T cells play an integral part in the regulation of the immune response, much work has been done on the association of MHC class II antigens and disease prevalence. This is demonstrated in studies of sympathetic ophthalmia, Vogt-Koyanagi-Harada syndrome, rheumatoid arthritis, insulin-dependent diabetes mellitus, and Sjögren’s disease, to name a few.[21] [22] Although it is apparent that the presence of certain haplotypes predisposes an individual to some diseases, it is still unclear how the disease process in these instances is initiated. Cross-reactivity between microbial proteins and certain HLA antigens might result in an autoimmune response. Although examples of such molecular mimicry have been noted, their significance in pathogenesis is unclear. Studies have analyzed disease induction by HLA peptides that have a homology in their sequence to certain retinal antigens.[23] Although the expression of a particular HLA gene product may not by itself be the cause

 

TABLE 159-1 — ASSOCIATION OF HUMAN LEUKOCYTE ANTIGEN (HLA) AND OCULAR INFLAMMATORY DISEASE

Disease

Associated HLA

Anterior uveitis

HLA-B27

Behçet’s syndrome

HLA-51

Birdshot retinopathy

HLA-A29

Intermediate uveitis

HLA-A28[22]

Vogt–Koyanagi–Harada syndrome

HLA-Dw53, HLA-DR4

 

 

 

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of any autoimmune disease, it may be one of several factors that contribute to the breakdown of tolerance.

INFLAMMATION AND HYPERSENSITIVITY REACTIONS

Inflammation is a response that causes the influx of leukocytes and plasma molecules to the site of an infection, antigenic challenge, or tissue damage. This process, which involves chemotactic factors and cell migration, adhesion molecules and vascular permeability, and the release of various inflammatory mediators both locally at the site of inflammation and at distant locations, is an exquisitely orchestrated sequence of events ( Fig. 159-3 ). The sequence at an inflammatory site is dependent on the cause of the inciting event. In the case of an infection, leukocytes arrive at the site and produce soluble mediators that regulate subsequent events such as cell accumulation and activation. The inflammation ideally resolves when the causative agent is removed. When the inflammation is elicited by the immune system itself, ensuing events are controlled by the antigen that initiated the original response. This situation usually results in a chronic inflammatory state, either in an infection or in an autoimmune response because the inciting antigen cannot be completely removed.

Chemokines

Cells migrate to sites of subsequent inflammation through the action of mediators known as chemokines released at these locations. Chemokines are chemotactic cytokines and belong to a family of small proteins.[24] [25] They can be divided into subgroups, named CC, CXC, C-x3-C, and C chemokines, on the basis of the location of the first two cysteines compared with other amino acids (X) in the structure of the protein. The CXC subgroup is also referred to as a-chemokines and the CC subgroup as ß-chemokines.

 

 

Figure 159-3 Steps in inflammation.

T lymphocytes produce the chemokine macrophage inflammatory protein 1a (MIP-1a), which causes chemotaxis of naive T cells, B cells, and NK cells.[24] The chemokine RANTES (regulated upon activation normal T cell expressed presumed secreted), which is produced by many cell types in response to specific stimuli, is preferentially chemotactic for memory T cells but also acts on NK cells and so causes activation of both these cell types. RANTES is a chemoattractant for eosinophils and macrophages and causes histamine release from basophils. It is found in very low amounts in normal adult human tissue, but its expression increases dramatically at inflammatory sites.[25]

MIP-1a, RANTES, and monocyte chemoattractant protein 1 (MCP-1) are examples of CC chemokines. IL-8, which is derived from monocytes, lymphocytes, fibroblasts, epithelial cells, and vascular endothelial cells, localizes PMNs and belongs to the CXC subgroup. Fractalkine or neurotactin is a chemokine belonging to the C-x3-C subgroup, and lymphotactin, which is chemotactic for T cells and NK cells, is a member of the C chemokine subgroup. Molecules that are chemotactic for PMNs and macrophages include the complement system–derived factor C5a and leukotriene B4 . In addition, studies have indicated that oxidized lipids derived from retinal membranes are chemotactic for PMNs.[26]

Extravasation of Cells

A circulating inflammatory cell must leave the blood in order to reach a tissue site. This transendothelial migration through the vessel wall includes tethering, triggering, and latching of leukocytes to the vascular endothelial cells. These events are mediated by adhesion molecules expressed on vascular endothelial cells, which bind to corresponding molecules on leukocytes ( Fig. 159-4 ). The major endothelial cell adhesion molecules include intercellular adhesion molecule-1 (ICAM-1), ICAM-2, and vascular cell adhesion molecule-1 (VCAM-1). The adhesion molecules present on leukocytes belong to a family of proteins referred to as integrins. Different types of integrins exist, broadly differentiated by their structure. The leukocyte functional

 

 

Figure 159-4 Steps in leukocyte adhesion. The initial step in adhesion involves the interaction of adhesion molecules (CD15) on leukocytes with molecules present on endothelium. The latter belong to a group of molecules termed selectins, E-selectin being one such transmembrane member, expressed on endothelium. This first stage of attachment is referred to as tethering. In the next phase, the tethered cell is activated or triggered by either direct or indirect signals from molecules on the epithelium or by the action of chemokines. This step is termed triggering. In the final phase, termed latching, there is up-regulation of various molecules, such as integrins, on the leukocytes, which then attach to adhesion molecules such as intercellular adhesion molecules (ICAM)-1 that are generated on the endothelium. The latching stage is induced by triggering. (Adapted from Figure 14.10 of Roitt I, Brostoff J, Male D. Immunology, 4th ed. London: Mosby; 1996.)

 

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antigens (LFAS) bind to ICAM-1 and ICAM-2 and are present on most leukocytes. The VLAs (very late antigen), which include the integrin VLA-4, are primarily involved in binding cells to extracellular matrix. In addition, VLA-4 binds to VCAM-1 on the vascular endothelium.

When inflammatory cells reach the tissue to which they are directed, a phenomenon referred to as homing, they participate in the local immune response. This process is controlled by the production of various soluble mediators, termed cytokines, which are proteins involved in the communication between cells. Cytokines have been shown to have autocrine, paracrine, and endocrine functions. Some of these cytokines induce perpetuation of the inflammation through the recruitment of other effector cells to the site, whereas others modulate the process by shutting down the operation of effector cells so that host tissue damage is minimized. A balance between these different cytokines is crucial to the outcome of the response. Ample evidence is available to support the fact that cytokines play a pivotal role in the pathogenesis of uveitis.[25]

Cytokines

Cytokines generated in uveitis include some of the IL family of proteins (IL-1, IL-2, IL-4, IL-6, IL-8, IL-10, and IL-12), IFN-? and TNF-a. [24] The ILs (IL-1 through IL-18 at the present time) are a major group of cytokines that have been found to have diverse functions in immune responses. Macrophages and B cells produce IL-1, which is important in the stimulation of macrophages, the activation of lymphocytes, and the adhesion of leukocytes to endothelial cells. IL-2 is a potent cytokine produced by T cells, and its primary known physiological effect is its action as a T-lymphocyte growth factor. T cells also produce IL-4, the principal targets of which are B cells and T cells. IL-4 also functions in B-cell growth. T and B cells and macrophages secrete IL-6, which is known to act on B cells during their differentiation. Finally, IL-10 is one of the T cell–derived mediators known to down-regulate an immune response by the inhibition of cytokine synthesis by TH1 cells, and IL-12, a monocyte product, is responsible for the induction of TH1 cells.

A product of T cells and NK cells, IFN-? is one of the earliest cytokines to be produced during inflammation. Its role in the induction of MHC class I and II antigens, the activation of macrophages, the increase in adhesion of endothelial cells and leukocytes, and the reduction of cytokine synthesis has been shown to be central to the inflammatory process. In experimental models, generation of IFN-? appears to be essential for T-cell passage to ocular tissues such as the retina. Activation of macrophages, granulocytes, and cytotoxic cells is caused by TNF-a, which also increases leukocyte and endothelial cell adhesion and enhances MHC class I production as well as many other functions. Transforming growth factor-ß is a multifunctional cytokine that has been found in normal and inflamed ocular fluids. [27] It is known to have immunosuppressive functions, such as inhibition of macrophages and inhibition of T- and B-cell proliferation, and is normally present in the aqueous humor. Results obtained after intravitreal injection of low doses of transforming growth factor-ß have led to the suggestion that this cytokine may interrupt the cascade of events that leads to ocular inflammation.

Although cytokines are major participants in an immune response, other cell-derived agents such as reactive oxygen metabolites and hydrolytic enzymes or proteases are equally important in determining the consequence of an inflammatory reaction. These oxygen free radicals and proteases are produced by phagocytic cells and are a normal part of the armament of these cells in their role against microbes. However, free radicals have also been implicated in uveitis.[28] A reactive radical given prominence in the pathogenesis of several inflammatory diseases is nitric oxide, which is produced endogenously in small amounts. However, certain cytokines can induce the production of large amounts of nitric oxide. Studies in an acute model of uveitis have indicated that nitric oxide may be a key mediator, playing a complex role in ocular inflammation.[29]

Immune Response and Hypersensitivity

A specific immune response involves the interaction of effector mechanisms such as complement, phagocytes, inflammatory cells, and cytokines. However, because these responses are not directed against the inciting agent only, injury to the surrounding host tissue usually occurs as well. Under normal conditions, because of the self-limiting nature of immune responses, these injurious reactions are minimal and dampened when the foreign antigen is eliminated. In addition, because of tolerance to autoantigens, immune responses to autologous tissues do not usually occur. In some cases, however, when a specific immune response is not appropriately controlled, a phenomenon termed hypersensitivity ensues. Hypersensitivity is the result of a beneficial immune response that has gone awry. Four main types of hypersensitivity reactions occur, classified as types I, II, III, and IV.

Type I reactions are the result of an IgE response to a particular antigen termed an allergen, which is the underlying mechanism in allergic reactions. They are mediated by mast cells and their mediators, in particular histamine. A type I reaction is the underlying mechanism for the development of conditions such as allergic conjunctivitis.

Type II hypersensitivity responses are observed in transfusion reactions, hemolytic anemia, Goodpasture’s syndrome, pemphigus, and myasthenia gravis. In this category of hypersensitivity, the reaction is a result of antibody binding to cell surface antigens on cells or tissues. As a result, damage is restricted to the cells or tissues that express those antigens. The initial antibody binding, which could be of the IgM or IgG type, leads to activation of the complement system and effector cells and results in ultimate damage to cells and tissues by cytotoxic effects or lysis, or both. Graves’ disease, which is associated with an ophthalmopathy, may be mediated by a type II reaction.

Reactions that involve type III hypersensitivity responses are also termed immune complex reactions. Under normal conditions, when an antibody and an antigen combine, an immune complex is formed. These complexes are usually cleared from the systemic circulation by phagocytes. Ineffectively cleared immune complexes that persist in the circulation can lead to systemic disease. Deposited complexes trigger a variety of processes such as complement activation, cytokine production, and release of vasoactive amines. Complement activation, in addition to producing other mediators, produces chemotactic factors, which in turn cause an influx of inflammatory cells to the site and lead to amplification of the response. Cytokines such as TNF-a and IL-1 are produced by macrophages and lead to enhancement of the inflammation. In addition, direct interaction of the complexes with basophils and platelets leads to the release of vasoactive amines. This type of hypersensitivity is responsible for diseases such as systemic lupus erythematosus, polyarteritis nodosa, serum sickness, and phacoanaphylaxis.

The hallmark of type IV hypersensitivity, which is often referred to as delayed-type hypersensitivity, is that it can be transferred in experimental animals from one to another by T cells and is regulated primarily by T cells. Type IV responses are subclassified into contact, tuberculin, and granulomatous hypersensitivity reactions. Contact hypersensitivity, mediated by Langerhans’ cells and keratinocytes, is primarily an epidermal reaction. Tuberculin-type hypersensitivity primarily involves monocytes and is a memory response of T cells that have previously encountered the inciting antigen. Granulomatous hypersensitivity results in the ultimate formation of epithelioid cell granulomas and is the most relevant type IV reaction in clinical settings. Diseases in which this type IV reaction is manifest include leprosy, tuberculosis, and sarcoidosis. A common trend in these diseases is that the causative agent persists, generating a

 

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chronic antigenic stimulus. The pathogenesis of sympathetic ophthalmia is mediated by a type IV reaction.

ANIMAL MODELS OF UVEITIS

Uveitis in humans is a complex intraocular condition. Notwithstanding the fact that much effort has been spent to elucidate the mechanism of pathogenesis in human cases of uveitis, to date most of the data regarding the possible pathways of disease induction and progression have been obtained from well-established animal models. These models not only allow study of the pathogenesis of uveitis but also in some cases are aimed at modulating the disease process. Investigators have striven to use various inducing agents in developing animal models of uveitis in an effort to mimic the inflammation observed in humans.

Experimental Autoimmune Uveitis

Experimental autoimmune uveitis has been induced in subhuman primates, rats, mice, and guinea pigs using a variety of intraocular antigens. Animals that under normal conditions do not spontaneously develop inflammation do so following immunization with a range of ocular autoantigens. These models provide an understanding of the prevailing mechanisms in the types of human uveitis that may have an autoimmune component. Among these is Vogt-Koyanagi-Harada disease.[30]

Although many ocular antigens have been used to induce uveitis in animal models, the best studied condition is that induced in rats and subhuman primates with the retinal soluble protein S-antigen ( Fig. 159-5 ). Most animal models of uveitis have been shown to be mediated by CD4+ cells. Some of the antigens that have been utilized are interphotoreceptor retinoid binding protein, melanin protein, rhodopsin, PEP-65 (a protein purified from retinal pigment epithelial cells), phosducin, recoverin (a calcium-binding protein identified in cancer-associated retinopathy), and those belonging to the tyrosinase family of proteins.[30] Whereas most of the antigens used produce a disease that mainly involves the posterior region of the eye, the disease produced with melanin protein is characterized by an inflammation that is predominantly anterior in nature, with essentially no retinal involvement ( Fig. 159-6 ). It is termed experimental autoimmune anterior uveitis and resembles the noninfectious iridocyclitis observed in humans.

S-Antigen Uveitis

The model of S-antigen uveitis is used here to discuss the disease process that occurs in experimental autoimmune uveitis, which

 

 

Figure 159-5 Histopathology of a rat eye with retinal S-antigen–induced experimental autoimmune uveitis (the prototype animal model for many human cases of uveitis). The intraocular inflammation consists of mononuclear cells and polymorphonuclear leukocytes present at the site of retinal damage, in the outer segment and in the choroid (hematoxylin & eosin).

develops when S-antigen–reactive lymphocytes are activated. The activation is initiated when TCRs recognize the peptide presented to them by the MHC class II molecule on APCs. It has been postulated that these APCs are present in the eye. [11] The uveoretinitis that ensues as a final consequence of this initial reaction is the result of an immune reaction that occurs when these activated lymphocytes reach the eye and further activate the immunological process. The sensitized lymphocytes trigger this cascade of events, which include the production of cytokines, expression of adhesion molecules, and the recruitment of inflammatory cells to ocular tissue. Studies have demonstrated an increase in the expression of cytokines IL-2, IL-4, lymphotoxin, and IFN-?.[25] Enhanced expression of adhesion molecules has also been noted in experimental autoimmune uveitis.[31] The expression of the adhesion molecule ICAM-1 occurred prior to the earliest detection of inflammatory cells, indicating a definite pattern in the sequence of events.

Arthus-Type Reaction

An animal model using lens antigen has been extensively analyzed to study the pathogenesis of phacoanaphylactic endophthalmitis in humans, and it has been reported that the inflammation observed in experimental lens-induced uveitis is due to an altered tolerance to lens proteins ( Fig. 159-7 ). Studies by Marak[32] and others have proved that the inflammation in lens-induced uveitis is a result of a type III hypersensitivity reaction. This indicates that the response is an Arthus-type reaction involving antigen-antibody complexes and the subsequent activation of the complement cascade.

IMMUNE PRIVILEGE OF THE EYE

An anatomical site in which immunogenic tissue survives for extended periods of time in an immunocompetent host is referred to as an immune-privileged site.[33] This privilege is thought to have evolved as a protective mechanism in highly specialized organs whose normal function would be disrupted as a consequence of a local immune response; this phenomenon is believed to occur only in organs whose maintenance of normal function is vital to host survival. It has long been known that the eye is such a site of immune privilege, which is evident in the anterior chamber (AC), vitreous cavity, and subretinal space. Some of the features that appear to play a role in this privilege include the presence of a tight blood-ocular barrier, the complete lack of an intraocular lymphatic system, and an intraocular microenvironment

 

 

Figure 159-6 Histopathology of a rat eye following induction of uveitis with melanin. The disease is characterized by an anterior uveitis with inflammatory cells predominantly infiltrating the iris and ciliary body (hematoxylin & eosin).

 

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Figure 159-7 Histopathology of a rat eye with lens-induced granulomatous uveitis. The inflammation is distinguished by the presence of multinucleated giant cells and epithelioid cells present around the extruded lens cortex material (hematoxylin & eosin).

that is immunosuppressive. The presence of an intraocular immunosuppressive microenvironment is most apparent in the AC. As a result of this feature, antigens experimentally placed in the eye elicit a stereotypic deviant form of systemic immunity termed AC-associated immune deviation (ACAID). It was originally believed that this phenomenon arose because antigens within the AC failed to escape and therefore could not induce an immune response in the host; the lack of direct access to the lymphatic system supported this belief. Although the human AC has no direct lymphatic drainage, a pathway has been demonstrated in monkeys.

Many studies have demonstrated that various cytokines, neuropeptides, and other factors appear to contribute to intraocular immunosuppression. Some of the better studied of these are transforming growth factor-ß1 and 2, a-melanocyte–stimulating hormone, calcitonin gene–related peptide, and cortisol. ACAID may develop through a mechanism in which parenchymal cells within the iris–ciliary body secrete immunosuppressive factors into the aqueous humor. This deviant systemic immunity is selectively deficient in T cells that mediate delayed hypersensitivity reactions as well as in complement-fixing antibodies. The phenomenon has been confirmed by the adoptive transfer of lymphoid cells. It is thought that the immunosuppressive microenvironment observed in the eye acts to limit or avert the local induction of immunogenic inflammation, which could lead to loss of vision.[33]

Although much of the work pertaining to immune privilege in the eye has been concerned with the AC, there are studies indicating that the vitreous cavity and subretinal space also may possess this feature. The vitreous cavity and aqueous chamber are similar in that both sites lack significant lymphatic drainage and have a tight blood-tissue barrier. In experimental studies in mice, newborn neural retinal grafts implanted in the subretinal space and vitreous cavity experienced immune privilege, as evidenced by lack of inflammation. No donor-specific delayed-type hypersensitivity occurred, and the responses resembled those seen in ACAID.[34] The results of such experiments may be useful in overcoming the barrier of immune rejection and lead to successful retinal transplantation in the future.

SUMMARY

Uveitis is a complex intraocular inflammatory process characterized by the classical features of inflammation. Clinically, the disease includes altered vascular permeability, cellular infiltration of the uveal tract and intraocular cavities, and, in severe cases, loss of vision. In the majority of cases the cause of the disease remains elusive. However, in instances in which the cause is known, infection, trauma, and autoimmunity appear to play a role.

In this chapter, the basic immune mechanisms of inflammation and their participation in uveitis, where relevant, are discussed. A better understanding of ocular immune responses would lead to elucidation of the cause and pathophysiology of many uveitides and result in more effective therapy for these potentially blinding diseases.

 

 

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