5Ocular immunity and inflammation

INTRODUCTION

Many posterior-segment diseases involve inflammatory and immune mechanisms. This chapter provides a basic summary of these mechanisms as a framework for understanding pharmacotherapeutics that may be useful in treating retinal disease.

HISTORY

Like the field of immunology, the field of ocular immunology and inflammation has grown rapidly over the past years. This chapter covers broad concepts in immunology important to ocular disease as well as basic concepts in diagnostic and therapeutic strategies used in ocular inflammatory disease.

KEY CONCEPTS AND FUNDAMENTALS IN MOLECULAR BIOLOGY AND BIOCHEMISTRY

INNATE IMMUNITY

Innate immunity and complement provide immediate protection from pathogens. Although most relevant to eye disease in the protection of the ocular surface from exogenous insults such as infection, the genetically determined intensity of innate complement activation is increasingly understood to be involved in the pathogenesis of age-related macular degeneration (AMD) and assumed to be involved in posterior and panuveitis as well.

Intact surface epithelium, low surface temperature, neutral pH, protective tear-borne enzymes and macrophages, and an intact blink response are important first-line defenses of the external eye. Microbial lipopolysaccharides, peptidoglycans, lipoteichoic acids, mannans, bacterial DNA, double-stranded RNA, and glucans elicit an immediate response from the innate (natural) immune system. In addition, the mammalian immune system recognizes hundreds of pathogenassociated molecular motifs through pattern recognition receptors expressed on macrophages, B lymphocytes, and dendritic cells.1 Activation of these cells stimulates antigen presentation and upregulation of T lymphocyte costimulatory molecules and results in the slower adaptive (acquired) immune response directed against the invading organism. Finally, pattern recognition molecules coat pathogens and increase recognition by complement and phagocytes. Among the bestcharacterized pattern recognition receptors are mannan-binding lectin, mannose receptor, Toll-like receptor (TLR), and lipopolysaccharidebinding protein. TLRs have been localized to lymphocytes, eosinophils, mast cells, and epithelial cells of the conjunctiva. More recently, innate immunity via TLRs on intraocular cells has been hypothesized to participate in the development of AMD (Chapter 18).

Over 20 proteins and protein fragments make up the complement system, including serum proteins and cell membrane receptors.

Complement activation is destructive to tissue and is rapidly amplified through a triggered-enzyme cascade; several mechanisms prevent its uncontrolled activation and may be particularly important in the retina and choroid. For example, factor H is an important down-regulator of complement located on cell membranes. A common single-nucleotide polymorphism in factor H (Tyr402His) has been strongly associated with AMD.2 The hypothesis is that this polymorphism results in poorer function of factor H, increased complement activity, and more tissue destruction. Immunologic reactions in the retinal pigment epithelium (RPE) and choroid due to routine immune surveillance of blood-borne infectious agents can be assumed to lead to subsequent damage to ocular tissues if not dampened by protective factors.

ADAPTIVE IMMUNITY

Although polymorphisms in immune response genes may modify the course of intraocular inflammation from any cause, autoimmune, noninfectious uveitis mainly involves adaptive (acquired) immunity through antigen recognition by lymphocytes and typically requires days to establish an effective clonal expansion. A primary response results from the initial contact with the antigen. Secondary, tertiary, and subsequent responses increase in magnitude.

The afferent arm of the immune response occurs in specialized immune organs. Lymphatic vessels are found predominantly in the eyelids and drain into the submandibular lymph nodes.3 Although when vascularized the cornea may form some lymphatic channels,4 the remainder of the ocular anatomical structures, with the exception of the conjunctiva and eyelid, do not contain lymphatic channels. Lymphatic drainage from intraocular tissues is minimal, which likely serves to reduce acquired immunity to intraocular proteins except during inadvertent exposure, such as globe rupture or surgery. Nevertheless, extracellular fluid may drain through perivenous spaces of the posterior segment, through the lamina cribrosa, and into the spaces of the optic nerve.5 There is evidence that antigen injected intravitreally reaches the preauricular lymph node.6

Specific cell surface markers distinguish the cells of the adaptive immune system (Table 5.1). Glycoproteins designated as major histocompatibility complex (MHC) molecules – known as human leukocyte antigen (HLA) molecules in humans – present antigens to immunocompetent cells, primarily T lymphocytes. Two classes of MHC molecules exist: class I molecules are found on all nucleated cells and present antigen derived from intracellular components, such as tumor or viral antigens, to cytotoxic cluster of differentiation (CD) 8+ T lymphocytes; class II molecules are found predominantly on professional antigenpresenting cells (APCs), such as dendritic cells, macrophages, and B cells. Class II-bearing cells present extracellular antigens that have been endocytosed, such as fungal or bacterial antigens, to helper CD4+ T lymphocytes (Figure 5.1).

Dendritic cells and macrophages are classic APCs since they constitutively express MHC class II molecules and present antigen to CD4+ T lymphocytes. They also express immunoglobulin G (IgG) receptors that engulf extracellular antigens after they are coated with IgG, a reaction known as opsonization. Moreover, APCs stimulate lymphocytes by cytokine production, such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-α), and by surface expression of costimulatory

inflammation and immunity• 5 chapterOcular

Class II histocompatibility molecule

Figure 5.1  Antigen presentation. Top row: A macrophage or other antigen presenting cell (purple) endocytoses extracellular antigens, processes them, and presents peptides derived from these antigens in a binding groove of the major histocompatibility complex (MHC) class II molecule on the cell surface. This complex is recognized by the T-cell receptor (CD3) and CD4 molecules on the helper T-cell surface. Bottom row: infected cells process intracellular antigens such as viral components into viral peptides which are presented in a binding groove of the MHC class I molecule on the cell surface. This complex is recognized by the T-cell receptor (CD3) and CD 8 molecules on the helper T-cell surface.

are presented with an antigen for which they have a specific T-cell receptor (CD3) by APCs, the T cells undergo clonal expansion, initiating an immune response targeted at the specific antigen. Expression of intraocular proteins in the thymus is necessary for the deletion of autoreactive T-cell clones, resulting in tolerance, and is an important aspect of the immune privilege of the eye.

Subsets of T cells react to antigen in two different ways depending on the co-receptor molecule (either CD4 or CD8) associated with their T-cell receptor (CD3). Cells bearing the CD4 co-receptor promote the immune response through secreted cytokines that activate surrounding lymphocytes and/or macrophages. These cells recognize extracellular antigens that have been processed in intracellular vesicles following ingestion by an APC and presented as peptide in the groove of MHC class II molecules. T cells bearing the CD8 co-receptor are cytocidal, destroying the cells harboring antigen. These cells, called cytotoxic T cells, recognize cytotoxic antigens, such as viral antigens, presented in the groove of MHC class I molecules. In addition to antigen-specific signals mediated through the T-cell receptor and co-receptor molecules, T cells also require antigen-nonspecific costimulation for activation (Figure 5.2). The B7 family of molecules on APCs, which include B7-1 (CD80) and B7-2 (CD86), are recognized by CD28 on T lymphocytes and play important roles in providing costimulatory signals required for development of antigen-specific immune responses.

CD4+ T cells can be divided into four groups: Th1, Th2, Th17 and regulatory T cells (Tregs) (Figure 5.3). Th1 cells are potent clearers of intracellular bacteria such as Mycobacterium tuberculosis and M. leprae by activating macrophages to fuse bacteria-containing vesicles with lysozymes. They also secrete cytokines that stimulate macrophage migration to the site of infection. Th2 helper cells activate B cells to secrete antibodies, and drive humoral immunity. The term “helper” T cell is

“Signal One”

Histocompatibility

molecule

T-cell

Antigen-Presenting cell (APC)

“Signal Two” T-cell B7 receptor

CD28

Figure 5.2  Antigen costimulation. In addition to major histocompatibility complex (MHC) class II presentation of antigen to the T-cell receptor (i.e., the first signal), professional antigen presentation also requires a second signal resulting from the engagement of specific coreceptor molecules on the cell surface, B7, and CD28 pictured.

Th17

IL-17

Th2

IL-4

IL-5

IL-10

IL-13

Figure 5.3  T-cell differentiation into Th1, Th2, or Th17 lymphocytes.

often used to describe both Th1 and Th2 cells since Th1 cells “help” activate macrophages and Th2 cells “help” activate B cells. A new subset of CD4+ cells that produce IL-17 have been identified and termed Th17. These cells are stimulated by the combination of IL-6 and transforming growth factor-β (TGF-β). The importance of these cells has yet to be worked out, but they appear to be crucial in the initiation of many autoimmune conditions, including experimental autoimmune uveitis (EAU) – an experimental uveitis following immunization with retinal antigens in animals models.7 Tregs are regulatory, anti-inflam- matory T lymphocytes that are often CD4+ and CD25+ and express the transcription factor forkhead box P3 (FoxP3).8 They are produced either centrally in the thymus or peripherally.

The differentiation of naïve CD4+ T cells into Th1, Th2, or Th17 cells occurs at the draining lymph node, largely determined by the surrounding cytokines. IL-12 and interferon-gamma (IFN-γ) tend to promote Th1 response, IL-4 and IL-6 promote Th2 differentiation, and IL-6 and TGF-β promote Th17 differentiation. An indication that the Th17 effector may play a role in pathogenesis is the ability to ameliorate inflammation with anti-IL-17 antibodies, and conversely, the enhanced IL-17 response to interphotoreceptor retinoid-binding protein (IRBP), a retinal antigen, observed in the highly EAU-susceptible IFN-γ knockout (KO) mice.9 IFN-γ KO mice are highly susceptible to EAU but lack a normal Th1 response; this was believed to result from a deviant Th2 response,10 but may indeed be a manifestation of Th-17 differentiation.

Retina in Sciences Basic • 1 section