Pathology in Scleritis

The histopathological aspects of scleral inßammation have been studied extensively in the past. Although certain inferences can be drawn from these descriptions, terms such as Òchronic nonspeciÞc inßammation,Ó or even Òchronic granulomatous inßammation,Ó only camoußaged our ignorance about the possible pathogenic roles that cells might play. Even sophisticated techniques, such as electron microscopy provided limited insights into the delineation of the distinctive composition of the cellular inÞltrates as well as into the components of the matrix during inßammation. It was not until the relatively recent development of immunohistochemistry, particularly the exploitation of monoclonal antibody technology, that the etiology and pathogenesis of scleral diseases have become less obscure. Our ability to characterize cells and determine extracellular matrix changes during inßammatory reactions contributes to the understanding of mechanisms of lesion production and thereby to the development of treatment options most likely to succeed. Thus, it is possible to distinguish between T and B lymphocytes, to determine mononuclear subsets (T-helper, T-cytotoxic/suppressor, natural killer, macrophages, and LangerhansÕ cells), and to detect HLA-DR glycoproteins in scleral inßammation; it is now possible to identify and further characterize different types of components of extracellular matrix (collagen, glycosamynoglycans, and glycoproteins) during inßammatory reactions; it is also possible to

localize speciÞc antigens (herpes simplex virus) in inßamed tissue.

This chapter has, as its primary focus, a description of the histopathologic, immunopathologic, and ultrastructural characteristics of sclera affected by inßammation of diverse origins. Although recognizing that a clear understanding of the etiology and pathogenesis of scleritis is not yet available, study of the pathology of scleritis may contribute to insights into the mechanism of inßammation as well as into the search for most appropriate forms of treatment.

5.1General Considerations of Connective Tissue Inflammation

Inßammation, deÞned as the reaction of tissue to a noxious stimulus, is characterized by increased vascular exudation of ßuid, plasma proteins, and inßammatory cells. Histologically, connective tissue diseases are characterized by the presence of chronic inßammation. Chronic inßammation may arise as a result of a persistent inciting stimulus following acute inßammation or directly, as a smoldering response. Two main forms of chronic inßammation are recognized: nongranulomatous and granulomatous. Chronic granulomatous inßammation surrounds a central area of extracellular matrix degradation known as Þbrinoid necrosis; in some instances, vasculitis may result in further necrosis.

5.1.1Chronic Nongranulomatous Inflammation

The histological hallmarks of chronic nongranulomatous or nonspeciÞc inßammation are inÞltration of mononuclear cells, including macrophages, lymphocytes, and plasma cells, proliferation of Þbroblasts, and sometimes blood vessels, and degeneration of the tissue affected by the inßammatory process. Macrophages are important components of chronic inßammation because of the great number of biologically active products they can produce [1, 2]. Some of these products cause proliferation of Þbroblasts and vessels (interleukin 1 and growth-promoting factors for Þbroblasts and blood vessels), some are toxic to tissues (acid proteases, collagenase, elastase, and reactive oxygen intermediates), some are chemotactic for other cell types (neutrophils and lymphocytes), and some produce coagulation and Þbrin accumulation (factor V and thromboplastin). Lymphocytes participate in both antibodyand cell-mediated hypersensitivity reactions. After contact with antigen, they produce lymphokines that are chemotactic for monocytes; lymphocytes also cause macrophage activation and differentiation (interferon g) [3, 4]. Macrophages produce monokines that activate both T- and B-lymphocytes (interleukin 1). B lymphocytes produce plasma cells that participate in antibody formation. Neutrophils are usually considered the hallmark of acute inßammation but they also participate in many forms of chronic inßammation as well. Since Þbroblasts and neutrophils may release collagenase and other proteases, they play prominent roles in tissue damage. Mast cells and eosinophils may also be present but their precise functions in chronic inßammation are not well understood.

5.1.2Chronic Granulomatous Inflammation

Chronic granulomatous inßammation is characterized by the presence of epithelioid cells, which are modiÞed macrophages with abundant, pale-pink, ill-deÞned cytoplasm, resembling an epithelial

cell. Their function is poorly understood, but the presence of numerous organelles, such as endoplasmic reticulum, Golgi apparatus, vesicles, and vacuoles suggests that they are particularly adapted to secretion of the biologically active products explained above, rather than phagocytosis, antigen processing, and antigen presentation [5, 6]. As the epithelioid cells coalesce and fuse, amitotic nuclear division may occur, thus forming a multinucleated giant cell containing a row of nuclei arranged along the periphery, the Langhans-type giant cell [7]. Multinucleated giant cells have been shown in vitro to secrete more collagenase than do cultures of unfused epithelioid cells. Their presence in tissues may be associated with accelerated rates of matrix degradation. Giant cells, lymphocytes, plasma cells, Þbroblasts, neutrophils, mast cells, eosinophils, as well as blood vessel proliferation, and destruction of the tissue, can be seen in a granuloma, but the detection of epithelioid cells is the only requirement for the diagnosis of granulomatous inßammation. Macrophages are chemotactically attracted into an area of inßammation in response to complement components (which may have been attracted by immune complexes) [2], and through interleukin 1 production they activate T lymphocytes. T cell activity then potentiates granuloma formation [2, 8] because lymphokines, such as interferon g and possibly interleukin 4, enhance the transformation of macrophages to epithelioid cells and multinucleated giant cells [9, 10]. If macrophages are intensively stimulated, they secrete acid proteases (cathepsins), neutral proteases (collagenase and elastase), and reactive (free radical) oxygen intermediates, thereby participating in the production of a central area of extracellular matrix degeneration known as Þbrinoid necrosis.

5.1.3Fibrinoid Necrosis

Fibrinoid necrosis is characterized by the histopathological appearance of a ÒsmudgyÓ material in association with tissue destruction [11, 12]. This material, which stains eosinophilic with eosin, is composed of a mixture of Þbrin, proteoglycan Þlaments, collagen Þbers in varying

stages of degradation, granular debris, and cell membranes. Accumulation of Þbrin is probably the result of coagulation factors (factor V and thromboplastin) [2] released by macrophages as well as of impairment of the normal mechanism for debris removal. The necrosis is probably the result of the enormous quantities of proteases produced by macrophages (including modiÞed macrophages, such as epithelioid cells and multinucleated giant cells) and Þbroblasts. Intracellular digestion by lysosomal acid hydrolases, and extracellular digestion by neutral proteases, is the main mechanism of proteoglycan, collagen, and glycoprotein degradation. Microinfarctions resulting from thrombosis of terminal vessels may also play a role, through ischemia, in the tissue destruction.

5.1.4Vascular Inflammation

Connective tissue vessels may show an inßammatory inÞltration that leads to endothelial damage and subsequent reparative proliferation responses of the vascular endothelium. Circulating immune complexes (CICs) may precipitate in a vessel wall and activate the complement system, which is chemotactic for neutrophils and macrophages. Endothelial cell damage exposes the subendothelium or basement membrane, which is composed of collagen, proteoglycans, and glycoproteins; these components can be degraded by proteolytic enzymes secreted by neutrophils, macrophages, and activated endothelial cells. Activated endothelial cells also enhance aggregation of platelets (platelet-activating factor) [13], Þbrinogen accumulation (tissue factor and factor V) [14, 15], polymerization of Þbrin, and Þbrin deposition, which in turn promote thrombus formation and infarction of the tissues supplied by the vessel. Because activated endothelial cells also express HLA-DR glycoproteins, they participate in the immune response through their interaction with T lymphocytes [16Ð19]. Macrophages activate Þbroblasts and together participate in extracellular matrix degradation through enzyme release. Further stimulation of macrophages, probably through a T-cell-related

delayed hypersensitivity response, results in granuloma formation.

Endothelial cells activated by chronic inßammation migrate through the openings of degraded basement membranes and generate new capillary blood vessels through the complex proliferative process of angiogenesis [20]. Vascular inßammation involves vessels usually present in the tissue and new formed vessels produced as a result of the inßammation.

5.2Specific Considerations

of Scleral Tissue Inflammation

The sclera, the dense connective tissue that covers about Þve-sixths of the eye, consists of Þbroblasts, collagen, proteoglycans, glycoproteins, and few blood vessels. These components are functionally and metabolically interdependent in maintaining tissue homeostasis. In scleral inßammation, normal sclera homeostasis breaks down, resulting in loss of scleral integrity. The destructive events in the sclera are consequent to a complex interaction of many cell types and their soluble products.

Episcleral and scleral vasculature has not been extensively studied and clearly deserves more careful morphologic characterization. Unlike elastic arteries (large-sized vessels), muscular arteries (medium-sized vessels), and arterioles (small-sized vessels), these vessels in the episclera and sclera appear to be capillaries and postcapillary venules, which do not possess a tunica media that consists chießy of smooth muscle cells. Episcleral and perforating scleral vessels appear to possess a simple wall composed of continuous endothelial cells and pericytes, without smooth muscle cells (see Figs. 1.23Ð1.25).

Classic vasculitis is a clinicopathological process characterized by inßammation and necrosis of blood vessels; the classically described histopathological hallmarks as deÞned in vessels containing a tunica media include neutrophilic inÞltration of the vessel wall and Þbrinoid degeneration leading to necrosis of the tunica media (Figs. 5.1 and 5.2). Vasculitic syndromes comprise a broad spectrum of disorders involving

Fig. 5.1 Vasculitis with neutrophil inÞltrate and degeneration of the vascular wall (MagniÞcation, ×100; hematoxylinÐeosin stain)

vessels of different types, sizes, and locations. Some examples include large-vessel vasculitis in temporal arteritis, mediumand small-vessel vasculitis in polyarteritis nodosa (PAN), and smallvessel vasculitis in connective tissue diseases, such as rheumatoid arthritis and systemic lupus erythematosus. However, because the types of vessels involved in some conditions vary widely, attempts to classify vasculitic syndromes appropriately have not been satisfactory. The subject is further confused by the fact that clinicopathological classiÞcations do not follow pure histopathological concepts: small-vessel vasculitis (hypersensitivity or leukocytoclastic vasculitis) refers to a heterogeneous group of clinical syndromes that have in common the predominant involvement of arterioles, capillaries, and venules; however, because capillaries do not have a tunica media, a pure classical histopathological deÞnition of vasculitis cannot be applied to them.

We, as ophthalmologists, frequently dealing with capillaries and postcapillary venules, have been frustated by this confused classiÞcation for years; because scleral blood vessel inßammation in scleritis can be associated with inßammation of vessels elsewhere in the body as part of systemic vasculitic syndromes, we have included scleral ÒvasculitisÓ as part of our clinicopathological classiÞcation. However, we freely admit that this term is histopathologically incomplete and imperfect as regards classic pathology deÞnitions, and have adopted an independent term, Òinßammatory

Fig. 5.2 (a) Elastin stain (magniÞcation, ×40). Note the large area of vessel wall destruction with Þbrinoid necrosis and the absence of elasticum (compare with the normal appearance of the large vessel shown in panel c) (HematoxylinÐeosin stain). (b) Elastin stain, higher magniÞcation (×100), vividly illustrating the area of vessel wall destruction and vascular lumen obliteration (HematoxylinÐeosin stain). (c) Elastin stain, normal artery. Note the continuous black-staining elasticum adjacent to the muscularis (MagniÞcation, ×40; hematoxylinÐ eosin stain)

microangiopathy,Ó to deÞne histopathological neutrophilic inÞltration in and around capillaries and postcapillary venules, such as conjunctival, episcleral, and perforating scleral vessels.