0099-2399/9011602-0054/$02 00/0 JCIURNAL OF ENOODONTICS Copynght 9 1990 by The American Association of Endodontist~,
Pnnted in U.S.A. VOL 16, NO. 2, FEBRUARY1990
Immunological Aspects of Chronic Inflammation and Repair Henry O. Trowbridge, DDS, PhD
Macrophages and lymphocytes play essential roles in cell-mediated mechanisms involved in chronic inflammation. During the development of specific immunity, macrophages are required to process and present antigen to immunocompetent T cells. They also serve as "accessory" cells to lymphocytes by releasing soluble factors involved in host defense. In order to carry out their role more effectively, macrophages have the property of being activated, a process that provides them with greater ability to phagocytose and kill ingested microorganisms. Activated T cells serve as key effector cells that are capable of producing lymphokines which modify the behavior of other cells. In this way, they are able to facilitate or suppress an immune response. Chronic inflammation is often associated with irreversible destruction of parenchymal tissue, and fibrous connective tissue fills the resultant defect. Proliferation of fibroblasts, collagen production, and neovascularization are enhanced by the secretion of cytokines by T cells and macrophages.
diseases such as these, chronic inflammation often results in the replacement of functional elements of the tissues with fibrous scarring. THE NATURE OF C t l R O N I C I N F L A M M A T I O N In addition to denoting the duration of inflammation, the terms acute and chronic arc used to signify the characteristics of an inflammatory reaction. Basically, an acu:e inflammatory reaction is exudative in nature in that small vessels become more permeable, thus allowing fluid and plasma proteins to leave the bloodstream and enter the tissues. Chronic inflammation, on the other hand, represents a pattern of inflammation in which there is proliferation of fibroblasts and angioblasts. In addition, increased nerve sprouting has been reported in pulps which have been subjected to chronic injury (1). In no lesion is the proliferative nature of chronic inflammation more apparent than in chronic hyperplastic pulpitis (2). This condition occurs almost exclusively in immature teeth with wide open apices. The pulps of such teeth have an abundance of blood vessels and flbroblasts. When carious exposure of the pulp creates a cavity through which drainage can occur, chronic inflammation supersedes acute inflammation, and chronic inflammatory tissue proliferates to form a pulp polyp (Fig. 1). An inflammatory response can also be classified according to the type of inflammatory cells that predominate in the inflamed tissue. Acute inflammation is characterized by an influx of neutrophils while in chronic inflammation the tissue becomes infiltrated by mononuclear inflammatory cells, often referred to as "round cells." These consist of macrophages, lymphocytes, and often plasma cells. ]hus, mononuclear inflammatory cells, fibroblasts, collagen, and small blood vessels are the primary elements of chronic inflammatory tissue (Fig. 2).
For the sake of convenience most teachers of pathology divide inflammation into three stages: acute inflammation, chronic inflammation, and repair. Actually, in many diseases there is no clear dividing line between acute and chronic inflammation. Similarly, there is a close relationship between chronic inflammation and repair, the presence of fibroblasts and collagen formation being common to both. Even in acute inflammation the healing process is discernible, as increased vascular permeability and phagocytosis are forerunners of repair. Any inflammatory reaction lasting more than a few clays or weeks is, in a temporal sense, a chronic process. If the cause of initial injury is not completely eliminated, acute inflammation gradually gives way to chronic inflammation. However, there is no set time sequence. A majority of the most crippling diseases that afflict humans involve chronic rather than acute inflammatory reactions. Examples include rheumatoid arthritis, silicosis, liver cirrhosis, gout, chronic pyelonephritis, and granulomatous diseases such as tuberculosis, leprosy, and sarcoidosis. In
"FRUSTRATED REPAIR" Chronic inflammation frequently involves a "smoldering" type of reaction in which tissue destruction and repair is a "seesaw" process, with periods of destruction alternating with periods of repair. One often observes fibroblasts and collagen intermingled with chronic inflammatory cells (Fig. 3). Like granulation tissue which mediates wound healing, chronic 54
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formed. Tissues and organs such as the endocrine glands, lung parenchyma, neurons of the central nervous system, and striated and cardiac muscle have limited or no ability to regenerate, so replacement by fibrous connective tissue is the usual outcome. Liver and kidney cells have a limited potential for regeneration, but if the stroma of the parenchyma is not preserved, the functional units of the liver and kidney, i.e. hepatic lobules, renal glomeruli, and tubules, cannot be restored9 C L I N I C A L M A N I F E S T A T I O N S OF C t t R O N I C INFLAMMATION
Fie 1. Chronic hyperplastic pulpitis showing dense infiltration of chronic inflammatory cells (hematoxylin and eosin; original magnification x20).
From a clinical standpoint, virtually all acute inflammatory reactions of any consequence evoke one or more of the cardinal signs of inflammation, that is, redness, swelling, heat, pain, and loss of function. As chronic inflammation supplants acute inflammation, these signs begin to fade and disappear. Consequently, chronic inflammation tends to be less dramatic than acute inflammation. Heat and redness become less noticeable and then vanish altogether9 Swelling persists for a
FIG 2. Chronic inflammatory lesion in pulp beneath deep cavity restored with zinc phosphate cement. Note presence of inflammatory cells, blood vessel (arrow), and collagen fibers (CF) (hematoxylin and eosin; onginal magnification x125).
inflammatory tissue is rich in small blood vessels. Consequently, all of the elements of repair are present in the lesion, but healing is held in abeyance until the causative agent has been completely eliminated. Hence, the term frustrated repair. Repair primarily involves the formation of fibrous scar tissue, and this ultimately leads to impairment of function of the affected organ. For example, in cirrhosis of the liver, fibrous tissue replaces irreversibly injured liver cells, and during this process scarfing may obliterate the central veins of the hepatic lobules (Fig. 4), thus giving rise to portal hypertension. In chronic glomerulonephritis, scarring of the glomeruli and interstitial fibrosis cause the kidney to become contracted and diffusely nodular. When both kidneys are affected, the eventual outcome is hypertension and uremia. In rheumatoid arthritis, fibrosis and calcification may cause permanent ankylosis. Tissue changes associated with chronic inflammation may be reversible or irreversible. If the cells which are destroyed have the ability to regenerate, the tissue can be restored to its original state once the injurious agent has been removed 9 But this will occur only if the supporting reticular framework of the tissue is preserved and not too much scar tissue has
FIG 3. Chronic inflammatory reaction in the pulp showing collagen fibers (arrows) intermixed with mononuclear inflammatory cells (hematoxylin and eosin; original magnification x125).
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FiG 4. Liver cirrhosis. Extensive scarring (arrow) has distorted the normal Iobular architecture (hematoxylin and eosin; original magnification x250).
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time, but eventually the lesion may become indurated as fibrous connective tissue forms. Loss of function, initially the result of pain and swelling, is later attributable to loss of parenchymal cells and scarring. Pain in acute inflammation is due largely to pressure from edema or suppuration on sensory nerve endings. Pressure plays a particularly important role in tissues such as the dental pulp, nail bed and bone marrow which are in a low compliance cnvironment. In chronic inflammation, pain is often absent until the advanced stages of disease. CAUSES OF C H R O N I C I N F L A M M A T I O N The basic cause of chronic inflammation is the persistence of etiologic factors that are difficult for the body to eliminate. Several major groups of causative agents are recognized.
Infectious Agents Extracellular organisms primarily evoke an acute inflammatory response in which the neutrophil is the predominant inflammatory cell. However, when infectious agents are persistent and the acute inflammatory process cannot be resolved, chronic inflammation supersedes acute inflammation. At times, chronic inflammation may develop either in the absence of acute inflammation or with only a brief initial acute inflammatory response. This is likely to be the case when the infectious agent is of low toxicity as compaed with organisms that are capable of initiating an acute inflammatory response. Examples of low toxicity organisms include intracellular organisms such as Treponema pallidum, Mycobacterium tuberculosis, Mycobacteriurn leprae, and certain fungi, notably Coccidioides immitis and ltistoplasrna capsulatum. Although they can be readily ingested by phagocytes, some intracellular parasites are able to resist the phagocyte's enzymatic and oxidative killing mechanisms. Such organisms usually cause chronic diseases in which cell-mediated immunity plays a vital role in host defense. The classical model of this form of chronic inflammation is tuberculosis. The tubercle bacillus is of relatively low toxicity and is not associated with any known endotoxin or exotoxin. The virulence of this organism seems to be largely, if not entirely, related to its ability to evoke a destructive immune hypersensitivity reaction. Hypersensitivity-evoked injury is also encountered in deep fungal infections such as histoplasmosis and coccidiomycosis.
Bacterial Components Under some circumstances it is not necessary for microorganisms to actually enter a tissue to evoke an inflammatory reaction. For example, it has been shown that in the absence of living organisms, bacterial components applied to exposed dentin are capable of evoking an inflammatory response in the underlying pulp (3). Pulpal inflammation associated with growth of bacteria beneath restorations provides another example of bacterial components as causative agents. Dental caries is a protracted process, and the evoked inflammatory reaction in the underlying pulp develops over a period of years. Inflammation develops insidiously as a lowgrade, chronic inflammatory response rather than an acute
response (4). Initially, lymphocytes, macrophages, and plasma cells infiltrate the tissue beneath the carious lesion. Presumably these cells accumulate in response to the influx of antigenic bacterial components diffusing to the pulp from the caries process. It is only in the later stages of caries when bacteria approximate the pulp that neutrophils become the predominant inflammatory cells in the lesion.
Remnants of Dead Organisms It appears that chronic inflammation can also be evoked by remnants of bacteria. Thus, injection of rats with streptococcal cell walls can induce a chronic polyarthritis (5). Some strains of streptococci can be easily phagocytosed and killed by neutrophils and macrophages, but it is not always possible for phagocytes to digest the cell walls of these organisms. Apparently bacterial components such as these are carried to tissues where they can act as antigenic stimuli. Following active infection normal constituents of tissues may cross-react with bacterial antigen and thus become the target of a cellular immune response.
Foreign Bodies Foreign materials such as splinters, thorns, talcum powder, metal objects, and suture material are difficult for the body to dispose of, as they cannot be readily degraded. Such substances evoke a foreign body response resulting in the accumulation of macrophages and possibly the formation of multinucleated giant cells. Inhalation of silica or beryllium dust over a long period of time ultimately leads to a chronic inflammatory reaction with fibrosis and impairment of pulmonary function. In the foreign body-type response, macrophages that ingest particles which they are unable to dispose of release their lysosomal enzymes into the surrounding tissue. These enzymes are capable of degrading host tissue, and their release frequently results in tissue damage. As a consequence of such injury, fibrobla.sts in the lesion proliferate and secrete collagen. The end result is fibrous scarring.
Products of Metabolism Under some circumstances, metabolic by-products may be the cause of inflammation. In gout, for example, inflammatory cells accumulate around urate crystals which precipitate in the tissues. Deposition of these crystals is a consequence of the basic genetic disorder in gout that leads to hyperuricemia. In the chronic arthritis of gout, large aggregations of urate crystals produce a "tophus," the pathognomonic lesion of this condition. The urate crystals are surrounded by an intense inflammatory reaction consisting primarily of macrophagcs, lymphocytes, fibroblasts, and foreign body-type giant cells. Cholesterol crystals which are deposited in the tissues can also initiate a foreign body-type reaction.
Immune Reactions Severe chronic inflammatory conditions such as rheumatoid arthritis and Hashimoto's thyroiditis are actually immunologic reactions to the patient's own tissues. As such, they
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are classified as autoimmune diseases. In disorders such as these, the presence of autoantibodies leads to i m m u n e reactions which inflict injury on the tissues of the host. Rheumatoid arthritis is characterized by chronic synovitis involving swelling and hypertrophy of synoviocytes and a mononuclear inflammatory cell infiltrate. In Hashimoto's thyroiditis there is extensive replacement of thyroid tissue by lymphocyles, plasma cells, and macrophages. A significant amount of fibrosis is characteristically present within the gland.
Unknown In some conditions associated with chronic inflammation the cause remains obscure. Such is the case in diseases like sarcoidosis, ulcerative colitis and regional enteritis (Crohn's disease). I M M U N O L O G I C A L ASPECTS O F C H R O N I C INFLAMMATION The immune system has the ability to recognize "self," i.e. the body's own tissues, and "non-self," substances that arc foreign to the body. The latter are known as antigens. To deal with antigens, the i m m u n e system has special cells consisting of B cells and T cells which can counteract foreign invaders. In addition, there arc cells which play a role in the recognition of antigen. Many chronic inflammatory, diseases are associated with the persistent presence of antigens. Since the primary, goal of the immune system is elimination of antigen from the body, the immune system plays a key role in most chronic inflammatory reactions. Even in foreign body reactions in which the causative agent does not possess the properties of an antigen. lymphocytes are usually present. We will now consider the various cells and chemical mediators that participate in immune reactions.
Accessory Cells, Lymphocytes, and Cytokines Cells which are classified as accessory cells include monocytes, macrophages, and ceils having dendritic processes such as Langerhans' cells of the epidermis and the dendritic cells found in lymphoid tissue. Accessory cells are particularly numerous in the lymphoid tissues where there are large numbers of lymphocytes with which to interact. However, these cells are found in most tissues, and recently it has been shown that dendritic-like cells are located in the odontoblast layer of human teeth (6). In addition to the epidermis, Langerhans" cells have been observed in apical periodontal cysts (7). Unlike macrophages, Langerhans' cells and dendritic cells are only weakly phagocytic and do not engage in microbicidal activities. Accessory cells are non-antigen-specific cells that ingest antigens and "present" them to lymphocytes. Within the accessory cell antigens are broken into smaller fragments which are termed epitopes (8). Epitopes are transported to the surface of the accessory cell where they combine with a protein component of the cell membrane of the accessory cell. The protein component is called a restriction element. Although B cells can be activated by free antigen, -I" cells are only able to recognize antigen in the form ofepitopes bound to restric-
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tion elements. When receptors on the cell membrane o f a T cell bind to epitope-restriction element complexes on an accessory cell, the T cell is presented with the antigen. Restriction elements are coded for by the major histocompatibility complex (9). There are two classes o f major histocompatibility complex molecules, Class 1 and Class II. With the exception of red blood cells, Class I antigens are found on the surface of all cells. Class II antigens, on the other hand, are found only on the surface of accessory, cells, B cells, and possibly endothelial cells. A cytokine, 3,-interferon, increases Class II major histocompatibility complex expression on these cells (8). Class I antigens serve as restriction elements for suppressor and cytotoxic T cells while Class II antigens serve as restriction elements tot helper/inducer T cells.
Macrophages Macrophages are motile phagocytes having a life-span of several months. They are derived from blood monocytes and belong to the mononuclear phagocyte system (formerly known as the reticuloendothelial system). Monocytes leave the bloodstream and enter the tissues where they undergo further differentiation to become macrophages (10). Macrophages respond to chemotactic stimuli such as lymphokines and the ('5a fragment of complement. They form the primary line of defense against microorganisms such as Mycobacterium tuberculosis and Mycobacterium leprae which are resistant to killing. In wound healing, macrophages play a key role m debriding the area of injured tissue so that fibroblasts and new capillaries can enter the wound. As pointed out earlier, they also process antigen. Of great importance in many chronic inflammatory reactions is the "'activation" of macrophages. The ordinary macrophagc may be unable to cope with microorganisms or substances that are resistant to destruction. In order to enhance their ability to deal with such agents, macrophages can become activated. Activation is mediated by T cell cytokines such as g a m m a interferon and granulocyte-macrophage colony-stimulating factor (11). Macrophages exposed to endotoxin release a cytokine which can itself activate macrophages, thus raising the possibility that macrophages coming in contact with endotoxin at sites of infection may activate themselves (12). As compared with nonactivated macrophages, activated macrophages have a number of properties which provide them with a greater ability to kill and digest engulfed organisms, degrade particulate matter, and participate in immune reactions (Table 1). Some of the numerous secretory products of activated macrophages are listed in Table 2. TABLE 1. Properties of activated macrophages, as compared with normal macrophages 1. 2. 3. 4. 5. 6. 7.
Increased cell size More active metabolism Increased chemotaxis Increased ruffling of surface membrane Increased capacity for adherence and spreading on surfaces Increased formation of pseudopods Greater ability to pl~agocytose microorganisms and particulate matter 8. Greater ability to k=ll ingested microorganisms 9. Increased ability to present antigen to lymphocytes
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TABLE 2. Secretory products of activated macrophages 1. Neutral proteases Collagenase Elastase 2. Acid hydrolases 3. Plasminogen activator 4. Complement components 5. Prostaglandins and leukotrienes 6. Fibronectin 7. Intedeukin 1 8. Angiogenesis factors 9. Interferons
The final developmental stage of macrophages is the multinucleated giant cell. Although the function of these cells is not altogether clear, they appear to be capable of phagocytosing erythrocytes and certain microorganisms such as candida. Basically, there are two types of giant cells that participate in chronic inflammatory reactions. One is the Langhans type that is often associated with granulomatous inflammation which will be discussed later. The other is the foreign body giant cell. Relatively undigestible foreign material that is too large for single macrophages to ingest often causes macrophages to fuse together to form a giant cell. Thus, foreign body giants cells tend to accumulate around material such as silica, talc, uric acid crystals, and fragments of glass or wood splinters.
TABLE 3. Selected functions of intedeukin 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Initiates fever (endogenous pyrogen) Stimulates the release of ACTH from the pituitary gland Stimulates secretion of acute phase reactants by the liver Stimulates the production of neutrophils in bone marrow Enhances neutrophil phagocytosis Activates T and B lymphocytes Increases the toxicity of cytotoxic T cells Stimulates fibroblasts to proliferate and produce collagen Stimulates collagenase production by fibroblasts Increases osteoclastic activity leading to bone resorption Controls endothelial cell growth Degrades muscie to provide a source of energy, leading possibly to cachexia
There are three types of lymphocytes: T cells, B cells, and "null" lymphocytes. B cells and T cells are programmed to respond to specific antigen whereas null lymphocytes do not require prior sensitization to perform their functions. The progeny of activated B cells are plasma cells which secrete antibodies. Null lymphocytes include killer and natural killer cells. These cells are capable of killing tumor cells as well as virus-infected and other cells. T cells play a primary role in nearly all chronic inflammatory reactions. The preponderance of circulating lymphocytes are T cells and they recirculate constantly from the bloodstream to the lymphatics. From the bloodstream they enter and patrol the tissues. When activated by antigen, they participate in cellular immune reactions which are of great importance in diseases such as tuberculosis and leprosy. T cells perform various functions. Some switch on different aspects of the immune response while others switch them off. Thus, there are helper and suppressor T cells. Some T lymphocytes are able to kill antigen-bearing cells such as tumor cells or virus-infected cells. These are referred to as cytotoxic T cells.
produced them. It is now recognized that other cells such as macrophages also secrete lymphokine-like products, so the more general term cylokinc may ultimately replace the term lymphokine. At the Sixth International Congress of Immunology held in 1986, it was agreed that a new cytokine would be named according to its biological properties, but once the amino acid sequence of the cytokine had been determined, it would be assigned an interleukin number (13). Thus, a cytokine formerly known as "B-cell stimulating factor 1'" is now called interleukin 4. If the amino acid sequence is still unknown, the cytokine is named according to its biological effect. For example, "T-cell growth factor" stimulates T cells to proliferate. B-ceU stimulating factor induces differentiation of activated B cells into plasma cells. However, since most cytokines have more than one biological effect, descriptive names can be misleading. Nearly all inflammatory and immune responses stimulate monocytes and macrophages to produce a certain cytokine, interleukin 1. In addition to affecting inflammatory and immunologically competent cells, interleukin 1 affects other targets including the liver, pancreas, bone, muscle, brain and fibroblasts (14). Selected functions of interleukin 1 are listed in Table 3. When they encounter the antigen toward which they are sensitized, T cells become activated. Once activated, they release cytokines having a wide array of biological properties (13). Among other things, cytokines can modify the behavior of other cells by causing them to proliferate and differentiate. lnterleukin 3, for example, is a growth factor for mast cells and promotes the growth of bone marrow stem cells. Interleukin 4 causes activated B cells to proliferate. Other cytokines promote chemotaxis of inflammatory cells, stimulate proliferation of nonsensitized T cells, promote osteoclastic resorption of bone, and inhibit macropbage migration out of inflammatory lesions. The interferons are a group ofcytokines with antiviral, immunomodulatory, and antitumor activities.
Cytokines
Fibroblasts
In response to antigen, certain cells of the immune system have the ability to release soluble polypeptide products known as cytokines (13). Unlike antibodies, the chemical composition of cytokines is not determined by the antigen that evoked their production. Cytokines have traditionally been called lymphokines when it was thought that only lymphocytes
Evidence suggests that a number of factors likely to be present in wounds are capable of chemotactically attracting fibroblasts. These include fibrin, fibronectin, cytokines, a complement fragment, native collagens I to V, and platelet derived growth factor (15). Interleukin 1 stimulates fibroblasts to proliferate and produce collagen (14).
Lymphocytes
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Neutrophils
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Neutrophils are quick to respond to injury and are the predominant inflammatory cell in acute inflammatory reactions. In chronic inflammatory reactions, the number of neutrophils present is generally reduced relative to the number of chronic inflammatory cells. This is pariticularly true in the case of inflammatory reactions evoked by persistent intracellular parasites. However, in chronic suppurative reactions, neutrophils continue to play a major role.
lysosomal enzymes are released, and this results in digestion of macromolecular components of the connective tissue, a process termed liquefaction necrosis. The designation "chronic abscess" is based solely on temporal considerations. If an abscess persists for more than a few days, it becomes a chronic lesion. Histologically, a chronic abscess can be distinguished from an acute abscess by the presence of chronic inflammatory tissue that has formed around it. Presumably, this is nature's way of confining the abscess and separating it from the surrounding healthy tissue.
Eosinophils
HYPERSENSITIVITY
It is known that eosinophils respond to chemotactic factors such as histamine and eosinophil chemotactic factor which are released by mast cells. However, the role of eosinophils in chronic inflammation has yet to be elucidated. It has been suggested that they are important in degrading certain mediators of inflammation such as histamine and immune complexes, and thus limit the inflammatory response.
Delayed-type hypersensitivity is a form of cell-mediated (Type IV) hypersensitivity that is characterized by the influx of antigen-sensitized T cells and macrophages. This reaction develops in response to intracellular pathogens such as the tubercle bacillus. The T cells are sensitized as a result of prior exposure to the pathogen. Sensitized T cells become immune memory cells which remain in circulation and patrol the tissues for long periods of time. When reexposed to the specific antigen these memory cells become activated, undergo blast transformation, and proliferate. This results in the production of effcctor T lymphocytes which secrete cytokines, one of which immobilizes macrophages at the site of antigen deposition, thus causing them to accumulate. Another cytokine activates the assembled macrophages while still others attract neutrophils and eosinophils. Contact dermatitis and poison ivy are examples of contact sensitivity. Sensitization involves binding of a hapten (a low molecular weight molecule that is too small to evoke an immune response by itself) to glycoproteins of the skin which act as carriers. When the hapten-carrier complex reaches the lymphoid tissues, T cells become sensitized to the complex and enter the circulation as immune memory cells. If they are reexposed to the hapten-carrier complex in the skin, they become activated, proliferate, and differentiate into effector T cells. Macrophages are drawn to the scene and activated. In an effort to eliminate the antigen from the skin, the activated macrophages release lysosomal enzymes and form oxygen-derived free radicals which mediate cell injury. This results in ulceration and necrosis of the skin.
Mast Cells There is an increase in the number of mast cells during chronic inflammation, but the significance of this is unclear. The anticoagulant heparin is found in the granules of mast cells, as is histamine. Mast cells can also produce slow-reacting substance of anaphylaxis which consists of leukotrienes C, and D,. Slow-reacting substance of anaphylaxis is a potent agent in increasing vascular permeability and causing bronchial smooth muscle contraction. Interleukin 3 and interleukin 4 are growth factors for mast cells (13).
Epithelial Ceils If present in an area of inflammation, epithelium is frequently stimulated to proliferate. This proliferative response is termed inflammatory hyperplasia. It is possible that the stimulus for proliferation is provided by growth-stimulatory factors such as interleukin l and epidermal growth factor. Inflammatory hyperplasia is responsible for the proliferation of sulcular epithelial cells, a feature of periodontal disease, and for the formation of cysts in apical granulomas.
DESTRUCI'IVE ASPECTS OF CEI,L-MEDIATED HYPERSENSITIVITY
CHRONIC SUPPURATION Suppurative infections involve bacteria that collectively are referred to as pyogenic, or pus-producing organisms. Pyogenic bacteria include staphylococci, many gram-negative bacilli, meningococci, pneumococci, and gonococci. When these organisms infect the skin or a solid organ, they produce an abscess. An abscess is a localized accumulation of pus consisting of dead leukocytes (predominantly neutrophils), inflammatory exudate, and dead bacteria and tissue. Basically, pyogenic bacteria evoke a massive emigration of neutrophils. In order for pus to form, the bacteria must be resistant to phagocytosis and killing, thus ensuring a continuing emigration of neutrophils. Neutrophils have a life span of from only a few hours to 1 or 2 days. When they die, their
While the development of hypersensitivity may have a beneficial outcome, such is frequently not the case. In the process of attempting to eliminate the injurious agents, lymphocytes and macrophages may destroy nearby cells and connective tissues of the host. Cells of the host killed in this fashion are referred to as "innocent bystander cells." Thus, the release of lysosomal enzymes and oxygen-derived free radicals such as superoxide anion and hydroxyl radical may have a devastating effect on host tissue. While parenchymal tissue is being destroyed, fibroblasts are stimulated to form scar tissue with resulting fibrosis and its sequelae. When bony tissue is involved, osteoclasts may be activated by agents such as interleukin-I and tumor necrosis factor and mediate bone resorption (16).
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GRANULOMAS Originally the term granuloma was used to denote the tiny granular bodies that could be seen in the tissues of patients with tuberculosis. They represented the hematogenous spread of tubercle bacilli and localized inflammatory lesions which the organisms evoked. Histologically, a granuloma is a rounded, circumscribed inflammatory lesion usually consisting o f a n accumulation of macrophages and lymphocytes. Therc may also be a variable number of plasma cells, neutrophils, giant cells, eosinophils, and mast cells. Some of the inflammatory cells in granulomas are engaged in phagocytosis while others play a role in immunity. Established granulomas are usually enclosed within a collagenous stroma. Several forms ofgranulomas exist, each with its own unique characteristics. Examples include the central giant cell granuloma, eosinophilic granuloma, pyogenic granuloma, tuberculous granuloma, and periapical granuloma. PERIAPICAL G R A N U L O M A The periapical granuioma is a localized mass of chronic inflammatory tissue that forms in response to infection of the dental pulp. The granuloma is initiated in the periodontal ligament and enlarges at the expense of bone supporting the tooth. The infected canal becomes a continuous source of antigenic material. When lymphocytes that have previously been sensitized contact these antigens, they become activated and produce cytokines that are capable of recruiting inflammatory cells, activating macrophages, and keeping them in the lesion. Several studies on periapical inflammatory lesions from humans have shown that the predominant cells are T lymphocytes and macrophages ( 17-19). While periapical granulomas enlarge as a result of bone resorption, both resorption and active bone apposition may occur within the .same lesion, suggesting that both osteoblasts and osteoclasts are being activated (Fig. 5). This again exemplifies the seesaw relationship between destruction and repair which is so characteristic of chronic inflammation. Foreign body giant cells are found in periapical granulomas in which cholesterol crystals have accumulated. Macrophages which ingest a large amount of lipid material have a frothy cytoplasm and are known as foam cells. As in other types of granulomas, fibroblasts and bundles of collagen are often found at the periphery of the lesion, frequently in a capsular arrangement. The presence of epithelium in periapical granulomas is a frequent finding. Presumably this epithelium arises from epithelial rests of Malassez, which appear to have the latent capacity to proliferate (20). Epithelial proliferation may in time result in the formation of a cyst. GRANULOMATOUS INFI,AMMATION Granulomatous inflammation is a special type of inflammation that is encountered in a limited number of diseases. As its name implies, this form of inflammation leads to the development of granulomas. The granulomatous diseases include tuberculosis, leprosy, sarcoidosis, syphilis, cat-scratch disease, and certain fungal diseases such as histoplasmosis
FIG 5. Bone adjacent to periapical granuloma. Note areas of I~one formation (small arrows) and bone resorption (large arrows) (hematoxylin and eosin; original magnification x 125).
and coccidioidomycosis. Similar granulomas are found in certain foreign body-type reactions such as chronic berylliosis. Granulomatous inflammation is a manifestation of delayed-type hypersensitivity which is evoked by the causative agent. Antigen-sensitized T cells are activated and release lymphokines which attract macrophages, immobilize them so that they remain in the lesion, and transform them into activated macrophages. The hallmark of granulomatous inflammation is the presence of epithelioid cells within the granulomas. Their appearance in the lesion coincides with the development of cellmediated immunity. Epithelioid cells are transformed macrophages with abundant granular cytoplasm which gives them the appearance of certain types of epithelial cells, hence the name epithelioid cells. Unlike the macrophage from which they are derived, these cells are only weakly phagocytic. An abundant endoplasmic reticulum and well-developed Golgi apparatus suggests that they have a secretory function. Material which is too large for single macrophages to ingest may cause multinucleated giant cells to lotto. Characteristically, 20 or more nuclei are arranged in a ring or horseshoe configuration at the periphery of the cell. This arrangement is typical of the Langhans giant cell seen in tuberculosis (Fig. 6). These cells may be found in any of the granulomatous diseases. Central necrosis is a feature of most granulomas formed as a result of infection by M. tuberculosis. This process, known
Vol. 16, No. 2, February 1990
FtG 6. Multinucleated giant cell of the Langhans type (hematoxylin and eosin; original magnification x500).
as caseation because of the cheesy appearance of the necrotic material, involves the death of cells in the center of the granuloma. The onset of caseation coincides with the development of hypersensitivity to the tubercle bacillus. Dr. Trowbndge is associated with the Department of Pathology, School of Dental Medione, University of Pennsylvania, Philadelphia, PA.
References 1. Taylor PE, Byers MR, Redd PE. Spro~Jting of CGRP nerve fibers in response to dentin injury in rat molars. Brain Res 1988;461:371-6. 2. Trowbridge HO. inflammation in the dental pulp and periapical tissues. In: Hardin JF. ed. Clark's clinical dentistry. Vol. 4, Chap. 5. Philadelphia: JB
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IJppincott, 1987:12 3. Bergenholtz G. Effects of bacterial products on inflammatory reactions in the dental pulp. Scaod J Dent Res 1977;85:122-9. 4. Trowbridge FtO. Pathogenesis of pulpitis resulting from dental canes. J Endodon 1981 ;7:52-60. 5. Cromartie WJ. Craddock JG, Schwab IH, Anderle SK, Yang CH. Arthritis in rats after systemic injectio~ of streptococcal cells or call walls. J Exp Med 1977;146:1585-602. 6. Jontell M, Gunraj MN, Becgenho~tz G. Immunocompetent cells in the normal dental pulp. J Dent Res 1987;66:1149-53. 7. Contos JG, Corcoran JF, LaTurno SAL, Chiego DJ, Regezi JA. Langerhans cells in apical periodontal cysts: an immunohistochemK~al study. J Endodon 1987;13:52-5. 8. Nossal GJV. The basic components of the immune system. N Engl J Med 1987;1320-5. 9. Benacerraf B. McDewtt HO. Histocompatibility4mked immune response genes. Science 1972; 175:273-9. 10. Johnston RB Jr. Monocytes and macrophages. N Engl J Meal 1988;318:747-52 11. We~ser WY. van Niel A, Clark SC, David JR, Remold HG Recombinant human granulocyte/macrophage colony-stimulation factor activates intracellular kiihng of Leishmania donovani by human monocyte-derived macrophages. J Exp Med 1987;166:1436-46. 12. de Titto EH, Catterall JR, Remington JS. Activity of recombinant tumor necrosis factor on Toxoplasma gondii and Trypanosoma cruzi. J Immunol 1986;137:1342-5. 13. Dinarello CA, Mier JW. Lymphokines. N Engl J Med 1987;317:940-5. 14. Dinarello CA. Interleukin-1 and the patho(j(mesis of the acute-phase response. N Engi J Med 1984;311:1413-8. 15. Dvorak HF Tumors: wounds that do not heal. N Engl J Meal 1986:315:1650-9. 16. Le J, Vilcek J. Tumor necrosts factor and interleukin-l: cytokines with murtiple ovedapl~ng biological activities. Lab Invest 1987;56:234-48. 17. Stem MH, Dre~zen S, Mackler BF, Levy BM. Is~ation and charactedzat~n of inflammatory catls from the human periapical granuloma J Dent Res 1982;61:1408-12. 18. Torabinelad M, Ketterlng JD. Identification and relative concentration B and T lymphocytes in human chronic penapical lesions. J Endodon 1985;11:122-5. 19. NJlsen R, Johannessen AC, Skaog N, Matre R. In situ characterizatK)n of mononuclear cells in human dental periapical inflammatory lesions using monoclonal antibodies. Oral Surg 1984;58:160-5. 20. Trowb~idge HO, Shibata F. Mitotic activity in epdhelial rests of Malassez. Penodont~cs 1967;5:109-12.
Pnnted in U.S.A. VOL 16, NO. 2, FEBRUARY1990
Immunological Aspects of Chronic Inflammation and Repair Henry O. Trowbridge, DDS, PhD
Macrophages and lymphocytes play essential roles in cell-mediated mechanisms involved in chronic inflammation. During the development of specific immunity, macrophages are required to process and present antigen to immunocompetent T cells. They also serve as "accessory" cells to lymphocytes by releasing soluble factors involved in host defense. In order to carry out their role more effectively, macrophages have the property of being activated, a process that provides them with greater ability to phagocytose and kill ingested microorganisms. Activated T cells serve as key effector cells that are capable of producing lymphokines which modify the behavior of other cells. In this way, they are able to facilitate or suppress an immune response. Chronic inflammation is often associated with irreversible destruction of parenchymal tissue, and fibrous connective tissue fills the resultant defect. Proliferation of fibroblasts, collagen production, and neovascularization are enhanced by the secretion of cytokines by T cells and macrophages.
diseases such as these, chronic inflammation often results in the replacement of functional elements of the tissues with fibrous scarring. THE NATURE OF C t l R O N I C I N F L A M M A T I O N In addition to denoting the duration of inflammation, the terms acute and chronic arc used to signify the characteristics of an inflammatory reaction. Basically, an acu:e inflammatory reaction is exudative in nature in that small vessels become more permeable, thus allowing fluid and plasma proteins to leave the bloodstream and enter the tissues. Chronic inflammation, on the other hand, represents a pattern of inflammation in which there is proliferation of fibroblasts and angioblasts. In addition, increased nerve sprouting has been reported in pulps which have been subjected to chronic injury (1). In no lesion is the proliferative nature of chronic inflammation more apparent than in chronic hyperplastic pulpitis (2). This condition occurs almost exclusively in immature teeth with wide open apices. The pulps of such teeth have an abundance of blood vessels and flbroblasts. When carious exposure of the pulp creates a cavity through which drainage can occur, chronic inflammation supersedes acute inflammation, and chronic inflammatory tissue proliferates to form a pulp polyp (Fig. 1). An inflammatory response can also be classified according to the type of inflammatory cells that predominate in the inflamed tissue. Acute inflammation is characterized by an influx of neutrophils while in chronic inflammation the tissue becomes infiltrated by mononuclear inflammatory cells, often referred to as "round cells." These consist of macrophages, lymphocytes, and often plasma cells. ]hus, mononuclear inflammatory cells, fibroblasts, collagen, and small blood vessels are the primary elements of chronic inflammatory tissue (Fig. 2).
For the sake of convenience most teachers of pathology divide inflammation into three stages: acute inflammation, chronic inflammation, and repair. Actually, in many diseases there is no clear dividing line between acute and chronic inflammation. Similarly, there is a close relationship between chronic inflammation and repair, the presence of fibroblasts and collagen formation being common to both. Even in acute inflammation the healing process is discernible, as increased vascular permeability and phagocytosis are forerunners of repair. Any inflammatory reaction lasting more than a few clays or weeks is, in a temporal sense, a chronic process. If the cause of initial injury is not completely eliminated, acute inflammation gradually gives way to chronic inflammation. However, there is no set time sequence. A majority of the most crippling diseases that afflict humans involve chronic rather than acute inflammatory reactions. Examples include rheumatoid arthritis, silicosis, liver cirrhosis, gout, chronic pyelonephritis, and granulomatous diseases such as tuberculosis, leprosy, and sarcoidosis. In
"FRUSTRATED REPAIR" Chronic inflammation frequently involves a "smoldering" type of reaction in which tissue destruction and repair is a "seesaw" process, with periods of destruction alternating with periods of repair. One often observes fibroblasts and collagen intermingled with chronic inflammatory cells (Fig. 3). Like granulation tissue which mediates wound healing, chronic 54
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Chronic Inflammation
55
formed. Tissues and organs such as the endocrine glands, lung parenchyma, neurons of the central nervous system, and striated and cardiac muscle have limited or no ability to regenerate, so replacement by fibrous connective tissue is the usual outcome. Liver and kidney cells have a limited potential for regeneration, but if the stroma of the parenchyma is not preserved, the functional units of the liver and kidney, i.e. hepatic lobules, renal glomeruli, and tubules, cannot be restored9 C L I N I C A L M A N I F E S T A T I O N S OF C t t R O N I C INFLAMMATION
Fie 1. Chronic hyperplastic pulpitis showing dense infiltration of chronic inflammatory cells (hematoxylin and eosin; original magnification x20).
From a clinical standpoint, virtually all acute inflammatory reactions of any consequence evoke one or more of the cardinal signs of inflammation, that is, redness, swelling, heat, pain, and loss of function. As chronic inflammation supplants acute inflammation, these signs begin to fade and disappear. Consequently, chronic inflammation tends to be less dramatic than acute inflammation. Heat and redness become less noticeable and then vanish altogether9 Swelling persists for a
FIG 2. Chronic inflammatory lesion in pulp beneath deep cavity restored with zinc phosphate cement. Note presence of inflammatory cells, blood vessel (arrow), and collagen fibers (CF) (hematoxylin and eosin; onginal magnification x125).
inflammatory tissue is rich in small blood vessels. Consequently, all of the elements of repair are present in the lesion, but healing is held in abeyance until the causative agent has been completely eliminated. Hence, the term frustrated repair. Repair primarily involves the formation of fibrous scar tissue, and this ultimately leads to impairment of function of the affected organ. For example, in cirrhosis of the liver, fibrous tissue replaces irreversibly injured liver cells, and during this process scarfing may obliterate the central veins of the hepatic lobules (Fig. 4), thus giving rise to portal hypertension. In chronic glomerulonephritis, scarring of the glomeruli and interstitial fibrosis cause the kidney to become contracted and diffusely nodular. When both kidneys are affected, the eventual outcome is hypertension and uremia. In rheumatoid arthritis, fibrosis and calcification may cause permanent ankylosis. Tissue changes associated with chronic inflammation may be reversible or irreversible. If the cells which are destroyed have the ability to regenerate, the tissue can be restored to its original state once the injurious agent has been removed 9 But this will occur only if the supporting reticular framework of the tissue is preserved and not too much scar tissue has
FIG 3. Chronic inflammatory reaction in the pulp showing collagen fibers (arrows) intermixed with mononuclear inflammatory cells (hematoxylin and eosin; original magnification x125).
-
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.
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FiG 4. Liver cirrhosis. Extensive scarring (arrow) has distorted the normal Iobular architecture (hematoxylin and eosin; original magnification x250).
56
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time, but eventually the lesion may become indurated as fibrous connective tissue forms. Loss of function, initially the result of pain and swelling, is later attributable to loss of parenchymal cells and scarring. Pain in acute inflammation is due largely to pressure from edema or suppuration on sensory nerve endings. Pressure plays a particularly important role in tissues such as the dental pulp, nail bed and bone marrow which are in a low compliance cnvironment. In chronic inflammation, pain is often absent until the advanced stages of disease. CAUSES OF C H R O N I C I N F L A M M A T I O N The basic cause of chronic inflammation is the persistence of etiologic factors that are difficult for the body to eliminate. Several major groups of causative agents are recognized.
Infectious Agents Extracellular organisms primarily evoke an acute inflammatory response in which the neutrophil is the predominant inflammatory cell. However, when infectious agents are persistent and the acute inflammatory process cannot be resolved, chronic inflammation supersedes acute inflammation. At times, chronic inflammation may develop either in the absence of acute inflammation or with only a brief initial acute inflammatory response. This is likely to be the case when the infectious agent is of low toxicity as compaed with organisms that are capable of initiating an acute inflammatory response. Examples of low toxicity organisms include intracellular organisms such as Treponema pallidum, Mycobacterium tuberculosis, Mycobacteriurn leprae, and certain fungi, notably Coccidioides immitis and ltistoplasrna capsulatum. Although they can be readily ingested by phagocytes, some intracellular parasites are able to resist the phagocyte's enzymatic and oxidative killing mechanisms. Such organisms usually cause chronic diseases in which cell-mediated immunity plays a vital role in host defense. The classical model of this form of chronic inflammation is tuberculosis. The tubercle bacillus is of relatively low toxicity and is not associated with any known endotoxin or exotoxin. The virulence of this organism seems to be largely, if not entirely, related to its ability to evoke a destructive immune hypersensitivity reaction. Hypersensitivity-evoked injury is also encountered in deep fungal infections such as histoplasmosis and coccidiomycosis.
Bacterial Components Under some circumstances it is not necessary for microorganisms to actually enter a tissue to evoke an inflammatory reaction. For example, it has been shown that in the absence of living organisms, bacterial components applied to exposed dentin are capable of evoking an inflammatory response in the underlying pulp (3). Pulpal inflammation associated with growth of bacteria beneath restorations provides another example of bacterial components as causative agents. Dental caries is a protracted process, and the evoked inflammatory reaction in the underlying pulp develops over a period of years. Inflammation develops insidiously as a lowgrade, chronic inflammatory response rather than an acute
response (4). Initially, lymphocytes, macrophages, and plasma cells infiltrate the tissue beneath the carious lesion. Presumably these cells accumulate in response to the influx of antigenic bacterial components diffusing to the pulp from the caries process. It is only in the later stages of caries when bacteria approximate the pulp that neutrophils become the predominant inflammatory cells in the lesion.
Remnants of Dead Organisms It appears that chronic inflammation can also be evoked by remnants of bacteria. Thus, injection of rats with streptococcal cell walls can induce a chronic polyarthritis (5). Some strains of streptococci can be easily phagocytosed and killed by neutrophils and macrophages, but it is not always possible for phagocytes to digest the cell walls of these organisms. Apparently bacterial components such as these are carried to tissues where they can act as antigenic stimuli. Following active infection normal constituents of tissues may cross-react with bacterial antigen and thus become the target of a cellular immune response.
Foreign Bodies Foreign materials such as splinters, thorns, talcum powder, metal objects, and suture material are difficult for the body to dispose of, as they cannot be readily degraded. Such substances evoke a foreign body response resulting in the accumulation of macrophages and possibly the formation of multinucleated giant cells. Inhalation of silica or beryllium dust over a long period of time ultimately leads to a chronic inflammatory reaction with fibrosis and impairment of pulmonary function. In the foreign body-type response, macrophages that ingest particles which they are unable to dispose of release their lysosomal enzymes into the surrounding tissue. These enzymes are capable of degrading host tissue, and their release frequently results in tissue damage. As a consequence of such injury, fibrobla.sts in the lesion proliferate and secrete collagen. The end result is fibrous scarring.
Products of Metabolism Under some circumstances, metabolic by-products may be the cause of inflammation. In gout, for example, inflammatory cells accumulate around urate crystals which precipitate in the tissues. Deposition of these crystals is a consequence of the basic genetic disorder in gout that leads to hyperuricemia. In the chronic arthritis of gout, large aggregations of urate crystals produce a "tophus," the pathognomonic lesion of this condition. The urate crystals are surrounded by an intense inflammatory reaction consisting primarily of macrophagcs, lymphocytes, fibroblasts, and foreign body-type giant cells. Cholesterol crystals which are deposited in the tissues can also initiate a foreign body-type reaction.
Immune Reactions Severe chronic inflammatory conditions such as rheumatoid arthritis and Hashimoto's thyroiditis are actually immunologic reactions to the patient's own tissues. As such, they
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are classified as autoimmune diseases. In disorders such as these, the presence of autoantibodies leads to i m m u n e reactions which inflict injury on the tissues of the host. Rheumatoid arthritis is characterized by chronic synovitis involving swelling and hypertrophy of synoviocytes and a mononuclear inflammatory cell infiltrate. In Hashimoto's thyroiditis there is extensive replacement of thyroid tissue by lymphocyles, plasma cells, and macrophages. A significant amount of fibrosis is characteristically present within the gland.
Unknown In some conditions associated with chronic inflammation the cause remains obscure. Such is the case in diseases like sarcoidosis, ulcerative colitis and regional enteritis (Crohn's disease). I M M U N O L O G I C A L ASPECTS O F C H R O N I C INFLAMMATION The immune system has the ability to recognize "self," i.e. the body's own tissues, and "non-self," substances that arc foreign to the body. The latter are known as antigens. To deal with antigens, the i m m u n e system has special cells consisting of B cells and T cells which can counteract foreign invaders. In addition, there arc cells which play a role in the recognition of antigen. Many chronic inflammatory, diseases are associated with the persistent presence of antigens. Since the primary, goal of the immune system is elimination of antigen from the body, the immune system plays a key role in most chronic inflammatory reactions. Even in foreign body reactions in which the causative agent does not possess the properties of an antigen. lymphocytes are usually present. We will now consider the various cells and chemical mediators that participate in immune reactions.
Accessory Cells, Lymphocytes, and Cytokines Cells which are classified as accessory cells include monocytes, macrophages, and ceils having dendritic processes such as Langerhans' cells of the epidermis and the dendritic cells found in lymphoid tissue. Accessory cells are particularly numerous in the lymphoid tissues where there are large numbers of lymphocytes with which to interact. However, these cells are found in most tissues, and recently it has been shown that dendritic-like cells are located in the odontoblast layer of human teeth (6). In addition to the epidermis, Langerhans" cells have been observed in apical periodontal cysts (7). Unlike macrophages, Langerhans' cells and dendritic cells are only weakly phagocytic and do not engage in microbicidal activities. Accessory cells are non-antigen-specific cells that ingest antigens and "present" them to lymphocytes. Within the accessory cell antigens are broken into smaller fragments which are termed epitopes (8). Epitopes are transported to the surface of the accessory cell where they combine with a protein component of the cell membrane of the accessory cell. The protein component is called a restriction element. Although B cells can be activated by free antigen, -I" cells are only able to recognize antigen in the form ofepitopes bound to restric-
Chronic Inflammation
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tion elements. When receptors on the cell membrane o f a T cell bind to epitope-restriction element complexes on an accessory cell, the T cell is presented with the antigen. Restriction elements are coded for by the major histocompatibility complex (9). There are two classes o f major histocompatibility complex molecules, Class 1 and Class II. With the exception of red blood cells, Class I antigens are found on the surface of all cells. Class II antigens, on the other hand, are found only on the surface of accessory, cells, B cells, and possibly endothelial cells. A cytokine, 3,-interferon, increases Class II major histocompatibility complex expression on these cells (8). Class I antigens serve as restriction elements for suppressor and cytotoxic T cells while Class II antigens serve as restriction elements tot helper/inducer T cells.
Macrophages Macrophages are motile phagocytes having a life-span of several months. They are derived from blood monocytes and belong to the mononuclear phagocyte system (formerly known as the reticuloendothelial system). Monocytes leave the bloodstream and enter the tissues where they undergo further differentiation to become macrophages (10). Macrophages respond to chemotactic stimuli such as lymphokines and the ('5a fragment of complement. They form the primary line of defense against microorganisms such as Mycobacterium tuberculosis and Mycobacterium leprae which are resistant to killing. In wound healing, macrophages play a key role m debriding the area of injured tissue so that fibroblasts and new capillaries can enter the wound. As pointed out earlier, they also process antigen. Of great importance in many chronic inflammatory reactions is the "'activation" of macrophages. The ordinary macrophagc may be unable to cope with microorganisms or substances that are resistant to destruction. In order to enhance their ability to deal with such agents, macrophages can become activated. Activation is mediated by T cell cytokines such as g a m m a interferon and granulocyte-macrophage colony-stimulating factor (11). Macrophages exposed to endotoxin release a cytokine which can itself activate macrophages, thus raising the possibility that macrophages coming in contact with endotoxin at sites of infection may activate themselves (12). As compared with nonactivated macrophages, activated macrophages have a number of properties which provide them with a greater ability to kill and digest engulfed organisms, degrade particulate matter, and participate in immune reactions (Table 1). Some of the numerous secretory products of activated macrophages are listed in Table 2. TABLE 1. Properties of activated macrophages, as compared with normal macrophages 1. 2. 3. 4. 5. 6. 7.
Increased cell size More active metabolism Increased chemotaxis Increased ruffling of surface membrane Increased capacity for adherence and spreading on surfaces Increased formation of pseudopods Greater ability to pl~agocytose microorganisms and particulate matter 8. Greater ability to k=ll ingested microorganisms 9. Increased ability to present antigen to lymphocytes
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TABLE 2. Secretory products of activated macrophages 1. Neutral proteases Collagenase Elastase 2. Acid hydrolases 3. Plasminogen activator 4. Complement components 5. Prostaglandins and leukotrienes 6. Fibronectin 7. Intedeukin 1 8. Angiogenesis factors 9. Interferons
The final developmental stage of macrophages is the multinucleated giant cell. Although the function of these cells is not altogether clear, they appear to be capable of phagocytosing erythrocytes and certain microorganisms such as candida. Basically, there are two types of giant cells that participate in chronic inflammatory reactions. One is the Langhans type that is often associated with granulomatous inflammation which will be discussed later. The other is the foreign body giant cell. Relatively undigestible foreign material that is too large for single macrophages to ingest often causes macrophages to fuse together to form a giant cell. Thus, foreign body giants cells tend to accumulate around material such as silica, talc, uric acid crystals, and fragments of glass or wood splinters.
TABLE 3. Selected functions of intedeukin 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Initiates fever (endogenous pyrogen) Stimulates the release of ACTH from the pituitary gland Stimulates secretion of acute phase reactants by the liver Stimulates the production of neutrophils in bone marrow Enhances neutrophil phagocytosis Activates T and B lymphocytes Increases the toxicity of cytotoxic T cells Stimulates fibroblasts to proliferate and produce collagen Stimulates collagenase production by fibroblasts Increases osteoclastic activity leading to bone resorption Controls endothelial cell growth Degrades muscie to provide a source of energy, leading possibly to cachexia
There are three types of lymphocytes: T cells, B cells, and "null" lymphocytes. B cells and T cells are programmed to respond to specific antigen whereas null lymphocytes do not require prior sensitization to perform their functions. The progeny of activated B cells are plasma cells which secrete antibodies. Null lymphocytes include killer and natural killer cells. These cells are capable of killing tumor cells as well as virus-infected and other cells. T cells play a primary role in nearly all chronic inflammatory reactions. The preponderance of circulating lymphocytes are T cells and they recirculate constantly from the bloodstream to the lymphatics. From the bloodstream they enter and patrol the tissues. When activated by antigen, they participate in cellular immune reactions which are of great importance in diseases such as tuberculosis and leprosy. T cells perform various functions. Some switch on different aspects of the immune response while others switch them off. Thus, there are helper and suppressor T cells. Some T lymphocytes are able to kill antigen-bearing cells such as tumor cells or virus-infected cells. These are referred to as cytotoxic T cells.
produced them. It is now recognized that other cells such as macrophages also secrete lymphokine-like products, so the more general term cylokinc may ultimately replace the term lymphokine. At the Sixth International Congress of Immunology held in 1986, it was agreed that a new cytokine would be named according to its biological properties, but once the amino acid sequence of the cytokine had been determined, it would be assigned an interleukin number (13). Thus, a cytokine formerly known as "B-cell stimulating factor 1'" is now called interleukin 4. If the amino acid sequence is still unknown, the cytokine is named according to its biological effect. For example, "T-cell growth factor" stimulates T cells to proliferate. B-ceU stimulating factor induces differentiation of activated B cells into plasma cells. However, since most cytokines have more than one biological effect, descriptive names can be misleading. Nearly all inflammatory and immune responses stimulate monocytes and macrophages to produce a certain cytokine, interleukin 1. In addition to affecting inflammatory and immunologically competent cells, interleukin 1 affects other targets including the liver, pancreas, bone, muscle, brain and fibroblasts (14). Selected functions of interleukin 1 are listed in Table 3. When they encounter the antigen toward which they are sensitized, T cells become activated. Once activated, they release cytokines having a wide array of biological properties (13). Among other things, cytokines can modify the behavior of other cells by causing them to proliferate and differentiate. lnterleukin 3, for example, is a growth factor for mast cells and promotes the growth of bone marrow stem cells. Interleukin 4 causes activated B cells to proliferate. Other cytokines promote chemotaxis of inflammatory cells, stimulate proliferation of nonsensitized T cells, promote osteoclastic resorption of bone, and inhibit macropbage migration out of inflammatory lesions. The interferons are a group ofcytokines with antiviral, immunomodulatory, and antitumor activities.
Cytokines
Fibroblasts
In response to antigen, certain cells of the immune system have the ability to release soluble polypeptide products known as cytokines (13). Unlike antibodies, the chemical composition of cytokines is not determined by the antigen that evoked their production. Cytokines have traditionally been called lymphokines when it was thought that only lymphocytes
Evidence suggests that a number of factors likely to be present in wounds are capable of chemotactically attracting fibroblasts. These include fibrin, fibronectin, cytokines, a complement fragment, native collagens I to V, and platelet derived growth factor (15). Interleukin 1 stimulates fibroblasts to proliferate and produce collagen (14).
Lymphocytes
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Neutrophils
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Neutrophils are quick to respond to injury and are the predominant inflammatory cell in acute inflammatory reactions. In chronic inflammatory reactions, the number of neutrophils present is generally reduced relative to the number of chronic inflammatory cells. This is pariticularly true in the case of inflammatory reactions evoked by persistent intracellular parasites. However, in chronic suppurative reactions, neutrophils continue to play a major role.
lysosomal enzymes are released, and this results in digestion of macromolecular components of the connective tissue, a process termed liquefaction necrosis. The designation "chronic abscess" is based solely on temporal considerations. If an abscess persists for more than a few days, it becomes a chronic lesion. Histologically, a chronic abscess can be distinguished from an acute abscess by the presence of chronic inflammatory tissue that has formed around it. Presumably, this is nature's way of confining the abscess and separating it from the surrounding healthy tissue.
Eosinophils
HYPERSENSITIVITY
It is known that eosinophils respond to chemotactic factors such as histamine and eosinophil chemotactic factor which are released by mast cells. However, the role of eosinophils in chronic inflammation has yet to be elucidated. It has been suggested that they are important in degrading certain mediators of inflammation such as histamine and immune complexes, and thus limit the inflammatory response.
Delayed-type hypersensitivity is a form of cell-mediated (Type IV) hypersensitivity that is characterized by the influx of antigen-sensitized T cells and macrophages. This reaction develops in response to intracellular pathogens such as the tubercle bacillus. The T cells are sensitized as a result of prior exposure to the pathogen. Sensitized T cells become immune memory cells which remain in circulation and patrol the tissues for long periods of time. When reexposed to the specific antigen these memory cells become activated, undergo blast transformation, and proliferate. This results in the production of effcctor T lymphocytes which secrete cytokines, one of which immobilizes macrophages at the site of antigen deposition, thus causing them to accumulate. Another cytokine activates the assembled macrophages while still others attract neutrophils and eosinophils. Contact dermatitis and poison ivy are examples of contact sensitivity. Sensitization involves binding of a hapten (a low molecular weight molecule that is too small to evoke an immune response by itself) to glycoproteins of the skin which act as carriers. When the hapten-carrier complex reaches the lymphoid tissues, T cells become sensitized to the complex and enter the circulation as immune memory cells. If they are reexposed to the hapten-carrier complex in the skin, they become activated, proliferate, and differentiate into effector T cells. Macrophages are drawn to the scene and activated. In an effort to eliminate the antigen from the skin, the activated macrophages release lysosomal enzymes and form oxygen-derived free radicals which mediate cell injury. This results in ulceration and necrosis of the skin.
Mast Cells There is an increase in the number of mast cells during chronic inflammation, but the significance of this is unclear. The anticoagulant heparin is found in the granules of mast cells, as is histamine. Mast cells can also produce slow-reacting substance of anaphylaxis which consists of leukotrienes C, and D,. Slow-reacting substance of anaphylaxis is a potent agent in increasing vascular permeability and causing bronchial smooth muscle contraction. Interleukin 3 and interleukin 4 are growth factors for mast cells (13).
Epithelial Ceils If present in an area of inflammation, epithelium is frequently stimulated to proliferate. This proliferative response is termed inflammatory hyperplasia. It is possible that the stimulus for proliferation is provided by growth-stimulatory factors such as interleukin l and epidermal growth factor. Inflammatory hyperplasia is responsible for the proliferation of sulcular epithelial cells, a feature of periodontal disease, and for the formation of cysts in apical granulomas.
DESTRUCI'IVE ASPECTS OF CEI,L-MEDIATED HYPERSENSITIVITY
CHRONIC SUPPURATION Suppurative infections involve bacteria that collectively are referred to as pyogenic, or pus-producing organisms. Pyogenic bacteria include staphylococci, many gram-negative bacilli, meningococci, pneumococci, and gonococci. When these organisms infect the skin or a solid organ, they produce an abscess. An abscess is a localized accumulation of pus consisting of dead leukocytes (predominantly neutrophils), inflammatory exudate, and dead bacteria and tissue. Basically, pyogenic bacteria evoke a massive emigration of neutrophils. In order for pus to form, the bacteria must be resistant to phagocytosis and killing, thus ensuring a continuing emigration of neutrophils. Neutrophils have a life span of from only a few hours to 1 or 2 days. When they die, their
While the development of hypersensitivity may have a beneficial outcome, such is frequently not the case. In the process of attempting to eliminate the injurious agents, lymphocytes and macrophages may destroy nearby cells and connective tissues of the host. Cells of the host killed in this fashion are referred to as "innocent bystander cells." Thus, the release of lysosomal enzymes and oxygen-derived free radicals such as superoxide anion and hydroxyl radical may have a devastating effect on host tissue. While parenchymal tissue is being destroyed, fibroblasts are stimulated to form scar tissue with resulting fibrosis and its sequelae. When bony tissue is involved, osteoclasts may be activated by agents such as interleukin-I and tumor necrosis factor and mediate bone resorption (16).
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GRANULOMAS Originally the term granuloma was used to denote the tiny granular bodies that could be seen in the tissues of patients with tuberculosis. They represented the hematogenous spread of tubercle bacilli and localized inflammatory lesions which the organisms evoked. Histologically, a granuloma is a rounded, circumscribed inflammatory lesion usually consisting o f a n accumulation of macrophages and lymphocytes. Therc may also be a variable number of plasma cells, neutrophils, giant cells, eosinophils, and mast cells. Some of the inflammatory cells in granulomas are engaged in phagocytosis while others play a role in immunity. Established granulomas are usually enclosed within a collagenous stroma. Several forms ofgranulomas exist, each with its own unique characteristics. Examples include the central giant cell granuloma, eosinophilic granuloma, pyogenic granuloma, tuberculous granuloma, and periapical granuloma. PERIAPICAL G R A N U L O M A The periapical granuioma is a localized mass of chronic inflammatory tissue that forms in response to infection of the dental pulp. The granuloma is initiated in the periodontal ligament and enlarges at the expense of bone supporting the tooth. The infected canal becomes a continuous source of antigenic material. When lymphocytes that have previously been sensitized contact these antigens, they become activated and produce cytokines that are capable of recruiting inflammatory cells, activating macrophages, and keeping them in the lesion. Several studies on periapical inflammatory lesions from humans have shown that the predominant cells are T lymphocytes and macrophages ( 17-19). While periapical granulomas enlarge as a result of bone resorption, both resorption and active bone apposition may occur within the .same lesion, suggesting that both osteoblasts and osteoclasts are being activated (Fig. 5). This again exemplifies the seesaw relationship between destruction and repair which is so characteristic of chronic inflammation. Foreign body giant cells are found in periapical granulomas in which cholesterol crystals have accumulated. Macrophages which ingest a large amount of lipid material have a frothy cytoplasm and are known as foam cells. As in other types of granulomas, fibroblasts and bundles of collagen are often found at the periphery of the lesion, frequently in a capsular arrangement. The presence of epithelium in periapical granulomas is a frequent finding. Presumably this epithelium arises from epithelial rests of Malassez, which appear to have the latent capacity to proliferate (20). Epithelial proliferation may in time result in the formation of a cyst. GRANULOMATOUS INFI,AMMATION Granulomatous inflammation is a special type of inflammation that is encountered in a limited number of diseases. As its name implies, this form of inflammation leads to the development of granulomas. The granulomatous diseases include tuberculosis, leprosy, sarcoidosis, syphilis, cat-scratch disease, and certain fungal diseases such as histoplasmosis
FIG 5. Bone adjacent to periapical granuloma. Note areas of I~one formation (small arrows) and bone resorption (large arrows) (hematoxylin and eosin; original magnification x 125).
and coccidioidomycosis. Similar granulomas are found in certain foreign body-type reactions such as chronic berylliosis. Granulomatous inflammation is a manifestation of delayed-type hypersensitivity which is evoked by the causative agent. Antigen-sensitized T cells are activated and release lymphokines which attract macrophages, immobilize them so that they remain in the lesion, and transform them into activated macrophages. The hallmark of granulomatous inflammation is the presence of epithelioid cells within the granulomas. Their appearance in the lesion coincides with the development of cellmediated immunity. Epithelioid cells are transformed macrophages with abundant granular cytoplasm which gives them the appearance of certain types of epithelial cells, hence the name epithelioid cells. Unlike the macrophage from which they are derived, these cells are only weakly phagocytic. An abundant endoplasmic reticulum and well-developed Golgi apparatus suggests that they have a secretory function. Material which is too large for single macrophages to ingest may cause multinucleated giant cells to lotto. Characteristically, 20 or more nuclei are arranged in a ring or horseshoe configuration at the periphery of the cell. This arrangement is typical of the Langhans giant cell seen in tuberculosis (Fig. 6). These cells may be found in any of the granulomatous diseases. Central necrosis is a feature of most granulomas formed as a result of infection by M. tuberculosis. This process, known
Vol. 16, No. 2, February 1990
FtG 6. Multinucleated giant cell of the Langhans type (hematoxylin and eosin; original magnification x500).
as caseation because of the cheesy appearance of the necrotic material, involves the death of cells in the center of the granuloma. The onset of caseation coincides with the development of hypersensitivity to the tubercle bacillus. Dr. Trowbndge is associated with the Department of Pathology, School of Dental Medione, University of Pennsylvania, Philadelphia, PA.
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