Immunosuppressive Therapy in the Treatment of Immune-mediated Disease

Immunosuppressive Therapy in the Treatment of Immune-mediated Disease

Ellen Miller, DVM, MS, ACVIM Diplomate

Immune-mediated diseases represent some of the most frustrating types of disorders that are diagnosed and treated in veterinary medicine. Drug-induced immunosuppression is an attempt to control the aberrant immune response against self antigens but the immunosuppression can result in sepsis or other unacceptable adverse effects. If the pathophysiology of immune-mediated and autoimmune disease is considered, the immune response can be divided into several components and attempts can be made to selectively deal with each component separately. The components of the immune response that can be manipulated by therapy include antibodies, effector cells, the mononuclear phagocytic system, and the peripheral manifestations of disease. This article reviews the therapy of immune-mediated and autoimmune diseases based on a pathophysiologic approach and discusses conventional as well as current therapies in the treatment of these devastating diseases. (Journal of Veterinary Internal Medicine 1992; 6~206-213)

THE IMMUNE SYSTEM is an organism’s primary de- fense against infectious microorganisms, toxins, and tu- mors. However, when uncontrolled, the immune system can damage reversibly or irreversibly various body components. Immune-mediated diseases (IMD) can be divided into primary or secondary forms. Primary or true autoimmunity implies the presence of self-reactive cells and autoantibody. Because immunoglobulin and T-cell receptor gene rearrangement occur before exposure to antigen, autoreactive cells and antibody develop in all individuals. ’ These autoreactive cells and their products are deleted early in life through a process called “clonal abortion” in the thymus (T cells) and elsewhere (B cells), and tolerance to self antigens develops. If these cells escape this surveillance mechanism, two other protective mechanisms prevent autoimmunity. First, antigen-specific T-suppressor cells act by suppressing antigen-specific T-helper cells, thereby limiting the immune response. In addition, the idiotypelanti-idiotype network is believed to regulate the immune response by negative feedback inhibition. Idiotypes are the specific protein structure of the antigen-combining site of the T-cell receptor or immunoglobulin molecule. They are antigens

From the Department of Clinical Sciences, Colorado State University, College of Veterinary Medicine and Biomedical Sciences, Fort Collins, Colorado.

Accepted for publication January 15, I99 I . Reprint requests: Ellen Miller, DVM, MS, Department of Clinical

Sciences, Colorado State University, College of Veterinary Medicine and Biomedical Sciences, Fort Collins, CO 80523.

in their own right and can induce an antibody (anti-idiotype) response. Jerne proposed that these anti-idiotypes have a regulating effect on the immune response (Figure 1) .2 True autoimmunity may develop when there is polyclonal activation of B cells, increased T-helper cell function, decreased T-suppressor cell function, or alterations in the idiotypelanti-idiotype network.

Specifically autoreactive immunocytes are not present in secondary autoimmunity; however, cells are destroyed as “innocent bystanders” by a variety of immunologic mechanisms. Secondary autoimmunity is a result of an immune response against a drug, chemical, or infectious agent that goes awry. Body cells may be destroyed if foreign antigens adhere to and alter the antigenic make-up ofthe cell so that it is perceived as foreign. Alternatively, exposure of previously hidden antigens may induce autoimmunity after cellular damage by other agents. The antigenic structure of a foreign antigen may be similar to a self antigen and induce a crossreac- tive antibody response. Lastly, polyclonal B-cell activation occurs as a sequela to a variety of infectious diseases. This amplification of the immune response might trigger clones of autoreactive B cells, thereby inducing autoimmunity.

Underlying IMD in each affected individual is a genetic predisposition to the development of an immune response against self antigens. This predisposition proba- bly lies in the major histocompatibility complex (MHC). With the right combination of genetics and triggering factors, autoimmunity develops.

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VOl. 6 . NO. 4. 1992 IMMUNOSUPPRESSIVE THERAPY AND IMMUNE-MEDIATED DISEASE 207

idiotype

Antibody 1

d Antibody 2

Antibody 3

anti-idiotype

anti-antiidiotype

FIG. 1. Idiotype/anti-idiotype network. Antibody I is specific for an antigen (not shown). Antibody 2 (the anti-idiotype) is synthesized in response to antibody I . It is postulated that antibody 2 downregulates the production ofantibody I , and thereby, plays a role in control ofthe immune response. Note that antibody 2, since it can combine with antibody I at the antigen-combining site, must have a structure similar to the original antigen. Antibody 3 (the anti-antiidiotype) is synthesized in response to antibody 2. It is postulated that antibody 3 downreg- dates the production of antibody 2. This network could go on ad infin- itzrm, however, it does not go past the level of antibody 3 frequently.

Whether the IMD is primary or secondary often has little bearing on the therapy unless an underlying disease (infectious or neoplastic) or chemical (drug) can be iden- tified and eliminated.

The goals of therapy of IMD are to suppress the aberrant immune response and decrease the secondary inflammatory response. Therapy may be directed at four levels: the genetic level, the level of immunosurveillance, the level of antibody production, and the level of peripheral disease manifestation^.^ In the future, gene therapy may be used to alter the genetic predisposition to IMD. In addition, immunosurveillance, or the body's ability to discriminate between self and nonself, may be able to be manipulated such that antiself immune responses may be prevented in individuals genetically predisposed to developing IMD.4 For the time being, therapy of IMD is directed at levels 3 and 4. This nonselective immunosuppressive approach has several negative aspects. Because conventional therapy suppresses protective as well as de- structive immune responses, the individual is predisposed to infectious diseases. Death due to overwhelming infection is a risk of immunosuppression. In addition, adverse effects of currently used immunosuppressive agents are considerable (Table 2). Glucocorticoids commonly cause iatrogenic hyperadrenocorticism and may possibly be associated with pulmonary thromboembo- lism, a serious sequela to immune-mediated hemolytic anemia.5 Cytotoxic agents can induce significant bone marrow suppression as well as gastrointestinal upset. Therefore, nonselective immunosuppression is a crude approach, and the current therapeutic strategies should be improved.

A Pathophysiologic Approach to the Treatment of Immune-mediated Disease

Table 1 summarizes the immunosuppressive effects of the drugs commonly used to treat IMDs. A discussion of the various components of the immune response important in the pathophysiology of IMDs and the drugs that affect each of these components follows.

Role ofAntibodies in IMD

In primary or secondary IMD, antibodies play an important role in the pathology of IMD. In true autoimmune responses, an autoantibody (antibody with activity against self antigens) is believed to be the inciting cause of the immunologic damage. Autoantibodies have been demonstrated for several autoimmune diseases in humans and dogs including myasthenia gravis (antibodies against the acetylcholine receptor), systemic lupus erythematosus (antibodies against a variety of nuclear and nucleolar proteins), and autoimmune thyroiditis.6-'' The role ofautoantibodies in the initiation ofautoimmu- nity is clouded by the fact that autoantibodies can be both a cause and a result of disease.

In addition to autoantibodies, antibodies in the form of immune complexes are also capable of initiating an inflammatory response and IMD. Immune complex, or type 111 hypersensitivity, disease is seen commonly in the dog in the form of glomerulonephritis. This can be a sequela to a variety of neoplastic diseases; infectious diseases such as pyometritis, bacterial endocarditis, or heartworm disease; or can be idiopathic in origin.' Joints, blood vessels, and skin may also be affected by immune complex deposition. In some forms oftrue autoimmunity such as systemic lupus erythematosus (SLE), for example, immune complexes play as important a role in the pathophysiology of the disease as the autoantibodies.'

Another source of pathogenic antibodies, the idiotype-anti-idiotype network, may be capable of initiating and/or perpetuating IMD. As discussed, Jerne originally proposed the network theory as a means of explaining immune-response regulation.2 The anti-idiotype antibody is proposed to downregulate the production of the original antibody (idiotype). This regulation has the potential to go awry, resulting in IMD. First, if the anti- idiotype is not synthesized at all or is synthesized in in- sufficient amounts, the original antibody is produced in an unregulated manner. If that antibody is specific for a self antigen, true autoimmunity will result. If, on the other hand, the original antibody is directed against a microorganism, drug, or chemical, then immune complex disease may result from idiotype/anti-idiotype immune complexes. Anti-idiotypes may also initiate autoimmunity if their production is u n ~ o n t r o l l e d . ~ ~ ~ ~ ' ~ The anti-idiotype will have a protein structure, which is iden-

208 MILLER Journal of Veterinary

Internal Medicine

TABLE 1. Dosages and Effects of Commonly Used Immunosuppressive Agents

Dose

Drug Dog Cat Effects

Glucocorticoids 2-4 mg/kg 2-4 mg/kg Decreased circulating lymphocytes; decreased lymphocyte division; decreased number of receptors for immunoglobulin on macrophage surfaces; stabilized lysosomal membranes, inhibited chemotaxis of neutrophils, inhibited production of prostaglandins and leukotrienes

Decreased the number of lymphocytes; decreased the production of antibody

Decreased the number of lymphocytes, decreased the production of antibody

blocked the amplification of the immune response, decreased production of antibody against T-cell dependent antigens

cells; alteration in helper versus suppressor T-cell ratios; decreased immunoglobulin receptors on phagocytic cells

Azathioprine

Cyclophosphamide

Cyclosporine A 20 mg/kg19 daily Not available Decreased production and action of interleukin-2;

2 mg/kg daily or every 48 hr

50 mg/m2 daily 4 days/week

I . 1-2.2 mg/kg or every 48 hrI6

50 mg/m2 daily 4 days/week

Danazol 4 mg/kg TID Not available Decrease in amount of immunoglobulin bound to

tical to the original antigen, i.e., they both can combine with antibody 1, in its antigen-combining site (Figure 1). Therefore, this anti-idiotype can perpetuate the immune response and amplify it even if the original antigen is no longer present. Plotz has proposed that anti-idiotype responses against viral antigens that interact with host cell structures are capable of causing autoimmunity.12 Evi- dence supporting this theory comes from many studies including an epidemiologic link between Coxsackie B virus infection and dermatomyositis in ~hi1dren.l~ In addition, SLE and other connective tissue diseases are associated with autoantibodies against nuclear proteins vital to viral replication.

What can we do to reduce the concentration of the offending antibody? Conventional therapy aimed at this arm of the immune response includes glucocorticoids, cytotoxic drugs, and cyclosporine A. Glucocorticoids are the mainstay of therapy of IMD.3 In large doses, they have been shown to decrease antibody production.' They do not decrease the concentration of the antibodies already present. This must be reduced by normal clearance mechanisms. How glucocorticoids decrease the production of new antibodies is not understood but may be a result of a distribution change in the T-lymphocyte ~ 0 0 1 . ~ 9 ' ~ The number of lymphocytes in the circulating pool is reduced, and therefore, these sequestered cells are not as likely to be exposed to antigen and be activated. T cells are not available for B-cell help. In addition, lymphocyte division is reduced by glucocorticoids, decreasing the number of cells capable of producing antibody. This effect of glucocorticoids on antibody concentration is seen with large doses. The importance of this aspect of glucocorticoid action is downplayed by the fact that autoantibody concentration is not always decreased even when an autoimmune disease is in remission. In addition, Nara et al. did not show a decrease in specific anti-

body in dogs treated with immunosuppressive doses of glucocorticoids and vaccinated with canine distemper vaccine versus untreated dogs."

Cytotoxic drugs such as azathioprine and cyclophosphamide are also used frequently in the control of IMD. Azathioprine is an antimetabolite that is metabolized in the liver to 6-mer~aptopurine.'~ Within cells, 6-mercap- topurine attaches to riboside-triphosphate and is trans- formed into thioinosinic acid. In this form, it interferes at several sites with DNA synthesis. This interference reduces the number of lymphocytes as well as their production of immunoglobulin. Cyclophosphamide is an alkylating agent that blocks the synthesis of DNA and RNA. Alkylation of guanine within the DNA molecule results in weakness in the glucoside bridge between the deoxyribosides and their alkaline base favoring disrup- tion of the DNA helix.3 Cyclophosphamide not only reduces cell division but also is directly cytotoxic to lymphocytes, especially T cells.17

The use of cyclosporine A for IMD in human and veterinary medicine has been reported. In people, cyclosporine A has been used successfully in the treatment of SLE, rheumatoid arthritis, pure red cell aplasia, and Sjo- gren's ~yndrome.~ Isolated reports in the veterinary literature suggest that it may be beneficial in the treatment of immune-mediated hemolytic anemia in dogs." A selective T-cell immunosuppressant that reduces the production of interleukin-2, cyclosporine A blocks the amplification of the immune response." Activated T cells are the most susceptible to its effects. Antibody formation against T-cell-dependent antigens is thus reduced.

Many drugs have been used in the therapy of IMD and the list is continually expanding. Depending on the mechanism of the underlying IMD, one or another drug may be more suitable. For instance, immunoglobulin G-mediated pathology is most amenable to corticoste-

Vol. 6 . NO. 4, 1992 IMMUNOSUPPRESSIVE THERAPY AND IMMUNE-MEDIATED DISEASE 209

roids in combination with antimetabolite^.^ Diseases in which immunoglobulin M is the primary antibody involved respond better to alkylating agents. This effect is potentiated by glucocorticoids, however.

Other therapeutic modalities that reduce antibody concentrations show promise in the future therapy of IMD. Plasmapheresis, or the removal of plasma and its constituents from whole blood, can be acutely effective in decreasing the antibody and immune complex concentrations within the blood.” A wide variety of IMDs have been treated using plasmapheresis with mixed results. In humans, immune-mediated thrombocytopenia (IMT), immune-mediated hemolytic anemia (IMHA), SLE, myasthenia gravis, rheumatoid arthritis, glomerulonephritis have all been treated successfully with plasmapheresis.21,” Dogs with SLE and myasthenia gravis have also been treated successfully with this m~dality.’~,’~ Plasmapheresis may be used as an emergency procedure before cytotoxic therapy has had time to take effect. Tem- porary stabilization of acute hemolysis occurred in response to plasmapheresis in two dogs with IMHA.” Plasmapheresis is usually an adjunctive therapy to glucocorticoids and cytotoxic drugs; however, it may allow reductions in the dosages of those drugs in certain case^.'^ Unfortunately, the availability of the equipment necessary to perform plasmapheresis is lacking in veterinary medicine, making this mode of therapy unavailable to most patients.

As previously mentioned, anti-idiotype antibodies are believed to be important in the regulation of the immune response and the development of IMD. With the advent of monoclonal antibody technology, innovative forms of therapy for IMD are on the horizon. Two of these are of special interest because of their ability to reduce the concentration of offending antibodies. A direct approach in- volves the development of a monoclonal antibody with specificity for the idiotype of the offending antibody. This anti-idiotype antibody could be used to block the offending antibody or destroy the B cell carrying the specific idiotype on its surface. Toxins such as ricin have been conjugated with monoclonal antibodies to target and kill specific autoreactive B cells.’6 The main drawback of this therapy is the lack of knowledge and purifica- tion of the relevant idiotype or antigen. An alternative approach is to direct the monoclonal antibody to MHC antigens. Because T cells “see” antigen in the context of MHC antigens presented on the macrophage cell surface, destruction of MHC antigens would limit antigen presentation and, therefore, reduce the number of activated T cells that could provide help to B cells. If B cells do not have appropriate help, they can not produce antibody. Anti-MHC class I1 monoclonal antibodies are capable of preventing, suppressing, and treating experimental allergic encephalitis in mice, suppressing experimental autoimmune myasthenia gravis in mice, and preventing type I diabetes mellitus in the diabetic rat.27

Role of the Eflector Cell

In most cases of IMD, there is an effector cell that is responsible for the ultimate damage to host cells. Macro- phages, neutrophils, and cytotoxic T cells are important because of their ability to directly phagocytize host cells, to release harmful chemicals, or to cause cellular lysis, respectively. Macrophages, as part of the mononuclear phagocytic system (MPS), clean up abnormal cells such as erythrocytes or thrombocytes coated with antibody in the case of IMHA and IMT.’ In this way, the number of those types of cells are reduced to dangerously decreased levels. Neutrophils can also phagocytize abnormal cells. In addition, neutrophils are involved in perpetuating the inflammatory response and causing damage to the host by releasing enzymes (lysozyme), oxygen radicals (superoxide anion, peroxides), and chemotactive agents.’ Cy- totoxic T cells may destroy host cells with or without antibody present, depending on the type of antigen involved.

Reducing the interaction of the effector cells with the offending antibody/antigen helps to curb IMD. Cortico- steroids decrease the number of receptors for immunoglobulin on the macrophage surface.” In addition, glucocorticoids reduce the function of phagocytic cells by stabilizing lysosomal membranes, inhibiting chemotaxis, and preventing the formation of prostaglandins and leukotrienes. l 4 As mentioned, by reducing the number of circulating lymphocytes, corticosteroids decrease the interaction of T cells with antigen.

Another therapeutic approach directed at this arm of the immune response is the administration of large doses of gamma-globulin intravenously. Pure red cell aplasia, IMT, and IMHA are a few of the diseases that have been treated successfully in humans with this m ~ d a l i t y . * ~ , ~ ~ , ~ ’ Although the exact mechanism by which this therapy works is unknown, it is believed that it induces a tran- sient dysfunction of cells of the mononuclear-phagocytic system by reducing the number of immunoglobulin receptors on their surface as well as blocking the receptors that are present. Intravenous gamma globulin may be exert its therapeutic effect by anti-idiotypic suppression of the patient’s cytotoxic antib~dy.~’

Danazol is an attenuated androgen, i.e., it has fewer masculinizing effects than other androgens, that has been used in humans and dogs with various IMDs including IMT and IMHA.3’-36 Danazol has been used in conjunction with other immunosuppressive therapies and may be better suited for long-term therapy than glu- cocor t i~oids .~~ A variety of mechanisms have been proposed to explain the effectiveness of danazol. It appears to reduce the number of immunoglobulin receptors on phagocytic cells as well as the amount of immunoglobulin bound to the cells being de~troyed.~’

Monoclonal antibodies, alone or complexed with a toxin, that have specificity for the Fc receptor of immu-


Internal Medicine

noglobulin on macrophages can be used to block the receptor or destroy the phagocytic cell, respectively. With this procedure, Clarkson et al. reported the success- ful treatment of ITP in a woman.38

Role of the Mononuclear Phagocytic System

The mononuclear phagocytic system is a large reservoir of phagocytic cells and is the site of destruction of host cells in IMT, IMHA and other IMDs. The lymph nodes, spleen, and liver are the primary organs that make up the MPS.39

Eliminating the site of destruction of host cells might be a useful form oftherapy for IMD. This is why splenectomy is used in the therapy for IMT and IMHA. Al- though all phagocytic cells cannot be removed from the body, a large portion is removed when the spleen is taken out. Splenectomy has been used as adjunctive therapy, with some success, in humans and dogs with IMHA and IMT. In one study, of nine dogs that underwent splenectomy for treatment of IMHA, IMT, or Evan's Syn- drome, medical therapy was reduced or discontinued in eight.40 In another study, therapy was discontinued after splenectomy in four of five dogs with IMT .41

An alternative approach is to target toxins toward the MPS. Phagocytic cells are believed to be destroyed when vincristine-loaded platelets are injected intravenously and localized in the MPS.42 This therapy has proven useful in refractory cases, of human IMHA and IMT.42

Role of the T Lymphocyte

The T cell in conjunction with antibody plays an important role in the cytotoxic or type I1 hypersensitivity reaction responsible for diseases such as SLE, IMT, and IMHA.' In addition to direct lysis of'host cells, T cells produce a multitude of factors that amplify the immune response. These factors are termed cytokines or lympho- kines and include the interleukins, tumor necrosis factor, and interferon^.^^ Increased amounts of tumor necrosis factor alpha and interleukin- 1, which contribute to joint destruction by causing bone and cartilage resorp- tion, are present in the serum and synovial fluid of patients with rheumatoid a r t h r i t i ~ . ~ ~ , ~ ~ Gamma interferon, which is produced by activated T cells, is increased in the serum of patients with various IMDs including SLE.45*46 This protein induces the differentiation of resting B cells and promotes active secretion of immunoglobulin. In conjunction with interleukins 1 and 2, interferon in- creases MHC antigen expression on many cells. This increased MHC antigen expression enhances tissue destruction by cytotoxic T lymphocyte^.^^ Decreased suppressor T-cell activity and facilitation of complement activation are also known effects of i n t e r f e r ~ n . ~ ~ Interleukin-2, the most studied of the interleukins, is secreted by activated

T lymphocytes and is of central importance for further recruitment of activated T cells. Interleukin-2-dependent mechanisms are responsible for activation of B cells and macrophages as well as cytotoxic and helper T cells. The summation of the effects of these factors tend to promote and perpetuate IMD.

Therapy directed at the T cell has been restricted to the use of corticosteroids and cytotoxic agents for the most part. Corticosteroids are lymphocytolytic in some species (rats and mice) but more importantly cause a redis- tribution of the lymphocytes into the nonrecirculating pool as previously mentioned. Suppression of T cell division is another effect of corticosteroids that ultimately reduces their numbers.14 Cytotoxic drugs such as azathioprine and cyclophosphamide also reduce the number of lymphocytes by decreasing cell division, whereas cyclosporine reduces the number of T cells through its effects on interleukin-2 production.

In the future, other forms of immunologic intervention directed at T cells will be available.26 Monoclonal antibodies can be targeted at T-cell surface antigens or antigen receptors or the interleukin-2 receptor. Monoclo- nal antibodies against the interleukin-2 receptor have been used in a rat model of acute graft rejection and were found to extend allograft survival.47 This therapy has also been found to suppress lupus nephritis in mice genetically predisposed to this disease.48 Using recombinant DNA technology, diphtheria toxin can be linked to the anti-interleukin-2 receptor antibody as a means of kill- ing interleukin-2 receptor positive cells.49

Role of Suppressor Versus Helper T Cells

Altered immune regulation is the underlying defect in the autoimmune response. One proposed mechanism of this altered control is an upset in the balance between helper/inducer and suppressor/cytotoxic T cells favoring enhancement of the immune response.

Enhancing the suppressor cell activity or altering the ratio of suppressor to helper T cells in favor of suppressor cells may be beneficial in the therapy of IMD. Andro- gens have been shown to enhance suppressor T cell activity whereas estrogens have the opposite effect.32 This may in part explain the predisposition of females to IMD. The beneficial effects of danazol are possibly mediated by this effect of androgens.

In humans and mice, helper T cells are antigenically distinct from suppressor T cells. Because the T-helper cell antigen plays an important role in the generation and amplification of the immune response, monoclonal antibodies directed against this antigen might be helpful in the therapy of IMD. Many studies in mice have shown that this type of therapy is effective. Treatment of lupus- prone mice with weekly injections of rat monoclonal antibodies against the T-helper cell antigen L3T4 from age 4 to 12 months prevented spontaneous a u t ~ i m m u n i t y . ~ ~

Vol. 6 . NO. 4, 1992 IMMUNOSUPPRESSIVE THERAPY AND IMMUNE-MEDIATED DISEASE 21 1

Peripheral Manifestations of Disease

The inflammatory response subsequent to IMD is responsible for many of the secondary disease manifestations associated with these conditions such as fever, arthralgia, myalgia, malaise, and anorexia. The inflammatory response is initiated by a specific immunologic reaction, i.e., autoreactive T and B cells, and is generated by neutrophils and macrophages and their products, complement, prostaglandins/leukotrienes, and various cytokines.’ Activation of complement by immune complexes results in the production of chemotaxins, C3a and C5a. Neutrophils, eosinophils, macrophages, and baso- phils migrate to the affected area.’ These cells produce superoxide anions, hydroxyl radicals, singlet oxygen, and hydrogen peroxide, which result in tissue destruction.’ Activation of phospholipase and subsequent release of arachidonic acid from membranes is followed by prostaglandin and leukotriene production by various inflammatory cells. Prostaglandins and leukotrienes exert such effects as chemotaxis of inflammatory cells, vasodi- lation, smooth muscle contraction, and increased vessel permeability. Both prostaglandins and leukotrienes have been reviewed in the veterinary l i t e r a t ~ r e . ~ ’ . ~ ~ Interleu- kin- 1, which plays a pivotal role in the acute inflammatory response, is a cytokine produced by macrophages. Some of the biological activities attributed to interleukin- 1 include fever, neutrophilia, fibroplasia and colla- gen production, amino-acid release, and T-cell activation and pr~liferation.’~ Again, these factors tend to promote and perpetuate the inflammatory response.

Corticosteroids are potent anti-inflammatory agents at smaller dosages than required for immunosuppression. Corticosteroids are potent inhibitors of phospholipase, thereby reducing the release of arachidonic acid and production of prostaglandins and leukotr iene~.~’ .~~ Glucocorticoids decrease adherence of neutrophils to vascular endothelium and thereby inhibit movement of neutrophils out of the vascular system to areas of inflam- mation. By reducing complement activation, decreasing prostaglandin and leukotriene production, stabilizing lysosomal membranes, reducing vascular permeability, and reducing chemotaxis, glucocorticoids nearly halt the inflammatory response.

There is a great potential for pharmacologic manipula- tion of this inflammatory component of IMDs. Block- ade of prostaglandin and leukotriene synthesis can be accomplished at several steps. Drugs such as chloroquine and gold salts may have their effect by inhibiting leukotriene production.” Synthetic analogues of arachidonic acid have shown promise in blocking the first enzyme in the pathway of leukotriene production from arachidonic acid.5’

Nutritional intervention may play a future role in the therapy of IMDs. Studies in mice and humans have shown that variations in any of the major nutrients in the

diet can bring about dramatic changes in the progressive, often fatal, course of IMD. The most well studied of these groups of nutrients are the fatty acids. Alterations in tissue fatty acid composition by changes in dietary lipid intake has been well d ~ c u r n e n t e d . ~ ~ Changes in polyunsaturated fatty acid composition of cells has also been shown to modify the synthesis of eicosanoids (prostaglandins and leukotrienes) in those cells. It has been postulated that dietary marine lipids, which have large concentrations of polyunsaturated fatty acids, may de- press the inflammatory response by their effects on prostaglandin and leukotriene prod~ction.’~ In fact, lupus- prone mice on diets high in marine lipids exhibit reduced disease severity and prolonged survival.55 In people with rheumatoid arthritis, those patients receiv- ing diets high in polyunsaturated fatty acids had signifi- cantly reduced morning stiffness, fewer tender joints, and increased grip strength compared with patients eat- ing a diet high in saturated fatty a ~ i d s . ’ ~ , ~ ~

Other nutrient manipulations that have been studied include selective amino-acid restriction, calorie restriction, mineral restriction, and various alterations in vita- min content.

Rationale of Combination Therapy

Because of the life-threatening and progressive nature of the IMDs, especially IMHA and SLE, aggressive therapy is warranted early in the course of disease. Glucocorti- coids remain the backbone of therapy because of their potent anti-inflammatory and immunosuppressive effects. Immunosuppressive drugs can be combined, however, for a synergistic effect. It is common to combine a cytotoxic drug with glucocorticoids in the therapy of IMDs. The rationale behind this is obvious when one looks at the various aspects of the immune response sup- pressed by each drug. The addition of a cytotoxic drug usually allows a reduction in dosage or more rapid taper- ing of the dosage of glucocorticoids. Since glucocorticoids are associated with multiple adverse effects, the benefit of combination therapy is apparent. Work by Ogilvie et al. suggests that azathioprine may be more effective in dogs than cyclophosphamide in the treatment of IMDs because of its T-cell suppressive effect^.^' Azathioprine appears to be well tolerated in dogs and is, therefore, a good choice for use in combination with glucocorticoids. As previously mentioned, azathioprine is believed to be more effective in IgG-mediated disease in people, whereas, cyclophosphamide might be a better choice for IgM-mediated disease. Because we rarely de- termine whether an IMD is IgG- or IgM-mediated in small animals, the differential effects of azathioprine and cyclophosphamide in IgG- versus IgM-mediated disease are unknown. Other therapies may be combined to further perturb the immune response. Splenectomy and danazol therapy may be of additional benefit in refrac-


Internal Medicine

TABLE 2. Adverse Effects of Commonly Used Immunosuppressive Agents

Drug Adverse Effects

Glucocorticoids Polyuria/polydipsia/polyphagia, weight gain, weakness, thin skin, alopecia, hyperpigmentation, infections, pancreatitis, hypertension, gastric ulceration, steroid hepatopathy, suppression of the hypothalamic- pituitary-adrenal axisI4

imtation, pancreatitis, hepatotoxicosis, infections, neuromuscular blockage in c a d 6

irritation, hemorrhagic cystitis, transitional cell carcinoma of the bladder, infertility, teratogenicity, alopecia”

hypertophy, hypertension, lymphoma, infections, gastrointestinal imtation”

Azathioprine Bone marrow suppression, gastrointestinal

Cyclophosphamide Bone marrow suppression, gastrointestinal

Cyclosporine A Nephrotoxicosis, hepatotoxicosis, gingival

Danazol Hepatopathy, cutaneous rash’

tory cases. Some severe forms of IMHA, especially intra- vascular hemolysis, may benefit from cyclosporine A therapy or plasmapheresis to rapidly reduce circulating autoantibody concentrations.

Adverse Effects of Immunosuppressive Therapy

The adverse effects of the commonly used immunosuppressive drugs are summarized in Table 2.

Adverse effects of monoclonal antibody therapy occur because these antibodies are foreign proteins that are usually generated in another species such as the mouse or rat. In addition to the possibility of causing anaphy- laxis, fever, and immune-complex disease, the most important drawback with this therapy isthat once antibodies are formed by the patient against the therapeutic antibody, the desired effect is no longer appreciated because the antibody is inactivated or blocked.26

Conclusion

Controlling the aberrant immune response without causing further harm to the patient is the goal of therapy for IMD. However, this goal is difficult or impossible to attain with conventional therapy. The addition of newer drugs with more specific suppressive effects on the immune system will hopefully improve the outcome in many cases of IMD. In addition, newer therapeutic modalities such as monoclonal antibodies directed at T- lymphocyte antigens, the interleukin-2 receptor, major histocompatibility antigens, the immunoglobulin receptor on macrophages and other phagocytic cells, and immunoglobulin on B-cell surfaces are promising for future control of these disorders.

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I I. Beale KM, Halliwell REW, Chen CL. Prevalence of antithyroglo- bulin antibodies detected by enzyme-linked immunosorbent assay of canine serum. J Am Vet Med Assoc 1990; 745-748.

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13. Christiansen ML, Pachman LM, Maryjowski MC, et al. Antibody to coxsackie B virus: increased incidence in children with re- cently diagnosed dermatomyositis. Arthrit Rheum 1983; 26:24.

14. Nelson AM, Conn DL. Glucocorticoids in rheumatic disease. Mayo Clin Proc 1980; 55:758-769.

15. Nara PL, Krakowka S, Powers TE. Effects of prednisolone on the development of immune responses to canine distemper virus in Beagle pups. Am J Vet Res 1979; 40: 1742- 1747.

16. Beale KM. Azathioprine for treatment of immune-mediated diseases of dogs and cats. J Am Vet Med Assoc 1988; 192: I3 16- 1318.

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Immunosuppressive Therapy in the Treatment of Immune-mediated Disease