Immunotherapy in autoimmune neuromuscular disorders

Future perspectives in target-specific immunotherapies of myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease caused by complement-fixing antibodies against acetylcholine receptors (AChR); antigen-specific CD4+ T cells, regulatory T cells (Tregs) and T helper (Th) 17+ cells are essential in antibody production. Target-specific therapeutic interventions should therefore be directed against antibodies, B cells, complement and molecules associated with T cell signaling. Even though the progress in the immunopathogenesis of the disease probably exceeds any other autoimmune disorder, MG is still treated with traditional drugs or procedures that exert a non-antigen specific immunosuppression or immunomodulation. Novel biological agents currently on the market, directed against the following molecular pathways, are relevant and specific therapeutic targets that can be tested in MG: (a) T cell intracellular signaling molecules, such as anti-CD52, anti-interleukin (IL) 2 receptors, anti- costimulatory molecules, and anti-Janus tyrosine kinases (JAK1, JAK3) that block the intracellular cascade associated with T-cell activation; (b) B cells and their trophic factors, directed against key B-cell molecules; (c) complement C3 or C5, intercepting the destructive effect of complement-fixing antibodies; (d) cytokines and cytokine receptors, such as those targeting IL-6 which promotes antibody production and IL-17, or the p40 subunit of IL-12/1L-23 that affect regulatory T cells; and (e) T and B cell transmigration molecules associated with lymphocyte egress from the lymphoid organs. All drugs against these molecular pathways require testing in controlled trials, although some have already been tried in small case series. Construction of recombinant AChR antibodies that block binding of the pathogenic antibodies, thereby eliminating complement and antibody-depended-cell-mediated cytotoxicity, are additional novel molecular tools that require exploration in experimental MG.

Antigen-specific immunotherapeutic vaccine for experimental autoimmune myasthenia gravis

Myasthenia gravis (MG) and experimental autoimmune myasthenia gravis (EAMG) are caused by Ab-mediated autoimmune responses to muscle nicotinic acetylcholine receptors (AChRs) that impair neuromuscular transmission, thereby causing muscle weakness. Previously, we discovered that i.p. injection of a therapeutic vaccine consisting of bacterially expressed cytoplasmic domains of human AChR subunits reduced the development of chronic EAMG in rats. In this article, we show that immunization with the therapeutic vaccine in adjuvants does not induce EAMG and, thus, is safe. The potency and efficacy of the therapeutic vaccine were greatly increased by s.c. administration of repeated low doses in IFA. Onset of chronic EAMG could be prevented. Established chronic EAMG could be rapidly reversed, modeling therapy of chronic MG. Therapy reduced pathological Abs assayed by immune precipitation of a main immunogenic region chimera. Successfully treated rats exhibited long-term resistance to reinduction of EAMG, suggesting a lasting cure of MG. A long-term effect of therapy was to change the isotype of the pathogenic Ab response from IgG2b, which fixes complement, to IgG1, which does not. Prevention and reversal of chronic EAMG was not caused by the isotype switch, but the isotype switch may contribute to resistance to reinduction of EAMG. Immunization with AChR cytoplasmic domains in adjuvant is promising as a safe, Ag-specific, potent, effective, rapidly acting, and long-lasting therapeutic approach to MG.

Treatment of chronic inflammatory demyelinating polyneuropathy: from molecular bases to practical considerations

Chronic inflammatory demyelinating polyneuropathy (CIDP) is an autoimmune disease of the peripheral nervous system, in which both cellular and humoral immune responses are involved. The disease is clinically heterogeneous with some patients displaying pure motor form and others also showing a variable degree of sensory dysfunction; disease evolution may also differ from patient to patient, since monophasic, progressive, and relapsing forms are reported. Underlying such clinical variability there is probably a broad spectrum of molecular dysfunctions that are and will be the target of therapeutic strategies. In this review we first explore the biological bases of current treatments and subsequently we focus on the practical management that must also take into account pharmacoeconomic issues.

Novel future therapeutic options in myasthenia gravis

BACKGROUND

Myasthenia Gravis (MG) is an autoimmune disease caused by complement-fixing antibodies against the acetylcholine receptors (AChR). Antigen-specific CD4+ T cells, Tregs and Th17+ are also necessary. Consequently, antibodies, B cells, molecules associated with signalling pathways on T helper cells, cytokines and complement are targets for more specific treatment options.

OBJECTIVES

Because available immunosuppressive therapies cause unacceptable side effects after long-term use or are not always effective in inducing remission, novel biological agents directed against the following targets might be options for future therapies in MG: 1) T cell Intracellular Signaling Pathways associated with T cell activation, such as monoclonal antibodies against CD52, Interleukin 2-receptor (IL-2 R), co-stimulatory molecules or compounds inhibiting Janus tyrosine kinases JAK1, JAK3; 2) B cells, against key B cell-surface molecules or trophic factors B cell activation factor (BAFF) and a proliferating inducing ligand (APRIL); 3) Complement, against C3 or C5 that intercept membranolytic attack complex formation; 4) Cytokines and cytokine receptors, including IL-6, IL-17, the p40 subunit of IL12/1L-23, and GM-CSF; and 5) Lymphocyte migration molecules. Construction of recombinant AChR antibodies that block the binding of the pathogenic antibodies, can be a future molecular tool.

CONCLUSION

New biological agents are in the offing for future therapies in MG. Their efficacy needs to be secured with vigorously controlled clinical trials and weighted against excessive cost and rare complications.

Progress in the treatment of myasthenia gravis

Substantial therapeutic progress has been made in myasthenia gravis (MG) even before the era of molecular medicine. Here we characterize modern treatment algorithms that are adapted to disease severity and introduce the principle of escalating treatment strategies for MG. In very mild cases and in some ocular forms of MG, treatment with acetylcholinesterase inhibitors may be sufficient, at least temporarily, but commonly some kind of immunologically active treatment is needed. In generalized MG, a wide array of immunosuppressive treatments has been established through observational studies, some prospective, but most of them have never been tested in a double-blind, prospective and randomized trial. Within the immunologically active drugs, glucocorticosteroids (GCS) and the immunosuppressive drug azathioprine (Aza) have been studied the longest. Aza is still the standard base-line treatment, in particular in cases where high doses of GCS would be needed to maintain remission. If Aza is not tolerated, several alternatives are available including cyclosporine A (Cic A), mycophenolate mofetil, cyclophosphamide, and methotrexate, all of them off-label in most western countries. Tacrolimus is under investigation. More severe cases may profit from drug combinations in which compounds with more rapidly acting drugs (GCS, Cic A) are combined with others showing a more delayed action (Aza). All such combination therapies need to be supervised by an experienced neuroimmunological center because of potentially serious adverse reactions. Serial measurements of anti-acetylcholine receptor antibodies, once these are elevated, is a useful adjunct for monitoring long-term treatment success and may help in weaning from higher to lower doses or to single drugs rather than combinations. For very severe and treatment-resistant cases, co-treatment with intravenous immunoglobulins or different modalities of plasmapheresis may be considered on the short term while the humanized monoclonal anti-CD 20 antibody (rituximab) is a candidate for the long term. In highly refractory cases also immuno-ablation via high-dose cyclophosphamide, followed by hematologic trophic factors such as G-CSF, has been tried successfully. Future developments may include other immunologically active monoclonal antibodies (e.g., anti-CD 52, Campath-1). Up to 10% of patients with MG are associated with a malignant thymoma, often referred to as paraneoplastic MG, as detected by CT scan or MRI, and these patients require thymomectomy and sometimes postsurgical chemotherapy and radiation treatment. In nonthymoma patients with generalised MG, including older children and adults up to the 5th decade, a complete transsternal thymectomy is recommended based on available open trials and expert opinion, preferentially during the first year of disease. Endoscopic surgery may also be effective. Before surgery, pretreatment with immunosuppressive medication or plasmapheresis is usually recommended to ameliorate MG and subsequently reduce perioperative morbidity and mortality which is now near zero in experienced centers. Myasthenic crisis is the life-threatening exacerbation of MG and is best treated by plasmapheresis, mostly combined with immunoadsorption techniques. Intravenous immunoglobulins are a reasonable alternative, but a shortage in supplies and high prices limit its use.

Therapeutic advances and future prospects in immune-mediated inflammatory myopathies

THE INFLAMMATORY MYOPATHIES INCLUDE THREE DISTINCT ENTITIES: polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM). A T-cell-mediated cytotoxic process in PM and IBM and a complement-mediated microangiopathy in DM are the hallmarks of the underlying autoimmune processes. The most consistent therapeutic problem remains the distinction of PM from the difficult-to-treat mimics such as s-IBM, necrotizing myopathies and inflammatory dystrophies. This review provides a step-by-step approach to the treatment of inflammatory myopathies, highlights the common pitfalls and mistakes in therapy, and identifies the emerging new therapies. In uncontrolled studies, PM and DM respond to prednisone to some degree and for some period of time, while a combination with one immu-nosuppressive drug (azathioprine, cyclosporine, mycophenolate, methotrexate) offers additional benefit or steroid-sparing effect. In contrast, IBM is resistant to most of these therapies, most of the time. Controlled studies have shown that IVIg is effective and safe for the treatment of DM, where is used as a second, and at times first, line therapy. IVIg seems to be also effective in the majority of patients with PM based on uncontrolled series, but it offers transient help to a small number of patients with IBM especially those with dysphagia. Bona fide patients with PM and DM who become resistant to the aforementioned therapies, may respond to rituximab, tacrolimus or rarely to an tumor necrosis factor alpha inhibitor. For IBM patients, experience with alemtuzumab, a T-cell-depleting monoclonal antibody, is encouraging.

Current and future standards in treatment of myasthenia gravis

Myasthenia gravis (MG) is a prototypic antibody-mediated neurological autoimmune disorder. Herein we characterize modern treatment algorithms that are adapted to disease severity, and introduce the current principles of escalating strategies for MG treatment. In non-thymoma patients younger than about 50 years of age and with generalized weakness, a complete early (but not urgent) thymectomy is considered as state-of-the-art on the basis of circumstantial evidence and expert opinion. In up to 10% of patients, MG is associated with a thymoma (i.e., is of paraneoplastic origin). The best surgical type of procedure is still under debate. Myasthenic crisis is best treated by plasmapheresis, mostly combined with immunoabsorption techniques. Intravenous immunoglobulins are a reasonable alternative, but a shortage in supplies and high prices limit their use. In generalized MG, a wide array of immunosuppressive treatments has been established, although not formally tested in double-blind, prospective trials. With regard to immunosuppression, azathioprine is still the standard baseline treatment, often combined with initial corticosteroids. In rare patients with an inborn hepatic enzyme deficiency of thiomethylation, azathioprine may be substituted by mycophenolate mofetil. Severe cases may benefit from combined immunosuppression with corticosteroids, cyclosporine A, and even with moderate doses of methotrexate or cyclophosphamide. Tacrolimus is under investigation. In refractory cases, immunoablation via high-dose cyclophosphamide followed by trophic factors such as granulocyte colony-stimulating factor has also been suggested. In the future we may face an increased use of novel, B-cell, or T-cell-directed monoclonal antibodies.

Therapy of immune neuropathies with intravenous immunoglobulins

Autoimmune-mediated disorders belong to the main causes of neuropathies worldwide. During recent years much progress has been achieved in the understanding of the underlying pathomechanisms, associated with implications for therapeutic approaches. Here we will briefly review the pathogenesis and discuss treatment options of the Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, multifocal motor neuropathy, and neuropathies associated with paraproteinemias. In most of these disease entities intravenous immunoglobulins play a major role as effective and safe treatment options.

Drug Insight: the use of intravenous immunoglobulin in neurology—therapeutic considerations and practical issues

Over the past few years, we have achieved increasing success in the treatment of a number of autoimmune-mediated disorders affecting nerves and muscles. This success is partly attributable to the use of high-dose polyclonal intravenous immunoglobulin (IVIg), which has dramatically changed our treatment options. On the basis of results from controlled, but non-FDA-approved, clinical trials, IVIg is now the treatment of choice for Guillain-Barré syndrome, chronic idiopathic inflammatory demyelinating polyneuropathy and multifocal motor neuropathy; IVIg offers rescue therapy for patients with rapidly worsening myasthenia gravis, and is a second-line therapy for dermatomyositis, stiff-person syndrome, and pregnancy-associated or postpartum multiple sclerosis attacks. The ability of IVIg to treat such immunologically diverse disorders effectively, coupled with its excellent safety profile, has led clinicians to use the drug more liberally, even in diseases for which the data are weak and not evidence-based and in patients with coexisting conditions. Use of IVIg for such indications can increase the risk of complications while raising the cost of the drug. Practical issues regarding dosing and frequency of infusions generate dilemmas in clinical practice. In this article, we review the current indications for IVIg treatment, address practical issues related to the use and costs of the drug, and summarize its mechanisms of action.

B cells in the pathophysiology of autoimmune neurological disorders: A credible therapeutic target

There is evidence that B cells are involved in the pathophysiology of many neurological diseases, either in a causative or contributory role, via production of autoantibodies, cytokine secretion, or by acting as antigen-presenting cells leading to T cell activation. Clonal expansion of B cells either in situ or intrathecally and circulating autoantibodies are critical elements in multiple sclerosis (MS), Devic's disease, paraneoplastic central nervous system disorders, stiff-person syndrome, myasthenia gravis, autoimmune demyelinating neuropathies and dermatomyositis. The pathogenic role of B cells and autoantibodies in central and peripheral nervous system disorders, as reviewed here, provides a rationale for investigating whether depletion of B cells with new agents can improve clinical symptomatology and, potentially, restore immune function. Preliminary results from several clinical studies and case reports suggest that B cell depletion may become a viable alternative approach to the treatment of autoimmune neurological disorders.

Treatment of inflammatory myopathies

Idiopathic inflammatory myopathies, notably polymyositis and dermatomyositis are comparatively uncommon diseases and few randomised, double blind placebo controlled trials have been done. Final validation of measures to assess outcome and response to treatment is awaited. Corticosteroids are an effective initial treatment, although rarely tested in randomised controlled trials. Unfortunately, not all patients respond to them and many develop undesirable side effects. There is thus a need for second line agents notably immunosuppressives or intravenous immunoglobulin. There are no defined guidelines or best treatment protocols agreed internationally and so the medical approach must be individualised, based on the severity of clinical presentation, disease duration, presence of extramuscular features, and prior therapy and contraindications to particular agents. There is still a significant percentage of non-responders (around 25%) and clinical relapses. Novel therapeutic approaches are now directed towards cytokine modulation and the use of monoclonal antibodies targeting B and T cells.

Neuromuscular junction autoimmune disease: muscle specific kinase antibodies and treatments for myasthenia gravis

PURPOSE OF REVIEW

Some of the 20% of myasthenia gravis patients who do not have antibodies to acetylcholine receptors (AChRs) have antibodies to muscle specific kinase (MuSK), but a full understanding of their frequency, the associated clinical phenotype and the mechanisms of action of the antibodies has not yet been achieved. Moreover, some patients do not respond well to conventional corticosteroid therapy. Here we review recent clinical and experimental studies on MuSK antibody associated myasthenia gravis, and summarize the results of newer treatments for myasthenia gravis.

RECENT FINDINGS

MuSK antibodies are found in a variable proportion of AChR antibody negative myasthenia gravis patients who are often, but not exclusively, young adult females, with bulbar, neck, or respiratory muscle weakness. The thymus histology is normal or only very mildly abnormal. Surprisingly, limb or intercostal muscle biopsies exhibit no reduction in AChR numbers or complement deposition. However, patients without AChR or MuSK antibodies appear to be similar to those with AChR antibodies and may have low-affinity AChR antibodies. A variety of treatments, often intended to enable corticosteroid doses to be reduced, have been used in all types of myasthenia gravis with some success, but they have not been subjected to randomized clinical trials.

SUMMARY

MuSK antibodies define a form of myasthenia gravis that can be difficult to diagnose, can be life threatening and may require additional treatments. An improved AChR antibody assay may be helpful in patients without AChR or MuSK antibodies. Clinical trials of drugs in other neuroimmunological diseases may help to guide the treatment of myasthenia gravis.

[Autoimmune neuropathies--current aspects of immunopathologic diagnostics and therapy]

The group of autoimmune neuropathies includes the Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuritis, multifocal motor neuropathy, neuropathies associated with monoclonal gammopathies, and vasculitic neuropathies. This educational review first addresses diagnostic pathways that facilitate more rational diagnostic decisions. Many therapies are effective for treating immune neuropathies. Unfortunately, none of the available therapies are specific. In the acute phase, glucocorticosteroids, plasmapheresis, and intravenous immunoglobulins play key roles. The list of long-term therapies includes azathioprine, cyclosporine, cyclophosphamide, and immunoglobulins. The therapeutic mechanisms involved are not clear for most of these compounds. Modern immunotherapy has to consider medical aspects, available therapeutic evidence, and long-term economic burden.

Treatment of immune-mediated, dysimmune neuropathies

This review focuses on the actual status and recent advances in the treatment of immune-mediated neuropathies, including: Guillain-Barre syndrome (GBS) with its subtypes acute inflammatory demyelinating polyradiculoneuropathy, acute motor axonal neuropathy, acute motor and sensory axonal neuropathy, Miller Fisher syndrome, and acute pandysautonomia; chronic inflammatory demyelinating polyneuropathy (CIDP) with its subtypes classical CIDP, CIDP with diabetes, CIDP/monoclonal gammopathy of undetermined significance (MGUS), sensory CIDP, multifocal motor neuropathy (MMN), multifocal acquired demyelinating sensory and motor neuropathy or Lewis-Sumner syndrome, multifocal acquired sensory and motor neuropathy, and distal acquired demyelinating sensory neuropathy; IgM monoclonal gammopathies with its subtypes Waldenstrom's macroglobulinemia, myelin-associated glycoprotein-associated gammopathy, polyneuropathy, organomegaly, endocrinopathy, M-protein, skin changes syndrome, mixed cryoglobulinemia, gait ataxia, late-onset polyneuropathy syndrome, and MGUS. Concerning the treatment of GBS, there is no significant difference between intravenous immunoglobulins (IVIG), plasma exchange or plasma exchange followed by IVIG. Because of convenience and absent invasiveness, IVIG are usually preferred. In treating CIDP corticosteroids, IVIG, or plasma exchange are equally effective. Despite the high costs and relative lack of availability, IVIG are preferentially used. For the one-third of patients, who does not respond, other immunosuppressive options are available. In MMN IVIG are the treatment of choice. Inadequate response in 20% of the patients requires adjunctive immunosuppressive therapies. Neuropathies with IgM monoclonal gammopathy may respond to various chemotherapeutic agents, although the long-term effects are unknown. In addition, such treatment may be associated with serious side effects. Recent data support the use of rituximab, a monoclonal antibody against the B-cell surface-membrane-marker CD20.

Comparative Analysis of Antigen Specificities in the Monomeric and Dimeric Fractions of Intravenous Immunoglobulin

Intravenous immunoglobulin (IVIG) preparations are derived from the pooled plasma of thousands of healthy donors and contain a complex mix of antibodies. Depending on the formulation, IVIG preparations contain variable amounts of monomeric and dimeric IgG. The biological and therapeutic significance of these IVIG fractions is still ill defined. Kinetic analysis of monomeric and dimeric IgG isolated by size-exclusion chromatography revealed a stable monomeric versus an unstable dimeric IgG fraction tending to dissociation. Biochemical analysis by 2D gel electrophoresis and isotype analysis showed no significant differences between the fractions. In contrast, comparative analysis by immunodot, ELISA, FACS, and immunohistology of monomeric and dimeric IgG fractions showed a preferential reactivity of the dimeric IgG on a variety of both self-antigens and exoantigens.

The use of intravenous immunoglobulin in the treatment of autoimmune neuromuscular diseases: evidence-based indications and safety profile

Intravenous immunoglobulin (i.v.Ig) has multiple actions on the immunoregulatory network that operate in concert with each other. For each autoimmune neuromuscular disease, however, there is a predominant mechanism of action that relates to the underlying immunopathogenetic cause of the respective disorder. The best understood actions of i.v.Ig include the following: (a) modulation of pathogenic autoantibodies, an effect relevant in myasthenia gravis (MG), Lambert-Eaton myasthenic syndrome (LEMS), Guillain-Barré syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), and stiff-person syndrome (SPS); (b) inhibition of complement activation and interception of membranolytic attack complex (MAC) formation, an action relevant to the complement-mediated mechanisms involved in GBS, CIDP, MG, and dermatomyositis (DM); (c) modulation of the inhibitory or activation Fc receptors on macrophages invading targeted tissues in nerve and muscle, as seen in CIDP, GBS, and inflammatory myopathies; (d) down-regulation of pathogenic cytokines and adhesion molecules; (e) suppression of T-cell functions; and (f) interference with antigen recognition. Controlled clinical trials have shown that i.v.Ig is effective as first-line therapy in patients with GBS, CIDP, and multifocal motor neuropathy (MMN), and as second-line therapy in DM, MG, LEMS, and SPS. In paraproteinemic IgM anti-MAG (myelin-associated glycoprotein) demyelinating polyneuropathies and inclusion body myositis (IBM), the benefit is variable, marginal, and not statistically significant. i.v.Ig has a remarkably good safety record for long-term administration, however, the following side effects have been observed: mild, infusion-rate-related reactions, such as headaches, myalgia, or fever; moderate but inconsequential events, such as aseptic meningitis and skin rash; and severe, but rare, complications, such as thromboembolic events and renal tubular necrosis. Future studies are needed to (a) find the appropriate dose and frequency of infusions that maintain a response; (b) address pharmacoeconomics, comparing the high cost of i.v.Ig to the cost of the other therapies, which, although less expensive, cause significantly more long-term side effects; (c) determine why some patients respond better than others; and (d) examine the merits of combining i.v.Ig with other immunosuppressive drugs.

Polymyositis and dermatomyositis

The inflammatory myopathies, commonly described as idiopathic, are the largest group of acquired and potentially treatable myopathies. On the basis of unique clinical, histopathological, immunological, and demographic features, they can be differentiated into three major and distinct subsets: dermatomyositis, polymyositis, and inclusion-body myositis. Use of new diagnostic criteria is essential to discriminate between them and to exclude other disorders. Dermatomyositis is a microangiopathy affecting skin and muscle; activation and deposition of complement causes lysis of endomysial capillaries and muscle ischaemia. In polymyositis and inclusion-body myositis, clonally expanded CD8-positive cytotoxic T cells invade muscle fibres that express MHC class I antigens, which leads to fibre necrosis via the perforin pathway. In inclusion-body myositis, vacuolar formation with amyloid deposits coexists with the immunological features. The causative autoantigen has not yet been identified. Upregulated vascular-cell adhesion molecule, intercellular adhesion molecule, chemokines, and their receptors promote T-cell transgression, and various cytokines increase the immunopathological process. Early initiation of therapy is essential, since both polymyositis and dermatomyositis respond to immunotherapeutic agents. New immunomodulatory agents currently being tested in controlled trials may prove promising for difficult cases.