Pathogenetic mechanisms of post-polio syndrome: morphological, electrophysiological, virological, and immunological correlations

To understand the mechanism of post-poliomyelitis muscular atrophy (PPMA) and the post-polio syndrome (PPS) in general, we performed the following studies: (1) histopathology in spinal cord sections from patients who died 9 days to 44 years after acute paralytic poliomyelitis; (2) enzyme histochemistry, immunocytochemistry (for lymphocyte subsets, MHC antigens and N-CAM) and polymerase chain reaction (PCR) for poliovirus RNA in the muscle biopsies from symptomatic or asymptomatic muscles of post-polio patients; (3) determination of lymphocyte subsets and circulating IgG or IgM antibodies against GM1 and poliovirus; (4) virological studies in the spinal fluid for oligoclonal bands and search for poliovirus genome with PCR; (5) electrophysiological studies including single fiber EMG, fiber density and macro-EMG; and (6) [31P] exercise MRS spectroscopy on previously affected muscles to search for a metabolic correlate of fatigue. These studies concluded that in PPS a continuing dysfunction is present in the spinal cord motor neurons, resulting in ongoing muscle denervation and reinnervation first evident at the axonal branch points. Symptoms are related to attrition of the oversprouting motor neurons which after a period of time cannot support all their axonal sprouts, resulting in failure of re-reinnervation. In some patients with PPS there is also an ongoing immune activation and presence of defective viral particles in the spinal fluid. However, their role in the pathogenesis of PPS is presently unknown.

How to design a therapeutic study in patients with the post-polio syndrome. Methodological concerns and status of present therapies

The innervation of muscles in patients with the post-polio syndrome (PPS) may differ from limb to limb or even within the same limb because of the segmental nature of the initial involvement and the varying degree of subsequent recovery. Consequently, the compensatory effort of the neighboring muscles varies even in the same limb. Clinicohistological studies have confirmed that in PPS the various muscle groups differ according to whether they were affected during the acute polio and have recovered (partially or completely), or whether they were clinically spared during the original disease, in spite of subclinical involvement. Because the impact of the late effects of polio is also variable in these muscle groups, the effect of therapies may be different not only from patient to patient and from limb to limb, but also from muscle to muscle within the same limb. These variables require careful statistical determination of the sample size at the design of a trial. Another problematic issue in the therapy of PPS is to define the end point of therapy. The two disabling PPS symptoms, excessive fatigue and new muscle weakness, can coexist. An experimental therapeutic design must focus separately on the fatigue, using validated fatigue scales, and on muscle weakness, using quantitative muscle testing. Another methodological concern is the placebo-controlled design. Patients with PPS, even those without depression, can be prone to a placebo effect; hence the need for controlled trials. Finally, the length of a trial remains unresolved because of the slow and unpredictable progression of PPS that varies from patient to patient. Until the natural history of PPS is defined, therapies aimed at arresting disease progression are not reliable.

Detection of poliovirus antibodies and poliovirus genome in patients with the post-polio syndrome

To investigate the role of poliovirus (PV) infection in the development of the post-polio syndrome (PPS), we studied the serum, spinal fluid, peripheral blood lymphocytes, and muscle from 47 patients with PPS. We found high titers of IgM PV antibodies (up to 1:250) in the serum of 6 patients, compared to very low titers (less than 1:50) in normal subjects or disease controls. By polymerase chain reaction, using primers of the replicase PV gene, we amplified PV sequences in the peripheral blood lymphocytes in 7 of 37 patients and in the CSF in 4 of 40 patients, but in none of the controls. Sequencing of the amplified product confirmed that it belonged to PV type 1 with a 99.3% homology. We conclude that some patients with PPS have in the serum high titers of IgM anti-PV antibodies, implying an ongoing antibody response to a viral antigen. The presence of PV-RNA in the CSF or lymphocytes suggests possible persistence of mutated virus or defective PV particles. The significance of these findings in the pathogenesis of PPS remains to be determined.

A double-blind, placebo-controlled trial of amantadine for the treatment of fatigue in patients with the post-polio syndrome

Because amantadine has been shown to reduce fatigue in patients with multiple sclerosis, we performed a double-blind, placebo-controlled study to assess its efficacy in the disabling symptom of post-polio fatigue. Twenty-three patients completed six weeks of therapy. Fatigue was measured by the patients using visual analogue scales (twice per day) and numerical fatigue severity scales (once per week) and by overall impression (at end of therapy). Formal neuropsychological testing and serum drug levels were performed to assess compliance. On all measures, no significant difference was found between treatment and placebo groups. Fifty-four percent of patients given amantadine and 43% given placebo reported a decrease in fatigue; however, the visual analogue scales and fatigue severity scales failed to reflect any improvement. Several patients in the treatment group elected to continue amantadine therapy after the study was completed. Our findings suggest that amantadine is not significantly better than placebo in reducing the sensation of fatigue in post-polio syndrome, and that the measures we employed were insensitive to capture the subjective response experienced by a few patients.

Expression of poliovirus receptor in human spinal cord and muscle

Because a prerequisite for infection of a cell with the poliovirus is the presence of poliovirus receptor (PVR), we examined its tissue localization in the human muscle, spinal cord, and muscle cultures using a specific monoclonal antibody against PVR in immunocytochemical studies on serial sections. We found weak expression of PVR in the motor neurons but not the axons. In normal muscle, PVR was expressed at the end plate as confirmed by immunolocalization in serial sections with alpha-bungarotoxin. In neurogenic conditions and in myopathies, PVR was found in occasional denervated muscle fibers and in several regenerating ones. Human myotubes expressed PVR and were susceptible to the poliovirus infection. We conclude that PVR is present at the motor end-plate that can serve as one of the routes of entry of the virus to the motor neurons. The presence of PVR in the regenerating muscle fibers is in accord with clinical observations that muscle injuries can predispose patients to paralytic poliomyelitis.

The post-polio syndrome as an evolved clinical entity. Definition and clinical description

Post-polio syndrome (PPS) refers to the new neuromuscular symptoms that occur at least 15 years after stability in patients with prior acute paralytic polio-myelitis. They include: (1) new muscle weakness and atrophy in the limbs, the bulbar or the respiratory muscles [post-poliomyelitis muscular atrophy (PPMA)] and (2) excessive muscle fatigue and diminished physical endurance. PPS is a clinical diagnosis that requires exclusion of all other medical, neurological, orthopedic or psychiatric diseases that could explain the cause of the new symptoms. Routine electromyography is useful to confirm chronic and ongoing denervation and exclude neuropathies. Muscle biopsy, single fiber electromyography (EMG), macro-EMG, serum antibody titers to polio virus, and spinal fluid studies are very useful research tools but they are rarely needed to establish the clinical diagnosis. PPS is a slowly progressive phenomenon with periods of stability that vary from 3 to 10 years. Current evidence indicates that PPS is the evolution of a subclinically ongoing motor neuron dysfunction that begins after the time of the acute polio. It is clinically manifested as PPS when the well-compensated reinnervating process crosses a critical threshold beyond which the remaining motor neurons cannot maintain the innervation to all the muscle fibers within their motor unit territory.

Mitochondrial toxicity of antiviral drugs

Long-term treatment with antiviral nucleoside analogue drugs, such as AZT, can give rise to delayed and at times severe mitochondrial toxicity. Although these toxic effects are manifest in many tissues, a common disease mechanism can explain the diverse clinical events. A better understanding of these disorders will shed light on genetic mitochondrial diseases and lead to the design of safer and more effective antiviral drugs.

Basic aspects of neuroimmunology as they relate to immunotherapeutic targets: present and future prospects

The neurological diseases with definite or putative immune pathogenesis include myasthenia gravis; Lambert-Eaton myasthenic syndrome; IgM monoclonal anti-myelin-associated glycoprotein-associated demyelinating polyneuropathy; Guillain-Barré syndrome; chronic inflammatory demyelinating polyneuropathy; multifocal motor neuropathy with or without GM1 antibodies; multiple sclerosis; inflammatory myopathies; stiff-man syndrome; autoimmune neuromyotonia; paraneoplastic neuronopathies and cerebellar degeneration; and neurological diseases associated with systemic autoimmune conditions, vasculitis, or viral infections. The events that lead to these autoimmune diseases are not clear but the following sequential steps are critical: (a) the breaking of tolerance, a process in which cytokines, molecular mimicry, or superantigens may play a role in rendering previously anergic T cells to recognize neural autoantigens; (b) antigen recognition by the T-cell receptor complex and processing of the antigen via the major histocompatibility complex class I or II; (c) costimulatory factors especially B7 and B7-binding proteins (CD28, CTLA-4) and intercellular adhesion molecule (ICAM)-1 and its leukocyte function-associated (LFA)-1 ligand; (d) traffic of the activated T cells across the blood-brain or blood-nerve barrier via a series of adhesion molecules that include selectins, leukocyte integrins (LFA-1, Mac-1, very late activating antigen [VLA]-4) and their counterreceptors (ICAM-1, vascular cell adhesion molecule [VCAM]) on the endothelial cells; and (e) tissue injury when the activated T cells, macrophages, or specific autoantibodies find their antigenic targets on glial cells, myelin, axon, calcium channels, or muscle. In designing specific immunotherapy, the main players involved in every step of the immune response need to be considered. Targets for specific therapy in neurological diseases include agents that (a) interfere or compete with antigen recognition or stimulation, (b) inhibit costimulatory signals or cytokines, (c) inhibit the traffic of the activated cells to tissues, and (d) intervene at the antigen recognition sites in the targeted organ. The various immunomodulating procedures and immunosuppressive drugs currently used for nonselective neuroimmunotherapy are discussed in the context of their interference with the above-described immune mediators.

Immunopathogenesis of inflammatory myopathies

Immune-mediated mechanisms appear to play a primary role in the pathogenesis of polymyositis (PM) and dermatomyositis (DM). The serum of patients with active DM has high levels of circulating complement fragments C3b, C4b, and C5b-9 membranolytic attack complex (MAC) and demonstrates a very high C3 uptake in an vitro assay system. The MAC and the immune complex-specific C3bNEO fragment are deposited on the endomysial capillaries early in the disease and lead sequentially to loss of capillaries, muscle ischemia, muscle fiber necrosis, and perifascicular atrophy. In contrast, in PM the muscle fiber injury is initiated by sensitized CD8+ cytotoxic T cells that recognize heretofore unknown and probably endogenous muscle antigens in the context of major histocompatibility complex (MHC) class I expression. A restricted (oligoclonal) pattern of T-cell receptor with prominence of Va1, Vb6, and Vb15 genes is noted within the endomysial infiltrates suggesting that the T-cell response is antigen driven. In both PM and DM, intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 are upregulated in the endomysial endothelial cells and function as ligands for the leukocyte integrins leukocyte function-associated antigen (LFA)-1 and very late activating antigen (VLA)-4, allowing activated lymphocytes to adhere to the endothelial cells and migrate to the muscle fibers. Among viruses, only the retroviruses human immunodeficiency virus (HIV) and human T-cell lymphotropic virus (HTLV)-1 have been convincingly shown to trigger PM, which is mediated by nonviral-specific, cytotoxic CD8+ cells. The treatment of inflammatory myopathies remains empirical. Many patients respond to steroids to some degree and for some period of time. Azathioprine, methotrexate, cyclosporine, cyclophosphamide, and plasmapheresis can be of mild to moderate benefit. High-dose intravenous immunoglobulin (IVIg) is a promising therapeutic modality for some patients resistant to therapies. In a controlled study, IVIg was effective in DM not only in improving the clinical symptoms but also in reversing the underlying immunopathology. The role of IVIg in PM and IBM is under study in control trials.

The effect of L-carnitine on the AZT-induced destruction of human myotubes. Part II: Treatment with L-carnitine improves the AZT-induced changes and prevents further destruction

BACKGROUND

Zidovudine (AZT) as used in the treatment of AIDS causes a mitochondrial myopathy characterized by enzymatic defects in the respiratory chain system, accumulation of lipid droplets, and carnitine deficiency. Human myotubes treated with AZT demonstrate abnormal mitochondria, accumulation of lipid, and increased lysosomes. Because L-carnitine plays a major role in the transport of long chain fatty acids across the inner mitochondrial membrane and facilitates the beta-oxidation of fatty acids, we examined whether L-carnitine can enhance the recovery of the affected myotubes after withdrawal of AZT and can improve the structural changes of the myotubes while AZT treatment continues.

EXPERIMENTAL DESIGN

Myotubes, prepared from human muscle biopsies, were exposed to 250 microM of AZT for 3 to 6 weeks. After 3 weeks of AZT treatment, the cultures were treated with L-carnitine or medium for 3 weeks, while AZT treatment was either withdrawn or continued for 3 more weeks. The cultures were evaluated with: (a) light microscopy; (b) immunocytochemistry, to count the number of myotubes stained with antibodies to Leu-19; (c) oil red O stain, to assess the lipid droplet accumulation; and (d) electron microscopy, to count all the organelles within representative sections of the myotubes, at x24,000, and to calculate the volumetric density (Vvi) of each organelle per unit volume of tissue.

RESULTS

In the post-AZT-treated cultures, L-carnitine increased the number of Leu-19-positive myotubes from 3.83 +/- 1.23 to 23 +/- 1.5 per field, normalized their mitochondria, decreased the lipid droplets, and increased the Vvi of the myofibrils. In the cultures treated with 3 weeks of L-carnitine while AZT treatment continued for 3 more weeks, the number of myotubes increased from 3.3 +/- 0.74 to 6.87 +/- 1.35; the absolute number of the mitochondria increased from 1.65 +/- 0.35 to 9.02 +/- 1.11 and their Vvi from 3.67 +/- 0.83 to 6.57 +/- 0.78 (p < 0.05); the Vvi of the myofibrils increased from 2.50 +/- 0.52 to 5.37 +/- 0.76 (p < 0.05); and the Vvi of the lipid droplets decreased from 5.06 +/- 1.44 to 2.72 +/- 0.72 (p < 0.05). In the AZT-treated cultures that did not receive L-carnitine, the mitochondria demonstrated extensive vacuolation, abnormal cristae, and paracrystalline inclusions; in contrast, in the L-carnitine-treated cultures, the mitochondria had substantially improved in spite of continuation of AZT.

CONCLUSIONS

L-carnitine enhances the pace and degree of recovery of the AZT-associated destruction of human myotubes, restores and preserves the structure of mitochondria, mobilizes the endomyotubular fat, and allows the regeneration of myofibrils, even if AZT treatment continues. The findings may have potential clinical implications in improving the myotoxicity of AZT in patients with AIDS when the administration of AZT treatment must continue.