The treatment of experimental allergic neuritis by plasma exchange

From bench to bedside--experimental rationale for immune-specific therapies in the inflamed peripheral nerve

Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy are autoimmune-mediated inflammatory diseases of the PNS. In recent years, substantial progress has been made towards understanding the immune mechanisms that underlie these conditions, in large part through the study of experimental models. Here, we review the available animal models that partially mimic human Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy, and discuss the wide range of therapeutic approaches that have been successfully established in these models of inflammatory neuropathies. Transfer of this preclinical knowledge to patients has been far less successful, and inflammatory neuropathies are still associated with significant morbidity and mortality. We will summarize successful therapeutic trials in human autoimmune neuropathies to provide a vantage point for the transfer of experimental treatment strategies to clinical practice in immune-mediated diseases of the peripheral nerve.

Treatment of inflammatory neuropathy

Experimental autoimmune neuritis (EAN) provides an accurate model for understanding the mechanism of acute and chronic inflammatory demyelinating polyradiculoneuropathy (AIDP and CIDP). Treatments aimed at every stage of the immune process in EAN have been effective in inhibiting or treating the disease, including antibodies directed against cell adhesion molecules on the endothelium, inhibition of T cells, removal or blockade of antibodies, depletion of complement, and interference with the release or action of macrophage effector molecules. In human disease the only proven treatments are plasma exchange and intravenous immunoglobulin (IVIg) in AIDP, and either of these regimens and also corticosteroids in CIDP. However the outcome from AIDP and CIDP remains unsatisfactory with existing immunosuppressive regimens. This problem arises from the fact that while demyelination appears to be effectively and promptly repaired by remyelination, it may be accompanied by axonal degeneration which can cause severe persistent disability. In addition to limiting demyelination, it will also be important to develop strategies to protect axons from degeneration and to enhance regeneration.

IgG immunoadsorption in experimental allergic neuritis: effect on antibody levels and clinical course

The effect of IgG immunoadsorption upon the course of chronic experimental allergic neuritis (EAN) is described. Miniature membrane plasma separators coupled with a Protein A (PA)-Sepharose immunoadsorbent column were used to perform upon conscious rabbits 5 IgG immunoadsorption treatments over 6 days. Quantitation of anti-myelin IgG and IgM by ELISA revealed that 55-65% of plasma IgG was removed per treatment. Rapid post-treatment antibody rebound was observed for anti-myelin IgG although no antibody overshoot above control levels could be observed. Anti-myelin IgM levels remained relatively unaffected by PA immunoadsorption. Comparisons of clinical scores between control and treatment animals showed that IgG immunoadsorption was significantly beneficial (day 1 post-treatment p less than 0.001; day 2 post-treatment p less than 0.05). However, rapid relapse was observed in all treatment animals such that by day 3 post-treatment no significant clinical difference between control and treatment groups could be observed. IgG immunoadsorption suppresses the clinical progression of chronic EAN in a manner similar to that seen with plasma exchange. This finding suggests that antibody modulates early disease pathogenesis.

In vivo CNS demyelination mediated by anti-galactocerebroside antibody

The mechanism of antibody-mediated central nervous system (CNS) demyelination in vivo was studied using rabbit eyes. Injection of anti-galactocerebroside (Gal C) antiserum alone into the normal rabbit vitreous body induced demyelination in the epiretinal myelinated fibers. This activity of the antiserum disappeared after heat treatment at 56 degrees C for 30 min and was restored by supplement of normal fresh serum, suggesting the complement dependency of the activity. Heated anti-Gal C antiserum could induce demyelination, however, when macrophages were introduced by injecting lymphocyte supernatants together with antiserum. Electron microscopic study revealed penetration of macrophage process between the myelin lamella. These findings suggest that the cooperation of anti-Gal C antibody and macrophage can result in the antibody-dependent cell-mediated demyelination in the absence of complement. Because oligodendrocyte generally appeared normal, myelin, not oligodendrocyte is suspected to be the primary target by anti-Gal C antiserum. In contrast, neither anti-MBP nor anti-gangliosides antiserum had the in vivo-demyelinating activity. In CNS demyelination by anti-Gal C antibody, complement-mediated and macrophage-mediated mechanisms may cooperate in varying degrees.

Membrane plasma exchange in experimental allergic neuritis: effect on antibody levels and clinical course

The effect of membrane plasma exchange on the course of chronic experimental allergic neuritis in rabbits is described. Using miniature membrane plasma separators conscious animals were treated with 4 exchanges over 5 days removing one plasma volume per procedure and using a non-immunogenic gelatin plasma solution as replacement. Comparisons of clinical scores between control and treated groups indicated that treated animals received significant benefit from plasma exchange (day 1 post-treatment P less than 0.002; day 3 post-treatment P less than 0.01). However, relapse was observed in all plasma exchanged animals such that by day 8 post-treatment there was no significant difference in clinical scores between the two groups. Quantitation of anti-myelin IgG and IgM by ELISA assay showed that 55-60% of circulating antibody was removed per exchange. Rapid post-exchange antibody rebound was observed for both IgG and IgM so that pre-exchange levels were re-established within 24-48 h. However, no significant overshoot in circulating levels of anti-myelin IgG nor IgM could be observed. It is probable that long-term remission as a result of therapeutic plasma exchange is a function of effective circulating plasma removal and if present, the suppression of ongoing antigenic stimuli.

Chronic experimental allergic neuritis. An electrophysiological and histological study in the rabbit

Ten adult outbred New Zealand white rabbits were inoculated with a single multiportal dose of purified bovine peripheral nerve myelin and Freund's adjuvant containing 500 mg of nerve antigen. Seven animals developed chronic relapsing or progressive disease which was followed by clinical examination for 14 months. Electrophysiological studies showed marked slowing of motor conduction velocity, dispersion of the evoked muscle action potential (MAP) and reduction in amplitude of the MAP derived from distal stimulation. Histological examination of the peripheral nervous system showed at 12 months a marked hypertrophic neuropathy in the nerve roots with well developed onion bulbs, active demyelination and a moderate nerve fibre loss. It is suggested that these animals provide a reliable and predictable model for human chronic inflammatory demyelinating polyneuropathy (CIDP) which should prove valuable for therapeutic trials and studies of pathogenetic mechanisms.

A critical review of therapies in acute and chronic inflammatory demyelinating polyneuropathies

Acute and chronic inflammatory demyelinating neuropathies are among the most common treatable neuropathies seen by neurologists. Evidence for effective therapy has only recently been provided by randomized or controlled trials. In the Guillain-Barré syndrome such evidence does not support the use of corticosteroids or immunosuppressive agents. However, when used early in the course, plasma exchange (PE) has been shown to lessen the severity and shorten the duration of the disease; it is indicated only in severely paralyzed patients or those whose rapid deterioration suggests the imminent need for ventilatory support. Some patients with chronic inflammatory demyelinating polyneuropathy (CIDP) respond to corticosteroid therapy or other immunosuppressive agents. PE is also effective in certain patients, but there is no sound evidence to date concerning combined immunosuppression and PE. The rationale of PE in these conditions and whether it is the removal of a toxic factor or the replacement fluid used that is beneficial remains to be clarified.

Effects of glucocorticoids on experimental allergic neuritis

Corticosteroids were administered to rats and guinea pigs with experimental allergic neuritis, from the time of inoculation with antigen or from the onset of signs of disease. No statistically significant effects were observed in guinea pigs. In rats, to which large doses of corticosteroids were administered, disease severity was slightly but significantly reduced in both groups and recovery was more rapid in the animals treated from the time of induction of disease. These results were comparable with those obtained in trials of corticosteroids in acute inflammatory polyneuropathy in man, which have also not demonstrated any striking effects.

Beneficial effects of plasma exchange in acute inflammatory polyradiculoneuropathy

The results of a controlled trial in which 38 patients with severe acute inflammatory polyradiculoneuropathy took part indicate that plasma exchange favourably influenced the course of the disease. Significant benefits were seen in time until onset of improvement, course of muscular weakness, improvement in disability grades over the first 2 months, and working capacity after 1 month. Cost-benefit analysis showed that the exchange treatment resulted in net financial savings. The results suggest that plasma exchange may have a role in the treatment of severe acute inflammatory polyradiculoneuropathy.

The effect of suppression of macrophage activity on the development of experimental allergic neuritis

The selective toxicity of silica dust for macrophages has been used to assess the role of these cells in experimental allergic neuritis (EAN). Inbred Lewis rats were inoculated with bovine dorsal roots in Freund's complete adjuvant (day 0). In two experiments, animals received 200 mg of silica dust in 1 cm3 of saline intraperitoneally (IP) at days 8 and 16. In another two experiments, animals received IP silica at days 3, 7, and 11. Control animals received 1 cm3 saline IP at corresponding times. Regular clinical assessment showed that in animals treated on days 8 and 16 there was a significant delay in the time taken to reach their maximum degree of illness. This delay was not seen in the animals treated on days 3, 7, and 11. Neither of the injection regimes reduced the final maximum severity of the disease. In three experiments recovery of the treated and control animals occurred concurrently, hence the duration of the disease was reduced in the animals treated at days 8 and 16. However, in one group of animals given silica at days 3, 7 and 11, there was a delay in the time taken to recover from the most severe phase of the disease but thereafter the treated animals improved more quickly to reach their best grade at the same time as the controls. If the silica blockade of macrophages is to be effective in delaying the onset of EAN, the timing of injections is critical.