Myasthenia gravis, thymectomy and serum thymic hormone activity

Different age-related effects of thymectomy in myasthenia gravis: role of thymoma, zinc, thymulin, IL-2 and IL-6

Different age-related immune pathogenetic mechanisms in myasthenia gravis (MG) have been suggested because of restoration after thymectomy (Tx) of altered zinc, thymulin (TH) and T-cell subsets exclusively in early-onset patients (younger <50 years), not in late-onset patients (older >50 years). In this context interleukin-2 (IL-2), interleukin-6 (IL-6) and thymoma are crucial because both involved in MG pathogenesis and correlated with acetylcholine receptors (AchRs) Ab production. Moreover, IL-2 and IL-6 are zinc-dependent, are altered in aging and related with zinc and TH age-dependent declines. Moreover, zinc is relevant for immune efficiency. In order to confirm these different age-related pathogenetic mechanisms further, the role of thymoma, zinc, TH, IL-2 and IL-6 is studied in MG patients with generalized MG with and without thymoma before and 1 month and 1 year after Tx. The high IL-2, IL-6, zinc, and AChR Ab levels observed before Tx are significantly correlated each other in younger MG patients (<50 years) independently by thymoma and in older MG patients (>50 years) with thymoma. No correlations exist in older MG patients without thymoma. Thymulin is not correlated with other parameters considered to be both in younger and older MG patients independently by the thymoma. Thymectomy restores zinc; immune parameters and AChR Ab are exclusively in the younger group, not in the older one. These findings suggest that IL-2 and IL-6, via zinc, rather than TH, may be involved in different age-related pathogenetic mechanisms mainly in early-onset MG. By contrast, thymoma may be involved in MG etiology in late-onset representing, as such, a useful discriminant tool for MG etiology between early and late-onset MG patients. Because autoimmune phenomena may rise in aging, a parallelism with altered immune functions during aging is discussed.

Altered thymic endocrine activity along with impairments of peripheral zinc metabolism and T-lymphocyte populations are associated with myasthenia gravis: a follow-up study

Thymic endocrine activity was assessed by a bioassay to determine the basal activity of thymulin (TH), a zinc dependent hormone, and its in vitro reactivation in two different age groups of patients with myasthenia gravis (MG). Before thymectomy, basal TH plasma levels were increased in patients over the age of 50 years. Plasma zinc levels were increased in all patients, this increment being very high in old patients. One year after thymectomy both TH and zinc plasma levels decreased. While zinc plasma levels were within the normal ranges for their respective ages, TH levels were lower in young and higher in old patients than in age comparable controls. Young patients with MG showed increased CD3,DR positive peripheral T-cells as well as lymphocytes with the CD16,CD56 phenotype. An increment of CD3 positive cells along with CD4 and CD16,CD56 positive cells were found in older patients. Thymectomy partially affected blood lymphocyte representation only in young patients, since CD3,DR T-cells decreased one year after surgery. No significant variations in T-cell representation were found in old patients after thymectomy. Immunosuppression in thymectomized patients did not significantly affected TH and zinc plasma levels. Very high levels of TH and the presence of additional alterations in T-lymphocyte subsets in old patients suggested that differential age related pathogenetic immunological mechanisms might be associated with the disease.

The calf thymus superoxide dismutase: a protein active on cholesterol metabolism

1. Recent studies on the new aspects of thymus physiology describing the correlation between thymus hormones and pituitary hormone secretion, are illustrated. 2. Subsequently, results of a series of experiments showing the effect of a calf thymus protein on cholesterol synthesis in rat hepatocyte cells are discussed. 3. The last part of this review is focused on the biochemical characteristics of this calf thymus protein that revealed an amino acid sequence that was found to be identical with regions of bovine erythrocyte superoxide dismutase. New perspectives of studies, focused on the isolation of possible superoxide dismutase membrane receptors, are described at the end of this review.

Thymopoietin, a thymic polypeptide, prevents nicotinic agonist-induced morphological changes in neonatal muscle cells in culture

Thymopoietin, a polypeptide hormone isolated from thymus and involved in immune function, potently inhibited [125I]alpha-bungarotoxin binding to neonatal muscle cells in culture (IC50 = 3.8 nM) and blocked carbachol-stimulated 22Na uptake with an IC50 of 1.9 +/- 0.2 nM and 23 +/- 7 nM at a half-maximal and maximal concentration of carbachol, respectively. Studies were subsequently done to evaluate potential long-term functional consequences of this interaction of thymopoietin at the nicotinic receptor. Exposure (1-3 days) of neonatal muscle cells in culture to nicotine (3 x 10(-6) M) or carbachol (1 x 10(-6) M) resulted in a decline in myotube branching and a decrease in myotube length. Thymopoietin did not appreciably alter myotube morphology on its own; however, it prevented the effects of nicotine and carbachol on muscle cell morphology at concentrations (1-10 nM) which corresponded well to those with which thymopoietin interacted at the receptor. The action of alpha-bungarotoxin on the myotubes was very similar to that of thymopoietin. These studies suggest that the endogenously occurring polypeptide, thymopoietin, has the potential to modulate muscle cell morphology through an interaction at the nicotinic receptor.

Combined effects of a thymic peptide, thymopoietin and myasthenic patient sera in rat myotube culture

We investigated in a rat myotube assay the combined effect of 26 myasthenic (MG) patient sera and a thymic peptide, thymopoietin (Tpo) which had previously been shown to bind Torpedo and human AChR and to compete with alpha-bungarotoxin (alpha-Bgt) binding. Cultures were first exposed to Tpo alone for 3 h (0.3, 7.5, 15 nM), then MG sera (5% final dilution) were added for an additional 18 h. Reduction in the amount of 125I-alpha-Bgt binding sites in the presence of various concentrations of Tpo were similar with control sera and in all the patients with low or undetectable anti-AChR Ab (11 cases). In cultures exposed to Tpo and sera with high anti-AChR Ab titre (15 cases), Tpo and anti-AChR Ab have an additive capacity to reduce the number of alpha-Bgt binding sites. The results are compatible with the hypothesis that anti-AChR Ab and Tpo could impair neuromuscular transmission by complementary mechanisms.

Thymopoietin interacts at the alpha-bungarotoxin site of and induces process formation in PC12 pheochromocytoma cells

Thymopoietin, a polypeptide isolated from thymus and involved in immune regulation, potently inhibited [125I]alpha-bungarotoxin binding in both pheochromocytoma (PC12) cells in culture (IC50 of 3.9 nM) and in PC12 cell membranes (IC50 of 2.2 nM). The degree of inhibition produced by thymopoietin was similar to that observed with alpha-bungarotoxin; in contrast, nicotinic receptor ligands affected alpha-bungarotoxin binding only at micromolar concentrations, in agreement with previous work. Binding of thymopoietin was reversible. Studies with PC12 cell membranes suggested that the interaction between alpha-bungarotoxin and thymopoietin at the receptor was competitive. The effect of thymopoietin was subsequently assessed on various morphological characteristics of PC12 cells in culture. Exposure of the cells to the polypeptide resulted in neurite extension, which was evident as early as 1-2 days in culture and was maximal after 4-6 days; this response was observed with concentrations of thymopoietin as low as 10(-8) M. Nerve growth factor also induced neurite extension in PC12 cells; however, the effects of nerve growth factor were qualitatively and quantitatively distinct from those which occurred with thymopoietin. Moreover, a monoclonal antibody to nerve growth factor completely prevented the nerve growth factor-induced process formation without affecting the thymopoietin-induced response. On the other hand, alpha-bungarotoxin resulted in the formation of processes which appeared morphologically similar to those induced by thymopoietin, although alpha-bungarotoxin appeared less potent than the thymic polypeptide. The effect of thymopoietin appeared to be specific; thysplenin, a polypeptide with approximately 80% homology with thymopoietin, did not elicit process formation. The thymopoietin-induced effect was reversed upon removal of the polypeptide from the culture medium. These results show that thymopoietin, a polypeptide endogenous to mammalian systems, potently interacted at the alpha-bungarotoxin site in a neuronal cell line. Furthermore, thymopoietin could elicit process formation in PC12 cells, suggesting that it may be a neuronotrophic factor.

Thymopoietin, a thymic polypeptide, specifically interacts at neuronal nicotinic alpha-bungarotoxin receptors

alpha-Bungarotoxin (alpha-BGT), a snake venom polypeptide, interacts potently and specifically with a nicotinic receptor population in neuronal tissue. However, the identity of this site is unclear, because, unlike at the neuromuscular junction and in electroplax, in nervous tissue the toxin does not block nicotinic cholinergic responses. Therefore, we sought endogenous compounds other than acetylcholine that could interact with the neuronal alpha-BGT site. In the present experiments, thymopoietin, a polypeptide isolated from the thymus, is shown to inhibit potently alpha-BGT binding to brain membranes in a dose-dependent manner (IC50 = 3.1 nM). This effect was not shared by a wide variety of other peptides, including thysplenin, a closely related polypeptide. Thymopoietin did not inhibit the binding of other radioligands known to interact with different populations of cholinergic receptors, such as [3H]nicotine and [3H]methylcarbachol, which bind to nicotinic receptors, or [3H]quinuclidinylbenzilate, which binds to muscarinic receptors. These results show that thymopoietin potently and specifically affects 125I-alpha-BGT binding to brain membranes and suggest that thymopoietin might be an endogenous ligand for alpha-BGT receptors in neuronal tissue.

Myasthenic sera recognize the human acetylcholine receptor bound to thymopoietin

The thymic hormone, thymopoietin (Tpo), from human (HTpo), bovine (BTpo) and from synthetic (sHTpo) origins bound to the acetylcholine receptor (AChR) solubilized by Triton 1.5% from human muscle. This binding was demonstrated either by inhibition of formation of radiolabeled alpha bungarotoxin (alpha Bgt)-AChR complexes measured after precipitation by ammonium sulfate or by a myasthenic serum containing a high concentration of anti-AChR antibodies, or directly by incubating the human AChR with radiolabeled sHTpo or BTpo. The 125I-labeled alpha Bgt-AChR complexes were totally inhibited by 10(-6) M sHTpo or BTpo. The complexes formed by AChR and the radiolabeled Tpo were recognized specifically by sera containing anti-AChR antibodies from myasthenic patients. The active pentapeptide derivative of Tpo, thymopentin, another thymic hormone, thymulin, as well as bovine insulin did not interfere with the specific binding of alpha Bgt to human AChR. Tpo and anti-AChR antibodies could participate together in the inhibition of neuromuscular conduction with Tpo modulating the depressive effect of the antibodies on the neuromuscular junction in myasthenia gravis.

Recurrence of thymic hyperplasia after trans-sternal thymectomy in myasthenia gravis

A 23-year-old woman with myasthenia gravis underwent thymectomy in September, 1975 by a trans-sternal approach for the removal of an hyperplastic gland. Surgical intervention was followed by marked improvement of her symptoms. Five years later, symptoms recurred. In March, 1982 diagnostic pneumomediastinum disclosed an anterosuperior mediastinal mass which, on excision, proved to be an hyperplastic thymus. This report confirms previous findings of thymus regrowth after surgery. It also demonstrates that surgical techniques using the trans-sternal approach do not guarantee complete removal of the thymus.

Binding of thymopoietin to the acetylcholine receptor

Thymopoietin is a polypeptide hormone of the thymus with physiological effects on the immune system and on acetylcholine-mediated transmission at the neuromuscular synapse. Elucidation of the structure and function of the nicotinic acetylcholine receptor has been facilitated by the use of the electric organs of Torpedo ray or Electrophorus eel as rich sources of the receptor and by the use of snake polypeptide toxins such as alpha-bungarotoxin as highly selective labels of the acetylcholine binding site. We now show that thymopoietin binds with high affinity (Ka approximately equal to 2.5 X 10(9) M-1) to the acetylcholine binding region of the acetylcholine receptor of Torpedo californica, as evidenced by similar and complete inhibition of the binding of radiolabeled thymopoietin or alpha-bungarotoxin by either of these polypeptides. These findings raise intriguing questions concerning the mechanisms whereby alpha-bungarotoxin and the thymopoietin affect acetylcholine receptor function, since these two polypeptides with such similar binding properties have very different functional effects.

Thymoma epithelial cells secrete thymic hormone but do not express class II antigens of the major histocompatibility complex

17 thymomas were studied by indirect immunofluorescence for the presence of thymic hormones and antigens of the major histocompatibility complex (MHC). The thymoma epithelial cells (specifically identified by their keratin content) contained thymic hormones (thymulin and thymosin alpha 1), a finding corroborated by the observation of elevated thymulin serum levels. In contrast with normal or hyperplastic thymuses, thymoma epithelial cells did not express HLA-DR and HLA-DC antigens as assessed by immunofluorescence as well as immunoblot analyses. Conversely, MHC class I antigens (HLA-ABC) were normally expressed. Thus, we conclude that thymoma epithelial cells are endocrinologically active but are defective for the expression of some MHC products (class II molecules) known to play an essential role in intrathymic T cell differentiation.

Thymic peptide hormones: basic properties and clinical applications in cancer

The manuscript will provide an in-depth and critical review of the nomenclature, biochemistry, biological properties, and a summary of published and on-going clinical trials with all reported thymic preparations, including both partially purified thymic factors (e.g., thymosin fraction 5, thymostimulin) as well as purified and synthesized thymic peptides (e.g., thymosin alpha 1, thymulin). Particular emphasis will be placed on which thymic peptides should be categorized as true hormones. In addition, the comparative biochemistry and biological activity in animals will be summarized and contrasted for all the currently available thymic factors. The effects, in vitro of thymic factors, on peripheral blood lymphocytes isolated from normal donors and patients with primary immunodeficiency disorders, autoimmune disorders, and neoplastic disorders will also be reviewed. Finally, a detailed critical summary of the clinical trials performed with each of the thymic preparations will be presented with an emphasis on treatment of patients with cancer.

Thymic hormone-containing cells VI. Immunohistologic evidence for the simultaneous presence of thymulin, thymopoietin and thymosin alpha 1 in normal and pathological human thymuses

The localization of the three best-defined thymic hormones, namely, thymulin, thymopoietin and thymosin alpha 1 was studied by immunofluorescence using antibodies directed against these three molecules. With both human thymus frozen sections and cultured cells, thymic hormones were found exclusively in the epithelial component (recognized by its keratin content), in normal as well as pathological thymuses. The double-labeling experiments using the different anti-thymic hormone antibodies showed that the same epithelial cells contained the three hormones. These results suggest that the production of different hormones in the thymus is accomplished by the same epithelial cells.

Thymectomy in myasthenia gravis. A review

The role of thymectomy in the management of myasthenia gravis is reviewed in the light of the published data and of a personal series. The patients in whom the operation is most successful are non thymomatous patients aged between 10 and 40 years with an MG history of less than 3 years. There is no sex prevalence. Lasting improvement may be expected. There are no proven correlations between biological indices like the germinal centers in the thymus and/or AChR antibody titers and the postoperative course of the disease. Complete removal of the thymus seems to be crucial and hence the transsternal approach is preferred. The operation, less effective in patients with thymoma than in those with an active thymus, is nonetheless necessary to in these patients prevent putative damage to surrounding organs from thymoma infiltration. Why thymectomy should be effective in patients with an active thymus and not in those with a thymoma may be revealed by in vitro studies of the interactions between thymic cells and peripheral B cells, now in progress.

Postoperative pericarditis following thymectomy for myasthenia gravis. A prospective study

Eight consecutive patients who underwent thymectomy for the treatment of myasthenia gravis were evaluated prospectively for the development of postoperative pericarditis. In four of the eight patients (50 percent) pericarditis developed within 48 hours after thymectomy. All four had a three-component pericardial friction rub, two of the four patients had a new postoperative pericardial effusion by echocardiography, and in two typical ECG diffuse concave ST segment elevation and evolutionary ST and T wave changes developed. In the four patients with postoperative pericarditis, a malignant thymoma adherent to the pericardium was found in two, while two patients had a normal thymus histology with no adherence to the pericardium. Thus, postoperative pericarditis was found in both of the patients with a thymoma and in two of six patients without a thymoma. This study demonstrates that a high incidence of pericarditis occurs following thymectomy for the treatment of myasthenia gravis.

Recurrence of thymic hyperplasia after thymectomy in myasthenia gravis. Its importance as a cause of failure of surgical treatment

Persistence of symptoms in patients with myasthenia gravis who have undergone previous thymectomy has been attributed to thymus remnants. Patients with partial or no recovery were studied 40 +/- 31 months (mean +/- SD) after surgery, which had been carried out by the transcervical approach in 20 and trans-sternal approach in four. Lateral x-ray tomography of the mediastinum after injection of air showed images compatible with residual thymus gland in 18 patients (75 percent). Thirteen of these underwent reoperation by the trans-sternal approach, and thymic tissue was found in 11 (85 percent). After repeated thymectomy, 67 percent of the patients improved clinically. Therefore, it is quite reasonable to infer that incomplete removal of the thymus was responsible, at least partly, for failure of the first procedure.

Induction of E-rosette-promoting factor in human plasma by levamisole: an assessment in a patient with partial DiGeorge syndrome

A male infant with partial DiGeorge syndrome responded to weekly administration of levamisole (2.5 mg/kg of body weight) with an increase of circulating E-rosette-forming T cells. Thymic hormone activity in plasma appeared to be elevated to a near-normal level of 11.6 ng thymopoietin equivalent/ml after levamisole administration. The in vitro incubation studies indicated that levamisole by itself had no E-rosette-promoting ability, but a dialysable and relatively heat-stable plasma factor induced by levamisole both in the patient and in healthy individuals had E-rosette-promoting activity for the patient's lymphocytes. Such a plasma factor, however, could not be induced in all four thymectomized myasthenic subjects examined, suggesting a thymus-dependent nature of the plasma factor. These results suggest that levamisole might mediate an increased secretion of humoral factor(s) with E-rosette-promoting activity, even from such a rudimentary thymus as in the partial DiGeorge syndrome.

Transcervical thymectomy and thymus remnants

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Immunocytochemical localization of thymosin-alpha 1 in thymic epithelial cells of normal and myasthenia gravis patients and in thymic cultures

Thymosin alpha 1 (alpha 1) is a potent thymic polypeptide hormone. With antibodies against synthetic thymosin alpha 1, indirect immunofluorescence was applied to human normal thymus and to hyperplastic, thymomatous or "involuted" thymus of myasthenia gravis (MG) patients. Alpha 1 was localized only in the epithelial cells, lying singly, grouped, in Hassall's corpuscles or proliferated in thymomas. In contrast to normal thymus, which had fewer and more weakly stained cells, MG hyperplastic thymus had many strongly positive epithelial cells: this was markedly evident in thymomas. "Involuted" MG thymus had a few but brightly stained cells lying within the fatty tissue. In tissue cultures of human thymus, anti-alpha 1 stained the epithelial cells, but not fibroblasts. These findings: (a) demonstrate the origin of the thymic hormone alpha 1 to be the thymic epithelial cell; (b) raise the possibility that excess alpha 1 may act pathologically to facilitate and perpetuate the dysimmune mechanism in MG; (c) may partially explain the beneficial effect of thymectomy in MG patients of any age; and (d) suggest that epithelial cells may be autonomous for the production of alpha 1 as evidenced by their positivity in tissue culture.

The current role of thymectomy for myasthenia gravis

Myasthenia gravis is an autoimmune disease characterized by muscle weakness and fatigability due to a reduction in available acetylcholine receptors at the neuromuscular junction. Treatment with anticholinesterase drugs and corticosteroids has improved the prognosis for patients with this disease. However, controversy continues concerning the indications for thymectomy. During a 9 year period, 27 patients who underwent thymectomy by median sternotomy were reviewed. Eighty-one percent of these patients benefited from the procedure. Clinical improvement did not correlate with age, sex, duration of symptoms, severity of disease or thymic histology, This suggests that the indications for thymectomy should be liberalized to include most patients with generalized myasthenia who fail to respond readily to conventional medical therapy. The importance of immunosuppression nd plasmapheresis in the therapy of myasthenia gravis awaits further delineation of the immune defect associated with the disease.

Plasmapheresis therapy in myasthenia gravis

Plasmapheresis in the treatment of myasthenia gravis has led to disparate results when applied by different investigators because of considerable differences in the volume, number, and tempo of plasmaphereses and in the type and amount of concomitant immunosuppressive drug therapy. Used as short-term crisis intervention, plasmapheresis produces temporary clinical improvement and reduction in titer of antibody to acetylcholine receptor, even without accompanying drug therapy. When applied as long-term primary therapy under optimal conditions, plasmapheresis is capable of generating stable improvement in most patients. This response appears to result from a synergistic action with immunosuppressive drugs, since it is characterized by a sustained reduction in titer of antibody to acetylcholine receptor. Where clinical circumstances warrant cytotoxic immunosuppression in patients with myasthenia gravis, consideration should be given to the simultaneous employment of plasmapheresis, in order to maximize benefit to the patient from a given exposure to drug therapy.

Controversies about the treatment of myasthenia gravis

Clinicians treating patients with myasthenia gravis must choose cholinergic drugs, corticosteroids, immunosuppressive drugs, thymectomy, or plasmapheresis. Clinicians must decide the sequence or combination of these therapies and when to deem lack of improvement a sign for a different therapeutic approach. Because controlled trials have not been done to evaluate therapies that may require months or years before benefit is evident, controversy abounds.

An inhibitor of thymic hormone activity in serum from patients with lymphoblastic leukemia

Serum from 21 patients with lymphoblastic leukemia, five with myeloblastic leukemia and 30 age matched control subjects tested for thymic hormone activity in an assay that measures the induction of T cell surface antigen. This activity was subnormal in serum from 10 of 16 patients with untreated lymphoblastic leukemia (p less than 0.001) but was within the normal range when the leukemia was in remission. Low inductive activity was associated with an inhibitor of T cell induction which was less than 30,000 daltons in molecular size and interfered with induction by purified thymopoietin plus a high concentration of ubiquitin or by normal serum alone.