The effect of thymectomy on autoreactive T- and B-lymphocytes in myasthenia gravis

Determinants of Suboptimal Outcome Following Thymectomy in Myasthenia Gravis

Background

Response to thymectomy in myasthenia gravis (MG) is influenced by various patient-, disease-, and therapy-related factors.

Methods

Retrospective analysis of 128 patients with MG who underwent maximal thymectomy over 15 years was done to identify the determinants of suboptimal clinical outcome.

Results

Among the 128 patients, 62 (48.4%) were females with a mean age of 38.97 (12.29) years. Thymomatous MG occurred in 66 (51.6%). Overall improvement from preoperative status was noted in 88 (68.8%) patients after mean follow-up of 51.68 (33.21) months. The presence of thymoma was the major predictor of suboptimal clinical outcome (P = 0.001), whereas age, gender, preoperative disease severity, and seropositive status did not attain significance. Patients with better outcome had received higher steroid dose preoperatively (P = 0.035).

Conclusions

Suboptimal response after thymectomy occurred in one-third of MG patients, more commonly with thymomatous MG. Relationship of preoperative steroid therapy to remission merits evaluation.

Causes and Consequences of miR-150-5p Dysregulation in Myasthenia Gravis

Autoimmune Myasthenia gravis (MG) is a chronic neuromuscular disease mainly due to antibodies against the acetylcholine receptor (AChR) at the neuromuscular junction that induce invalidating muscle weaknesses. In early-onset MG, the thymus is the effector organ and is often characterized by B-cell infiltrations leading to ectopic germinal center (GC) development. The microRNA miR-150-5p has been previously characterized as a biomarker in MG due to its increase in the serum of patients and its decrease after thymectomy, correlated with an improvement of symptoms. Here, we investigated the causes and consequences of the miR-150 increase in the serum of early-onset MG patients. We observed that miR-150 expression was upregulated in MG thymuses in correlation with the presence of thymic B cells and showed by in situ hybridization experiments, that miR-150 was mainly expressed by cells of the mantle zone of GCs. However, we did not observe any correlation between the degree of thymic hyperplasia and the serum levels in MG patients. In parallel, we also investigated the expression of miR-150 in peripheral blood mononuclear cells (PBMCs) from MG patients. We observed that miR-150 was down-regulated, especially in CD4⁺ T cells compared to controls. These results suggest that the increased serum levels of miR-150 could result from a release from activated peripheral CD4⁺ T cells. Next, we demonstrated that the in vitro treatment of PBMCs with miR-150 or antimiR-150 oligonucleotides, respectively, decreased or increased the expression of one of its major target gene: the proto-oncogene MYB, a well-known actor of hematopoiesis. These results revealed that increased serum levels of miR-150 in MG patients could have a functional effect on PBMCs. We also showed that antimiR-150 caused increased cellular death of CD4⁺ and CD8⁺ T cells, along with the overexpression of pro-apoptotic genes targeted by miR-150 suggesting that miR-150 controlled the survival of these cells. Altogether, these results showed that miR-150 could play a role in MG both at the thymic level and in periphery by modulating the expression of target genes and peripheral cell survival.

[Thymus function and autoimmunity]

PURPOSE

Thymus is the site of T-cell development and is essential for the induction of self-tolerance, by deletion of autoreactive T lymphocytes (negative selection) and by generation of regulatory T cells. Defect of the selection mechanism of both types of lymphocytes lead to autoimmune diseases.

CURRENT KNOWLEDGE AND KEY POINTS

Elimination of potentially self-reactive T cells in the thymus requires the intrathymic expression of ubiquitous and "tissue-specific" antigens. Some thymic antigen expressions are dependent on AIRE expression. Mutations in the AIRE gene that are associated with the absence of autoantigen expression in the thymus, defects in the peptide presentation or in apoptosis can allow autoreactive T cells to escape negative selection, and are associated with autoimmune diseases. Recent data are now available concerning the thymic selection of autoreactive regulatory T cells. The Foxp3 gene was recently shown to be predominantly expressed in regulatory T cells and could be a more specific marker of regulatory T cells than phenotypic markers.

FUTURE PROSPECTS AND PROJECTS

Animal models show that regulatory T cells injection or intrathymic inoculation of antigen lead to immunological tolerance in autoimmunity and transplantation. These novel strategies could be used in human.

Circulating CD4+CD25+ and CD4+CD25+ T cells in myasthenia gravis and in relation to thymectomy

Studies in experimental animal models of human autoimmune diseases have revealed that CD4(+)CD25(+) T regulatory (Tr) cells are of thymic origin and have potentials in preventing auto-aggressive immunity. Myasthenia gravis (MG) is the best-characterized autoimmune disease. Changes in the thymus are found in a majority of patients with MG. Thymectomy has beneficial effects on the disease severity and course in a substantial proportion of MG patients. But the occurrence and characteristics of Tr cells have not yet been defined in MG. We determined the frequencies and properties of circulating CD4(+)CD25(+) versus CD4(+)CD25(-) cells in MG patients and healthy controls (HCs), with special focus on the effect of thymectomy on CD4(+)CD25(+) cells. CD4(+)CD25(high) cells comprise only about 2% of blood lymphocytes in both MG patients and HCs. Frequencies of CD4(+)CD25(high) cells were similar in MG patients irrespective of treatment with thymectomy. CD4(+)CD25(+) cells in both MG patients and HCs are mainly memory T cells and are activated to a greater extent than CD4(+)CD25(-) cells, as reflected by high levels of CD45RO and human leucocyte antigen (HLA)-DR-positive cells. In both MG patients and HCs, CD4(+)CD25(+) cells also contained a high proportion of CD95-expressing cells as possible evidence of apoptosis-proneness. Upon stimulation with anti-CD3/CD28 monoclonal antibodies, CD4(+)CD25(+) cells responded more vigorously than CD4(+)CD25(-) cells in MG, irrespective of treatment with thymectomy, as well as in HCs. Although CD4(+)CD25(-) cells are mainly naïve T cells, in non-thymectomized MG patients, they are activated to a greater extent as reflected by higher expression of HLA-DR and CD95 on the surface compared to HCs. The data thus show that there is no deficiency of CD4(+)CD25(+) cells in MG, nor is the proportion of CD4(+)CD25(+) cells influenced by thymectomy.

In myasthenia gravis, clinical and immunological improvement post-thymectomy segregate with results of in vitro antibody secretion by immunocytes

In myasthenia gravis (MG), presence of pathogenic acetylcholine receptor antibodies (AchRAb) and thymic pathology are related. Our study assesses the long-term clinical outcome of thymectomy as a function of thymic pathologies, AchRAb levels in blood and the ability of immunocytes to secrete AchRAb ex vivo. We found that those patients who have thymocytes that secrete AchRAb in vitro have a greater decrease in their need for Mestinon over the 60 months of follow-up than the non-secretors (p = 0.04). Those patients whose cultured peripheral blood mononuclear cells (PBMC) secreted AchRAb also had a greater decrease in Mestinon requirements over the 60 months of follow-up than the non-secretors (p = 0.05), however, differences between the two groups at each individual assessment were not statistically significant. Serial post-op AchRAb measurements in sera of thymocyte and PBMC non-secretors showed no change from pre-op levels (< 10 nmol/l), while secretors had a reduction of more than 80% from pre-op levels, starting 6 months post-op. We also found that results of PBMC culture at time of thymectomy is a better predictor of response to thymectomy than serum AchRAb levels but inferior to results of thymocytes cultures. Patients with thymoma had the highest proportion of individuals secreting AchRAb from thymocytes in culture (78%), the shortest duration of disease (9 months) and showed the greatest trend toward improvement in clinical signs and symptoms post-op. We have thus demonstrated the long held suspicion that removal of actively secreting cells from the thymus at an early time in the disease confers the greatest benefit.

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.

Decreased mRNA expression of TNF-alpha and IL-10 in non-stimulated peripheral blood mononuclear cells in myasthenia gravis

Myasthenia gravis (MG) is characterized by T cell-dependent autoantibodies to the acetylcholine receptor on the post-synaptic membrane of the neuromuscular junction. The production of autoantibodies is regulated by T cells via cytokines. To investigate the involvement of interferon gamma (IFN-gamma), tumour necrosis factor alpha (TNF-alpha), interleukin-4 (IL-4) and interleukin-10 (IL-10) in MG, the mRNA levels of these cytokines in peripheral blood mononuclear cells without stimulation in vitro were quantified by competitive reverse transcription polymerase chain reaction technique. The mRNA level of TNF-alpha was significantly lower (P = 0.0004) in the overall group of MG patients compared with controls. IL-10 was also lower in MG patients (P = 0.008), most markedly in non-thymectomized patients (P = 0.016). There were no significant differences in IFN-gamma and IL-4 between patients and healthy controls, but the mRNA levels of IL-4 in non-thymectomized patients was significantly lower than in controls (P = 0.02) and in thymectomized patients (P = 0.03). These results, reflecting the in vivo expression pattern of these cytokines in the periphery, suggest an altered cytokine expression at the systemic level in MG.