The in vivo effects of corticosteroids on thymocyte subsets in myasthenia gravis

Comparison of juvenile and adult myasthenia gravis in a French cohort with focus on thymic histology

Myasthenia gravis (MG) is an autoimmune disease characterized by muscle fatigability due to acetylcholine receptor (AChR) autoantibodies. To better characterize juvenile MG (JMG), we analyzed 85 pre- and 132 post-pubescent JMG (with a cutoff age of 13) compared to 721 adult MG patients under 40 years old using a French database. Clinical data, anti-AChR antibody titers, thymectomy, and thymic histology were analyzed. The proportion of females was higher in each subgroup. No significant difference in the anti-AChR titers was observed. Interestingly, the proportion of AChR+ MG patients was notably lower among adult MG patients aged between 30 and 40 years, at 69.7%, compared to over 82.4% in the other subgroups. Thymic histological data were examined in patients who underwent thymectomy during the year of MG onset. Notably, in pre-JMG, the percentage of thymectomized patients was significantly lower (32.9% compared to more than 42.5% in other subgroups), and the delay to thymectomy was twice as long. We found a positive correlation between anti-AChR antibodies and germinal center grade across patient categories. Additionally, only females, particularly post-JMG patients, exhibited the highest rates of lymphofollicular hyperplasia (95% of cases) and germinal center grade. These findings reveal distinct patterns in JMG patients, particularly regarding thymic follicular hyperplasia, which appears to be exacerbated in females after puberty.

Thymic Germinal Centers and Corticosteroids in Myasthenia Gravis: an Immunopathological Study in 1035 Cases and a Critical Review

The most common form of Myasthenia gravis (MG) is due to anti-acetylcholine receptor (AChR) antibodies and is frequently associated with thymic pathology. In this review, we discuss the immunopathological characteristics and molecular mechanisms of thymic follicular hyperplasia, the effects of corticosteroids on this thymic pathology, and the role of thymic epithelial cells (TEC), a key player in the inflammatory thymic mechanisms. This review is based not only on the literature data but also on thymic transcriptome results and analyses of pathological and immunological correlations in a vast cohort of 1035 MG patients without thymoma. We show that among patients presenting a thymic hyperplasia with germinal centers (GC), 80 % are females, indicating that thymic follicular hyperplasia is mainly a disease of women. The presence of anti-AChR antibodies is correlated with the degree of follicular hyperplasia, suggesting that the thymus is a source of anti-AChR antibodies. The degree of hyperplasia is not dependent upon the time from the onset, implying that either the antigen is chronically expressed and/or that the mechanisms of the resolution of the GC are not efficiently controlled. Glucocorticoids, a conventional therapy in MG, induce a significant reduction in the GC number, together with changes in the expression of chemokines and angiogenesis. These changes are likely related to the acetylation molecular process, overrepresented in corticosteroid-treated patients, and essential for gene regulation. Altogether, based on the pathological and molecular thymic abnormalities found in MG patients, this review provides some explanations for the benefit of thymectomy in early-onset MG patients.

Thymopoiesis, regulatory T cells, and TCRVbeta expression in thymoma with and without myasthenia gravis, and modulatory effects of steroid therapy

We analyzed thymocyte and thymic regulatory T cell (CD4SPCD25+Foxp3+cells, Treg) development in thymoma with and without myasthenia gravis (MG, MG-thymoma, non-MG-thymoma) and in MG-associated non-neoplastic thymus (MG-NNT). An increased number of immature CD4+CD8(-)CD3(-) thymocytes through the CD4+CD8+ to CD4+CD8(-) transition and an abnormal T cell receptor Vbeta (TCRVbeta) development through the CD4+CD8+ to CD4(-)CD8+ transition were seen both in MG-and non-MG-thymomas. Terminal thymopoiesis, i.e., CD45RA+ cells within the CD4+CD8(-)CD3+ and CD8+CD4(-)CD3+ subsets, was skewed towards the CD4+ compartment in MG-thymoma and CD8+ compartment in non-MG-thymoma, but thymic export was increased only in the latter in keeping with the hypothesis that CD8+ lymphocytes may play a role in the initial stages of autosensitization and in disagreement with the relevance of an increased output of CD4+ T lymphocytes in paraneoplastic MG. Treg level in normal thymus and MG-NNT and both MG- and non-MG-thymoma was similar, and TCRVbeta development in Treg cells was slightly altered in thymoma but irrespective of MG presence. Thus, the relevance of a defective Treg development in MG context remains to be established. Most alterations in thymopoiesis were corrected by therapeutic corticosteroid administration, and the effects of steroid administration may be mediated by thymic microenvironment.

Altered intrathymic T-cell repertoire in human myasthenia gravis

In myasthenia gravis, the thymus is thought to be the primary site of autosensitization. We investigated the V beta T-cell repertoire at different intrathymic differentiation stages in 17 patients with myasthenia gravis and 8 age-matched control subjects by tricolor immunofluorescence, using a panel of six anti-V beta antibodies. We observed an increased expression of V beta 5.1 and V beta 8 subfamilies in the patients compared to the control subjects. These increases were observed not only in mature cells but also in the latest thymic precursors of mature cells (double-positive CD3 high), while there was no change in intermediate precursors (double-positive CD3 low), pointing to biased selection during intrathymic differentiation. In addition, there was a strong correlation between the percentage of V beta 5.1+ and V beta 8+ cells among both the CD4 and CD8 subsets in the patients, but not in control subjects, suggesting that thymic events relevant to the disease lead to these selected populations. Finally, location studies of V beta 5.1+ cells on thymic sections indicated that these cells were overrepresented both in the core of germinal centers and in perifollicular areas of hyperplastic thymuses, suggesting a role in the autoimmune response. Taken together, these findings are compatible with the hypothesis of a biased intrathymic selection in myasthenia gravis.

Evidence of enhanced recombinant interleukin-2 sensitivity in thymic lymphocytes from patients with myasthenia gravis: possible role in autoimmune pathogenesis

We evaluated the activation state of thymic lymphocytes in patients with myasthenia gravis (MG) by cytofluorographic analysis of CD25 expression and by testing their sensitivity to recombinant interleukin-2 (rIL-2) in the absence of any known previous stimulation. We detected no phenotypic signs of activation in fresh MG thymic lymphocyte suspensions, while functional signs of activation were reflected in a significantly higher sensitivity to rIL-2 in MG patients than in controls. The responses to rIL-2 were time- and dose-dependent, were inhibited by a blocking anti-IL-2 receptor antibody, and were associated with an increase in CD25+ T cells in both patients and controls. The T cells with functional signs of previous activation may represent autoreactive cells involved in the autoimmune process and confirm thymus gland hyperactivity in MG. These cells could result from primary autosensitization against the thymic acetylcholine receptor (AChR)-like molecule or from altered migration of peripheral activated cells into an abnormal thymic environment. Our results also provide a clue for understanding the effect of thymectomy in myasthenia gravis.

In vitro anti-acetylcholine receptor antibody synthesis by myasthenia gravis patient lymphocytes: correlations with thymic histology and thymic epithelial-cell interactions

In vitro anti-acetylcholine receptor antibody (anti-AChR Ab) production by peripheral blood lymphocytes (PBL) and thymic lymphocytes was investigated in 52 patients with myasthenia gravis (MG). There was a positive correlation between in vitro anti-AChR Ab synthesis and in vivo titers. A relationship between the rates of synthesis by PBL and histological abnormalities of the thymus was also observed. Patients with hyperplastic thymus tended to produce the largest amounts in vitro, while those with an involuted thymus produced little or none. Production in thymoma patients is likely to correlate with the histological nature of the thymus associated with the tumor. In vitro Ab synthesis was modulated by the depletion of a cell subset for half of the patients tested. Finally, anti-AChR Ab production by thymocytes but not by PBL is enhanced by the addition of autologous or allogeneic thymic epithelial cells, suggesting a possible role of thymic epithelial cells in the autosensitization against AChR occurring in the thymus.