Pokeweed mitogen-induced immunoglobulin secretory responses of thymic B cells in myasthenia gravis: selective secretion of IgG versus IgM cannot be explained by helper functions of thymic T cells

Modeling the intrathymic pathogenesis of myasthenia gravis

Myasthenia gravis is (MG) a prototypic autoimmune disease; the immune effector mechanisms and autoantigenic target have been delineated. However, the events that lead to the abrogation of self-tolerance to neuromuscular acetylcholine receptors (nAChRs) remain a mystery. The thymus gland has long been considered to hold the key to solving this mystery, although the nature of its involvement remains to be elucidated. The nAChR was one of the first self-proteins associated with a defined autoimmune disease that was found to be expressed on thymic stromal populations. The studies described herein represent our efforts to determine how this "promiscuous" autoantigen expression may be involved in the immunopathogenesis of MG. We review our work, characterizing the expression of the nAChR alpha subunit in the thymus, and advance a hypothesis and experimental model, which explore how intrathymic expression of this autoantigen may contribute to the immunopathogenesis of this autoimmune disease.

Two-color flow cytometric analysis of thymic lymphocytes from patients with myasthenia gravis and/or thymoma

Phenotypic characteristics of lymphocyte components of the thymus were determined by two-color flow cytometry using monoclonal antibodies to T cell-differentiation antigens and activation antigens, and B cells in eight myasthenia gravis (MG) thymuses and eight thymomas including four associated with MG to clarify the roles of thymus in the diseases. Fifteen normal thymuses and four thymuses from non-MG patients were used as controls. Phenotypes of lymphoid components in thymoma resembled those in normal thymuses, in which the majority of lymphoid cells were immature (common) thymocytes (CD4+CD8+ or CD1+CD3-). The proportions of immature thymocytes, however, were relatively higher and those of mature thymocytes (CD4+CD8-, CD4-CD8+, CD1-CD3+ or CD2+CD3+) were lower in thymomas than normal thymuses. The results speculate that functions which support further differentiations of immature thymocytes into mature thymocytes are deficient in neoplastic epithelial cells of thymoma. In thymomas, therefore, occasional T cells might escape from the thymic surveillance system which eliminates auto-reactive T cells. Between thymomas associated with MG and those without, however, no significant difference was found in the proportions of thymocytes at various stages of maturation. In nonthymomatous thymuses from MG patients, an increase of both B cells (CD2-DR+ or CD2-CD21+) and activated T cells (CD2+DR+ or CD2+IL-2R+) was observed. Furthermore, immature thymocytes were significantly decreased and mature thymocytes were increased in the MG thymuses, as compared with normal thymuses, non-MG thymuses, and thymomas. These alterations were relevant to the histological findings observed in MG thymuses such as thymic involutions and germinal center formations: decreased immature thymocytes are considered to reflect thymic involutions, while germinal centers, in which lymphocytes from peripheral lymphoid organs are activated, are responsible for the increase of B cells, activated T cells, and mature T cells. In conclusions, our results suggest that neoplastic epithelial cells of thymomas are functionally deficient and that MG thymuses are immunologically active and resemble peripheral lymphoid organs.