Paraneoplastic autoimmunity in thymus tumors.

Autoimmune phenomena are more frequent in thymic epithelial tumors (TET) than in any other human tumor. Mysthenia gravis (MG) is by far the most common autoimmune disease in thymoma patients. MG is characterized by muscle weakness due to autoantibodies against the acetylcholine receptor (AChR), and CD4+ AChR-specific T cells play a pivotal role for the production of these autoantibodies. About 10% of MG patients have a thymoma and, interestingly, only such thymomas exhibit an MG association that maintains thymuslike morphological and functional features with respect to the homing and differentiation of immature T cells. Since AChR protein is not expressed in thymomas, the specificity of the autoimmunity in thymoma-associated MG is thought to be determined by nonreceptor proteins with AChR epitopes. Such proteins are overexpressed in cortical-type MG-associated thymomas, and medullary thymomas express these proteins at barely detectable levels. Aside from this quantitative difference, the pathogenesis of anti-AChR autoimmunity might be qualitatively different in these thymoma subtypes. Our findings suggest that an antigen-specific abnormal T-cell selection by cortical-type TET may contribute to the pathogenesis of paraneoplastic MG. In contrast, an abnormal (intratumorous) activation of autoreactive T cells may be operative in medullary thymomas.

Abnormal thymocyte development and generation of autoreactive T cells in mixed and cortical thymomas.

To gain insight into the pathogenesis of thymoma-associated myasthenia gravis, thymocyte maturation was investigated in mixed and cortical thymomas by three-color flow cytometry. Although we detected cells at all recognizable stages, we noted an unusual increased percentage of early CD4+/CD3- thymocytes--especially in mixed thymoma--and a pronounced decreased percentage of mature CD4+/CD3+ cells in cortical thymomas as well. The percentage of CD3+/CD69+ cells that arose after positive selection was reduced in both thymoma subtypes compared with control thymuses, which suggests differences in the rate or efficiency of positive selection particularly in mixed thymomas. Mature T cells in 10 of 11 thymomas were not activated in situ as shown by the absence of CD25 expression. After stimulation with recombinant human acetylcholine receptor alpha-subunit fragments, thymocytes from 8 of 11 thymomas of both subtypes proliferated more strongly than those from controls, regardless of whether the donors were myasthenic. Responses of residual thymus cells to tetanus toxoid correlated well with those of autologous blood T cells, whereas those from the thymomas clearly did not--implying minimal colonization of thymomas by mature recirculating T cells. In conclusion, our results show that cortical and mixed thymomas exhibited differences in thymocyte maturation. Nevertheless, both thymoma subtypes seem to contribute to the pathogenesis of paraneoplastic myasthenia gravis by generating naive but potentially autoaggressive T cells; in some thymomas, these cells may then be actively immunized inside the tumor.

Pathogenesis of myasthenia gravis.

Various studies over the last 25 years in Man and animal models have revealed many steps in the pathogenesis of myasthenia gravis (MG) which is now considered the classical organ specific, autoantibody mediated and T cell dependent human autoimmune disease. Though not a disease entity, MG is associated with pathological alterations of the thymus in about 80% of cases. These are described here with reference to distinct models of autoimmunization against the acetylcholine receptor (AChR). In MG with thymitis, intrathymic production of AChR-specific autoantibodies is the result of a classical antigen-driven immune reaction that occurs completely inside the thymus and probably involves AChR on myoid cells as the triggering (myasthenogenic) antigen. Genetic factors contribute essentially to the pathogenesis of this form of MG. In thymoma-associated MG genetic factors are probably of marginal significance. Neither intratumour autoantibody production nor T cell activation seem to occur and the AChR is not the myasthenogenic antigen. Instead, abnormal neurofilaments that share epitopes with the AChR and other auto-antigen targets in paraneoplastic MG are expressed in thymomas and may trigger autoantigen-specific, non-tolerogenic T cell selection by molecular mimicry. These data support the hypothesis that initial steps in the pathogenesis of most MG cases take place within abnormal thymic microenvironments, be they inflammatory or neoplastic. Where these initial steps occur in MG cases without thymic pathology is not known. Likewise, the factors involved in the initial triggering of MG remain enigmatic in all MG subtypes.

Image analysis detects lineage-specific morphologic markers in leukemic blast cells.

This report outlines the morphologic classification of acute myeloid (AML: French-American-British FAB classification: M1) and lymphoid (ALL) leukemia by automatic image analysis and the correlation to immunologic and cytochemical classification. The investigation was carried out on Romanowsky-Giemsa stained bone marrow (n = 15) and blood smears (n = 10) from 25 patients with primary acute leukemia. The cases had been classified as of myeloid or lymphoid origin by three hematologic centers using immunochemistry or cytochemistry, but the specimens were submitted to the authors' laboratory without the diagnosis. The nuclear and cytoplasmic pattern of the blast cells were analyzed by a high resolution image analysis system and the measured and calculated cell features were sorted by means of a classifier program (CART). The image analysis classification was then compared with the immunophenotypical and cytochemical classification. Blood blast cells showed nuclear features that were significantly correlated to a myeloid or lymphoid immunophenotype. In contrast, bone marrow blast cells displayed overlapping and therefore nondiscriminating nuclear features. However, by generating a learning data set using the immunophenotypes the classifier program found specific cytoplasmic features that eventually permitted a differentiation into myeloid or lymphoid subtypes. In summary, the authors suggest that high resolution image analysis of leukemic blast cells detect nuclear and cytoplasmic features that are associated with the immunophenotype and therefore with the lineage determination of the cell. With this new objective and reproducible approach of morphologic cell analysis, it might not only be possible to classify blast cells with minimal cellular differentiation, but furthermore to discover prognostic features because the remarkable difference in classification quality between blood and bone marrow blast cells reported in this study, might be of biologic relevance and requires further investigation.

[CD40 as a mediator of proliferation in normal ans neoplastic thymic epithelium]. / CD40 als Vermittler von Proliferation in normalem und neoplastischem Thymusepithel.

AIMS AND METHODS: While CD40 is a costimulatory molecule of utmost importance for B-cell proliferation and Ig class switch its role has not been elucidated in the human thymus and in thymic epithelial tumors. To investigate, whether CD40 is functionally active there, we investigated the proliferative response by 3H-Thymidin incorporation of primary normal and neoplastic thymic epithelial cell cultures to CD40 triggering by soluble CD40 ligand (CD40L). RESULTS: Normal thymic epithelial cells exhibited proliferation in response to CD40 ligand in a dose dependent manner, while a CD40 ligand specific monoclonal antibody prevented this effect. This response was not significantly different from the response of neoplastic thymic epithelial cells derived from myasthenia gravis-associated thymomas. CONCLUSION: Our data demonstrate that CD40 is expressed on the surface of normal and neoplastic epithelial cells of the thymus and is a functional molecule in terms of epithelial cell proliferation in vitro. Given the differential expression of CD40 in the various histological thymoma subtypes in vivo, the finding suggests a role of CD40 in the different mechanisms of tolerance breakdown in thymomas.

[Abnormal T cell maturation in myasthenia gravis associated thymomas]. / Abnormale T-Zellreifung bei Myasthenia gravis assoziierten Thymomen.

AIMS AND METHODS: Paraneoplastic myasthenia gravis is a thymoma-associated autoimmune disease, characterized by autoantibodies against the acetylcholine receptor (AChR). CD4+ AChR-specific T cells play a pivotal role in the production of autoantibodies. However, it is not known how the tumor is involved in the pathogenesis of autoimmunity. To address this question thymocyte maturation was followed in medullary, mixed and cortical thymomas using three-color FACS analysis. RESULTS: Compared to normal thymuses medullary thymomas were almost devoid of immature T cells and most intratumorous lymphocytes appeared to be of peripheral origin. In mixed and cortical thymomas maturation of thymocytes took place from the most immature precursors to thymomcytes with a mature phenotype. Only in the latter tumor subtypes immature CD4+/CD8-/CD3-thymocytes were significantly increased. In addition, a reduced production of mature CD4+ T cells suggests ineffective positive selection. Activated T cells were present in one medullary but none of the mixed or cortical thymomas. CONCLUSION: Each histological thymoma subtype is associated with a characteristic defect of T cell maturation. The contribution of medullary thymomas to the pathogenesis of myasthenia gravis (MG) might be a pathological intratumorous T cell activation. By contrast, an abnormal antigen-specific T cell selection may be the contribution of cortical and mixed thymomas to anti-AChR autoimmunity.

Myasthenia gravis.

Myasthenia gravis (MG) is the classical organ specific, autoantibody mediated and T cell dependent human autoimmune disease. It is almost invariably associated with pathological alterations of the thymus. These are described here with reference to distinct models of autoimmunization against the acetylcholine receptor (AChR). In MG with thymitis B cells are increased in the medulla forming germinal centers or diffuse B cell infiltrates. Intrathymic production of AChR-specific autoantibodies is the result of a classical antigen-driven immune reaction that occurs completely inside the thymus and involves AChR on myoid cells as the triggering (myasthenogenic) antigen. In thymomas no intratumorous immune reaction occurs and the AChR is not the myasthenogenic antigen. Instead, an abnormal neurofilament that shares epitopes with the AChR is expressed in thymomas and may trigger AChR-specific, non-tolerogenic T cell selection by molecular mimicry. These data support the hypothesis that initial steps in the pathogenesis of MG take place within abnormal thymic microenvironments, be they inflammatory or neoplastic. The etiology of MG remains enigmatic.