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This article reports eight primary mediastinal tumors occurring in young adults (19 to 43 years, mean 29.4 years), predominantly female (six of eight) adults. Most patients responded badly to aggressive therapy. Progression is presently noted in one patient; five patients died 10, 11, 13, 18, and 22 months after diagnosis. No patient developed leukemia. The tumors were highly proliferative, had a diffuse growth pattern, and comprised clear cells of variable size. They could not be classified histologically, but could, however, be immunohistologically characterized as B cell lymphomas. In all cases, the immunophenotype was LC+, cALLa-, CD19+, CD20+, CD21-, Ig (surface/cytoplasm)-, and PC-1+. In addition, the neoplastic cells exhibited variable defects in the expression of HLA-A,B,C and HLA-DR and inconstant expression of other B cell-restricted/associated antigens. This combination of immunophenotypical and clinical features suggests that the mediastinal clear cell lymphoma (MCCL) is a previously undescribed type of B cell lymphoma corresponding to the terminal steps of B cell differentiation.

Reactive germinal-center B cells completely lack vimentin. This observation made by using monoclonal vimentin antibody V9 and a sensitive immunoperoxidase technique led to the investigation of vimentin expression in an unselected series of 73 immunohistochemically proven B cell lymphomas. While small lymphocytic lymphomas regularly expressed vimentin, the neoplastic population of mixed, small and large, and large cell lymphomas often contained various proportions of vimentin-negative tumor cells or failed to express it at all. Against the background of the Lukes-Collins classification and the Kiel classification, both defining and subdividing follicular center-cell lymphomas, there were statistically highly significant correlations between the absence of vimentin and the histology, suggesting a germinal center origin. Our data suggest that loss of vimentin expression is indicative of the transient, follicular center-cell stage of normal and malignant B lymphocytic transformation. The complete absence of vimentin within the neoplastic population of all Burkitt's lymphoma cases examined may serve as an argument for its follicular center stage of differentiation, as was asserted by Lukes and Collins.