B-cells in thymic epithelial tumours. An immunohistochemical analysis of intra- and extraepithelial B-cell compartments.

A total of 26 thymomas and thymic carcinomas were studied by immunohistochemistry to determine the presence and distribution of intratumoural B-cells. Double staining experiments revealed two distinct B-cell populations in the thymic epithelial tumours. One was found within the perivascular space (PVS), which is separated from the neoplastic epithelium by a basement membrane. In all tumours the PVS contained lymphocytes with the immunophenotype of peripheral B-cells. Large numbers of B-cells with germinal centre formation were found almost exclusively in myasthenia gravis (MG)-associated tumours, mainly in cortical thymomas and well differentiated thymic carcinomas. A second population of B-cells was located in the neoplastic epithelial meshwork, mostly in areas of organoid medullary differentiation characterized by epidermoid cells or Hassall's corpuscules. This population frequently comprised large, CD23+ cells with dendritic features resembling the special type of intramedullary B-cells of the normal human thymus. In contrast, B-cells were uncommon in areas of mixed thymoma showing spindle celled medullary differentiation, and were almost completely absent from tumour areas composed of cortical type epithelium. Hence a medullary microenvironment with epidermoid cells corresponding to Hassall's corpuscules seems to be necessary for specific intrathymic B-cell homing.