Expression of cortical and medullary thymic epithelial antigens in thymomas. An immunohistological study of 14 cases including a characterization of the lymphocytic compartment.

Four monoclonal antibodies against antigens expressed differentially by the normal thymus epithelium, which define the cortical, medullary and subcapsular compartments, were used for immunohistological characterization of the epithelial cells in 14 thymomas. Furthermore, thymoma-associated lymphocytes were studied with monoclonal antibodies directed against T-lymphocyte differentiation antigens (CD1a, CD3, T-cell antigen receptor). Only four of the 14 thymomas could be classified into either medullary or cortical type thymoma based on the immunophenotype of epithelial cells. Ten cases escaped immunophenotypical classification due to co-expression of medullary and cortical antigens by the tumour cells. This aberration from the normal phenotype might indicate the failure of differentiation of such tumours. The immunophenotype of the associated lymphocytes, on the other hand, made it possible to classify the tumours as cortical (5 cases), mixed (2) and medullary (3) thymomas. Four thymomas escaped this classification scheme due to the absence of lymphocytes (2) or to a hybrid immunophenotype (2). Nevertheless, thymocytes of cortical type clearly predominated and were seen in all thymomas with associated lymphocytes. This feature may constitute a good diagnostic tool in differential diagnosis since, in 28 mediastinal or extramediastinal metastasis of tumours not derived from thymic epithelium and associated with various numbers of lymphocytes, none of them were found to contain CD1a positive lymphocytes.