The role of CD5 expression in thymic carcinoma: possible mechanism for interaction with CD5+ lymphoid stroma (microenvironment)

Aims Most thymic carcinomas express the lymphocyte marker CD5 aberrantly. This study was performed to examine the role of the self-reactive CD5 antigen in thymic carcinoma. Methods and results We examined CD5 expression in thymic carcinoma in relation to the lymphoid stroma. All cases of thymic carcinoma examined expressed CD5. A number of CD5+ lymphocytes were also present in the stroma of thymic carcinoma. The CD5+ tumour areas were predominantly in contact with the lymphoid stroma, and the expression level was significantly lower in tumour cells than lymphocytes. Although p53 and Bcl-2 expression levels were significantly higher in thymic carcinoma than normal thymic epithelial cells (TECs), they did not differ between CD5+ and CD5− areas. E-cadherin expression in thymic carcinoma was comparable with that of normal TECs, and it also did not differ between these areas. In contrast, both Ki-67 index and mitotic activity were significantly higher in thymic carcinoma than normal TECs, and they were significantly higher in CD5+ than CD5− areas. Conclusions CD5 may be induced by interaction with CD5+ lymphoid stroma, and may be related to tumour proliferation. CD5 induction may also be a significant and/or specific effect of the tumour microenvironment of the thymus.