The role of CD 5 expression in thymic carcinoma: possible mechanism for interaction with CD 5 + lymphoid stroma (microenvironment)

Aims Most thymic carcinomas express the lymphocyte marker CD 5 aberrantly. This study was performed to examine the role of the self‐reactive CD 5 antigen in thymic carcinoma. Methods and results We examined CD 5 expression in thymic carcinoma in relation to the lymphoid stroma. All cases of thymic carcinoma examined expressed CD 5. A number of CD 5 + lymphocytes were also present in the stroma of thymic carcinoma. The CD 5 + 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 B cl‐2 expression levels were significantly higher in thymic carcinoma than normal thymic epithelial cells ( TEC s), they did not differ between CD 5 + and CD 5 − areas. E‐cadherin expression in thymic carcinoma was comparable with that of normal TEC s, and it also did not differ between these areas. In contrast, both K i‐67 index and mitotic activity were significantly higher in thymic carcinoma than normal TEC s, and they were significantly higher in CD 5 + than CD 5 − areas. Conclusions CD 5 may be induced by interaction with CD 5 + lymphoid stroma, and may be related to tumour proliferation. CD 5 induction may also be a significant and/or specific effect of the tumour microenvironment of the thymus.