Immunoreactivity of a New CD5 Antibody with Normal Epithelium and Malignant Tumors Including Thymic Carcinoma

CD5, first recognized on subsets of lymphocytes, also is detected in thymic carcinoma but not in thymoma or other malignant tumors. We studied CD5 expression in 73 cases of malignant tumors of various organs, 22 cases of thymoma, and 7 cases of thymic carcinoma by immunohistochemistry using the new monoclonal anti-CD5 antibody, NCL-CD5-4C7, with a pressure cooker antigen retrieval method. All cases of thymic carcinoma showed positive staining for CD5, predominantly on the cell membrane. Two of 4 cases of atypical thymoma also showed focal positivity, whereas the other types of thymoma were negative. CD5 was detected in cases of malignant tumors other than squamous cell carcinoma and in the normal epithelium of their counterparts. Squamous cell carcinomas of various organs were negative for CD5. Malignant mesothelioma showed peculiar intracytoplasmic staining in contrast to the other tumors. The NCL-CD5-4C7 positivity in thymic epithelial tumors may support the hypothesis suggesting progression of atypical thymoma to thymic carcinoma. NCL-CD5-4C7 may be useful in the differential diagnosis of mediastinal tumors, especially between thymic carcinoma and metastatic squamous cell carcinoma of various primary sites, and for distinguishing malignant mesothelioma from adenocarcinoma of the lung by the different staining pattern.