Utility of anti-L523S antibody in the diagnosis of benign and malignant serous effusions.

BACKGROUND: Immunohistochemistry is helpful in distinguishing metastatic carcinoma from atypical mesothelial cells; however, it is not useful in differentiating atypical mesothelial cells from malignant mesothelial cells. K homolog domain containing protein overexpressed in cancer (KOC), a member of the insulin-like growth factor mRNA-binding protein (IMP) family, also known as L523S and IMP3, is expressed during embryogenesis and in various malignancies. Using a mouse monoclonal antibody (L523S) against KOC, KOC expression was investigated in malignant tumors and reactive mesothelial cells in serous effusions.

METHODS: Seventy-six cases with paraffin-embedded pleural, pericardial, and peritoneal serous effusion cell blocks including 60 malignant serous effusions (11 malignant pleural mesotheliomas and 49 metastatic carcinomas) and benign pleural effusions (14 cases with reactive mesothelial cells and 2 cases with atypical cells with uncertain significance) were selected for immunohistochemical analysis with L523S, calretinin, and CK5/6.

RESULTS: Immunohistochemical studies showed that positive staining for KOC of variable degrees of intensity was observed in 47 of 60 cases in malignant serous effusions including 10 of 11 mesotheliomas and 36 of 49 metastatic carcinomas. The associated reactive mesothelial cells were negative for KOC but positive for calretinin and CK5/6. All 11 malignant mesotheliomas exhibited positivity for calretinin, and 9 of 11 cases had CK5/6 staining. In addition, 16 cases that were originally diagnosed either as pleural effusions with reactive mesothelial cells (14) or atypical cells with uncertain significance (2) were also tested for KOC expression. Interestingly, 3 of 16 cases exhibited various degrees of positivity for KOC, 2 of which were diagnosed as lung adenocarcinoma with a recurrence after tumor resection and 1 as malignant pleural mesothelioma.

CONCLUSIONS: Anti-L523S antibody is a useful marker for the detection of malignant cells in serous effusions and it can have significant utility in differentiating reactive mesothelial cells from malignant mesothelioma and metastatic carcinoma in combination with calretinin and CK5/6 staining.