The Immunohistochemical Diagnosis of Mesothelioma : A Comparative Study of Epithelioid Mesothelioma and Lung Adenocarcinoma

Tissue-specific gene expression is mediated largely by transcription factors, and a master regulatory gene is thus a potential marker of cellular lineage. Using normal fetal through adult pulmonary tissues and 64 consecutive lung adenocarcinomas, we examined the expression of thyroid transcription factor (TTF-1), which plays a crucial role in normal lung function and morphogenesis. TTF-1 was expressed consistently throughout the life stages and uniformly in the terminal respiratory unit, which is comprised of peripheral airway cells and small-sized bronchioles. Furthermore, the expression was maintained in 72% of adenocarcinomas that exhibited high correlation with surfactant apoprotein (p <0.001) and morphologic resemblance to terminal respiratory unit cells (p <0.001). The staining pattern was also uniform in the adenocarcinomas despite histologic and microenvironmental diversity in individual tumors and their metastatic foci. This consistency and uniformity, therefore, suggested that TTF-1 expression could be used as a lineage marker of terminal respiratory unit. We also identified interesting distinctions between TTF-1-positive and -negative adenocarcinomas based on their clinicopathologic features and expression of various cancer-associated genes. TTF-1-positive adenocarcinomas had statistically significant prevalence of female (p <0.01), nonsmoker (p <0.05), negative p53 staining (p <0.01), less frequent RB loss (p <0.05), and preserved expression of p27 (p <0.01). The results supported the TTF-1 lineage marker and suggested that molecular pathogenesis may in part be characterized by cellular lineage.

We have isolated a new monoclonal antibody (MAb), K1, that reacts with an epitope on the surface of human ovarian carcinoma cells. This antibody was generated by immunization of mice with periodate-treated human ovarian carcinoma (OVCAR-3) cells. These mice had been previously made tolerant with normal human kidney membranes. Spleen lymphocytes from these mice were selected prior to fusion using a panning purification method on living OVCAR-3 cells. Initial screening of surface-reactive clones was performed in a single day using immunofluorescence on living OVCAR-3 cells, and secondary screening was performed using immunoperoxidase histochemistry on cryostat sections of normal human tissues and human tumors. The K1 clone was subcloned and identified as an IgM isotype, but was subsequently isotype-switched to IgG1K using a panning selection method. When evaluated by immunohistochemistry, the antigen reactive with K1 was found in many ovarian non-mucinous tumors, as well as in squamous tumors of the esophagus, and cervical cancer. The only normal adult human tissues showing uniform reactivity with K1 were the mesothelia of the peritoneal, pleural and pericardial cavities. There was also limited reactivity with epithelia of the trachea, tonsil and Fallopian tube. A similar tissue reactivity for K1 was found in tissues from cynomolgus monkeys. K1 reacted with many of the same tissues and tumors as the previously identified antibody OC125, but several lines of evidence indicate that K1 reacts with a different epitope and probably a different molecule, when compared to OC125. This evidence included assays employing immunofluorescence competition, double-label immunofluorescence, and solid-phase and live-cell radioimmunoassays. Since our data indicate that the antigen reactive with the K1 antibody is a new molecular species, we have named the antigen CAK1. Unlike the shed antigen CA125, CAK1 was only cell-associated and was not found in the supernatant of cultured OVCAR-3 cells or in the blood of ovarian cancer patients. The K1 antibody may be useful as a targeting agent for therapy and in the diagnosis of ovarian carcinoma, as well as some other human cancers.

Immunohistochemistry is a powerful diagnostic adjunct in the differential diagnosis between malignant mesothelioma (especially of the epithelial type) and adenocarcinoma metastatic to the serous membranes. Most of the immunological probes commonly used, however, recognize antigens expressed by the epithelial malignancies and absent from mesothelial cells and mesotheliomas. Probes suitable for the positive identification of mesotheliomas are comparatively scarce and much less commonly used because of their reduced sensitivity and specificity, their unsuitability for staining routinely fixed and embedded tissues, or their lack of commercial availability. We now document that two different polyclonal antisera to calretinin consistently immunostain mesothelial cells and malignant mesotheliomas both in routinely fixed and embedded tissue sections and in cytological preparations of serous effusions. The diagnostic sensitivity of this novel immunocytochemical approach reached 100%, allowing immunostaining of all 44 mesotheliomas investigated, which included five biphasic and three sarcomatoid types. The specificity of calretinin immunoreactivity was checked against 294 adenocarcinomas of different origin (19 serosal metastases and 275 primary tumors potentially able to metastatize to serosal membranes) relevant for the discussion of the differential diagnosis with malignant mesothelioma: only 28 cases showed focal immunoreactivity for calretinin. We conclude that calretinin is a most useful marker for the positive identification of malignant mesotheliomas.