Purpose: Malignant pleural mesothelioma is a rare malignancy with major environmental implications regarding passive asbestos exposure. We have conducted an immunohistochemical and functional study to address three questions: (1) What is the representation of CD44 on tumor cells as detected by immunohistochemistry? (2) Do cultured cell lines derived from malignant pleural mesothelioma tissue express the same CD44 phenotypes as the original tumors, and can they serve as a model for the study of CD44 in mesotheliomas? (3) What is the functional status of the CD44 expressed on mesotheliomas, with regard to hyaluronan anchorage?
Materials and methods: Thirty-seven samples of pleural mesothelioma were obtained from patients entered on phase I/II protocols conducted in the Surgery Branch of the National Cancer Institute since 1991. The diagnosis was confirmed in all 37 patients by means of a battery of immunohistochemical tests for markers differentiating malignant pleural mesothelioma from adenocarcinoma. Tumor-positive lymph nodes and distant metastases were also examined in six of the patients. Cell lines, established from tumor tissue of six of the patients described above, were used in these experiments. Four (H2596, H2461, H2373, H2452) were derived from primary solid tumors and two (HP-1 and HP-3) were derived from effusions.
Results: Immunohistochemical staining with a monoclonal antibody (H4C4) that recognizes a constant region of human CD44 demonstrated that 34 (92%) of the malignant pleural mesotheliomas examined expressed CD44 on 50% to 100% of their cells. The extent of CD44 expression was apparently related to histologic subtype with the highest expression seen in epithelioid mesotheliomas and the least in sarcomatoid tumors. Tumor cell lines established from the primary tumors or effusions of six of the malignant pleural mesothelioma patients showed high expression of the hematopoietic form of CD44. Four of these cell lines exhibited strong attachment to hyaluronan in an in vitro attachment assay, indicating that their CD44 was functional with respect to hyaluronan anchorage. Hyaluronan attachment was specific in that it could be abolished by preincubation with epitope-specific anti-CD44 antibodies or soluble substrate or by hyaluronidase treatment of attachment surfaces.
Conclusions: We conclude that CD44 is highly expressed on human mesotheliomas, that cell lines adequately represent tumor expression, and that CD44 mediates association with hyaluronan, a major component of pleural fluid.