Loss of mesothelin expression by mesothelioma cells grown in vitro determines sensitivity to anti-mesothelin immunotoxin SS1P

Background/Aim: To determine if early passage tumor cells obtained from patients with mesothelioma continue to express the tumor differentiation antigen mesothelin and their sensitivity to the anti-mesothelin immunotoxin SS1P. Materials and Methods: Cell cultures were established from ascites or pleural effusion of 6 peritoneal and 3 pleural mesothelioma patients, respectively. These cells were evaluated for mesothelin expression by immunohistochemistry and flow cytometry. Results: Although mesothelin was highly expressed in tumor biopsies of all patients, only 3 out of 9 malignant effusions from these patients when grown in short-term culture showed strong mesothelin positivity by IHC. By flow cytometry, the number of mesothelin sites per cell was variable ranging from 580 to 210,000 sites/cell. Cells with strong mesothelin expression by IHC and increased number of mesothelin sites/cell were sensitive to SS1P. Conclusions: Most mesothelioma tumors loose mesothelin when grown in vitro and the sensitivity of these cells to SS1P is dependent on the number of mesothelin sites/cell.