Mesothelin expression in human lung cancer

Purpose: To investigate mesothelin as a new target for immunotherapy in lung cancer. Experimental Design: Mesothelin mRNA and protein expression were assessed by reverse transcription-PCR, immunoblotting, and immunohistochemistry in human lung cancer specimens. Expression was also characterized in human lung cancer cell lines by flow cytometry and immunoblotting. The SS1P immunotoxin specific for mesothelin was assessed for its cytotoxic activity against lung cancer cells. Results: We found that mesothelin mRNA was expressed in 83% of lung adenocarcinomas (10 of 12 patients). The mesothelin precursor protein was detected in 82% of lung adenocarcinoma (9 of 11 patients), and its mature form was detected in 55% (6 of 11 patients). Immunohistochemistry showed strong and diffuse mesothelin staining in human lung adenocarcinomas and weak or modest staining in squamous cell carcinomas. We detected mesothelin mRNA in 78% of lung cancer cell lines (7 of 9) of the NCl-60 cell line panel. Mesothelin mRNA and proteins were expressed at a high level in non-small cell lung cancer lines EKVX, NCl-H460, NCl-H322M, and NCl-H522. Flow cytometric analysis showed high surface expression of mesothelin in NCl-H322M and EKVX cell lines. Immunotoxin SS1P showed high cytotoxic activity on NCl-H322M and EKVX cells with IC50 values ranging from 2 to 5 ng/mL. Conclusions: Mesothelin is expressed on the surface of most lung adenocarcinoma cells. Immunotoxin SS1P is cytotoxic against mesothelin-expressing lung cancer cell lines and merits evaluation as a new therapeutic agent in treating non-small cell lung cancer.