Distinctive microvillous brush border staining with HBME-1 distinguishes pleural mesotheliomas from pulmonary adenocarcinomas.

HBME-1 is an antimesothelial monoclonal antibody that recognises an unknown antigen on microvilli of mesothelioma cells. The aim of this study was to evaluate the staining pattern with respect to antibody dilution, cellular distribution and intensity of immunohistochemical staining with HBME-1 in pleural mesotheliomas compared with pulmonary adenocarcinomas. A total of 27 pulmonary adenocarcinomas and 26 mesotheliomas were stained with commercially available HBME-1 at various antibody dilutions and evaluated for the site (membranous, +/- microvillous brush border or cytoplasmic), intensity and percentage of cells staining. On light microscopy, 23 mesotheliomas showed distinctive microvillous brush border staining with HBME-1 (three mesotheliomas--two sarcomatoid and one poorly differentiated--were negative). Twenty-five adenocarcinomas showed membranous +/- cytoplasmic staining but lacked the distinctive microvillous brush border staining. In a subgroup of tumours studied by electron microscopy following immunogold labelling by HBME-1, all of 16 mesothelioma cases showed strong immunogold labelling in the membranes of the long microvilli. In contrast, the 12 cases of pulmonary adenocarcinomas showed minimal labelling in the membranes of the short microvilli, but staining was seen within vesicles, often near the surface of the cells. This study shows that the presence of a distinctive microvillous brush border by immunohistochemical staining with HBME-1 allows distinction between pleural mesotheliomas and pulmonary adenocarcinomas (sensitivity of 88%, specificity of 100%). The difference in the ultrastructural distribution of immunogold labelling with HBME-1 between mesotheliomas and adenocarcinomas underscores the light microscopy findings.