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Differential diagnostic significance of the paucity of HLA- I antigens on metastatic breast carcinoma cells in effusions

Distinction between benign reactive mesothelial cells and metastatic breast adenocarcinoma cells in effusions from patients with a known prior history of breast cancer is not the easiest task in diagnostic pathology. Here, we report the usefulness of testing the expression of class I HLA antigens (HLA A, B, C) in this respect. Cytospins were prepared from effusions of patients without the history of breast cancer (5 cases) and from effusions of patients with infiltrating ductal carcinoma (11 cases). Three effusions from cancerous patients were not malignant cytologically. The expression of HLA-A, B, C, HLA-DR and β2-microglobulin as well as the macrophage antigen, CD14, was evaluated by immunocytochemistry. In 10 of 11 effusions the cytologically malignant cells expressed very weak or undetectable HLA-A,B,C as compared to the mesothelial cells and macrophages. The paucity of expression of HLA-A, B, C was detectable in those 3 cases where a definitive cytological diagnosis of malignancy could not be established. In contrast, mesothelial cells and macrophages from all samples were uniformly and strongly positive for both HLA-A, B, C and β2-microglobulin. We conclude that the paucity of HLA-I antigens provides a marker helpful in distinguishing metastatic breast carcinoma cells from reactive mesothelial cells in effusions.