Levels of Complement Regulatory Molecules in Lung Cancer: Disappearance of the D17 Epitope of CD55 in Small‐cell Carcinoma

The levels of complement‐regulatory molecules (complement receptor type one [CR1], decay‐accelerating factor [DAF], membrane cofactor protein [MCP], and an inhibitor of membrane attack complex [CD59]) in lung cancer cells were analyzed to investigate the relation between their expression and histological subtypes, and the possibility of homologous complement deposition on cancer cells. In 25 cell lines (10 adenocarcinoma, 3 large‐cell carcinoma, 7 small‐cell lung cancer [SCLC], and 5 squamous cell carcinoma), flow cytometric analysis revealed that MCP was expressed in all cell lines, whereas none of the cell lines was CR1‐positive. CD59 was detected in all cells. The DAF epitope defined by IA10 was expressed in all cells except one large cell carcinoma cell line. However, another epitope for anti‐DAF monoclonal antibody, D17, was not detected in 5 (71.4%) SCLC and in 4 (22.2%) non‐small‐cell lung cancer. This disparity was seen in most cell lines, irrespective of histological subtypes. The loss of D17 reactivity seemed to be pertinent to malignant phenotype, because most of the normal pulmonary cells possessed the D17 epitope. Furthermore, a cell line lacking DAF (IA10(−)/D17(−)) allowed alternative pathway‐mediated homologous complement (C3) deposition after pretreatment with anti‐MCP antibody. This raises a new possibility for immuno‐targeting of cancer. These cell lines should be useful in studying the biology of lung cancer.