After immunization of mice with the human hepatocellular carcinoma (HCC) cell line PLC/PRF/5, we produced monoclonal antibody KM-2, which allowed us to characterize a new HCC-associated antigen (KM-2 antigen) and to develop a sandwich-type radioimmunoassay. The KM-2 antigen was strongly expressed on the cell surface of HCC cell lines. Immunofluorescence staining of frozen sections of different tissues and tumors confirmed its specific expression on the cell surface of a group of HCC. The antigen was also detected in the bile canaliculi of normal liver. Its biochemical characterization revealed a high molecular weight (Mr ∼900,000) glycoprotein with an N-linked carbohydrate chain close to the peptide epitope recognized by the KM-2 monoclonal antibody. By the radioimmunoassay for the KM-2 antigen, the antigen was detected in sera of 72 (47%) of 154 patients with HCC and 3 (3%) of 102 patients with liver cirrhosis; it was not detected in 96 patients with chronic hepatitis or in 100 healthy control individuals. The positive rate of than KM-2 antigen (72 of 154, 47%) was significantly (P < 0.01) higher that (51 of 154, 33%) of α-fetoprotein (AFP) when the cut-off level of AFP was taken as the widely accepted 400 ng/ml. No significant correlation was recognized between serum levels of the KM-2 antigen and AFP (r = 0.15; P > 0.05). In addition, among 103 patients with HCC whose AFP levels were less than 400 ng/ml, 31 (30%) were positive for the KM-2 antigen. Determination of the serum KM-2 antigen would be useful for the serodiagnosis of patients with HCC, particularly in cases with normal or low AFP levels.

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