Human IgG3 monoclonal antibody directed to an unbranched repeating type 2 chain (Gal beta 1----4GlcNAc beta 1----3Gal beta 1----4GlcNAc beta 1----3Gal beta 1----R) which is highly expressed in colonic and hepatocellular carcinoma.
Previously established human monoclonal antibodies (MAbs) directed to carbohydrate antigens are essentially all IgM class, and show relatively low affinity and low reactivity at 37 degrees C. We report here the establishment of a human IgG3 MAb displaying high affinity antigen-binding activity at 37 degrees C and efficiently activating cellular cytotoxicity directed to human tumor cell lines expressing the polylactosamine antigen. The IgG3 MAb (MH21-134) reacted with the repeated unbranched polylactosamine structure Gal beta 1----4GlcNAc beta 1----3Gal beta 1----4GlcNAc beta 1----3Gal beta 1----R, i.e., nLc6, nLc8, etc., but did not react with sialyl 2----3 or 2----6 substituted derivatives at the terminal Gal. This specificity differs from that of several anti-i antibodies, or human anti-i-like MAbs which react with sialyl 2----3 substituted structures. Directly biotinylated MH21-134 antibody was used in immunohistochemical staining of 154 formalin-fixed, paraffin-embedded tissue sections to study distribution of the antigen. High incidence of positive staining was found in colon cancer (11/17; 65%) and hepatocellular carcinoma (8/12; 67%), followed by large cell and squamous cell carcinoma of lung cancer (10/13; 59%, and 14/26; 54%, respectively). TLC immunostaining of glycolipid extracts from a variety of tumor tissues showed the presence of nLc6 and/or nLc8 in over 50% of cases. The antigens nLc6 and nLc8 were found to be absent from normal colonic epithelia, kidney, and pancreas. Only a weak band corresponding to nLc8 and one corresponding to nLc6 were found in liver and spleen, although all these normal tissues, including gastrointestinal epithelia, lung, liver, spleen, erythrocytes, and lymphocytes, were essentially negative on immunohistology. However, the antigen was found to be highly expressed in myelocytes and weakly in bronchial glands of lung and pancreatic duct epithelia. Nevertheless, expression of unsubstituted, unbranched polylactosamine antigen could be an important basis for induction of humoral immune response against certain types of human cancer, despite its limited expression in normal cells.