Tk, a new colon tumor-associated antigen resulting from altered O-glycosylation

Erythrocyte polyagglutination antigens T and Tn are truncated O-glycan chai ns that are also carcinoma-associated antigens, We investigated whether Tk polyagglutination antigen could similarly be a carcinoma-associated marker and a target of immunotherapy. Monoclonal antibody LM389 was raised against Tk erythrocytes and tested by immunohistochemistry, LM389 strongly reacted with 48% human colorectal carcinomas. Labeling of normal tissues was visib le on epithelial cells, mainly digestive, but was confined at a supranuclea r level. Expression of the antigen on cloned human carcinoma cells correlat ed with sialosyl-Tn expression. O-Sialoglycoprotein endopeptidase treatment revealed that on carcinomas and cell lines, the epitope was present on O-g lycans. Antibody specificity was determined using synthetic carbohydrates. Direct binding and inhibition studies indicated that LM389 best ligands wer e terminated by two branched N-acetylglucosamine units. Screening of murine cellular cell lines with LM389 allowed development of an experimental mode l with Tk-positive and -negative cells in syngeneic BDIX rats. Vaccination of rats with Tk erythrocytes provided a protection against growth of rat Tk -positive, but not of Tk-negative, tumor cells in association with the deve lopment of antibodies. Taken together, the results indicate that Tk polyagg lutination antigen is a new colorectal carcinoma-associated antigen, absent from the normal cell surface, resulting from alteration of O-glycans biosy nthesis and with potential as a target of immunotherapy.