IDENTIFICATION OF TUMOR-ASSOCIATED ANTIGEN THAT IS EXPRESSED ON BOVINE LEUKEMIA VIRUS-INDUCED LYMPHOSARCOMA CELLS AND EXPRESSION OF ITS HUMAN HOMOLOG IN HUMAN T-CELL LYMPHOTROPHIC VIRUS I-INFECTED CELL

By using the monoclonal antibody (MAb) c143 against tumor-associated a ntigen that is expressed in tumor cells of cattle with bovine leukemia virus (BLV)-induced enzootic bovine leukosis (EBL), we found a novel bovine MHC class II-related antigen which consists of alpha chain (36- 37 kDa) and beta chain (32 and 34 kDa). The nature of the c143 antigen was different from previously identified class II antigens, such as D R and DQ, as indicated by test for reactivities with mouse L cell tran sfectants expressing human class II antigens, sequential immunoprecipi tation and tryptic peptide mapping. With the progression of EBL, the n umber of cells carrying the c143 antigen increased, and the beta chain was specifically phosphorylated al serine residue(s) in lymphosarcoma cells and the cell lines derived from them. A marked difference betwe en expression patterns of the c143 antigen and the class II antigens w as observed in lymph nodes from BLV-free and -infected animals. Althou gh bovine mixed lymphocyte reaction (MLR) was inhibited by the additio n of MAbs against class II antigens, the c143 MAb did not inhibit a ly mphoproliferative response of T cells in the MLR, suggesting that the function of this antigen is distinct from those of classical class II molecules. Significantly, although the human homologue of the c143 ant igen is expressed little if at all in human T-cell lines, such as CEM, Molt-4, Jurkat and HPB-ALL, its expression become apparent in T-cell lines infected with human T-cell lymphotrophic virus I (HTLV-I).