Phenotype and ontogeny of cells carrying a tumor-associated antigen that is expressed on bovine leukemia virus-induced lymphosarcoma.

The phenotype and ontogeny of cells carrying the tumor-associated antigen (TAA), identified in tumors of cattle with enzootic bovine leukosis (EBL) by use of the monoclonal antibody (MAb) c143, were analyzed by flow cytometry and immunohistochemistry. The TAA recognized by the c143 MAb (c143 TAA) was mainly expressed on B-cells, macrophages, reticular cells, and a minor population of BoCD4-positive T-cells in bovine leukemia virus (BLV)-free normal cattle. When the peripheral blood mononuclear cells from normal cattle were activated in vitro, some populations of BoCD8-positive T- and non-T/non-B-cells also showed expression. Moreover, B-cells expressed the c143 TAA until the antigen was lost from cells at the final stage of B-cell differentiation, namely, the plasma cell stage. The c143 MAb-positive cells in blood of BLV-free normal cattle form heterologous subpopulations, and these cells coexpress other surface markers such as BoCD2, BoCD5, BoCD6, and B-cell-specific molecules B1 low, B1 bright, and B2. In BLV-infected cattle, the proportion of peripheral blood mononuclear cells that express the c143 TAA increased with the progression of EBL, and, in addition, BLV-infected cattle that had no lymphosarcomas showed increased proportions of c143 MAb-positive cells that coexpressed surface IgM (sIgM), BoCD5, B1 low, and B2 but not BoCD2, BoCD4, or BoCD6. Furthermore, most of the c143 MAb-positive tumors from all cattle with EBL appeared to be a monoclonal population derived from a single B-cell and to be divided into two types, c143 TAA+BoCD5+B1 low+ B2+ sIgM+ or sIgM-. Collectively, these results show that the c143 TAA is not only a useful surface marker for identification of EBL but also a marker of differentiation of lymphoid cells.