Individual Epitopes of an 85,000 MW Membrane Adherence Molecule Are Variably Expressed on Cells of Different Lineage

The recently reported 85,000 MW membrane activation antigen, provisionally termed 7F7 antigen, is involved in the initiation of T-cell responses, in the binding of C3b-coated sheep erythrocytes to B cells and in the phorbol ester-induced aggregation of U937, suggesting that it represents a membrane molecule important for cell-cell adherence. Three new monoclonal antibodies were raised against the purified antigen and used to examine the expression of individual epitopes on cells of different lineage. All antibodies reacted with the purified membrane antigen on Western blots. Antibodies 7G2 and 7C6 reacted only weakly with activated T cells, although the 85,000 MW 7F7 antigen is recognized on T cells by antibodies 7F7 and 8B9. Whereas 7F7 and 8B9 inhibit the phorbol ester-induced aggregation of U937 and the attachment of phytohaemagglutinin (PHA)-activated T cells to fibroblasts, antibody 7G2 was without inhibitory effect. Four antibodies raised against the P3.58 glycoprotein, a melanoma-associated antigen with biochemical features similar to the 7F7 antigen were shown to react with the purified 7F7 antigen on Western blots. These four antibodies also failed to react with activated T cells. In addition, 7F7 bound to a L cell transfectant prepared with human chromosomal DNA which expressed the P3.58 antigen. The results indicate that the 7F7 antigen and the P3.58 melanoma antigen are identical, that some epitopes of this adherence molecule are differentially expressed on individual cell types and that one epitope, which is only weakly expressed on activated T cells, does not contribute to the role of the 7F7 antigen in some examples of cell-cell adherence.