The monoclonal antibodies Trop-4 and 4F2 detect the same membrane antigen that is expressed at an early stage of lymphocyte activation and is retained on secondary lymphocytes.

The monoclonal antibodies Trop-4 and 4F2 recognize cell surface antigens present on peripheral blood monocytes, activated lymphocytes, and on continuous cell lines, but not on resting lymphocytes in blood. The membrane antigens detected by antibodies Trop-4 and 4F2 were compared by serial immunoprecipitations from membrane lysates of surface labeled T lymphoid cells and by parallel polyacrylamide gel electrophoretic analysis. It is shown that both to these antibodies recognize the heavy subunit of the heterodimeric membrane complex of an 85,000 m.w. glycoprotein disulphide linked to a light subunit of 41,000 m.w. The kinetics of the expression of the antigen was studied by indirect immunofluorescence on peripheral blood T lymphocytes during blast transformation induced by concanavalin A in vitro and during reversion of the lymphocytes back to small "secondary" lymphocytes. Upon activation of T lymphocytes with concanavalin A, the first blast cells staining with the antibodies appear within 6 hr after the initiation of the culture. After 18 hr, all blast cells displayed strong expression of the antigen. Inhibition of DNA synthesis by hydroxyurea treatment did not affect the early blast transformation and the expression of the antigen. When the mitogen-induced blast cells reverted back to small secondary lymphocytes during prolonged culturing for up to 18 days, these cells retained the expression of the antigen detected by antibodies Trop-4 and 4F2, whereas another membrane marker of activation, the transferrin receptor, rapidly disappeared. These findings demonstrate a phenotypical difference between primed, secondary T lymphocytes and resting, unstimulated cells.