Target Antigen of Monoclonal Reagent S5.7: Comparison with T 3 Antigen

S5.7 recognizes a 20 kD cell surface protein which is present on T lymphocytes. S5.7 binds to a nonglycosylated protein, which can be labeled by cell-surface radioiodination and by a hydrophobic reagent [¹²⁵I]-iodo-5-naphthyl-1-azide (INA). As the T-lymphocyte-specific T₃ complex was found to contain a nonglycosylated 20 kD species, and since this 20 kD T₃ form can be labeled preferentially by INA,⁽¹⁾ a comparison between T₃ and S5.7 was made. Isoelectric focusing experiments showed, however, that the two proteins are different. Moreover, the S5.7 monoclonal antibody does not block CML, is not mitogenic, reacts with immature cells of several hemopoietic lineages, and differs in that respect from anti-T₃ monoclonal antibodies.