Biochemical identification of a direct physical interaction between the CD4: p56^lck and T_i(TcR)/CD3 complexes

The CD4 and CD8 antigens function in synergy with the TcR/CD3 complex in the generation of intracellular signals leading to T cell proliferation. The association of the protein‐tyrosine kinase p56^lck with CD4 and CD8 provides a potential mechanism in the generation of intracellular signals. Several studies have shown that CD4 can co‐modulate with TcR/CD3 suggesting that these receptor complexes may associate on the surface of the T cell. Nevertheless, it has proven difficult to formally demonstrate a direct physical interaction between the CD4 and TcR/CD3 complexes using biochemical techniques. In this study, we have used the sensitivity of the in vitro kinase assay to show a direct physical linkage between the CD4: p56^lck complex and various CD3 subunits. Immunoprecipitation of CD4 from cell lysates derived from the T lymphoblastoid line HPB‐ALL results in the co‐purification of p56^lck with an additional polypeptide at 20 kDa. Re‐precipitation analysis and isoelectric focusing demonstrated that this band corresponds to the CD3ϵ chain. An alternative approach which involves the labeling of microsomal membranes with [γ⁻³²P]ATP revealed the presence of CD3ϵ and ζ chains in anti‐CD4 immunoprecipitates. By contrast, we were unable to demonstrate the association of the CD4: p56^lck and TcR/CD3 complex in resting peripheral blood lymphocytes. These data indicate that the CD4: p56^lck and TcR/CD3 complexes have the ability to form stable complexes on the surface of certain T cell lines.