The sheep erythrocyte receptor and both α and β chains of the human T-lymphocyte antigen receptor bind the mitogenic lectin (phytohaemagglutinin) from Phaseolus vulgaris

We have studied the interaction of mitogenic lectins such as phytohaemagglutinin (PHA) and concanavalin A (Con A) with both surface molecules which, by the use of monoclonal antibodies, are known to trigger T-cell mitogenesis. Monoclonal antibodies recognizing the T-lymphocyte receptor for antigen (Ti) and/or its associated structure, CD3, activate T cells. More recently, a second pathway of activation has been described which involves the sheep erythrocyte binding glycoprotein CD2, a surface molecule distinct from Ti-CD3. Lysates from surface-iodinated T-leukaemia cell lines were treated with lectin and affinity purified anti-lectin antibodies coupled to protein A-Sepharose. We have shown that eluates from Con A/anti-Con A or PHA/anti-PHA immunoprecipitates contained Ti, since a rabbit anti-Tα serum, which recognizes the native and denatured forms of the constant region of the α chain, immunoprecipitated Ti from these eluates. Furthermore, Ti immunoprecipitated by anti-Tα serum from lysates of surface iodinated E⁺ lymphocytes was binding to PHA after elution from the immunoprecipitate. When the purified Ti molecule was reduced and alkylated, allowing the permanent dissociation of its α and β subunits, PHA interacted with both chains, whereas anti-Tα serum immunoprecipitated the α chain only. Altogether, these results demonstrate that PHA interacts with both chains of the T cell receptor for antigen on human peripheral T lymphocytes. With the HPB-ALL tumour line, a similar approach showed that both α and β chains of Ti bind to Con A and Ulex europaeus 1 but not Helix pomatia. Affinity chomatography on immobilized lectins and immunoprecipitation with lectin/anti-lectin antibodies were employed to test whether CD2 binds to PHA and Con A. The results show that CD2 from human peripheral T lymphocytes binds both lectins but with a lower affinity for PHA than Con A.