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

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Abstract
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 eluatcs from Con A/anti‐Con A or PHA/anti‐PHA immunoprecipitates contained Ti. since;a rabbit anti‐Tα serum, which recognises the native and denatured forms of the constant region of the α chain, immuno‐precipitated Ti from these eluates. Furthermore, Ti immunoprecipitalcd bv anti‐Tα serum from lysates of surface iodinated E lymphocvtes 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 immunoprecipitalcd the α chain onlv. 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 europtaeus‐ 1 but not Helix pomatia. Affinity chomatographv on immobilized lectins and immunoprecipitation with lectin/anti‐leelin antibodies were employed to test whether CD2 binds to PHA and Con A. The results show that CD2 from human peripheral T lymphoevtes binds both lectins but with a lower affinity for PHA than Con A.

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