A monoclonal mouse anti-human IL-2 receptor antibody (anti-Tac) will recognize molecules on the surface of Xenopus laevis immunocytes which specifically bind rIL-2 and are only slightly larger than the human Tac protein

Abstract We have made visible the binding of a mouse monoclonal anti-human interleukin 2 ( IL-2) receptor(anti-Tac) antibody on the surface of phytohemagglutinin(PHA)-stimulated Xenopus thymocytesusing a colloidal gold-conjugated goat anti-mouse antibody and transmission electron microscopy. No binding was found when a different mouse monoclonal antibody (mAb) of the same isotypeand subclass was tested, or when the anti-Tac antibody was omitted from the procedure. After metabolic radiolabeling of the IL-2 receptorswith [ 35 S]methionine using PHA-stimulated thymocytesof Xenopuslaevis , the South African clawed toad, we show that a concentrated preparation of the mouse anti-human Tac antibody will immunoprecipitate a radiolabeled molecule just slightly larger than 55 kDa. Phorbol dibutyrate (PDB), an effective T cell mitogen, and cyclosporin A, an inhibitor of T cell mitogenesis in this species, are both capable of regulating the expression of this IL-2-binding molecule on Xenopusimmunocytes [1]. Here, we use the calcium ionophore A23187 to show that the relationship between IL-2 receptorexpression and mitogenesis, which was previously established in X. laevis [1], is associated with a calciumion flux. Flow cytometry is used for assaying alterations in epitope expression after binding the lectin-stimulated cells under test with a fluorescence (Fl * ) conjugate of the anti-Tac antibody or a control mAb, which is either anti-DNP or anti- keyhole limpet hemocyanin(KLH) in specificity, but of the same mouse isotypeand subclass as the anti- IL-2 receptorantibody.