Toad splenocytes bind human IL-2 and anti-human IL-2 receptor antibody specifically

Human r-DNA IL-2 and fluorescent (Fl) mouse anti-human IL-2 receptor antibody have been tested separately and in competition with each other for their capacities to bind to the splenocytes of Xenopus laevis, the South African clawed toad. Binding by Fl*-mouse anti-DNP antibody of the same subclass (IgG1, kappa) was used as a control. The results of visual tests using rIL-2 coated fluorescent Covaspheres demonstrate that the human mediator will bind cells of the toad spleen. Moreover, the mediator inhibits binding of the antibody against the human IL-2 receptor, as detected by cytofluorimetry. Some of the IL-2 receptors on the toad cells appear to be constitutive, since they are expressed on freshly biopsied lymphocytes. Activation of these cells in vitro will increase the percentage of those cells able to bind both the anti-receptor antibody and rIL-2. Since the human mediator is only able to modulate in vivo immune activity in antigen-activated toads, it appears that in spite of having some constitutive IL-2 receptors, a quantitative increase in receptor expression is required before immunological behavior can be effected. More stringent controls of receptor expression may have provided an additional regulatory level as mammalian mechanisms evolved.