Characterization of the bovine receptor(s) for interleukin-2

The binding properties of the bovine receptors (IL-2R) for the lymphokine interleukin-2 (IL-2) have been examined using activated bovine lymphoid cells and human recombinant [125I] IL-2. The results of these binding studies indicate that the bovine IL-2R in many ways resembles the receptors for human and mouse IL-2, but with some differences. Equilibrium binding experiments revealed the presence of two classes of bovine IL-2R, one with a KD approximately 20 pM, representing approximately 400-1300 sites per cell, the other with a KD approximately 6 nM, representing approximately 20,000-50,000 sites per cell. A study of the time course of IL-2R appearance on the cell surface of activated bovine lymphocytes showed that both high- and low-affinity receptors appear rapidly following stimulation, with maximum levels of expression being reached within about 48-96 hr. Lymphoid cell proliferation, as monitored by [3H]thymidine [( 3H]TdR) incorporation, increased in parallel with the expression of high-affinity IL-2R. Measurements of the association/dissociation kinetics showed that IL-2 binds to (t1/2 approximately 10 seconds), and dissociates from (t1/2 approximately 20 seconds) the low-affinity bovine IL-2R very rapidly. In contrast, IL-2 binds rapidly to (t1/2 approximately 40 seconds), but dissociates slowly from (t1/2 approximately 8.5 hr) the high-affinity bovine IL-2R. In previous work, our laboratory has molecularly cloned the cDNA coding for the bovine IL-2 and IL-2R (p55, Tac) proteins. Comparisons of the deduced amino acid sequences of these bovine proteins with those of the homologous human and mouse proteins revealed a high degree of evolutionary conservation among these mammalian proteins. Our present IL-2/IL-2R binding studies are also consistent with such a close evolutionary relationship, but leave unresolved the molecular basis for the previously observed species specificity of the bovine IL-2/IL-2R receptor-ligand system.