Kinetic analysis of high affinity forms of interleukin (IL)-13 receptors: Suppression of IL-13 binding by IL-2 receptor gamma chain

Interleukin-13 (IL-13) is a pleiotropic cytokine that controls growth, diff erentiation, and apoptosis of immune and tumor cells. To understand the mec hanisms of interaction between IL-13 and IL-13 receptors (IL-13R), and the role of the IL-2 receptor common gamma chain (gamma(c)) in IL-13 binding an d processing, we have examined IL-13 binding kinetics, dissociation/sheddin g, and internalization in renal cell carcinoma (RCC) cell lines. We observe d a new phenomena in that the apparent rate of association, but not the dis sociation, was strongly related to IL-13 concentration. We also observed co operativity phenomena in IL-13 and IL-13R interaction in control RCC (MLneo ) cells, but not in cells transfected with gamma(c) chain (MLgamma c). The number of IL-13 binding sites, the effective rate of ligand association, an d the dissociation rate constants were reduced in gamma(c)-transfected cell s compared to control RCC cells. Two forms of IL-13R were detected in these cell lines, which differed in the kinetics of endocytosis and dissociation /exocytosis. Only a small fraction of bound receptors (14-24%) was rapidly internalized and the same fraction of the ligand-receptor complexes was she d and/or dissociated. The expression of gamma(c) chain did not change any o f these processes. A two independent high-affinity and moderate-affinity re ceptor model fit the kinetic observations in gamma(c)-transfected cells. Ho wever, in control cells, the binding kinetics were more complicated. A math ematical model that fit a set of kinetic and steady state data in control c ells was selected from a set of possible models. This best-fit model predic ts that 1) two different IL-13R are expressed on the cell membrane, 2) a mi nor fraction of IL-13R exist as microclusters (homodimers and/or heterodime rs) without exogenous IL-13, 3) high morphological complexity of the gamma( c) negative control cell membrane affects the cooperativity phenomena of IL -13 binding, and 4) a large number of co-receptor molecules is present, whi ch helps keep the ligand on the cell surface for a long period of time afte r fast IL-13 binding and provides a negative control for ligand binding via production of the high affinity inhibitor bound to IL-13. Our data demonst rate that gamma(c) exerts dramatic changes in the kinetic mechanisms of IL- 13 binding.