Metabolism of Tac (IL2Ralpha): physiology of cell surface shedding and renal catabolism, and suppression of catabolism by antibody binding.

Abstract

SummaryThe interleukin 2 receptor 0t (IL2Rcl; CD25; Tac) is the prototypic model for soluble receptor studies. It exists in vivo as a transmembrane complete molecule (TM-Tac) on cell surfaces and as a truncated soluble form (sTac; slL2Rot), sTac has been used as a serum marker ofT cell activation in immune disorders and of tumor burden in Tac-expressing malignancies. In vivo, serum levels of all soluble proteins depend on the balance between production and catabolism, but little is known about the metabolic features of this class of molecules. We have developed a model for Tac metabolism that incorporates new insights in its production and catabolism. Tac was shed from the surface of malignant and activated human T cells with a modal half-life (tl/2) of 2-6 h, but which was prolonged under certain circumstances. The rate of shedding is first order overall and nonsaturable over a two order of magnitude range of substrate (TM-Tac) expression. Once shed from cells, sTac is subject to catabolic activities in the host. In vivo studies in mice showed that 90% ofsTac was catabolized by the kidney with a h/2 of 1 h and a filtration fraction of 0.11 relative to creatinine. The remaining 10% of catabolism was mediated by other tissues with a tl/2 of 10 h. Approximately 1-3% ofsTac is excreted intact as proteinuria with the remaining 97-99% catabolized to amino acids. Antibody to the receptor induced a marked delay in sTac catabolism by preventing filtration of the smaller protein through the renal glomerulus and additionally suppressing other nonrenal catabolic mechanisms. A discrepancy between the catabolic rates for Tac and anti-Tac in the same complex was interpreted as a previously unrecognized differential catabolic mechanism, suggesting features of the Brambell hypothesis and immunoglobulin G transport and catabolism, in which the antigen-in-complex in intracellular vesicles is relatively less protected from catabolism than the associated antibody. In light of the pivotal role played by the kidney in sTac catabolism and the impact of administered antibody, the serum concentration of Tac in the settings of renal dysfunction or antibody therapy is not a suitable surrogate of activated T cells or of the body burden of tumor. These results provide parameters for assessing soluble receptor-ligand interactions generally.

Soluble interleukin-2 receptor and metalloproteinase-9 expression in head and neck cancer: prognostic value and analysis of their relationships

“…However, in two models, a leukaemic cell line which expressed and spontaneously released CD25 and PBMC activated by PHA, we failed to demonstrate a physiological role of MMP-9 in the cleavage of IL-2Ra using a specific inhibitor of MMP-9 at doses up to 800 times the IC50 of the inhibitor. A previous study by Junghans et al also reported that zinc chelators known to inhibit metalloproteinase activity [60] did not influence shedding of sIL-2Ra [61]. The absence of change in sIL-2Ra release in MMP-9-deficient mice also supports these results, as although it does not formally exclude a physiological role of MMP-9 in the regulation of sIL-2Ra, it demonstrates that other proteases may replace MMP-9 for cleavage of sIL2Ra.…”

Impact of antigenemia on the bioactivity of infused anti-Tac antibody: Implications for dose selection in antibody immunotherapies

“…Our work with animal models showed that Tac is catabolized rapidly in vivo by mainly renal mechanisms, and this catabolism is delayed markedly by antibody binding with no impact on survival of the antibody itself (10). Therefore, the progressive decline of antibody bioactivity in these patients implies a continued production of antigen during therapy that can be judged by the time course of the bindability change.…”

“…k ϭ ln2͞t 1͞2 is the antibody beta phase rate constant for IgG, which is undisturbed by antigen binding (10) and provisionally is assigned also as the clearance rate constant for antigen bound to antibody. A 0 and B 0 are moles of A and B at time ϭ 0. q, the sTac production rate, is selected to provide the best fit of the data.…”

“…Tac is up-regulated markedly during normal T cell activation and in several immune disorders and malignant states (8). Accompanying this upregulation is a concomitant active shedding of Tac from the membrane surface (7,9,10). In normal physiology, this shedding of receptors is thought to mediate rapid down-regulation of activated T cells when the activating stimulus is removed.…”

Epitope Blocking: Positive and Negative Effects on the Biodistribution of 125I‐Labeled Anti‐Tac Disulfide‐stabilized Fv Fragment of Two Antibodies against Different Epitopes of the Circulating Antigen

“…2 was subjected to HPLC analyses. The amount of radioactivity in each fraction was plotted for mice receiving -----------------------lism of sIL-2Rα, 48) in which administration of anti-Tac to mice with increased sIL-2Rα levels in serum resulted in longer retention of the sIL-2Rα, but did not decrease the half-life of the anti-Tac IgG. Furthermore, in our previous study we demonstrated that preinjection of HuTac resulted in formation of complexes with sIL-2Rα that competed with and prevented the binding of the radiolabeled dsFv to sIL-2Rα.…”