IL-2 Internalization Research Articles

Low-Dose Cyclophosphamide and Continuous-Infusion Interleukin-2 with Famotidine in Previously Treated Metastatic Melanoma or Kidney Cancer

The cytotoxic ability of T-cells against tumor cells may be increased by interleukin (IL)-2. The infiltration of tumors by these cytotoxic T-cells may be enhanced by low-dose cyclophosphamide, which may also serve to deplete regulatory T-cells. Famotidine may increase IL-2 internalization by the IL-2 receptor on T-lymphocytes. We have treated 14 patients with either metastatic melanoma or kidney cancer, using CY 350 mg/M(2) intravenously (i.v.) over 1 hour followed by a continuous infusion IL-2 9 MIU/M(2)/24 hours for 72 hours and famotidine 20 mg i.v. twice per day. All patients had received prior systemic therapy. Cycles were repeated every 4 weeks until disease progression. 8 with melanoma, 8 males, median age, 64, median Eastern Cooperative Oncology Group performance status, 1; most common metastatic sites: lungs, lymph nodes, bone, and liver. Median number of cycles received=2 (range, 2-4). Most common toxicities were fever, nausea/emesis, hypomagnesemia, hypophosphatemia, hyponatremia, and rigors. All patients were treated on an oncology inpatient unit. One (1) patient with kidney cancer has had a partial response in lung and lymph nodes for 5 months, while 1 patient with melanoma had a partial response in pulmonary metastases. Cyclophosphamide and IL-2 with famotidine has evidence of antitumor activity in previously treated kidney cancer and melanoma.

Anti-CD25 Targeted Killing of Bicistronically Transduced Cells: A Novel Safety Mechanism Against Retroviral Genotoxicity

Gene therapy for Fabry disease, a deficiency in alpha-galactosidase A (alpha-gal A) activity, has the potential to provide a cure for the disorder with a single treatment. Despite modifications to existing vectors, concerns have arisen regarding the risk of genotoxicity associated with the use of retroviruses. To address safety concerns, we propose that expression of a cell surface protein, human CD25 (huCD25) in a bicistronic format, with any therapeutic gene such as alpha-gal A can provide a target that can be used to kill transduced cells selectively should transformative events occur. We show that an anti-CD25 antibody and immunotoxin can specifically target and eliminate transduced leukemia cells expressing CD25. In a murine leukemia model, antibody treatment reduced tumor burden 32-fold and increased survival compared with untreated mice. Furthermore, after a bone marrow transplant of therapeutically transduced cells into Fabry mice, antibody treatment reduced the number of retrovirally transduced huCD25-expressing cells in the peripheral blood. A systemic loss of transduced cells with functional consequences was also evident in the liver and spleen. This proof-of-principle study demonstrates that a targeted antibody can reduce tumor burden and selectively clear bicistronically transduced hematopoietic cells that express a target antigen, thus acting as a built-in safety mechanism.

Anti-CD25 Targeted Killing of Bicistronically Transduced Cells: A Novel Safety Mechanism Against Retroviral Genotoxicity

Anti-CD25 Targeted Killing of Bicistronically Transduced Cells: A Novel Safety Mechanism Against Retroviral Genotoxicity

Efficient internalization of IL-2 depends on the distal portion of the cytoplasmic tail of the IL-2R common gamma-chain and a lymphoid cell environment.

The common gamma-chain (gammac), a subunit of the IL-2R, is essential for high affinity ligand binding and signal transduction due to Jak3 association to gammac. Another consequence of IL-2/IL-2R interaction is rapid receptor-mediated endocytosis of the receptor-ligand complex. In the present study, we establish that this rapid endocytosis of IL-2 in a T cell tumor line is dependent upon the cytoplasmic tail of gammac. Deletion mutants of the cytoplasmic tail mapped this activity to 9 aa of gammac, 45-54 aa distal to the transmembrane region. In contrast, ligand-independent constitutive endocytosis of gammac occurred more slowly and was dependent upon a PEST sequence in a more membrane-proximal region of the cytoplasmic tail of gammac. Thus, this receptor subunit may use distinct sorting signals for its constitutive regulation and ligand-induced endocytosis. Rapid endocytosis of IL-2 was inhibited by the tyrosine kinase inhibitor genistein, implicating a role for a signal transduction pathway in IL-2 internalization. However, one T cell line bearing a mutant gammac exhibited impaired endocytosis of IL-2, despite normal IL-2-induced Jak/STAT activation. Furthermore, inefficient endocytosis of IL-2 was noted after transfection of the COS7 epithelial cell line with the IL-2R, and further reconstitution of these cells with Jak/STAT proteins did not enhance this internalization. Collectively, these latter findings indicate that rapid endocytosis of IL-2 is dependent upon cellular signaling in lymphoid cell environment that is not solely a consequence of the presence of the Jak/STAT pathway.

Synthetic Peptides from the Region 30-54 of the Interleukin-2 Receptor β-Chain (IL-2Rβ)Affect IL-2 Binding to Human T-Cells

The interleukin-2 receptor β-chain (IL-2Rβ) is responsible for the IL-2 internalization and signal transduction. To identify domains from the IL-2Rβ extracellular region which are involved in ligand receptor interactions, eight peptides of 8 to 25 amino acids corresponding to IL-2Rβ sequence 30-54 were synthesized. Their biological effect was tested with [125I]IL-2 equilibrium binding to PHA-blasts. Preincubation of [125I]IL-2 with peptides IL-2Rβ (35-54) and (30-42) affects IL-2 binding to both high and intermediate affinity receptors by revealing a greater number of active sites with lower affinity.

Requirement of thiol compounds as reducing agents for IL-2-mediated induction of LAK activity and proliferation of human NK cells.

Thiol-related compounds, such as L-cystine, 2-ME or reduced glutathione (GSH), are important in many lymphoid cell activation pathways. We investigated their role in IL-2-generated lymphokine-activated killer (LAK) activity in human NK (CD16+ and/or CD56+) cells. Depletion of GSH and L-cystine, but not L-leucine or glycine, from medium used during culture of NK cells with IL-2 inhibited LAK and proliferative activities, whereas IL-2-independent lytic function of NK cells remained intact. Addition of L-cysteine, 2-ME or GSH, but not methylated analogs of these compounds (which cannot function as proton donors) to L-cystine/GSH-depleted medium restored proliferative response of NK cells and LAK generation. In the presence of L-buthionine-(S,R)-sulfoximine, an inhibitor of GSH synthesis, IL-2-induced LAK activity and proliferation of NK cells in medium without L-cystine and GSH, could be restored, at least in part, by addition of GSH, but not 2-ME or L-cystine. Furthermore, intracellular GSH levels were depressed in cells cultured in L-cystine/GSH-depleted medium but could be restored by the addition of 2-ME. The results suggest that (1) L-cystine or thiol containing compounds such as L-cysteine, 2-ME, or GSH are necessary for effective IL-2-activation of human NK cells, (2) these compounds must be functional proton-donors, i.e., reducing agents, implying regulation of the IL-2 activation pathway by oxidation-reduction, and (3) GSH synthesis is necessary for the activation. Experiments were performed to begin to dissect the redox-sensitive step(s) in LAK development. Depletion of reducing agents had no effect on internalization of rIL-2. In contrast, intracellular granzyme A activity was significantly depressed in NK cells cultured with rIL-2 in L-cystine/GSH-depleted medium compared with those cultured in medium in which L-cystine levels had been replenished. The findings suggest that step(s) in the transduction of the IL-2 signal in NK cells, between the internalization of IL-2 and the maturation of the lytic mechanism, are subject to regulation by oxidation-reduction.

Cloning of the Mouse Interleukin 2 Receptor γ Chain: Demonstration of Functional Differences Between the Mouse and Human Receptors

We isolated a cDNA clone for the γ chain of the mouse interleukin 2 receptor. Introduction of the mouse γ chain cDNA clone into a mouse fibroblast cell line, L929, expressing the mouse αβ heterodimer IL-2 receptor converted pseudohigh affinity of the IL-2 receptor into functional high, resulting in internalization of IL-2 and induction of the c-myc, cfos and c-jun genes. The mouse βγ heterodimer, however, failed to bind IL-2 unlike the human βγ heterodimer intermediate-affinity receptor. These results indicate that the mouse functional IL-2 receptor is a complex comprising three distinct subunits, α, β and γ chains, but the βγ heterodimer is not functional and different from the human heterodimer.

Selenium Supplementation Enhances the Expression of Interleukin 2 Receptor Subunits and Internalization of Interleukin 2

Selenium (Se) is an essential nutritional factor that was shown previously by us to alter the kinetics of expression of high affinity (p55/p75) interleukin 2 receptors (IL-2R). This study shows that dietary (2 ppm for 8 weeks) or in vitro (1 x 10(-7) M) supplementation with Se (as sodium selenite) results in a significant upregulation of the expression of both the p55 and p70/75 IL-2 binding sites on the surface of concanavalin A-stimulated lymphocytes from C57BL/6J mice. This resulted in the formation of significantly higher numbers of high affinity IL-2R/cell with preservation of the normal ratio of high affinity to total IL-2 binding sites/cell. The high affinity IL-2R on cells from Se-supplemented animals functioned normally in terms of ligand binding and kinetics of IL-2 internalization, but their greater numbers/cell resulted in the internalization of significantly larger amounts of IL-2/cell. As Se supplementation results in an earlier expression of greater numbers of high affinity IL-2R, the presence of Se in the cell environment can result in an accelerated clonal expansion of activated lymphocytes.

The interleukin-2 receptor gamma chain maps to Xq13.1 and is mutated in X-linked severe combined immunodeficiency, SCIDX1.

The gene encoding the gamma chain of the lymphocyte interleukin-2 receptor has been cloned and shown to be required to associate with the beta chain in order for IL-2 internalization and cell activation to occur (1). We considered this gene, IL2RG, a candidate for the X-linked form of severe combined immunodeficiency at the SCIDX1 locus, in which affected males have impaired lymphocyte development. Using fluorescence in situ hybridization and PCR amplification of somatic cell hybrid DNAs, we mapped IL2RG to human Xq13.1, a location within the SCIDX1 critical region established by linkage analysis. The 4.2 kb IL2RG gene was sequenced, and its genomic organization was elucidated. Seven of 19 transformed B-lymphocyte cell lines with independent SCIDX1 mutations had absent or minimal IL2RG mRNA. Unique point mutations were documented to be specifically associated with the disease and the carrier state in four unrelated affected males and their family members: one in a boy with no detectable IL2RG mRNA, in which the mutation ablated a splice donor site; one causing premature chain termination; and two causing distinct amino acid changes. The demonstration of impaired IL2RG mRNA expression in males with X-linked SCID and of unique point mutations in SCIDX1 pedigrees constitutes powerful evidence that the SCIDX1 gene is IL2RG. Noguchi et al. (2) have independently published IL2RG mapping to Xq13 and discovery of mutations in three affected males. The specific pathogenesis of IL2RG mutations and approaches to gene therapy can now be addressed in the X-linked form of SCID.

Effects of glutathione on the synthesis and turnover of interleukin-2 receptors

Internalization of IL-2 is important for its biological activities. The internalization of IL-2 was regulated by the duration of glutathione (GSH) treatment in CTLL-2 and CT-4R cells. Flow cytometric studies showed that the level of surface IL-2 receptors was not increased by GSH treatment. Northern blot analysis showed that the mRNA of IL-2Rp55 and IL-2Rp70, the two major components of the high-affinity IL-2 receptors, was increased 6 hr after GSH treatment. The appearance rate of membrane IL-2 receptors in GSH-treated cells was faster than that of the untreated cells. GSH also shortened the half-life (from 5 to ⩽3 hr) and thus increased the turnover of the surface high-affinity IL-2 receptors. These results suggest that although GSH does not affect the level of surface IL-2 receptors, GSH may regulate the internalization of IL-2 by enhancing the synthesis and turnover of surface IL-2 receptors.

IL-2-dependent proliferation of murine T cells requires expression of both the p55 and p70 subunits of the IL-2 receptor.

An IL-4-dependent T cell clone (LD8) was isolated from the murine IL-2-dependent cytotoxic T cell line C30.1. This clone has lost the capacity to proliferate in response to IL-2 after long-term culture in IL-4. LD8 cells express the p70, but not the p55, subunit of the IL-2R on their cell surface. The number of p70 IL-2R molecules on LD8 cells is comparable with the number of high-affinity IL-2R on the parental C30.1 cell line. LD8 cells can efficiently internalize IL-2 through the p70 IL-2R subunit. Following stimulation by IL-2, LD8 cells up-regulate p70 IL-2R mRNA, but do not express p55 IL-2R mRNA. IL-2-dependent proliferation of LD8 cells was reconstituted after introduction and expression of a human p55 IL-2R cDNA. To further investigate the role of p70 IL-2R, we have measured IL-2-induced proliferation of C30.1 cells in the presence of three anti-p55 IL-2R mAb (5A2, PC61, and 7D4) that recognize different epitopes. Under the experimental conditions used, the combination of anti-p55 IL-2R mAb prevents the formation of high-affinity IL-2R, but does not affect the binding of IL-2 to p70 IL-2R or IL-2 internalization. However, these three mAb inhibit proliferation of C30.1 cells even in the presence of IL-2 concentrations sufficient to saturate p70 IL-2R. Together these results demonstrate that p70 IL-2R alone is not sufficient to transmit IL-2-induced growth signals and that formation of p55-p70 IL-2R complex is required for IL-2-dependent proliferation of murine T cells.

Characterization of the heterodimeric complex of human IL-2 receptor alpha.beta chains reconstituted in a mouse fibroblast cell line, L929.

The receptor for IL-2 has been known to exist in three forms on the basis of their affinities to IL-2: high, intermediate, and low affinity forms. Two IL-2R components have been identified as IL-2R alpha (p55, Tac Ag) and IL-2R beta (p70-75) chains, both bind IL-2 with low and intermediate affinities, respectively. Recently, we cloned human IL-2R beta chain cDNA and demonstrated that the cDNA product binds IL-2 with intermediate affinity and forms high affinity IL-2R with coexpressed IL-2R alpha chain in a human T cell line, Jurkat. In this study, we report the establishment of the mouse fibroblast transformants expressing either the IL-2R beta chain alone or both the IL-2R alpha and IL-2R beta chains. In contrast to lymphoid cells, significant IL-2 binding was not detected in the transformants expressing the IL-2R beta chain alone at IL-2 concentrations (50 pM to 10 nM) generally utilized. Nonetheless, the transformants expressing both IL-2R alpha and IL-2R beta chains displayed two forms of the IL-2R with high and low affinities to IL-2. However, neither IL-2 internalization nor signal transduction via the high affinity IL-2R complex were observed in the L929 transformants. Those findings suggest that the interaction of the IL-2R beta chain with the IL-2R alpha chain occurs in the absence of additional lymphoid specific component(s) to form high affinity IL-2R, but that this interaction is insufficient for IL-2 internalization and signal transduction just as observed in lymphoid cells. The experimental approach described here may allow further dissection of the molecular architecture of the IL-2R complex in the ligand binding, internalization, and signal transduction.

IL-5 up-regulates but IL-4 down-regulates IL-2R expression on a cloned B lymphoma line.

Both IL-4 and IL-5 demonstrate B cell growth activity. IL-5 can render a cloned neoplastic B cell line, BCL1-CL-3 cells, responsive to IL-2, whereas IL-4 has no such activity. The response to IL-5 and IL-2 proceeds in two phases: the first phase which clearly depends upon IL-5 is the obvious increase in number of high affinity IL-2R (3.1-fold) with modest increase of low affinity IL-2R (1.2-fold) and gain of the ability of facilitated IL-2-binding and internalization of IL-2, and the second phase which is induced by IL-2 in the IL-5-stimulated CL-3 cells comprises the striking increase of low affinity IL-2R (8.5-fold). Kinetic study has revealed that high affinity IL-2R expressed on CL-3 cells begins to increase at 6 h and reaches to maximum at 12 h after stimulation with IL-5 or IL-5 plus IL-2, whereas low affinity IL-2R expression increases at 18 h and becomes maximal at 24 h after stimulation of CL-3 cells with IL-5 and IL-2. However, in the presence of IL-4, IL-5 cannot induce an increase in number of high affinity IL-2R on CL-3 cells. Thus, CL-3 cells stimulated with the mixture of IL-5 and IL-4 cannot respond to IL-2, and fail to show up-regulated expression of low affinity IL-2R. IL-4 also has a capacity to modestly interfere with the action of IL-2 to up-regulate low affinity IL-2R expression on IL-5-pretreated CL-3 cells. Thus, this monoclonal B cell system provides an excellent model system to define the roles of IL-5 and IL-4 involved in the B cell differentiation and to characterize the properties of B cells competent to IL-2 stimulation and the signal transduction mechanism which operates through IL-2/IL-2R system.

Phorbol myristate acetate induces both high affinity and low affinity interleukin 2-receptors on a pre-B leukemic cell line

The human pre-B leukemic cell line Reh6 does not constitutively express the Tac molecule (p55), which is one of the two polypeptide chains of the interleukin 2-receptor (IL2-R). Cell incubation with Phorbol Myristate Acetate (PMA) induces expression of the Tac molecule in a dose and time-dependent manner. Binding experiments with radiolabelled recombinant IL2 (rIL2) revealed both high and low affinity IL2-R (225 ± 105 sites/cell with a K d of 130 ± 51 pM and 24060 sites/cell with a K d of 17.3 nM respectively), whereas unstimulated Reh6 cells only expressed intermediate affinity IL2-R (58 sites per cell with a K d of 460 pM). Cross-linking of radiolabelled rIL2 to PMA-incubated Reh6 cells revealed the presence of two polypeptide chains of mol. wts 55,000 (Tac molecule) and 70,000, as in normal activated T cells, while the 70,000 mol. wt chain alone was observed in unstimulated Reh6 cells. IL2-R-bearing Reh6 cells could absorb rIL2 in a dose-dependent manner and this absorption was inhibited by a monoclonal antibody against the Tac molecule (anti-Tac). Moreover, partial internalization of IL2 bound under high affinity conditions occurred at 37°C. IL2-R expressed on PMA-induced cells were functional since rIL2 specifically enhanced the proliferation in vitro of PMA-treated cells in semi-solid but not liquid cultures. These findings thus demonstrate an IL2-dependent mechanism of proliferation in vitro of pre-B leukemic cells induced by PMA, which can express high affinity, functional IL2-R.

Interleukin 2 receptors

Interleukin 2 receptors (IL-2-R) are composed of (at least) two chains: the α chain (or p55 or Tac) and the β chain (or p70 or p75). The association of both chains constitutes high affinity IL2-R complexes ( K d ⋍ 10–100 pM ), whereas the α-chain or the β chain alone bind IL-2 with low ( K d ⋍ 5–50 nM ) and intermediate ( K d ⋍ 1 nM ) affinities, respectively. The β chain (but not the α chain) mediates IL-2 internalization and signal transduction (proliferation or differentiation) to the cell, thus only high and intermediate affinity IL-2-R are functional. High affinity IL-2-R are expressed on activated normal and leukemic T and B lymphoid cells. Low affinity IL-2-R (Tac alone) are seen on stimulated T cells, B cells, and monocytes, as well as on various leukemic cells. The p70 chain alone is constitutively expressed by subsets of normal resting T and B cells, LGL and NK cells and we have recently demonstrated its presence on leukemic cells from various hemopoietic lineages. The finding of IL-2-R on non-lymphoid cells may disclose new functions for IL-2.

Trypanosoma cruzi reduces the number of high-affinity IL-2 receptors on activated human lymphocytes by suppressing the expression of the p55 and p70 receptor components.

We previously established that Trypanosoma cruzi, the causative agent of Chagas' disease, has the ability to suppress expression of the p55 component of the IL-2R by activated human PBMC. We explored in this work whether the parasite alters the expression of high affinity IL-2R, responsible for the internalization of IL-2 and signal transduction. Radiobinding measurements revealed that the trypanosome indeed inhibited the expression of high affinity IL-2R. Thus, a considerably smaller number of 125I-IL-2 molecules was necessary to saturate the IL-2R on PHA-stimulated PBMC cocultured with T. cruzi than those of control PBMC that had not been exposed to the organisms. Scatchard analysis of equilibrium binding data showed that, in the presence of T. cruzi, the number of high affinity IL-2R per cell was reduced by approximately 80%. The Kd for IL-2 binding to the fewer IL-2R expressed on PBMC exposed to T. cruzi was not significantly different from that of IL-2R on nonsuppressed PBMC. Independent measurements made after cross-linking 125I-IL-2 to its specific receptors with disuccinimidylsuberate showed that both the p55 and p70 components of the IL-2R were markedly suppressed and to comparable extents. These results demonstrate for the first time that T. cruzi suppresses the expression of high affinity IL-2R by human cells, including the p70 chain of the heterodimeric IL-2R. It is noteworthy that the in vitro model system we used in this work to study the mechanisms whereby T. cruzi may induce the immunosuppression that accompanies acute Chagas' disease also lends itself to the exploration of the regulatory mechanisms governing the expression of IL-2R by human PBMC.

Interactions between receptors for interleukin 2 and interleukin 4 on lines of helper T cells (HT-2) and B lymphoma cells (BCL1).

IL-2 and IL-4 induce a synergistic proliferative response in HT-2 cells, suggesting that IL-2Rs and IL-4Rs may interact. The purpose of this study was to examine the effect of IL-4 on the expression and function of IL-2Rs. Preincubation of HT-2 and BCL1-3B3 cells with IL-4 for 60 min at 4 degrees C or 37 degrees C resulted in a partial decrease in the number, but not the affinity of high affinity IL-2Rs as evidenced by Scatchard analysis of binding data. The decrease in the number of high affinity receptors correlated with decreased internalization of IL-2. After preincubation with IL-4, crosslinking of 125I-IL-2 to high affinity IL-2Rs also demonstrated a approximately 50% reduction in the number of high affinity IL-2Rs. Another lymphokine, IL-1, which acts on HT-2 cells, had no measurable effect on the affinity or number of IL-2Rs. Taken together, these results indicate that IL-4 downregulates the expression of high affinity IL-2Rs on some cells. It is not known whether this occurs by a direct ligand-mediated receptor interaction, by the sharing of a common receptor subunit, or by interaction of the two receptors with another membrane molecule or cytoskeletal component.

Age-related changes of expression of IL-2 receptor subunits and kinetics of IL-2 internalization in T cells after mitogenic stimulation

The age-associated changes of the expression of IL-2 binding molecules p55/ Tac(α chain) and p70/75(β chain) were examined after phytohemagglutinin (PHA) stimulation. The expressions of both p55/Tac molecules and p70/75 molecules were significantly reduced in the aged compared with those in the young persons. The amounts of p55/Tac and p70/75 molecules on T cells from the aged were 55% and 59% of those on young ones, respectively. The ratio of the amount of p70/75 to that of p55/Tac in aged T cells was 0.28 and that in young ones was 0.26. The ratio was somewhat higher in the aged but not significantly. We also examined the kinetics of IL-2 internalization mediated by its receptor. The calculated t 1 2 of receptor-mediated IL-2 internalization was 17 min in the aged and 16 min in the young, respectively. There was no kinetic difference between the 2 groups. The percentage of the internalized IL-2 to the sum total was 58.2% in the aged and 73.4% in the young ( P < 0.02). The amount of internalized IL-2 in T cells from the aged was 48.6% of that from the young ( P< 0.01).

Novel receptor-mediated internalization of interleukin 2 in B cells.

Abstract Novel IL-2-binding molecules (p70 and p75) mediating internalization and degradation of IL-2 were examined by employing a human B lymphoblastoid line, SKW6-4 cells. High concentrations of IL-2 induced IgM secretion in these cells through a receptor distinct from Tac antigen. The acid-wash technique revealed that more than 60% of 125I-labeled IL-2 bound to the cells became acid-unremovable in the first 30 min of incubation at 37 degrees C and degradation products of 125I-IL-2 increased after 30 min of incubation. Treatment of the cells with NaN3 buffer inhibited the appearance of acid-unremovable 125I-IL-2, suggesting that acid-unremovable 125I-IL-2 was not due to fluid-phase pinocytosis but due to internalization. Loss of labeled bands by incubation of cells with 125I-IL-2 at 37 degrees C before affinity cross-linking demonstrated that 125I-IL-2 was internalized via novel IL-2-binding molecules. These results suggest that novel IL-2-binding molecules are responsible for internalization and may mediate signal transduction in B cells in the absence of Tac antigen.

Internalization of interleukin 2 is mediated by the beta chain of the high-affinity interleukin 2 receptor.

High-affinity IL-2-R correspond to a membrane receptor complex composed of two different IL-2-binding proteins, the Tac antigen (alpha chain) and a 70-75 kD beta chain. Using cell lines that express either the alpha or the beta protein, we demonstrate that IL-2 internalization occurs when ligand is bound to the isolated beta chain, but not when it is bound to the isolated alpha chain. The kinetics of IL-2 internalization mediated by the intermediate-affinity beta chain were nearly identical to those of the high-affinity alpha/beta heterodimer (t1/2 of 10-15 min), and each type of receptor targeted the bound IL-2 for intracellular degradation in lysosomes. The beta chain thus appeared to provide the essential element necessary for ligand internalization by both types of IL-2-R.