Endocytosis of the common cytokine receptor gammac chain. Identification of sequences involved in internalization and degradation

Daclizumab: Mechanisms of Action, Therapeutic Efficacy, Adverse Events and Its Uncovering the Potential Role of Innate Immune System Recruitment as a Treatment Strategy for Relapsing Multiple Sclerosis

Daclizumab (DAC) is a humanized, monoclonal antibody that blocks CD25, a critical element of the high-affinity interleukin-2 receptor (IL-2R). DAC HYP blockade of CD25 inhibits effector T cell activation, regulatory T cell expansion and survival, and activation-induced T-cell apoptosis. Because CD25 blockade reduces IL-2 consumption by effector T cells, it increases IL-2 bioavailability allowing for greater interaction with the intermediate-affinity IL-2R, and therefore drives the expansion of CD56^bright natural killer (NK) cells. Furthermore, there appears to be a direct correlation between CD56^bright NK cell expansion and DAC HYP efficacy in reducing relapses and MRI evidence of disease activity in patients with RMS in phase II and phase III double-blind, placebo- and active comparator-controlled trials. Therapeutic efficacy was maintained during open-label extension studies. However, treatment was associated with an increased risk of rare adverse events, including cutaneous inflammation, autoimmune hepatitis, central nervous system Drug Reaction with Eosinophilia Systemic Symptoms (DRESS) syndrome, and autoimmune Glial Fibrillary Acidic Protein (GFAP) alpha immunoglobulin-associated encephalitis. As a result, DAC HYP was removed from clinical use in 2018. The lingering importance of DAC is that its use led to a deeper understanding of the underappreciated role of innate immunity in the potential treatment of autoimmune disease.

A Generic Mechanism for Enhanced Cytokine Signaling via Cytokine-Neutralizing Antibodies

Enhancement or inhibition of cytokine signaling and corresponding immune cells responses are critical factors in various disease treatments. Cytokine signaling may be inhibited by cytokine-neutralizing antibodies (CNAs), which prevents further activation of cytokine receptors. However, CNAs may result in enhanced—instead of inhibitory—cytokine signaling (an “agonistic effect”) in various in vitro and in vivo experiments. This may lead to lack of efficacy or adverse events for cytokine-inhibiting based medicines. Alternatively, cytokine-antibody complexes may produce stronger signaling vs. cytokine alone, thereby increasing the efficacy of stimulating cytokine-based drugs, at equal or lower cytokine doses. In this paper, the effect of cytokine signaling enhancement by a CNA was studied in a generic mathematical model of interleukin-4 (IL-4) driven T-cell proliferation. The occurrence of the agonistic effect depends upon the antibody-to-cytokine binding affinity and initial concentrations of antibody and cytokine. Model predictions were in agreement with experimental studies. When the cytokine receptor consists of multiple subunits with substantially differing affinities (e.g., IL-4 case), the choice of the receptor chain to be blocked by the antibody is critical, for the agonistic effect to appear. We propose a generic mechanism for the effect: initially, binding of the CNA to the cytokine reduces free cytokine concentration; yet, cytokine molecules bound within the cytokine-CNA complex—and released later and over time—are “rescued” from earlier clearance via cellular internalization. Hence, although free cytokine-dependent signalling may be less potent initially, it will also be more sustained over time; and given non-linear dynamics, it will lead ultimately to larger cellular effector responses, vs. the same amount of free cytokine in the absence of CNA. We suggest that the proposed mechanism is a generic property of {cytokine, CNA, receptor} triads, both in vitro and in vivo, and can occur in a predictable fashion for a variety of cytokines of the immune system.

Renal IL-17 expression in human ANCA-associated glomerulonephritis

Interleukin-17A (IL-17) promotes inflammatory renal tissue damage in mouse models of crescentic glomerulonephritis, including murine experimental autoimmune anti-myeloperoxidase glomerulonephritis, which most likely depends on IL-17-producing Th17 cells. In human anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis, however, the cellular sources of IL-17 remain to be elucidated. Therefore, we analyzed human kidney biopsies of active necrotizing and crescentic ANCA-associated glomerulonephritis by immunohistochemistry using an IL-17-specific antibody and by immunofluorescent colocalization with cell type markers. We detected numerous IL-17-expressing (IL-17(+)) cells in the glomeruli and in the tubulointerstitium. Unexpectedly, most of these IL-17(+) cells were polymorphonuclear neutrophilic granulocytes, while IL-17(+) T cells and IL-17(+) mast cells were present at significantly lower frequencies. IL-17 was not detected in other infiltrating or resident kidney cells. In those patients who had not received immunosuppressive treatment before biopsy, serum creatinine levels were positively correlated with tubulointerstitial IL-17(+) neutrophils as well as IL-17(+) T cells. Furthermore, we could demonstrate that purified human blood neutrophils expressed IL-17 protein and released it upon stimulation in vitro. In conclusion, these results support a pathogenic role for IL-17 in human ANCA-associated glomerulonephritis. Our data suggest that in the acute stage of the disease neutrophils may act as an important immediate-early innate source of IL-17 and may thereby initiate and promote ongoing renal inflammation. IL-17 may thus be a target for treating acute ANCA-associated glomerulonephritis.

The influence of HIV on CD127 expression and its potential implications for IL-7 therapy

Interleukin-7 (IL-7) is critical for early T-cell development and plays an important role in T-cell homeostasis, differentiation and function. Signalling via the IL-7 receptor is dependent on the expression of its components, IL-7Rα (CD127) and IL-2Rγ (CD132) and is mediated in part by alterations in CD127 expression levels in different cell subsets. Naïve and memory T-cells express high levels of CD127, while effector cells are CD127(lo) and retention of the receptor is thought to influence the development of memory cells. Reduced expression of CD127 has been associated with markers of disease severity in HIV infection and other chronic viral infections as well as in various cancers. In HIV infection, decreased CD127 expression on T-cells is correlated with reduced CD4(+) T-cell counts, increased viral replication and immune activation. The loss of IL-7 activity, due to decreased CD127 expression, may contribute to the observed loss of CD8(+) cytotoxic T lymphocyte (CTL) activity in HIV infection. The downregulation of CD127 expression in HIV infection may be due to host (e.g. IL-7, IL-4, immune activation) and/or viral (e.g. HIV-tat) factors and mechanisms of receptor regulation may differ by cell type. In addition, the expression of a soluble form of CD127 (sCD127) has been shown to be increased in HIV infection. This protein may affect IL-7 activity in vivo and therefore may have implications for IL-7-based therapies which are currently being tested in clinical trials. Understanding how CD127 is regulated during HIV infection will provide insight for the development of novel therapeutics to improve immune function and anti-viral T-cell activity.

Expression of γ-chain cytokine receptors on CD8+ T cells in HIV infection with a focus on IL-7Rα (CD127)

When interleukin-2 (IL-2) receptor γ-chain (γ(C))-sharing cytokine receptors on T cells bind their specific ligands (IL-2, -4, -7, -9, -15 or -21), they initiate a variety of cell signals that promote survival, differentiation or antiviral or antitumor cytolytic functions. Although expression of the γ(C) is constitutive across T-cell subsets, the varying expression of other receptor complex components can regulate cytokine signalling and function. Impaired γ(C) cytokine activity in HIV infection, and the role of γ(C) cytokines in CD8(+) T-cell function and homeostasis, implicates these molecules among potential contributors to the observed decline of cytolytic activity (CTL) in HIV disease. In particular, this review will be highlighting information about the IL-7 receptor (IL-7R) complex, which is composed of the γ(C) and the IL-7Rα (CD127) chains. There has been an abundance of HIV-related CD127 research and its important role in CD8(+) T-cell survival and function. The expression of CD127 undergoes dramatic changes throughout the course of T-cell responses in HIV infection. The expression of CD127 is significantly decreased in progressive HIV disease, whereas effective antiretroviral therapy results in its recovery. Observations of impaired IL-7 activity in HIV(+) individuals have suggested that CD127 has an important role in HIV immunopathogenesis. In addition, a soluble form of CD127 (sCD127) is upregulated in the plasma of HIV(+) individuals. Hence, CD127 is being increasingly considered as a marker of disease prognosis, and related information may provide insight into understanding the expression and role of other γ(C) receptors in HIV disease and contribute to the development of novel cytokine-based therapeutics.

Cellular level models as tools for cytokine design

Cytokines and growth factors are critical regulators that connect intracellular and extracellular environments through binding to specific cell-surface receptors. They regulate a wide variety of immunological, growth, and inflammatory response processes. The overall signal initiated by a population of cytokine molecules over long time periods is controlled by the subtle interplay of binding, signaling, and trafficking kinetics. Building on the work of others, we abstract a simple kinetic model that captures relevant features from cytokine systems as well as related growth factor systems. We explore a large range of potential biochemical behaviors, through systematic examination of the model's parameter space. Different rates for the same reaction topology lead to a dramatic range of biochemical network properties and outcomes. Evolution might productively explore varied and different portions of parameter space to create beneficial behaviors, and effective human therapeutic intervention might be achieved through altering network kinetic properties. Quantitative analysis of the results reveals the basis for tensions among a number of different network characteristics. For example, strong binding of cytokine to receptor can increase short-term receptor activation and signal initiation but decrease long-term signaling due to internalization and degradation. Further analysis reveals the role of specific biochemical processes in modulating such tensions. For instance, the kinetics of cytokine binding and receptor activation modulate whether ligand-receptor dissociation can generally occur before signal initiation or receptor internalization. Beyond analysis, the same models and model behaviors provide an important basis for the design of more potent cytokine therapeutics by providing insight into how binding kinetics affect ligand potency.

NEDD4-2 associates with gamma(c) and regulates its degradation rate

Interleukin-2 (IL-2) is a cytokine that regulates proliferation, differentiation and survival of various lymphoid cell subsets. Its actions are mediated through its binding to the IL-2 receptor which is composed of three subunits (IL-2Ralpha, IL-2Rbeta and gamma(c)). Only beta and gamma(c) have been shown to transduce intra cellular signals. The gamma(c) chain is shared by the interleukin-2, 4, 7, 9, 15 and 21 receptors, and is essential for lymphocyte functions. The regulation of gamma(c) expression level is therefore critical for the ability of cells to respond to these cytokines. In the present work, we show that the IL-2R constitutively associates with the ubiquitin ligase NEDD4-2, and to a lesser extent NEDD4-1. We identified the specific binding site on gamma(c). And we show that the loss of NEDD4 association on gamma(c) is accompanied by a dramatic increase of the half-life of the receptor subunit.

Clathrin- and caveolae-independent entry of feline infectious peritonitis virus in monocytes depends on dynamin

Feline infectious peritonitis virus (FIPV), a coronavirus that causes a lethal chronic disease in cats, enters feline monocytes via endocytosis. In this study, the pathway of internalization is characterized by evaluating the effect of chemical inhibitors and/or expression of dominant-negative (DN) proteins on the percentage of internalized virions per cell and infection. Further, co-localization studies were performed to determine the involvement of certain cellular internalization proteins. FIPV is not internalized through a clathrin-mediated pathway, as chlorpromazine, amantadine and DN eps15 did not influence virus uptake and FIPV did not co-localize with clathrin. The caveolae-mediated pathway could be excluded based on the inability of genistein and DN caveolin-1 to inhibit virus uptake and lack of co-localization between FIPV and caveolin-1. Dynamin inhibitory peptide and DN dynamin effectively inhibited virus internalization. The inhibitor strongly reduced uptake to 20.3+/-1.1% of uptake in untreated cells. In the presence of DN dynamin, uptake was 58.7+/-3.9% relative to uptake in untransduced cells. Internalization of FIPV was slightly reduced to 85.0+/-1.4 and 87.4+/-6.1% of internalization in control cells by the sterol-binding drugs nystatin and methyl-beta-cyclodextrin, respectively. Rho GTPases were inhibited by Clostridium difficile toxin B, but no effect was observed. These results were confirmed with infection studies showing that infection was not influenced by chlorpromazine, amantadine and genistein, but was significantly reduced by dynamin inhibition and nystatin. In conclusion, these results indicate that FIPV enters monocytes through a clathrin- and caveolae-independent pathway that strongly depends on dynamin and is slightly sensitive to cholesterol depletion.

IL-7 decreases IL-7 receptor alpha (CD127) expression and induces the shedding of CD127 by human CD8+ T cells

IL-7 receptor alpha (CD127) signaling is essential for T-cell development and regulation of naive and memory T-cell homeostasis. Fewer CD8(+) T cells from HIV-infected patients express CD127 compared with healthy individuals, suggesting that specific host and/or viral factors regulate IL-7 receptor expression. Factors relevant to HIV infection that could potentially decrease CD127 expression on human CD8(+) T cells and the mechanisms by which this occurs were therefore evaluated. IL-7, but not HIV gp120, IL-1-beta, IL-6, IL-10, IL-13, transforming growth factor-beta or tumor necrosis factor-alpha, reduced CD127-surface expression and did so without altering CD127 mRNA expression. Furthermore, IL-7 did not increase the amount of cytoplasmic CD127 in CD8(+) T cells. Interestingly, IL-7 induced the shedding of CD127 from CD8(+) T cells, suggesting a mechanism that may contribute to the increased concentration of CD127 in the plasma of HIV(+) individuals, a novel finding reported here. Naive CD8(+) T cells are more sensitive to IL-7 that mediated the down-regulation of CD127, suggesting that these effects may have particular significance early in T-cell life cycle. Since CD127 down-regulation may be an important contributor to HIV-associated T-cell dysfunction, determining the mechanism thereof may prove to be of considerable significance.

Constitutive internalization of murine MHC class I molecules

The total number of cell surface glycoprotein molecules at the plasma membrane results from a balance between their constitutive internalization and their egress to the cell surface from intracellular pools and/or biosynthetic pathway. Constitutive internalization is net result of constitutive endocytosis and endocytic recycling. In this study we have compared spontaneous internalization of murine major histocompatibility complex (MHC) class I molecules (K(d), D(d), full L(d), and empty L(d)) after depletion of their egress to the cell surface (Cycloheximide [CHX], brefeldin A [BFA]) and internalization after external binding of monoclonal antibody (mAb). MHC class I alleles differ regarding their cell surface stability, kinetics, and in the way of internalization and degradation. K(d) and D(d) molecules are more stable at the cell surface than L(d) molecules and, thus, constitutively internalized more slowly. Although the binding of mAbs to cell surface MHC class I molecules results in faster internalization than depletion of their egress, it is still slow and, thereby, can serve as a model for tracking of MHC class I endocytosis. Internalization of fully conformed MHC class I molecules (K(d), D(d), and L(d)) was neither inhibited by chlorpromazine (CP) (inhibitor of clathrin endocytosis), nor with filipin (inhibitor of lipid raft dependent endocytosis), indicating that fully conformed MHC class I molecules are internalized via the bulk pathway. In contrast, internalization of empty L(d) molecules was inhibited by filipin, indicating that non-conformed MHC class I molecules require intact cholesterol-rich membrane microdomains for their constitutive internalization. Thus, conformed and non-conformed MHC class I molecules use different endocytic pathways for constitutive internalization.

Ubiquitination of the common cytokine receptor gammac and regulation of expression by an ubiquitination/deubiquitination machinery

The common cytokine receptor gamma(c) is shared by the interleukin-2, -4, -7, -9, -15, and -21 receptors, and is essential for lymphocyte proliferation and survival. The regulation of gamma(c) receptor expression level is therefore critical for the ability of cells to respond to these cytokines. We previously reported that gamma(c) is efficiently constitutively internalized and addressed towards a degradation endocytic compartment. We show that gamma(c) is ubiquitinated and also associated to ubiquitinated proteins. We report that the ubiquitin-ligase c-Cbl induces gamma(c) down-regulation. In addition, the ubiquitin-hydrolase, DUB-2, counteracts the effect of c-Cbl on gamma(c) expression. We show that an increase in DUB-2 expression correlates with an increased gamma(c) half-life, resulting in the up-regulation of the receptor. Altogether, we show that gamma(c) is the target of an ubiquitination mechanism and its expression level can be regulated through the activities of a couple of ubiquitin-ligase/ubiquitin-hydrolase enzymes, namely c-Cbl/DUB-2.

Cortactin and dynamin are required for the clathrin-independent endocytosis of gammac cytokine receptor

Endocytosis is critical for many cellular functions. We show that endocytosis of the common gammac cytokine receptor is clathrin independent by using a dominant-negative mutant of Eps15 or RNA interference to knock down clathrin heavy chain. This pathway is synaptojanin independent and requires the GTPase dynamin. In addition, this process requires actin polymerization. To further characterize the function of dynamin in clathrin-independent endocytosis, in particular its connection with the actin cytoskeleton, we focused on dynamin-binding proteins that interact with F-actin. We compared the involvement of these proteins in the clathrin-dependent and -independent pathways. Thus, we observed that intersectin, syndapin, and mAbp1, which are necessary for the uptake of transferrin (Tf), a marker of the clathrin route, are not required for gammac receptor endocytosis. Strikingly, cortactin is needed for both gammac and Tf internalizations. These results reveal the ubiquitous action of cortactin in internalization processes and suggest its role as a linker between actin dynamics and clathrin-dependent and -independent endocytosis.

Regulation of gene expression by biotin (review)

In mammals, biotin serves as coenzyme for four carboxylases, which play essential roles in the metabolism of glucose, amino acids, and fatty acids. Biotin deficiency causes decreased rates of cell proliferation, impaired immune function, and abnormal fetal development. Evidence is accumulating that biotin also plays an important role in regulating gene expression, mediating some of the effects of biotin in cell biology and fetal development. DNA microarray studies and other gene expression studies have suggested that biotin affects transcription of genes encoding cytokines and their receptors, oncogenes, genes involved in glucose metabolism, and genes that play a role in cellular biotin homeostasis. In addition, evidence has been provided that biotin affects expression of the asialoglycoprotein receptor and propionyl-CoA carboxylase at the post-transcriptional level. Various pathways have been identified by which biotin might affect gene expression: activation of soluble guanylate cyclase by biotinyl-AMP, nuclear translocation of NF-kappaB (in response to biotin deficiency), and remodeling of chromatin by biotinylation of histones. Some biotin metabolites that cannot serve as coenzymes for carboxylases can mimic biotin with regard to its effects on gene expression. This observation suggests that biotin metabolites that have been considered "metabolic waste" in previous studies might have biotin-like activities. These new insights into biotin-dependent gene expression are likely to lead to a better understanding of roles for biotin in cell biology and fetal development.

Jak3-independent trafficking of the common gamma chain receptor subunit: chaperone function of Jaks revisited

Janus kinases (Jaks) play an essential role in cytokine signaling and have been reported to regulate plasma membrane expression of their cognate receptors. In this study, we examined whether Jak3 and the common gamma chain (gamma(c)) reciprocally regulate their plasma membrane expression. In contrast to interleukin-2Ralpha, gamma(c) localized poorly to the plasma membrane and accumulated in endosomal-lysosomal compartments. However, gamma(c) was expressed at comparable levels on the surface of cells lacking Jak3, and plasma membrane turnover of gamma(c) was independent of Jak3. Nonetheless, overexpression of Jak3 enhanced accumulation of gamma(c) at the plasma membrane. Without gamma(c), Jak3 localized in the cytosol, whereas in the presence of the receptor, it colocalized with gamma(c) in endosomes and at the plasma membrane. Although the Jak FERM domain is necessary and sufficient for receptor binding, the requirement for full-length Jak3 in gamma(c) membrane trafficking was remarkably stringent; using truncation and deletion mutants, we showed that the entire Jak3 molecule was required, although kinase activity was not. Thus, unlike other cytokine receptors, gamma(c) does not require Jak3 for receptor membrane expression. However, full-length Jak3 is required for normal trafficking of this cytokine receptor/Jak pair, a finding that has important structural and clinical implications.

Diaminobiotin and desthiobiotin have biotin-like activities in Jurkat cells

In mammals, biotin serves as a coenzyme for carboxylases such as propionyl-CoA carboxylase. The expression of genes encoding interleukin-2 (IL-2) and IL-2 receptor (IL-2R)gamma also depends on biotin. Biotin metabolites are structurally similar to biotin, and their concentrations in tissues are quantitatively important. Here, the hypothesis was tested that biotin metabolites can mimic the effects of biotin on gene expression and thus have biotin-like activities. A human T-cell line (Jurkat cells) was used to model effects of biotin and synthetic metabolites (diaminobiotin and desthiobiotin) on the expression of genes encoding IL-2 and IL-2Rgamma. Cells were cultured in biotin-deficient medium (0.025 nmol/L biotin) for 35 d; controls were cultured in medium containing 10 nmol/L biotin. The biotin-deficient medium was supplemented with 10 nmol/L of diaminobiotin, desthiobiotin, biotin or no biotin 24 h before gene expression analyses. Transcriptional activities of genes encoding IL-2 and IL-2Rgamma were increased up to 43% in cells supplemented with diaminobiotin, desthiobiotin or biotin compared with biotin-deficient cells, as judged by luciferase activities after transfection with reporter-gene constructs. These findings are consistent with the hypothesis that diaminobiotin and desthiobiotin mimic the effects of biotin on gene expression and thus have biotin-like activities. Supplementation of cells with diaminobiotin and desthiobiotin did not affect abundances of holocarboxylases and activities of propionyl-CoA carboxylase, suggesting that effects of synthetic biotin metabolites on gene expression are not mediated by carboxylase-dependent pathways. It is not known whether naturally occurring biotin metabolites also have biotin-like activities.

Interleukin-2 receptor-gamma -dependent endocytosis depends on biotin in Jurkat cells

Biotin has been credited with having beneficial effects on immune function despite observations that biotin supplementation causes decreased secretion of interleukin-2. Here this paradox was addressed by determining whether receptor-dependent internalization of interleukin-2 by immune cells depends on biotin. Theoretically, this would be consistent with both decreased net secretion of interleukin-2 by biotin-supplemented cells (causing increased endocytosis) and beneficial effects of biotin on immune function (causing increased receptor signaling). Jurkat cells were cultured in biotin-defined media (25, 250, or 10,000 pM). Secretion of interleukin-2 correlated negatively with biotin supply, but transcriptional activity of the interleukin-2 gene correlated positively with biotin supply, suggesting that decreased secretion of interleukin-2 by biotin-supplemented cells was not caused by decreased gene expression. Expression of the interleukin-2 receptor-gamma gene was greater at 10,000 pM than 25 pM biotin, mediating increased endocytosis of interleukin-2 in biotin-supplemented medium. Inhibition of endocytosis by genistein and overexpression of interleukin-2 receptor-gamma abolished the effect of biotin. These findings suggest that endocytosis of interleukin-2 depends on biotin.

Signaling domains of the interleukin 2 receptor

Interleukin (IL-)2 and its receptor (IL-2R) constitute one of the most extensively studied cytokine receptor systems. IL-2 is produced primarily by activated T cells and is involved in early T cell activation as well as in maintaining homeostatic immune responses that prevent autoimmunity. This review focuses on molecular signaling pathways triggered by the IL-2/IL-2R complex, with an emphasis on how the IL-2R physically translates its interaction with IL-2 into a coherent biological outcome. The IL-2R is composed of three subunits, IL-2Ralpha, IL-2Rbeta and gammac. Although IL-2Ralpha is an important affinity modulator that is essential for proper responses in vivo, it does not contribute to signaling due a short cytoplasmic tail. In contrast, IL-2Rbeta and gammac together are necessary and sufficient for effective signal transduction, and they serve physically to connect the receptor complex to cytoplasmic signaling intermediates. Despite an absolute requirement for gammac in signaling, the majority of known pathways physically link to the receptor via IL-2Rbeta, generally through phosphorylated cytoplasmic tyrosine residues. This review highlights work performed both in cultured cells and in vivo that defines the functional contributions of specific receptor subdomains-and, by inference, the specific signaling pathways that they activate-to IL-2-dependent biological activities.

A single internalization signal from the di-leucine family is critical for constitutive endocytosis of the type II TGF-(β) receptor

Endocytosis has an important contribution to the regulation of the surface expression levels of many receptors. In spite of the central role of the transforming growth factor (β) (TGF-(β)) receptors in numerous cellular and physiological processes, their endocytosis is largely unexplored. Current information on TGF-(β) receptor endocytosis relies exclusively on studies with chimeric constructs containing the extracellular domain of the GM-CSF receptors, following the internalization of the GM-CSF ligand; the conformation and interactions of the chimeric receptors (and therefore their endocytosis) may differ considerably from those of the native TGF-(β) receptors. Furthermore, there are no data on the potential endocytosis motif(s) of the TGF-(β) receptors or other receptor Ser/Thr kinases. Here, we report the use of type II TGF-(β) receptors, myc-tagged at their extracellular terminus, to investigate their endocytosis. Employing fluorescent antibody fragments to label exclusively the cell surface myc-tagged receptors exposed to the external milieu, made it possible to follow the internalization of the receptors, without the complications that render labeling with TGF-(β) (which binds to many cellular proteins) unsuitable for such studies. The results demonstrate that the full-length type II TGF-(β) receptor undergoes constitutive endocytosis via clathrin-coated pits. Using a series of truncation and deletion mutants of this receptor, we identified a short peptide sequence (I(218)I(219)L(220)), which conforms to the consensus of internalization motifs from the di-leucine family, as the major endocytosis signal of the receptor. The functional importance of this sequence in the full-length receptor was validated by the near complete loss of internalization upon mutation of these three amino acids to alanine.

IL-15 and IL-2: a matter of life and death for T cells in vivo

Interleukin (IL)-2 and IL-15 are redundant in stimulating T-cell proliferation in vitro. Their precise role in vivo in governing T-cell expansion and T-cell homeostasis is less clear. Each may have distinct functions and regulate distinct aspects of T-cell activation. The functional receptors for IL-2 and IL-15 consist of a private alpha-chain, which defines the binding specificity for IL-2 or IL-15, and shared IL-2 receptor beta- and gamma-chains. The gamma-chain is also a critical signaling component of IL-4, IL-7 and IL-9 receptors. Thus, the gamma-chain is called the common gamma or gamma-c. As these receptor subunits can be expressed individually or in various combinations resulting in the formation of receptors with different affinities, distinct signaling capabilities or both, we hypothesized that differential expression of IL-2 and IL-15 receptor subunits on cycling T cells in vivo may direct activated T cells to respond to IL-2 or IL-15, thereby regulating the homeostasis of T-cell response in vivo. By observing in vivo T-cell divisions and expression of IL-2 and IL-15 receptor subunits, we demonstrate that IL-15 is a critical growth factor in initiating T cell divisions in vivo, whereas IL-2 limits continued T-cell expansion via downregulation of the gamma-c expression. Decreased gamma-c expression on cycling T cells reduced sustained Bcl-2 expression and rendered cells susceptible to apoptotic cell death. Our study provides data that IL-2 and IL-15 regulate distinct aspects of primary T-cell expansion in vivo.

Identification of two intracellular mechanisms leading to reduced expression of oncoretrovirus envelope glycoproteins at the cell surface

All retrovirus glycoproteins have a cytoplasmic domain that plays several roles in virus replication. We have determined whether and how the cytoplasmic domains of oncoretrovirus glycoproteins modulate their intracellular trafficking, by using chimeric proteins that combined the alpha-chain of the interleukin-2 receptor with the glycoprotein cytoplasmic domains of five oncoretroviruses: human T-cell leukemia virus type 1 (HTLV-1), Rous sarcoma virus (RSV), bovine leukemia virus (BLV), murine leukemia virus (MuLV), and Mason-Pfizer monkey virus (MPMV). All of these proteins were synthesized and matured in the same way as a control protein with no retrovirus cytoplasmic domain. However, the amounts of all chimeric proteins at the cell surface were smaller than that of the control protein. The protein appearing at and leaving the cell surface and endocytosis were measured in stable transfectants expressing the chimera. We identified two groups of proteins which followed distinct intracellular pathways. Group 1 included chimeric proteins that reached the cell surface normally but were rapidly endocytosed afterwards. This group included the chimeric proteins with HTLV-1, RSV, and BLV cytoplasmic domains. Group 2 included chimeric proteins that were not detected at the cell surface, despite normal intracellular concentrations, and were accumulated in the Golgi complex. This group included the chimeric proteins with MuLV and MPMV cytoplasmic domains. Finally, we verified that the MuLV envelope glycoproteins behaved in the same way as the corresponding chimeras. These results indicate that retroviruses have evolved two distinct mechanisms to ensure a similar biological feature: low concentrations of their glycoproteins at the cell surface.

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.

Increased endosomal sorting of ligand to recycling enhances potency of an interleukin-2 analog

An interleukin-2 (IL-2) variant containing adjacent point mutations (L18M/L19S, termed 2D1) displaying binding affinity to the heterotrimeric IL-2 receptor similar to that of wild-type IL-2 (WT) had been previously found to surprisingly exhibit increased bioactivity in a peripheral blood lymphocyte proliferation assay. In order to provide an explanatory mechanism for this unexpected potency enhancement, we hypothesize that altered endocytic trafficking of the 2D1 variant might be responsible by increasing the number of ligand-receptor complexes. We demonstrate here that the internalization kinetics of 2D1 via the high affinity IL-2 receptor are equivalent to those of WT but that a significantly increased fraction of internalized 2D1 is sorted to recycling instead of to lysosomal degradation. We further find a reduced pH sensitivity of binding to IL-2 receptor alpha relative to IL-2 receptor beta compared with WT, which could be responsible for the altered sorting behavior of 2D1 in the acidic endosomal compartment. Accordingly, the 2D1 variant displays a half-life 36 h longer than that of IL-2 in T-lymphocyte culture at concentrations equal to the K(D) of the IL-2 receptor. The extended half-life of intact 2D1 provides enhanced mitogenesis as compared with IL-2. In addition, 2D1 stimulates natural killer cells to a lesser degree than IL-2 at equal concentrations. We conclude that this IL-2 variant provides increased mitogenic stimulation that could not be easily predicted from its cell surface receptor binding affinity while minimizing undesired stimulation of natural killer cells. This concept of altering trafficking dynamics may offer a generalizable approach to generating improvements in the pharmacological efficacy of therapeutic cytokines.

Role of the common cytokine receptor gamma chain (gammac) in thymocyte selection

During thymocyte development, T-cell receptor (TCR) alphabeta-mediated intracellular signals can elicit two entirely different cellular responses: positive selection (resulting in rescue from death and maturation or differentiation) and negative selection (induction of apoptosis). Here, Hiroshi Nakajima and colleagues discuss how survival signals that are dependent on the common cytokine receptor gamma chain (gammac) might affect the TCR-driven selection process in thymocytes, underscoring the potential role of cytokines in this process.

The two subunits of the interleukin-4 receptor mediate independent and distinct patterns of ligand endocytosis

Interleukin-4 (IL-4) triggers cellular responses by interaction with the bipartite interleukin-4 receptor (IL-4R). IL-4-responsive cells specifically endocytose IL-4. We studied the ligand internalization properties of the human IL-4R and analyzed the specific functions of its two subunits IL-4Ralpha and gammac in this process. IL-4 mutant RY, which binds to IL-4Ralpha but does not recruit gammac into the receptor complex was used as a tool to show that IL-4Ralpha can promote independent ligand uptake in human T cells. Internalization was limited, however, by rapid IL-4 dissociation, suggesting that one important function of gammac in IL-4 endocytosis is to retain the ligand sufficiently long within the ternary receptor complex. We then measured IL-4 internalization by murine Ba/F3 cells that were stably transfected with various human IL-4R constructs. Efficient IL-4 uptake required the cytoplasmic section of the receptor. The intracellular domains of IL-4Ralpha and gammac were responsible for independent endocytosis processes with distinct kinetics. IL-4Ralpha-mediated internalization resulted in long-term intracellular maintainance of IL-4, whereas gammac directed the associated radioligand to intracellular breakdown and rapid release in the form of degraded protein. Mutants of either IL-4R subunit deficient in Janus kinase activation were not impaired in internalization, indicating that IL-4 endocytosis is not functionally connected to signal transduction.

The CD3ε Subunit of the TCR Contains Endocytosis Signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3γ and CD3δ contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3ε and CDζ display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3ε bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3ε that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3ε displays endocytosis determinants. These data indicate that CD3ε could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3ε subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

A cytosolic domain of the erythropoietin receptor contributes to endoplasmic reticulum-associated degradation

The erythropoietin receptor (EPO-R) is the cellular target for erythropoietin (EPO), the primary hormone that mediates the proliferation of immature erythroblasts and their differentiation into mature erythrocytes. Unusual features of the EPO-R are its short half-life (t(1/2) 1-2 h), its degradation via multiple pathways and the fact that less than 1% of total cellular EPO-R molecules are found on the cell surface. The contribution of EPO-R structural determinants to the regulation of its intracellular metabolism is still unclear. The epidermal growth factor receptor (EGF-R), unlike the EPO-R, is efficiently transported to the cell surface and displays a much longer metabolic half-life. To determine which EPO-R cytosolic domains are involved in intracellular degradation, we studied chimeric receptor molecules constructed of EGF-R extracellular and transmembrane parts, linked to the full length or truncated cytosolic part of the EPO-R. The chimeras were expressed in transiently transfected COS 7 cells and stably expressed in Ba/F3 cells. Our experiments indicate that the cytosolic part of the EPO-R contains determinants that mark it for rapid degradation, in association with the endoplasmic reticulum (ER). This degradation was insensitive to brefeldin A and was inhibited by specific proteasomal inhibitors. A truncated EGF-R/EPO-R chimera containing only 50 amino acids of the EPO-R membrane-proximal cytosolic part was also rapidly degraded suggesting that these 50 amino acids are involved in receptor degradation.

Molecular characterization of the signal responsible for the targeting of the interleukin 2 receptor beta chain toward intracellular degradation

During receptor-mediated endocytosis, most growth factor receptors are transported to late endocytic compartments and degraded. This process is important to control their expression on the cell surface and requires sorting in early endocytic compartments. Little is known about the mechanisms and the signals involved. We have studied the signal involved in targeting the interleukin 2 receptor beta chain (IL2Rbeta), a member of the cytokine receptor superfamily, toward degradation after internalization. We show that a motif of 8 amino acids in the cytosolic tail of IL2Rbeta is sufficient to target a normally recycling receptor toward degradation. Deletion of this signal strongly impairs IL2Rbeta degradation. Further molecular characterization of the motif shows that it does not resemble the well documented tyrosine and dileucine families of trafficking signals.