Ligand binding analysis of soluble interleukin-2 receptor complexes by surface plasmon resonance

Molecular reshaping of phage-displayed Interleukin-2 at beta chain receptor interface to obtain potent super-agonists with improved developability profiles

Interleukin-2 (IL-2) engineered versions, with biased immunological functions, have emerged from yeast display and rational design. Here we reshaped the human IL-2 interface with the IL-2 receptor beta chain through the screening of phage-displayed libraries. Multiple beta super-binders were obtained, having increased receptor binding ability and improved developability profiles. Selected variants exhibit an accumulation of negatively charged residues at the interface, which provides a better electrostatic complementarity to the beta chain, and faster association kinetics. These findings point to mechanistic differences with the already reported superkines, characterized by a conformational switch due to the rearrangement of the hydrophobic core. The molecular bases of the favourable developability profile were tracked to a single residue: L92. Recombinant Fc-fusion proteins including our variants are superior to those based on H9 superkine in terms of expression levels in mammalian cells, aggregation resistance, stability, in vivo enhancement of immune effector responses, and anti-tumour effect.

IL-2Rβ abundance differentially tunes IL-2 signaling dynamics in CD4+ and CD8+ T cells

Interleukin-2 (IL-2) stimulates both activated CD4⁺ and CD8⁺ T cells to proliferate. IL-2 signals through an identical receptor complex and promotes the same dose-dependent phosphorylation of the canonical transcription factor STAT5 in both cell types. Despite this, CD8⁺ T cells enter the S phase earlier and proliferate to a greater extent than do CD4⁺ T cells in response to IL-2. We identified distinct IL-2 signaling dynamics in CD4⁺ and CD8⁺ T cells. In IL-2-stimulated CD8⁺ T cells, STAT5 phosphorylation increased rapidly and was sustained for 6 hours. In contrast, CD4⁺ T cells had a biphasic response, with maxima at 15 min and 2 to 4 hours after stimulation. Both cell types required vesicular trafficking, but only CD4⁺ T cells required new protein synthesis to maintain high phosphorylation of STAT5. Two subunits of the IL-2 receptor, IL-2Rβ and IL-2Rγ, were twice as abundant in CD8⁺ T cells than in CD4⁺ T cells. Reduction of IL-2Rβ abundance by 50% was sufficient to convert CD8⁺ T cells to a CD4⁺ T cell-like signaling pattern and delay S phase entry. These results suggest that the larger pool of IL-2Rβ chains in CD8⁺ T cells is required to sustain IL-2 signaling and contributes to the quantitatively greater proliferative response to IL-2 relative to that of CD4⁺ T cells. This cell type-specific difference in IL-2Rβ abundance appears to tune responses, potentially preventing extensive, autoimmune proliferation of CD4⁺ T cells, while still enabling sufficient proliferation of CD8⁺ T cells to control viral infections.

Modeling the receptor pharmacology, pharmacokinetics, and pharmacodynamics of NKTR-214, a kinetically-controlled interleukin-2 (IL2) receptor agonist for cancer immunotherapy

Cytokines are potent immune modulating agents but are not ideal medicines in their natural form due to their short half-life and pleiotropic systemic effects. NKTR-214 is a clinical-stage biologic that comprises interleukin-2 (IL2) protein bound by multiple releasable polyethylene glycol (PEG) chains. In this highly PEG-bound form, the IL2 is inactive; therefore, NKTR-214 is a biologic prodrug. When administered in vivo, the PEG chains slowly release, creating a cascade of increasingly active IL2 protein conjugates bound by fewer PEG chains. The 1-PEG-IL2 and 2-PEG-IL2 species derived from NKTR-214 are the most active conjugated-IL2 species. Free-IL2 protein is undetectable in vivo as it is eliminated faster than formed. The PEG chains on NKTR-214 are located at the region of IL2 that contacts the alpha (α) subunit of the heterotrimeric IL2 receptor complex, IL2Rαβγ, reducing its ability to bind and activate the heterotrimer. The IL2Rαβγ complex is constitutively expressed on regulatory T cells (Tregs). Therefore, without the use of mutations, PEGylation reduces the affinity for IL2Rαβγ to a greater extent than for IL2Rβγ, the receptor complex predominant on CD8 T cells. NKTR-214 treatment in vivo favors activation of CD8 T cells over Tregs in the tumor microenvironment to provide anti-tumor efficacy in multiple syngeneic models. Mechanistic modeling based on in vitro and in vivo kinetic data provides insight into the mechanism of NKTR-214 pharmacology. The model reveals that conjugated-IL2 protein derived from NKTR-214 occupy IL-2Rβγ to a greater extent compared to free-IL2 protein. The model accurately describes the sustained in vivo signaling observed after a single dose of NKTR-214 and explains how the properties of NKTR-214 impart a unique kinetically-controlled immunological mechanism of action.

Structural and biophysical characterization of the interactions between the death domain of Fas receptor and calmodulin

The extrinsic apoptotic pathway is initiated by cell surface death receptors such as Fas. Engagement of Fas by Fas ligand triggers a conformational change that allows Fas to interact with adaptor protein Fas-associated death domain (FADD) via the death domain, which recruits downstream signaling proteins to form the death-inducing signaling complex (DISC). Previous studies have shown that calmodulin (CaM) is recruited into the DISC in cholangiocarcinoma cells, suggesting a novel role of CaM in Fas-mediated signaling. CaM antagonists induce apoptosis through a Fas-related mechanism in cholangiocarcinoma and other cancer cell lines possibly by inhibiting Fas-CaM interactions. The structural determinants of Fas-CaM interaction and the underlying molecular mechanisms of inhibition, however, are unknown. Here we employed NMR and biophysical techniques to elucidate these mechanisms. Our data show that CaM binds to the death domain of Fas (FasDD) with an apparent dissociation constant (Kd) of ~2 μM and 2:1 CaM:FasDD stoichiometry. The interactions between FasDD and CaM are endothermic and entropically driven, suggesting that hydrophobic contacts are critical for binding. We also show that both the N- and C-terminal lobes of CaM are important for binding. NMR and surface plasmon resonance data show that three CaM antagonists (N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide, tamoxifen, and trifluoperazine) greatly inhibit Fas-CaM interactions by blocking the Fas-binding site on CaM. Our findings provide the first structural evidence for Fas-CaM interactions and mechanism of inhibition and provide new insight into the molecular basis for a novel role of CaM in regulating Fas-mediated apoptosis.

Kinetic analysis of cytokine-mediated receptor assembly using engineered FC heterodimers

A method for analyzing ligand-receptor binding kinetics is described, which is based on an engineered FC domain (FChk) that forms a covalent heterodimer. To validate the system, the type I IFN receptors (IFNAR1 and IFNAR2) were expressed as IFNAR1-FChk, IFNAR2-FCkh, and IFNAR1/IFNAR2-FChk fusion proteins. Surface plasmon resonance (SPR) analysis of binary IFNα2a/IFNAR interactions confirmed prior affinity measurements, while the affinity of the IFNα2a/IFNAR1/IFNAR2-FChk interaction reproduced the affinity of IFNα2a binding to living cells. In cellular assays, IFNAR1/IFNAR2-FChk potently neutralized IFNα2a bioactivity with an inhibitory concentration equivalent to the KD measured by SPR. These studies suggest that FChk provides a simple reagent to evaluate the binding kinetics of multiple ligand-receptor signaling systems that control cell growth, development, and immunity.

Modeling the role of IL2 in the interplay between CD4+ helper and regulatory T cells: studying the impact of IL2 modulation therapies

Several reports in the literature have drawn a complex picture of the effect of treatments aiming to modulate IL2 activity in vivo. They seem to promote indistinctly immunity or tolerance, probably depending on the specific context, dose and timing of their application. Such complexity might derives from the dual role of IL2 on T-cell dynamics. To theoretically address the latter possibility, we develop a mathematical model for helper, regulatory and memory T-cells dynamics, which account for most well-known facts relative to their relationship with IL2. We simulate the effect of three types of therapies: IL2 injections, IL2 depletion using anti-IL2 antibodies and IL2/anti-IL2 immune complexes injection. We focus in the qualitative and quantitative conditions of dose and timing for these treatments which allow them to potentate either immunity or tolerance. Our results provide reasonable explanations for the existent pre-clinical and clinical data and further provide interesting practical guidelines to optimize the future application of these types of treatments. Particularly, our results predict that: (i) Immune complexes IL2/anti-IL2 mAbs, using mAbs which block the interaction of IL2 and CD25 (the alpha chain of IL2 receptor), is the best option to potentate immunity alone or in combination with vaccines. These complexes are optimal when a 1:2 molar ratio of mAb:IL2 is used and the mAbs have the largest possible affinity; (ii) Immune complexes IL2/anti-IL2 mAbs, using mAbs which block the interaction of IL2 and CD122 (the beta chain of IL2 receptor), are the best option to reinforce preexistent natural tolerance, for instance to prevent allograft rejection. These complexes are optimal when a 1:2 molar ratio of mAb:IL2 is used and the mAbs have intermediate affinities; (iii) mAbs anti-IL2 can be successfully used alone to treat an ongoing autoimmune disorder, promoting the re-induction of tolerance. The best strategy in this therapy is to start treatment with an initially high dose of the mAbs (one capable to induce some immune suppression) and then scales down slowly the dose of mAb in subsequent applications.

Structural biology of shared cytokine receptors

Recent structural information for complexes of cytokine receptor ectodomains bound to their ligands has significantly expanded our understanding of the macromolecular topology and ligand recognition mechanisms used by our three principal shared cytokine signaling receptors-gp130, gamma(c), and beta(c). The gp130 family receptors intricately coordinate three structurally unique cytokine-binding sites on their four-helix bundle cytokine ligands to assemble multimeric signaling complexes. These organizing principles serve as topological blueprints for the entire gp130 family of cytokines. Novel structures of gamma(c) and beta(c) complexes show us new twists, such as the use of a nonstandard sushi-type alpha receptors for IL-2 and IL-15 in assembling quaternary gamma(c) signaling complexes and an antiparallel interlocked dimer in the GM-CSF signaling complex with beta(c). Unlike gp130, which appears to recognize vastly different cytokine surfaces in chemically unique fashions for each ligand, the gamma(c)-dependent cytokines appear to seek out some semblance of a knobs-in-holes shape recognition code in order to engage gamma(c) in related fashions. We discuss the structural similarities and differences between these three shared cytokine receptors, as well as the implications for transmembrane signaling.

The effect of covalently immobilized rhIL-1ra-ELP fusion protein on the inflammatory profile of LPS-stimulated human monocytes

The objective of this research was to investigate whether immobilized anti-inflammatory cytokines will signal changes in the inflammatory profile of cultured monocytes. A fusion protein of recombinant human IL-1 receptor antagonist and elastin-like peptide (IL-1ra-ELP) was expressed in Escherichia coli. THP-1 human monocytes were cultured on either carboxyl-terminated self-assembled monolayers (SAMs), or SAMs with either covalently immobilized or soluble IL-1ra-ELP. LPS-stimulated monocytes exposed to either soluble or immobilized IL-1ra-ELP were prevented from cell differentiation, showed attenuated expression of pro-inflammatory cytokines, and had increased production of anti-inflammatory and pro-wound healing cytokines. These results suggest that immobilized anti-inflammatory cytokines have the potential to be immunomodulatory biomaterials.

The structure of IL2 bound to the three chains of the IL2 receptor and how signaling occurs

The interleukin-2 molecule and receptor were the first of the interleukins to be discovered and characterized at the molecular level. Now after 20 years of effort, two groups have succeeded in determining the structure of IL2 bound to the external domains of the three receptor chains in a quaternary complex. What do we know now that we did not know before this structural information was available, and how do these new data help us to develop new therapies?

Compensatory energetic mechanisms mediating the assembly of signaling complexes between interleukin-2 and its alpha, beta, and gamma(c) receptors

Interleukin-2 is a key immuno-regulatory cytokine whose actions are mediated by three different cell surface receptors: the alpha, beta and the "common gamma" (gamma(c)) chains. We have undertaken a complete thermodynamic characterization of the stepwise assembly cycle for multiple possible combinations of the receptor-ligand, and receptor-receptor interactions that are necessary for formation of the high-affinity IL-2/alphabetagamma(c) signaling complex. We find an entropically favorable high affinity interaction between IL-2 and its alpha receptor, a moderately entropically favorable low affinity interaction between IL-2 and its beta receptor, and no interaction between IL-2 and the shared receptor, gamma(c). Formation of the stable intermediate trimolecular complexes of IL-2 with alpha and beta receptors, as well as IL-2 with beta and gamma(c) receptors proceeds through enthalpy-entropy compensation mechanisms. Surprisingly, we see a moderate affinity interaction between the unliganded receptor alpha and beta chains, suggesting that a preformed alphabeta complex may serve as the initial interaction complex for IL-2. Reconstitution of the IL-2/Ralphabetagamma(c) high-affinity quaternary signaling complex shows it to be assembled through cooperative energetics to form a 1:1:1:1 assembly. Collectively, the favorable entropy of the bimolecular interactions appears to be offset by the loss in rigid body entropy of the receptor components in the higher-order complexes, but overcome by the formation of increasingly enthalpically favorable composite interfaces. This enthalpic mechanism utilized by gamma(c) contrasts with the favorable entropic mechanism utilized by gp130 for degenerate cytokine interaction. In conclusion, we find that several energetically redundant pathways exist for formation of IL-2 receptor signaling complexes, suggesting a more complex equilibrium on the cell surface than has been previously appreciated.

Binding of small peptides to immobilized antibodies: kinetic analysis by surface plasmon resonance

This unit describes a method for screening small viral peptides as specific antigens using a surface plasmon resonance (SPR) biosensor. The basic protocol in this unit is suited for direct single-step SPR analysis of small ligand-large receptor interactions, where small peptides are used as analytes (injected in the continuous buffer flow) and monoclonal antibodies (MAbs) are immobilized on the SPR sensor chip surface. An alternate protocol is included for situations where kinetic analysis is not possible and uses a surface competition assay to indirectly measure the kinetics of small analyte binding.

Biospecific interaction analysis: a tool for drug discovery and development

The recent development of surface plasmon resonance (SPR)-based biosensor technologies for biospecific interaction analysis (BIA) enables the monitoring of a variety of molecular reactions in real-time. The biomolecular interactions occur at the surface of a flow cell of a sensor chip between a ligand immobilized on the surface and an injected analyte. SPR-based BIA offers many advantages over most of the other methodologies available for the study of biomolecular interactions, including full automation, no requirement for labeling, and the availability of a large variety of activated sensor chips that allow immobilization of DNA, RNA, proteins, peptides and cells. The assay is rapid and requires only small quantitities of both ligand and analyte in order to obtain informative results. In addition, the sensor chip can be re-used many times, leading to low running costs. Aside from the analysis of all possible combinations of peptide, protein, DNA and RNA interactions, this technology can also be used for screening of monoclonal antibodies and epitope mapping, analysis of interactions between low molecular weight compounds and proteins or nucleic acids, interactions between cells and ligands, and real-time monitoring of gene expression. Applications of SPR-based BIA in medicine include the molecular diagnosis of viral infections and genetic diseases caused by point mutations. Future perspectives include the combinations of SPR-based BIA with mass spectrometry, the use of biosensors in proteomics, and the application of this technology to design and develop efficient drug delivery systems.

Analysis of the role of the interleukin-2 receptor gamma chain in ligand binding

Interleukin-2 is the primary T cell growth factor secreted by activated T cells. IL-2 is an alpha-helical cytokine that binds to a multisubunit receptor expressed on the surface of a variety of cell types. IL-2Ralpha, IL-2Rbeta, and IL-2Rgammac receptor subunits expressed on the surface of cells may aggregate to form distinct binding sites of differing affinities. IL-2Rgammac was the last receptor subunit to be identified. It has since been shown to be shared by at least five other cytokine receptors. In this study, we have probed the role of IL-2Rgammac in the assembly of IL-2R complexes and in ligand binding. We demonstrate that in the absence of ligand IL-2Rgammac does not possess detectable affinity for IL-2Ralpha, IL-2Rbeta, or the pseudo-high-affinity binding site composed of preformed IL-2Ralpha/beta. We also demonstrate that IL-2Rgammac possesses an IL-2-dependent affinity for IL-2Rbeta and IL-2Ralpha/beta. We performed a detailed biosensor analysis to examine the interaction of soluble IL-2Rgammac with IL-2-bound IL-2Rbeta and IL-2-bound IL-2Ralpha/beta. The kinetic and equilibrium constants for sIL-2Rgammac binding to these two different liganded complexes were similar, indicating that IL-2Ralpha does not play a role in recruitment of IL-2Rgammac. We also determined that the binding of IL-2 to the isolated IL-2Rgammac was very weak (approximate K(D) = 0.7 mM). The experimental methodologies and principles derived from these studies can be extended to at least five other cytokines that share IL-2Rgammac as a receptor subunit.

Direct kinetic assay of interactions between small peptides and immobilized antibodies using a surface plasmon resonance biosensor

A surface plasmon resonance (SPR) protocol is described for the direct kinetic analysis of small antigenic peptides interacting with immobilized monoclonal antibodies (mAb). High peptide concentrations (up to 2.5 microM) and medium mAb surface densities (about 1.5 ng/mm(2)) are needed to ensure measurable binding levels, and fast buffer flow rates (60 microl/min) are required to minimize diffusion-controlled kinetics. Good reproducibility levels in the kinetic constants are obtained under these analysis conditions (standard deviations below 10% of the mean values). Application of this protocol to determine the antigenic ranking of viral peptides shows an excellent agreement between SPR and previous competition enzyme-linked immunosorbent assays (ELISA) on the same peptide/antibody systems.

The use of coiled-coil interactions for the analysis and micropreparative isolation of adenomatous polyposis coli protein complexes

The coiled coil is a common structural motif found both as the dominant structure in fibrous proteins and as an oligomerization domain in a variety of cytoskeletal and extracellular matrix proteins. Coiled-coils typically consist of two to four helices that are supercoiled around one another in either parallel or antiparallel orientations. In the past few years our knowledge of the structure and specificity of coiled coil interactions has increased, allowing the de novo design and preparation of coiled-coils with well-defined structure and specificity. Indeed, the design and synthesis of a peptide that binds specifically to a single coiled-coil-containing protein, adenomatous polyposis coli (APC) has been reported. We have optimized solid-phase synthesis techniques to produce a modified form of the anti-APC peptide that contains a biotin moiety specifically placed so as to allow selective orientation onto the surface of a biosensor or affinity support. These peptide surfaces have been used to both monitor and purify APC and APC complexes from cellular extracts.

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.

Increased serum-soluble interleukin-2 receptor in burn patients: characterization and effects on the immune system

The consequences of high serum concentrations of the interleukin (IL)-2 receptor alpha chain (sIL-2Ralpha) in several diseases are poorly understood. The objective of this study was to determine the form of sIL-2Ralpha in burn patients and its biological role. sIL-2Ralpha was measured in 18 severely burned individuals who received nutritional support with a normal or low fat content. sIL-2Ralpha was elevated throughout the study and it was notably lower in patients fed a low fat diet. Serum IL-6 and sIL-2Ralpha significantly correlated (r = 0.74, p < 0.05) in burn patients. The presence of sIL-2Ralpha was associated with a decrease in DR molecules in the CD2(-) and CD11b(+) cells of these patients. Western blot analysis of serum protein with N-terminal or C-terminal specific antibodies indicated that sIL-2Ralpha represents the extracellular domain of this molecule. Patient serum inhibited specifically murine, but not human IL-2-dependent T-cell proliferation. To determine the significance of sIL-2Ralpha, recombinant sIL-2Ralpha was used in different cellular model involving IL-2. sIL-2Ralpha inhibited natural killer cell activity by 50% in the presence of IL-2. The basal proliferation of peripheral blood mononuclear cells was inhibited by sIL-2Ralpha, but phytohemagglutinin-induced proliferation was unaffected by this form of receptor. Interferon (INF)-gamma production induced by OKT-3 on peripheral blood mononuclear cells was not altered by sIL-2Ralpha, but IL-2 induced increase in INF-gamma production was suppressed. The decreasing production of INF-gamma in the presence of IL-4 was significantly increased in the presence of sIL-2Ralpha in media. These results show that the large amount of sIL2-Ralpha circulating in burn patients is related to the inflammatory response. The amount of dietary fat modulates sIL2Ralpha concentration in burn patients, confirming the beneficial effect of low fat administration after burn trauma. Inhibition of T-cell activation in burn patients is not directly related to sIL-2Ralpha, although the presence of sIL-2Ralpha in serum can inhibit some IL-2 mediated response, such as the emergence of TH1 and TH2 cells.

Biosensor analysis of the interleukin-2 receptor complex

Surface plasmon resonance (SPR) biosensor technology has been a significant addition to the evolution and refinement of methods to study macromolecular interactions. Prior to the advent of SPR, we employed a variety of biochemical and biological techniques to study the interleukin-2/interleukin-2 receptor system (IL-2/IL-2R). By combining site-directed mutagenesis, equilibrium and kinetic radioligand binding, and competitive biological assays, we and others had begun to understand many aspects of the structure-activity relationships of the IL-2/IL-2R system. Due to the complexity of the IL-2R, cell-based assays proved limited in their ability to provide quantitative information on the binding characteristics of subclasses of the IL-2 receptor. SPR technology promised to be a new and powerful approach to the quantitative analysis of complex receptor systems. To demonstrate the feasibility of this technology, we employed Biacore analysis to investigate the ligand binding characteristics of novel, pre-assembled, IL-2R coiled-coil complexes. The results of these studies, although limited by instrumentation and data analysis, clearly established the utility of this method. Subsequently, by incorporating advancements in both of these areas, we have been able to carry out detailed kinetic analyses of the binding properties of individual IL-2R subunits as well as heteromeric complexes on the surface of a biosensor. Therefore, SPR biosensor analysis combined with other established analytical methods has proven to be a powerful tool for the analysis of complex hematopoietic receptor systems. Published in 1999 by John Wiley & Sons, Ltd.

The use of biosensor technology for the engineering of antibodies and enzymes

Recently developed scientific instrumentation featuring surface plasmon resonance detection allows the detection of biomolecular interactions in real time and without chemical modification of the binding partners. These biosensors are proving invaluable tools in protein engineering, particularly in research aimed at the isolation and improvement of protein binders and catalysts from macromolecular repertoires containing billions of individual members. This article reviews the use of biosensor technology for the isolation and characterization of engineered antibodies and enzymes.

Solution assembly of the pseudo-high affinity and intermediate affinity interleukin-2 receptor complexes

The high affinity interleukin-2 receptor is composed of three cell surface subunits, IL-2Ralpha, IL-2Rbeta, and IL-2Rgamma. Functional forms of the IL-2 receptor exist, however, that enlist only two of the three subunits. On activated T-cells, the alpha- and beta-subunits combine as a preformed heterodimer (the pseudo-high affinity receptor) that serves to capture IL-2. On a subpopulation of natural killer cells, the beta- and gamma-subunits interact in a ligand-dependent manner to form the intermediate affinity receptor site. Previously, we have demonstrated the feasibility of employing coiled-coil molecular recognition for the solution assembly of a heteromeric IL-2 receptor complex. In that study, although the receptor was functional, the coiled-coil complex was a trimer rather than the desired heterodimer. We have now redesigned the hydrophobic heptad sequences of the coiled-coils to generate soluble forms of both the pseudo-high affinity and the intermediate affinity heterodimeric IL-2 receptors. The properties of these complexes were examined and their relevance to the physiological IL-2 receptor mechanism is discussed.

Partial signaling by cytokines: cytokine regulation of cell cycle and Fas-dependent, activation-induced death in CD4+ subsets

Fas-dependent, activation-induced death (AID) of T cells has been implicated in the regulation of peripheral T cell populations. We have previously reported that IL-2 plays a unique role in regulating sensitivity to AID in primary CD4+ cells. In this report we have compared the capacity of IL-2, IL-4, and IL-7 to increase entry into cell cycle vs their capacity to increase sensitivity to AID. Our data indicate that IL-2 plays a unique role in the regulation of AID in both Th1 and Th2 subsets and that with a given AID stimulus, cell cycle progression is necessary, but not sufficient, for AID. Interestingly, induction of cell cycle entry and sensitivity to AID can be dissociated (partial signaling) not only with different cytokines, but even with point mutations in IL-2 itself. This provides the first evidence that cytokine variants or pharmacological agents that mimic their action will be useful in enhancing selective elements of pleiotropic cytokine actions.

Analysis of a peptide hormone-receptor interaction in the yeast two-hybrid system

Interaction between a peptide hormone and extracellular domains of its receptor is a crucial step for initiation of hormone action. We have developed a modification of the yeast two-hybrid system to study this interaction and have used it to characterize the interaction of insulin-like growth factor 1 (IGF-1) with its receptor by using GAL4 transcriptional regulation with a beta-galactosidase assay as readout. In this system, IGF-1 and proIGF-1 bound to the cysteine-rich domain, extracellular domain, or entire IGF-1 proreceptor. This interaction was specific. Thus, proinsulin showed no significant interaction with the IGF-1 receptor, while a chimeric proinsulin containing the C-peptide of IGF-1 had an intermediate interaction, consistent with its affinity for the IGF-1 receptor. Over 2000 IGF-1 mutants were generated by PCR and screened for interaction with the color assay. About 40% showed a strong interaction, 20% showed an intermediate interaction, and 40% give little or no signal. Of 50 mutants that were sequenced, several (Leu-5 --> His, Glu-9 --> Val, Arg-37 --> Gly, and Met-59 --> Leu) appeared to enhance receptor association, others resulted in weaker receptor interaction (Tyr-31 --> Phe and Ile-43 --> Phe), and two gave no detectable signal (Leu-14 --> Arg and Glu-46 --> Ala). Using PCR-based mutagenesis with proinsulin, we also identified a gain of function mutant (proinsulin Leu-17 --> Pro) that allowed for a strong IGF-1-receptor interaction. These data demonstrate that the specificity of the interaction between a hormone and its receptor can be characterized with high efficiency in the two-hybrid system and that novel hormone analogues may be found by this method.

Biosensors in immunology: the story so far

In conclusion, biosensors are versatile tools with a range of applications. With a thorough knowledge of possible artefacts and limitations, it possible to perform assays that were heretofore not practicable in immunology.

The functional display of interleukin-2 on filamentous phage

We report the novel display of interleukin-2 (IL-2) and an IL-2 analog, D126, on the surface of filamentous bacteriophage using a phagemid vector system. A synthetic human IL-2 gene and its D126 analog were fused to the carboxyl-terminal domain of the gene III minor phage coat protein. Expression of IL-2 and D126 was verified by their reactivity with an IL-2-specific antibody. Biological response of IL-2 phage on murine CTLL-2 cells was comparable to that of recombinant soluble IL-2, while the D126 phage displayed a reduced biological response similar to that previously measured by soluble D126 protein. Biosensor surface plasmon resonance was employed to verify binding of the IL-2 and D126 phage to the IL-2 alpha beta cc receptor complex. A 41-fold enrichment of IL-2 phage over R408 helper phage was demonstrated in biopanning affinity selection studies employing biotinylated alpha beta cc receptor complex. These biopanning studies are the first reports of affinity selection of IL-2 phage and demonstrate a novel use for the alpha beta cc receptor complex. Together, these studies confirm that the structural integrity of IL-2 and D126 is maintained when they are displayed as a gIIIp fusion protein on phage particles and provide the foundation for further selection studies employing IL-2 analog phage libraries.

Kinetic analysis of macromolecular interactions using surface plasmon resonance biosensors

Surface plasmon resonance based biosensors are being used to define the kinetics of a wide variety of macromolecular interactions. As the popularity of this approach grows, experimental design and data analysis methods continue to evolve. These advances are making it possible to accurately define the assembly mechanisms and rate constants associated with macromolecular interactions.

Use of surface plasmon resonance to probe the equilibrium and dynamic aspects of interactions between biological macromolecules

Surface plasmon resonance biosensors have become increasingly popular for the qualitative and quantitative characterization of the specific binding of a mobile reactant to a binding partner immobilized on the sensor surface. This article reviews the use of this new technique to measure the binding affinities and the kinetic constants of reversible interactions between biological macromolecules. Immobilization techniques, the most commonly employed experimental strategies, and various analytical approaches are summarized. In recent years, several sources of potential artifacts have been identified: immobilization of the binding partner, steric hindrance of binding to adjacent binding sites at the sensor surface, and finite rate of mass transport of the mobile reactant to the sensor surface. Described here is the influence of these artifacts on the measured binding kinetics and equilibria, together with suggested control experiments.

Kinetic analysis of ligand binding to interleukin-2 receptor complexes created on an optical biosensor surface

The interleukin-2 receptor (IL-2R) is composed of at least three cell surface subunits, IL-2R alpha, IL-2R beta, and IL-2R gamma c. On activated T-cells, the alpha- and beta-subunits exist as a preformed heterodimer that simultaneously captures the IL-2 ligand as the initial event in formation of the signaling complex. We used BIAcore to compare the binding of IL-2 to biosensor surfaces containing either the alpha-subunit, the beta-subunit, or both subunits together. The receptor ectodomains were immobilized in an oriented fashion on the dextran matrix through unique solvent-exposed thiols. Equilibrium analysis of the binding data established IL-2 dissociation constants for the individual alpha- and beta-subunits of 37 and 480 nM, respectively. Surfaces with both subunits immobilized, however, contained a receptor site of much higher affinity, suggesting the ligand was bound in a ternary complex with the alpha- and beta-subunits, similar to that reported for the pseudo-high-affinity receptor on cells. Because the binding responses had the additional complexity of being mass transport limited, obtaining accurate estimates for the kinetic rate constants required global fitting of the data sets from multiple surface densities of the receptors. A detailed kinetic analysis indicated that the higher-affinity binding sites detected on surfaces containing both alpha- and beta-subunits resulted from capture of IL-2 by a preformed complex of these subunits. Therefore, the biosensor analysis closely mimicked the recognition properties reported for these subunits on the cell surface, providing a convenient and powerful tool to assess the structure-function relationships of this and other multiple subunit receptor systems.

cDNA cloning and characterization of the human interleukin 13 receptor alpha chain

We have cloned cDNAs corresponding to the human interleukin 13 receptor alpha chain (IL-13Ralpha). The protein has 76% homology to murine IL-13Ralpha, with 95% amino acid identity in the cytoplasmic domain. Only weak IL-13 binding activity was found in cells transfected with only IL-13Ralpha; however, the combination of both IL-13Ralpha and IL-4Ralpha resulted in substantial binding activity, with a Kd of approximately 400 pM, indicating that both chains are essential components of the IL-13 receptor. Whereas IL-13Ralpha serves as an alternative accessory protein to the common cytokine receptor gamma chain (gammac) for IL-4 signaling, it could not replace the function of gammac in allowing enhanced IL-2 binding activity. Nevertheless, the overall size and length of the cytoplasmic domain of IL-13Ralpha and gammac are similar, and like gammac, IL-13Ralpha is located on chromosome X.

A point mutation in interleukin-2 that alters ligand internalization

In previous studies, we have identified an interleukin-2 (IL-2) analog containing a point mutation at position 51 (T51P) that expresses nearly wild-type bioactivity, yet has approximately 10-fold lower receptor binding affinity. Since ligand-dependent receptor internalization may be the rate-limiting step controlling the duration of IL-2 receptor signaling, a reduction in the receptor internalization rate could contribute to the observed response enhancement for this analog. To evaluate this possibility, we compared the internalization of IL-2 and T51P in three separate assays. While the internalization rate for IL-2 agreed with values determined by others, the internalization of T51P was markedly reduced. The receptor binding rate constants for this analog were only slightly different; thus, altered binding kinetics could not explain the decreased internalization rate. The effects of reduced internalization were also observable in bioassays, where T51P maintained T-cell proliferation for a longer period compared with IL-2. These results indicate that the T51P point mutation reduces the receptor internalization rate compared with IL-2 in a fashion that is independent of the dissociation rate. This analog may represent a new approach to the preparation of cytokine analogs with potentiated agonist and antagonist properties.

Solution assembly of a soluble, heteromeric, high affinity interleukin-2 receptor complex

In this study, we report the use of coiled-coil (leucine zipper) molecular recognition for the solution assembly of stable, high affinity, heteromeric interleukin-2 receptor complexes. Co-expression of interleukin-2 receptor alpha and beta extracellular domains (ectodomains), each fused to seven coiled-coil heptad repeats, resulted in the formation of heteromeric complexes that bound interleukin-2 in a cooperative fashion and with much higher affinity than similar homomeric complexes. The dissociation constants for these solution complexes are within the range of values reported for the comparable cell surface "pseudo high affinity" interleukin-2 receptor. Ligand-induced cross-linking of homomeric or heteromeric receptor subunits is the common signal transmission mechanism employed by hematopoietin receptors. Individual receptor ectodomains, however, often do not bind ligand with measurable affinity. This is the first study to demonstrate the feasibility of coiled-coil mediated preassembly of cytokine receptor complexes.