A human interleukin 3 analog with increased biological and binding activities

Structural and evolutionary exploration of the IL-3 family and its alpha subunit receptors

Interleukin-3 (IL-3) is a cytokine belonging to the family of common β (βc) and is involved in various biological systems. Its activity is mediated by the interaction with its receptor (IL-3R), a heterodimer composed of two distinct subunits: IL-3Rα and βc. IL-3 and its receptor, especially IL-3Rα, play a crucial role in pathologies like inflammatory diseases and therefore are interesting therapeutic targets. Here, we have performed an analysis of these proteins and their interaction based on structural and evolutionary information. We highlighted that IL-3 and IL-3Rα structural architectures are conserved across evolution and shared with other proteins belonging to the same βc family interleukin-5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The IL-3Rα/IL-3 interaction is mediated by a large interface in which most residues are surprisingly not conserved during evolution and across family members. In spite of this high variability, we suggested small regions constituted by few residues conserved during the evolution in both proteins that could be important for the binding affinity.

GM-CSF, IL-3, and IL-5 Family of Cytokines: Regulators of Inflammation

The β common chain cytokines GM-CSF, IL-3, and IL-5 regulate varied inflammatory responses that promote the rapid clearance of pathogens but also contribute to pathology in chronic inflammation. Therapeutic interventions manipulating these cytokines are approved for use in some cancers as well as allergic and autoimmune disease, and others show promising early clinical activity. These approaches are based on our understanding of the inflammatory roles of these cytokines; however, GM-CSF also participates in the resolution of inflammation, and IL-3 and IL-5 may also have such properties. Here, we review the functions of the β common cytokines in health and disease. We discuss preclinical and clinical data, highlighting the potential inherent in targeting these cytokine pathways, the limitations, and the important gaps in understanding of the basic biology of this cytokine family.

In silico mutational analysis and identification of stability centers in human interleukin-4

Interleukin-4 (IL-4) is a multifunctional cytokine that plays a critical role in apoptosis, differentiation and proliferation. The intensity of IL4 response depends upon binding to its receptor, IL-4R. The therapeutic efficiency of interleukins can be increased by generating structural mutants having greater stability. In the present work, attempts were made to increase the stability of human IL-4 using in-silico site directed mutagenesis. Different orthologous sequences of IL4 from Pan troglodytes, Aotusnigriceps, Macacamulatta, Papiohamadryas, Chlorocebusaethiops, Vicugnapacos, Susscrofa and Homo sapiens were aligned using Clustal Omega that revealed the conserved and non-conserved positions. For each non-conserved position, possible favorable and stabilizing mutations were found using CUPSAT with predicted ΔΔG (kcal/mol). The one with highest ΔΔG (kcal/mol) among all possible mutations, for each non-conserved position was selected and introduced manually in human IL-4 sequence resulting in multiple mutants of IL-4. Mutant proteins were modeled using structure of IL4 (PDB ID: 2B8U) as a template by SWISS MODEL. The mutants A49L and Q106T were identified to have stability centre using SCide. Molecular dynamics and docking analysis also confirmed the mutants stability and binding respectively. Mutants A49L and Q106T had -7.580079 kcal/mol and -39.418124 kcal/mol respectively lesser energy value than the wild type IL4. The result suggested that, the stability of human IL-4 has been increased by mutation.

A dual role for the N-terminal domain of the IL-3 receptor in cell signalling

The interleukin-3 (IL-3) receptor is a cell-surface heterodimer that links the haemopoietic, vascular and immune systems and is overexpressed in acute and chronic myeloid leukaemia progenitor cells. It belongs to the type I cytokine receptor family in which the α-subunits consist of two fibronectin III-like domains that bind cytokine, and a third, evolutionarily unrelated and topologically conserved, N-terminal domain (NTD) with unknown function. Here we show by crystallography that, while the NTD of IL3Rα is highly mobile in the presence of IL-3, it becomes surprisingly rigid in the presence of IL-3 K116W. Mutagenesis, biochemical and functional studies show that the NTD of IL3Rα regulates IL-3 binding and signalling and reveal an unexpected role in preventing spontaneous receptor dimerisation. Our work identifies a dual role for the NTD in this cytokine receptor family, protecting against inappropriate signalling and dynamically regulating cytokine receptor binding and function.

The N-terminal domain (NTD) of interleukin-3 receptor α-subunit (IL3Rα) is involved in IL-3 recognition but the underlying mechanism is unknown. Here, the authors present crystal structures of the IL3Rα complex and provide biochemical evidence that the NTD regulates IL-3 binding and signalling complex assembly.

AntiCD3Fv fused to human interleukin-3 deletion variant redirected T cells against human acute myeloid leukemic stem cells

Background

Leukemic stem cells (LSCs) are frequently seen as a cause of treatment failure and relapse in patients with acute myeloid leukemia (AML). Thus, successful new therapeutic strategies for the treatment of AML should aim at eradicating LSCs. The identification of targets on the cell surface of LSCs is getting more and more attention. Among these, CD123, also known as the interleukin-3 (IL3)-receptor α chain, has been identified as a potential immunotherapeutic target due to its overexpression on LSCs in AML as well as on AML blasts, rather than normal hematopoietic stem cells.

Methods

We constructed a CD123-targeted fusion protein antiCD3Fv-⊿IL3, with one binding site for T cell antigen receptor (TCRCD3) and the other for CD123, by recombinant gene-engineering technology. Cysteine residues were introduced into the V domains of the antiCD3Fv segment to enhance its stability by locking the two chains of Fv together with disulfide covalent bonds. The stability and cytotoxicity of the two fusion proteins were detected in vitro and in vivo.

Results

Both fusion proteins were produced and purified from Escherichia coli 16C9 cells with excellent yields in fully active forms. High-binding capability was observed between these two fusion proteins and human IL3R, leading to the specific lysis of CD123-expressing cell lines KG1a; also, mononuclear cells from primary AML patients were inhibited in a colony forming assay in vitro, presumably by redirecting T lymphocytes in vitro. In addition, they displayed an antileukemic activity against KG1a xenografts in non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice, especially disulfide-stabilized (ds)-antiCD3Fv-⊿IL3 for its improved stability.

Conclusions

These results suggest that both fusion proteins display the antileukemic activity against CD123-expressing cell lines as well as leukemic progenitors in vitro and in vivo, especially ds-antiCD3Fv-⊿IL3. They could be the promising candidates for future immunotherapy of AML.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0109-5) contains supplementary material, which is available to authorized users.

The GM-CSF/IL-3/IL-5 cytokine receptor family: from ligand recognition to initiation of signaling

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are members of a discrete family of cytokines that regulates the growth, differentiation, migration and effector function activities of many hematopoietic cells and immunocytes. These cytokines are involved in normal responses to infectious agents, bridging innate and adaptive immunity. However, in certain cases, the overexpression of these cytokines or their receptors can lead to excessive or aberrant initiation of signaling resulting in pathological conditions, with chronic inflammatory diseases and myeloid leukemias the most notable examples. Recent crystal structures of the GM-CSF receptor ternary complex and the IL-5 binary complex have revealed new paradigms of cytokine receptor activation. Together with a wealth of associated structure-function studies, they have significantly enhanced our understanding of how these receptors recognize cytokines and initiate signals across cell membranes. Importantly, these structures provide opportunities for structure-based approaches for the discovery of novel and disease-specific therapeutics. In addition, recent biochemical evidence has suggested that the GM-CSF/IL-3/IL-5 receptor family is capable of interacting productively with other membrane proteins at the cell surface. Such interactions may afford additional or unique biological activities and might be harnessed for selective modulation of the function of these receptors in disease.

Characterization of variant diphtheria toxin-interleukin-3 fusion protein, DTIL3K116W, for phase I clinical trials

We have produced clinical grade of DTIL3K116W, a variant diphtheria toxin-interleukin-3 fusion protein, for treatment of acute myeloid leukemia. The product was filter sterilized, aseptically vialed, and stored at -80 degrees C. It was characterized by Coomassie-stained SDS-PAGE, endotoxin assay, cytotoxicity assay, sterility, mass spectroscopy, receptor binding affinity, ADP-ribosylation, inhibition of normal human CFU-GM, disulfide bond analysis, immunoblots, stability, size exclusion chromatography-HPLC, sequencing, and immunohistochemistry. Vialed product was sterile in 0.25 M NaCl/5 mM Tris, pH 7.9, and had a protein concentration of 1.08 mg/ml. Purity by SDS-PAGE was >99%. Aggregates by HPLC were <1%. Endotoxin levels were 0.296EU/mg. Peptide mapping and mass spectroscopy confirmed its composition and molecular weight. The vialed drug kept reactivity with anti-IL3 and DT antibodies. Potency study revealed a 48-h EC(50) of 0.5 pM on TF1/H-ras cell. Its binding properties were confirmed by competitive experiments showing IC(50) of 1.4 nM. ADP-ribosylation activity was equivalent to DTGM-CSF. Drug did not react with tested frozen human tissue sections by immunohistochemistry. There was no evidence of loss of solubility, proteolysis aggregation, or loss of potency over 6 months at -80 degrees C. Further, the drug was stable at 4 and 25 degrees C in the plastic syringe and administration tubing for 48 h.

A cytokine-cytokine interaction in the assembly of higher-order structure and activation of the interleukine-3:receptor complex

Interleukine-3 (IL-3) binds its receptor and initiates a cascade of signaling processes that regulate the proliferation and differentiation of hematopoietic cells. To understand the detailed mechanisms of IL-3 induced receptor activation, we generated a homology model of the IL-3:receptor complex based on the closely related crystal structure of the GM-CSF:receptor complex. Model-predicted interactions between IL-3 and its receptor are in excellent agreement with mutagenesis data, which validate the model and establish a detailed view of IL-3:receptor interaction. The homology structure reveals an IL-3:IL-3 interaction interface in a higher-order complex modeled after the dodecamer of the GM-CSF:receptor complex wherein an analogous GM-CSF:GM-CSF interface is also identified. This interface is mediated by a proline-rich hydrophobic motif (PPLPLL) of the AA' loop that is highly exposed in the structure of isolated IL-3. Various experimental data suggest that this motif is required for IL-3 function through receptor-binding independent mechanisms. These observations are consistent with structure-function studies of the GM-CSF:receptor complex showing that formation of the higher-order cytokine:receptor complex is required for signaling. However, a key question not answered from previous studies is how cytokine binding facilitates the assembly of the higher-order complex. Our studies here reveal a potential cytokine-cytokine interaction that participates in the assembly of the dodecamer complex, thus linking cytokine binding to receptor activation.

Diphtheria toxin fused to variant interleukin-3 provides enhanced binding to the interleukin-3 receptor and more potent leukemia cell cytotoxicity

Chemoresistance is a common cause of treatment failure in patients with acute myeloid leukemia (AML). We generated a diphtheria toxin (DT) fusion protein composed of the catalytic and translocation domains of DT (DT388) fused to interleukin-3 (IL-3). IL-3 receptors (IL-3R) are overexpressed on blasts from many AML patients. DT388IL-3 showed cytotoxicity to leukemic blasts in vitro and in vivo and minimal damage to normal tissues in nonhuman primate models. However, only a fraction of patient leukemic samples were sensitive to the agent. To enhance the potency and specificity of the DT388IL-3 molecule, we constructed variants with altered residues in the IL-3 moiety. Two of these variants, DT388IL-3[K116W] and DT388IL-3[Delta125-133], were produced and partially purified from Escherichia coli with excellent yields. They showed enhanced binding to the human IL-3R and greater cytotoxicity to human leukemia cell lines relative to wild-type DT388IL-3. Interestingly, the results support a previously hypothesized model for interaction of the C-terminal residues of IL-3 with a hydrophobic patch on the alpha-subunit of IL-3R. Rational modification of the targeting domain based on structural analysis can produce a fusion toxin with increased ability to kill tumor cells. One or both of these variant fusion proteins merit further development for therapy of chemotherapy refractory AML.

Evidence for a lectin activity for human interleukin 3 and modeling of its carbohydrate recognition domain

We demonstrate that human interleukin 3 (IL-3) is a lectin recognizing specifically the glycosaminoglycan part of a chondroitin sulfate proteoglycan (PGS3; Normand, G., Kuchler, S., Meyer, A., Vincendon, G., and Zanetta, J. P. (1988) J. Neurochem. 51, 665-676) isolated from the adult rat brain. The specificity of the interaction of this particular proteoglycan with IL-3 is due to the abundance of GlcA(2S)beta 1,3GalNAc(4S)beta 1 disaccharide units as suggested by (1)H NMR. Computational docking experiments of the lower energy conformers of the different disaccharides from chondroitin sulfates reveal a privileged binding site for GlcA(2S)beta 1,3GalNAc(4S)beta 1 (involving His-26, Arg-29, Asn-70, and Trp-104) localized in an area of IL-3 different from the receptor-binding domain previously identified by others (Bagley, C. J., Phillips, J., Cambareri, B., Vadas, M. A., and Lopez, A. F. (1996) J. Biol. Chem. 271, 31922-31928). Molecular modeling of the mutation P33G, described as increasing the biological activity of IL-3 without affecting its receptor binding (Lokker, N. A., Movva, N. R., Strittmatter, U., Fagg, B., and Zenke, G. (1991) J. Biol. Chem. 266, 10624-10631) provokes a change of the three-dimensional structure of IL-3, especially in the area of the putative carbohydrate recognition domain defined above. Computational docking experiments of the different disaccharides of chondroitin sulfates indicate a loss of affinity for the previous ligand but a higher affinity for the classic disaccharide of chondroitin-4-sulfate. This change from a rare and specific ligand to a more abundant constituent of proteoglycans could induce an increased quantitative association between the IL-3 receptors and its ligands and, consequently, an increased signaling.

Receptor binding kinetics of human IL-3 variants with altered proliferative activity

The binding kinetics of native IL-3 and a set of truncated IL-3 variants to the alpha subunit of the IL-3 receptor (IL-3Ralpha) were studied using surface plasmon resonance. These variants, with amino acid substitutions at residues, 22, 42, 43, 45, 46, 113, or 116, have previously been identified to have altered capacity to stimulate cell proliferation compared to native IL-3(1-133). In this study, variants E43N and F113Y exhibited >100-fold slower association rates than IL-3(15-125) consistent with residues 43 and 113 being essential for the binding of IL-3 to the IL-3Ralpha. Variants G42A, G42D, Q45V, D46S, K116V, and K116W exhibited increased association rates (up to 15-fold relative to IL-3(15-125)) and decreased dissociation rates (up to 7-fold). The results demonstrate that both the association and dissociation rates for the binding of IL-3 to the IL-3Ralpha are altered by truncation and by amino acid substitution at individual sites. Intracellular signaling studies using K116W and E43N demonstrate that differences in the IL-3alpha binding characteristics are reflected in magnitude and kinetics of STAT5 phosphorylation.

The receptor binding site of human interleukin-3 defined by mutagenesis and molecular modeling

Interleukin-3 (IL-3) is a member of the cytokine superfamily that promotes multi-potential hematopoietic cell growth by interacting with a cell surface receptor composed of alpha and beta chains. The newly available three-dimensional structure of a variant of human (h) IL-3 allowed us to evaluate new and existing mutagenesis data and to rationally interpret the structure-function relationship of hIL-3 on a structural basis. The amino acid residues that were identified to be important for hIL-3 activity are grouped into two classes. The first class consists of largely hydrophobic residues required for the structural integrity of the protein, including the residues in IL-3 that are largely conserved among 10 mammalian species. These residues form the core of a scaffold for the second class of more rapidly diverging solvent-exposed residues, likely to be required for interaction with the receptor. Ten important and solvent-exposed residues, Asp21, Gly42, Glu43, Gln45, Asp46, Met49, Arg94, Pro96, Phe113, and Lys116, map to one side of the protein and form a putative binding site for the alpha subunit of the receptor. A model of the IL-3.IL-3 receptor complex based on the human growth hormone (hGH).hGH soluble receptor complex structure suggests that the interface between IL-3 and the IL-3 receptor alpha subunit consists of a cluster of hydrophobic residues flanked by electrostatic interactions. Although the IL-3/IL-3 receptor beta subunit interface cannot be uniquely located due to the lack of sufficient experimental data, several residues of the beta subunit that may interact with Glu22 of IL-3 are proposed. The role of these residues can be tested in future mutagenesis studies to define the interaction between IL-3 and IL-3 receptor beta subunit.

Roles of the N and C Terminal Domains of the Interleukin-3 Receptor α Chain in Receptor Function

The interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and IL-5 receptor α chains are each composed of three extracellular domains, a transmembrane domain and a short intracellular region. Domains 2 and 3 constitute the cytokine receptor module (CRM), typical of the cytokine receptor superfamily; however, the function of the N-terminal domain is not known. We have investigated the functions of the N-terminal and C-terminal domains of the IL-3 receptor (IL-3R) α chain. We find that cells transfected with the receptor β chain (hβc) and a truncated IL-3Rα that is devoid of the intracellular region fail to proliferate or to activate STAT5 in response to human IL-3, despite binding the IL-3 with affinity indistiguishable from that of full-length receptor. In addition, IL-3–induced phosphorylation of hβc was not detected. Thus, the IL-3Rα intracellular region does not contribute detectably to stabilization of the receptor/ligand complex, but is essential for signal propagation. In contrast, a truncated IL-3Rα with the N-terminal domain deleted interacts functionally with the β chain; mouse cells transfected with these receptor chains proliferate in response to human IL-3 and STAT5 transcription factor is activated. High- and low-affinity binding sites are retained, although the affinity for IL-3 is decreased 15-fold, indicating a significant role for the N-terminal domain in IL-3 binding.

A discontinuous eight-amino acid epitope in human interleukin-3 binds the alpha-chain of its receptor

We have previously reported that, within the first helix of human interleukin (IL)-3, residues Asp21 and Glu22 are important for interaction with the alpha- and beta-chains of the IL-3 receptor, respectively. In order to define more precisely the sites of interaction with the receptor, we have performed molecular modeling of the helical core of IL-3 and single amino acid substitution mutagenesis of residues predicted to lie on the surfaces of the A, C, and D helices. The resulting analogues were characterized for their abilities to stimulate proliferation of TF-l cells and for binding to the high affinity (alpha- and beta-chain; IL-3Ralpha/Rbeta) or low affinity (alpha-chain alone; IL-3Ralpha) IL-3 receptor. We found that in addition to Asp21, residues Ser17, Asn18, and Thr25 within the A helix and Arg108, Phe113, Lys116, and Glu119 within the D helix of IL-3 were important for biological activity. Analysis of their binding characteristics revealed that these analogues were deficient in binding to both the IL-3Ralpha/Rbeta and the IL-3Ralpha forms of the receptor, consistent with a selective impairment of interaction with IL-3Ralpha. Molecular modeling suggests that these eight amino acid residues are adjacent in the tertiary structure, consistent with a discontinuous epitope interacting selectively with IL-3Ralpha. On the other hand, Glu22 of IL-3 was found to interact preferentially with the beta-chain with bulky and positively charged substitutions causing greater than 10,000-fold reduction in biological activity. These results show fundamental differences between IL-3 and granulocyte-macrophage colony-stimulating factor in the structural basis for recognition of their receptors that has implications for the construction of novel analogues and our understanding of receptor activation.

Binding in the growth hormone receptor complex

Binding reactions between human growth hormone (hGH) and its receptor provide a detailed account of how a polypeptide hormone activates its receptor and more generally how proteins interact. Through high-resolution structural and functional studies it is seen that hGH uses two different sites (site 1 and site 2) to bind two identical receptor molecules. This sequential dimerization reaction activates the receptor, presumably by bringing the intracellular domains into close proximity so they may activate cytosolic components. As a consequence of this mechanism it is possible to build antagonists to the receptor by introducing mutations in hGH that block binding at site 2 and to build even more potent antagonists by combining these with mutants that enhance binding at site 1. Alanine-scanning mutagenesis of all contact residues at the site 1 interface shows that only a small and complementary set of side chains clustered near the center of the interface affects binding. The most important contacts are hydrophobic, and these are surrounded by polar and charged interactions of lesser importance. Kinetic analysis shows for the most part that the important side chains function to maintain the complex, not to guide the hormone to the receptor. Hormone-induced homodimerization or heterodimerization reactions are turning out to be pervasive mechanisms for signal transduction. Moreover, the molecular recognition principles seen in the hGH-receptor complex are likely to generalize to other protein-protein complexes.

Saturation mutagenesis of human interleukin-3

A deletion variant of human interleukin-3, hIL-3(15-125), was produced in the periplasmic space of Escherichia coli and had full activity in an AML193.1.3 cell proliferation assay. Libraries of random single-amino acid substitutions were constructed at each of 105 positions in the gene for hIL-3(15-125). Approximately eight single-site substitutions at each position were produced in osmotic shock fractions and screened for activity. 15 mutants were found with bioactivity of 5-26-fold greater than that of native hIL-3. The majority of amino acids in hIL-3(15-125) could be substituted without substantial loss of activity. Substitution of residues predicted to be in the hydrophobic core of the protein often resulted in reduced activity and/or low accumulation levels. Only five residues predicted to be on the surface of the protein were intolerant of substitution and hence are candidates for sites of interaction with the receptor. We therefore propose that the majority of residues in hIL-3 serve a structural role and permit the display of a few key residues in the correct configuration for recognition by the receptor.

Molecular modeling of the GM-CSF and IL-3 receptor complexes

A model for the structure of the cytokine interleukin-3 (IL-3) is presented based on the structural homology of the hematopoietic cytokines and utilizing the crystal structures of interleukin-5 and granulocyte macrophage colony stimulating factor (GM-CSF). In addition, models of the receptor complexes of GM-CSF and IL-3 are presented based on the structural homology of the hematopoietic receptors to growth hormone. Several key interactions between the ligands and their receptors are discovered, some in agreement with previous mutagenesis studies and others that have not yet been the subject of mutagenesis studies. The models provide insights into the binding of GM-CSF and IL-3 to their receptors.

Identification of key charged residues of human interleukin-5 in receptor binding and cellular activation

Interleukin-5 (IL-5) is a cytokine that plays a major role in the differentiation and activation of eosinophils. In order to identify which charged residues of human IL-5 are important in binding to its receptor and subsequent cellular activation, we have systematically replaced all of the clusters of charged amino acids with alanine residues. The mutants have been expressed in Escherichia coli, renatured, and purified. They were assayed for ability to cause proliferation of the erythroleukaemic cell line TF-1 and the up-regulation of eosinophil adhesion to ICAM-1. In addition, we studied receptor binding using either immobilized recombinant IL-5 receptor alpha-chain or the alpha/beta-receptor complex expressed on TF-1 cells. The key charged residue involved in binding to the beta-chain of the receptor is Glu-12. This residue is in an identical position to those previously identified in IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) involved in binding to the receptor beta-chain. The alpha-chain binding site is shown to involve the side chains Arg-90 and Glu-109, located in the second beta sheet and after the end of the fourth helix, respectively. It is unique to IL-5 and does not occur in IL-3 or GM-CSF. Understanding the topology of the interaction of IL-5 with its receptor chains will help in the search for rationally designed antagonists of IL-5 function.

Identification of receptor-binding domains on human interleukin 5 and design of an interleukin 5-derived receptor antagonist

A detailed structure-function analysis of human interleukin 5 (hIL5) has been performed. The hIL5 receptor is composed of two different polypeptide chains, the alpha and beta subunits. The alpha subunit alone is sufficient for ligand binding, but association with the beta subunit leads to a 2- to 3-fold increase in binding affinity. The beta chain is shared with the receptors for IL3 and granulocyte/macrophage-colony-stimulating factor--hence the descriptor beta C (C for common). All hIL5 mutants were analyzed in a solid-phase binding assay for hIL5R alpha interaction and in a proliferation assay using IL5-dependent cell lines for receptor-complex activation. Most residues affecting binding to the receptor alpha subunit were clustered in a loop connecting beta-strand 1 and helix B (mutants H38A, K39A, and H41A), in beta-strand 2 (E89A and R91A; weaker effect for E90A) and close to the C terminus (T109A, E110A, W111S, and I112A). Mutations at one position, E13 (Glu13), caused a reduced activation of the hIL5 receptor complex. In the case of E13Q, only 0.05% bioactivity was detected on a hIL5-responsive subclone of the mouse promyelocytic cell line FDC-P1. Moreover, on hIL5-responsive TF1 cells, the same mutant was completely inactive and proved to have antagonistic properties. Interactions of this mutant with both receptor subunits were nevertheless indistinguishable from those of nonmutated hIL5 by crosslinking and Scatchard plot analysis of transfected COS-1 cells.

Homology model of human interferon-alpha 8 and its receptor complex

Human interferon-alpha 8 (HuIFN alpha 8), a type I interferon (IFN), is a cytokine belonging to the hematopoietic super-family that includes human growth hormone (HGH). Recent data identified two human type I IFN receptor components. One component (p40) was purified from human urine by its ability to bind to immobilized type I IFN. A second receptor component (IFNAR), consisting of two cytokine receptor-like domains (D200 and D200'), was identified by expression cloning. Murine cells transfected with a gene encoding this protein were able to produce an antiviral response to human IFN alpha 8. Both of these receptor proteins have been identified as members of the immunoglobulin superfamily of which HGH receptor is a member. The cytokine receptor-like structural motifs present in p40 and IFNAR were modeled based on the HGH receptor X-ray structure. Models of the complexes of HuIFN alpha 8 with the receptor subunits were built by superpositioning the conserved C alpha backbone of the HuIFN alpha 8 and receptor subunit models with HGH and its receptor complex. The HuIFN alpha 8 model was constructed from the C alpha coordinates of murine interferon-beta crystal structure. Electrostatic potentials and hydrophobic interactions appear to favor the model of HuIFN alpha 8 interacting with p40 at site 1 and the D200' domain of IFNAR at site 2 because there are regions of complementary electrostatic potential and hydrophobic interactions at both of the proposed binding interfaces. Some of the predicted receptor binding residues within HuIFN alpha 8 correspond to functionally important residues determined previously for human IFN alpha 1, IFN alpha 2, and IFN alpha 4 subtypes by site-directed mutagenesis studies. The models predict regions of interaction between HuIFN alpha 8 and each of the receptor proteins, and provide insights into interactions between other type I IFNs (IFN-alpha subtypes and IFN-beta) and their respective receptor components.

Structure/function analysis of human interleukin 5 and its receptor

We have performed a detailed structure-function analysis of human interleukin 5 (hIL5) and its receptor. By testing a hIL5 mutant panel in a solid phase binding assay and a proliferation assay using hIL5 dependent cell-lines, areas on hIL5 involved in either the receptor alpha-subunit interaction or in receptor activation were identified. Epitope mapping data of a neutralizing and a non-neutralizing monoclonal antibody were in agreement with the mutant analysis. hIL5 binding areas on the IL5R alpha-subunit were identified by interspecies chimaera analysis. Finally, hIL5 mutants with reduced receptor activation potential have antagonistic properties.

Involvement of the fourth alpha-helix of mouse granulocyte-macrophage colony-stimulating factor in binding to the alpha-subunit of the receptor complex

Mouse granulocyte-macrophage colony-stimulating factor (mGM-CSF) proteins with substitutions for residues located within alpha-helix D were examined for biological activity and receptor binding properties. Alanine substitutions of the surface exposed positions indicated that several residues contribute to the ligand-receptor interface. Position K108 and particularly D102 appeared to dominate the binding epitope recognized by mGM-R alpha. Several amino acid substitutions were made for K108 which reduced binding with concommitant losses in bioactivity. Substitutions for D102 resulted in binding affinities less than 0.1% that of the wild-type mGM-CSF and bioactivity decreased to 1.0%. Comparative analysis using high and low affinity binding conditions indicated that mGM-R beta binding was unaffected by these mutations.

Molecular evolution of interleukin-3

Chimpanzee, tamarin, and marmoset interleukin-3 (IL-3) genes were cloned, sequenced, and expressed. Western blot analysis demonstrated that functional genes were isolated. IL-3 sequences were compared with those of mouse, rat, rhesus monkey, gibbon, and man. Multiple alignment of the IL-3 coding regions showed that only a few regions had been conserved during mammalian evolution, which are likely associated with functional domains of the IL-3 protein. Substitution rates for the various lineages were calculated and the numbers of synonymous and nonsynonymous substitutions were estimated separately. Distance matrices of the IL-3 coding regions were used to construct phylogenetic trees which revealed large differences in IL-3 evolution rate as well as a more rapid substitution rate for rodents and a rate slowdown during hominoid evolution. Extremes were rhesus monkey IL-3, which accumulated few synonymous substitutions, and gibbon IL-3, which had almost exclusively synonymous substitutions. In rhesus monkey IL-3, nonsynonymous substitutions outnumbered synonymous substitutions, which could not be readily explained by a random process of substitutions. We assume that during evolution of IL-3, the majority of the amino acid replacements and the impaired interspecies functional cross-reactivity originate from selection mechanisms with the most likely selective force being the structure of the heterodimeric IL.3 cell-surface receptor. Insight into IL-3 architecture and structural analysis of the IL-3 receptor are needed to analyze the unusually fast evolution of IL-3 in more detail.

Native electrophoresis to monitor chemical modifications of human interleukin-3

Native electrophoresis on the Pharmacia PhastSystem using a new buffer system is used to monitor chemical modifications of human interleukin-3. The method is faster, more gentle and more convenient than amino acid analysis. Micrograms of protein suffice in contrast with milligrams required in spectrophotometric methods. As compared to generally used native electrophoresis, the combination of a changed conductivity, lower pH and the low density matrix results in a much better separation and detection of both the reaction products containing a high charge and the reaction products containing a low charge. This enables monitoring of extensive modifications as well as mild modifications. Our method also provides indications for denaturation of the protein. It has proven to be highly reproducible for at least 10 different modification reactions. Information can be obtained about the presence of the various distinct forms of the modified molecule, for instance leading to the average number of groups on the molecule. It also detects modifications not detectable with generally used native electrophoresis.

Identification of functional domains of interleukin-3 by construction of primate interspecies chimera

Interleukin-3 (IL-3) is involved in regulation of proliferation and differentiation of multipotent hemopoietic cells and stimulates the production of most blood cell types. The observed functional specificity across species concurs with an extreme rate of IL-3 amino acid substitutions during mammalian evolution. Tamarin IL-3 exhibited 70.5% sequence identity with human IL-3 and was severely impaired in supporting proliferation of human IL-3-dependent cells. In contrast, chimpanzee IL-3 displayed high amino acid sequence homology (98.5%) and could substitute for human IL-3. A panel of interspecies chimera between the chimpanzee and tamarin IL-3 genes has been constructed and expressed in Escherichia coli and eukaryotic cells to investigate the role of substitutions in different protein domains on the functional species specificity. Our analyses show that substitutions at residues encoded by the first two exons appear crucial in the functional species specificity, whereas C-terminal alterations show only moderate effects.

Homodimeric murine interleukin-3 agonists indicate that ligand dimerization is important for high-affinity receptor complex formation

Homodimeric murine interleukin 3 (mIL-3) agonists were generated by intermolecular cystine-bonding. Steady-state binding assays and association kinetics performed at 4 degrees C using these agonists revealed specific binding to both the high- and low-affinity receptor. DSS-mediated crosslinking studies performed at 4 degrees C with agonist concentrations compatible with high-affinity receptor complex formation allowed to detect protein complexes of the alpha chain, the beta chain(s) and the high-affinity receptor complex migrating with apparent molecular weights of 90 kDa, 140 kDa, and above 180 kDa, respectively. In contrast, monomeric mIL-3 was crosslinked to the alpha chain receptor only unless high concentrations were used. Binding studies performed at 4 degrees C revealed a positive cooperative interaction of monomeric mIL-3 with the low-affinity receptor. Proliferation studies and association kinetics performed at 37 degrees C showed that under physiological conditions these agonists were at least 2- to 3-fold more potent than monomeric mIL-3. We therefore propose that dimerization of mIL-3 may be involved in high-affinity receptor complex formation.