Development of a human interleukin-6 receptor antagonist

Identification of single amino acid residues of human IL-6 involved in receptor binding and signal initiation

The pleiotropic cytokine interleukin-6 (IL-6) has been predicted to be a protein with four antiparallel alpha-helices. On target cells, IL-6 interacts with a specific ligand binding receptor subunit (IL-6R), and this complex associates with the signal-transducing subunit gp130. Human IL-6 acts on human and murine cells, whereas murine IL-6 is only active on murine cells. The construction of chimeric human/murine IL-6 proteins has allowed us to define a region (residues 77-95, region 2c) within the human IL-6 protein that is important for IL-6R binding and a region (residues 50-55, region 2a2) that is important for IL-6R dependent gp130 interaction. Guided by sequence alignment and molecular modeling, we have constructed several IL-6 variants with point mutations in these regions and have tested them for receptor binding and signal initiation. Within region 2c, phenylalanine 78 was involved in receptor binding, whereas lysine 54 within region 2a2 participated in gp130 activation. Furthermore, some IL-6 variants with lysine 54 replacements could be used to construct muteins that retained receptor binding but failed to activate gp130. Such IL-6 muteins were efficient IL-6 receptor antagonists.

Molecular cloning and sequencing of interleukin 6 cDNA fragments from the harbor seal (Phoca vitulina), killer whale (Orcinus orca), and Southern sea otter (Enhydra lutris nereis)

Using polymerase chain reaction, interleukin-6 (IL-6) cDNA fragments from harbor seal (Phoca vitulina), killer whale (Orcinus orca), and Southern sea otter (Enhydra lutris nereis) were cloned and sequenced. For all three species, a continuous open reading frame encoding 203 residues for harbor seal, 199 residues for killer whale, and 201 residues for sea otter with stop codons located at analogous positions were identified. These fragments correspond to nucleotides 71 - 753 of the human IL-6 transcript and represent 96% of the complete coding nucleotides. Comparison of these marine mammal sequences with other published mammalian IL-6 cDNA demonstrated that both harbor seal and sea otter IL-6 had most similarity to that of other terrestrial carnivores (Mustelidae and Canidae), while killer whale had highest identity with ruminants (Bovidae and Ovidae). Among the three marine mammal species characterized, as well as cDNA sequences from nine other species, 40 invariant amino acids, including a number of residues situated at the putative gp80 and gp130 receptor binding sites, were identified. The presence of invariant amino acids within the receptor-binding portion of IL-6 for twelve different species suggests these positions are essential for biological activity of IL-6 and, moreover, likely account for the cross-reactivity among different mammalian IL-6-like activities in mouse bioassays. An additional significant finding was the presence of several variant residues only within the mouse putative IL-6 receptor binding region, which may account for observations of restricted cross-reactivity of mouse IL-6-like activity in human bioassays. Together, these findings provide insights into the evolution of the mammalian IL-6 gene and additional valuable information regarding amino acid residues essential for the biological activity of mammalian IL-6.

Immunohistochemical localization of interleukin-6-like immunoreactivity to peripheral nerve-like structures in normal and inflamed human skin

Interleukin-6-like (IL-6-like) immunoreactivity was sought in inflamed and normal human skin using the same immunohistochemical technique as for detection of neuropeptides. Such immunoreactivity was found in dermal and in a few intraepidermal nerve-like fibres in biopsy specimens from inflamed skin from patients with positive epicutaneous patch-test reactions to nickel sulphate, and in skin specimens from patients with atopic dermatitis and prurigo nodularis. However, IL-6-like immunoreactivity was also found in nerve-like fibres in specimens from nonlesional skin. In skin from patients with positive epicutaneous patch-test reactions there was a statistically significantly (P < 0.01) higher number of IL-6-positive nerve fibres in the epidermis than in normal skin, in contrast to the papillary dermis, in which no difference was found. Moreover, there were clusters of nerve-like fibres with IL-6-like immunoreactivity in the dermis of prurigo nodularis lesions. In these nerve-like fibres, the colocalization of the immunoreactivities for IL-6 and calcitonin gene-related peptide was indicated. Localization of immunoreactivity to nerve-like structures surrounding the eccrine sweat glands indicates that IL-6 is present in autonomic as well as in sensory nerve fibres.

Participation of two Ser-Ser-Phe-Tyr repeats in interleukin-6 (IL-6)-binding sites of the human IL-6 receptor

The alpha-subunit of interleukin-6 (IL-6) receptor is a member of the hematopoietin receptor family. The alignment of its amino acid sequence with those of other members of this family (human somatotropin receptor/murine IL-3 receptor beta and human IL-2 receptor beta) has suggested that amino acids included in two SSFY repeats found in each of its hematopoietin receptor domains, contribute to the binding of the ligand. The involvement of these amino acids in IL-6 binding and signal transduction was studied by site-directed mutagenesis and molecular modelling. We present a computer-derived three-dimensional model of the IL-6/IL-6 receptor complex based on the structure of the human somatotropin/human somatotropin receptor complex. This model allowed the location of distinct regions important for IL-6 and gp130 binding. We show that some of the residues included in the SSFY repeats located in our IL-6 receptor model in the loops between beta-strands E and F of domain-I and B' and C', of domain-II, participate in the formation of a major IL-6-binding site. These residues are necessary for IL-6 and gp130 binding and for signal transduction. Using our IL-6 receptor mutants we mapped the epitopes of our anti-(IL-6 receptor) neutralising monoclonal antibodies to these residues. Our results demonstrate that a generic hematopoietin receptor family structural module can be used for the study of both alpha and beta receptor subunits belonging to this family.

Ligand-induced dimerization of growth factor receptors: variations on the theme

Growth, differentiation, apoptosis and movement of cells are regulated, in part, by polypeptide growth factors, or cytokines, which exert their effects by binding to cell surface receptors on the target cells. Recent observations have indicated that a common mechanism of activation of several classes of such receptors is ligand-induced dimerization or oligomerization of the receptors.

Activating mechanism of CNTF and related cytokines

Ciliary neurotrophic factor (CNTF) shares structural and functional properties with members of the hematopoietic cytokine family. It is composed of a four-helix bundle structure and shares the transmembrane signal transducing proteins, glycoprotein-130 (gp130) and leukemia inhibitory factor receptor (LIF-R). Structure-function analysis showed that the gp130-interactive proteins bind in a similar manner to that of growth hormone (site I and II). In addition, gp130-interactive proteins and granulocyte colony-stimulating factor (G-CSF) utilize another binding site (site III) at the boundary between CD loop and helix D. CNTF triggers the association of receptor components, resulting in activation of a signal transduction cascade mediated by specific intracellular protein tyrosine kinases. The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) and Ras/mitogen-activated protein kinase (MAPK) signaling pathways have been characterized in terms of gp130-interactive protein, and there should be other pathways and some crosstalk between them to enhance, prolong, or specify the signals.

The interleukin-6 (IL-6) partial antagonist (Q159E,T162P)IL-6 interacts with the IL-6 receptor and gp130 but fails to induce a stable hexameric receptor complex

The extracellular "soluble" domains of the IL-6 receptor (sIL-6R) and gp130 (sgp130) form a hexameric ternary receptor complex together with IL-6, consisting of two molecules of each component. In this report we have investigated the interactions of the partial IL-6 antagonist (Q159E,T162P)IL-6 ((QT)IL-6), with the sIL-6R and sgp130. The kinetic rate constants of the binding of sIL-6R to immobilized monomeric (QT)IL-6 or IL-6 were obtained using an optical biosensor with analysis of the primary data by linear and nonlinear regression. Both methods of analysis showed that, due to a higher off-rate, sIL-6R has lower apparent affinity for (QT)IL-6 than IL-6. The lower affinity of (QT)IL-6 was further confirmed by equilibrium binding measurements at the sensor surface and in solution. Using the biosensor it was also shown that the (QT)IL-6 complex interacts with sgp130, supporting the notion that the biological activity of (QT)IL-6 is mediated via gp130. However, the IL-6 mutant, when incubated with sIL-6R and sgp130, failed to induce a stable hexameric receptor complex, as shown by narrowbore size exclusion chromatography.

Extension of recombinant human RANTES by the retention of the initiating methionine produces a potent antagonist

Extension of recombinant human RANTES by a single residue at the amino terminus is sufficient to produce a potent and selective antagonist. RANTES is a proinflammatory cytokine that promotes cell accumulation and activation in chronic inflammatory diseases. When mature RANTES was expressed heterologously in Escherichia coli, the amino-terminal initiating methionine was not removed by the endogenous amino peptidases. This methionylated protein was fully folded but completely inactive in RANTES bioassays of calcium mobilization and chemotaxis of the promonocytic cell line THP-1. However, when assayed as an antagonist of both RANTES and macrophage inflammatory polypeptide-1 alpha (MIP-1 alpha) in these assays, the methionylated RANTES (Met-RANTES) inhibited the actions of both chemokines. T cell chemotaxis was similarly inhibited. The antagonistic effect was selective since Met-RANTES had no effect on interleukin-8- or monocyte chemotractant protein-1-induced responses in these cells. Met-RANTES can compete with both [125I]RANTES and [125I]IMP-1 alpha binding to THP-1 cells or to stably transfected HEK cells recombinantly expressing their common receptor, CC-CKR-1. These data show that the integrity of the amino terminus of RANTES is crucial to receptor binding and cellular activation.

Definition of a composite binding site for gp130 in human interleukin-6

The helical cytokine interleukin-6 (IL-6) assembles a multiprotein receptor complex. The starting event in the activation of intracellular signaling is the binding of the IL-6/IL-6R alpha subcomplex to two gp130 chains. The homodimerization of gp130 is triggered by two distinct and independent regions of IL-6 called sites 2 and 3. Several IL-6 antagonists have been obtained that affect signaling, but not IL-6 IL-6R alpha subcomplex formation. In this paper, we analyze in detail the impact of these antagonists on gp130 binding and dimerization and show that each signaling variant affects gp130 dimerization in vitro and that biological activity on cells decreases in precise parallel to the decrease in gp130 dimerization in vitro. All IL-6 antagonists can be classified into two groups, mapping at either site 2 or 3 in correspondence to their mode of interaction with gp130. We found that site 3 is a large region, which includes residues at the beginning of helix D spatially flanked by residues in the putative AB loop and located at one extremity of the cytokine 4-helix bundle. Interestingly, in leukemia inhibitory factor, another cytokine that signals through gp130, site 3, is topologically conserved but has evolved to bind leukemia inhibitory factor receptor.

Monovalent phage display of human interleukin (hIL)-6: selection of superbinder variants from a complex molecular repertoire in the hIL-6 D-helix

Phage display of proteins can be used to study ligand-receptor interaction and for the affinity-maturation of binding sites in polypeptide hormones and/or cytokines. We have expressed human interleukin-6 (hIL-6) on M13 phage in a monovalent fashion as a fusion protein with the phage coat protein, pIII. Phage-displayed hIL-6 is correctly folded, as judged by its ability to interact with conformation-specific anti-hIL-6 monoclonal antibodies (mAb) and with the hIL-6 receptor complex in vitro. We set up an experimental protocol for the efficient affinity selection of hIL-6 phage using the extracellular portion of the hIL-6 receptor alpha (hIL-6R alpha) fixed on a solid phase. This system was used to affinity-purify from a library of hIL-6 variants, in which four residues in the predicted D-helix of the cytokine were fully randomized, mutants binding hIL-6R alpha with higher efficiency than the wild type. When the best-binder variant Q175I/Q183A was combined with a previously identified superbinder S176R [Savino et al., Proc. Natl. Acad. Sci. 90 (1993) 4067-4071], a triple-substitution mutant Q175I/S176R/Q183A (hIL-6IRA) was obtained with a fivefold increased hIL-6R alpha binding and a 2.5-fold enhanced biological activity.

Mediators and vascular effects in response to endotoxin

Recent experimental findings indicate that endotoxin (i.e. lipopolysaccharide) interacts with specific membrane receptors localized to mononuclear phagocytic cells and neutrophils. Binding of endotoxin to these cells, together with endotoxin-induced activation of host vascular endothelium, initiates a series of signal transduction events that culminate in release of numerous biochemical mediators. The latter include cytokines, platelet-activating factor, thromboxane A2, prostaglandins, leukotrienes, nitric oxide, proteases, toxic O2 radicals, and vasoactive amines. These mediators orchestrate complex biological interactions and amplification signals that lead to cardiopulmonary dysfunction and multi-organ failure within 4-6 h of experimental infusion of endotoxin into animals. The pathophysiological changes include decreased cardiac output, systemic hypotension, decreased blood flow and O2 delivery to tissues, intense pulmonary vasoconstriction and hypertension, bronchoconstriction, increased permeability, pulmonary oedema, ventilation-to-perfusion inequalities, hypoxaemia, and haemoconcentration. Metabolic alterations include increased blood lactate and pyruvate, metabolic acidosis, hyperkalaemia and hypoglycaemia. Potential therapeutic modalities for treatment of endotoxaemia/septic shock include specific antagonists directed against lipopolysaccharide, cytokine, and platelet-activating factor receptors, monoclonal antibodies directed against cytokines and lipid A/core polysaccharides of endotoxin, antiproteases, and agents that block release of toxic O2 and arachidonic acid metabolites.

Leucine-58 in the putative 5th helical region of human interleukin (IL)-6 is important for activation of the IL-6 signal transducer, gp130

A model of the tertiary structure of human IL-6, derived from the crystal-structure of granulocyte-colony stimulating factor, reveals a 5th helical region in the loop between the first and second alpha-helix. To investigate the importance of this region for biological activity of IL-6, residues Glu-52, Ser-53, Ser-54, Lys-55, Glu-56, Leu-58, and Glu-60 were individually replaced by alanine. IL-6.Leu-58Ala displayed a 5-fold reduced biological activity on the IL-6 responsive human cell lines XG-1 and A375. This reduction in bioactivity was shown to be due to a decreased capacity of the mutant protein to trigger IL-6 receptor-alpha-chain-dependent binding to the IL-6 signal transducer, gp130.

Alanine-scanning mutagenesis of human interleukin-11: identification of regions important for biological activity

We have identified functionally important regions of human interleukin-11 (hIL-11) by means of alanine-scanning mutagenesis. A total of 61 mutated forms of hIL-11 were produced in E. coli as thioredoxin fusion proteins and tested in a murine T10 plasmacytoma proliferation assay. Mutations made at several positions proximal to the hIL-11 C-terminus caused substantial reduction in biological activity. In addition a number of other mutations in this region affected either protein folding or stability. Both effects displayed a characteristic periodicity with respect to the primary sequence which suggested that residues close to the C-terminus of hIL-11 adopt a helical conformation. Mutations made proximal to the N-terminus of hIL-11 also exhibited reduced bioactivity, although no effects on protein folding or stability were observed. The N-terminal mutations with reduced activity also mapped with a periodicity suggestive of a helical conformation. We previously have proposed a four-helix bundle topology for the hIL-11 structure based on physical studies, selective chemical modifications, positions of intron/exon boundaries, limit proteolysis experiments and site-directed mutagenesis. The alanine-scanning mutagenesis data we report here provide additional support for this model.

Development of human IL-6 receptor antagonists

We have shown that through mutagenesis of IL-6 it is possible to separate receptor binding from signal transduction of the cytokine. Mutations in residues important for signal transduction via gp130 result in IL-6 variants that can competitively inhibit wtIL-6 activity in vitro. The differential effects of these signaling deficient mutants on various cell lines of human origin suggest that receptor composition and/or signal transduction pathways may vary between cells of different origin. The observations that three sites have been identified which are important for gp130 interaction raises the question what the role of each region is in the stepwise formation of the active IL-6 receptor complex. The overall tertiary conformation of the beta-site mutants is intact, as judged from their binding characteristics to conformation specific mAbs and IL-6R alpha. As can be deduced from Figure 1, beta-site mutations may therefore affect a direct interaction with gp130, dimerization of IL-6, or maybe a conformational change in IL-6R alpha, important for gp130 interaction. A future challenge will therefore be to determine the function of each of the beta-sites in IL-6 receptor interaction.

Growth hormone, interferon-gamma, and leukemia inhibitory factor promoted tyrosyl phosphorylation of insulin receptor substrate-1

The identification of JAK2 as a growth hormone (GH) receptor-associated, GH-activated tyrosine kinase has established tyrosyl phosphorylation as a signaling mechanism for GH. In the present study, GH is shown to stimulate tyrosyl phosphorylation of insulin receptor substrate 1 (IRS-1), the principle substrate of the insulin receptor. Tyrosyl phosphorylation of IRS-1 is a critical step in insulin signaling and provides binding sites for proteins with the appropriate Src homology 2 domains, including the 85-kDa regulatory subunit of phosphatidylinositol (PI) 3'-kinase. In 3T3-F442A fibroblasts, GH-dependent tyrosyl phosphorylation of IRS-1 was detected by 1 min and at GH concentrations as low as 5 ng/ml (0.23 nM). Tyrosyl phosphorylation of IRS-1 was transient, with maximal stimulation detected at 30 min and diminished signal detected at 60 min. The ability of GH receptor (GHR) to transduce the signal for IRS-1 tyrosyl phosphorylation is mediated by the intracellular region of GHR between amino acids 295 and 380 by a mechanism not involving the two tyrosines in this region. This region of GHR is required for GH-dependent JAK2 association and activation (VanderKuur, J. A., Wang, X., Zhang, L., Campbell, G. S., Allevato, G., Billestrup, N., Norstedt, G., and Carter-Su, C. (1994) J. Biol. Chem. 269, 21709-21717). When other cytokines that activate JAK2 were tested for the ability to stimulate the tyrosyl phosphorylation of IRS-1, stimulation was detected with interferon-gamma and leukemia inhibitory factor. The correlation between JAK2 tyrosyl phosphorylation and IRS-1 tyrosyl phosphorylation in response to GH, interferon-gamma, and leukemia inhibitory factor and in cells expressing different GHR mutants, provides evidence that IRS-1 may interact with JAK2 or an auxiliary molecule that binds to JAK2. GH is also shown to stimulate binding of IRS-1 to the 85-kDa regulatory subunit of PI 3'-kinase. The ability of GH to stimulate tyrosyl phosphorylation of IRS-1 and its association with PI 3'-kinase provides a biochemical basis for responses shared by insulin and GH including the well characterized insulin-like metabolic effects of GH observed in a variety of cell types.

Localization of functional receptor epitopes on the structure of ciliary neurotrophic factor indicates a conserved, function-related epitope topography among helical cytokines

By rational mutagenesis, receptor-specific functional analysis, and visualization of complex formation in solution, we identified individual amino acid side chains involved specifically in the interaction of ciliary neurotrophic factor (CNTF) with CNTFR alpha and not with the beta-components, gp130 and LIFR. In the crystal structure, the side chains of these residues, which are located in helix A, the AB loop, helix B, and helix D, are surface accessible and are clustered in space, thus constituting an epitope for CNTFR alpha. By the same analysis, a partial epitope for gp130 was also identified on the surface of helix A that faces away from the alpha-epitope. Superposition of the CNTF and growth hormone structures showed that the location of these epitopes on CNTF is analogous to the location of the first and second receptor epitopes on the surface of growth hormone. Further comparison with proposed binding sites for alpha- and beta-receptors on interleukin-6 and leukemia inhibitory factor indicated that this epitope topology is conserved among helical cytokines. In each case, epitope I is utilized by the specificity-conferring component, whereas epitopes II and III are used by accessory components. Thus, in addition to a common fold, helical cytokines share a conserved order of receptor epitopes that is function related.

Site-directed mutagenesis of human CNTF: functional analysis of recombinant variants

Ciliary neurotrophic factor (CNTF), interleukin-6 (IL-6), leukemia inhibitory factor (LIF), and oncostatin M (OSM) share functional properties, a predicted common helical framework, and partially identical receptor components. CNTF is a survival promoting factor for various types of neurons in vitro and in vivo. In the present study, structural features essential for the biological function of human CNTF were investigated. Several recombinant CNTF variants were constructed by PCR and expressed in E. coli. Their survival promoting activities were determined using cultures of embryonic chick and newborn rat dorsal root ganglion cells. Deletion of 14 N-terminal and 18 C-terminal amino acids significantly increased bioactivity compared to wild-type (wt) CNTF. Further truncation of the CNTF molecule at the N- or C-terminus resulted in a significant reduction or complete loss of activity. Substitution of two amino acids (Lys154Glu and Trp157Pro) abolished the survival promoting effect. Recently described analogous substitutions in IL-6 had resulted in a partial IL-6 receptor antagonist. However, the double substitution variant had no significant inhibitory effect on wtCNTF activity in assays with both wt and mutant factor. The CNTF variants constructed had almost identical effects on both chick and rat neurons indicating a close similarity of the avian and the mammalian CNTF receptor complex. The present results also demonstrate that a core segment of the CNTF molecule is indispensable for biological function. Analogous segments important for activity have already been identified in the related molecules IL-6, LIF, and OSM. Thus, our data confirm the close structural relationship of CNTF to these "neuropoietic" cytokines. In addition, they demonstrate that site-directed mutagenesis of recombinant human CNTF can yield molecules which show increased survival promoting activity on mammalian neurons.

Combining two mutations of human interleukin-6 that affect gp130 activation results in a potent interleukin-6 receptor antagonist on human myeloma cells

The pleiotropic cytokine interleukin-6 (IL-6) interacts with the specific ligand binding subunit (IL-6R alpha) of the IL-6 receptor, and this complex associates with the signal-transducing subunit gp130 (IL-6R beta). Human IL-6 acts on human and murine cells, whereas murine IL-6 is only active on murine cells. The construction of a set of chimeric human/murine IL-6 proteins has recently allowed us to define a region (residues 43-55) within the human IL-6 protein, which is important for the interaction with gp130. Subdividing this region shows that mainly residues 50-55 of the human IL-6 are necessary for this interaction. Recently, another human IL-6 double mutant (Q159E and T162P) showed reduced affinity to gp130 but residual activity on the human myeloma cell line XG-1. Into this IL-6 mutant we introduced the murine residues 43-49 or 50-55 together with two point mutations, F170L and S176A, which had been reported to increase the affinity of IL-6 to the IL-6R alpha. The resulting IL-6 molecule, which contained the murine residues 50-55, was inactive on human myeloma cells and in addition completely inhibited wild type IL-6 activity on these cells. Such an antagonist may be used as a specific inhibitor of IL-6 activity in vivo.

Immunotherapy of multiple myeloma

In 1994, an estimated 12,700 new cases of multiple myeloma (MM) will be diagnosed in the USA and 9,800 patients will die from this disease. At present, a cure for MM has not been achieved with any chemotherapeutic regimen. Therefore, it is important to develop novel therapeutic approaches to treat this fatal disease. This review focuses on new concepts in the immunotherapy of MM. Thus far, interferons and anti-human interleukin (IL)-6 monoclonal anti-bodies (MAbs) have been used to treat patients with this disease. Bone marrow transplantation using autologous marrow purged with MAbs and complement, with anti-myeloma immunotoxins (ITs), or MAb-magnetic bead conjugates has been reported. Adoptive cellular therapy, in vivo with anti-CD3 and IL-2, as well as transplantation of purified autologous CD34+ peripheral blood stem cells, is now being evaluated in clinical trials. Anti-human IL-6 receptor (IL-6R) and anti-CD54 (ICAM-1) MAbs have shown promising results in the therapy of human myeloma cell lines in SCID mice, while an IL-6 antagonist protein, anti-gp130 MAbs, recombinant soluble gp130, anti-B7, anti-HLA-DR, and recombinant soluble CD16 also inhibit the growth of myeloma cell lines in vitro. These experimental therapeutic modalities hold promise for use in humans and may also provide further insights into the pathogenesis of MM.

Four-helix bundle growth factors and their receptors: protein-protein interactions

Many growth factors and cytokines promote receptor clustering on binding. At least three different protein-protein interaction sites are involved: cytokine-receptor I, cytokine-receptor II and receptor I-receptor II. Although structural data on these complexes are limited, recent structural and mutagenesis studies of the four-helix bundle class of cytokines are clarifying the nature of the complexes formed.

Influence of interleukin-6 (IL-6) autoantibodies on IL-6 binding to cellular receptors

Neutralizing autoantibodies to interleukin-6 (aAb-IL-6) have been reported in healthy individuals, in patients with autoimmune diseases, and in pharmaceutically prepared pooled IgG (IVIg). We investigated the ability of aAb-IL-6 derived from IVIg to interfere with IL-6 binding to the undifferentiated monocytic cell line U-937. High-affinity aAb-IL-6, primarily of the IgG1 subclass, constituted approximately 1:10(6) of the total IgG in IVIg preparations. IL-6 binding to cellular receptors was strongly inhibited by one class of aAb-IL-6. These antibodies recognized epitope(s) on IL-6 essential for the binding of IL-6 to the alpha subunit of the IL-6 receptor (IL-6R). Another class of aAb-IL-6 recognized epitope(s) on IL-6, which is not essential for the binding to IL-6R but nevertheless important for the formation of high-affinity cellular IL-6 binding. These antibodies presumably interfered with the association of IL-6 receptor beta chains (gp130) with IL-6/IL-6R complexes, implicating that small IL-6/aAb-IL-6 immune complexes bound saturably (low affinity/high capacity) to cellular IL-6 receptors. There was no detectable binding of IL-6 through aAb-IL-6 and Fc receptors on U-937, and IVIg had no direct IL-6 receptor antagonizing activity. Dissociation kinetics of IL-6/aAb-IL-6 complexes at 37 degrees C revealed that IL-6 was liberated from 75% of the aAb-IL-6 with a half-time (t/2) approximately 4 h but bound almost irreversibly to the remaining aAb-IL-6 (t/2 > 20 h). Cellular IL-6 uptake and degradation was suppressed by aAb-IL-6. Taken together, the data suggest that loss of immunologic tolerance against IL-6 might be a novel physiological mechanism by which IL-6 activities are effectively attenuated. Finally, binding of IL-6 in complex with IgG1 aAb-IL-6 on cells expressing IL-6 receptors implicates that such cells could be targets of antibody-dependent immunological reactions, including cytotoxic reactions.

Structure-function analysis of human IL-6: identification of two distinct regions that are important for receptor binding

Interleukin-6 (IL-6) is a multifunctional cytokine that plays an important role in host defense. It has been predicted that IL-6 may fold as a 4 alpha-helix bundle structure with up-up-down-down topology. Despite a high degree of sequence similarity (42%) the human and mouse IL-6 polypeptides display distinct species-specific activities. Although human IL-6 (hIL-6) is active in both human and mouse cell assays, mouse IL-6 (mIL-6) is not active on human cells. Previously, we demonstrated that the 5 C-terminal residues of mIL-6 are important for activity, conformation, and stability (Ward LD et al., 1993, Protein Sci 2:1472-1481). To further probe the structure-function relationship of this cytokine, we have constructed several human/mouse IL-6 hybrid molecules. Restriction endonuclease sites were introduced and used to ligate the human and mouse sequences at junction points situated at Leu-62 (Lys-65 in mIL-6) in the putative connecting loop AB between helices A and B, at Arg-113 (Val-117 in mIL-6) at the N-terminal end of helix C, at Lys-150 (Asp-152 in mIL-6) in the connecting loop CD between helices C and D, and at Leu-178 (Thr-180 in mIL-6) in helix D. Hybrid molecules consisting of various combinations of these fragments were constructed, expressed, and purified to homogeneity. The conformational integrity of the IL-6 hybrids was assessed by far-UV CD. Analysis of their biological activity in a human bioassay (using the HepG2 cell line), a mouse bioassay (using the 7TD1 cell line), and receptor binding properties indicates that at least 2 regions of hIL-6, residues 178-184 in helix D and residues 63-113 in the region incorporating part of the putative connecting loop AB through to the beginning of helix C, are critical for efficient binding to the human IL-6 receptor. For human IL-6, it would appear that interactions between residues Ala-180, Leu-181, and Met-184 and residues in the N-terminal region may be critical for maintaining the structure of the molecule; replacement of these residues with the corresponding 3 residues in mouse IL-6 correlated with a significant loss of alpha-helical content and a 200-fold reduction in activity in the mouse bioassay. A homology model of mIL-6 based on the X-ray structure of human granulocyte colony-stimulating factor is presented.

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.