Epstein-Barr virus IL-10 engages IL-10R1 by a two-step mechanism leading to altered signaling properties.

Human interleukin-10 (hIL-10) is a pleiotropic cytokine that is able to suppress or activate cellular immune responses to protect the host from invading pathogens. Epstein-Barr virus (EBV) encodes a viral IL-10 (ebvIL-10) in its genome that has retained the immunosuppressive activities of hIL-10 but lost the ability to induce immunostimulatory activities on some cells. These functional differences are at least partially due to the ∼1000-fold difference in hIL-10 and ebvIL-10 binding affinity for the IL-10R1·IL-10R2 cell surface receptors. Despite weaker binding to IL-10R1, ebvIL-10 is more active than hIL-10 in inducing B-cell proliferation. To explore this counterintuitive observation further, a series of monomeric and dimeric ebvIL-10·hIL-10 chimeric proteins were produced and characterized for receptor binding and cellular proliferation on TF-1/hIL-10R1 cells that express high levels of the IL-10R1 chain. On this cell line, monomeric chimeras elicited cell proliferation in accordance with how tightly they bound to the IL-10R1 chain. In contrast, dimeric chimeras exhibiting the highest affinity for IL-10R1 exhibited reduced proliferative activity. These distinct activity profiles are correlated with kinetic analyses that reveal that the ebvIL-10 dimer is impaired in its ability to form a 1:2 ebvIL-10·IL-10R1 complex. As a result, the ebvIL-10 dimer functions like a monomer at low IL-10R1 levels, which prevents efficient signaling. At high IL-10R1 levels, the ebvIL-10 dimer is able to induce signaling responses greater than hIL-10. Thus, the ebvIL-10 dimer scaffold is essential to prevent activation of cells with low IL-10R1 levels but to maintain or enhance activity on cells with high IL-10R1 levels.

Structure and mechanism of IFN-gamma antagonism by an orthopoxvirus IFN-gamma-binding protein.

Ectromelia virus (ECTV) encodes an IFN-gamma-binding protein (IFN-gammaBP(ECTV)) that disrupts IFN-gamma signaling and its ability to induce an antiviral state within cells. IFN-gammaBP(ECTV) is an important virulence factor that is highly conserved (>90%) in all orthopoxviruses, including variola virus, the causative agent of smallpox. The 2.2-A crystal structure of the IFN-gammaBP(ECTV)/IFN-gamma complex reveals IFN-gammaBP(ECTV) consists of an IFN-gammaR1 ligand-binding domain and a 57-aa helix-turn-helix (HTH) motif that is structurally related to the transcription factor TFIIA. The HTH motif forms a tetramerization domain that results in an IFN-gammaBP(ECTV)/IFN-gamma complex containing four IFN-gammaBP(ECTV) chains and two IFN-gamma dimers. The structure, combined with biochemical and cell-based assays, demonstrates that IFN-gammaBP(ECTV) tetramers are required for efficient IFN-gamma antagonism.

Structure of IL-22 bound to its high-affinity IL-22R1 chain.

IL-22 is an IL-10 family cytokine that initiates innate immune responses against bacterial pathogens and contributes to immune disease. IL-22 biological activity is initiated by binding to a cell-surface complex composed of IL-22R1 and IL-10R2 receptor chains and further regulated by interactions with a soluble binding protein, IL-22BP, which shares sequence similarity with an extracellular region of IL-22R1 (sIL-22R1). IL-22R1 also pairs with the IL-20R2 chain to induce IL-20 and IL-24 signaling. To define the molecular basis of these diverse interactions, we have determined the structure of the IL-22/sIL-22R1 complex. The structure, combined with homology modeling and surface plasmon resonance studies, defines the molecular basis for the distinct affinities and specificities of IL-22 and IL-10 receptor chains that regulate cellular targeting and signal transduction to elicit effective immune responses.

Crystallization and preliminary X-ray diffraction analysis of human IL-22 bound to the extracellular IL-22R1 chain.

Interleukin-22 (IL-22) is a potent mediator of cellular inflammatory responses. Crystals of IL-22 bound to the extracellular high-affinity cell-surface receptor sIL-22R1 have been grown from polyethylene glycol solutions. Crystals suitable for X-ray diffraction analysis were only obtained with mutants of IL-22 and sIL-22R1 that removed the N-linked glycosylation sites found in the wild-type amino-acid sequences. The crystals belonged to space group P2(1), with unit-cell parameters a = 50.43, b = 76.33, c = 114.92 A, beta = 92.45 degrees , and diffracted X-rays to 3.2 A resolution. The crystallographic asymmetric unit contained two IL-22-sIL-22R1 complexes, corresponding to a solvent content of approximately 52%.

Same structure, different function crystal structure of the Epstein-Barr virus IL-10 bound to the soluble IL-10R1 chain.

Human IL-10 (hIL-10) is a cytokine that modulates diverse immune responses. The Epstein-Barr virus (EBV) genome contains an IL-10 homolog (vIL-10) that shares high sequence and structural similarity with hIL-10. Although vIL-10 suppresses inflammatory responses like hIL-10, it cannot activate many other immunostimulatory functions performed by the cellular cytokine. These functional differences have been correlated with the approximately 1000-fold lower affinity of vIL-10, compared to hIL-10, for the IL-10R1 receptor chain. To define the structural basis for these observations, crystal structures of vIL-10 and a vIL-10 point mutant were determined bound to the soluble IL-10R1 receptor fragment (sIL-10R1) at 2.8 and 2.7 A resolution, respectively. The structures reveal that subtle changes in the conformation and dynamics of the vIL-10 AB and CD loops and an orientation change of vIL-10 on sIL-10R1 are the main factors responsible for vIL-10's reduced affinity for sIL-10R1 and its distinct biological profile.

Noncompetitive antibody neutralization of IL-10 revealed by protein engineering and x-ray crystallography.

IL-10 is a dimeric cytokine that must engage its high-affinity cell surface receptor, IL-10R1, to induce multiple cellular activities. Here we report the 1.9 A crystal structure of an engineered IL-10 monomer (IL-10M1) in complex with a neutralizing Fab fragment (9D7Fab). 9D7Fab and IL-10R1 bind distinct nonoverlapping surfaces on IL-10M1. Antagonism of the IL-10M1/IL-10R1 interaction is the result of 9D7Fab-induced conformational changes in the CD loop of IL-10M1 that indirectly alter the structure of the IL-10R1 binding site. A single mutation (Ile87Ala) in the same CD loop region of the Epstein-Barr virus IL-10 (ebvIL-10) also reduces IL-10R1 binding affinity, suggesting that ebvIL-10 and 9D7Fab use similar allosteric mechanisms to modulate IL-10R1 affinity and biological activity.

The IL-10R2 binding hot spot on IL-22 is located on the N-terminal helix and is dependent on N-linked glycosylation.

IL-22 is a class 2 alpha-helical cytokine involved in the generation of inflammatory responses. These activities require IL-22 to engage the cell surface receptors IL-22R1 and the low-affinity signaling molecule IL-10R2. IL-10R2 also interacts with five other class 2 cytokines: IL-10, IL-26, and the interferon-like cytokines IL-28A, IL-28B, and IL-29. Here, we define the IL-10R2 binding site on IL-22 using surface plasmon resonance (SPR) and site-directed mutagenesis. Surprisingly, the binding hot spot on IL-22 includes asparagine 54 (N54), which is post-translationally modified by N-linked glycosylation. Further characterization of the glycosylation reveals that only a single fucosylated N-acetyl glucosamine on N54 is required for maximal IL-10R2 binding. Biological responses of IL-22 mutants measured in cell-based luciferase assays correlate with the in vitro SPR studies. Together, these data suggest that IL-22 activity may be modulated via changes in the glycosylation state of the ligand during inflammation.

Comparison of interleukin-22 and interleukin-10 soluble receptor complexes.

Interleukin-22 (IL-22) is a cellular homolog of IL-10 that stimulates the production of acute-phase reactants. IL-22 and IL-10 require different ligand-specific receptor chains (IL-22R and IL-10R1) but share a second receptor chain (IL-10R2) to initiate cellular responses. The quaternary structures and the ability of IL-22 and IL-10 to engage soluble (s) IL-10R1, IL-22R, IL-10R2 receptor chains were analyzed using size exclusion chromatography and surface plasmon resonance techniques. In contrast to IL-10, which is a homodimer, IL-22 is a monomer in solution that forms a 1:1 interaction with sIL-22R. Kinetic binding data reveal sIL-22R and sIL-10R1 exhibit specific nanomolar binding constants for IL-22 (k(on)/k(off) = 14.9 nM) and a monomeric isomer of IL-10 (IL-10M1) (k(on)/k(off) = 0.7 nM), respectively. In contrast, IL-10R2 exhibits essentially no affinity for IL-22 (K(eq) approximately 1 mM) or IL-10M1 (K(eq) approximately 2 mM) alone but displays a substantial increase in affinity for the IL-10/sIL-10R1 (K(eq) approximately 350 microM) and IL-22/sIL-22R (K(eq) approximately 45 microM) complexes. Three-dimensional models of IL-22 and IL-10 receptor complexes suggest two receptor residues (Gly-44 and Arg-96) are largely responsible for the marked differences in ligand affinity observed for sIL-10R1 and sIL-22R vs. sIL-10R2.

Structure and mechanism of receptor sharing by the IL-10R2 common chain.

IL-10R2 is a shared cell surface receptor required for the activation of five class 2 cytokines (IL-10, IL-22, IL-26, IL-28, and IL-29) that play critical roles in host defense. To define the molecular mechanisms that regulate its promiscuous binding, we have determined the crystal structure of the IL-10R2 ectodomain at 2.14 A resolution. IL-10R2 residues required for binding were identified by alanine scanning and used to derive computational models of IL-10/IL-10R1/IL-10R2 and IL-22/IL-22R1/IL-10R2 ternary complexes. The models reveal a conserved binding epitope that is surrounded by two clefts that accommodate the structural and chemical diversity of the cytokines. These results provide a structural framework for interpreting IL-10R2 single nucleotide polymorphisms associated with human disease.

Crystal structure of human cytomegalovirus IL-10 bound to soluble human IL-10R1.

Human IL-10 (hIL-10) modulates critical immune and inflammatory responses by way of interactions with its high- (IL-10R1) and low-affinity (IL-10R2) cell surface receptors. Human cytomegalovirus exploits the IL-10 signaling pathway by expressing a functional viral IL-10 homolog (cmvIL-10), which shares only 27% sequence identity with hIL-10 yet signals through IL-10R1 and IL-10R2. To define the molecular basis of this virus-host interaction, we determined the 2.7-A crystal structure of cmvIL-10 bound to the extracellular fragment of IL-10R1 (sIL-10R1). The structure reveals cmvIL-10 forms a disulfide-linked homodimer that binds two sIL-10R1 molecules. Although cmvIL-10 and hIL-10 share similar intertwined topologies and sIL-10R1 binding sites, their respective interdomain angles differ by approximately 40 degrees. This difference results in a striking re-organization of the IL-10R1s in the putative cell surface complex. Solution binding studies show cmvIL-10 and hIL-10 share essentially identical affinities for sIL-10R1 whereas the Epstein-Barr virus IL-10 homolog (ebvIL-10), whose structure is highly similar to hIL-10, exhibits a approximately 20-fold reduction in sIL-10R1 affinity. Our results suggest cmvIL-10 and ebvIL-10 have evolved different molecular mechanisms to engage the IL-10 receptors that ultimately enhance the respective ability of their virus to escape immune detection.