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

Abstract

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.

Figure 1. Schematic diagram of protein complexes formed by IL-22 receptors

Although not shown, IL-10R2 also forms IL-20R1/IL-10R2 and IL-28R1/IL-10R2 cell surface complexes that activate IL-26, IL-28, and IL-29 signaling.

Figure 2. Structure of the IL-22/IL-22R1 complex

(A) Ribbon diagram of IL-22/IL-22R1 complex. Helix A/AB loop and helix F that form the sIL-22R1 binding site are colored red and blue, respectively. Boxes show the location of Figures 2B and 2E, respectively. (B) IL-22/sIL-22R1 site 1 interface. (C) Superpositionof IL-22/sIL-22R1 and one half of the IL-10/sIL-10R1 dimer structure. IL-22 is colored red, sIL-22R1 is magenta, IL-10 is colored green, sIL-10R1 is cyan. (D) Structural differences, including the role of carbohydrate, between sIL-22R1 and sIL-10R1 D2 domains. (E) Putative orientation of IL-22/IL-22R1 and IL-10/IL-10R1 cell surface complexes. The IL-10/IL-10R1 complex was orientated so that the IL-10 dimer twofold axis was perpendicular to the membrane. The helix A/AB loop and helix F residues of IL-10, which form the IL-10R1 binding site are colored yellow and blue.

Figure 3. IL-22 and IL-10 exhibit distinct site 1 structures and binding surfaces

(A) Superposition of IL-22 and IL-10 site 1 binding epitopes colored as described in Figure 2C. (B) Bar graph of IL-22 and IL-10 surface area buried in the site 1 interfaces of IL-22/sIL-22R1 and IL-10/sIL-10R1. Space filling models colored according to the amount of surface area buried (red greatest, dark blue least) at each residue position in IL-22 (C) and IL-10 (D) site 1 interfaces.

Figure 4. Comparison of IL-22/sIL-22R1 and IL-10/sIL-10R1 interfaces

(A) Enlarged picture of IL-22 and IL-10 site 1 interfaces, as shown in Figure 2A, except the D1 domains of the receptors are superimposed. Important AB loop hydrogen bonds formed in IL-22/sIL-22R1 site 1a (B) and IL-10/sIL-10R1 site 1a (C). (D) Sequence alignment of IL-10 family cytokine AB loops. Interactions in IL-22/sIL-22R1 site 1b (E) and IL-10/sIL-10R1 site 1b (F) are also shown. Receptor residue labels are distinguished from the ligand labels by underlining.

Figure 5. Binding and structure of IL-22BP isoforms

(A) Short (7 minute) and long (22 hour, inset) period sensorgrams used to derive the affinity of the IL-22/IL-22BPI2 interaction (See Table 1). (B) Sensorgram performed using IL-22BPI3-his6 captured to anti-his Ab surfaces, followed by the injection of IL-22. Larger versions of the sensorgram are shown in Figure S1. (C) Homology model of IL-22BPI2 colored to show the location of residue differences and similarities with sIL-22R1. Residues conserved between IL-22BPI2 and sIL-22R1 are colored cyan, and differences are colored magenta. The orange residue, Pro-111^IL-22BP highlights the one amino change in D1 that alters contacts with IL-22. (D) Site 1b interface observed in the IL-22/IL-22BPI2 homology model. (E) Model of the IL-22BPI3 colored as described in C. Unique C-terminal residues of IL-22BPI3 are shown in yellow. (F) Sequence alignment of sIL-22R1 and IL-22BP isoforms. sIL-22R1 residues that bind IL-22 are labeled with red dots.

Figure 6. Models of the IL-22/IL-22R1/IL-10R2 and IL-10/IL-10R1/IL-10R2 ternary complexes

Overall IL-22 (A) and IL-10 (B) ternary complex structures. Enlarged pictures of the IL-22 (C) and IL-10 (D) site 2a/2b and IL-22 (E) and IL-10 (F) site 2c are also shown.