Interleukin‐25 recognition by its unique receptor IL‐17Rb via two discrete linear and cyclic epitopes

Abstract Interleukin‐17 (IL‐17) is a family of pro‐inflammatory cytokines and has been involved in the pathogenesis of chronic inflammatory and autoimmune diseases. The IL‐17E, also known as IL‐25, is a distinct member of this family that binds to its unique receptor IL‐17Rb to induce the activation of nuclear factor kappa‐light‐chain enhancer of activated B cells. Here, we systematically examined the intermolecular recognition and association of IL‐25 with IL‐17Rb and demonstrated that the IL‐25 primarily adopts two discrete linear and cyclic epitopes to interact with IL‐17Rb. The two epitopes are separately located in the monomers 1 and 2 of IL‐25 homodimer and cover sequences 125 DPRGNSELLYHN 136 and 77 ELDRDLNRLPQDLY 90 . They totally contribute 71.6% binding energy to the full‐length IL‐25. The linear epitope targets a site spanning over the extracellular fnIIID1 and fnIIID2 domains of IL‐17Rb, while the cyclic epitope primarily binds at the fnIIID1 domain. In addition, we also found that the linear and cyclic epitopes are natively folded into ordered single‐stranded and double‐stranded conformations in IL‐25 protein context, respectively, but would become largely disordered when splitting from the context to be free peptides, which, however, cannot bind effectively to IL‐17Rb as them in the native state. In this respect, we extended the cyclic epitope to cover the whole IL‐25 double‐stranded region and added a disulfide bridge across its two strands at three selected anchor residue pairs. It is revealed that the disulfide‐stapled peptides can be constrained into a native‐like conformation and thus exhibit an improved binding potency to IL‐17Rb as compared to their unstapled counterpart.