Structural basis of activation and antagonism of receptor signaling mediated by interleukin-27.

Interleukin-27 (IL-27) uniquely assembles p28 and EBI3 subunits to a heterodimeric cytokine that signals via IL-27Rα and gp130. To provide the structural framework for receptor activation by IL-27 and its emerging therapeutic targeting, we report here crystal structures of mouse IL-27 in complex with IL-27Rα and of human IL-27 in complex with SRF388, a monoclonal antibody undergoing clinical trials with oncology indications. One face of the helical p28 subunit interacts with EBI3, while the opposite face nestles into the interdomain elbow of IL-27Rα to juxtapose IL-27Rα to EBI3. This orients IL-27Rα for paired signaling with gp130, which only uses its immunoglobulin domain to bind to IL-27. Such a signaling complex is distinct from those mediated by IL-12 and IL-23. The SRF388 binding epitope on IL-27 overlaps with the IL-27Rα interaction site explaining its potent antagonistic properties. Collectively, our findings will facilitate the mechanistic interrogation, engineering, and therapeutic targeting of IL-27.

Revisiting the combinatorial potential of cytokine subunits in the IL-12 family.

The four core members of the Interleukin-12 (IL-12) family of cytokines, IL-12, IL-23, IL-27 and IL-35 are heterodimers which share α- and ß-cytokine subunits. All four cytokines are immune modulators and have been proposed to play divergent roles in inflammatory arthritis. In recent years additional combinations of α- and ß-cytokine subunits belonging to the IL-12 family have been proposed to form novel cytokines such as IL-39. However, the actual extent of the combinatorial potential of the cytokine subunits in the human IL-12 family is not known. Here, we identify several combinations of subunits that form secreted heterodimeric assemblies based on a systematic orthogonal approach. The heterodimers are detected in the conditioned media harvested from mammalian cell cultures transfected with unfused pairs of cytokine subunits. While certain previously reported subunit combinations could not be recapitulated, our approach showed robustly that all four of the canonical members could be secreted. Furthermore, we provide evidence for the interaction between Cytokine Receptor Like Factor 1 (CRLF1) and Interleukin-12 subunit alpha (p35). Similar to IL-27 and IL-35 this novel heterodimer is not abundantly secreted rendering isolation from the conditioned medium very challenging, unlike IL-12 and IL-23. Our findings set the stage for fine-tuning approaches towards the biochemical reconstitution of IL-12 family cytokines for biochemical, cellular, and structural studies.

Pathophysiology and inhibition of IL-23 signaling in psoriatic arthritis: A molecular insight.

Psoriatic arthritis (PsA) is a chronic inflammatory arthritis of unknown etiology, and currently the cellular and molecular interactions that dictate its pathogenesis remain elusive. A role of the interleukin-23 (IL-23)/IL-23R (IL-23 receptor) interaction in the development of psoriasis and PsA is well established. As IL-23 regulates the differentiation and activation of innate and adaptive immunity, it pertains to a very complex pathophysiology involving a plethora of effectors and transducers. In this review, we will discuss recent advances on the cellular and molecular pathophysiological mechanisms that regulate the initiation and progression of PsA as well as new therapeutic approaches for IL-23/IL-23R targeted therapeutics.

Structural Activation of Pro-inflammatory Human Cytokine IL-23 by Cognate IL-23 Receptor Enables Recruitment of the Shared Receptor IL-12Rß1.

Interleukin-23 (IL-23), an IL-12 family cytokine, plays pivotal roles in pro-inflammatory T helper 17 cell responses linked to autoimmune and inflammatory diseases. Despite intense therapeutic targeting, structural and mechanistic insights into receptor complexes mediated by IL-23, and by IL-12 family members in general, have remained elusive. We determined a crystal structure of human IL-23 in complex with its cognate receptor, IL-23R, and revealed that IL-23R bound to IL-23 exclusively via its N-terminal immunoglobulin domain. The structural and functional hotspot of this interaction partially restructured the helical IL-23p19 subunit of IL-23 and restrained its IL-12p40 subunit to cooperatively bind the shared receptor IL-12Rß1 with high affinity. Together with structural insights from the interaction of IL-23 with the inhibitory antibody briakinumab and by leveraging additional IL-23:antibody complexes, we propose a mechanistic paradigm for IL-23 and IL-12 whereby cognate receptor binding to the helical cytokine subunits primes recruitment of the shared receptors via the IL-12p40 subunit.