The IL-23p19/EBI3 heterodimeric cytokine termed IL-39 remains a theoretical cytokine in man

Objective The heterodimeric IL-12 family member cytokines including, IL-12, IL-23, IL-27, and IL-35 and have multiple roles in regulating innate and adaptive immunity with crucial functions in inflammatory disorders such as psoriasis. Chain pairing promiscuity is a feature of the IL-12 family. Recently, based on murine data, a new family member, IL-39, was proposed, consisting of IL23p19 (shared with IL-23) and EBI3 (shared with IL-27 and IL-35). IL-39 has subsequently been implicated in experimental murine lupus. Given the success of IL-23p19 therapeutic targeting in diseases including psoriasis, it is of great interest to confirm the presence of IL-39 in man. Human IL-39 is yet to be either detected or expressed, which has halted research in this area. Methods Using a disulphide-linked human chimera protein composing of IL-23p19 and EBI3 human chains, we stimulated human leukocytes, and analysed cytokine secretion and STAT3 phosphorylation. Results and Conclusion We report that this cytokine shows no activity in human cells. IL-39 chimera protein failed to induce either IL-6, IL-8, TNF, or IL-17A from leukocytes or STAT3 phosphorylation and thus, remains a ‘theoretical cytokine' in humans.