Interleukin newcomers creating new numbers in rheumatology: IL-34 to IL-38

The development of innovative technologies is steadily increasing the body of knowledge on molecules involved in physiological functions. Thus, several interleukins (ILs) have been identified and characterized in the past few years. Here, we detail the structural and functional characteristics of IL-34 to IL-38 with special attention to their involvement in inflammatory joint disease. IL-34 chiefly increases osteoclast activation and proliferation and therefore, it plays a direct role in bone destruction as seen in rheumatoid arthritis (RA). Regulatory T-cells (Tregs) express IL-35, which therefore exerts anti-inflammatory effects by restoring Treg suppressive capabilities and by inhibiting the Th17 pathway. IL-37 has anti-inflammatory effects mediated by a negative feedback loop that decreases the release of pro-inflammatory cytokines. IL-36 belongs to the IL-1 family and has three different forms. Although this cytokine has been chiefly studied in psoriasis and psoriatic arthritis, it also exerts pro-inflammatory effects in RA. The specific IL-36 antagonist, IL-36Ra binds to the IL-36 receptor, thereby, preventing signal transduction. Finally, IL-38 is a recently identified cytokine whose effect may resemble that of IL-36Ra as it binds to the IL-36 receptor and inhibits its effects, particularly the Th17-response. Although the exact roles for these cytokines awaits elucidation, the current improvements in our knowledge of the mechanisms that regulate chronic inflammatory conditions, such as RA may lead to the identification of new treatment targets.