IL-37 is an extraordinarily potent anti-inflammatory IL-1 family member effective against a broad spectrum of inflammatory assaults in cell lines, PBMC and mice transgenic for IL-37 (IL-37tg). One mechanism of this beneficial effect is via intracellular association with Smad3; however, a cell-surface receptor for IL-37 is unknown.
Here, we show that to block inflammation, IL-37 requires IL-18 receptor alpha (IL-18Rα) and SIGIRR, a so-far orphaned member of the IL-1R family.
In PBMC, which naturally express IL-37, knockdown of SIGIRR resulted in a 3-fold increase in IL-1β and IL-6. Though IL-1β- and LPS-induced pro-inflammatory cytokines were markedly reduced in THP-1 cells transfected with IL-37, simultaneous transfection of IL-37 and siRNA to SIGIRR significantly attenuated IL-37-mediated cytokine reduction (83% inhibition with IL-37 in the presence of SIGIRR, 34% with SIGIRR silenced). The anti-inflammatory function of IL-37 was also attenuated when IL-18Rα was silenced or blocked in A549 cells, PBMC and MEF. To eliminate SIGIRR´s contribution to IL-37´s functions in vivo, we created SIGIRR-/- mice expressing IL-37. When these IL-37tg-SIGIRR-/- mice and IL-37tg animals were injected with LPS, the protective effect observed in IL-37tg was markedly reduced in the IL-37tg-SIGIRR-/- strain.
By immunofluorescence, we observed that IL-37 associates with SIGIRR and IL-18Rα in LPS-stimulated RAW macrophages and IL-1β-treated A549 cells, both transfected with IL-37. Using proximity ligation assays in PBMC, we demonstrated sub-40nm co-localization of the three pairs IL-37-SIGIRR, IL-37-IL-18Rα and SIGIRR-IL-18Rα. FRET analysis in PBMC revealed that IL-37-SIGIRR interaction peaks 30min after LPS stimulation, decreasing to non-stimulated levels by ~4h.
We conclude that IL-37 limits inflammation by associating with SIGIRR and IL-18Rα, suggesting that SIGIRR-IL-18Rα functions as the cell-surface receptor for IL-37.