IL-1 family cytokines and monocytes/macrophages in the regulation of inflammation.

The IL-1 family encompasses eleven ligands and ten receptors, which have a key role in different phases of the acute resolving inflammatory defence responses, as well as in the persistent inflammation characteristic of chronic inflammatory diseases. Both inflammatory and anti-inflammatory cytokines are present among the ligands of the IL-1 family, including natural inhibitors/antagonists. IL-1 family receptors include ligand-binding activating receptors, non-binding accessory chains, and ligand-binding non-signalling "decoy" receptors, the latter being able to subtract the agonist ligands from interaction with activating receptors. The proper quantitative and temporal modulation of all these molecules underlies the physiological resolving inflammatory defensive responses, while an imbalance may result in the transition to chronic/pathological inflammation. The human innate/inflammatory response in a tissue can be modelled in vitro by culturing human normal monocytes isolated from blood in conditions that resemble the recruitment from blood into the inflamed site, then the encounter with the inflammatory agents and the development of an inflammatory reaction, and eventually the conditions promoting tissue repair and homeostatic regulation upon resolution. In order to simulate this sequence of events, CD14+ monocytes were exposed sequentially in culture to CCL2 (from 0 to 2 h, 37°C, normoxia), LPS-displaying bacterial vesicles (from 2 to 14 h, 39°C, hypoxia), TNF-? (from 3 to 14 h, 39°C, hypoxia), IFN-? (from 7 to 14 h, 39°C, hypoxia), IL-10 (from 14 to 24 h, 37°C, normoxia), and TGF-? (from 24 to 48 h, 37°C, normoxia). Moreover, by changing the culture conditions, we also reproduced the conditions of persistent/chronic inflammation, such as in rheumatoid arthritis, by exposing CD14+ monocytes in culture to CCL2 (from 0 to 2 h, 37°C, normoxia), a cocktail of bacterial and viral stimuli (LPS, PDG, poly:IC, CpG, from 2 to 7 h, 39°C, hypoxia) and IFN-?, ACPA-PA immune complexes, Survivin, M-CSF and GM-CSF (from 7 to 96 h, 39°C, hypoxia). The modulation of the IL-1 family members in monocytes during the different phases of acute and chronic inflammatory reactions was profiled by transcriptome analysis using RNA-Seq. Expression of the genes for six cytokines (IL1B, IL1A, IL1RN, IL18, IL18BP, IL36G) and four receptors (IL1R1, IL1R2, IL1RAP, SIGIRR) are similarly regulated during the early phase of the acute and chronic inflammatory responses, with the peak of expression between 2 and 14 h. The expression of most of these genes returned to the low basal level during the resolution phase in the acute model (except IL18 that was less expressed than in basal conditions), while it was higher than in fresh monocytes in the chronic model from 24 h on. Exceptions are the genes of the two inhibitors IL18BP and SIGIRR. The former significantly increased from 24 to 96 h, while the latter decreased throughout the entire course of the inflammatory response. The GSEA analysis revealed that in both acute and chronic inflammation the expression of all the IL-1 family members is mainly modulated during the first fourteen hours of stimulation, a period in which it is possible to observe the largest variation of expression. Moreover, the analysis of differentially expressed genes between two consecutive time points confirmed that IL-1 family members are modulated during the entire course of the inflammatory reactions, except between 72 and 96 h in the chronic model where they remain practically unchanged. In conclusion, our results confirm the strong involvement of the IL-1 family genes, especially in the early phases of the monocyte inflammatory response, to underline the central role of IL-1 related cytokines and receptors in initiating innate/nflammatory defensive reactions.