Interleukin‐21 enhances Toll‐like receptor 2/4‐mediated cytokine production via phosphorylation in the STAT3, Akt and p38 MAPK signalling pathways in human monocytic THP‐1 cells

: Interleukin-21 (IL-21) is a type I cytokine produced by activated T cells that promotes cytokine production in monocytes. Monocytes are activated by Toll-like receptors (TLRs) to produce pro-inflammatory mediators. However, little is known about the regulatory effect of IL-21 on TLR-mediated inflammation in human monocytes. This study investigated the potential association between IL-21 and TLR2/4-mediated inflammation in human monocytic THP-1 cells. First, the expression of the IL-21 receptor (IL-21R) in human monocytic THP-1 cells was examined by flow cytometry. Then, THP-1 cells were treated with either the TLR2 ligand peptidoglycan (PGN) or the TLR4 ligand lipopolysaccharide (LPS) with or without IL-21. Then, the production of several cytokines (IL-6, IL-8, TNF-α, IFN-γ and IL-10), expression of TLR2/4, and activation of the downstream signaling pathways of mitogen-activated protein kinase (MAPK), Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3), phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), and nuclear factor-kappa B (NF-κB) were determined. We found that IL-21R was highly expressed in human monocytic THP-1 cells and that IL-21 induced TLR2 and TLR4 expression and further enhanced PGN/LPS-mediated TLR2/4 expression. In addition, IL-21 also upregulated the expression of IL-6, IL-8, TNF-α, IFN-γ and IL-10 and enhanced TLR2/4-mediated cytokine production in THP-1 cells via phosphorylation of the STAT3, Akt and p38 MAPK signalling pathways. Our study suggests, for the first time that IL-21 enhances TLR2/4-mediated cytokine production in human monocytic THP-1 cells by activating the STAT3, PI3K/Akt and p38 MAPK signalling pathways.