3.1. IL-6/TGF-β
IL-6 production is triggered by the transcription factor nuclear factor kappa B (NF-κB) upon toll-like receptor (TLR) activation, a response that is often accompanied by the concomitant production of pro-inflammatory cytokines such as interleukin-1 (IL-1) and tumour necrosis factor alpha (TNFα)[21]. It has been shown that continuous IL-6 is required for the maintenance of Th17 cells and that loss of IL-6 signalling is a major contributor to Th17 plasticity[22]. It is unknown whether continuous IL-6 signalling contributes to Th17 maintenance or whether there are additional factors that antagonise lineage destabilising signals such as IL-12 and IL-23 induced transcription factor 4 (STAT4) to maintain the Th17 phenotype[23]. While IL-6 alone is unable to induce Th17 cells, culture of IL-6 in combination with TGF-β has been reported to promote murine and human naive T cells to become Th17 and to inhibit conversion into Tregs[24]. However, TGF-β is considered to be an anti-inflammatory factor and is now largely established as an essential factor for Th17 responses[25]. The nuclear receptor (NR) retinoic acid receptor-related orphan receptor gamma t (RORγt; NR1F3) is considered a lineage-defining transcription factor of Th17 cells. When TGF-β and IL-6 are present together, IL-6 activates the transcription factor 3 (STAT3), and TGF-β induces RORγt expression prior to the polarisation of naive T cells polarization into Th17 cells[26,27]. In vitro stimulation of CD4+CD25-CD62L+CD44-T cells from wild-type or RORγt deficient mice under IL-6 and TGF-β polarisation conditions revealed a significant increase in IL-17 production in wild-type CD4+ T cells. While CD4+ T cells from RORγ-/- mice (lacking RORγ and RORγt) showed a significant reduction in IL-17 cells[28]. RORγt promotes the differentiation of Th17 cells, while inhibiting T cells from producing IL-10, which makes Th17 cells pathogenic[29]. Smad2/3 and STAT3, together with other transcription factors activated by TCR signalling, induce the expression of RORγt and Th17 differentiation[30].