TLR-2 Signaling Promotes IL-17A Production in CD4+CD25+Foxp3+ Regulatory Cells during Oropharyngeal Candidiasis

Abstract

Recent studies show that CD4+CD25+Foxp3+ regulatory cells (Tregs) produce effector cytokines under inflammatory conditions. However, the direct role of microbial agents that serve as toll-like receptor (TLR) ligands in the induction of effector cytokines in Tregs is less clear. Here we show that CD4+Foxp3+Tregs produce the effector cytokine IL-17A during oropharyngeal candidiasis (OPC) and inflammatory bowel disease in a TLR-2/Myd88 signaling dependent manner. TLR-2 ligands promote proliferation in Tregs in the presence and absence of TCR signals and inflammatory cytokines in vitro. The proliferation is directly dependent on TLR-2 expression in Tregs. Consistent with this, Tlr2−/− mice harbor fewer thymically derived Tregs and peripheral Tregs under homeostatic conditions in vivo. However, under Th17 inducing conditions, IL-6 and TLR-2 signaling both in Tregs as well as antigen presenting cells (APC) are critical for maximal ROR-γt and IL-17A up-regulation in Foxp3+ Tregs. The minimal and transient loss of Foxp3 expression and suppressive properties are due to the presence of IL-6 in the milieu, but not the direct effect of TLR-2 signaling in Tregs. Taken together, our data reveal that TLR-2 signaling promotes not only proliferation, but also IL-17A in Tregs, depending on the cytokine milieu. These IL-17A producing Tregs may be relevant in mucosal infections and inflammation.

Origin and functions of pro-inflammatory cytokine producing Foxp3+ regulatory T cells

“…These data support the idea that during an infection, Th1 environment can subvert regulatory networks and become pathogenic by promoting Th1-like T regs [101]. In the context of oropharyngeal candidiasis (OPC), a Th17 infection model, Th17-like T regs arise transiently in infected mice [78,102]. The increase is pronounced among T regs in mouse oral lamina propria and intraepithelial cells (MOIL) isolated from tongue and palatal tissues of the infected mice [103].…”

“…These cells suppress T-cell proliferation in vitro , supporting the interpretation that these are not reprogrammed to lose their suppressive capacity [8,75]. Similarly, in the mouse model, although a fraction of T regs produce IL-17A under Th17 conditions in vitro , when adoptively transferred into immunodeficient mice, they still modulate IBD and weight loss during Th17 IBD inflammation in vivo [78,79]. This is observed despite some of these T regs may have additionally lost Foxp3 upon transfer into Rag−/− mice.…”

“…The increase is pronounced among T regs in mouse oral lamina propria and intraepithelial cells (MOIL) isolated from tongue and palatal tissues of the infected mice [103]. Both Th17 effectors and nT regs contribute to the induction of Th17-like T regs in vitro , likely reflecting the same scenario during candidal infection in vivo [78]. Functionally, it is clear that the development of these cells does not contribute to pathology during acute candida infection in mice.…”

“…In contrast, during Th17 inducing conditions in the presence of TGF-β and IL-6, although Foxp3 induction is substantially reduced because of the inhibitory effect of IL-6, a few cells express Foxp3. Interestingly, a small proportion of these induced Foxp3 + cells also co-express IL-17A [78]. Using mixed cultures where CD45.2 nT regs are co-cultured with CD45.1 naïve cells under Th17 inducing conditions, it has been shown that both nT regs and induced Foxp3+ cells are capable of producing IL-17A in a IL-6 dependent manner [78].…”

Transforming growth factor-β1 sustains the survival of Foxp3+ regulatory cells during late phase of oropharyngeal candidiasis infection

“…We and others have shown that during infections, Toll-like receptor-2 signaling can promote proliferation and accumulation of T regs in the infected tissues and the draining lymph nodes, and is required to limit immunopathology. 31,45 Although T regs underwent proliferation, their expansion is less than that of the effector cells, and could not have contributed to increased viability compared with effector cells (Supplementary Figure S1A). It is conceivable that although most effector cells die after infection clearance, T regs have adopted a survival mechanism to be retained longer in the tissues and limit inflammation and tissue damage.…”