Natarajan Bhaskaran

Recent studies show that CD4⁺CD25⁺Foxp3⁺ regulatory cells (T_regs) 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 T_regs is less clear. Here we show that CD4⁺Foxp3⁺T_regs 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 T_regs in the presence and absence of TCR signals and inflammatory cytokines in vitro. The proliferation is directly dependent on TLR-2 expression in T_regs. Consistent with this, Tlr2^−/− mice harbor fewer thymically derived T_regs and peripheral T_regs under homeostatic conditions in vivo. However, under Th17 inducing conditions, IL-6 and TLR-2 signaling both in T_regs as well as antigen presenting cells (APC) are critical for maximal ROR-γt and IL-17A up-regulation in Foxp3⁺ T_regs. 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 T_regs. Taken together, our data reveal that TLR-2 signaling promotes not only proliferation, but also IL-17A in T_regs, depending on the cytokine milieu. These IL-17A producing T_regs may be relevant in mucosal infections and inflammation.

Utilizing mouse models provides excellent immunological and experimental tools to study oral immune responses. However for functional assays, isolating T lymphocytes from the oral tissues has proved to be challenging due to the absence of reliable methods that yield viable cells with consistency. To study adaptive immune cell interactions in the oral mucosal tissues, it is necessary to isolate T cells with a good viability and study them at the single cell level.