Changes to the symbiotic microbiota early in life, or the absence of it, can lead to exacerbated type 2 immunity and allergic inflammations. Although it is unclear how the microbiota regulates type 2 immunity, it is a strong inducer of proinflammatory T helper 17 (T(H)17) cells and regulatory T cells (T(regs)) in the intestine. Here, we report that microbiota-induced T(regs) express the nuclear hormone receptor RORγt and differentiate along a pathway that also leads to T(H)17 cells. In the absence of RORγt(+) T(regs), T(H)2-driven defense against helminths is more efficient, whereas T(H)2-associated pathology is exacerbated. Thus, the microbiota regulates type 2 responses through the induction of type 3 RORγt(+) T(regs) and T(H)17 cells and acts as a key factor in balancing immune responses at mucosal surfaces.

D-tryptophan is a newly identified product from probiotic bacteria. Our findings support the concept that defined bacterial products can be exploited in novel preventative strategies for chronic immune diseases.

Medullary thymic epithelial cells (mTECs) contribute to self-tolerance by expressing and presenting peripheral tissue antigens for negative selection of autoreactive T cells and differentiation of natural regulatory T cells. The molecular control of mTEC development remains incompletely understood. We here demonstrate by TEC-specific gene manipulation in mice that the NF-κB transcription factor subunit RelB, which is activated by the alternative NF-κB pathway, regulates development of mature mTECs in a dose-dependent manner. Mice with conditional deletion of Relb lacked mature mTECs and developed spontaneous autoimmunity. In addition, the NF-κB subunits RelA and c-Rel, which are both activated by classical NF-κB signaling, were jointly required for mTEC differentiation by directly regulating the transcription of Relb. Our data reveal a crosstalk mechanism between classical and alternative NF-κB pathways that tightly controls the development of mature mTECs to ensure self-tolerance.