Intestinal microbes provide multicellular hosts with nutrients and confer resistance to infection. The delicate balance between pro- and anti-inflammatory mechanisms, essential for gut immune homeostasis, is affected by the composition of the commensal microbial community. Regulatory T (Treg) cells expressing transcription factor Foxp3 play a key role in limiting inflammatory responses in the intestine1. Although specific members of the commensal microbial community have been found to potentiate the generation of anti-inflammatory Treg or pro-inflammatory Th17 cells2-6, the molecular cues driving this process remain elusive. Considering the vital metabolic function afforded by commensal microorganisms, we hypothesized that their metabolic by-products are sensed by cells of the immune system and affect the balance between pro- and anti-inflammatory cells. We found that a short-chain fatty acid (SCFA), butyrate, produced by commensal microorganisms during starch fermentation, facilitated extrathymic generation of Treg cells. A boost in Treg cell numbers upon provision of butyrate was due to potentiation of extrathymic differentiation of Treg cells as the observed phenomenon was dependent upon intronic enhancer CNS1, essential for extrathymic, but dispensable for thymic Treg cell differentiation1, 7. In addition to butyrate, de novo Treg cell generation in the periphery was potentiated by propionate, another SCFA of microbial origin capable of HDAC inhibition, but not acetate, lacking this activity. Our results suggest that bacterial metabolites mediate communication between the commensal microbiota and the immune system, affecting the balance between pro- and anti-inflammatory mechanisms.

Innate lymphoid cells (ILC) contribute to barrier immunity, tissue homeostasis, and immune regulation at various anatomical sites throughout the body. How ILCs maintain their presence in lymphoid and peripheral tissues is currently unknown. We found that in the lymphoid and non-lymphoid organs of adult mice, ILC are tissue-resident cells that were maintained and expanded locally under physiologic conditions, upon systemic perturbation of immune homeostasis, and during acute helminth infection. However, at later time points post-infection, cells from hematogenous sources helped to partially replenish the pool of resident ILCs. Thus, ILC are maintained by self-renewal in broadly different microenvironments and physiological settings. Such an extreme “sedentary” lifestyle is consistent with the proposed roles of ILCs as sentinels and local keepers of tissue function.

Regulatory T (Treg) cells, expressing abundant amounts of the IL-2 receptor (IL-2R), are reliant on IL-2 produced by activated T cells. This feature implied a key role for a simple network based on IL-2 consumption by Treg cells in their suppressor function. However, congenital deficiency in IL-2R results in reduced expression of the Treg cell lineage specification factor Foxp3, confounding experimental efforts to understand the role of IL-2R expression and signaling in Treg suppressor function. Using genetic gain and loss of function approaches, we demonstrate that IL-2 capture is dispensable for control of CD4+ T cells, but is important for limiting CD8+ T cell activation, and that IL-2R dependent STAT5 transcription factor activation plays an essential role in Treg cell suppressor function separable from T cell receptor signaling.