Trail⁺DX5⁻Eomes⁻ natural killer (NK) cells arise in the mouse fetal liver and persist in the adult liver. Their relationships with Trail⁻DX5⁺ NK cells remain controversial. We generated a novel Eomes-GFP reporter murine model to address this question. We found that Eomes⁻ NK cells are not precursors of classical Eomes⁺ NK cells but rather constitute a distinct lineage of innate lymphoid cells. Eomes⁻ NK cells are strictly dependent on both T-bet and IL-15, similarly to NKT cells. We observed that, in the liver, expression of T-bet in progenitors represses Eomes expression and the development of Eomes⁺ NK cells. Reciprocally, the bone marrow (BM) microenvironment restricts T-bet expression in developing NK cells. Ectopic expression of T-bet forces the development of Eomes⁻ NK cells, demonstrating that repression of T-bet is essential for the development of Eomes⁺ NK cells. Gene profile analyses show that Eomes⁻ NK cells share part of their transcriptional program with NKT cells, including genes involved in liver homing and NK cell receptors. Moreover, Eomes⁻ NK cells produce a broad range of cytokines, including IL-2 and TNF in vitro and in vivo, during immune responses against vaccinia virus. Thus, mutually exclusive expression of T-bet and Eomes drives the development of different NK cell lineages with complementary functions.

Foxp3 (forkhead box P3 transcription factor)-expressing regulatory T cells (Tregs) are essential for immunological tolerance, best illustrated by uncontrolled effector T-cell responses and autoimmunity upon loss of Foxp3 expression. Tregs can adopt specific effector phenotypes upon activation, reflecting the diversity of functional demands in the different tissues of the body. Here, we report that Foxp3⁺CD4⁺ T cells coexpressing retinoic acid-related orphan receptor-γt (RORγt), the master transcription factor for T helper type 17 (Th17) cells, represent a stable effector Treg lineage. Transcriptomic and epigenetic profiling revealed that Foxp3⁺RORγt⁺ T cells display signatures of both Tregs and Th17 cells, although the degree of similarity was higher to Foxp3⁺RORγt⁻ Tregs than to Foxp3⁻RORγt⁺ T cells. Importantly, Foxp3⁺RORγt⁺ T cells were significantly demethylated at Treg-specific epigenetic signature genes such as Foxp3, Ctla-4, Gitr, Eos, and Helios, suggesting that these cells have a stable regulatory rather than inflammatory function. Indeed, adoptive transfer of Foxp3⁺RORγt⁺ T cells in the T-cell transfer colitis model confirmed their Treg function and lineage stability in vivo, and revealed an enhanced suppressive capacity as compared with Foxp3⁺RORγt⁻ Tregs. Thus, our data suggest that RORγt expression in Tregs contributes to an optimal suppressive capacity during gut-specific immune responses, rendering Foxp3⁺RORγt⁺ T cells as an important effector Treg subset in the intestinal system.

The devastating effect of ischemic stroke is attenuated in mice lacking conventional and unconventional T cells, suggesting that inflammation enhances tissue damage in cerebral ischemia. We explored the functional role of αβ and γδ T cells in a murine model of stroke and distinguished 2 different T cell–dependent proinflammatory pathways in ischemia-reperfusion injury. IFN-γ produced by CD4+ T cells induced TNF-α production in macrophages, whereas IL-17A secreted by γδ T cells led to neutrophil recruitment. The synergistic effect of TNF-α and IL-17A on astrocytes resulted in enhanced secretion of CXCL-1, a neutrophil chemoattractant. Application of an IL-17A–blocking antibody within 3 hours after stroke induction decreased infarct size and improved neurologic outcome in the murine model. In autoptic brain tissue of patients who had a stroke, we detected IL-17A–positive lymphocytes, suggesting that this aspect of the inflammatory cascade is also relevant in the human brain. We propose that selective targeting of IL-17A signaling might provide a new therapeutic option for the treatment of stroke.