Matthias Lochner

Microbial Flora Drives Interleukin 22 Production in Intestinal NKp46+ Cells that Provide Innate Mucosal Immune Defense

Natural killer (NK) cells are innate lymphocytes with spontaneous antitumor activity, and they produce interferon-gamma (IFN-gamma) that primes immune responses. Whereas T helper cell subsets differentiate from naive T cells via specific transcription factors, evidence for NK cell diversification is limited. In this report, we characterized intestinal lymphocytes expressing the NK cell natural cytotoxicity receptor NKp46. Gut NKp46+ cells were distinguished from classical NK cells by limited IFN-gamma production and absence of perforin, whereas several subsets expressed the nuclear hormone receptor retinoic acid receptor-related orphan receptor t (RORgammat) and interleukin-22 (IL-22). Intestinal NKp46+IL-22+ cells were generated via a local process that was conditioned by commensal bacteria and required RORgammat. Mice lacking IL-22-producing NKp46+ cells showed heightened susceptibility to the pathogen Citrobacter rodentium, consistent with a role for intestinal NKp46+ cells in immune protection. RORgammat-driven diversification of intestinal NKp46+ cells thereby specifies an innate cellular defense mechanism that operates at mucosal surfaces.

In vivo equilibrium of proinflammatory IL-17+ and regulatory IL-10+ Foxp3+ RORγt+ T cells

The nuclear hormone receptor retinoic acid receptor–related orphan receptor γt (RORγt) is required for the generation of T helper 17 cells expressing the proinflammatory cytokine interleukin (IL)-17. In vivo, however, less than half of RORγt+ T cells express IL-17. We report here that RORγt+ Tαβ cells include Foxp3+ cells that coexist with IL-17–producing RORγt+ Tαβ cells in all tissues examined. The Foxp3+ RORγt+ Tαβ express IL-10 and CCL20, and function as regulatory T cells. Furthermore, the ratio of Foxp3+ to IL-17–producing RORγt+ Tαβ cells remains remarkably constant in mice enduring infection and inflammation. This equilibrium is tuned in favor of IL-10 production by Foxp3 and CCL20, and in favor of IL-17 production by IL-6 and IL-23. In the lung and skin, the largest population of RORγt+ T cells express the γδ T cell receptor and produce the highest levels of IL-17 independently of IL-6. Thus, potentially antagonistic proinflammatory IL-17–producing and regulatory Foxp3+ RORγt+ T cells coexist and are tightly controlled, suggesting that a perturbed equilibrium in RORγt+ T cells might lead to decreased immunoreactivity or, in contrast, to pathological inflammation.

Lineage Relationship Analysis of RORγt+ Innate Lymphoid Cells

Innate Innit? Innate lymphocytes (ILCs) are a recently described population of immune cells that produce cytokines like those associated with T helper cells, but lack the recombined antigen receptors characteristic of T cells. Again, like some T helper cell lineages, a proportion of ILCs express the transcription factor RORγt. These include lymphoid tissue inducer (LTi) cells required for fetal lymphoid tissue organogenesis and a population of natural killer (NK)–like cells that function in gut immune responses. Sawa et al. (p. 665; see the Perspective by Veldhoen and Withers) wondered whether the RORγt-expressing ILCs all develop from the same progenitor population. Indeed, they found a fetal liver progenitor that gave rise to several phenotypically distinct populations. However, the LTi cells were not progenitors for the NK-like cells. It seems the trajectory of different ILC populations is developmentally regulated, and postnatally ILCs are favored that play a role in intestinal defense before the gut is fully colonized by intestinal microbiota. Immune cells develop to preempt intestinal colonization by microbial symbionts. Lymphoid tissue–inducer (LTi) cells initiate the development of lymphoid tissues through the activation of local stromal cells in a process similar to inflammation. LTi cells express the nuclear hormone receptor RORγt, which also directs the expression of the proinflammatory cytokine interleukin-17 in T cells. We show here that LTi cells are part of a larger family of proinflammatory RORγt+ innate lymphoid cells (ILCs) that differentiate from distinct fetal liver RORγt+ precursors. The fate of RORγt+ ILCs is determined by mouse age, and after birth, favors the generation of cells involved in intestinal homeostasis and defense. Contrary to RORγt+ T cells, however, RORγt+ ILCs develop in the absence of microbiota. Our study indicates that RORγt+ ILCs evolve to preempt intestinal colonization by microbial symbionts.

DC activated via dectin-1 convert Treg into IL-17 producers

Th cells producing IL‐17 play a pro‐inflammatory role at mucosal surfaces. Treg at the same sites dampen inflammation and prevent immunopathology. Th cells producing IL‐17 (Th17) and Treg are thought to be distinct populations defined by expression of the transcription factors ROR‐γt and Foxp3, respectively. Here, we show that mouse CD25+Foxp3+ Treg can be converted into a hybrid T‐cell population characterized by the expression of Foxp3 and ROR‐γt and the production of IL‐17. Conversion was observed upon coculture with DC selectively activated via dectin‐1, a C‐type lectin receptor involved in fungal recognition, and depended on IL‐23 produced by DC. Within the Foxp3+ population, only Foxp3+ROR‐γt+ T cells but not Foxp3+ROR‐γt‐–T cells become Foxp3+IL‐17+ T cells. These results indicate that some Foxp3+ T cells can produce IL‐17 while retaining Foxp3 expression and suggest that Treg could play an unexpected pro‐inflammatory role in some settings.

Critical role of ROR-γt in a new thymic pathway leading to IL-17-producing invariant NKT cell differentiation

Invariant natural killer T (iNKT) cells constitute a subpopulation of T cells that recognize glycolipids presented by CD1d molecules. They are characterized by their prompt production of interleukin-4 (IL-4) and interferon-γ (IFN-γ), which enables them to modulate diverse immune responses. Recently, we enlarged this concept by identifying a distinct IL-17-producing iNKT cell subset, named iNKT17 cells. The mechanisms leading to the acquisition of this new iNKT cell activity are unknown. Herein we show that IL-17-producing iNKT cells are already present in the thymus, predominantly among a subset regarded so far as an immature stage of thymic iNKT cell development, the CD1d tetramerposCD44posNK1.1negCD4neg cells. Using EGFP reporter mice, we demonstrate that the transcription factor ROR-γt is critical for the thymic differentiation of this subset because only ROR-γtpos iNKT cells are capable of massively secreting IL-17. Moreover, IL-17-producing CD1d tetramerposCD44posNK1.1negCD4neg thymic iNKT cells have reached a mature differentiation stage because they fail to generate other cell subsets in fetal thymic organ culture. Conversely, thymic ROR-γtneg iNKT cell precursors give rise to progeny, but acquire neither ROR-γt expression nor the ability to secrete IL-17. In conclusion, our findings demonstrate an alternative thymic pathway leading to the development of iNKT17 cells that requires ROR-γt expression.

Microbiota-induced tertiary lymphoid tissues aggravate inflammatory disease in the absence of RORγt and LTi cells

Microbiota drive tertiary lymphoid tissue formation in mice lacking the nuclear hormone receptor Rorγt, leading to intestinal inflammation and wasting disease.

Fatty acid metabolism in the regulation of T cell function

The specific regulation of cellular metabolic processes is of major importance for directing immune cell differentiation and function. We review recent evidence indicating that changes in basic cellular lipid metabolism have critical effects on T cell proliferation and cell fate decisions. While induction of de novo fatty acid (FA) synthesis is essential for activation-induced proliferation and differentiation of effector T cells, FA catabolism via β-oxidation is important for the development of CD8(+) T cell memory as well as for the differentiation of CD4(+) regulatory T cells. We consider the influence of lipid metabolism and metabolic intermediates on the regulation of signaling and transcriptional pathways via post-translational modifications, and discuss how an improved understanding of FA metabolism may reveal strategies for manipulating immune responses towards therapeutic outcomes.

Inflammation Recapitulates the Ontogeny of Lymphoid Stromal Cells

Stromal cells in lymphoid tissues regulate lymphocyte recruitment and survival through the expression of specific chemokines and cytokines. During inflammation, the same signals recruit lymphocytes to the site of injury; however, the “lymphoid” stromal (LS) cells producing these signals remain poorly characterized. We find that mouse inflammatory lesions and tumors develop gp38+ LS cells, in recapitulation of the development of LS cells early during the ontogeny of lymphoid organs and the intestine, and express a set of genes that promotes the development of lymphocyte-permissive tissues. These gp38+ LS cells are induced by a robust pathway that requires myeloid cells but not known Toll- or NOD-like receptors, the inflammasome, or adaptive immunity. Parabiosis and inducible genetic cell fate mapping experiments indicate that local precursors, presumably resident fibroblasts rather that circulating precursors, massively proliferate and give rise to LS cells during inflammation. Our results show that LS cells are both programmed during ontogeny and reinduced during inflammation.

Restricted Microbiota and Absence of Cognate TCR Antigen Leads to an Unbalanced Generation of Th17 Cells

Retinoic acid-related orphan receptor (ROR)γt+ TCRαβ+ cells expressing IL-17, termed Th17 cells, are most abundant in the intestinal lamina propria. Symbiotic microbiota are required for the generation of Th17 cells, but the requirement for microbiota-derived Ag is not documented. In this study, we show that normal numbers of Th17 cells develop in the intestine of mice that express a single TCR in the absence of cognate Ag, whereas the microbiota remains essential for their development. However, such mice, or mice monocolonized with the Th17-inducing segmented filamentous bacteria, fail to induce normal numbers of Foxp3+ RORγt+ T cells, the regulatory counterpart of IL-17+RORγt+ T cells. These results demonstrate that a complex microbiota and cognate Ag are required to generate a properly regulated set of RORγt+ T cells and Th17 cells.

Foxp3 + T cells expressing RORγt represent a stable regulatory T-cell effector lineage with enhanced suppressive capacity during intestinal inflammation

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.