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International audience; ILCs and T cells are closely related functionally but they significantly differ in their ability to circulate, expand, and renew. Cooperation and reciprocal functional regulation suggest that these cell types are more complementary than simply redundant during immune responses. How ILCs shape T-cell responses is strongly dependent on the tissue and inflammatory context. Likewise, indirect regulation of ILCs by adaptive immunity is induced by environmental cues such as the gut microbiota. Here, we review shared requirements for the development and function of both cell types and divergences in the orchestration of prototypic immune functions. We discuss the diversity of functional interactions between T cells and ILCs during homeostasis and immune responses. Identifying the location and the nature of the tissue microenvironment in which these interactions are taking place may uncover the remaining mysteries of their close encounters.

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.

Terminal deoxynucleotidyl transferase (TdT) expression is controlled at the transcriptional level, however, the TdT core promoter combining D, D′, an initiator (Inr) and downstream basal elements (DBE) does not recapitulate the whole complex regulation of TdT expression. We hypothesized that important cis-regulatory elements of the gene are located outside of the TdT promoter. In an attempt to identify these elements, we performed DNase I hypersensitivity assays over 24 kb including a 10 kb region located upstream of the transcription start site (+1) and a 14 kb region spanning exons and introns I to VI. Hypersensitive sites (HS) HS1 and HS2 were localized 8.5 and 8 kb upstream of the transcription start site, respectively, and were exclusively detected in TdT + cell types. HS3, HS4 and HS5 were mapped at positions −7, −3.4 and −3 kb, respectively, and detected in both TdT negative and positive cells. HS6, HS7 and HS8 were detected immediately upstream of the TdT promoter. HS10 and HS11 were localized in the first and third intron of the gene. Luciferase reporter assays revealed that HS1, HS2 and HS3 synergize with the TdT promoter to activate transcription in a TdT + pre-T cell line but not in a TdT + pro-B cell line. In summary novel cis-regulatory elements have been identified in the 5′ region of the TdT locus that synergize with the promoter to activate gene expression and our results suggest these elements may be more active in T cells.

The nuclear hormone receptor retinoic acid receptor-related orphan receptor gamma t (RORgamma t) is required for the generation of T helper 17 cells expressing the proinflammatory cytokine interleukin (IL)-17. In vivo, however, less than half of RORgamma t(+) T cells express IL-17. We report here that RORgamma t(+) T alphabeta cells include Foxp3(+) cells that coexist with IL-17-producing RORgamma t(+) T alphabeta cells in all tissues examined. The Foxp3(+) RORgamma t(+) T alphabeta express IL-10 and CCL20, and function as regulatory T cells. Furthermore, the ratio of Foxp3(+) to IL-17-producing RORgamma t(+) T alphabeta 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 RORgamma t(+) T cells express the gammadelta 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(+) RORgamma t(+) T cells coexist and are tightly controlled, suggesting that a perturbed equilibrium in RORgamma t(+) T cells might lead to decreased immunoreactivity or, in contrast, to pathological inflammation.

The initial B cell repertoire contains a considerable proportion of autoreactive specificities. The first major B cell tolerance checkpoint is at the stage of the immature B cell, where receptor editing is the primary mode of eliminating self-reactivity. The cells that emigrate from the bone marrow have a second tolerance checkpoint in the transitional compartment in the spleen. Although it is known that the second checkpoint is defective in lupus, it is not clear whether there is any breakdown in central B cell tolerance in the bone marrow. We demonstrate that receptor editing is less efficient in the lupus-prone strain MRL/lpr. In an in vitro system, when receptor-editing signals are given to bone marrow immature B cells by antiidiotype antibody or after in vivo exposure to membrane-bound self-antigen, MRL/lpr 3-83 transgenic immature B cells undergo less endogenous rearrangement and up-regulate recombination activating gene messenger RNA to a lesser extent than B10 transgenic cells. CD19, along with immunoglobulin M, is down-regulated in the bone marrow upon receptor editing, but the extent of down-regulation is fivefold less in MRL/lpr mice. Less efficient receptor editing could allow some autoreactive cells to escape from the bone marrow in lupus-prone mice, thus predisposing to autoimmunity.

International audience; RORγt(+) innate lymphoid cells (ILCs), or ILC3, play a fundamental role in the development of lymphoid tissues, as well as in homeostasis and defence of mucosal tissues. These cells produce IL-22, IL-17A and LTα1β2, key cytokines for the activation of epithelial defences and the recruitment of polymorphonuclear phagocytes. In the absence of ILC3, the early defence to infection and resistance to injury are compromised. Given the importance of ILC3 in mucosal immunity, significant efforts are made to discover their multiple functions and decipher their mode of action and regulation.

ILC have recently emerged as new subsets of innate lymphoid effectors devoid of antigen receptors and harboring strikingly diverse functions. Three main sub-types of ILC can be distinguished based on their developmental requirements and functions. ILC1 include cytotoxic natural killer cells and T-bet dependent IFNγ producing cells. ILC2 are generated from bone marrow progenitors expressing GATA-3 and produce type 2 cytokines such as IL-5 and IL-13 in response to infections by parasitic worms and Influenza virus. Type 3 ILC are dependent on RORγt, they are involved in the maintenance of the gut homeostasis and in the defense against intestinal pathogens. Given their close relationship with the gut and airway epithelial barriers, ILC are in the first line of defense against a number of pathogens but they also need to be tightly regulated in order to avoid chronic inflammation.

International audience; Innate lymphoid cells (ILCs) are generated from common lymphoid precursors, like lymphocytes, but do not express an antigen receptor. ILCs include Natural Killer (NK) cells, first described 38 years ago, as well as the more recently discovered lymphoid tissue inducer (LTi) cells, NK(22) cells and ILC2s. ILCs reflect many functions of CD4(+) T helper cells by expressing IFNγ, IL-17, IL-22 or IL-13. However, in contrast to T cells, they are not selected on the basis of antigen specificity, and expand and act shortly after stimulation. Therefore, ILCs play fundamental roles early in responses to infection and injury, in the maintenance of homeostasis, and possibly in the regulation of adaptive immunity. Here, we review the recent data on the development and role of RORγt(+) ILCs and ILC2s in intestinal homeostasis and defense.

Notch signaling is required for the generation of α4β7+RORγt− fetal progenitors, but must then be turned off to allow RORγt expression and LTi cell maturation., Lymphoid tissue development is initiated during embryogenesis by the migration of lymphoid tissue inducer (LTi) cells from the fetal liver to the periphery, where they induce the formation of lymph nodes and Peyer’s patches. In the fetal liver, a subset of common lymphoid progenitors (CLPs) that expresses the integrin α4β7 gives rise to LTi cells, a process strictly dependent on the expression of the transcriptional repressor Id2 and the nuclear hormone receptor retinoic acid–related orphan receptor γ t (RORγt). In this study, we show that Id2 and RORγt are sequentially up-regulated during LTi cell development, matching two waves of differentiation with opposite requirements for Notch signaling. Both the expression of Id2 and Notch are required for the generation of α4β7+ RORγt− fetal progenitors, but Notch subsequently blocks progression to the RORγt+ stage and final maturation of LTi cells. Notch is therefore a necessary switch to engage the LTi developmental pathway, but needs to be turned off later to avoid diversion to the T cell fate.

International audience; Lymphoid tissue inducer (LTi) cells are programmed by the mammalian fetus to induce the development of lymph nodes and Peyer's patches. LTi cells share a pro-inflammatory profile with Th17 cells, as well as their requirement for the transcription factor RORγt. We discuss here the latest data on the fetal and post-natal development of LTi cells, and their relationship with the larger family of innate lymphoid cells (ILCs). We suggest that the re-programming of RORγt in a subset of common lymphoid progenitors allowed mammals to develop lymphoid organs before birth, whereas other vertebrates only develop such organs in response to infection or injury.

Early B cell factor (EBF)1 is essential for B lineage specification. Previously, we demonstrated the synergistic activation of Cd79a ( mb-1 ) genes by EBF1 and its functional partner, RUNX1. Here, we identified consequences of Ebf1 haploinsufficiency together with haploinsufficiency of Runx1 genes in mice. Although numbers of “committed” pro-B cells were maintained in Ebf1 +/− Runx1 +/− ( ER het ) mice, activation of B cell-specific gene transcription was depressed in these cells. Expression of genes encoding Aiolos, κ 0 sterile transcripts, CD2 and CD25 were reduced and delayed in ER het pro-B cells, whereas surface expression of BP-1 was increased on late pro-B cells in ER het mice. Late pre-B and immature and mature B cells were decreased in the bone marrow of Ebf1 +/− ( E het ) mice and were nearly absent in ER het mice. Although we did not observe significant effects of haploinsuficiencies on IgH or Ig κ rearrangements, a relative lack of Ig λ rearrangements was detected in E het and ER het pre-B cells. Together, these observations suggest that B cell lineage progression is impaired at multiple stages in the bone marrow of E het and ER het mice. Furthermore, enforced expression of EBF1 and RUNX1 in terminally differentiated plasmacytoma cells activated multiple early B cell-specific genes synergistically. Collectively, these studies illuminate the effects of reduced Ebf1 dosage and the compounding effects of reduced Runx1 dosage. Our data confirm and extend the importance of EBF1 in regulating target genes and Ig gene rearrangements necessary for B cell lineage specification, developmental progression, and homeostasis.

N region diversity in antigen receptors is a developmentally regulated process in B and T lymphocytes, which correlates with the differential expression of terminal deoxynucleotidyl transferase (TdT). To precisely determine the onset of TdT gene activation during T cell differentiation and thymic ontogeny, TdT expression was directly detected at the cellular level by in situ hybridization and TdT function was assessed by analyzing the distribution of N additions in alpha and beta TCR genes at early stages of development. Even though TdT transcripts were undetectable at birth, substantial N additions were observed in ValphaJalpha junctions and 3 days later in VbetaDbetaJbeta junctions, indicating that TdT expression could be induced in immature thymocytes much earlier than expected. Indeed low TdT expression level was found in TN3/4 and DP from fetal day 17, suggesting that the onset of TdT expression occurs simultaneously in both populations and may depend on microenvironmental cues. Moreover significant increase in the proportion of thymocytes expressing high levels of TdT mRNA during the first week after birth without a similar increase in the level of N diversity suggests that TdT expression and TdT function in the generation of N diversity are not strictly correlated.