Search

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.

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; 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.