Lucille C. Rankin

The transcription factor T-bet is essential for the development of NKp46+ innate lymphocytes via the Notch pathway

NKp46+ innate lymphoid cells (ILCs) serve important roles in regulating the intestinal microbiota and defense against pathogens. Whether NKp46+ ILCs arise directly from lymphoid tissue–inducer (LTi) cells or represent a separate lineage remains controversial. We report here that the transcription factor T-bet (encoded by Tbx21) was essential for the development of NKp46+ ILCs but not of LTi cells or nuocytes. Deficiency in interleukin 22 (IL-22)-producing NKp46+ ILCs resulted in greater susceptibility of Tbx21−/− mice to intestinal infection. Haploinsufficient T-bet expression resulted in lower expression of the signaling molecule Notch, and Notch signaling was necessary for the transition of LTi cells into NKp46+ ILCs. Furthermore, NKp46+ ILCs differentiated solely from the CD4− LTi population, not the CD4+ LTi population. Our results pinpoint the regulation of Notch signaling by T-bet as a distinct molecular pathway that guides the development of NKp46+ ILCs.

Nfil3 is required for the development of all innate lymphoid cell subsets

Loss of Nfil3 selectively reduces Peyer’s patch formation, impairing recruitment and distribution of lymphocytes and compromising immune responses to inflammatory and infectious agents.

Complementarity and redundancy of IL-22-producing innate lymphoid cells

Intestinal T cells and group 3 innate lymphoid cells (ILC3 cells) control the composition of the microbiota and gut immune responses. Within the gut, ILC3 subsets coexist that either express or lack the natural cytoxicity receptor (NCR) NKp46. We identified here the transcriptional signature associated with the transcription factor T-bet–dependent differentiation of NCR− ILC3 cells into NCR+ ILC3 cells. Contrary to the prevailing view, we found by conditional deletion of the key ILC3 genes Stat3, Il22, Tbx21 and Mcl1 that NCR+ ILC3 cells were redundant for the control of mouse colonic infection with Citrobacter rodentium in the presence of T cells. However, NCR+ ILC3 cells were essential for cecal homeostasis. Our data show that interplay between intestinal ILC3 cells and adaptive lymphocytes results in robust complementary failsafe mechanisms that ensure gut homeostasis.

TCF-1 Controls ILC2 and NKp46+RORγt+ Innate Lymphocyte Differentiation and Protection in Intestinal Inflammation

Innate lymphocyte populations play a central role in conferring protective immunity at the mucosal frontier. In this study, we demonstrate that T cell factor 1 (TCF-1; encoded by Tcf7), a transcription factor also important for NK and T cell differentiation, is expressed by multiple innate lymphoid cell (ILC) subsets, including GATA3+ nuocytes (ILC2) and NKp46+ ILCs (ILC3), which confer protection against lung and intestinal inflammation. TCF-1 was intrinsically required for the differentiation of both ILC2 and NKp46+ ILC3. Loss of TCF-1 expression impaired the capacity of these ILC subsets to produce IL-5, IL-13, and IL-22 and resulted in crippled responses to intestinal infection with Citrobacter rodentium. Furthermore, a reduction in T-bet expression required for Notch-2–dependent development of NKp46+ ILC3 showed a dose-dependent reduction in TCF-1 expression. Collectively, our findings demonstrate an essential requirement for TCF-1 in ILC2 differentiation and reveal a link among Tcf7, Notch, and Tbx21 in NKp46+ ILC3 development.

The Helix-Loop-Helix Protein ID2 Governs NK Cell Fate by Tuning Their Sensitivity to Interleukin-15

The inhibitor of DNA binding 2 (Id2) is essential for natural killer (NK) cell development with its canonical role being to antagonize E-protein function and alternate lineage fate. Here we have identified a key role for Id2 in regulating interleukin-15 (IL-15) receptor signaling and homeostasis of NK cells by repressing multiple E-protein target genes including Socs3. Id2 deletion in mature NK cells was incompatible with their homeostasis due to impaired IL-15 receptor signaling and metabolic function and this could be rescued by strong IL-15 receptor stimulation or genetic ablation of Socs3. During NK cell maturation, we observed an inverse correlation between E-protein target genes and Id2. These results shift the current paradigm on the role of ID2, indicating that it is required not only to antagonize E-proteins during NK cell commitment, but constantly required to titrate E-protein activity to regulate NK cell fitness and responsiveness to IL-15.

Transforming growth factor–β and Notch ligands act as opposing environmental cues in regulating the plasticity of type 3 innate lymphoid cells

Opposing signals in the tissue microenvironment balance the number of intestinal group 3 innate lymphoid cells. Plasticity in innate lymphoid cell function Like the T cells and B cells of the adaptive immune system, cells in the innate immune system are key to organismal health. Innate lymphoid cells (ILCs) are a heterogeneous type of innate immune cell that regulates immune responses and tolerance at mucosal surfaces, such as in the gut, by rapidly secreting cytokines. Group 3 ILCs (ILC3s) are characterized by the presence or absence of a cell surface natural cytotoxicity receptor (NCR). Two studies now provide evidence of heterogeneity and plasticity within ILC3s. Chea et al. found that a substantial proportion of mouse NCR− ILC3s differentiated into NCR+ ILC3s in response to stimulation of the receptor Notch2. In mice with defective Notch signaling specifically in lymphoid cells, NCR+ ILC3s were reduced in number and showed impaired cytokine secretion. Viant et al. showed that Notch signaling was required for the maintenance of NCR+ ILC3s. Furthermore, signaling by the cytokine transforming growth factor–β (TGF-β) antagonized Notch signaling, resulting in reduced numbers of NCR+ ILC3s. Together, these studies indicate that ILC3 subset composition in vivo depends on the balance between different signals found in tissue microenvironments, which has implications for whether immune responses or tolerance will prevail. Group 3 innate lymphoid cells (ILC3s) are composed of subsets that are either positive or negative for the natural cytotoxicity receptor (NCR) NKp46 (encoded by Ncr1). ILC3s are located at mucosal sites, such as in the intestine and lung, where they are exposed to billions of commensal microbes and potentially harmful pathogens. Together with T cells, the various ILC3 subsets maintain the balance between homeostasis and immune activation. Through genetic mapping, we identified a previously uncharacterized subset of NCR− ILC3s in mice that transiently express Ncr1, demonstrating previously undescribed heterogeneity within the ILC3 population. In addition, we showed that sustained Notch signaling was required for the maintenance of the NCR+ phenotype and that the cytokine transforming growth factor–β (TGF-β) impaired the development of NCR+ ILC3s. Thus, the plasticity of ILC3s is regulated by the balance between the opposing effects of Notch and TGF-β signaling, maintaining homeostasis in the face of continual challenges.

Diversity, function, and transcriptional regulation of gut innate lymphocytes

The innate immune system plays a critical early role in host defense against viruses, bacteria, and tumor cells. Until recently, natural killer (NK) cells and lymphoid tissue inducer (LTi) cells were the primary members of the innate lymphocyte family: NK cells form the front-line interface between the external environment and the adaptive immune system, while LTi cells are essential for secondary lymphoid tissue formation. More recently, it has become apparent that the composition of this family is much more diverse than previously appreciated and newly recognized populations play distinct and essential functions in tissue protection. Despite the importance of these cells, the developmental relationships between different innate lymphocyte populations remain unclear. Here we review recent advances in our understanding of the development of different innate immune cell subsets, the transcriptional programs that might be involved in driving fate decisions during development, and their relationship to NK cells.

Diverse Roles of Inhibitor of Differentiation 2 in Adaptive Immunity

The helix-loop-helix (HLH) transcription factor inhibitor of DNA binding 2 (Id2) has been implicated as a regulator of hematopoiesis and embryonic development. While its role in early lymphopoiesis has been well characterized, new roles in adaptive immune responses have recently been uncovered opening exciting new directions for investigation. In the innate immune system, Id2 is required for the development of mature natural killer (NK) cells, lymphoid tissue-inducer (LTi) cells, and the recently identified interleukin (IL)-22 secreting nonconventional innate lymphocytes found in the gut. In addition, Id2 has been implicated in the development of specific dendritic cell (DC) subsets, decisions determining the formation of αβ and γδ T-cell development, NK T-cell behaviour, and in the maintenance of effector and memory CD8+ T cells in peripheral tissues. Here, we review the current understanding of the role of Id2 in lymphopoiesis and in the development of the adaptive immune response required for maintaining immune homeostasis and immune protection.

Characterization of Blimp-1 function in effector regulatory T cells

Regulatory T (Treg) cells maintain immunological tolerance in steady-state and after immune challenge. Activated Treg cells can undergo further differentiation into an effector state that highly express genes critical for Treg cell function, including ICOS, TIGIT and IL-10, although how this process is controlled is poorly understood. Effector Treg cells also specifically express the transcriptional regulator Blimp-1 whose expression overlaps with many of the canonical markers associated with effector Treg cells, although not all ICOS+TIGIT+ Treg cells express Blimp-1 or IL-10. In this study, we addressed the role of Blimp-1 in effector Treg cell function. Mice lacking Blimp-1 specifically in Treg cells mature normally, but succumb to a multi-organ inflammatory disease later in life. Blimp-1 is not required for Treg cell differentiation, with mutant mice having increased numbers of effector Treg cells, but regulated a suite of genes involved in cell signaling, communication and survival, as well as being essential for the expression of the immune modulatory cytokine IL-10. Thus, Blimp-1 is a marker of effector Treg cells in all contexts examined and is required for the full functionality of these cells during aging.

Innate Lymphoid Cells Type 3

Retinoic acid receptor-related (RAR) orphan receptor γ (Rorγt + ) innate lymphoid cells or group 3 innate lymphoid cells (ILC3s) play key roles in the development of lymphoid tissues, maintenance of epithelial tissue homeostasis, and repair and defense against pathogens encountered at the bodys barriers. These cells produce interleukin (IL)-22, IL-17A, and lymphotoxin (LT)α 1 β 2 which are critical cytokines necessary for induction of the protective pathways at epithelial and mucosal surfaces. The ILC3 are a heterogeneous group of cells. Significant efforts have been made to decipher the transcriptional program that regulates their development and to understand the functions and mechanisms of actions of the individual subsets in providing immune protection.