Transcription Factor PU.1 Promotes Conventional Dendritic Cell Identity and Function via Induction of Transcriptional Regulator DC-SCRIPT.

Dendritic cells (DCs) are can be broadly divided into conventional (cDC) and plasmacytoid (pDC) subsets. Despite the importance of this lineage diversity, its genetic basis is not fully understood. We found that conditional ablation of the Ets-family transcription factor PU.1 in DC-restricted progenitors led to increased pDC production at the expense of cDCs. PU.1 controlled many of the cardinal functions of DCs, such as antigen presentation by cDCs and type I interferon production by pDCs. Conditional ablation of PU.1 de-repressed the pDC transcriptional signature in cDCs. The combination of genome-wide mapping of PU.1 binding and gene expression analysis revealed a key role for PU.1 in maintaining cDC identity through the induction of the transcriptional regulator DC-SCRIPT. PU.1 activated DC-SCRIPT expression, which in turn promoted cDC formation, particularly of cDC1s, and repressed pDC development. Thus, cDC identity is regulated by a transcriptional node requiring PU.1 and DC-SCRIPT.

Effector Regulatory T Cell Differentiation and Immune Homeostasis Depend on the Transcription Factor Myb.

FoxP3-expressing regulatory T (Treg) cells are essential for maintaining immune homeostasis. Activated Treg cells undergo further differentiation into an effector state that highly expresses genes critical for Treg cell function, although how this process is coordinated on a transcriptional level is poorly understood. Here, we demonstrate that mice lacking the transcription factor Myb in Treg cells succumbed to a multi-organ inflammatory disease. Myb was specifically expressed in, and required for the differentiation of, thymus-derived effector Treg cells. The combination of transcriptome and genomic footprint analyses revealed that Myb directly regulated a large proportion of the gene expression specific to effector Treg cells, identifying Myb as a critical component of the gene regulatory network controlling effector Treg cell differentiation and function.

C-Myb(+) erythro-myeloid progenitor-derived fetal monocytes give rise to adult tissue-resident macrophages.

Although classified as hematopoietic cells, tissue-resident macrophages (MFs) arise from embryonic precursors that seed the tissues prior to birth to generate a self-renewing population, which is maintained independently of adult hematopoiesis. Here we reveal the identity of these embryonic precursors using an in utero MF-depletion strategy and fate-mapping of yolk sac (YS) and fetal liver (FL) hematopoiesis. We show that YS MFs are the main precursors of microglia, while most other MFs derive from fetal monocytes (MOs). Both YS MFs and fetal MOs arise from erythro-myeloid progenitors (EMPs) generated in the YS. In the YS, EMPs gave rise to MFs without monocytic intermediates, while EMP seeding the FL upon the establishment of blood circulation acquired c-Myb expression and gave rise to fetal MOs that then seeded embryonic tissues and differentiated into MFs. Thus, adult tissue-resident MFs established from hematopoietic stem cell-independent embryonic precursors arise from two distinct developmental programs.

Deconstructing deployment of the innate immune lymphocyte army for barrier homeostasis and protection.

The study of the immune system has shifted from a purely dichotomous separation between the innate and adaptive arms to one that is now highly complex and reshaping our ideas of how steady-state health is assured. It is now clear that immune cells do not neatly fit into these two streams and immune homeostasis depends on continual dialogue between multiple lineages of the innate (including dendritic cells, innate lymphoid cells, and unconventional lymphocytes) and adaptive (T and B lymphocytes) arms together with a finely tuned synergy between the host and microbes which is essential to ensure immune homeostasis. Innate lymphoid cells are critical players in this new landscape. Here, we discuss recent studies that have elucidated in detail the development of ILCs from their earliest progenitors and examine factors that influence their identification and ability to drive immune homeostasis and long-term immune protection.

Maternal retinoids control type 3 innate lymphoid cells and set the offspring immunity.

The impact of nutritional status during fetal life on the overall health of adults has been recognized; however, dietary effects on the developing immune system are largely unknown. Development of secondary lymphoid organs occurs during embryogenesis and is considered to be developmentally programmed. Secondary lymphoid organ formation depends on a subset of type 3 innate lymphoid cells (ILC3) named lymphoid tissue inducer (LTi) cells. Here we show that mouse fetal ILC3s are controlled by cell-autonomous retinoic acid (RA) signalling in utero, which pre-sets the immune fitness in adulthood. We found that embryonic lymphoid organs contain ILC progenitors that differentiate locally into mature LTi cells. Local LTi cell differentiation was controlled by maternal retinoid intake and fetal RA signalling acting in a haematopoietic cell-autonomous manner. RA controlled LTi cell maturation upstream of the transcription factor RORγt. Accordingly, enforced expression of Rorgt restored maturation of LTi cells with impaired RA signalling, whereas RA receptors directly regulated the Rorgt locus. Finally, we established that maternal levels of dietary retinoids control the size of secondary lymphoid organs and the efficiency of immune responses in the adult offspring. Our results reveal a molecular link between maternal nutrients and the formation of immune structures required for resistance to infection in the offspring.

TCF-1 limits the formation of Tc17 cells via repression of the MAF-RORγt axis.

Interleukin (IL)-17-producing CD8+ T (Tc17) cells have emerged as key players in host-microbiota interactions, infection, and cancer. The factors that drive their development, in contrast to interferon (IFN)-γ-producing effector CD8+ T cells, are not clear. Here we demonstrate that the transcription factor TCF-1 (Tcf7) regulates CD8+ T cell fate decisions in double-positive (DP) thymocytes through the sequential suppression of MAF and RORγt, in parallel with TCF-1-driven modulation of chromatin state. Ablation of TCF-1 resulted in enhanced Tc17 cell development and exposed a gene set signature to drive tissue repair and lipid metabolism, which was distinct from other CD8+ T cell subsets. IL-17-producing CD8+ T cells isolated from healthy humans were also distinct from CD8+IL-17- T cells and enriched in pathways driven by MAF and RORγt Overall, our study reveals how TCF-1 exerts central control of T cell differentiation in the thymus by normally repressing Tc17 differentiation and promoting an effector fate outcome.

A point mutation in the Ncr1 signal peptide impairs the development of innate lymphoid cell subsets.

NKp46 (CD335) is a surface receptor shared by both human and mouse natural killer (NK) cells and innate lymphoid cells (ILCs) that transduces activating signals necessary to eliminate virus-infected cells and tumors. Here, we describe a spontaneous point mutation of cysteine to arginine (C14R) in the signal peptide of the NKp46 protein in congenic Ly5.1 mice and the newly generated NCRB6C14R strain. Ly5.1C14R NK cells expressed similar levels of Ncr1 mRNA as C57BL/6, but showed impaired surface NKp46 and reduced ability to control melanoma tumors in vivo. Expression of the mutant NKp46C14R in 293T cells showed that NKp46 protein trafficking to the cell surface was compromised. Although Ly5.1C14R mice had normal number of NK cells, they showed an increased number of early maturation stage NK cells. CD49a+ILC1s were also increased but these cells lacked the expression of TRAIL. ILC3s that expressed NKp46 were not detectable and were not apparent when examined by T-bet expression. Thus, the C14R mutation reveals that NKp46 is important for NK cell and ILC differentiation, maturation and function. Significance Innate lymphoid cells (ILCs) play important roles in immune protection. Various subsets of ILCs express the activating receptor NKp46 which is capable of recognizing pathogen derived and tumor ligands and is necessary for immune protection. Here, we describe a spontaneous point mutation in the signal peptide of the NKp46 protein in congenic Ly5.1 mice which are widely used for tracking cells in vivo. This Ncr1 C14R mutation impairs NKp46 surface expression resulting in destabilization of Ncr1 and accumulation of NKp46 in the endoplasmic reticulum. Loss of stable NKp46 expression impaired the maturation of NKp46+ ILCs and altered the expression of TRAIL and T-bet in ILC1 and ILC3, respectively.

Deciphering the Innate Lymphoid Cell Transcriptional Program.

Innate lymphoid cells (ILCs) are enriched at mucosal surfaces, where they provide immune surveillance. All ILC subsets develop from a common progenitor that gives rise to pre-committed progenitors for each of the ILC lineages. Currently, the temporal control of gene expression that guides the emergence of these progenitors is poorly understood. We used global transcriptional mapping to analyze gene expression in different ILC progenitors. We identified PD-1 to be specifically expressed in PLZF+ ILCp and revealed that the timing and order of expression of the transcription factors NFIL3, ID2, and TCF-1 was critical. Importantly, induction of ILC lineage commitment required only transient expression of NFIL3 prior to ID2 and TCF-1 expression. These findings highlight the importance of the temporal program that permits commitment of progenitors to the ILC lineage, and they expand our understanding of the core transcriptional program by identifying potential regulators of ILC development.

Identification of a novel lymphoid population in the murine epidermis.

T cell progenitors are known to arise from the foetal liver in embryos and the bone marrow in adults; however different studies have shown that a pool of T cell progenitors may also exist in the periphery. Here, we identified a lymphoid population resembling peripheral T cell progenitors which transiently seed the epidermis during late embryogenesis in both wild-type and T cell-deficient mice. We named these cells ELCs (Epidermal Lymphoid Cells). ELCs expressed Thy1 and CD2, but lacked CD3 and TCRαß/γδ at their surface, reminiscent of the phenotype of extra- or intra- thymic T cell progenitors. Similarly to Dendritic Epidermal T Cells (DETCs), ELCs were radioresistant and capable of self-renewal. However, despite their progenitor-like phenotype and expression of T cell lineage markers within the population, ELCs did not differentiate into conventional T cells or DETCs in in vitro, ex vivo or in vivo differentiation assays. Finally, we show that ELC expressed NK markers and secreted IFN-γ upon stimulation. Therefore we report the discovery of a unique population of lymphoid cells within the murine epidermis that appears related to NK cells with as-yet-unidentified functions.