Macrophage Development Rewritten Macrophages provide protection against a wide variety of infections and critically shape the inflammatory environment in many tissues. These cells come in many flavors, as determined by differences in gene expression, cell surface phenotype and specific function. Schulz et al. (p. 86, published online 22 March) investigated whether adult macrophages all share a common developmental origin. Immune cells, including most macrophages, are widely thought to arise from hematopoietic stem cells (HSCs), which require the transcription factor Myb for their development. Analysis of Myb-deficient mice revealed that a population of yolk-sac–derived, tissue-resident macrophages was able to develop and persist in adult mice in the absence of HSCs. Importantly, yolk sac–derived macrophages also contributed substantially to the tissue macrophage pool even when HSCs were present. In mice, a population of tissue-resident macrophages arises independently of bone marrow–derived stem cells. Macrophages and dendritic cells (DCs) are key components of cellular immunity and are thought to originate and renew from hematopoietic stem cells (HSCs). However, some macrophages develop in the embryo before the appearance of definitive HSCs. We thus reinvestigated macrophage development. We found that the transcription factor Myb was required for development of HSCs and all CD11bhigh monocytes and macrophages, but was dispensable for yolk sac (YS) macrophages and for the development of YS-derived F4/80bright macrophages in several tissues, such as liver Kupffer cells, epidermal Langerhans cells, and microglia—cell populations that all can persist in adult mice independently of HSCs. These results define a lineage of tissue macrophages that derive from the YS and are genetically distinct from HSC progeny.

A lineage of myeloid cells independent of Myb and hematopoietic stem cells

Lymphocytes must adapt to a wide array of environmental stressors as part of their normal development, during which they undergo a dramatic metabolic remodeling process. Research in this area has yielded surprising findings on the roles of diverse metabolic pathways and metabolites, which have been found to regulate lymphocyte signaling and influence differentiation, function and fate. In this review, we integrate the latest findings in the field to provide an up-to-date resource on lymphocyte metabolism.

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T cell metabolism drives immunity

T cell subsets and their signature cytokines in autoimmune and inflammatory diseases

Recent evidence indicates that mitochondria lie at the heart of immunity. Mitochondrial DNA acts as a danger-associated molecular pattern (DAMP), and the mitochondrial outer membrane is a platform for signaling molecules such as MAVS in RIG-I signaling, and for the NLRP3 inflammasome. Mitochondrial biogenesis, fusion and fission have roles in aspects of immune-cell activation. Most important, Krebs cycle intermediates such as succinate, fumarate and citrate engage in processes related to immunity and inflammation, in both innate and adaptive immune cells. These discoveries are revealing mitochondrial targets that could potentially be exploited for therapeutic gain in inflammation and cancer.

Mitochondria are the powerhouses of immunity.

Transcription factors define tissue T cells The immune system fights microbial invaders by maintaining multiple lines of defense. For instance, specialized memory T cells [resident memory T cells (Trms)] colonize portals of pathogen entry, such as the skin, lung, and gut, to quickly halt reinfections. Mackay et al. now report that in mice, Trms as well as other tissue-dwelling lymphocyte populations such as natural killer cells share a common transcriptional program driven by the related transcription factors Hobit and Blimp1. Tissue residency and retention of lymphocytes require expression of Hobit and Blimp1, which, among other functions, suppress genes that promote tissue exit. Science, this issue p. 459 Tissue-dwelling lymphocyte populations share a common transcriptional signature. Tissue-resident memory T (Trm) cells permanently localize to portals of pathogen entry, where they provide immediate protection against reinfection. To enforce tissue retention, Trm cells up-regulate CD69 and down-regulate molecules associated with tissue egress; however, a Trm-specific transcriptional regulator has not been identified. Here, we show that the transcription factor Hobit is specifically up-regulated in Trm cells and, together with related Blimp1, mediates the development of Trm cells in skin, gut, liver, and kidney in mice. The Hobit-Blimp1 transcriptional module is also required for other populations of tissue-resident lymphocytes, including natural killer T (NKT) cells and liver-resident NK cells, all of which share a common transcriptional program. Our results identify Hobit and Blimp1 as central regulators of this universal program that instructs tissue retention in diverse tissue-resident lymphocyte populations.

Hobit and Blimp1 instruct a universal transcriptional program of tissue-residency in lymphocytes

NKT cells and neutrophils collaborate in graft rejection

for protection against the intra­ cellular parasite Toxoplasma gondii. But the parasite­derived peptides that are recognized by the protective T cells have remained elusive. Now, Shastri and colleagues show that a single parasite peptide presented by the mouse MHC class I allele H2­Ld is the target of an immunodominant CD8+ T­cell response, and mice that fail to process this peptide correctly are susceptible to toxoplasmosis. T. gondii establishes productive infection within a specialized intracellular vacuole, existing either as rapidly growing tachyzoites or as bradyzoites in semidormant cysts in the brain or muscles. To identify parasite proteins that might access the MHC class I processing pathway and stimulate T­cell responses, the authors generated T. gondii­specific T­cell hybridomas and assessed their reactivity against H2­Ld­expressing cells transfected with a cDNA library produced from tachyzoites. One T­cell hybridoma responded to several cDNAs, which were all found to encode (full length or a truncated version of) the parasite protein GRA6. Further studies confirmed that GRA6 contained the T­cell­ stimulating peptide that bound to H2­Ld, a decapeptide termed HF10. To establish whether the HF10 peptide was naturally processed from GRA6 in T. gondii­infected cells, the authors tested the antigenic activity of peptide fractions derived from infected cells. Indeed, the peptide fraction that stimulated the T­cell hybridomas eluted at the same time point as synthetic HF10. In addition, HF10­specific CD8+ T cells could be detected in the spleen and brain of mice that were infected with T. gondii. Surprisingly, given that the large T. gondii genome encodes thousands of potential antigens, the entire T­cell population that was responding to the parasite was found to be specific for the HF10 peptide, which is consistent with HF10 being an immunodominant epitope. Finally, the observation that immunization with bone­marrow­derived dendritic cells pulsed with HF10, but not with an irrelevant control peptide, pro­ tected mice from an otherwise lethal challenge with T. gondii confirmed that the CD8+ T­cell response to this epitope confers protection against T. gondii infection. A clue to the natural processing of the HF10 peptide was provided by the finding that mice lacking ERAAP (endoplasmic reticulum amino­ peptidase associated with antigen processing; which is known to be important for customizing peptides for presentation by MHC molecules) were more susceptible to T. gondii infection. Cells from these mice were considerably compromised in their ability to activate T. gondii­specific T­cell hybridomas following infection owing to a defect in the generation of HF10–H2­Ld complexes, which was shown to depend on the proteasome and TAP (transporter associated with antigen processing), as well as ERAAP. So, these data indicate that the ability of exogenous or vacuolar antigens to access the cytosolic MHC class I processing pathway determines whether they serve as sources for peptide–MHC ligands that elicit CD8+ T­cell responses. Lucy Bird

Parasite Immunity: How T cells see Toxo

10.1038/nri2086 How does the same antigenic peptide elicit different responses by T cells that are at different developmental stages? Might there be some intrinsic component that tunes a T cell’s sensitivity to peptide antigens and that influences the strength of the signal delivered by the T-cell receptor (TCR)? In a recent study published in Cell, researchers say there is. They show that the microRNA miR-181a governs T-cell sensitivity by suppressing the expression levels of multiple phosphatases that negatively regulate TCR signalling, thereby lowering the excitation threshold of a T cell. MicroRNAs (which are RNAs of ~22 nucleotides in length) are emerging as important post-transcriptional regulators of gene expression, targeting cognate messenger RNA for degradation or translational repression. To investigate whether microRNAs have a role in regulating the differential sensitivity of T cells during maturation, Li et al. measured the levels of a microRNA that is known to be highly expressed in the thymus, miR-181a. They found that miR-181a expression levels varied during T-cell maturation, with T cells at the early stages of differentiation, such as CD4+CD8+ double-positive thymocytes, expressing the highest levels and effector T cells expressing the lowest levels. To determine whether high expression of miR-181a had an effect on TCR signalling, the authors performed experiments with TCR-transgenic T cells transfected with miR-181a. They observed that increased miR-181a expression caused a significant increase in intracellular calcium concentration and interleukin-2 (IL-2) production following stimulation of the T cells with their cognate peptide. Moreover, compared with untransfected cells, fewer peptide–MHC complexes were needed to produce a half-maximal calcium response, indicating that miR-181a may increase the sensitivity and strength of TCR signalling. Consistent with this, miR-181a expression enabled cells to respond (by calcium flux and IL-2 production) to an otherwise non-stimulatory antagonist peptide as if it were an agonist peptide. This finding provides insight into how a T cell might sense quantitative differences in antigenic affinity for the TCR and elicit different responses. So how might miR-181a mediate this effect on T-cell reactivity? After failing to observe any changes in TCR and CD4 expression levels by miR-181a, the authors turned their attention to tyrosine phosphatases involved in proximal TCR signalling. SHP2 (SRC-homology-2-domaincontaining protein tyrosine phosphatase 2), PTPN22 (protein tyrosine phosphatase non-receptor type 22), DUSP5 (dual-specificity protein phosphatase 5) and DUSP6 were all shown to contain miR-181a pairing sites and to be downregulated in cells expressing miR-181a. Downregulation of such phosphatases is likely to reduce negative feedback and favour prolonged TCR signalling. Accordingly, even before and after TCR stimulation, miR-181a-expressing cells had higher levels of phosphorylated LCK (a target of PTPN22) and phosphorylated extracellular-signal-regulated kinase 1 (ERK1)/ERK2 in the nucleus (a target of DUSP5) and in the cytosol (a target of DUSP6) compared with control cells. So, increased basal levels of these activated signalling intermediates in miR-181a-expressing T cells reduces the amount of signal that is required for T-cell activation. Finally, the authors confirmed a role for miR-181a in increasing T-cell sensitivity, by showing that knockdown of miR-181a using a specific antisense oligonucleotide lowered T-cell sensitivity to both agonist and antagonist peptides, as indicated by calcium flux and IL-2 production. Importantly, the high level of miR-181a expression by double-positive thymocytes was shown to enable these cells to respond to low-affinity peptides necessary for proper positive and negative selection. Therefore, miR-181a acts as a ‘rheostat’ to fine-tune T-cell sensitivity to antigen — a function that is likely to be important in regulating the selection of the T-cell repertoire, as well as in regulating tolerance versus effector-cell function. Lucy Bird

Lucy Bird

Papers

NKT cells and neutrophils collaborate in graft rejection

Parasite Immunity: How T cells see Toxo

Tuning a T cell's excitation threshold

Mucosal immunology: APRIL drives switch to IgA 2

Neuroimmunology: No pain, all gain