Hot off the presses! Aug 01 Nat Immunol

The Aug 01 issue of the Nat Immunol is now up on Pubget (About Nat Immunol): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• Hyping research
  - Nat Immunol 10(8):795 (2009)
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• Jean Dausset 1916–2009
  - Nat Immunol 10(8):797 (2009)
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• RNAi screening: tips and techniques
  - Nat Immunol 10(8):799-804 (2009)
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• Themis imposes new law and order on positive selection
  - Nat Immunol 10(8):805-806 (2009)
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• TH2 bias: Mina tips the balance
  - Nat Immunol 10(8):806-808 (2009)
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• New insight into the everlasting host-pathogen arms race
  - Nat Immunol 10(8):808-809 (2009)
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• Dead man walking: how thymocytes scan the medulla
  - Nat Immunol 10(8):809-811 (2009)
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• One problem, two solutions
  - Nat Immunol 10(8):811-813 (2009)
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• Research Highlights
  - Nat Immunol 10(8):815 (2009)
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• RAG: a recombinase diversified
  - Nat Immunol 10(8):817-821 (2009)
  During B cell and T cell development, the lymphoid-specific proteins RAG-1 and RAG-2 act together to initiate the assembly of antigen receptor genes through a series of site-specific somatic DNA rearrangements that are collectively called variable-diversity-joining (V(D)J) recombination. In the past 20 years, a great deal has been learned about the enzymatic activities of the RAG-1–RAG-2 complex. Recent studies have identified several new and exciting regulatory functions of the RAG-1–RAG-2 complex. Here we discuss some of these functions and suggest that the RAG-1–RAG-2 complex nucleates a specialized subnuclear compartment that we call the 'V(D)J recombination factory'.
• The impact of negative selection on thymocyte migration in the medulla
  - Nat Immunol 10(8):823-830 (2009)
  Developing thymocytes are screened for self-reactivity before they exit the thymus, but how thymocytes scan the medulla for self antigens is unclear. Using two-photon microscopy, we observed that medullary thymocytes migrated rapidly and made frequent, transient contacts with dendritic cells. In the presence of a negative selecting ligand, thymocytes slowed, became confined to areas of approximately 30 m in diameter and had increased contact with dendritic cells surrounding confinement zones. One third of polyclonal medullary thymocytes also showed confined, slower migration and may correspond to autoreactive thymocytes. Our data suggest that many autoreactive thymocytes do not undergo immediate arrest and death after encountering a negative selecting ligand but instead adopt an altered migration program while remaining in the medullary microenvironment.
• Themis is a member of a new metazoan gene family and is required for the completion of thymocyte positive selection
  - Nat Immunol 10(8):831-839 (2009)
  T cell antigen receptor (TCR) signaling in CD4+CD8+ double-positive thymocytes determines cell survival and lineage commitment, but the genetic and molecular basis of this process is poorly defined. To address this issue, we used ethylnitrosourea mutagenesis to identify a previously unknown T lineage–specific gene, Themis, which is critical for the completion of positive selection. Themis contains a tandem repeat of a unique globular domain (called 'CABIT' here) that includes a cysteine motif that defines a family of five uncharacterized vertebrate proteins with orthologs in most animal species. Themis-deficient thymocytes showed no substantial impairment in early TCR signaling but did show altered expression of genes involved in the cell cycle and survival before and during positive selection. Our data suggest a unique function for Themis in sustaining positive selection.
• Themis, a T cell–specific protein important for late thymocyte development
  - Nat Immunol 10(8):840-847 (2009)
  During positive selection, thymocytes transition through a stage during which T cell antigen receptor (TCR) signaling controls CD4-versus-CD8 lineage 'choice' and subsequent maturation. Here we describe a previously unknown T cell–specific protein, Themis, that serves a distinct function during this stage. In Themis-/- mice, thymocyte selection was impaired and the number of transitional CD4+CD8int thymocytes as well as CD4+ or CD8+ single-positive thymocytes was lower. Notably, although we detected no overt TCR-proximal signaling deficiencies, Themis-/- CD4+CD8int thymocytes showed developmental defects consistent with attenuated signaling that were reversible by TCR stimulation. Our results identify Themis as a critical component of the T cell developmental program and suggest that Themis functions to sustain and/or integrate signals required for proper lineage commitment and maturation.
• Themis controls thymocyte selection through regulation of T cell antigen receptor–mediated signaling
  - Nat Immunol 10(8):848-856 (2009)
  Themis (thymocyte-expressed molecule involved in selection), a member of a family of proteins with unknown functions, is highly conserved among vertebrates. Here we found that Themis had high expression in thymocytes between the pre–T cell antigen receptor (pre-TCR) and positive-selection checkpoints and low expression in mature T cells. Themis-deficient thymocytes showed defective positive selection, which resulted in fewer mature thymocytes. Negative selection was also impaired in Themis-deficient mice. A greater percentage of Themis-deficient T cells had CD4+CD25+Foxp3+ regulatory and CD62LloCD44hi memory phenotypes than did wild-type T cells. In support of the idea that Themis is involved in TCR signaling, this protein was phosphorylated quickly after TCR stimulation and was needed for optimal TCR-driven calcium mobilization and activation of the kinase Erk.
• Production of interleukin 22 but not interleukin 17 by a subset of human skin-homing memory T cells
  - Nat Immunol 10(8):857-863 (2009)
  Interleukin 22 (IL-22) is a cytokine produced by the TH-17 lineage of helper T cells and NK-22 subset of natural killer cells that acts on epithelial cells and keratinocytes and has been linked to skin homeostasis and inflammation. Here we characterize a population of human skin-homing memory CD4+ T cells that expressed the chemokine receptors CCR10, CCR6 and CCR4 and produced IL-22 but neither IL-17 nor interferon- (IFN-). Clones isolated from this population produced IL-22 only and had low or undetectable expression of the TH-17 and T helper type 1 (TH1) transcription factors RORt and T-bet. The differentiation of T cells producing only IL-22 was efficiently induced in naive T cells by plasmacytoid dendritic cells in an IL-6- and tumor necrosis factor–dependent way. Our findings delineate a previously unknown subset of human CD4+ effector T cells dedicated to skin pathophysiology.
• Identification of a human helper T cell population that has abundant production of interleukin 22 and is distinct from TH-17, TH1 and TH2 cells
  - Nat Immunol 10(8):864-871 (2009)
  Interleukin 22 (IL-22) is a member of the IL-10 cytokine family that is involved in inflammatory and wound healing processes. Originally considered a T helper type 1 (TH1)-associated cytokine, IL-22 has since been shown to be produced mainly by IL-17-producing helper T cells (TH-17 cells). Here we describe a previously uncharacterized IL-22-producing human helper T cell population that coexpressed the chemokine receptor CCR6 and the skin-homing receptors CCR4 and CCR10. These cells were distinct from both TH-17 cells and TH1 cells. Downregulation of either the aryl hydrocarbon receptor (AHR) or the transcription factor RORC by RNA-mediated interference affected IL-22 production, whereas IL-17 production was affected only by downregulation of RORC by RNA-mediated interference. AHR agonists substantially altered the balance of IL-22- versus IL-17-producing cells. This subset of IL-22-producing cells may be important in skin homeostasis and pathology.
• Mina, an Il4 repressor, controls T helper type 2 bias
  - Nat Immunol 10(8):872-879 (2009)
  T helper type 2 (TH2) bias, which is the propensity of naive CD4+ T cells to differentiate into interleukin 4 (IL-4)-secreting TH2 cells, is a genetic trait that affects susceptibility to infectious, autoimmune and allergic diseases. TH2 bias correlates with the amount of IL-4 initially secreted by newly activated helper T cells that feeds back positively through the pathway of the IL-4 receptor and the transcription factors STAT6 and GATA-3 to drive TH2 development. Here we identify Mina, a member of the jumonji C (JmjC) protein family, as a genetic determinant of TH2 bias. Mina specifically bound to and repressed the Il4 promoter. Mina overexpression in transgenic mice impaired Il4 expression, whereas its knockdown in primary CD4+ T cells led to Il4 derepression. Our findings collectively provide mechanistic insight into an Il4-regulatory pathway that controls helper T cell differentiation and genetic variation in TH2 bias.
• Essential function for the GTPase TC21 in homeostatic antigen receptor signaling
  - Nat Immunol 10(8):880-888 (2009)
  T cell antigen receptors (TCRs) and B cell antigen receptors (BCRs) transmit low-grade signals necessary for the survival and maintenance of mature cell pools. We show here that TC21, a small GTPase encoded by Rras2, interacted constitutively with both kinds of receptors. Expression of a dominant negative TC21 mutant in T cells produced a rapid decrease in cell viability, and Rras2-/- mice were lymphopenic, possibly as a result of diminished homeostatic proliferation and impaired T cell and B cell survival. In contrast, TC21 was overexpressed in several human lymphoid malignancies. Finally, the p110 catalytic subunit of phosphatidylinositol-3-OH kinase (PI(3)K) was recruited to the TCR and BCR in a TC21-dependent way. Consequently, we propose TC21 directly links antigen receptors to PI(3)K-mediated survival pathways.
• Immunoglobulin D enhances immune surveillance by activating antimicrobial, proinflammatory and B cell–stimulating programs in basophils
  - Nat Immunol 10(8):889-898 (2009)
  Immunoglobulin D (IgD) is an enigmatic antibody isotype that mature B cells express together with IgM through alternative RNA splicing. Here we report active T cell–dependent and T cell–independent IgM-to-IgD class switching in B cells of the human upper respiratory mucosa. This process required activation-induced cytidine deaminase (AID) and generated local and circulating IgD-producing plasmablasts reactive to respiratory bacteria. Circulating IgD bound to basophils through a calcium-mobilizing receptor that induced antimicrobial, opsonizing, inflammatory and B cell–stimulating factors, including cathelicidin, interleukin 1 (IL-1), IL-4 and B cell–activating factor (BAFF), after IgD crosslinking. By showing dysregulation of IgD class–switched B cells and 'IgD-armed' basophils in autoinflammatory syndromes with periodic fever, our data indicate that IgD orchestrates an ancestral surveillance system at the interface between immunity and inflammation.
• Mycobacterium tuberculosis evades macrophage defenses by inhibiting plasma membrane repair
  - Nat Immunol 10(8):899-906 (2009)
  Induction of macrophage necrosis is a strategy used by virulent Mycobacterium tuberculosis (Mtb) to avoid innate host defense. In contrast, attenuated Mtb causes apoptosis, which limits bacterial replication and promotes T cell cross-priming by antigen-presenting cells. Here we show that Mtb infection causes plasma membrane microdisruptions. Resealing of these lesions, a process crucial for preventing necrosis and promoting apoptosis, required translocation of lysosomal and Golgi apparatus–derived vesicles to the plasma membrane. Plasma membrane repair depended on prostaglandin E2 (PGE2), which regulates synaptotagmin 7 (Syt-7), the calcium sensor involved in the lysosome-mediated repair mechanism. By inducing production of lipoxin A4 (LXA4), which blocks PGE2 biosynthesis, virulent Mtb prevented membrane repair and induced necrosis. Thus, virulent Mtb impairs macrophage plasma membrane repair to evade host defenses.
• Targeting of the GTPase Irgm1 to the phagosomal membrane via PtdIns(3,4)P2 and PtdIns(3,4,5)P3 promotes immunity to mycobacteria
  - Nat Immunol 10(8):907-917 (2009)
  Vertebrate immunity to infection enlists a newly identified family of 47-kilodalton immunity-related GTPases (IRGs). One IRG in particular, Irgm1, is essential for macrophage host defense against phagosomal pathogens, including Mycobacterium tuberculosis (Mtb). Here we show that Irgm1 targets the mycobacterial phagosome through lipid-mediated interactions with phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P2) and PtdIns(3,4,5)P3. An isolated Irgm1 amphipathic helix conferred lipid binding in vitro and in vivo. Substitutions in this region blocked phagosome recruitment and failed to complement the antimicrobial defect in Irgm1-/- macrophages. Removal of PtdIns(3,4,5)P3 or inhibition of class I phosphatidylinositol-3-OH kinase (PI(3)K) mimicked this effect in wild-type cells. Cooperation between Irgm1 and PI(3)K further facilitated the engagement of Irgm1 with its fusogenic effectors at the site of infection, thereby ensuring pathogen-directed responses during innate! immunity.
• A TNF- and c-Cbl-dependent FLIPS-degradation pathway and its function in Mycobacterium tuberculosis–induced macrophage apoptosis
  - Nat Immunol 10(8):918-926 (2009)
  Apoptosis is central to the interaction between pathogenic mycobacteria and host macrophages. Caspase-8-dependent apoptosis of infected macrophages, which requires activation of the mitogen-activated protein (MAP) kinase p38, lowers the spread of mycobacteria. Here we establish a link between the release of tumor necrosis factor (TNF) and mycobacteria-mediated macrophage apoptosis. TNF activated a pathway involving the kinases ASK1, p38 and c-Abl. This pathway led to phosphorylation of FLIPS, which facilitated its interaction with the E3 ubiquitin ligase c-Cbl. This interaction triggered proteasomal degradation of FLIPS, which promoted activation of caspase-8 and apoptosis. Our findings identify a previously unappreciated signaling pathway needed for Mycobacterium tuberculosis–triggered macrophage cell death.