Essential requirement for IRF8 and SLC15A4 implicates plasmacytoid dendritic cells in the pathogenesis of lupus.

In vitro evidence suggests that plasmacytoid dendritic cells (pDCs) are intimately involved in the pathogenesis of lupus. However, it remains to be determined whether these cells are required in vivo for disease development, and whether their contribution is restricted to hyperproduction of type I IFNs. To address these issues, we created lupus-predisposed mice lacking the IFN regulatory factor 8 (IRF8) or carrying a mutation that impairs the peptide/histidine transporter solute carrier family 15, member 4 (SLC15A4). IRF8-deficient NZB mice, lacking pDCs, showed almost complete absence of anti-nuclear, anti-chromatin, and anti-erythrocyte autoantibodies, along with reduced kidney disease. These effects were observed despite normal B-cell responses to Toll-like receptor (TLR) 7 and TLR9 stimuli and intact humoral responses to conventional T-dependent and -independent antigens. Moreover, Slc15a4 mutant C57BL/6-Fas(lpr) mice, in which pDCs are present but unable to produce type I IFNs in response to endosomal TLR ligands, also showed an absence of autoantibodies, reduced lymphadenopathy and splenomegaly, and extended survival. Taken together, our results demonstrate that pDCs and the production of type I IFNs by these cells are critical contributors to the pathogenesis of lupus-like autoimmunity in these models. Thus, IRF8 and SLC15A4 may provide important targets for therapeutic intervention in human lupus.

ENU-induced phenovariance in mice: inferences from 587 mutations.

BACKGROUND: We present a compendium of N-ethyl-N-nitrosourea (ENU)-induced mouse mutations, identified in our laboratory over a period of 10 years either on the basis of phenotype or whole genome and/or whole exome sequencing, and archived in the Mutagenetix database. Our purpose is threefold: 1) to formally describe many point mutations, including those that were not previously disclosed in peer-reviewed publications; 2) to assess the characteristics of these mutations; and 3) to estimate the likelihood that a missense mutation induced by ENU will create a detectable phenotype. FINDINGS: In the context of an ENU mutagenesis program for C57BL/6J mice, a total of 185 phenotypes were tracked to mutations in 129 genes. In addition, 402 incidental mutations were identified and predicted to affect 390 genes. As previously reported, ENU shows strand asymmetry in its induction of mutations, particularly favoring T to A rather than A to T in the sense strand of coding regions and splice junctions. Some amino acid substitutions are far more likely to be damaging than others, and some are far more likely to be observed. Indeed, from among a total of 494 non-synonymous coding mutations, ENU was observed to create only 114 of the 182 possible amino acid substitutions that single base changes can achieve. Based on differences in overt null allele frequencies observed in phenotypic vs. non-phenotypic mutation sets, we infer that ENU-induced missense mutations create detectable phenotype only about 1 in 4.7 times. While the remaining mutations may not be functionally neutral, they are, on average, beneath the limits of detection of the phenotypic assays we applied. CONCLUSIONS: Collectively, these mutations add to our understanding of the chemical specificity of ENU, the types of amino acid substitutions it creates, and its efficiency in causing phenovariance. Our data support the validity of computational algorithms for the prediction of damage caused by amino acid substitutions, and may lead to refined predictions as to whether specific amino acid changes are responsible for observed phenotypes. These data form the basis for closer in silico estimations of the number of genes mutated to a state of phenovariance by ENU within a population of G3 mice.

Slc15a4, a gene required for pDC sensing of TLR ligands, is required to control persistent viral infection.

Plasmacytoid dendritic cells (pDCs) are the major producers of type I IFN in response to viral infection and have been shown to direct both innate and adaptive immune responses in vitro. However, in vivo evidence for their role in viral infection is lacking. We evaluated the contribution of pDCs to acute and chronic virus infection using the feeble mouse model of pDC functional deficiency. We have previously demonstrated that feeble mice have a defect in TLR ligand sensing. Although pDCs were found to influence early cytokine secretion, they were not required for control of viremia in the acute phase of the infection. However, T cell priming was deficient in the absence of functional pDCs and the virus-specific immune response was hampered. Ultimately, infection persisted in feeble mice. We conclude that pDCs are likely required for efficient T cell priming and subsequent viral clearance. Our data suggest that reduced pDC functionality may lead to chronic infection.

Hypermorphic mutation of the voltage-gated sodium channel encoding gene Scn10a causes a dramatic stimulus-dependent neurobehavioral phenotype.

The voltage-gated sodium channel Na(v)1.8 is known to function in the transmission of pain signals induced by cold, heat, and mechanical stimuli. Sequence variants of human Na(v)1.8 have been linked to altered cardiac conduction. We identified an allele of Scn10a encoding the α-subunit of Na(v)1.8 among mice homozygous for N-ethyl-N-nitrosourea-induced mutations. The allele creates a dominant neurobehavioral phenotype termed Possum, characterized by transient whole-body tonic immobility induced by pinching the skin at the back of the neck ("scruffing"). The Possum mutation enhanced Na(v)1.8 sodium currents and neuronal excitability and heightened sensitivity of mutants to cold stimuli. Striking electroencephalographic changes were observed concomitant with the scruffing-induced behavioral change. In addition, electrocardiography demonstrated that Possum mice exhibited marked sinus bradycardia and R-R variability upon scruffing, abrogated by infusion of atropine. However, atropine failed to prevent or mitigate the tonic immobility response. Hyperactive sodium conduction via Na(v)1.8 thus leads to a complex neurobehavioral phenotype, which resembles catatonia in schizophrenic humans and tonic immobility in other mammals upon application of a discrete stimulus; no other form of mechanosensory stimulus could induce the immobility phenotype. Our data confirm the involvement of Na(v)1.8 in transducing pain initiated by cold and additionally implicate Na(v)1.8 in previously unknown functions in the central nervous system and heart.

Slc15a4, AP-3, and Hermansky-Pudlak syndrome proteins are required for Toll-like receptor signaling in plasmacytoid dendritic cells.

Despite their low frequency, plasmacytoid dendritic cells (pDCs) produce most of the type I IFN that is detectable in the blood following viral infection. The endosomal Toll-like receptors (TLRs) TLR7 and TLR9 are required for pDCs, as well as other cell types, to sense viral nucleic acids, but the mechanism by which signaling through these shared receptors results in the prodigious production of type I IFN by pDCs is not understood. We designed a genetic screen to identify proteins required for the development and specialized function of pDCs. One phenovariant, which we named feeble, showed abrogation of both TLR-induced type I IFN and proinflammatory cytokine production by pDCs, while leaving TLR responses intact in other cells. The feeble phenotype was mapped to a mutation in Slc15a4, which encodes the peptide/histidine transporter 1 (PHT1) and has not previously been implicated in pDC function. The identification of the feeble mutation led to our subsequent observations that AP-3, as well as the BLOC-1 and BLOC-2 Hermansky-Pudlak syndrome proteins are essential for pDC signaling through TLR7 and TLR9. These proteins are not necessary for TLR7 or TLR9 signaling in conventional DCs and thus comprise a membrane trafficking pathway uniquely required for endosomal TLR signaling in pDCs.

Flt3 permits survival during infection by rendering dendritic cells competent to activate NK cells.

A previously unappreciated signal necessary for dendritic cell (DC)-mediated activation of natural killer (NK) cells during viral infection was revealed by a recessive N-ethyl-N-nitrosourea-induced mutation called warmflash (wmfl). Wmfl homozygotes displayed increased susceptibility to mouse cytomegalovirus (MCMV) infection. In response to MCMV infection in vivo, delayed NK cell activation was observed, but no intrinsic defects in NK cell activation or function were identified. Rather, coculture experiments demonstrated that NK cells are suboptimally activated by wmfl DCs, which showed impaired cytokine production in response to MCMV or synthetic TLR7 and TLR9 ligands. The wmfl mutation was identified in the gene encoding the Fms-like tyrosine kinase 3 (Flt3). Flt3 ligand (Flt3L) is transiently induced in the serum upon infection or TLR activation. However, antibody blockade reveals no acute requirement for Flt3L, suggesting that the Flt3L --> Flt3 axis programs the development of DCs, making them competent to support NK effector function. In the absence of Flt3 signaling, NK cell activation is delayed and survival during MCMV infection is markedly compromised.

Intracellular toll-like receptors.

Foreign nucleic acids, the signature of invading viruses and certain bacteria, are sensed intracellularly. The nucleic acid-specific Toll-like receptors (TLRs) detect and signal within endolysosomal compartments, triggering the induction of cytokines essential for the innate immune response. These cytokines include proinflammatory molecules produced mainly by macrophages and conventional dendritic cells, as well as type I interferons, which are produced in great quantities by plasmacytoid dendritic cells. The cellular and molecular pathways by which nucleic acids and TLRs meet within the endosome assure host protection yet also place the host at risk for the development of autoimmunity. Here, we review the latest findings on the intracellular TLRs, with special emphasis on ligand uptake, receptor trafficking, signaling, and regulation.

The Tpl2 mutation Sluggish impairs type I IFN production and increases susceptibility to group B streptococcal disease.

Sluggish was identified in a population of third generation mice descended from N-ethyl-N-nitrosourea-mutagenized sires. Macrophages from homozygotes exhibited impaired TNF-alpha production in response to all TLR ligands tested and displayed impaired type I IFN production in response to TLR7 and TLR9 stimulations. The phenotype was confined to a critical region on mouse chromosome 18 and then ascribed to a T to A transversion in the acceptor splice site of intron 4 at position 13346 of the Map3k8 gene, resulting in defective splicing. The Map3k8(Sluggish) mutation does not result in susceptibility to viral infections, but Sluggish mice displayed high susceptibility to group B streptococcus infection, with impaired TNF-alpha and type I IFN production in infected macrophages. Our data demonstrate that the encoded protein kinase Tpl2 plays an essential role in cell signaling in the immune response to certain pathogens.

Mice with mutations of Dock7 have generalized hypopigmentation and white-spotting but show normal neurological function.

The classical recessive coat color mutation misty (m) arose spontaneously on the DBA/J background and causes generalized hypopigmentation and localized white-spotting in mice, with a lack of pigment on the belly, tail tip, and paws. Here we describe moonlight (mnlt), a second hypopigmentation and white-spotting mutation identified on the C57BL/6J background, which yields a phenotypic copy of m/m coat color traits. We demonstrate that the 2 mutations are allelic. m/m and mnlt/mnlt phenotypes both result from mutations that truncate the dedicator of cytokinesis 7 protein (DOCK7), a widely expressed Rho family guanine nucleotide exchange factor. Although Dock7 is transcribed at high levels in the developing brain and has been implicated in both axon development and myelination by in vitro studies, we find no requirement for DOCK7 in neurobehavioral function in vivo. However, DOCK7 has non-redundant role(s) related to the distribution and function of dermal and follicular melanocytes.

In vivo conversion of BM plasmacytoid DC into CD11b+ conventional DC during virus infection.

DC are a highly heterogeneous population that plays a critical role in host defense. We previously demonstrated that virus infection induces BM plasmacytoid DC (pDC) differentiation into CD11b(+) conventional DC (cDC) upon in vitro culture with Fms-like tyrosine kinase 3 ligand (Flt3L). Here we use immunoglobulin D-J rearrangements and pDC adoptive transfer to provide definitive proof supporting BM pDC conversion into CD11b(+) cDC during in vivo viral infection. We show that in vivo BM pDC conversion into CD11b(+) cDC relates to enhanced ability to prime virus-specific T cells. Furthermore, we demonstrate that in vivo pDC conversion does not rely on viral infection of BM pDC, but instead is mediated by type I IFN signaling. Finally, by exploiting recently identified pDC-specific Ab, we provide further characterizations of the BM pDC fraction that exhibits this broader developmental plasticity. Collectively, these data indicate that BM pDC actively contribute to the CD11b(+) cDC pool during in vivo viral infection and delineates molecular, functional, and phenotypic features of this novel developmental pathway.

Inflammation and autoimmunity caused by a SHP1 mutation depend on IL-1, MyD88, and a microbial trigger.

A recessive phenotype called spin (spontaneous inflammation) was induced by N-ethyl-N-nitrosourea (ENU) mutagenesis in C57BL/6J mice. Homozygotes display chronic inflammatory lesions affecting the feet, salivary glands and lungs, and antichromatin antibodies. They are immunocompetent and show enhanced resistance to infection by Listeria monocytogenes. TLR-induced TNF and IL-1 production are normal in macrophages derived from spin mice. The autoinflammatory phenotype of spin mice is fully suppressed by compound homozygosity for Myd88(poc), Irak4(otiose), and Il1r1-null mutations, but not Ticam1(Lps2), Stat1(m1Btlr), or Tnf-null mutations. Both autoimmune and autoinflammatory phenotypes are suppressed when spin homozygotes are derived into a germ-free environment. The spin phenotype was ascribed to a viable hypomorphic allele of Ptpn6, which encodes the tyrosine phosphatase SHP1, mutated in mice with the classical motheaten alleles me and me-v. Inflammation and autoimmunity caused by SHP1 deficiency are thus conditional. The SHP1-deficient phenotype is driven by microbes, which activate TLR signaling pathways to elicit IL-1 production. IL-1 signaling via MyD88 elicits inflammatory disease.

Development and function of murine B220+CD11c+NK1.1+ cells identify them as a subset of NK cells.

Lymphoid organs contain a B220(+)CD11c(+)NK1.1(+) cell population that was recently characterized as a novel dendritic cell (DC) subset that functionally overlaps with natural killer (NK) cells and plasmacytoid DCs (PDCs). Using Siglec-H and NK1.1 markers, we unambiguously dissected B220(+)CD11c(+) cells and found that PDCs are the only professional interferon (IFN)-alpha-producing cells within this heterogeneous population. In contrast, B220(+)CD11c(+)NK1.1(+) cells are a discrete NK cell subset capable of producing higher levels of IFN-gamma than conventional NK cells. Unlike DCs, only a minute fraction of B220(+)CD11c(+)NK1.1(+) cells in the spleen expressed major histocompatibility complex class II ex vivo or after stimulation with CpG. Consistent with being a NK cell subset, B220(+)CD11c(+)NK1.1(+) cells depended primarily on interleukin 15 and common cytokine receptor gamma chain signaling for their development. In terms of function, expression of distinctive cell surface receptors, and location in lymphoid organs, NK1.1(+)B220(+)CD11c(+) appear to be the murine equivalent of human CD56(bright) NK cells.

Bone marrow stromal cell antigen 2 is a specific marker of type I IFN-producing cells in the naive mouse, but a promiscuous cell surface antigen following IFN stimulation.

Type I IFN-producing cells (IPC) are sentinels of viral infections. Identification and functional characterization of these cells have been difficult because of their small numbers in blood and tissues and their complex cell surface phenotype. To overcome this problem in mice, mAbs recognizing IPC-specific cell surface molecules have been generated. In this study, we report the identification of new Abs specific for mouse IPC, which recognize the bone marrow stromal cell Ag 2 (BST2). Interestingly, previously reported IPC-specific Abs 120G8 and plasmacytoid dendritic cell Ag-1 also recognize BST2. BST2 is predominantly specific for mouse IPC in naive mice, but is up-regulated on most cell types following stimulation with type I IFNs and IFN-gamma. The activation-induced promiscuous expression of BST2 described in this study has important implications for the use of anti-BST2 Abs in identification and depletion of IPC. Finally, we show that BST2 resides within an intracellular compartment corresponding to the Golgi apparatus, and may be involved in trafficking secreted cytokines in IPC.

Sampling and signaling in plasmacytoid dendritic cells: the potential roles of Siglec-H.

Plasmacytoid dendritic cells (pDCs) detect viruses through toll-like receptor (TLR)7 and TLR9 and respond by secreting type I interferons (IFNs). Because TLR7 and TLR9 are present in endosomes, a mechanism is required to capture and deliver viruses to TLRs. A member of the sialic acid binding Ig-like lectin (Siglec) family, Siglec-H, has recently been identified as a specific surface marker for pDCs in mice. Siglec-H is endocytosed and can mediate the uptake of antigens for processing and presentation. Thus, Siglec-H might have a role in the capture of viruses or other pathogens for their delivery to intracellular TLRs. Paradoxically, Siglec-H also transmits intracellular signals through the associated adaptor DAP12, which reduces pDC responses to TLR ligands. In this review, we discuss models to explain the potential outcomes of Siglec-H engagement in the pDC secretion of type I IFN.

Siglec-H is an IPC-specific receptor that modulates type I IFN secretion through DAP12.

Natural interferon (IFN)-producing cells are the primary cell type responsible for production of type I IFN in response to viruses. Herein we report the identification of the first molecular marker of mouse natural interferon-producing cells (IPCs), a novel member of the sialic acid-binding immunoglobulin (Ig)-like lectin (Siglec) family termed Siglec-H. Siglec-H is expressed exclusively on IPCs and is unique among Siglec proteins in that it associates with the adaptor protein DAP12. Moreover, we show that DAP12 modulates the type I IFN response of IPCs to a Toll-like receptor 9 (TLR9) agonist. This observation explains our previous finding that stimulation of IPCs with 440c, a Siglec-H-specific antibody, reduces IPC secretion of type I IFN. Moreover, it supports a model in which engagement of DNAX-activation protein 12 (DAP12)-associated receptors with antibodies or low avidity endogenous ligands interferes with TLR-mediated cellular activation.

Adhesive mechanisms governing interferon-producing cell recruitment into lymph nodes.

Natural interferon-producing cells (IPCs) are found in peripheral lymph nodes (PLNs), where they support NK cell, T cell, and B cell responses to pathogens. However, their route of entry and the adhesive mechanisms used to gain access to PLNs remain poorly defined. We report that IPCs can enter PLNs via a hematogenous route, which involves a multistep adhesive process, and that transmigration is enhanced by inflammation. Results indicate that L-selectin on IPCs is required for efficient attachment and rolling on high endothelial venules in vivo in both nonstimulated and inflamed PLNs. IPCs, however, also possess functional ligands for E-selectin that contribute to this process only in the latter case. In conjunction with selectin-mediated adhesion, both beta(1)- and beta(2)-integrins participate in IPC attachment to the inflamed vessel wall, whereas chemotaxis relies in part on the chemokine receptor CCR5. Identification of the adhesive machinery required for IPC trafficking into PLNs may provide opportunities to regulate immune responses reliant on the activity of these cells.