Plasmacytoid dendritic cells: sensing nucleic acids in viral infection and autoimmune diseases

Accumulation of plasmacytoid DC: Roles in disease pathogenesis and targets for immunotherapy

Plasmacytoid DC (pDC) secrete type I IFN in response to viruses and RNA/DNA/immunocomplexes. Type I IFN confer resistance to viral infections and promote innate and adaptive immune responses. pDC also produce cytokines and chemokines that influence recruitment and function of T cells and differentiation of B cells. Thus, pDC have been implicated both in protective immune responses and in induction of tolerance. In this Viewpoint, we discuss how the recruitment and accumulation of pDC may impact pathogenesis of several diseases and how pDC can be targeted for therapeutic interventions.

Some active areas of DC research and their medical potential

This Viewpoint series provides authoritative and detailed outlines of exciting areas of DC research. Some of the subjects that frequently come up include development of DC; distribution of DC in lymphoid and non-lymphoid tissues such as skin, intestine and lung; different forms or subsets of DC; and the role of DC in initiating tolerance and immunity. In this Preface, I will introduce the Viewpoints and consider some future challenges as well as the medical relevance of DC research.

Plasmacytoid dendritic cell dichotomy: identification of IFN-α producing cells as a phenotypically and functionally distinct subset

Plasmacytoid dendritic cells (pDC) produce large amounts of type I IFN in response to invading pathogens, but can also suppress immune responses and promote tolerance. In this study, we show that in mice, these functions are attributable to two distinct pDC subsets, one of which gives rise to the other. CD9(pos)Siglec-H(low) pDC secrete IFN-α when stimulated with TLR agonists, induce CTLs, and promote protective antitumor immunity. By contrast, CD9(neg)Siglec-H(high) pDC secrete negligible amounts of IFN-α, induce Foxp3(+) CD4(+) T cells, and fail to promote antitumor immunity. Although newly formed pDC in the bone marrow are CD9(pos) and are capable of producing IFN-α, after these cells traffic to peripheral tissues, they lose CD9 expression and the ability to produce IFN-α. We propose that newly generated pDC mobilized from the bone marrow, rather than tissue-resident pDC, are the major source of IFN-α in infected hosts.

Tolerogenic plasmacytoid DC

Plasmacytoid DC (pDC) are type-I IFN-producing cells known for their capacity to promote anti-viral innate and adaptive immune responses. Despite their potent anti-viral function, when compared with conventional DC, pDC exhibit poor immunostimulatory ability and their interaction with T cells often favors the generation of Treg. pDC are activated primarily in response to ssRNA and ssDNA through TLR7 and TLR9, respectively, but also through TLR-independent mechanisms. Non-lymphoid tissue pDC, such as those residing in the airways, gut, and liver, play a significant role in regulating mucosal immunity and are critical for the development of tolerance to inhaled or ingested antigens. Herein we discuss properties that define tolerogenic pDC and how their unique characteristics translate into an ability to regulate immunity and promote the development of tolerance. We cover the importance of pDC during intrathymic Treg development and the maintenance of peripheral tolerance, as well as their regulatory role in transplantation, autoimmunity, and cancer. We highlight recent findings regarding danger-associated molecular pattern and PAMP signaling in the regulation of pDC function, and how the ability of pDC to promote tolerance translates into the potential clinical applications of these cells as therapeutic targets to regulate immune reactivity.

Immune complex-mediated cell activation from systemic lupus erythematosus and rheumatoid arthritis patients elaborate different requirements for IRAK1/4 kinase activity across human cell types

IL-1R-associated kinases (IRAKs) are important mediators of MyD88-dependent signaling by the TLR/IL-1R superfamily and facilitate inflammatory responses. IRAK4 and IRAK1 function as active kinases and as scaffolds for protein-protein interactions. We report that although IRAK1/4 kinase activity is essential for human plasmacytoid dendritic cell (pDC) activation, it is dispensable in B, T, dendritic, and monocytic cells, which is in contrast with an essential active kinase role in comparable mouse cell types. An IRAK1/4 kinase inhibitor abrogated TLR7/9-induced IFN-α responses in both mouse and human pDCs, but other human immune cell populations activated via TLR7/9 or IL-1R were refractory to IRAK4 kinase inhibition. Gene ablation experiments using small interfering RNA demonstrated an essential scaffolding role for IRAK1 and IRAK4 in MyD88-dependent signaling. Finally, we demonstrate that autoimmune patient (systemic lupus erythematosus and rheumatoid arthritis) serum activates both pDC and B cells, but IRAK1/4 kinase inhibition affects only the pDC response, underscoring the differential IRAK1/4 functional requirements in human immune cells. These data reveal important species differences and elaborate cell type requirements for IRAK1/4 kinase activity.

The STING pathway and regulation of innate immune signaling in response to DNA pathogens

The innate immune system has evolved a variety of sensing mechanisms to detect and counter microbial invasion. These include the Toll-like receptor (TLR), cytoplasmic, nucleotide binding oligomerization domain (NOD)-like receptor and RIG-I-like helicase (RLH) pathways. However, how the cell detects pathogen-associated DNA to trigger host defense, including the production of interferon, remains to be fully clarified. Understanding these processes could have profound implications into how we understand and treat a variety of microbial-related disease, including viral-associated cancers, as well as autoimmune disorders. Recently, an endoplasmic reticulum-associated molecule referred to as STING (for stimulator of interferon genes) was isolated and shown to be critical for regulating the production of IFN in response to cytoplasmic DNA. Here, we review recent discoveries relating to the detection of foreign DNA, including the importance of the STING and inflammasome pathways and the triggering of innate signaling processes.

Current views of toll-like receptor signaling pathways

On microbial invasion, the host immediately evokes innate immune responses. Recent studies have demonstrated that Toll-like receptors (TLRs) play crucial roles in innate responses that lead not only to the clearance of pathogens but also to the efficient establishment of acquired immunity by directly detecting molecules from microbes. In terms of intracellular TLR-mediated signaling pathways, cytoplasmic adaptor molecules containing Toll/IL-1R (TIR) domains play important roles in inflammatory immune responses through the production of proinflammatory cytokines, nitric oxide, and type I interferon, and upregulation of costimulatory molecules. In this paper, we will describe our current understanding of the relationship between TLRs and their ligands derived from pathogens such as viruses, bacteria, fungi, and parasites. Moreover, we will review the historical and current literature to describe the mechanisms behind TLR-mediated activation of innate immune responses.

Continuous expression of the transcription factor e2-2 maintains the cell fate of mature plasmacytoid dendritic cells

The interferon-producing plasmacytoid dendritic cells (pDCs) share common progenitors with antigen-presenting classical dendritic cells (cDCs), yet they possess distinct morphology and molecular features resembling those of lymphocytes. It is unclear whether the unique cell fate of pDCs is actively maintained in the steady state. We report that the deletion of transcription factor E2-2 from mature peripheral pDCs caused their spontaneous differentiation into cells with cDC properties. This included the loss of pDC markers, increase in MHC class II expression and T cell priming capacity, acquisition of dendritic morphology, and induction of cDC signature genes. Genome-wide chromatin immunoprecipitation revealed direct binding of E2-2 to key pDC-specific and lymphoid genes, as well as to certain genes enriched in cDCs. Thus, E2-2 actively maintains the cell fate of mature pDCs and opposes the "default" cDC fate, in part through direct regulation of lineage-specific gene expression programs.

Plasmacytoid dendritic cell ablation impacts early interferon responses and antiviral NK and CD8(+) T cell accrual

Plasmacytoid dendritic cells (pDCs) mediate type I interferon (IFN-I) responses to viruses that are recognized through the Toll-like receptor 7 (TLR7) or TLR9 signaling pathway. However, it is unclear how pDCs regulate the antiviral responses via innate and adaptive immune cells. We generated diphtheria toxin receptor transgenic mice to selectively deplete pDCs by administration of diphtheria toxin. pDC-depleted mice were challenged with viruses known to activate pDCs. In murine cytomegalovirus (MCMV) infection, pDC depletion reduced early IFN-I production and augmented viral burden facilitating the expansion of natural killer (NK) cells expressing the MCMV-specific receptor Ly49H. During vesicular stomatitis virus (VSV) infection, pDC depletion enhanced early viral replication and impaired the survival and accumulation of virus-specific cytotoxic T lymphocytes. We conclude that pDCs mediate early antiviral IFN-I responses and influence the accrual of virus-specific NK or CD8(+) T cells in a virus-dependent manner.

Emerging small molecule and biological therapeutic approaches for the treatment of autoimmunity

IMPORTANCE OF THE FIELD

Biological therapeutics targeting TNF-α, IL-6, CD20 and CD80/86 is proving to be an important weapon in the clinicians' armory to fight autoimmunity alongside long-standing small molecule therapeutics such as methotrexate and glucocorticoids. However, there still remains a high unmet clinical need in the field of autoimmunity and many researchers are continuing to discover and develop new therapeutics to address this.

AREAS COVERED IN THIS REVIEW

A new wave of small molecule and biological therapeutics targeting different pathways is being developed which could generate exciting new options for clinicians. This review aims to highlight those emerging therapies that are most advanced in clinical development.

WHAT THE READER WILL GAIN

The reader will gain an appreciation of new approaches being developed to address the high unmet clinical need in the field of autoimmunity.

TAKE HOME MESSAGE

Despite recent success in the development of therapeutics to treat autoimmunity, new therapeutic strategies are being developed to address the remaining areas of a high unmet clinical need.

The immune inhibitory receptor LAIR-1 is highly expressed by plasmacytoid dendritic cells and acts complementary with NKp44 to control IFNα production

Plasmacytoid dendritic cells (pDCs) are a subset of dendritic cells endowed with the capacity of producing large amounts of IFNα. Here we show that the Leukocyte-Associated Ig-like Receptor-1 (LAIR-1) is abundantly expressed on pDCs (the highest expression among all leukocytes) and its cross-linking inhibits IFNα production in response to Toll-like receptor ligands. Remarkably, LAIR-1 expression in pDCs is down-regulated in the presence of interleukin (IL)-3, thus indicating coordinated functions with NKp44, another pDC inhibitory receptor, which is conversely induced by IL-3. Nevertheless, the expression of NKp44 in pDCs isolated from secondary lymphoid organs, which is thought to be influenced by IL-3, is not coupled to a decreased expression of LAIR-1. Interestingly, pDCs isolated from peripheral blood of systemic lupus erithematosus (SLE) patients express lower levels of LAIR-1 while displaying slight but consistent expression of NKp44, usually undetectable on pDCs derived from healthy donors. Using sera derived from SLE patients, we show that LAIR-1 and NKp44 display synergistic inhibitory effects on IFNα production by interleukin IL-3 cultured pDCs stimulated with DNA immunocomplexes. In conclusion, our results indicate that the inhibitory function of LAIR-1 may play a relevant role in the mechanisms controlling IFNα production by pDCs both in normal and pathological innate immune responses.

Plasmacytoid dendritic cells sense skin injury and promote wound healing through type I interferons

Cutaneous injury in mice drives transient TLR7- and TLR9-mediated production of type I interferon by plasmacytoid dendritic cells, which is required for re-epithelialization of the skin.

Plasmacytoid dendritic cells (pDCs) are specialized type I interferon (IFN-α/β)–producing cells that express intracellular toll-like receptor (TLR) 7 and TLR9 and recognize viral nucleic acids in the context of infections. We show that pDCs also have the ability to sense host-derived nucleic acids released in common skin wounds. pDCs were found to rapidly infiltrate both murine and human skin wounds and to transiently produce type I IFNs via TLR7- and TLR9-dependent recognition of nucleic acids. This process was critical for the induction of early inflammatory responses and reepithelization of injured skin. Cathelicidin peptides, which facilitate immune recognition of released nucleic acids by promoting their access to intracellular TLR compartments, were rapidly induced in skin wounds and were sufficient but not necessary to stimulate pDC activation and type I IFN production. These data uncover a new role of pDCs in sensing tissue damage and promoting wound repair at skin surfaces.

The interferon-alpha signature of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic multisystem autoimmune disorder where interplay of environmental and genetic risk factors leads to progressive loss of tolerance to nuclear antigens over time, finally culminating in clinical disease. The heterogeneity of clinical manifestations and the disease's unpredictable course characterized by flares and remissions are very likely a reflection of heterogeneity at the origin of disease, with a final common pathway leading to loss of tolerance to nuclear antigens. Impaired clearance of immune complexes and apoptotic material and production of autoantibodies have long been recognized as major pathogenic events in this disease. Over the past decade the type I interferon cytokine family has been postulated to play a central role in SLE pathogenesis, by promoting feedback loops progressively disrupting peripheral immune tolerance and driving disease activity. The identification of key molecules involved in the pathogenesis of SLE will not only improve our understanding of this complex disease, but also help to identify novel targets for biological intervention.

Bortezomib suppresses function and survival of plasmacytoid dendritic cells by targeting intracellular trafficking of Toll-like receptors and endoplasmic reticulum homeostasis

Dendritic cells (DCs) play a pivotal role in the pathogenesis of inflammatory disorders, so suppressing the activity of DCs is instrumental in treating such diseases. In the present study, we show that a proteasome inhibitor, bortezomib, suppresses the survival and immunostimulatory function of human plasmacytoid DCs (pDCs) by targeting 2 critical points, intracellular trafficking of nucleic acid-sensingToll-like receptors (TLRs) and endoplasmic reticulum (ER) homeostasis. Among the immune cells in blood, pDCs were the most susceptible to the killing effect of bortezomib. This correlates with a decrease in the spliced form of a transcription factor XBP1, which rescues cells from apoptosis by maintaining ER homeostasis. Bortezomib suppressed the production of interferon-α and interleukin-6 by pDCs activated with a TLR9-stimulating CpG DNA and a TLR7-stimulating influenza virus, which appears to be partially independent of apoptosis. Bortezomib inhibited translocation of TLR9 from the ER to endolysosomes but not of an ER membrane protein, Unc93B1, that delivers TLR9 to endolysosomes. Thus, bortezomib suppresses the activity of pDCs by inhibiting intracellular trafficking of TLRs through disrupting the coordinated translocation of TLRs and Unc93B1 and by disturbing ER homeostasis. This study suggests that proteasome inhibitors may alleviate inflammatory disorders such as lupus and psoriasis that involve pDCs.

Role of PIR-B in autoimmune glomerulonephritis

PIR-B, an inhibitory receptor expressed on murine B cells and myeloid cells, regulates humoral and cellular immune responses via its constitutive binding to the ligand, MHC class I molecules, on the same cells (cis) or on different cells (trans). Although it has been speculated that PIR-B is important for maintaining peripheral tolerance, PIR-B single deficiency does not cause overt autoimmune diseases. Recently, however, the combination of its deficiency with the Fas lpr mutation was found to result in augmented production of autoantibodies such as IgG rheumatoid factor and anti-DNA IgG, leading to glomerulonephritis in mice. Although the precise molecular mechanism for the overall scenario is unclear, PIR-B was found to suppress TLR9-mediated production of naturally autoreactive antibodies by innate B cells or B-1 cells by inhibiting the activation of Bruton's tyrosine kinase. Thus, PIR-B is an important regulator of innate immunity mediated by TLR9 in B-1 cells, which can otherwise provoke autoimmunity when overactivated.

Toll-like receptor expression and function in human dendritic cell subsets: implications for dendritic cell-based anti-cancer immunotherapy

Dendritic cells (DCs) are central players of the immune response. To date, DC-based immunotherapy is explored worldwide in clinical vaccination trials with cancer patients, predominantly with ex vivo-cultured monocyte-derived DCs (moDCs). However, the extensive culture period and compounds required to differentiate them into DCs may negatively affect their immunological potential. Therefore, it is attractive to consider alternative DC sources, such as blood DCs. Two major types of naturally occurring DCs circulate in peripheral blood, myeloid DCs (mDCs) and plasmacytoid (pDCs). These DC subsets express different surface molecules and are suggested to have distinct functions. Besides scavenging pathogens and presenting antigens, DCs secrete cytokines, all of which is vital for both the acquired and the innate immune system. These immunological functions relate to Toll-like receptors (TLRs) expressed by DCs. TLRs recognize pathogen-derived products and subsequently provoke DC maturation, antigen presentation and cytokine secretion. However, not every TLR is expressed on each DC subset nor causes the same effects when activated. Considering the large amount of clinical trials using DC-based immunotherapy for cancer patients and the decisive role of TLRs in DC maturation, this review summarizes TLR expression in different DC subsets in relation to their function. Emphasis will be given to the therapeutic potential of TLR-matured DC subsets for DC-based immunotherapy.

Interferon-α and interleukin-12 are induced, respectively, by double-stranded DNA and single-stranded RNA in human myeloid dendritic cells

Dendritic cells (DCs) are initiators of innate immunity and acquired immunity as cells linking these two bio-defence systems through the production of cytokines such as interferon-α (IFN-α) and interleukin-12 (IL-12). Nucleic acids such as DNA from damaged cells or pathogens are important activators not only for anti-microbial innate immune responses but also in the pathogenesis of IFN-related autoimmune diseases. Plasmacytoid DCs are regarded as the main effectors for the DNA-mediated innate immunity by possessing DNA-sensing toll-like receptor 9 (TLR9). We here found that double-stranded DNA (dsDNA) complexed with lipotransfectants triggered activation of human monocyte-derived DCs (moDCs), leading to the preferential production of IFN-α but not IL-12. This indicates that myeloid DCs also function as supportive effectors against the invasion of pathogenic microbes through the DNA-mediated activation in innate immunity. The dsDNA with lipotransfectants can be taken up by moDCs without co-localization of endosomal LAMP1 staining, and the dsDNA-mediated IFN-α production was not impaired by chloroquine. These findings indicate that moDC activation by dsDNA does not involve the endosomal TLR pathway. In contrast, single-stranded RNA (ssRNA) stimulated moDCs to secrete IL-12 but not IFN-α. This process was inhibited by chloroquine, suggesting an involvement of the TLR pathway in ssRNA-mediated moDC activation. As might be inferred from our findings, myeloid DCs may function as a traffic control between innate immunity via IFN-α production and acquired immunity via IL-12 production, depending on the type of nucleic acids. Our results provide a new insight into the biological action of myeloid DCs underlying the DNA-mediated activation of protective or pathogenic immunity.

miR-155 and its star-form partner miR-155* cooperatively regulate type I interferon production by human plasmacytoid dendritic cells

The recent discovery of microRNAs (miRNAs) has revealed a new layer of gene expression regulation, affecting the immune system. Here, we identify their roles in regulating human plasmacytoid dendritic cell (PDC) activation. miRNA profiling showed the significantly differential expression of 19 miRNAs in PDCs after Toll-like receptor 7 (TLR7) stimulation, among which miR-155* and miR-155 were the most highly induced. Although they were processed from a single precursor and were both induced by TLR7 through the c-Jun N-terminal kinase pathway, miR-155* and miR-155 had opposite effects on the regulation of type I interferon production by PDC. Further study indicated that miR-155* augmented interferon-α/β expression by suppressing IRAKM, whereas miR-155 inhibited their expression by targeting TAB2. Kinetic analysis of miR-155* and miR-155 induction revealed that miR-155* was mainly induced in the early stage of stimulation, and that miR-155 was mainly induced in the later stage, suggesting their cooperative involvement in PDC activation. Finally, we demonstrated that miR-155* and miR-155 were inversely regulated by autocrine/paracrine type I interferon and TLR7-activated KHSRP at the posttranscriptional level, which led to their different dynamic induction by TLR7. Thus, our study identified and validated novel miRNA-protein networks involved in regulating PDC activation.

Molecular mechanisms for plasmacytoid dendritic cell function and development

Dendritic cells (DCs) are activated by immune adjuvants in a DC subset-specific manner. Plasmacytoid DC (pDC) is a unique subset that senses nucleic acid immune adjuvants only through two Toll-like receptors (TLRs), TLR7 and TLR9 and produces large amounts of type I interferons. Here I review recent trend in pDC function and development.

Plasmacytoid dendritic cells in multiple sclerosis: chemokine and chemokine receptor modulation by interferon-beta

Plasmacytoid dendritic cells (pDCs) are present in peripheral blood, leptomeninges and demyelinating lesions in patients with multiple sclerosis (MS). The ability of pDCs to produce chemokines and express the chemokine receptor CCR7 in MS is not known. We studied pDCs in MS patients and healthy subjects. The ability of pDCs to up-regulate CCR7 was significantly increased in untreated MS patients as compared to healthy subjects. IFN-beta treatment significantly inhibited TLR9 agonist-specific secretion of chemokines, which are ligands for CCR5-positive Th1 cells (CCL3, CCL4, and CCL5), and impaired TLR9 agonist-induced up-regulation of CCR7 and IFN-alpha in MS patients. This finding represents a new immunomodulatory effect of IFN-beta in patients with multiple sclerosis.

Impaired Toll-like receptor 7 and 9 signaling: from chronic viral infections to cancer

HIV-1, hepatitis B virus, hepatitis C virus, and human papillomavirus type 16 cause persistent infections that frequently precede cancer development. Virions of these viruses are weak inducers of interferon-α and impair Toll-like receptor (TLR)9 function. Loss of TLR9 responsiveness also occurs in tumors without viral etiology such as breast, ovary, and head and neck carcinomas. Recent reports have suggested that viruses and components of the tumor microenviroment interact with regulatory receptors on plasmacytoid dendritic cells (pDCs) to impair TLR7 and TLR9 signaling, and to downregulate TLR9 gene expression. The limited responsiveness of pDCs might contribute to reduced innate immune responses during chronic viral infections and oncogenesis, and represent a target for new therapeutic approaches based on TLR agonists.

Arsenic modulates APOBEC3G-mediated restriction to HIV-1 infection in myeloid dendritic cells

DC are major targets of HIV-1 during the early events of infection. Yet, HIV-1 infects these cells only inefficiently in vitro as compared with CD4+T lymphocytes. Accordingly, we have previously identified a strong post-entry block to HIV-1 replication in MDDC as a result of the cellular restriction factor A3G. Furthermore, we have demonstrated that As₂O₃, a drug used to treat acute promyelocytic leukemia, can fully eliminate the potent post-entry restriction of HIV-1 infection in MDDC and in blood-derived MyDC by mechanisms that were unclear. We are now exploring the interplay between As₂O₃ and A3G-mediated restriction in primary DC subsets. Here, we report that As₂O₃ counteracts A3G-mediated restriction in MyDC but not in MDDC. RNAi of A3G in MyDC indicated that the As₂O₃-mediated increase of HIV-1 infection was largely dependent on the presence of the cellular restriction factor. This study reveals an unexpected interplay between As₂O₃ and A3G-mediated restriction to HIV-1 infection in primary human MyDC.

A versatile role of mammalian target of rapamycin in human dendritic cell function and differentiation

The mammalian target of rapamycin (mTOR) regulates cell growth and survival and exists as rapamycin-sensitive mTOR complex (mTORC) 1 and as rapamycin-insensitive mTORC2. Although mTOR is a well-known regulator of diverse immune cells, its detailed role in human dendritic cell (DC) function and differentiation is only incompletely understood. In this study, we demonstrate divergent roles of mTOR during activation and differentiation of myeloid DCs (mDCs) and monocyte-derived DCs (moDCs). Inhibition of mTORC1 in mDCs activated with TLR-dependent or -independent stimuli increased proinflammatory cytokines and NF-κB, whereas IL-10 and STAT3 were blocked. Rapamycin regulated the costimulatory/surface molecules CD86, programmed death ligand-1, and CD25 on mDCs and significantly increased the T cell allostimulatory potential of mDCs. In contrast, rapamycin suppressed immunostimulatory molecules and the allostimulatory potential of LPS-stimulated moDCs by an inability to augment NF-κB signaling. In differentiating moDCs, the PI3K/Akt-dependent mTOR pathway was constitutively activated by GM-CSF to induce DC differentiation in an mTORC1-dependent manner. Inhibition of mTORC1 or mTORC1/2 during moDC differentiation decreased moDC survival and markedly hampered its immunostimulatory phenotype. Analyzing the fate of DCs in vivo, we found that kidney transplant patients treated with rapamycin displayed an increased immunostimulatory potential of mDCs compared with patients treated with calcineurin inhibitors. Furthermore, rapamycin did not interfere with mDC differentiation in these patients. Collectively, mTOR exerts divergent immunoregulatory functions during DC activation and differentiation depending on the DC type that lead to opposing T cell responses, which might be of clinical importance in transplantation, cancer, and also for novel vaccination strategies.

Sex-specific association of X-linked Toll-like receptor 7 (TLR7) with male systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a multisystem, autoimmune disease that predominantly affects women. Previous findings that duplicated Toll-like receptor 7 (Tlr7) promotes lupus-like disease in male BXSB mice prompted us to evaluate TLR7 in human SLE. By using a candidate gene approach, we identified and replicated association of a TLR7 3'UTR SNP, rs3853839 (G/C), with SLE in 9,274 Eastern Asians (P(combined) = 6.5 x 10(-10)), with a stronger effect in male than female subjects [odds ratio, male vs. female = 2.33 (95% CI = 1.64-3.30) vs. 1.24 (95% CI = 1.14-1.34); P = 4.1 x 10(-4)]. G-allele carriers had increased TLR7 transcripts and more pronounced IFN signature than C-allele carriers; heterozygotes had 2.7-fold higher transcripts of G-allele than C-allele. These data established a functional polymorphism in type I IFN pathway gene TLR7 predisposing to SLE, especially in Chinese and Japanese male subjects.

The lambda interferons: guardians of the immune-epithelial interface and the T-helper 2 response

The type-III interferons (IFNs) are the most recently discovered IFNs in the human immune system and have important, but as yet poorly characterized, functions in innate and adaptive immunity that complement their antiviral functions. It is now becoming clear that these type-III IFNs have a functional niche where epithelial surfaces interact with the adaptive immune system, that their antiviral capability is not as highly developed as that of the type-I IFNs, and that they have their own profile of immunomodulatory functions; specifically, they are key modulators of the T-helper (Th)2 response.

Novel strategies and approaches to develop the next generation of vaccines against porcine reproductive and respiratory syndrome virus (PRRSV)

Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important swine pathogen. Since its discovery in the early 1990s, tremendous progresses have been made in understanding the molecular biology and pathogenesis of PRRSV. Although modified live-attenuated vaccines (MLVs) and inactivated vaccines against PRRSV have been available for more than a decade, the disease remains difficult to control. The efficacies of these vaccines especially against heterologous strains remain questionable: the MLVs were generally effective against homologous strains but variable in success against heterologous strains, and the outcomes of inactivated vaccines in the field are not very promising. With the development of PRRSV reverse genetics systems and the acquisition of new understanding on anti-PRRSV immunity, rational design of the next generation of PRRSV vaccines can now be explored. In this review, we discussed the recent advances in anti-PRRSV immunity and vaccinology, the recent progresses in PRRSV vaccine development particularly the reverse genetics system-based vaccine development, and provided a perspective on potential novel strategies and approaches that may be applicable to the development of the next generation of PRRSV vaccines.

Induction and evasion of innate antiviral responses by hepatitis C virus

Persistent hepatitis C virus infection is associated with progressive hepatic fibrosis and liver cancer. Acute infection evokes several distinct innate immune responses, but these are partially or completely countered by the virus. Hepatitis C virus proteins serve dual functions in replication and immune evasion, acting to disrupt cellular signaling pathways leading to interferon synthesis, subvert Jak-STAT signaling to limit expression of interferon-stimulated genes, and block antiviral activities of interferon-stimulated genes. The net effect is a multilayered evasion of innate immunity, which negatively influences the subsequent development of antigen-specific adaptive immunity, thereby contributing to virus persistence and resistance to therapy.

Respiratory virus-induced TLR7 activation controls IL-17-associated increased mucus via IL-23 regulation

The response to respiratory syncytial virus (RSV), negative strand ssRNA virus, depends upon the ability to recognize specific pathogen-associated targets. In the current study, the role of TLR7 that recognizes ssRNA was examined. Using TLR7(-/-) mice, we found that the response to RSV infection in the lung was more pathogenic as assessed by significant increases in inflammation and mucus production. Although there appeared to be no effect of TLR7 deficiency on type I IFN, the pathology was associated with an alteration in T cell responses with increases in mucogenic cytokines IL-4, IL-13, and IL-17. Examination of dendritic cells from TLR7(-/-) animals indicated a preferential activation of IL-23 (a Th17-promoting cytokine) and a decrease in IL-12 production. Neutralization of IL-17 in the TLR7(-/-) mice resulted in a significant decrease in the mucogenic response in the lungs of the RSV-infected mice. Thus, without TLR7-mediated responses, an altered immune environment ensued with a significant effect on airway epithelial cell remodeling and goblet cell hyper/metaplasia, leading to increased mucus production.

TLR-mediated up-regulation of serum retroviral gp70 is controlled by the Sgp loci of lupus-prone mice

The endogenous retroviral envelope glycoprotein, gp70, implicated in murine systemic lupus erythematosus (SLE), has been considered to be a product of xenotropic, polytropic (PT) and modified PT (mPT) endogenous retroviruses. It is secreted by hepatocytes like an acute phase protein, but its response is under a genetic control. Given critical roles of TLR7 and TLR9 in the pathogenesis of SLE, we assessed their contribution to the acute phase expression of serum gp70, and defined a pivotal role of the Sgp3 (serum gp70 production 3) and Sgp4 loci in this response. Our results demonstrated that serum levels of gp70 were up-regulated in lupus-prone NZB mice injected with TLR7 or TLR9 agonist at levels comparable to those induced by injection of IL-1, IL-6 or TNF. In addition, studies of C57BL/6 Sgp3 and/or Sgp4 congenic mice defined the major roles of these two loci in up-regulated production of serum gp70 during acute phase responses. Finally, the analysis of Sgp3 congenic mice strongly suggests the presence of at least two distinct genetic factors in the Sgp3 interval, one of which controlled the basal-level expression of xenotropic, PT and mPT gp70 and the other which controlled the up-regulated production of xenotropic and mPT gp70 during acute phase responses. Our results uncovered an additional pathogenic role of TLR7 and TLR9 in murine lupus nephritis by promoting the expression of nephritogenic gp70 autoantigen. Furthermore, they revealed the involvement of multiple regulatory genes for the expression of gp70 autoantigen under steady-state and inflammatory conditions in lupus-prone mice.

Enemy at the gates: dendritic cells and immunity to mucosal pathogens

Dendritic cells (DC) are diverse and specialized hematopoietic cells serving as an essential bridge between innate and adaptive immunity. DC exist in all lymphoid and nonlymphoid organs and are amongst the first responders to infection at epithelial surfaces including mucosal tissues. DC of the lung, gut, and vaginal mucosa display different phenotypes and functions reflecting each unique tissue and, in contrast to their counterparts in spleen and lymph nodes, are constantly exposed to both harmful and benign factors of their environments. Mucosal DC recognize and respond to pathogens through engagement of pattern recognition receptors, and activated DC migrate to draining lymph nodes to induce adaptive immune responses. The specialized function of DC aids in the induction of immunity and pathogen control at the mucosa. Such specialization includes the potent antiviral interferon response of plasmacytoid DC to viral nucleic acids, the ability of mucosal DC to capture organisms in the gut lumen, the capacity of DC to cross-present antigen from other infected cells, and the ability of mucosal DC to initiate IgA class switching in B cells. DC plasticity is also critical in the immune response to mucosal pathogens, as DC can respond to the microenvironment and sense pathogen-induced stress in neighboring epithelial cells. Finally, DC interact with each other through crosstalk to promote antigen presentation and T-cell immunity. Together, these processes condition mucosal DC for the induction of a tailored immune response to pathogens.

Toll-like receptor agonists: are they good adjuvants?

Therapeutic immunization leading to cancer regression remains a significant challenge. Successful immunization requires activation of adaptive immunity, including tumor specific CD4 T cells and CD8 T cells. Generally, the activation of T cells is compromised in patients with cancer because of immune suppression, loss of tumor antigen expression, and dysfunction of antigen-presenting cells. Antigen-presenting cells such as dendritic cells (DCs) are key for the induction of adaptive antitumor immune responses. Recently, attention has focused on novel adjuvants that enhance dendritic cell function and their ability to prime T cells. Agonists that target toll-like receptors are being used clinically either alone or in combination with tumor antigens and showing initial success both in terms of enhancing immune responses and eliciting antitumor activity. This review summarizes the application of these adjuvants to treat cancer and the potential for boosting responses in vivo.

Trafficking properties of plasmacytoid dendritic cells in health and disease

Plasmacytoid dendritic cells (PDCs) represent a subset of circulating leukocytes characterized by the ability to release high levels of type I interferon (IFN). Under homeostatic conditions PDCs are confined to primary and secondary lymphoid organs. This is consistent with the restricted profile of functional chemotactic receptors expressed by circulating PDCs (i.e. CXCR4 and ChemR23). Accumulation of PDCs in non-lymphoid tissue is, however, observed in certain autoimmune diseases, allergic reactions and tumors. Indeed, PDCs are now considered to be involved in the pathogenesis of diseases characterized by a type I IFN-signature and are considered as a promising target for new intervention strategies. Here, current knowledge of the molecular mechanisms involved in the recruitment of PDCs under homeostatic and pathological conditions are summarized.

Cellular receptors, differentiation and endocytosis requirements are key factors for type I IFN response by human epithelial, conventional and plasmacytoid dendritic infected cells by measles virus

While the antiviral response during measles virus (MeV) infection is documented, the contribution of the hosting cell type to the type I interferon (IFN-alpha/beta) response is still not clearly established. Here, we report that a signature heterogeneity of the IFN-alpha/beta response according to the cell type. The MeV tropism dictated by the expression of appropriate cellular receptor appeared to be crucial for epithelial cells. For conventional DCs (cDCs), the maturation state played a prominent role. In response to both wild type MeV isolates and laboratory/vaccine strains, immature cDCs produced higher levels of IFN-alpha than mature cDCs, despite the reduced expression levels of both CD46 and CD150 receptors by the former ones. While in epithelial cells and cDCs the MeV transcription was required to activate the IFN-alpha/beta response, plasmacytoid DCs (pDCs) rapidly produced large amounts of IFN-alpha mostly independently of the viral infection cycle. This argues for a significant contribution of pDCs in response to MeV infection and/or vaccination.

Plasmacytoid dendritic cells accumulate and secrete interferon alpha in lymph nodes of HIV-1 patients

Circulating plasmacytoid dendritic cells (pDC) decline during HIV-1 infection, but at the same time they express markedly higher levels of interferon alpha (IFNalpha), which is associated with HIV-1 disease progression. Here we show an accumulation of pDC in lymph nodes (LN) of treatment-naïve HIV-1 patients. This phenomenon was associated with elevated expression of the LN homing marker, CCR7, on pDC in peripheral blood of HIV-1 patients, which conferred increased migratory capacity in response to CCR7 ligands in ex vivo functional assays. LN-homed pDC of HIV-1 patients presented higher CD40 and lower BDCA2 levels, but unchanged CD83 and CD86 expression. In addition, these cells expressed markedly higher amounts of IFNalpha compared to uninfected individuals, and were undergoing faster rates of cell death. These results demonstrate for the first time that in asymptomatic, untreated HIV-1 patients circulating pDC up-regulate CCR7 expression, accumulate in lymph nodes, and express high amounts of IFNalpha before undergoing cell death. Since IFNalpha inhibits cell proliferation and modulates immune responses, chronically high levels of this cytokine in LN of HIV-1 patients may impair differentiation and immune function of bystander CD4(+) T cells, thus playing into the mechanisms of AIDS immunopathogenesis.

Trichostatin A blocks type I interferon production by activated plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDC) represent the main type I interferon (IFN-I) producing cells. Emerging evidence supports a role for IFN-I in autoimmune diseases. Given the central role of PDC in the pathogenesis of systemic lupus erythematosus (SLE), we investigated the effect of Trichostatin A (TSA), a prototypic histone deacetylase inhibitor, on PDC activation. TSA inhibited the production of IFN-I, TRAIL and of the pro-inflammatory cytokines TNFalpha and IL-6 by CpG-activated PDC. These effects were associated with the inhibition of IFN Regulatory Factor (IRF)-7 nuclear translocation. Furthermore, TSA was also effective in inhibiting the production of IFNalpha by PDC cultured in vitro in the presence of serum obtained from SLE patients. This study describes a new level of regulation of immune responses by histone deacetylase inhibitors and defines the molecular basis for new strategies to be exploited in the treatment of autoimmune diseases.

Rotavirus structural proteins and dsRNA are required for the human primary plasmacytoid dendritic cell IFNalpha response

Rotaviruses are the leading cause of severe dehydrating diarrhea in children worldwide. Rotavirus-induced immune responses, especially the T and B cell responses, have been extensively characterized; however, little is known about innate immune mechanisms involved in the control of rotavirus infection. Although increased levels of systemic type I interferon (IFNalpha and beta) correlate with accelerated resolution of rotavirus disease, multiple rotavirus strains, including rhesus rotavirus (RRV), have been demonstrated to antagonize type I IFN production in a variety of epithelial and fibroblast cell types through several mechanisms, including degradation of multiple interferon regulatory factors by a viral nonstructural protein. This report demonstrates that stimulation of highly purified primary human peripheral plasmacytoid dendritic cells (pDCs) with either live or inactivated RRV induces substantial IFNalpha production by a subset of pDCs in which RRV does not replicate. Characterization of pDC responses to viral stimulus by flow cytometry and Luminex revealed that RRV replicates in a small subset of human primary pDCs and, in this RRV-permissive small subset, IFNalpha production is diminished. pDC activation and maturation were observed independently of viral replication and were enhanced in cells in which virus replicates. Production of IFNalpha by pDCs following RRV exposure required viral dsRNA and surface proteins, but neither viral replication nor activation by trypsin cleavage of VP4. These results demonstrate that a minor subset of purified primary human peripheral pDCs are permissive to RRV infection, and that pDCs retain functionality following RRV stimulus. Additionally, this study demonstrates trypsin-independent infection of primary peripheral cells by rotavirus, which may allow for the establishment of extraintestinal viremia and antigenemia. Importantly, these data provide the first evidence of IFNalpha induction in primary human pDCs by a dsRNA virus, while simultaneously demonstrating impaired IFNalpha production in primary human cells in which RRV replicates. Rotavirus infection of primary human pDCs provides a powerful experimental system for the study of mechanisms underlying pDC-mediated innate immunity to viral infection and reveals a potentially novel dsRNA-dependent pathway of IFNalpha induction.

Type I interferon signaling contributes to chronic inflammation in a murine model of silicosis

Lung disorders induced by inhaled inorganic particles such as crystalline silica are characterized by chronic inflammation and pulmonary fibrosis. Here, we demonstrate the importance of type I interferon (IFN) in the development of crystalline silica-induced lung inflammation in mice, revealing that viruses and inorganic particles share similar signaling pathways. We found that instillation of silica is followed by the upregulation of IFN-beta and IRF-7 and that granulocytes (GR1(+)) and macrophages/dendritic cells (CD11c(+)) are major producers of type I IFN in response to silica. Two months after silica administration, both IFNAR- and IRF-7-deficient mice produced significantly less pulmonary inflammation and chemokines (KC and CCL2) than competent mice but developed similar lung fibrosis. Our data indicate that type I IFN contributes to the chronic lung inflammation that accompanies silica exposure in mice. Type I IFN is, however, dispensable in the development of silica-induced acute lung inflammation and pulmonary fibrosis.

Characterization of human DNGR-1+ BDCA3+ leukocytes as putative equivalents of mouse CD8alpha+ dendritic cells

In mouse, a subset of dendritic cells (DCs) known as CD8α⁺ DCs has emerged as an important player in the regulation of T cell responses and a promising target in vaccination strategies. However, translation into clinical protocols has been hampered by the failure to identify CD8α⁺ DCs in humans. Here, we characterize a population of human DCs that expresses DNGR-1 (CLEC9A) and high levels of BDCA3 and resembles mouse CD8α⁺ DCs in phenotype and function. We describe the presence of such cells in the spleens of humans and humanized mice and report on a protocol to generate them in vitro. Like mouse CD8α⁺ DCs, human DNGR-1⁺ BDCA3^hi DCs express Necl2, CD207, BATF3, IRF8, and TLR3, but not CD11b, IRF4, TLR7, or (unlike CD8α⁺ DCs) TLR9. DNGR-1⁺ BDCA3^hi DCs respond to poly I:C and agonists of TLR8, but not of TLR7, and produce interleukin (IL)-12 when given innate and T cell–derived signals. Notably, DNGR-1⁺ BDCA3⁺ DCs from in vitro cultures efficiently internalize material from dead cells and can cross-present exogenous antigens to CD8⁺ T cells upon treatment with poly I:C. The characterization of human DNGR-1⁺ BDCA3^hi DCs and the ability to grow them in vitro opens the door for exploiting this subset in immunotherapy.

Inhibitor of IkappaB kinase activity, BAY 11-7082, interferes with interferon regulatory factor 7 nuclear translocation and type I interferon production by plasmacytoid dendritic cells

Introduction

Plasmacytoid dendritic cells (pDCs) play not only a central role in the antiviral immune response in innate host defense, but also a pathogenic role in the development of the autoimmune process by their ability to produce robust amounts of type I interferons (IFNs), through sensing nucleic acids by toll-like receptor (TLR) 7 and 9. Thus, control of dysregulated pDC activation and type I IFN production provide an alternative treatment strategy for autoimmune diseases in which type I IFNs are elevated, such as systemic lupus erythematosus (SLE). Here we focused on IκB kinase inhibitor BAY 11-7082 (BAY11) and investigated its immunomodulatory effects in targeting the IFN response on pDCs.

Methods

We isolated human blood pDCs by flow cytometry and examined the function of BAY11 on pDCs in response to TLR ligands, with regards to pDC activation, such as IFN-α production and nuclear translocation of interferon regulatory factor 7 (IRF7) in vitro. Additionally, we cultured healthy peripheral blood mononuclear cells (PBMCs) with serum from SLE patients in the presence or absence of BAY11, and then examined the inhibitory function of BAY11 on SLE serum-induced IFN-α production. We also examined its inhibitory effect in vivo using mice pretreated with BAY11 intraperitonealy, followed by intravenous injection of TLR7 ligand poly U.

Results

Here we identified that BAY11 has the ability to inhibit nuclear translocation of IRF7 and IFN-α production in human pDCs. BAY11, although showing the ability to also interfere with tumor necrosis factor (TNF)-α production, more strongly inhibited IFN-α production than TNF-α production by pDCs, in response to TLR ligands. We also found that BAY11 inhibited both in vitro IFN-α production by human PBMCs induced by the SLE serum and the in vivo serum IFN-α level induced by injecting mice with poly U.

Conclusions

These findings suggest that BAY11 has the therapeutic potential to attenuate the IFN environment by regulating pDC function and provide a novel foundation for the development of an effective immunotherapeutic strategy against autoimmune disorders such as SLE.

Innate immune receptors and autophagy: implications for autoimmune kidney injury

Inflammation is the immune system's response to infectious or noninfectious sources of danger. Danger recognition is facilitated by various innate immune receptor families including the Toll-like receptors (TLRs), which detect danger signals in extracellular and intracellular compartments. It is an evolving concept that renal damage triggers intrarenal inflammation by immune recognition of molecules that are being released by dying cells. Such danger-associated molecules act as immunostimulatory agonists to TLRs and other innate immune receptors and induce cytokine and chemokine secretion, leukocyte recruitment, and tissue remodeling. As a new entry to this concept, autophagy allows stressed cells to reduce intracellular microorganisms, protein aggregates, and cellular organelles by moving and subsequently digesting them in autophagolysosomes. Within the autophagolysosome, endogenous molecules and danger-associated molecules may be presented to TLRs or loaded onto the major histocompatibility complex and presented as autoantigens. Here we discuss the current evidence for the danger signaling concept in autoimmune kidney injury and propose that autophagy-related processing of self-proteins provides a source of immunostimulatory molecules and autoantigens. A better understanding of danger signaling should enable us to unravel yet unknown triggers for renal immunopathology and progressive kidney disease.

Murine toll-like receptor 2 activation induces type I interferon responses from endolysosomal compartments

Background

Toll-like receptors (TLRs) are among the first-line sentinels for immune detection and responsiveness to pathogens. The TLR2 subfamily of TLRs (TLR1, TLR2, TLR6) form heterodimers with each other and are thus able to recognize a broad range of components from several microbes such as yeast, Gram-positive bacteria and protozoa. Until now, TLR2 activation by bacterial ligands has long been associated with pro-inflammatory cytokines but not type I interferon responses.

Methodology/Principal Findings

Using a variety of transgenic mice, here we provide in vivo and in vitro data showing that TLR2 activation does in fact induce interferon-beta and that this occurs via MyD88-IRF1 and -IRF7 pathways. Interestingly, by microscopy we demonstrate that although a cell surface receptor, TLR2 dependent induction of type I interferons occurs in endolysosomal compartments where it is translocated to upon ligand engagement. Furthermore, we could show that blocking receptor internalization or endolysosomal acidification inhibits the ability of TLR2 to trigger the induction type I interferon but not pro-inflammatory responses.

Conclusion/Significance

The results indicate that TLR2 activation induces pro-inflammatory and type I interferon responses from distinct subcellular sites: the plasma membrane and endolysosomal compartments respectively. Apart from identifying and characterizing a novel pathway for induction of type I interferons, the present study offers new insights into how TLR signaling discriminates and regulates the nature of responses to be elicited against extracellular and endocytosed microbes. These findings may also have clinical implication. Excessive production of pro-inflammatory cytokines and type I IFNs following activation of TLRs is a central pathologic event in several hyper-inflammatory conditions. The discovery that the induction of pro-inflammatory and type I IFN responses can be uncoupled through pharmacological manipulation of endolysosomal acidification suggests new avenues for potential therapeutic intervention against inflammations and sepsis.

Potential role of IFNα in adult lupus

Patients with lupus have a continuous production of IFNα and display an increased expression of IFNα-regulated genes. The reason for the ongoing IFNα synthesis in these patients seems to be an activation of plasmacytoid dendritic cells (pDCs) by immune complexes (ICs), consisting of autoantibodies in combination with DNA-containing or RNA-containing autoantigens. The mechanisms behind the activation of pDCs by such ICs have to some extent been elucidated during the last years. Thus, interferogenic ICs are internalized via the FcγRIIa expressed on pDCs, reach the endosomes and stimulate Toll-like receptor (TLR) 7 or 9, which subsequently leads to IFNα gene transcription. Variants of genes involved in both the IFNα synthesis and response have been linked to an increased risk to develop lupus. Among these genes are interferon regulatory factor 5 (IRF5), which is involved in TLR signaling, and the signal transducer and activator of transcription 4 (STAT4) that interacts with the type I interferon receptor. Produced IFNα may at least partially be responsible for several of the observed alterations in the immune system of lupus patients and contribute to the autoimmune disease process, which will be discussed in the present review. How produced IFNα can contribute to some clinical manifestations will briefly be described, as well as the possible consequences of this knowledge in clinical practice for disease monitoring and therapy.

Overview of the immune response

The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter through mucosal surfaces. Central to the immune system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic microbes, and both of these mechanisms include self-nonself discrimination. This overview identifies key mechanisms used by the immune system to respond to invading microbes and other exogenous threats and identifies settings in which disturbed immune function exacerbates tissue injury.