Multiple mechanisms contribute to impairment of type 1 interferon production during chronic lymphocytic choriomeningitis virus infection of mice

Metabolic Deficiencies Underlie Plasmacytoid Dendritic Cell Exhaustion After Viral Infection

Type I Interferons (IFN-I) are central to host protection against viral infections 1 . While any cell can produce IFN-I, Plasmacytoid Dendritic Cells (pDCs) make greater quantities and more varieties of these cytokines than any other cell type 2 . However, following an initial burst of IFN- I, pDCs lose their exceptional IFN-I production capacity and become "exhausted", a phenotype that associates with enhanced susceptibility to secondary infections 3-5 . Despite this apparent cost for the host, pDC exhaustion is conserved across multiple species and viral infections, but the underlying mechanisms and the potential evolutionary advantages are not well understood. Here we characterize pDC exhaustion and demonstrate that it is associated with a reduced capacity of pDCs to engage both oxidative and glycolytic metabolism. Mechanistically, we identify lactate dehydrogenase B (LDHB) as a novel positive regulator of pDC IFN-I production in mice and humans, show that LDHB deficiency is associated with suppressed IFN-I production, pDC metabolic capacity, and viral control following a viral infection, and demonstrate that preservation of LDHB expression is sufficient to partially restore exhausted pDC function in vitro and in vivo . Furthermore, restoring LDHB in vivo in exhausted pDCs increased IFNAR dependent infection- associated pathology. Therefore, our work identifies a novel and conserved mechanism for balancing immunity and pathology during viral infections, while also providing insight into the highly preserved but previously unexplained phenomenon of pDC exhaustion.

The role of CD4+ T cells in tumor and chronic viral immune responses

Immunotherapies are mainly aimed to promote a CD8+ T cell response rather than a CD4+ T cell response as cytotoxic T lymphocytes (CTLs) can directly kill target cells. Recently, CD4+ T cells have received more attention due to their diverse roles in tumors and chronic viral infections. In antitumor and antichronic viral responses, CD4+ T cells relay help signals through dendritic cells to indirectly regulate CD8+ T cell response, interact with B cells or macrophages to indirectly modulate humoral immunity or macrophage polarization, and inhibit tumor blood vessel formation. Additionally, CD4+ T cells can also exhibit direct cytotoxicity toward target cells. However, regulatory T cells exhibit immunosuppression and CD4+ T cells become exhausted, which promote tumor progression and chronic viral persistence. Finally, we also outline immunotherapies based on CD4+ T cells, including adoptive cell transfer, vaccines, and immune checkpoint blockade. Overall, this review summarizes diverse roles of CD4+ T cells in the antitumor or protumor and chronic viral responses, and also highlights the immunotherapies based on CD4+ T cells, giving a better understanding of their roles in tumors and chronic viral infections.

Immunomodulatory Role of Interferons in Viral and Bacterial Infections

Interferons are a group of immunomodulatory substances produced by the human immune system in response to the presence of pathogens, especially during viral and bacterial infections. Their remarkably diverse mechanisms of action help the immune system fight infections by activating hundreds of genes involved in signal transduction pathways. In this review, we focus on discussing the interplay between the IFN system and seven medically important and challenging viruses (herpes simplex virus (HSV), influenza, hepatitis C virus (HCV), lymphocytic choriomeningitis virus (LCMV), human immunodeficiency virus (HIV), Epstein-Barr virus (EBV), and SARS-CoV coronavirus) to highlight the diversity of viral strategies. In addition, the available data also suggest that IFNs play an important role in the course of bacterial infections. Research is currently underway to identify and elucidate the exact role of specific genes and effector pathways in generating the antimicrobial response mediated by IFNs. Despite the numerous studies on the role of interferons in antimicrobial responses, many interdisciplinary studies are still needed to understand and optimize their use in personalized therapeutics.

The pathogenic role of IFN-α in thyroiditis mouse models

AIM

Patients with chronic hepatitis C are frequently treated with interferon (IFN)-α. Autoimmune thyroid disease occurs in 20% ~ 40% of IFN-α-treated patients. In this study, the effects of IFN-α administration on triggering and regulating autoimmune thyroiditis in various animal models were evaluated.

MAIN METHODS

Exogenous IFN-α was given to naive CBA mice, and both thyroglobulin (TG) immunization-induced (CBA) and spontaneous autoimmune thyroiditis (NOD·H-2 h4) models. Thyroid function, and anti-thyroglobulin antibody (ATA) and B-cell-activating factor (BAFF) levels were measured. Alterations in transcriptome profiles were analyzed.

KEY FINDINGS

In the TG-induced thyroiditis model, IFN-α administration reduced plasma free thyroxine levels but did not alter ATA titers, BAFF levels, or the severity of histological changes. Interestingly, even without changes in thyroid functions, four of eight mice in the IFN-α alone group exhibited thyroiditis compared to the control group. Immunologically, mice in the IFN-α group exhibited profound CD3⁺ cell infiltration in the thyroid and higher plasma BAFF levels compared to the control group. Meanwhile, pathological and serological alterations after IFN-α administration were not observed in the NOD·H-2 h4 model. An RNA sequencing analysis revealed that immunoregulatory signatures were not excited by IFN-α treatment in naive CBA mice. Meanwhile, innate and adaptive immunity, inflammatory cytokine, chemokine, and cell-killing signaling pathways were all stimulated by IFN-α administration after TG immunization of CBA mice.

SIGNIFICANCE

We confirmed the remarkable effects of IFN-α in both initiating thyroid immunity and modulating thyroid function and immunoregulatory signatures in established autoimmune thyroiditis. We suggest that IFN-α should be administered with caution in clinical settings.

Type I Interferon Induction and Exhaustion during Viral Infection: Plasmacytoid Dendritic Cells and Emerging COVID-19 Findings

Type I Interferons (IFN-I) are a family of potent antiviral cytokines that act through the direct restriction of viral replication and by enhancing antiviral immunity. However, these powerful cytokines are a caged lion, as excessive and sustained IFN-I production can drive immunopathology during infection, and aberrant IFN-I production is a feature of several types of autoimmunity. As specialized producers of IFN-I plasmacytoid (p), dendritic cells (DCs) can secrete superb quantities and a wide breadth of IFN-I isoforms immediately after infection or stimulation, and are the focus of this review. Notably, a few days after viral infection pDCs tune down their capacity for IFN-I production, producing less cytokines in response to both the ongoing infection and unrelated secondary stimulations. This process, hereby referred to as "pDC exhaustion", favors viral persistence and associates with reduced innate responses and increased susceptibility to secondary opportunistic infections. On the other hand, pDC exhaustion may be a compromise to avoid IFN-I driven immunopathology. In this review we reflect on the mechanisms that initially induce IFN-I and subsequently silence their production by pDCs during a viral infection. While these processes have been long studied across numerous viral infection models, the 2019 coronavirus disease (COVID-19) pandemic has brought their discussion back to the fore, and so we also discuss emerging results related to pDC-IFN-I production in the context of COVID-19.

Infection and cancer suppress pDC derived IFN-I

Plasmacytoid dendritic cells (pDCs) are specialized producers of Type I interferon (IFN-I) that promote anti-viral and anti-tumor immunity. However, chronic infections and cancer inhibit pDC-derived IFN-I. While the mechanisms of this inhibition are multifarious they can be classified broadly into two categories: i) reduction or ablation of pDC IFN-I-production capacity (functional exhaustion) and/or ii) decrease in pDC numbers (altered population dynamics). Recent work has identified many processes that contribute to suppression of pDC-derived IFN-I during chronic infections and cancer, including sustained stimulation through Toll Like Receptors (TLRs), inhibitory microenvironments, inhibitory receptor ligation, and reduced development from bone marrow progenitors and apoptosis. Emerging success leveraging pDCs in treatment of disease through TLR activation illustrates the therapeutic potential of targeting pDCs. Deeper understanding of the systems that limit pDC-derived IFN-I has the potential to improve these emerging therapies as well as help devising new approaches that harness the outstanding IFN-I-production capacity of pDCs.

Distinct Molecular Mechanisms of Host Immune Response Modulation by Arenavirus NP and Z Proteins

Endemic to West Africa and South America, mammalian arenaviruses can cross the species barrier from their natural rodent hosts to humans, resulting in illnesses ranging from mild flu-like syndromes to severe and fatal haemorrhagic zoonoses. The increased frequency of outbreaks and associated high fatality rates of the most prevalent arenavirus, Lassa, in West African countries, highlights the significant risk to public health and to the socio-economic development of affected countries. The devastating impact of these viruses is further exacerbated by the lack of approved vaccines and effective treatments. Differential immune responses to arenavirus infections that can lead to either clearance or rapid, widespread and uncontrolled viral dissemination are modulated by the arenavirus multifunctional proteins, NP and Z. These two proteins control the antiviral response to infection by targeting multiple cellular pathways; and thus, represent attractive targets for antiviral development to counteract infection. The interplay between the host immune responses and viral replication is a key determinant of virus pathogenicity and disease outcome. In this review, we examine the current understanding of host immune defenses against arenavirus infections and summarise the host protein interactions of NP and Z and the mechanisms that govern immune evasion strategies.

Rapid expansion of Treg cells protects from collateral colitis following a viral trigger

Foxp3⁺ regulatory T (Treg) cells are essential for maintaining peripheral tolerance and preventing autoimmunity. While genetic factors may predispose for autoimmunity, additional environmental triggers, such as viral infections, are usually required to initiate the onset of disease. Here, we show that viral infection with LCMV results in type I IFN-dependent Treg cell loss that is rapidly compensated by the conversion and expansion of Vβ5⁺ conventional T cells into iTreg cells. Using Vβ5-deficient mice, we show that these Vβ5⁺ iTreg cells are dispensable for limiting anti-viral immunity. Rather, the delayed replenishment of Treg cells in Vβ5-deficient mice compromises suppression of microbiota-dependent activation of CD8⁺ T cells, resulting in colitis. Importantly, recovery from clinical symptoms in IBD patients is marked by expansion of the corresponding Vβ2⁺ Treg population in humans. Collectively, we provide a link between a viral trigger and an impaired Treg cell compartment resulting in the initiation of immune pathology.

Chronic Lymphocytic Choriomeningitis Infection Causes Susceptibility to Mousepox and Impairs Natural Killer Cell Maturation and Function

Chronic viral infections. like those of humans with cytomegalovirus, human immunodeficiency virus (even when under antiretroviral therapy), and hepatitis C virus or those of mice with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13), result in immune dysfunction that predisposes the host to severe infections with unrelated pathogens. It is known that C57BL/6 (B6) mice are resistant to mousepox, a lethal disease caused by the orthopoxvirus ectromelia virus (ECTV), and that this resistance requires natural killer (NK) cells and other immune cells. We show that most B6 mice chronically infected with CL13 succumb to mousepox but that most of those that recovered from acute infection with the LCMV Armstrong (Arm) strain survive. We also show that B6 mice chronically infected with CL13 and those that recovered from Arm infection have a reduced frequency and a reduced number of NK cells. However, at steady state, NK cells in mice that have recovered from Arm infection mature normally and, in response to ECTV, get activated, become more mature, proliferate, and increase their cytotoxicity in vivo Conversely, in mice chronically infected with CL13, NK cells are immature and residually activated, and following ECTV infection, they do not mature, proliferate, or increase their cytotoxicity. Given the well-established importance of NK cells in resistance to mousepox, these data suggest that the NK cell dysfunction caused by CL13 persistence may contribute to the susceptibility of CL13-infected mice to mousepox. Whether chronic infections similarly affect NK cells in humans should be explored.IMPORTANCE Infection of adult mice with the clone 13 (CL13) strain of lymphocytic choriomeningitis virus (LCMV) is extensively used as a model of chronic infection. In this paper, we show that mice chronically infected with CL13 succumb to challenge with ectromelia virus (ECTV; the agent of mousepox) and that natural killer (NK) cells in CL13-infected mice are reduced in numbers and have an immature and partially activated phenotype but do respond to ECTV. These data may provide additional clues why humans chronically infected with certain pathogens are less resistant to viral diseases.

Loss of Resistance to Mousepox during Chronic Lymphocytic Choriomeningitis Virus Infection Is Associated with Impaired T-Cell Responses and Can Be Rescued by Immunization

It is well established that chronic viral infections can cause immune suppression, resulting in increased susceptibility to other infectious diseases. However, the effects of chronic viral infection on T-cell responses and vaccination against highly pathogenic viruses are not well understood. We have recently shown that C57BL/6 (B6) mice lose their natural resistance to wild-type (WT) ectromelia virus (ECTV) when chronically infected with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13). Here we compared the T-cell response to ECTV in previously immunologically naive mice that were chronically infected with CL13 or that were convalescent from acute infection with the Armstrong (Arm) strain of LCMV. Our results show that mice that were chronically infected with CL13 but not those that had recovered from Arm infection have highly defective ECTV-specific CD8⁺ and CD4⁺ T-cell responses to WT ECTV. These defects are at least partly due to the chronic infection environment. In contrast to mice infected with WT ECTV, mice chronically infected with CL13 survived without signs of disease when infected with ECTV-Δ036, a mutant ECTV strain that is highly attenuated. Strikingly, mice chronically infected with CL13 mounted a strong CD8⁺ T-cell response to ECTV-Δ036 and survived without signs of disease after a subsequent challenge with WT ECTV. Our work suggests that enhanced susceptibility to acute viral infections in chronically infected individuals can be partly due to poor T-cell responses but that sufficient T-cell function can be recovered and resistance to acute infection can be restored by immunization with highly attenuated vaccines.IMPORTANCE Chronic viral infections may result in immunosuppression and enhanced susceptibility to infections with other pathogens. For example, we have recently shown that mice chronically infected with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13) are highly susceptible to mousepox, a disease that is caused by ectromelia virus and that is the mouse homolog of human smallpox. Here we show chronic CL13 infection severely disrupts the expansion, proliferation, activation, and cytotoxicity of T cells in response due at least in part to the suppressive effects of the chronic infection milieu. Notably, despite this profound immunodeficiency, mice chronically infected with CL13 could be protected by vaccination with a highly attenuated variant of ECTV. These results demonstrate that protective vaccination of immunosuppressed individuals is possible, provided that proper immunization tools are used.

Molecular characterization and expression analysis of rockfish (Sebastes schlegelii) viperin, and its ability to enervate RNA virus transcription and replication in vitro

Viperin, also known as RSAD2 (Radical S-adenosyl methionine domain containing 2), is an interferon-induced endoplasmic reticulum-associated antiviral protein. Previous studies have shown that viperin levels are elevated in the presence of viral RNA, but it has rarely been characterized in marine organisms. This study was designed to functionally characterize rockfish viperin (SsVip), to examine the effects of different immune stimulants on its expression, and to determine its subcellular localization. SsVip is a 349 amino acid protein with a predicted molecular mass of 40.24 kDa. It contains an S-adenosyl l-methionine binding conserved domain with a CNYKCGFC sequence. Unchallenged tissue expression analysis using quantitative real time PCR (qPCR) revealed SsVip expression to be the highest in the blood, followed by the spleen. When challenged with poly I:C, SsVip was upregulated by approximately 60-fold in the blood after 24 h, and approximately 50-fold in the spleen after 12 h. Notable upregulation was detected throughout the poly I:C challenge experiment in both tissues. Significant expression of SsVip was detected in the blood following Streptococcus iniae and lipopolysaccharide challenge, and viral hemorrhagic septicemia virus (VHSV) gene transcription was significantly downregulated during SsVip overexpression. Furthermore, cell viability assay and virus titer quantification with the presence of SsVip revealed a significant reduction in virus replication. As with previously identified viperin counterparts, SsVip was localized in the endoplasmic reticulum. Our findings show that SsVip is an antiviral protein crucial to innate immune defense.

Plasmacytoid Dendritic Cells: Development, Regulation, and Function

Plasmacytoid dendritic cells (pDCs) are a unique sentinel cell type that can detect pathogen-derived nucleic acids and respond with rapid and massive production of type I interferon. This review summarizes our current understanding of pDC biology, including transcriptional regulation, heterogeneity, role in antiviral immune responses, and involvement in immune pathology, particularly in autoimmune diseases, immunodeficiency, and cancer. We also highlight the remaining gaps in our knowledge and important questions for the field, such as the molecular basis of unique interferon-producing capacity of pDCs. A better understanding of cell type-specific positive and negative control of pDC function should pave the way for translational applications focused on this immune cell type.

Self-Renewal and Toll-like Receptor Signaling Sustain Exhausted Plasmacytoid Dendritic Cells during Chronic Viral Infection

Although characterization of T cell exhaustion has unlocked powerful immunotherapies, the mechanisms sustaining adaptations of short-lived innate cells to chronic inflammatory settings remain unknown. During murine chronic viral infection, we found that concerted events in bone marrow and spleen mediated by type I interferon (IFN-I) and Toll-like receptor 7 (TLR7) maintained a pool of functionally exhausted plasmacytoid dendritic cells (pDCs). In the bone marrow, IFN-I compromised the number and the developmental capacity of pDC progenitors, which generated dysfunctional pDCs. Concurrently, exhausted pDCs in the periphery were maintained by self-renewal via IFN-I- and TLR7-induced proliferation of CD4^- subsets. On the other hand, pDC functional loss was mediated by TLR7, leading to compromised IFN-I production and resistance to secondary infection. These findings unveil the mechanisms sustaining a self-perpetuating pool of functionally exhausted pDCs and provide a framework for deciphering long-term exhaustion of other short-lived innate cells during chronic inflammation.

Review of Mammarenavirus Biology and Replication

The family Arenaviridae is divided into three genera: Mammarenavirus, Reptarenavirus, and Hartmanivirus. The Mammarenaviruses contain viruses responsible for causing human hemorrhagic fever diseases including New World viruses Junin, Machupo, Guanarito, Sabia, and Chapare virus and Old World viruses Lassa, and Lujo virus. These two groups of arenaviruses share the same genome organization composed of two ambisense RNA segments. These segments contain four open reading frames that encode for four proteins: the nucleoprotein, glycoprotein precursor, L protein, and Z. Despite their genome similarities, these groups exhibit marked differences in their replication life cycles. This includes differences in attachment, entry, and immune evasion. By understanding the intricacy of replication in each of these viral species we can work to develop counter measures against human diseases. This includes the development of vaccines and antivirals for these emerging viral threats. Currently only the vaccine against Junin virus, Candid#1, is in use as well as Ribavirin for treatment of Lassa Fever. In addition, small molecule inhibitors can be developed to target various aspects of the virus life cycle. In these ways an understanding of the arenavirus replication cycle can be used to alleviate the mortality and morbidity of these infections worldwide.

CD4+ T Cell Differentiation in Chronic Viral Infections: The Tfh Perspective

CD4+ T cells play a critical role in the response to chronic viral infections during the acute phase and in the partial containment of infections once chronic infection is established. As infection persists, the virus-specific CD4+ T cell response begins to shift in phenotype. The predominant change described in both mouse and human studies of chronic viral infection is a decrease in detectable T helper type (Th)1 responses. Some Th1 loss is due to decreased proliferative potential and decreased cytokine production in the setting of chronic antigen exposure. However, recent data suggest that Th1 dysfunction is accompanied by a shift in the differentiation pathway of virus-specific CD4+ T cells, with enrichment for cells with a T follicular helper cell (Tfh) phenotype. A Tfh-like program during chronic infection has now been identified in virus-specific CD8+ T cells as well. In this review, we discuss what is known about CD4+ T cell differentiation in chronic viral infections, with a focus on the emergence of the Tfh program and the implications of this shift with respect to Tfh function and the host-pathogen interaction.

Regulatory T cells characterized by low Id3 expression are highly suppressive and accumulate during chronic infection

Foxp3⁺ regulatory T (Treg) cells are broadly divided into naive-like and activated Treg cells, however recent studies suggest further Treg cell heterogeneity. Treg cells contribute to impaired T cell responses in chronic infections, but the role of specific Treg cell subpopulations in viral infections is not well defined. Here, we report that activated Treg cells are separated into two transcriptionally distinct subpopulations characterized by low or high expression of the transcriptional regulator Id3. Id3^lo Treg cells are a highly suppressive Treg cell subpopulation, expressing elevated levels of immunomodulatory molecules and are capable of broadly targeting T cell responses. Viral infection and interleukin-2 promote the differentiation of Id3^hi into Id3^lo Treg cells and during chronic infection Id3^lo Treg cells are the predominant Treg cell population. Thus, our report provides a framework, in which different activated Treg cell subpopulations specifically affect immune responses, possibly contributing to T cell dysfunction in chronic infections.

The association between clinical outcome and CD8+ lymphocytic infiltration in advanced stages of colorectal cancer differs by latent virus infection in tumour tissue

AIMS

In the near future, an immunoscore based on the quantification of lymphocytic populations can be expected as a fundamental supplement of colorectal cancer (CRC) classification. This study explored whether latent viral infection has an influence on prognostically relevant host immunity in CRC.

METHODS AND RESULTS

CD8⁺ lymphocytic infiltration in three tumour compartments of 121 CRC was compared with clinical data and occurrence of latent infection with herpes simplex virus (HSV1, HSV2), cytomegalovirus (CMV), human papillomavirus (HPV16 and HPV18) in the tumour tissue, which was determined by polymerase chain reaction (PCR). Intraepithelial CD8⁺ lymphocytic infiltration (IE_CD8+ ) showed a trend towards correlation with clinical stage (P = 0.073), significant differences between CRC with and without metastases (P = 0.001) and a significant correlation with overall survival (OS, P = 0.001). Each of these three clinical parameters showed a significant link to IE_CD8+ in the virus DNA-negative (P-values: 0.001-0.036), but no significant differences in the virus DNA-positive subgroup, which is consistent with a moderating effect of virus DNA on these associations. A significant correlation of CD8⁺ infiltration in the invasive margin (IM_CD8+ ) with OS (P = 0.016) was also moderated by virus DNA.

CONCLUSION

Our data suggest a possible influence of latent viral infection on the association between clinical outcome and CD8⁺ lymphocytic infiltration in CRC tissue. After confirmation of these results by large cohort studies, a potential interaction between microbial pathogens and host immunity in CRC and its impact on prognostic immunoscores and/or new therapeutic strategies should be investigated further.

E-NPP3 controls plasmacytoid dendritic cell numbers in the small intestine

Extracellular adenosine 5'-triphosphate (ATP) performs multiple functions including activation and induction of apoptosis of many cell types. The ATP-hydrolyzing ectoenzyme ecto-nucleotide pyrophosphatase/phosphodiesterase 3 (E-NPP3) regulates ATP-dependent chronic allergic responses by mast cells and basophils. However, E-NPP3 is also highly expressed on epithelial cells of the small intestine. In this study, we showed that E-NPP3 controls plasmacytoid dendritic cell (pDC) numbers in the intestine through regulation of intestinal extracellular ATP. In Enpp3-/- mice, ATP concentrations were increased in the intestinal lumen. pDC numbers were remarkably decreased in the small intestinal lamina propria and Peyer's patches. Intestinal pDCs of Enpp3-/- mice showed enhanced cell death as characterized by increases in annexin V binding and expression of cleaved caspase-3. pDCs were highly sensitive to ATP-induced cell death compared with conventional DCs. ATP-induced cell death was abrogated in P2rx7-/- pDCs. Accordingly, the number of intestinal pDCs was restored in Enpp3-/- P2rx7-/- mice. These findings demonstrate that E-NPP3 regulates ATP concentration and thereby prevents the decrease of pDCs in the small intestine.

Making Mouse Models That Reflect Human Immune Responses

Humans are infected with a variety of acute and chronic pathogens over the course of their lives, and pathogen-driven selection has shaped the immune system of humans. The same is likely true for mice. However, laboratory mice we use for most biomedical studies are bred in ultra-hygienic environments, and are kept free of specific pathogens. We review recent studies that indicate that pathogen infections are important for the basal level of activation and the function of the immune system. Consideration of these environmental exposures of both humans and mice can potentially improve mouse models of human disease.

Chronic Type I IFN Is Sufficient To Promote Immunosuppression through Accumulation of Myeloid-Derived Suppressor Cells

Failure of the immune system to eradicate viruses results in chronic viral infections, which are associated with increased susceptibility to secondary infections. Pathogenic HIV or lymphocytic choriomeningitis virus chronic infections display a persistent type I IFN signature. In chronic lymphocytic choriomeningitis virus infection, blockade of type I IFN signaling partially restores antiviral responses. In a mouse model, we tested whether chronic administration of type I IFN, at doses mimicking chronic viral infection, induced immunosuppression. Chronic exposure of mice to IFN-α alone was sufficient to strongly suppress specific CD8⁺ T cells responses to subsequent vaccinia virus infection. It resulted in the accumulation of Ly6C^hi monocytes. These monocytes were similar, phenotypically and functionally, to the myeloid-derived suppressor cells found in cancer because they exerted a potent suppression on CD8⁺ T cell responses in vitro. They acted at least partly through the l-arginine pathway. In vivo, their elimination restored antiviral CD8⁺ T cell responses. Our work provides a specific mechanism accounting for the role of IFN-α in immunosuppression and predicts that type I IFN modulation will be pivotal to cure human chronic infections, cancer, or autoimmune diseases.

Activation of the RLR/MAVS Signaling Pathway by the L Protein of Mopeia Virus

The family Arenaviridae includes several important human pathogens that can cause severe hemorrhagic fever and greatly threaten public health. As a major component of the innate immune system, the RLR/MAVS signaling pathway is involved in recognizing viral components and initiating antiviral activity. It has been reported that arenavirus infection can suppress the innate immune response, and NP and Z proteins of pathogenic arenaviruses can disrupt RLR/MAVS signaling, thus inhibiting production of type I interferon (IFN-I). However, recent studies have shown elevated IFN-I levels in certain arenavirus-infected cells. The mechanism by which arenavirus infection induces IFN-I responses remains unclear. In this study, we determined that the L polymerase (Lp) of Mopeia virus (MOPV), an Old World (OW) arenavirus, can activate the RLR/MAVS pathway and thus induce the production of IFN-I. This activation is associated with the RNA-dependent RNA polymerase activity of Lp. This study provides a foundation for further studies of interactions between arenaviruses and the innate immune system and for the elucidation of arenavirus pathogenesis.

IMPORTANCE

Distinct innate immune responses are observed when hosts are infected with different arenaviruses. It has been widely accepted that NP and certain Z proteins of arenaviruses inhibit the RLR/MAVS signaling pathway. The viral components responsible for the activation of the RLR/MAVS signaling pathway remain to be determined. In the current study, we demonstrate for the first time that the Lp of MOPV, an OW arenavirus, can activate the RLR/MAVS signaling pathway and thus induce the production of IFN-I. Based on our results, we proposed that dynamic interactions exist among Lp-produced RNA, NP, and the RLR/MAVS signaling pathway, and the outcome of these interactions may determine the final IFN-I response pattern: elevated or reduced. Our study provides a possible explanation for how IFN-I can become activated during arenavirus infection and may help us gain insights into the interactions that form between different arenavirus components and the innate immune system.

Inhibition of Innate Immune Responses Is Key to Pathogenesis by Arenaviruses

Mammalian arenaviruses are zoonotic viruses that cause asymptomatic, persistent infections in their rodent hosts but can lead to severe and lethal hemorrhagic fever with bleeding and multiorgan failure in human patients. Lassa virus (LASV), for example, is endemic in several West African countries, where it is responsible for an estimated 500,000 infections and 5,000 deaths annually. There are currently no FDA-licensed therapeutics or vaccines available to combat arenavirus infection. A hallmark of arenavirus infection (e.g., LASV) is general immunosuppression that contributes to high viremia. Here, we discuss the early host immune responses to arenavirus infection and the recently discovered molecular mechanisms that enable pathogenic viruses to suppress host immune recognition and to contribute to the high degree of virulence. We also directly compare the innate immune evasion mechanisms between arenaviruses and other hemorrhagic fever-causing viruses, such as Ebola, Marburg, Dengue, and hantaviruses. A better understanding of the immunosuppression and immune evasion strategies of these deadly viruses may guide the development of novel preventative and therapeutic options.

CD28 Deficiency Enhances Type I IFN Production by Murine Plasmacytoid Dendritic Cells

Type I IFNs (IFN-I) are key innate mediators that create a profound antiviral state and orchestrate the activation of almost all immune cells. Plasmacytoid dendritic cells (pDCs) are the most powerful IFN-I-producing cells and play important roles during viral infections, cancer, and autoimmune diseases. By comparing gene expression profiles of murine pDCs and conventional DCs, we found that CD28, a prototypic T cell stimulatory receptor, was highly expressed in pDCs. Strikingly, CD28 acted as a negative regulator of pDC IFN-I production upon TLR stimulation but did not affect pDC survival or maturation. Importantly, cell-intrinsic CD28 expression restrained pDC (and systemic) IFN-I production during in vivo RNA and DNA viral infections, limiting antiviral responses and enhancing viral growth early after exposure. Finally, CD28 also downregulated IFN-I response upon skin injury. Our study identified a new pDC regulatory mechanism by which the same CD28 molecule that promotes stimulation in most cells that express it is co-opted to negatively regulate pDC IFN-I production and limit innate responses.

Immunoactivation induced by chronic viral infection inhibits viral replication and drives immunosuppression through sustained IFN-I responses

Acute or chronic viral infections can lead to generalized immunosuppression. Several mechanisms, such as immunopathology of CD8⁺ T cells, inhibitory receptors, or regulatory T (Treg) cells, contribute to immune dysfunction. Moreover, patients with chronic viral infections usually do not respond to vaccination, a finding that has not been previously explained. Recently, we reported that CD169⁺ macrophages enforce viral replication, which is essential for guaranteeing antigen synthesis and efficient adaptive immune responses. In the present study, we used a chronic lymphocytic choriomeningitis virus infection mouse model to determine whether this mechanism is affected by chronic viral infection, which may impair the activation of adaptive immunity. We found that enforced viral replication of a superinfecting virus is completely blunted in chronically infected mice. This absence of enforced viral replication in CD169⁺ macrophages is not explained by CD8⁺ T‐cell‐mediated immunopathology but rather by prolonged IFN‐I responses. Consequently, the absence of viral replication impairs both antigen production and the adaptive immune response against the superinfecting virus. These findings indicate that chronic infection leads to sustained IFN‐I action, which is responsible for the absence of an antiviral immune response against a secondary viral infection.

Innate and Adaptive Immune Regulation During Chronic Viral Infections

Chronic viral infections represent a unique challenge to the infected host. Persistently replicating viruses outcompete or subvert the initial antiviral response, allowing the establishment of chronic infections that result in continuous stimulation of both the innate and adaptive immune compartments. This causes a profound reprogramming of the host immune system, including attenuation and persistent low levels of type I interferons, progressive loss (or exhaustion) of CD8(+) T cell functions, and specialization of CD4(+) T cells to produce interleukin-21 and promote antibody-mediated immunity and immune regulation. Epigenetic, transcriptional, posttranscriptional, and metabolic changes underlie this adaptation or recalibration of immune cells to the emerging new environment in order to strike an often imperfect balance between the host and the infectious pathogen. In this review we discuss the common immunological hallmarks observed across a range of different persistently replicating viruses and host species, the underlying molecular mechanisms, and the biological and clinical implications.

The Role of Virus Infection in Deregulating the Cytokine Response to Secondary Bacterial Infection

Proinflammatory cytokines are produced by macrophages and dendritic cells (DCs) after infection to stimulate T helper (Th) cells, linking innate and adaptive immunity. Virus infections can deregulate the proinflammatory cytokine response like tumor necrosis factor-α and interleukin (IL)-2, making the host more susceptible to secondary bacterial infections. Studies using various viruses such as lymphocytic choriomeningitis virus, influenza A virus, and human immunodeficiency virus have revealed several intriguing mechanisms that account for the increased susceptibility to several prevalent bacterial infections. In particular, type I interferons induced during a virus infection have been observed to play a role in suppressing the production of some key antibacterial proinflammatory cytokines such as IL-23 and IL-17. Other suppressive mechanisms as a result of cytokine deregulation by viral infections include reduced function of immune cells such as DC, macrophage, natural killer, CD4(+), and CD8(+) T cells leading to impaired clearance of secondary bacterial infections. In this study, we highlight some of the immune mechanisms that become deregulated by viral infections, and can thus become defective during secondary bacterial infections.

New insights into type I interferon and the immunopathogenesis of persistent viral infections

Most viruses generate potent T cell responses that rapidly control infection. However, certain viruses can subvert the immune response to establish persistent infections. The inability to clear virus induces an immunosuppressive program leading to the sustained expression of many immunoregulatory molecules that down-regulate T cell responses. Further, viral persistence is associated with multiple immune dysfunctions including lymphoid disorganization, defective antigen presentation, aberrant B cell responses and hypergammaglobulinemia. Although best known for its antiviral activity, recent data has highlighted the role of type I IFN (IFN-I) signaling as a central mediator of immunosuppression during viral persistence. It is also becoming increasingly apparent that many of the immune dysfunctions during persistent virus infection can be attributed directly or indirectly to the effects of chronic IFN-I signaling. This review explores the increasingly complex role of IFN-I in the regulation of immunity against persistently replicating virus infections and examines current and potential uses of IFN-I and blockade of IFN-I signaling to dampen chronic inflammation and activation in the clinic.

Inflammatory monocytes recruited to the liver within 24 hours after virus-induced inflammation resemble Kupffer cells but are functionally distinct

Due to a scarcity of immunocompetent animal models for viral hepatitis, little is known about the early innate immune responses in the liver. In various hepatotoxic models, both pro- and anti-inflammatory activities of recruited monocytes have been described. In this study, we compared the effect of liver inflammation induced by the Toll-like receptor 4 ligand lipopolysaccharide (LPS) with that of a persistent virus, lymphocytic choriomeningitis virus (LCMV) clone 13, on early innate intrahepatic immune responses in mice. LCMV infection induces a remarkable influx of inflammatory monocytes in the liver within 24 h, accompanied by increased transcript levels of several proinflammatory cytokines and chemokines in whole liver. Importantly, while a single LPS injection results in similar recruitment of inflammatory monocytes to the liver, the functional properties of the infiltrating cells are dramatically different in response to LPS versus LCMV infection. In fact, intrahepatic inflammatory monocytes are skewed toward a secretory phenotype with impaired phagocytosis in LCMV-induced liver inflammation but exhibit increased endocytic capacity after LPS challenge. In contrast, F4/80(high)-Kupffer cells retain their steady-state endocytic functions upon LCMV infection. Strikingly, the gene expression levels of inflammatory monocytes dramatically change upon LCMV exposure and resemble those of Kupffer cells. Since inflammatory monocytes outnumber Kupffer cells 24 h after LCMV infection, it is highly likely that inflammatory monocytes contribute to the intrahepatic inflammatory response during the early phase of infection. Our findings are instrumental in understanding the early immunological events during virus-induced liver disease and point toward inflammatory monocytes as potential target cells for future treatment options in viral hepatitis.

IMPORTANCE

Insights into how the immune system deals with hepatitis B virus (HBV) and HCV are scarce due to the lack of adequate animal model systems. This knowledge is, however, crucial to developing new antiviral strategies aimed at eradicating these chronic infections. We model virus-host interactions during the initial phase of liver inflammation 24 h after inoculating mice with LCMV. We show that infected Kupffer cells are rapidly outnumbered by infiltrating inflammatory monocytes, which secrete proinflammatory cytokines but are less phagocytic. Nevertheless, these recruited inflammatory monocytes start to resemble Kupffer cells on a transcript level. The specificity of these cellular changes for virus-induced liver inflammation is corroborated by demonstrating opposite functions of monocytes after LPS challenge. Overall, this demonstrates the enormous functional and genetic plasticity of infiltrating monocytes and identifies them as an important target cell for future treatment regimens.

c-FLIP-Short reduces type I interferon production and increases viremia with coxsackievirus B3

Cellular FLIP (c-FLIP) is an enzymatically inactive paralogue of caspase-8 and as such can block death receptor-induced apoptosis. However, independent of death receptors, c-FLIP-Long (c-FLIP_L) can heterodimerize with and activate caspase-8. This is critical for promoting the growth and survival of T lymphocytes as well as the regulation of the RIG-I helicase pathway for type I interferon production in response to viral infections. Truncated forms of FLIP also exist in mammalian cells (c-FLIP_S) and certain viruses (v-FLIP), which lack the C-terminal domain that activates caspase-8. Thus, the ratio of c-FLIP_L to these short forms of FLIP may greatly influence the outcome of an immune response. We examined this model in mice transgenically expressing c-FLIP_S in T cells during infection with Coxsackievirus B3 (CVB3). In contrast to our earlier findings of reduced myocarditis and mortality with CVB3 infection of c-FLIP_L-transgenic mice, c-FLIP_S-transgenic mice were highly sensitive to CVB3 infection as manifested by increased cardiac virus titers, myocarditis score, and mortality compared to wild-type C57BL/6 mice. This observation was paralleled by a reduction in serum levels of IL-10 and IFN-α in CVB3-infected c-FLIP_S mice. In vitro infection of c-FLIP_S T cells with CVB3 confirmed these results. Furthermore, molecular studies revealed that following infection of cells with CVB3, c-FLIP_L associates with mitochondrial antiviral signaling protein (MAVS), increases caspase-8 activity and type I IFN production, and reduces viral replication, whereas c-FLIP_S promotes the opposite phenotype.

Molecular and transcriptional basis of CD4⁺ T cell dysfunction during chronic infection

T cell exhaustion is common during chronic infections. Although CD4(+) T cells are critical for controlling viral load during chronic viral infections, less is known about their differentiation and transcriptional program. We defined the phenotypic, functional, and molecular profiles of exhausted CD4(+) T cells. Global transcriptional analysis demonstrated a molecular profile distinct from effector and memory CD4(+) T cells and also from exhausted CD8(+) T cells, though some common features of CD4(+) and CD8(+) T cell exhaustion were revealed. We have demonstrated unappreciated roles for transcription factors (TFs) including Helios, type I interferon (IFN-I) signaling, and a diverse set of coinhibitory and costimulatory molecules during CD4(+) T cell exhaustion. Moreover, the signature of CD4(+) T cell exhaustion was found to be distinct from that of other CD4(+) T cell lineage subsets and was associated with TF heterogeneity. This study provides a framework for therapeutic interventions targeting exhausted CD4(+) T cells.

Functional limitations of plasmacytoid dendritic cells limit type I interferon, T cell responses and virus control in early life

Infant mortality from viral infection remains a major global health concern: viruses causing acute infections in immunologically mature hosts often follow a more severe course in early life, with prolonged or persistent viral replication. Similarly, the WE strain of lymphocytic choriomeningitis virus (LCMV-WE) causes acute self-limiting infection in adult mice but follows a protracted course in infant animals, in which LCMV-specific CD8⁺ T cells fail to expand and control infection. By disrupting type I IFNs signaling in adult mice or providing IFN-α supplementation to infant mice, we show here that the impaired early life T cell responses and viral control result from limited early type I IFN responses. We postulated that plasmacytoid dendritic cells (pDC), which have been identified as one major source of immediate-early IFN-I, may not exert adult-like function in vivo in the early life microenvironment. We tested this hypothesis by studying pDC functions in vivo during LCMV infection and identified a coordinated downregulation of infant pDC maturation, activation and function: despite an adult-like in vitro activation capacity of infant pDCs, the expression of the E2-2 pDC master regulator (and of critical downstream antiviral genes such as MyD88, TLR7/TLR9, NF-κB, IRF7 and IRF8) is downregulated in vivo at baseline and during LCMV infection. A similar pattern was observed in response to ssRNA polyU, a model ligand of the TLR7 viral sensor. This suggests that the limited T cell-mediated defense against early life viral infections is largely attributable to / regulated by infant pDC responses and provides incentives for novel strategies to supplement or stimulate immediate-early IFN-α responses.

Human plasmacytoid dendritic cells sense lymphocytic choriomeningitis virus-infected cells in vitro

We previously reported that exosomal transfer of hepatitis C virus (HCV) positive-strand RNA from human Huh-7 hepatoma cells to human plasmacytoid dendritic cells (pDCs) triggers pDC alpha/beta interferon (IFN-α/β) production in a Toll-like receptor 7 (TLR7)-dependent, virus-independent manner. Here we show that human pDCs are also activated by a TLR7-dependent, virus-independent, exosomal RNA transfer mechanism by human and mouse hepatoma and nonhepatoma cells that replicate the negative-strand lymphocytic choriomeningitis virus (LCMV).

Negative regulation of type I IFN expression by OASL1 permits chronic viral infection and CD8⁺ T-cell exhaustion

The type I interferons (IFN-Is) are critical not only in early viral control but also in prolonged T-cell immune responses. However, chronic viral infections such as those of human immunodeficiency virus (HIV) and hepatitis C virus (HCV) in humans and lymphocytic choriomeningitis virus (LCMV) in mice overcome this early IFN-I barrier and induce viral persistence and exhaustion of T-cell function. Although various T-cell-intrinsic and -extrinsic factors are known to contribute to induction of chronic conditions, the roles of IFN-I negative regulators in chronic viral infections have been largely unexplored. Herein, we explored whether 2'-5' oligoadenylate synthetase-like 1 (OASL1), a recently defined IFN-I negative regulator, plays a key role in the virus-specific T-cell response and viral defense against chronic LCMV. To this end, we infected Oasl1 knockout and wild-type mice with LCMV CL-13 (a chronic virus) and monitored T-cell responses, serum cytokine levels, and viral titers. LCMV CL-13-infected Oasl1 KO mice displayed a sustained level of serum IFN-I, which was primarily produced by splenic plasmacytoid dendritic cells, during the very early phase of infection (2-3 days post-infection). Oasl1 deficiency also led to the accelerated elimination of viremia and induction of a functional antiviral CD8 T-cell response, which critically depended on IFN-I receptor signaling. Together, these results demonstrate that OASL1-mediated negative regulation of IFN-I production at an early phase of infection permits viral persistence and suppresses T-cell function, suggesting that IFN-I negative regulators, including OASL1, could be exciting new targets for preventing chronic viral infection.

T-cell exhaustion due to persistent antigen: quantity not quality?

T-cell exhaustion is characterized by failure to respond to a persistent antigen and is a hallmark of chronic infections and cancer. Here, we discuss several recent reports on T-cell exhaustion, including one in this issue of the European Journal of Immunology where Richter et al. [Eur. J. Immunol. 42:2290-2304] examine the importance of the amount of persistent antigen versus the cell type presenting it to induce CD8(+) T-cell exhaustion, and the consequences for host survival during chronic viral infection.

CD8+ T cell-derived IFN-γ prevents infection by a second heterologous virus

Persistent viral infection is often associated with dysfunctional immune responses against unrelated pathogens. Lymphocytic choriomeningitis virus (LCMV) can establish acute or chronic infections in mice and is widely used as a model for persistent virus infections in humans. Mice infected with LCMV develop a transient defect in Ag-specific immunity against heterologous viral infection. Although it has been proposed that LCMV infection induces an immunosuppressed state within the host, our data show that infected mice successfully clear vaccinia virus through a mechanism that involves CD8(+) T cell-derived IFN-γ. This observation demonstrates that chronic LCMV infection does not impair protective immunity against heterologous viral challenge. Rather, a natural sterilizing immunity is induced following a primary infection that prevents a secondary infection. Our findings suggest a need to re-evaluate current thoughts about the immune suppression that might occur during a persistent infection.

Plasmacytoid dendritic cells are productively infected and activated through TLR-7 early after arenavirus infection

The antiviral response is largely mediated by dendritic cells (DCs), including conventional (c) DCs that function as antigen-presenting cells, and plasmacytoid (p) DCs that produce type I interferons, making them an attractive target for viruses. We find that the Old World arenaviruses lymphocytic choriomeningitis virus clone 13 (LCMV Cl13) and Lassa virus bind pDCs to a greater extent than cDCs. Consistently, LCMV Cl13 targets pDCs early after in vivo infection of its natural murine host and establishes a productive and robust replication cycle. pDCs coproduce type I interferons and proinflammatory cytokines, with the former being induced in both infected and uninfected pDCs, demonstrating a dissociation from intrinsic virus replication. TLR7 globally mediates pDC responses, limits pDC viral load, and promotes rapid innate and adaptive immune cell activation. These early events likely help dictate the outcome of infections with arenaviruses and other DC-replicating viruses and shed light on potential therapeutic targets.

Timing and magnitude of type I interferon responses by distinct sensors impact CD8 T cell exhaustion and chronic viral infection

Type I interferon (IFN-I) promotes antiviral CD8(+)T cell responses, but the contribution of different IFN-I sources and signaling pathways are ill defined. While plasmacytoid dendritic cells (pDCs) produce IFN-I upon TLR stimulation, IFN-I is induced in most cells by helicases like MDA5. Using acute and chronic lymphocytic choriomeningitis virus (LCMV) infection models, we determined that pDCs transiently produce IFN-I that minimally impacts CD8(+)T cell responses and viral persistence. Rather, MDA5 is the key sensor that induces IFN-I required for CD8(+)T cell responses. In the absence of MDA5, CD8(+)T cell responses to acute infection rely on CD4(+)T cell help, and loss of both CD4(+)T cells and MDA5 results in CD8(+)T cell exhaustion and persistent infection. Chronic LCMV infection rapidly attenuates IFN-I responses, but early administration of exogenous IFN-I rescues CD8(+)T cells, promoting viral clearance. Thus, effective antiviral CD8(+)T cell responses depend on the timing and magnitude of IFN-I production.

Wild-type coxsackievirus infection dramatically alters the abundance, heterogeneity, and immunostimulatory capacity of conventional dendritic cells in vivo

In vitro studies have shown that enteroviruses employ strategies that may impair the ability of DCs to trigger T cell immunity, but it is unclear how these viruses affect DCs in vivo. Here, we evaluate the effects of wild-type (wt) coxsackievirus B3 on DCs in vitro and in a murine model in vivo. Although CVB3 does not productively infect the vast majority of DCs, virus infection profoundly reduces splenic conventional DC numbers and diminishes their capacity to prime naïve CD8(+) T cells in vitro. In contrast to recombinant CVB3, highly pathogenic wt virus infection significantly diminishes the host's capacity to mount T cell responses, which is temporally associated with the loss of CD8α(+) DCs. Our findings demonstrate that enterovirus infection substantially alters the number, heterogeneity, and stimulatory capacity of DCs in vivo, and these dramatic immunomodulatory effects may weaken the host's capacity to mount antiviral T cell responses.

Plasmacytoid dendritic cells control T-cell response to chronic viral infection

Infections with persistent viruses are a frequent cause of immunosuppression, autoimmune sequelae, and/or neoplastic disease. Plasmacytoid dendritic cells (pDCs) are innate immune cells that produce type I interferon (IFN-I) and other cytokines in response to virus-derived nucleic acids. Persistent viruses often cause depletion or functional impairment of pDCs, but the role of pDCs in the control of these viruses remains unclear. We used conditional targeting of pDC-specific transcription factor E2-2 to generate mice that constitutively lack pDCs in peripheral lymphoid organs and tissues. The profound impact of pDC deficiency on innate antiviral responses was revealed by the failure to control acute infection with the cytopathic mouse hepatitis virus. Furthermore, pDC-deficient animals failed to clear lymphocytic choriomeningitis virus (LCMV) from hematopoietic organs during persistent LCMV infection. This failure was associated with reduced numbers and functionality of LCMV-specific CD4(+) helper T cells and impaired antiviral CD8(+) T-cell responses. Adoptive transfer of LCMV-specific T cells revealed that both CD4(+) and CD8(+) T cells required IFN-I for expansion, but only CD4(+) T cells required the presence of pDCs. In contrast, mice with pDC-specific loss of MHC class II expression supported normal CD4(+) T-cell response to LCMV. These data suggest that pDCs facilitate CD4(+) helper T-cell responses to persistent viruses independently of direct antigen presentation. Thus pDCs provide an essential link between innate and adaptive immunity to chronic viral infection, likely through the secretion of IFN-I and other cytokines.

Type I interferons: diversity of sources, production pathways and effects on immune responses

Type I interferons (IFN-I) were first described over 50 years ago as factors produced by cells that interfere with virus replication and promote an antiviral state. Innate and adaptive immune responses to viruses are also greatly influenced by IFN-I. In this article we discuss the diversity of cellular sources of IFN-I and the pathways leading to IFN-I production during viral infections. Finally, we discuss the effects of IFN-I on cells of the immune system with emphasis on dendritic cells.

Type I interferon negatively controls plasmacytoid dendritic cell numbers in vivo

Production of type I interferon in response to virus infection reduces plasmacytoid dendritic cell numbers, thus creating a negative feedback loop.

Plasmacytoid dendritic cells (pDCs) specialize in the secretion of type I interferons (IFN-I) and thus are considered critical mediators of antiviral responses. We recently reported that pDCs have a very early but limited and transient capacity to curtail viral infections. Additionally, pDC numbers are not sustained in human infections caused by Hepatitis B or C viruses (HBV and HCV) and HIV. Thus, the numbers and/or function of pDCs appear to be regulated during the course of viral infection. In this study, we show that splenic pDCs are reduced in vivo during several systemic viral infections and after administration of synthetic toll-like receptor ligands. We demonstrate that IFN-I, regardless of the source, contributes to this decline and mediates pDC death via the intrinsic apoptosis pathway. These findings demonstrate a feedback control mechanism by which IFN-I modulates pDC numbers, thus fine-tuning systemic IFN-I response to viruses. IFN-I–mediated control of pDCs may explain the loss of pDCs during human infections caused by HBV, HCV, or HIV and has important therapeutic implications for settings in which IFN-I is used to treat infections and autoimmune diseases.

Systemic responses during local viral infections: type I IFNs sound the alarm

Type I IFNs are well known for their role in controlling virus replication and spread. Type I IFNs produced by the infected tissue also signal beyond the boundaries of the infection to regulate different elements of the anti-viral immune response. Recent reports show that type I IFNs directly condition naive monocytes residing in the distal bone marrow (BM) and induce the expression of effector molecules in memory T cells, before their recruitment to the infected site. In addition, hematopoietic stem cells (HSCs) were shown to enter the cell cycle in response to systemically distributed type I IFNs. These discoveries expand our understanding of the pleiotropic effects of type I IFNs during infection and highlight the critical role of systemic signals in the development of an effective response to a localized viral infection.

Plasmacytoid dendritic cells: recent progress and open questions

Plasmacytoid dendritic cells (pDCs) are specialized in rapid and massive secretion of type I interferon (IFN-α/β) in response to foreign nucleic acids. Combined with their antigen presentation capacity, this powerful functionality enables pDCs to orchestrate innate and adaptive immune responses. pDCs combine features of both lymphocytes and classical dendritic cells and display unique molecular adaptations to nucleic acid sensing and IFN production. In the decade since the identification of the pDC as a distinct immune cell type, our understanding of its molecular underpinnings and role in immunity has progressed rapidly. Here we review select aspects of pDC biology including cell fate establishment and plasticity, specific molecular mechanisms of pDC function, and the role of pDCs in T cell responses, antiviral immunity, and autoimmune diseases. Important unresolved questions remain in these areas, promising exciting times in pDC research for years to come.

Type I interferon drives dendritic cell apoptosis via multiple BH3-only proteins following activation by PolyIC in vivo

Background

DC are activated by pathogen-associated molecular patterns (PAMPs), and this is pivotal for the induction of adaptive immune responses. Thereafter, the clearance of activated DC is crucial to prevent immune pathology. While PAMPs are of major interest for vaccine science due to their adjuvant potential, it is unclear whether and how PAMPs may affect DC viability. We aimed to elucidate the possible apoptotic mechanisms that control activated DC lifespan in response to PAMPs, particularly in vivo.

Methodology/Principal Findings

We report that polyinosinic:polycytidylic acid (PolyIC, synthetic analogue of dsRNA) induces dramatic apoptosis of mouse splenic conventional DC (cDC) in vivo, predominantly affecting the CD8α subset, as shown by flow cytometry-based analysis of splenic DC subsets. Importantly, while Bim deficiency conferred only minor protection, cDC depletion was prevented in mice lacking Bim plus one of three other BH3-only proteins, either Puma, Noxa or Bid. Furthermore, we show that Type I Interferon (IFN) is necessary and sufficient for DC death both in vitro and in vivo, and that TLR3 and MAVS co-operate in IFNß production in vivo to induce DC death in response to PolyIC.

Conclusions/Significance

These results demonstrate for the first time in vivo that apoptosis restricts DC lifespan following activation by PolyIC, particularly affecting the CD8α cDC subset. Such DC apoptosis is mediated by the overlapping action of pro-apoptotic BH3-only proteins, including but not solely involving Bim, and is driven by Type I IFN. While Type I IFNs are important anti-viral factors, CD8α cDC are major cross-presenting cells and critical inducers of CTL. We discuss such paradoxical finding on DC death with PolyIC/Type I IFN. These results could contribute to understand immunosuppression associated with chronic infection, and to the optimization of DC-based therapies and the clinical use of PAMPs and Type I IFNs.

Inhibition of the type I interferon antiviral response during arenavirus infection

Arenaviruses merit interest both as tractable experimental model systems to study acute and persistent viral infections, and as clinically-important human pathogens. Several arenaviruses cause hemorrhagic fever (HF) disease in humans. In addition, evidence indicates that the globally-distributed prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a human pathogen of clinical significance in congenital infections, and also poses a great danger to immunosuppressed individuals. Arenavirus persistence and pathogenesis are facilitated by their ability to overcome the host innate immune response. Mammalian hosts have developed both membrane toll-like receptors (TLR) and cytoplasmic pattern recognition receptors (PRRs) that recognize specific pathogen-associated molecular patterns (PAMPs), resulting in activation of the transcription factors IRF3 or IRF7, or both, which together with NF-κB and ATF-2/c-JUN induce production of type I interferon (IFN-I). IFN-I plays a key role in host anti-microbial defense by mediating direct antiviral effects via up-regulation of IFN-I stimulated genes (ISGs), activating dendritic cells (DCs) and natural killer (NK) cells, and promoting the induction of adaptive responses. Accordingly, viruses have developed a plethora of strategies to disrupt the IFN-I mediated antiviral defenses of the host, and the viral gene products responsible for these disruptions are often major virulence determinants. IRF3- and IRF7-dependent induction of host innate immune responses is frequently targeted by viruses. Thus, the arenavirus nucleoprotein (NP) was shown to inhibit the IFN-I response by interfering with the activation of IRF3. This NP anti-IFN activity, together with alterations in the number and function of DCs observed in mice chronically infected with LCMV, likely play an important role in LCMV persistence in its murine host. In this review we will discuss current knowledge about the cellular and molecular mechanisms by which arenaviruses can subvert the host innate immune response and their implications for understanding HF arenaviral disease as well as arenavirus persistence in their natural hosts.

Type I interferon modulates the battle of host immune system against viruses

Type I interferon (IFN), as its name implies, 'interferes' with virus replication by activating numerous genes. Further, virus-induced type I IFN regulates the magnitude and functions of cells directing the host immune system. Importantly, recent exploration into how type I IFN operates following virus infection has advanced our understanding of its role with respect to modulation of host innate and adaptive immune responses. Such activities include the activation of antigen-presenting dendritic cells and the localization, expansion or differentiation of virus-specific T lymphocytes and antibody-producing B lymphocytes. However, type I IFN not only benefits the host but can also induce unnecessary or extremely pathogenic immune responses. This review focuses on such interactions and the manner in which type I IFN induces dynamic changes in the host immune network, particularly adaptive immune responses to viral invasion. Manipulating the type I IFN-mediated host immune response during virus infections could provide new immunotherapeutic interventions to remedy viral diseases and implement more effective and sustainable type I IFN therapy.

Induction and inhibition of type I interferon responses by distinct components of lymphocytic choriomeningitis virus

Type I interferons (IFNs) play a critical role in the host defense against viruses. Lymphocytic choriomeningitis virus (LCMV) infection induces robust type I IFN production in its natural host, the mouse. However, the mechanisms underlying the induction of type I IFNs in response to LCMV infection have not yet been clearly defined. In the present study, we demonstrate that IRF7 is required for both the early phase (day 1 postinfection) and the late phase (day 2 postinfection) of the type I IFN response to LCMV, and melanoma differentiation-associated gene 5 (MDA5)/mitochondrial antiviral signaling protein (MAVS) signaling is crucial for the late phase of the type I IFN response to LCMV. We further demonstrate that LCMV genomic RNA itself (without other LCMV components) is able to induce type I IFN responses in various cell types by activation of the RNA helicases retinoic acid-inducible gene I (RIG-I) and MDA5. We also show that expression of the LCMV nucleoprotein (NP) inhibits the type I IFN response induced by LCMV RNA and other RIG-I/MDA5 ligands. These virus-host interactions may play important roles in the pathogeneses of LCMV and other human arenavirus diseases.

Viperin is highly induced in neutrophils and macrophages during acute and chronic lymphocytic choriomeningitis virus infection

Although most cells are thought to respond to IFNs, there is limited information regarding specific cells that respond in vivo. Viperin is an IFN-induced antiviral protein and, therefore, is an excellent marker for IFN-responsive cells. In this study, we analyzed viperin expression in vivo during acute lymphocytic choriomeningitis virus Armstrong infection, which induces high levels of type I IFNs, and in persistently infected lymphocytic choriomeningitis virus carrier mice, which contain low levels of type I IFNs. Viperin was induced in lymphoid cells and dendritic cells (DCs) during acute infection and highly induced in neutrophils and macrophages. The expression kinetics in neutrophils, macrophages, and T and B cells paralleled IFN-alpha levels, but DCs expressed viperin with delayed kinetics. In carrier mice, viperin was expressed in neutrophils and macrophages but not in T and B cells or DCs. For acutely infected and carrier mice, viperin expression was IFN dependent, because treating type I IFNR knockout mice with IFN-gamma-neutralizing Abs inhibited viperin expression. Viperin localized to the endoplasmic reticulum and lipid droplet-like vesicles in neutrophils. These findings delineate the kinetics and cells responding to IFNs in vivo and suggest that the profile of IFN-responsive cells changes in chronic infections. Furthermore, these data suggest that viperin may contribute to the antimicrobial activity of neutrophils.