Cell-Cell Sensing of Viral Infection by Plasmacytoid Dendritic Cells

Sensing of an HIV-1-Derived Single-Stranded RNA-Oligonucleotide Induces Arginase 1-Mediated Tolerance

Small synthetic oligodeoxynucleotides (ODNs) can mimic microbial nucleic acids by interacting with receptor systems and promoting immunostimulatory activities. Nevertheless, some ODNs can act differently on the plasmacytoid dendritic cell (pDC) subset, shaping their immunoregulatory properties and rendering them suitable immunotherapeutic tools in several clinical settings for treating overwhelming immune responses. We designed HIV-1-derived, DNA- and RNA-based oligonucleotides (gag, pol, and U5 regions) and assessed their activity in conferring a tolerogenic phenotype to pDCs in skin test experiments. RNA-but not DNA-oligonucleotides are capable of inducing tolerogenic features in pDCs. Interestingly, sensing the HIV-1-derived single-stranded RNA-gag oligonucleotide (RNA-gag) requires both TLR3 and TLR7 and the engagement of the TRIF adaptor molecule. Moreover, the induction of a suppressive phenotype in pDCs by RNA-gag is contingent upon the induction and activation of the immunosuppressive enzyme Arginase 1. Thus, our data suggest that sensing of the synthetic RNA-gag oligonucleotide in pDCs can induce a suppressive phenotype in pDCs, a property rendering RNA-gag a potential tool for therapeutic strategies in allergies and autoimmune diseases.

Targeting the dendritic cell-T cell axis to develop effective immunotherapies for glioblastoma

Glioblastoma is an aggressive primary brain tumor that has seen few advances in treatments for over 20 years. In response to this desperate clinical need, multiple immunotherapy strategies are under development, including CAR-T cells, immune checkpoint inhibitors, oncolytic viruses and dendritic cell vaccines, although these approaches are yet to yield significant clinical benefit. Potential reasons for the lack of success so far include the immunosuppressive tumor microenvironment, the blood-brain barrier, and systemic changes to the immune system driven by both the tumor and its treatment. Furthermore, while T cells are essential effector cells for tumor control, dendritic cells play an equally important role in T cell activation, and emerging evidence suggests the dendritic cell compartment may be deeply compromised in glioblastoma patients. In this review, we describe the immunotherapy approaches currently under development for glioblastoma and the challenges faced, with a particular emphasis on the critical role of the dendritic cell-T cell axis. We suggest a number of strategies that could be used to boost dendritic cell number and function and propose that the use of these in combination with T cell-targeting strategies could lead to successful tumor control.

Severe COVID-19 patients have impaired plasmacytoid dendritic cell-mediated control of SARS-CoV-2

Type I and III interferons (IFN-I/λ) are important antiviral mediators against SARS-CoV-2 infection. Here, we demonstrate that plasmacytoid dendritic cells (pDC) are the predominant IFN-I/λ source following their sensing of SARS-CoV-2-infected cells. Mechanistically, this short-range sensing by pDCs requires sustained integrin-mediated cell adhesion with infected cells. In turn, pDCs restrict viral spread by an IFN-I/λ response directed toward SARS-CoV-2-infected cells. This specialized function enables pDCs to efficiently turn-off viral replication, likely via a local response at the contact site with infected cells. By exploring the pDC response in SARS-CoV-2 patients, we further demonstrate that pDC responsiveness inversely correlates with the severity of the disease. The pDC response is particularly impaired in severe COVID-19 patients. Overall, we propose that pDC activation is essential to control SARS-CoV-2-infection. Failure to develop this response could be important to understand severe cases of COVID-19.

Tracking Plasmacytoid Dendritic Cell Response to Physical Contact with Infected Cells

Dendritic cells (DCs) are key regulators of both innate and adaptive immunity via varied functions, including cytokine production and antigen presentation. Plasmacytoid DC (pDC) is a DC subset specialized in the production of type I and III interferons (IFNs). They are thus pivotal players of the host antiviral response during the acute phase of infection by genetically distant viruses. The pDC response is primarily triggered by the endolysosomal sensors Toll-like receptors, which recognize nucleic acids from pathogens. In some pathologic contexts, pDC response can also be triggered by host nucleic acids, hereby contributing to the pathogenesis of autoimmune diseases, such as, e.g., systemic lupus erythematosus. Importantly, recent in vitro studies from our laboratory and others uncovered that pDCs sense viral infections when a physical contact is established with infected cells. This specialized synapse-like feature enables a robust type I and III IFN secretion at the infected site. Therefore, this concentrated and confined response likely limits the correlated deleterious impacts of excessive cytokine production to the host, notably due to tissue damages. Here we provide a pipeline of methods for ex vivo studies of pDC antiviral functions, designed to address how pDC activation is regulated by cell-cell contact with virally infected cells and the current approaches enabling to decipher the underlying molecular events leading to an efficient antiviral response.

Surviving the Storm: Cytokine Biosignature in SARS-CoV-2 Severity Prediction

Simple Summary

The world has been stricken mentally, physically, and economically by the COVID-19 virus. However, while SARS-CoV-2 viral infection results in mild flu-like symptoms in most patients, a number of those infected develop severe illness. These patients require hospitalization and intensive care. The severe disease can spiral downwards with eventual severe damage to the lungs and failure of multiple organs, leading to the individual’s demise. It is necessary to identify those who are developing a severe form of illness to provide early management. Therefore, it is crucial to learn about the mechanisms and chemical mediators that lead to critical conditions in SARS-CoV-2 infection. This paper reviews studies regarding the individual chemical mediators, pathways, and means that contribute to worsening health conditions in SARS-CoV-2 infection.

Abstract

A significant part of the world population has been affected by the devastating SARS-CoV-2 infection. It has deleterious effects on mental and physical health and global economic conditions. Evidence suggests that the pathogenesis of SARS-CoV-2 infection may result in immunopathology such as neutrophilia, lymphopenia, decreased response of type I interferon, monocyte, and macrophage dysregulation. Even though most individuals infected with the SARS-CoV-2 virus suffer mild symptoms similar to flu, severe illness develops in some cases, including dysfunction of multiple organs. Excessive production of different inflammatory cytokines leads to a cytokine storm in COVID-19 infection. The large quantities of inflammatory cytokines trigger several inflammation pathways through tissue cell and immune cell receptors. Such mechanisms eventually lead to complications such as acute respiratory distress syndrome, intravascular coagulation, capillary leak syndrome, failure of multiple organs, and, in severe cases, death. Thus, to devise an effective management plan for SARS-CoV-2 infection, it is necessary to comprehend the start and pathways of signaling for the SARS-CoV-2 infection-induced cytokine storm. This article discusses the current findings of SARS-CoV-2 related to immunopathology, the different paths of signaling and other cytokines that result in a cytokine storm, and biomarkers that can act as early signs of warning for severe illness. A detailed understanding of the cytokine storm may aid in the development of effective means for controlling the disease’s immunopathology. In addition, noting the biomarkers and pathophysiology of severe SARS-CoV-2 infection as early warning signs can help prevent severe complications.

TP53 Gene Therapy as a Potential Treatment for Patients with COVID-19

SGT-53 is a novel investigational agent that comprises an immunoliposome carrying a plasmid vector driving expression of the human TP53 gene that encodes wild-type human p53. SGT-53 is currently in phase II human trials for advanced pancreatic cancer. Although p53 is best known as a tumor suppressor, its participation in both innate and adaptive immune responses is well documented. It is now clear that p53 is an important component of the host response to various viral infections. To facilitate their viral life cycles, viruses have developed a diverse repertoire of strategies for counteracting the antiviral activities of host immune system by manipulating p53-dependent pathways in host cells. Coronaviruses reduce endogenous p53 levels in the cells they infect by enhancing the degradation of p53 in proteasomes. Thus, interference with p53 function is an important component in viral pathogenesis. Transfection of cells by SGT-53 has been shown to transiently produce exogenous p53 that is active as a pleiotropic transcription factor. We herein summarize the rationale for repurposing SGT-53 as a therapy for infection by SARS-CoV-2, the pathogen responsible for the COVID-19 pandemic. Because p53 regulation was found to play a crucial role in different infection stages of a wide variety of viruses, it is rational to believe that restoring p53 function based on SGT-53 treatment may lead to beneficial therapeutic outcomes for infectious disease at large including heretofore unknown viral pathogens that may emerge in the future.

Lactococcus lactis Strain Plasma Intake Suppresses the Incidence of Dengue Fever-like Symptoms in Healthy Malaysians: A Randomized, Double-Blind, Placebo-Controlled Trial

Dengue fever (DF) is a mosquito-borne disease still with no effective treatment or vaccine available. A randomized, placebo-controlled, double-blinded, parallel-group trial was undertaken to evaluate the efficacy of oral intake of Lactococcus lactis strain plasma (LC-Plasma) on the presentation and severity of DF-like symptoms among healthy volunteers. Study participants (320) were assigned into two groups, and consumed either placebo or LC-Plasma tablets (approximately 100 billion cells/day) for 8 weeks. The clinical symptoms of DF were self-recorded through questionnaires, and exposure to DENV was determined by serum antibody and/or DENV antigen tests. No significant differences between groups were observed for exposure to DENV, or the symptomatic ratio. Results obtained showed that participants from the LC-Plasma group reported a significant reduction in the cumulative incidence days of DF-like symptoms, which include fever (p < 0.001), muscle pain (p < 0.005), joint pain (p < 0.001), and pain behind the eyes (p < 0.001), compared to that of the placebo group. Subgroup analysis revealed a significantly (p < 0.05) reduced severity score in the LC-Plasma group when study sites were separately analyzed. Overall, our findings suggest that LC-Plasma supplementation reduces the cumulative days with DF-like symptoms, and the severity of the symptoms. Daily oral intake of LC-Plasma, hence, is shown to mitigate the DF-like symptoms.

CXCR4 blockade reduces the severity of murine heart allograft rejection by plasmacytoid dendritic cell-mediated immune regulation

Allograft-specific regulatory T cells (T_reg cells) are crucial for long-term graft acceptance after transplantation. Although adoptive T_reg cell transfer has been proposed, major challenges include graft-specificity and stability. Thus, there is an unmet need for the direct induction of graft-specific T_reg cells. We hypothesized a synergism of the immunotolerogenic effects of rapamycin (mTOR inhibition) and plerixafor (CXCR4 antagonist) for T_reg cell induction. Thus, we performed fully-mismatched heart transplantations and found combination treatment to result in prolonged allograft survival. Moreover, fibrosis and myocyte lesions were reduced. Although less CD3⁺ T cell infiltrated, higher T_reg cell numbers were observed. Noteworthy, this was accompanied by a plerixafor-dependent plasmacytoid dendritic cells-(pDCs)-mobilization. Furthermore, in vivo pDC-depletion abrogated the plerixafor-mediated T_reg cell number increase and reduced allograft survival. Our pharmacological approach allowed to increase T_reg cell numbers due to pDC-mediated immune regulation. Therefore pDCs can be an attractive immunotherapeutic target in addition to plerixafor treatment.

Differential infection of murine and human dendritic cell subsets by oncolytic vesicular stomatitis virus variants

Oncolytic viruses (OVs) can eradicate tumor cells and elicit antitumor immunity. VSV-GP, a chimeric vesicular stomatitis virus (VSV) with the glycoprotein (GP) of the lymphocytic choriomeningitis virus, is a promising new OV candidate. However, the interaction of VSV-GP with host immune cells is not fully understood. Dendritic cells (DCs) are essential for inducing efficient antitumor immunity. Thus, we aimed to investigate the interaction of VSV-GP with different murine and human DCs subsets in direct comparison to the less cytopathic variant VSV-dM51-GP and wild type VSV. Immature murine bone marrow-derived DCs (BMDCs) were equally infected and killed by VSV and VSV-GP. Human monocyte-derived DCs (moDCs) were more permissive to VSV. Interestingly, VSV-dM51-GP induced maturation instead of killing in both BMDCs and moDCs as well as a pronounced release of pro-inflammatory cytokines. Importantly, matured BMDCs and moDCs were no longer susceptible to VSV-GP infection. Mouse splenic conventional DC type 1 (cDC1) could be infected ex vivo by VSV and VSV-GP to a higher extent than cDC2. Systemic infection of mice with VSV-GP and VSV-dM51-GP resulted in strong activation of cDCs despite low infection rates in spleen and tumor tissue. Human blood cDC1 were equally infected by VSV and VSV-GP, whereas cDC2 showed preferential infection with VSV. Our study demonstrated differential DC infection, activation, and cytokine production after the treatment with VSV and VSV-GP variants among species and subsets, which should be taken into account when investigating immunological mechanisms of oncolytic virotherapy in mouse models and human clinical trials.

The signal pathways and treatment of cytokine storm in COVID-19

The Coronavirus Disease 2019 (COVID-19) pandemic has become a global crisis and is more devastating than any other previous infectious disease. It has affected a significant proportion of the global population both physically and mentally, and destroyed businesses and societies. Current evidence suggested that immunopathology may be responsible for COVID-19 pathogenesis, including lymphopenia, neutrophilia, dysregulation of monocytes and macrophages, reduced or delayed type I interferon (IFN-I) response, antibody-dependent enhancement, and especially, cytokine storm (CS). The CS is characterized by hyperproduction of an array of pro-inflammatory cytokines and is closely associated with poor prognosis. These excessively secreted pro-inflammatory cytokines initiate different inflammatory signaling pathways via their receptors on immune and tissue cells, resulting in complicated medical symptoms including fever, capillary leak syndrome, disseminated intravascular coagulation, acute respiratory distress syndrome, and multiorgan failure, ultimately leading to death in the most severe cases. Therefore, it is clinically important to understand the initiation and signaling pathways of CS to develop more effective treatment strategies for COVID-19. Herein, we discuss the latest developments in the immunopathological characteristics of COVID-19 and focus on CS including the current research status of the different cytokines involved. We also discuss the induction, function, downstream signaling, and existing and potential interventions for targeting these cytokines or related signal pathways. We believe that a comprehensive understanding of CS in COVID-19 will help to develop better strategies to effectively control immunopathology in this disease and other infectious and inflammatory diseases.

New insights into HDV persistence: The role of interferon response and implications for upcoming novel therapies

Chronic hepatitis D (CHD), a global health problem, manifests as the most severe form of viral hepatitis. The causative agent, HDV, is the smallest known human virus; it replicates its circular single-stranded RNA genome in the nucleus of hepatocytes. HDV requires HBV-encoded envelope proteins for dissemination and de novo cell entry. However, HDV can also spread through cell division. Following entry into hepatocytes, replicative intermediates of HDV RNA are sensed by the pattern recognition receptor MDA5 (melanoma differentiation antigen 5) resulting in interferon (IFN)-β/λ induction. This IFN response strongly suppresses cell division-mediated spread of HDV genomes, however, it only marginally affects HDV RNA replication in already infected, resting hepatocytes. Monotherapy with IFN-α/λ shows efficacy but rarely results in HDV clearance. Recent molecular insights into key determinants of HDV persistence and the accelerated development of specifically acting antivirals that interfere with the replication cycle have revealed promising new therapeutic perspectives. In this review, we briefly summarise our knowledge on replication/persistence of HDV, the newly discovered HDV-like agents, and the interplay of HDV with the IFN response and its consequences for persistence. Finally, we discuss the possible role of IFNs in combination with upcoming therapies aimed at HDV cure.

Innate immune responses in RNA viral infection

RNA viruses cause a multitude of human diseases, including several pandemic events in the past century. Upon viral invasion, the innate immune system responds rapidly and plays a key role in activating the adaptive immune system. In the innate immune system, the interactions between pathogen-associated molecular patterns and host pattern recognition receptors activate multiple signaling pathways in immune cells and induce the production of pro-inflammatory cytokines and interferons to elicit antiviral responses. Macrophages, dendritic cells, and natural killer cells are the principal innate immune components that exert antiviral activities. In this review, the current understanding of innate immunity contributing to the restriction of RNA viral infections was briefly summarized. Besides the main role of immune cells in combating viral infection, the intercellular transfer of pathogen and host-derived materials and their epigenetic and metabolic interactions associated with innate immunity was discussed. This knowledge provides an enhanced understanding of the innate immune response to RNA viral infections in general and aids in the preparation for the existing and next emerging viral infections.

Interplay between Hepatitis D Virus and the Interferon Response

Chronic hepatitis D (CHD) is the most severe form of viral hepatitis, with rapid progression of liver-related diseases and high rates of development of hepatocellular carcinoma. The causative agent, hepatitis D virus (HDV), contains a small (approximately 1.7 kb) highly self-pairing single-strand circular RNA genome that assembles with the HDV antigen to form a ribonucleoprotein (RNP) complex. HDV depends on hepatitis B virus (HBV) envelope proteins for envelopment and de novo hepatocyte entry; however, its intracellular RNA replication is autonomous. In addition, HDV can amplify HBV independently through cell division. Cellular innate immune responses, mainly interferon (IFN) response, are crucial for controlling invading viruses, while viruses counteract these responses to favor their propagation. In contrast to HBV, HDV activates profound IFN response through the melanoma differentiation antigen 5 (MDA5) pathway. This cellular response efficiently suppresses cell-division-mediated HDV spread and, to some extent, early stages of HDV de novo infection, but only marginally impairs RNA replication in resting hepatocytes. In this review, we summarize the current knowledge on HDV structure, replication, and persistence and subsequently focus on the interplay between HDV and IFN response, including IFN activation, sensing, antiviral effects, and viral countermeasures. Finally, we discuss crosstalk with HBV.

Mobilization and Activation of the Innate Immune Response to Dengue Virus

Dengue virus is an important human pathogen, infecting an estimated 400 million individuals per year and causing symptomatic disease in a subset of approximately 100 million. Much of the effort to date describing the host response to dengue has focused on the adaptive immune response, in part because of the well-established roles of antibody-dependent enhancement and T cell original sin as drivers of severe dengue upon heterotypic secondary infection. However, the innate immune system is a crucial factor in the host response to dengue, as it both governs the fate and vigor of the adaptive immune response, and mediates the acute inflammatory response in tissues. In this review, we discuss the innate inflammatory response to dengue infection, focusing on the role of evolutionarily conserved innate immune cells, their effector functions, and clinical course.

Zika Virus Inhibits IFN-α Response by Human Plasmacytoid Dendritic Cells and Induces NS1-Dependent Triggering of CD303 (BDCA-2) Signaling

Zika virus (ZIKV) dramatically emerged in French Polynesia and subsequently in the Americas where it has been associated with severe neurological complications in adults and newborns, respectively. Although plasmacytoid dendritic cells (pDCs) are a key sensor of viral infection and are critical for initiating an antiviral response, little is known about the impact of ZIKV infection on pDCs. Here, we investigated the susceptibility of human pDCs to infection with multiple strains of ZIKV and further investigated the impact of infection on pDCs functions. We observed that pDCs were refractory to cell-free ZIKV virions but were effectively infected when co-cultured with ZIKV-infected cells. However, exposure of pDCs to ZIKV-infected cells resulted in limited maturation/activation with significant down regulation of CD303 expression, a severe impairment of inflammatory cytokine production, and an inability to mount an IFN-α response. We show that ZIKV developed a strategy to inhibit the IFN-α response in primary human pDCs likely mediated through NS1-dependent CD303 signaling, thus suggesting a new mechanism of immune evasion.

Cellular immune dysregulation in the pathogenesis of immune thrombocytopenia

: Immune thrombocytopenia (ITP) is an acquired autoimmune hemorrhagic disease characterized by immune-mediated increased platelet destruction and decreased platelet production, resulting from immune intolerance to autoantigen. The pathogenesis of ITP remains unclear, although dysfunction of T and B lymphocytes has been shown to be involved in the pathogenesis of ITP. More recently, it is found that dendritic cells, natural killer, and myeloid-derived suppressor cells also play an important role in ITP. Elucidating its pathogenesis is expected to provide novel channels for the targeted therapy of ITP. This article will review the role of different immune cells in ITP.

Should we stimulate or suppress immune responses in COVID-19? Cytokine and anti-cytokine interventions

The coronavirus disease-19 pandemic (COVID-19), which appeared in China in December 2019 and rapidly spread throughout the world, has forced clinicians and scientists to take up extraordinary challenges. This unprecedented situation led to the inception of numerous fundamental research protocols and many clinical trials. It quickly became apparent that although COVID-19, in the vast majority of cases, was a benign disease, it could also develop a severe form with sometimes fatal outcomes. Cytokines are central to the pathophysiology of COVID-19; while some of them are beneficial (type-I interferon, interleukin-7), others appear detrimental (interleukin-1β, -6, and TNF-α) particularly in the context of the so-called cytokine storm. Yet another characteristic of the disease has emerged: concomitant immunodeficiency, notably involving impaired type-I interferon response, and lymphopenia. This review provides an overview of current knowledge on COVID-19 immunopathology. We discuss the defective type-I IFN response, the theoretical role of IL-7 to restore lymphocyte repertoire, as well as we mention the two patterns observed in severe COVID-19 (i.e. interleukin-1β-driven macrophage activation syndrome vs. interleukin-6-driven immune dysregulation). Next, reviewing current evidence drawn from clinical trials, we examine a number of cytokine and anti-cytokine therapies, including interleukin-1, -6, and TNF inhibitors, as well as less targeted therapies, such as corticosteroids, chloroquine, or JAK inhibitors.

Interplay between SARS-CoV-2 and the type I interferon response

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is responsible for the current COVID-19 pandemic. An unbalanced immune response, characterized by a weak production of type I interferons (IFN-Is) and an exacerbated release of proinflammatory cytokines, contributes to the severe forms of the disease. SARS-CoV-2 is genetically related to SARS-CoV and Middle East respiratory syndrome-related coronavirus (MERS-CoV), which caused outbreaks in 2003 and 2013, respectively. Although IFN treatment gave some encouraging results against SARS-CoV and MERS-CoV in animal models, its potential as a therapeutic against COVID-19 awaits validation. Here, we describe our current knowledge of the complex interplay between SARS-CoV-2 infection and the IFN system, highlighting some of the gaps that need to be filled for a better understanding of the underlying molecular mechanisms. In addition to the conserved IFN evasion strategies that are likely shared with SARS-CoV and MERS-CoV, novel counteraction mechanisms are being discovered in SARS-CoV-2-infected cells. Since the last coronavirus epidemic, we have made considerable progress in understanding the IFN-I response, including its spatiotemporal regulation and the prominent role of plasmacytoid dendritic cells (pDCs), which are the main IFN-I-producing cells. While awaiting the results of the many clinical trials that are evaluating the efficacy of IFN-I alone or in combination with antiviral molecules, we discuss the potential benefits of a well-timed IFN-I treatment and propose strategies to boost pDC-mediated IFN responses during the early stages of viral infection.

Immune-profiling of ZIKV-infected patients identifies a distinct function of plasmacytoid dendritic cells for immune cross-regulation

Zika virus (ZIKV) is a mosquito-borne pathogen with increasing public health significance. To characterize immune responses to ZIKV, here we examine transcriptional signatures of CD4 T, CD8 T, B, and NK cells, monocytes, myeloid dendritic cells (mDCs), and plasmacytoid dendritic cells (pDCs) from three individuals with ZIKV infection. While gene expression patterns from most cell subsets display signs of impaired antiviral immune activity, pDCs from infected host have distinct transcriptional response associated with activation of innate immune recognition and type I interferon signaling pathways, but downregulation of key host factors known to support ZIKV replication steps; meanwhile, pDCs exhibit a unique expression pattern of gene modules that are correlated with alternative cell populations, suggesting collaborative interactions between pDCs and other immune cells, particularly B cells. Together, these results point towards a discrete but integrative function of pDCs in the human immune responses to ZIKV infection.

Should we stimulate or suppress immune responses in COVID-19? Cytokine and anti-cytokine interventions

The coronavirus disease-19 pandemic (COVID-19), which appeared in China in December 2019 and rapidly spread throughout the world, has forced clinicians and scientists to take up extraordinary challenges. This unprecedented situation led to the inception of numerous fundamental research protocols and many clinical trials. It quickly became apparent that although COVID-19, in the vast majority of cases, was a benign disease, it could also develop a severe form with sometimes fatal outcomes. Cytokines are central to the pathophysiology of COVID-19; while some of them are beneficial (type-I interferon, interleukin-7), others appear detrimental (interleukin-1β, -6, and TNF-α) particularly in the context of the so-called cytokine storm. Yet another characteristic of the disease has emerged: concomitant immunodeficiency, notably involving impaired type-I interferon response, and lymphopenia. This review provides an overview of current knowledge on COVID-19 immunopathology. We discuss the defective type-I IFN response, the theoretical role of IL-7 to restore lymphocyte repertoire, as well as we mention the two patterns observed in severe COVID-19 (i.e. interleukin-1β-driven macrophage activation syndrome vs. interleukin-6-driven immune dysregulation). Next, reviewing current evidence drawn from clinical trials, we examine a number of cytokine and anti-cytokine therapies, including interleukin-1, -6, and TNF inhibitors, as well as less targeted therapies, such as corticosteroids, chloroquine, or JAK inhibitors.

HTLV-1 infection of myeloid cells: from transmission to immune alterations

Human T cell leukemia virus type 1 (HTLV-1), the etiological agent of adult T-cell leukemia/lymphoma (ATLL) and the demyelinating neuroinflammatory disease known as HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP), was the first human retrovirus to be discovered. T-cells, which represent the main reservoir for HTLV-1, have been the main focus of studies aimed at understanding viral transmission and disease progression. However, other cell types such as myeloid cells are also target of HTLV-1 infection and display functional alterations as a consequence. In this work, we review the current investigations that shed light on infection, transmission and functional alterations subsequent to HTLV-1 infection of the different myeloid cells types, and we highlight the lack of knowledge in this regard.

Dendritic Cells (DCs) as "Fire Accelerants" of Hantaviral Pathogenesis

Hantaviruses are widespread zoonotic pathogens found around the globe. Depending on their geographical location, hantaviruses can cause two human syndromes, haemorrhagic fever with renal syndrome (HFRS) or hantavirus pulmonary syndrome (HPS). HPS and HFRS have many commonalities amongst which excessive activation of immune cells is a prominent feature. Hantaviruses replicate in endothelial cells (ECs), the major battlefield of hantavirus-induced pathogenesis, without causing cytopathic effects. This indicates that a misdirected response of human immune cells to hantaviruses is causing damage. As dendritic cells (DCs) orchestrate antiviral immune responses, they are in the focus of research analysing hantavirus-induced immunopathogenesis. In this review, we discuss the interplay between hantaviruses and DCs and the immunological consequences thereof.

SLAMF9 regulates pDC homeostasis and function in health and disease

SLAMF9 belongs to the conserved lymphocytic activation molecule family (SLAMF). Unlike other SLAMs, which have been extensively studied, the role of SLAMF9 in the immune system remained mostly unexplored. By generating CRISPR/Cas9 SLAMF9 knockout mice, we analyzed the role of this receptor in plasmacytoid dendritic cells (pDCs), which preferentially express the SLAMF9 transcript and protein. These cells display a unique capacity to produce type I IFN and bridge between innate and adaptive immune response. Analysis of pDCs in SLAMF9^-/- mice revealed an increase of immature pDCs in the bone marrow and enhanced accumulation of pDCs in the lymph nodes. In the periphery, SLAMF9 deficiency resulted in lower levels of the transcription factor SpiB, elevation of pDC survival, and attenuated IFN-α and TNF-α production. To define the role of SLAMF9 during inflammation, pDCs lacking SLAMF9 were followed during induced experimental autoimmune encephalomyelitis. SLAMF9^-/- mice demonstrated attenuated disease and delayed onset, accompanied by a prominent increase of immature pDCs in the lymph node, with a reduced costimulatory potential and enhanced infiltration of pDCs into the central nervous system. These results suggest the crucial role of SLAMF9 in pDC differentiation, homeostasis, and function in the steady state and during experimental autoimmune encephalomyelitis.

Plasmacytoid Dendritic Cells and Infected Cells Form an Interferogenic Synapse Required for Antiviral Responses

Type I interferon (IFN-I) is critical for antiviral defense, and plasmacytoid dendritic cells (pDCs) are a predominant source of IFN-I during virus infection. pDC-mediated antiviral responses are stimulated upon physical contact with infected cells, during which immunostimulatory viral RNA is transferred to pDCs, leading to IFN production via the nucleic acid sensor TLR7. Using dengue, hepatitis C, and Zika viruses, we demonstrate that the contact site of pDCs with infected cells is a specialized platform we term the interferogenic synapse, which enables viral RNA transfer and antiviral responses. This synapse is formed via α_Lβ₂ integrin-ICAM-1 adhesion complexes and the recruitment of the actin network and endocytic machinery. TLR7 signaling in pDCs promotes interferogenic synapse establishment and provides feed-forward regulation, sustaining pDC contacts with infected cells. This interferogenic synapse may allow pDCs to scan infected cells and locally secrete IFN-I, thereby confining a potentially deleterious response.

Sensing of cell-associated HTLV by plasmacytoid dendritic cells is regulated by dense β-galactoside glycosylation

Human T Lymphotropic virus (HTLV) infection can persist in individuals resulting, at least in part, from viral escape of the innate immunity, including inhibition of type I interferon response in infected T-cells. Plasmacytoid dendritic cells (pDCs) are known to bypass viral escape by their robust type I interferon production. Here, we demonstrated that pDCs produce type I interferons upon physical cell contact with HTLV-infected cells, yet pDC activation inversely correlates with the ability of the HTLV-producing cells to transmit infection. We show that pDCs sense surface associated-HTLV present with glycan-rich structure referred to as biofilm-like structure, which thus represents a newly described viral structure triggering the antiviral response by pDCs. Consistently, heparan sulfate proteoglycans and especially the cell surface pattern of terminal β-galactoside glycosylation, modulate the transmission of the immunostimulatory RNA to pDCs. Altogether, our results uncover a function of virus-containing cell surface-associated glycosylated structures in the activation of innate immunity.

Imbalance in the ratio of CpG and polyG contributes to impaired interferon-α expression

The secretion of interferon-α (IFN-α) is impaired during hepatitis B virus (HBV) infection. DNA sequences purified from distinct viruses, for example, HBV versus members of Herpesviridae, have been shown to differ in their IFN-α signaling properties. The present study found that DNA from HBV inhibited, while DNA from members of Herpesviridae induced, the expression of IFN-α. Furthermore, stimulatory cytosine-phosphate-guanosine (CpG) sequences derived from these DNA viruses could induce the secretion of IFN-α, while inhibitory guanosine-rich oligodeoxynucleoti (polyG) oligonucleotide sequences derived from these DNA viruses could inhibit CpG-induced IFN-α secretion. Using a computational analysis of genomic DNA sequences, the discrimination between the genomes of HBV and those of other DNA viruses that can also cause inflammation of the liver is based on different frequencies of the CpG and polyG motifs. The underrepresentation of stimulatory CpG motifs and overrepresentation of inhibitory polyG motifs were documented in HBV genomes, whereas the DNA from other viral genomes displayed the opposite trend. Moreover, it was demonstrated that HBV could suppress the activation of IFN-α via its own DNA through the high proportion of polyG motifs. To our knowledge, this is the first demonstration of a specific role for polyG motifs in the inhibition of the IFN-α response following DNA virus infection.

Human Plasmacytoid and Monocyte-Derived Dendritic Cells Display Distinct Metabolic Profile Upon RIG-I Activation

Recent advances reveal that metabolic reprogramming is required for adequate antiviral responses of dendritic cells (DCs) that possess the capacity to initiate innate and adaptive immune responses. Several reports indicate that Toll-like receptor (TLR) stimulation of DCs is accompanied by a rapid induction of glycolysis; however, the metabolic requirements of retinoic-acid inducible gene I (RIG-I)-like receptor (RLR) activation have not defined either in conventional DCs (cDCs) or in plasmacytoid DCs (pDCs) that are the major producers of type I interferons (IFN) upon viral infections. To sense viruses and trigger an early type I IFN response, pDCs rely on endosomal TLRs, whereas cDCs employ cytosolic RIG-I, which is constitutively present in their cytoplasm. We previously found that RIG-I is upregulated in pDCs upon endosomal TLR activation and contributes to the late phase of type I IFN responses. Here we report that TLR9-driven activation of human pDCs leads to a metabolic transition to glycolysis supporting the production of type I IFNs, whereas RIG-I-mediated antiviral responses of pDCs do not require glycolysis and rather rely on oxidative phosphorylation (OXPHOS) activity. In particular, TLR9-activated pDCs show increased extracellular acidification rate (ECAR), lactate production, and upregulation of key glycolytic genes indicating an elevation in glycolytic flux. Furthermore, administration of 2-deoxy-D-glucose (2-DG), an inhibitor of glycolysis, significantly impairs the TLR9-induced secretion of type I IFNs by human pDCs. In contrast, RIG-I stimulation of pDCs does not result in any alterations of ECAR, and type I IFN production is not inhibited but rather promoted by 2-DG treatment. Moreover, pDCs activated via TLR9 but not RIG-I in the presence of 2-DG are impaired in their capacity to prime allogeneic naïve CD8+ T cell proliferation. Interestingly, human monocyte-derived DCs (moDC) triggered via RIG-I show a commitment to glycolysis to promote type I IFN production and T cell priming in contrast to pDCs. Our findings reveal for the first time, that pDCs display a unique metabolic profile; TLR9-driven but not RIG-I-mediated activation of pDCs requires glycolytic reprogramming. Nevertheless, the metabolic signature of RIG-I-stimulated moDCs is characterized by glycolysis suggesting that RIG-I-induced metabolic alterations are rather cell type-specific and not receptor-specific.

Toll-Like Receptor 7 (TLR-7) and TLR-9 Agonists Improve Hepatitis C Virus Replication and Infectivity Inhibition by Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDCs) are innate immune cells with high antiviral activity triggered by Toll-like receptor 7 (TLR-7) and TLR-9 stimulation. Moreover, they are important mediators between innate and adaptive immunity. Although nowadays there is available an effective therapeutic arsenal against hepatitis C virus (HCV), a protective vaccine is not available. We have analyzed the pDCs' response to HCV infection in a hepatitis C virus (HCV)-Huh7.5 virus-cell system, which allows completion of the virus infectious cycle. pDCs were cocultured following human immunodeficiency virus (HIV) aldrithiol-2 (AT-2 [TLR-7 agonist]) inactivation and CpG (TLR-9 agonist) stimulation. We employed three virus derivatives-wild-type Jc1, interferon (IFN)-resistant virus IR, and high-replicative-fitness virus P100-in order to explore additional IFN-α-related virus inhibition mechanisms. pDCs inhibited HCV infectivity and replication and produced IFN-α. After TLR-7 and TLR-9 stimulation, inhibition of infectivity and IFN-α production by pDCs were enhanced. TLR-7 stimulation drove higher TNF-related apoptosis-inducing ligand (TRAIL) expression in pDCs. Additionally, TLR-7- and TLR-9-stimulated pDCs exhibited a mature phenotype, improving the antigen presentation and lymph node homing-related markers. In conclusion, pDCs could serve as a drug target against HCV in order to improve antiviral activity and as an enhancer of viral immunization.IMPORTANCE We implemented a coculture system of pDCs with HCV-infected hepatoma cell line, Huh7.5. We used three HCV derivatives in order to gain insight into pDCs' behavior against HCV and associated antiviral mechanisms. The results with this cell coculture system support the capacity of pDCs to inhibit HCV replication and infectivity mainly via IFN-α, but also through additional mechanisms associated with pDC maturation. We provided evidence that TLR agonists can enhance antiviral pDCs' function and can induce phenotypic changes that may facilitate the interplay with other immune cells. These findings suggest the possibility of including TLR agonists in the strategies of HCV vaccine development.

Interferon-stimulated genes: new platforms and computational approaches

Interferon-stimulated genes (ISGs) are the effectors of interferon (IFN) actions and play major roles in innate immune defense against microbial infection. During virus infection, ISGs impart antiviral actions to control virus replication and spread but can also contribute to disease pathology if their expression is unchecked. Antiviral ISGs have been identified by a variety of biochemical, genetic, and virologic methods. New computational approaches are expanding and redefining ISGs as responders to a variety of stimuli beyond IFNs, including virus infection, stress, and other events that induce cytokines. These studies reveal that the expression of ISG subsets link to interferon regulatory factors (IRF)s, NF-kB, and other transcription factors that impart gene expression in specific cell types independently of IFNs, including stem cells and other cell types where ISGs are constitutively expressed. Here, we provide a broad overview of ISGs, define virus-induced genes (VSG)s, and discuss the application of computational approaches and bioinformatics platforms to evaluate the functional role of ISGs in epigenetics, immune programming, and vaccine responses.

Immature particles and capsid-free viral RNA produced by Yellow fever virus-infected cells stimulate plasmacytoid dendritic cells to secrete interferons

Plasmacytoid dendritic cells (pDCs) are specialized in the production of interferons (IFNs) in response to viral infections. The Flaviviridae family comprises enveloped RNA viruses such as Hepatitis C virus (HCV) and Dengue virus (DENV). Cell-free flaviviridae virions poorly stimulate pDCs to produce IFN. By contrast, cells infected with HCV and DENV potently stimulate pDCs via short-range delivery of viral RNAs, which are either packaged within immature virions or secreted exosomes. We report that cells infected with Yellow fever virus (YFV), the prototypical flavivirus, stimulated pDCs to produce IFNs in a TLR7- and cell contact- dependent manner. Such stimulation was unaffected by the presence of YFV neutralizing antibodies. As reported for DENV, cells producing immature YFV particles were more potent at stimulating pDCs than cells releasing mature virions. Additionally, cells replicating a release-deficient YFV mutant or a YFV subgenomic RNA lacking structural protein-coding sequences participated in pDC stimulation. Thus, viral RNAs produced by YFV-infected cells reach pDCs via at least two mechanisms: within immature particles and as capsid-free RNAs. Our work highlights the ability of pDCs to respond to a variety of viral RNA-laden carriers generated from infected cells.

Plasmacytoid dendritic cells control dengue and Chikungunya virus infections via IRF7-regulated interferon responses

Type I interferon (IFN-I) responses are critical for the control of RNA virus infections, however, many viruses, including Dengue (DENV) and Chikungunya (CHIKV) virus, do not directly activate plasmacytoid dendritic cells (pDCs), robust IFN-I producing cells. Herein, we demonstrated that DENV and CHIKV infected cells are sensed by pDCs, indirectly, resulting in selective IRF7 activation and IFN-I production, in the absence of other inflammatory cytokine responses. To elucidate pDC immunomodulatory functions, we developed a mouse model in which IRF7 signaling is restricted to pDC. Despite undetectable levels of IFN-I protein, pDC-restricted IRF7 signaling controlled both viruses and was sufficient to protect mice from lethal CHIKV infection. Early pDC IRF7-signaling resulted in amplification of downstream antiviral responses, including an accelerated natural killer (NK) cell-mediated type II IFN response. These studies revealed the dominant, yet indirect role of pDC IRF7-signaling in directing both type I and II IFN responses during arbovirus infections.

Viruses, like the ones responsible for the tropical diseases dengue and chikungunya, are parasites of living cells. As they cannot multiply on their own, these microbes need to infect a host cell and hijack its machinery to make more of themselves.

When a cell is invaded, it can sense the viral particles, and defend itself by releasing antiviral molecules. Some of these molecules, such as interferons, also help recruit immune cells that can fight the germs. However, viruses often evolve mechanisms to escape being detected by the cell they occupy.

Plasmacytoid dendritic cells are a rare group of immune cells, and they are able to detect when another cell is infected by the dengue virus. When they are in close physical contact with an invaded cell, these sentinels can recognize immature viral particles and release large amounts of antiviral molecules. However, it is unclear how important plasmacytoid dendritic cells are in clearing a viral infection.

Here, Webster, Werneke et al. confirmed that plasmacytoid dendritic cells were able to sense cells infected by dengue, but also by chikungunya. When this happened, the dendritic cells primarily produced interferon, rather than other defense molecules.

In addition, mice were genetically engineered so that the production of interferon was restricted to the plasmacytoid dendritic cells. When infected with dengue or chikungunya, the modified rodents resisted the diseases. These results show that, even though they are only a small percentage of all immune cells, plasmacytoid dendritic cells have an outsize role as first responders and as coordinators of the immune response.

Finally, Webster, Werneke et al. showed that when low doses of interferon are added, , the plasmacytoid dendritic cells respond more quickly to cells infected by dengue. Together these findings could potentially be leveraged to create new treatments to fight dengue. These would be of particular interest because interferons are not as damaging to the body compared to other types of defense molecules. The issue is timely since climate change is allowing the mosquitos that transmit dengue and chikungunya to live in new places, exposing more people to these serious infections.

The involvement of plasmacytoid cells in HIV infection and pathogenesis

Plasmacytoid dendritic cells (pDCs) are a unique dendritic cell subset that are specialized in type I interferon (IFN) production. pDCs are key players in the antiviral immune response and serve as bridge between innate and adaptive immunity. Although pDCs do not represent the main reservoir of the Human Immunodeficiency Virus (HIV), they are a crucial subset in HIV infection as they influence viral transmission, target cell infection and antigen presentation. pDCs act as inflammatory and immunosuppressive cells, thus contributing to HIV disease progression. This review provides a state of art analysis of the interactions between HIV and pDCs and their potential roles in HIV transmission, chronic immune activation and immunosuppression. A thorough understanding of the roles of pDCs in HIV infection will help to improve therapeutic strategies to fight HIV infection, and will further increase our knowledge on this important immune cell subset.

Activation of innate immunity in primary human cells using a plant virus derived nanoparticle TLR7/8 agonist

Rod-shaped virus-like nanoparticles (VLNP) made of papaya mosaic virus (PapMV) coat proteins (CP) self-assembled around a single stranded RNA (ssRNA) were showed to be a TLR7 agonist. Their utilization as an immune modulator in cancer immunotherapy was shown to be promising. To establish a clinical relevance in human for PapMV VLNP, we showed that stimulation of human peripheral blood mononuclear cells (PBMC) with VLNP induces the secretion of interferon alpha (IFNα) and other pro-inflammatory cytokines and chemokines. Plasmacytoid dendritic cells (pDCs) were activated and secreted IFN-α upon VLNP exposure. Monocyte-derived dendritic cells upregulate maturation markers and produce IL-6 in response to PapMV VLNP stimulation, which suggests the activation of TLR8. Finally, when co-cultured with NK cells, PapMV induced pDCs promoted the NK cytolytic activity against cancer cells. These data obtained with primary human immune cells further strengthen the clinical relevance of PapMV VLNPs as a cancer immunotherapy agent.

Epigenetic regulator CXXC5 recruits DNA demethylase Tet2 to regulate TLR7/9-elicited IFN response in pDCs

Ma and colleagues identify CXXC5 as an epigenetic regulator required for maintaining the hypomethylation of a subset of CGIs, thereby promoting the expression of transcriptional factors such as IRF7 in pDCs to enable robust IFN response to viral infection.

TLR7/9 signals are capable of mounting massive interferon (IFN) response in plasmacytoid dendritic cells (pDCs) immediately after viral infection, yet the involvement of epigenetic regulation in this process has not been documented. Here, we report that zinc finger CXXC family epigenetic regulator CXXC5 is highly expressed in pDCs, where it plays a crucial role in TLR7/9- and virus-induced IFN response. Notably, genetic ablation of CXXC5 resulted in aberrant methylation of the CpG-containing island (CGI) within the Irf7 gene and impaired IRF7 expression in steady-state pDCs. Mechanistically, CXXC5 is responsible for the recruitment of DNA demethylase Tet2 to maintain the hypomethylation of a subset of CGIs, a process coincident with active histone modifications and constitutive transcription of these CGI-containing genes. Consequently, CXXC5-deficient mice had compromised early IFN response and became highly vulnerable to infection by herpes simplex virus and vesicular stomatitis virus. Together, our results identify CXXC5 as a novel epigenetic regulator for pDC-mediated antiviral response.

Adenosine Deaminases That Act on RNA (ADARs)

Inosine is one of the most common modifications found in human RNAs and the Adenosine Deaminases that act on RNA (ADARs) are the main enzymes responsible for its production. ADARs were first discovered in the 1980s and since then our understanding of ADARs has advanced tremendously. For instance, it is now known that defective ADAR function can cause human diseases. Furthermore, recently solved crystal structures of the human ADAR2 deaminase bound to RNA have provided insights regarding the catalytic and substrate recognition mechanisms. In this chapter, we describe the occurrence of inosine in human RNAs and the newest perspective on the ADAR family of enzymes, including their substrate recognition, catalytic mechanism, regulation as well as the consequences of A-to-I editing, and their relation to human diseases.

Lethal Experimental Tick-Borne Encephalitis Infection: Influence of Two Strains with Similar Virulence on the Immune Response

Tick-borne encephalitis virus (TBEV) is a tick-transmitted arbovirus that causes serious diseases in humans in Europe and Northern Asia. About 6000–10,000 cases are registered annually, and one-third of them lead to sequela with different degrees of severity. Two TBEV strains (Absettarov and EK-328) similar in virulence rate in laboratory mice were used to study pathogenesis and immune response upon lethal infection in mice. The strains differed in the dynamics of appearance of virus, IFNs and other cytokines in blood of mice, and ability to induce a cytokine storm in the terminal stages of disease and a non-sterile immunity. Moreover, the TBEV strains differed in characteristics of their interactions with DCs: level of reproduction in these cells, virus dose triggering IFN-α production, and impact on DCs' maturation. Infection of DCs with Absettarov strain led to IFN-α induction only at high multiplicity of infection (MOI), and an increased amount of the mature DCs with high adhesion activity and low-level of MHCII positive cells. While reproduction of the EK-328 strain in DCs was less efficient, a low dose of the virus induced IFN-α production and stimulated maturation of DCs with relatively low adhesive capacity, but with the high percentage of cells expressing MHCII molecules. Thus, the studied strains differed significantly in the impact on DCs' maturation and antigen presentation to CD4⁺ lymphocytes. Injection of low (10³ PFU) and high (10⁶ PFU) doses of both TBEV strains caused a lethal infection in mice. At the same time, the dose of the virus in the inoculum, regardless of the strain properties, affected the following virulence characteristics: the time of virus appearance in brain (day 4–5 vs. day 1 p.i.), time of IFN-α appearance in blood (10 h vs. 5 h p.i.), concentration of IFN-α in blood, and induction of IFN-α during infection of DCs. Therefore, virulent TBEV strains during lethal infection can interact differently with the host immune system, and the infectious dose has an impact on both: virus spread in the infected organism and immune response activation.