Cutting Edge: Antibody-mediated TLR7-dependent recognition of viral RNA

Toll-Like Receptor 7-Expressed Macrophages Are Involved in the Pathogenesis of Esophageal Achalasia and Esophagogastric Junction Outflow Obstruction

INTRODUCTION

Esophageal achalasia is a typical esophageal motility disorder (EMD). Although viral infections have been hypothesized to play a role in the pathogenesis of esophageal achalasia, its etiology remains unclear. This study used esophageal muscle layer specimens collected during per-oral endoscopic myotomy (POEM) procedures to investigate the association between esophageal achalasia and esophagogastric junction outflow obstruction (EGJOO) and pattern recognition receptors.

METHODS

Patients with esophageal achalasia and EGJOO who underwent POEM were allocated to the EMD group. Biopsies of the inner circular muscle were conducted during the POEM procedure. The control group comprised individuals diagnosed with esophageal squamous cell carcinoma who underwent surgical resection. Expression of pattern recognition receptors, including Toll-like receptor (TLR) 7, was examined by polymerase chain reaction. Immunohistochemical staining was performed to determine TLR7 expression sites in the esophageal muscle layer, and the relationship between TLR7 mRNA expression and clinical score was investigated.

RESULTS

Our analysis revealed a notable upregulation of TLR7 mRNA levels within the muscle layer of esophageal achalasia and EGJOO, in contrast to those of control specimens. In contrast, the correlation between TLR7 and clinical score was not significant. Immunohistochemical staining revealed increased numbers of TLR7-expressing macrophages between the muscle layers.

CONCLUSIONS

TLR7-expressing macrophages are involved in the innate immune response underlying esophageal achalasia and EGJOO. This result will lead to the elucidation of new pathogenetic mechanisms and the development of novel therapeutic targets.

The interplay between the immune system and viruses

The human immune response can be divided into two arms: innate and adaptive immunity. The innate immune system consists of "hard-wired" responses encoded by host germline genes. In contrast, the adaptive response consists of gene elements that are somatically rearranged to assemble antigen-binding molecules with specificity for individual foreign structures. In contrast to the adaptive immune system, which depends upon T and B lymphocytes, innate immune protection is a task performed by cells of both hematopoietic and non-hematopoietic origin. Hematopoietic cells involved in innate immune responses include macrophages, dendritic cells, mast cell, neutrophils, eosinophils, natural killer (NK) cells and natural killer T cells. The induction of an adaptive immune response begins when a pathogen is ingested by an Antigen Presenting Cell (APC), such as the Dendritic cell (DC), in the infected tissue. DCs bridge the gap between first line innate responses and powerful adaptive immune responses, by internalizing, processing and presenting antigens on Major Histocompatibility Complex (MHC) and MHC-like molecules to the adaptive immune cells In addition to DCs, macrophages and B cells are deemed antigen presenting cells (Llewelyn & Cohen, 2002).

Tradeoffs for a viral mutant with enhanced replication speed

RNA viruses exist as genetically heterogeneous populations due to high mutation rates, and many of these mutations reduce fitness and/or replication speed. However, it is unknown whether mutations can increase replication speed of a virus already well adapted to replication in cultured cells. By sequentially passaging coxsackievirus B3 in cultured cells and collecting the very earliest progeny, we selected for increased replication speed. We found that a single mutation in a viral capsid protein, VP1-F106L, was sufficient for the fast-replication phenotype. Characterization of this mutant revealed quicker genome release during entry compared to wild-type virus, highlighting a previously unappreciated infection barrier. However, this mutation also reduced capsid stability in vitro and reduced replication and pathogenesis in mice. These results reveal a tradeoff between overall replication speed and fitness. Importantly, this approach-selecting for the earliest viral progeny-could be applied to a variety of viral systems and has the potential to reveal unanticipated inefficiencies in viral replication cycles.

Engineering of Fc Multimers as a Protein Therapy for Autoimmune Disease

The success of Intravenous Immunoglobulin in treating autoimmune and inflammatory processes such as immune thrombocytopenia purpura and Kawasaki disease has led to renewed interest in developing recombinant molecules capable of recapitulating these therapeutic effects. The anti-inflammatory properties of IVIG are, in part, due to the Fc region of the IgG molecule, which interacts with activating or inhibitory Fcγ receptors (FcγRs), the neonatal Fc Receptor, non-canonical FcRs expressed by immune cells and complement proteins. In most cases, Fc interactions with these cognate receptors are dependent upon avidity—avidity which naturally occurs when polyclonal antibodies recognize unique antigens on a given target. The functional consequences of these avid interactions include antibody dependent cell-mediated cytotoxicity, antibody dependent cell phagocytosis, degranulation, direct killing, and/or complement activation—all of which are associated with long-term immunomodulatory effects. Many of these immunologic effects can be recapitulated using recombinant or non-recombinant approaches to induce Fc multimerization, affording the potential to develop a new class of therapeutics. In this review, we discuss the history of tolerance induction by immune complexes that has led to the therapeutic development of artificial Fc bearing immune aggregates and recombinant Fc multimers. The contribution of structure, aggregation and N-glycosylation to human IgG: FcγR interactions and the functional effect(s) of these interactions are reviewed. Understanding the mechanisms by which Fc multimers induce tolerance and attempts to engineer Fc multimers to target specific FcγRs and/or specific effector functions in autoimmune disorders is explored in detail.

Human Plasmacytoid Dendritic Cells and Cutaneous Melanoma

The prognosis of metastatic melanoma (MM) patients has remained poor for a long time. However, the recent introduction of effective target therapies (BRAF and MEK inhibitors for BRAFV600-mutated MM) and immunotherapies (anti-CTLA-4 and anti-PD-1) has significantly improved the survival of MM patients. Notably, all these responses are highly dependent on the fitness of the host immune system, including the innate compartment. Among immune cells involved in cancer immunity, properly activated plasmacytoid dendritic cells (pDCs) exert an important role, bridging the innate and adaptive immune responses and directly eliminating cancer cells. A distinctive feature of pDCs is the production of high amount of type I Interferon (I-IFN), through the Toll-like receptor (TLR) 7 and 9 signaling pathway activation. However, published data indicate that melanoma-associated escape mechanisms are in place to hijack pDC functions. We have recently reported that pDC recruitment is recurrent in the early phases of melanoma, but the entire pDC compartment collapses over melanoma progression. Here, we summarize recent advances on pDC biology and function within the context of melanoma immunity.

Interferon-beta expression and type I interferon receptor signaling of hepatocytes prevent hepatic necrosis and virus dissemination in Coxsackievirus B3-infected mice

During Coxsackievirus B3 (CVB3) infection hepatitis is a potentially life threatening complication, particularly in newborns. Studies with type I interferon (IFN-I) receptor (IFNAR)-deficient mice revealed a key role of the IFN-I axis in the protection against CVB3 infection, whereas the source of IFN-I and cell types that have to be IFNAR triggered in order to promote survival are still unknown. We found that CVB3 infected IFN-β reporter mice showed effective reporter induction, especially in hepatocytes and only to a minor extent in liver-resident macrophages. Accordingly, upon in vitro CVB3 infection of primary hepatocytes from murine or human origin abundant IFN-β responses were induced. To identify sites of IFNAR-triggering we performed experiments with Mx reporter mice, which upon CVB3 infection showed massive luciferase induction in the liver. Immunohistological studies revealed that during CVB3 infection MX1 expression of hepatocytes was induced primarily by IFNAR-, and not by IFN-III receptor (IFNLR)-triggering. CVB3 infection studies with primary human hepatocytes, in which either the IFN-I or the IFN-III axis was inhibited, also indicated that primarily IFNAR-, and to a lesser extent IFNLR-triggering was needed for ISG induction. Interestingly, CVB3 infected mice with a hepatocyte-specific IFNAR ablation showed severe liver cell necrosis and ubiquitous viral dissemination that resulted in lethal disease, as similarly detected in classical IFNAR^-/- mice. In conclusion, we found that during CVB3 infection hepatocytes are major IFN-I producers and that the liver is also the organ that shows strong IFNAR-triggering. Importantly, hepatocytes need to be IFNAR-triggered in order to prevent virus dissemination and to assure survival. These data are compatible with the hypothesis that during CVB3 infection hepatocytes serve as important IFN-I producers and sensors not only in the murine, but also in the human system.

CVB3 belongs to human enteroviruses and is transmitted through the fecal-oral route. Infections with CVB3 are mostly unnoticed or cause flu-like symptoms, however, they can also cause severe disease, such as myocarditis, pancreatitis, and hepatitis. Although CVB3 does not efficiently trigger plasmacytoid dendritic cells, which are the main IFN-I producers in many other virus infections, IFNAR signaling plays a crucial role in CVB3 control. Therefore, we investigated which cells are stimulated to produce IFN-I following CVB3 infection and which cell types have to be IFNAR-triggered in order to confer anti-viral protection. We found that upon CVB3 infection IFN-β was mainly expressed within the liver, especially by hepatocytes and not by liver resident macrophages. This was corroborated by in vitro CVB3 infection experiments with primary murine and human hepatocytes. Interestingly, IFNAR signaling of hepatocytes was required to control the virus. Collectively, our data indicate that hepatocytes, and not immune cells, are the key innate effector cells that are relevant for the control of CVB3 infection.

Functional Consequences of RNA 5'-Terminal Deletions on Coxsackievirus B3 RNA Replication and Ribonucleoprotein Complex Formation

Group B coxsackieviruses are responsible for chronic cardiac infections. However, the molecular mechanisms by which the virus can persist in the human heart long after the signs of acute myocarditis have abated are still not completely understood. Recently, coxsackievirus B3 strains with 5'-terminal deletions in genomic RNAs were isolated from a patient suffering from idiopathic dilated cardiomyopathy, suggesting that such mutant viruses may be the forms responsible for persistent infection. These deletions lacked portions of 5' stem-loop I, which is an RNA secondary structure required for viral RNA replication. In this study, we assessed the consequences of the genomic deletions observed in vivo for coxsackievirus B3 biology. Using cell extracts from HeLa cells, as well as transfection of luciferase replicons in two types of cardiomyocytes, we demonstrated that coxsackievirus RNAs harboring 5' deletions ranging from 7 to 49 nucleotides in length can be translated nearly as efficiently as those of wild-type virus. However, these 5' deletions greatly reduced the synthesis of viral RNA in vitro, which was detected only for the 7- and 21-nucleotide deletions. Since 5' stem-loop I RNA forms a ribonucleoprotein complex with cellular and viral proteins involved in viral RNA replication, we investigated the binding of the host cell protein PCBP2, as well as viral protein 3CD^pro, to deleted positive-strand RNAs corresponding to the 5' end. We found that binding of these proteins was conserved but that ribonucleoprotein complex formation required higher PCBP2 and 3CD^pro concentrations, depending on the size of the deletion. Overall, this study confirmed the characteristics of persistent CVB3 infection observed in heart tissues and provided a possible explanation for the low level of RNA replication observed for the 5'-deleted viral genomes-a less stable ribonucleoprotein complex formed with proteins involved in viral RNA replication.IMPORTANCE Dilated cardiomyopathy is the most common indication for heart transplantation worldwide, and coxsackie B viruses are detected in about one-third of idiopathic dilated cardiomyopathies. Terminal deletions at the 5' end of the viral genome involving an RNA secondary structure required for RNA replication have been recently reported as a possible mechanism of virus persistence in the human heart. These mutations are likely to disrupt the correct folding of an RNA secondary structure required for viral RNA replication. In this report, we demonstrate that transfected RNAs harboring 5'-terminal sequence deletions are able to direct the synthesis of viral proteins, but not genomic RNAs, in human and murine cardiomyocytes. Moreover, we show that the binding of cellular and viral replication factors to viral RNA is conserved despite genomic deletions but that the impaired RNA synthesis associated with terminally deleted viruses could be due to destabilization of the ribonucleoprotein complexes formed.

Antiviral Functions of Human Immunodeficiency Virus Type 1 (HIV-1)-Specific IgG Antibodies: Effects of Antiretroviral Therapy and Implications for Therapeutic HIV-1 Vaccine Design

Contemporary antiretroviral therapy (ART) is effective and tolerable for long periods of time but cannot eradicate human immunodeficiency virus type 1 (HIV-1) infection by either elimination of viral reservoirs or enhancement of HIV-1-specific immune responses. Boosting "protective" HIV-1-specific immune responses by active or passive immunization will therefore be necessary to control or eradicate HIV-1 infection and is currently the topic of intense investigation. Recently reported studies conducted in HIV patients and non-human primate (NHP) models of HIV-1 infection suggest that HIV-1-specific IgG antibody responses may contribute to the control of HIV-1 infection. However, production of IgG antibodies with virus neutralizing activity by vaccination remains problematic and while vaccine-induced natural killer cell-activating IgG antibodies have been shown to prevent the acquisition of HIV-1 infection, they may not be sufficient to control or eradicate established HIV-1 infection. It is, therefore, important to consider other functional characteristics of IgG antibody responses. IgG antibodies to viruses also mediate opsonophagocytic antibody responses against virions and capsids that enhance the function of phagocytic cells playing critical roles in antiviral immune responses, particularly conventional dendritic cells and plasmacytoid dendritic cells. Emerging evidence suggests that these antibody functions might contribute to the control of HIV-1 infection. In addition, IgG antibodies contribute to the intracellular degradation of viruses via binding to the cytosolic fragment crystallizable (Fc) receptor tripartite motif containing-21 (TRIM21). The functional activity of an IgG antibody response is influenced by the IgG subclass content, which affects binding to antigens and to Fcγ receptors on phagocytic cells and to TRIM21. The IgG subclass content and avidity of IgG antibodies is determined by germinal center (GC) reactions in follicles of lymphoid tissue. As HIV-1 infects cells in GCs and induces GC dysfunction, which may persist during ART, strategies for boosting HIV-1-specific IgG antibody responses should include early commencement of ART and possibly the use of particular antiretroviral drugs to optimize drug levels in lymphoid follicles. Finally, enhancing particular functions of HIV-1-specific IgG antibody responses by using adjuvants or cytokines to modulate the IgG subclass content of the antibody response might be investigated in NHP models of HIV-1 infection and during trials of therapeutic vaccines in HIV patients.

Control of early HIV-1 infection associates with plasmacytoid dendritic cell-reactive opsonophagocytic IgG antibodies to HIV-1 p24

OBJECTIVES

We have previously demonstrated that HIV-1 p24-specific plasmacytoid dendritic cell-reactive opsonophagocytic antibody (PROAb) responses associate with control of chronic HIV infection. Here, we examined whether HIV-1 p24-specific PROAbs associate with control of early HIV infection and their relationship with HIV-1 p24-specific IgG subclasses.

METHODS

Plasma collected at 8 and 52 weeks following primary HIV-1 infection was obtained from antiretroviral therapy-naïve patients who were classified as 'good' (plasma HIV-1 RNA < 5000 copies/ml; n = 17) or 'poor' (HIV-1 RNA > 50 000 copies/ml; n = 15) controllers at week 52. HIV-1 p24-specific PROAb responses were assayed using a plasmacytoid dendritic cell line (Gen2.2), and HIV-1 p24-specific IgG3, IgG1 and IgG2 levels were assayed by ELISA.

RESULTS

HIV-1 p24-specific PROAb responses increased in 'good controllers' (P = 0.01) but remained unchanged in 'poor controllers' between weeks 8 and 52. Of the HIV-1 p24-specific IgG subclasses measured, only IgG1 increased over this time period in 'good controllers' alone (P = 0.003), which correlated with the increase in HIV-1 p24-specific PROAb responses (r = 0.83, P < 0.0001). Depletion of IgG1 from IgG preparations of 'good controllers' resulted in the inhibition of HIV-1 p24-specific PROAb responses. In the total patient cohort, plasma HIV-1 RNA levels at week 52 correlated inversely with changes in HIV-1 p24-specific PROAb responses (r = -0.37, P = 0.04) and IgG1 (r = -0.51, P = 0.003) levels between weeks 8 and 52.

CONCLUSION

Control of early HIV-1 infection was associated with an increase in HIV-1 p24-specific PROAb responses, which was mediated by HIV-1 p24-specific IgG1 antibodies. These findings provide further evidence that antibodies to HIV core proteins may contribute to control of HIV-1 infection.

Enterovirus Exposure Uniquely Discriminates Type 1 Diabetes Patients with a Homozygous from a Heterozygous Melanoma Differentiation-Associated Protein 5/Interferon Induced with Helicase C Domain 1 A946T Genotype

In children at risk for type 1 diabetes, innate immune activity is detected before seroconversion. Enterovirus infections have been linked to diabetes development, and a polymorphism (A946T) in the innate immune sensor recognizing enterovirus RNA, interferon-induced with helicase C domain 1/melanoma differentiation-associated protein 5, predisposes to disease. We hypothesized that the strength of innate antienteroviral responses is affected in autoimmune type 1 diabetes patients and linked to the A946T polymorphism. We compared induction of interferon-stimulated genes (ISGs) in peripheral blood mononuclear cells (PBMCs) and dendritic cells (DCs) in healthy individuals and diabetes patients upon stimulation with enterovirus, enterovirus-antibody complexes, or ligands mimicking infection in relation to the A946T polymorphism. Overall, PBMCs of diabetes patients and healthy donors showed comparable ISG induction upon stimulation. No differences were observed in DCs. Interestingly, the data imply that the magnitude of responses to enterovirus and enterovirus-antibody complexes in PBMCs is critically influenced by the A946T polymorphism and elevated in heterozygotes compared to TT homozygous individuals in autoimmune diabetes patients, but not healthy controls. These data imply an intrinsic difference in the responses to enterovirus and enterovirus-antibody complexes in diabetes patients carrying a TT risk genotype compared to heterozygotes that may influence control of enterovirus clearance.

Innate Immunity Evasion by Enteroviruses: Insights into Virus-Host Interaction

Enterovirus genus includes multiple important human pathogens, such as poliovirus, coxsackievirus, enterovirus (EV) A71, EV-D68 and rhinovirus. Infection with EVs can cause numerous clinical conditions including poliomyelitis, meningitis and encephalitis, hand-foot-and-mouth disease, acute flaccid paralysis, diarrhea, myocarditis and respiratory illness. EVs, which are positive-sense single-stranded RNA viruses, trigger activation of the host antiviral innate immune responses through pathogen recognition receptors such as retinoic acid-inducible gene (RIG-I)-likeand Toll-like receptors. In turn, EVs have developed sophisticated strategies to evade host antiviral responses. In this review, we discuss the interplay between the host innate immune responses and EV infection, with a primary focus on host immune detection and protection against EV infection and viral strategies to evade these antiviral immune responses.

Viremic HIV Controllers Exhibit High Plasmacytoid Dendritic Cell-Reactive Opsonophagocytic IgG Antibody Responses against HIV-1 p24 Associated with Greater Antibody Isotype Diversification

Identifying the mechanisms of natural control of HIV-1 infection could lead to novel approaches to prevent or cure HIV infection. Several studies have associated natural control of HIV-1 infection with IgG Abs against HIV-1 Gag proteins (e.g., p24) and/or production of IgG2 Abs against HIV-1 proteins. These Abs likely exert their effect by activating antiviral effector cell responses rather than virus neutralization. We hypothesized that an opsonophagocytic IgG Ab response against HIV-1 p24 that activates plasmacytoid dendritic cells (pDCs) through FcγRIIa would be associated with control of HIV and that this would be enhanced by Ab isotype diversification. Using the Gen2.2 pDC cell line, we demonstrated that pDC-reactive opsonophagocytic IgG Ab responses against HIV-1 p24 were higher in HIV controllers (HIV RNA < 2000 copies/ml) than noncontrollers (HIV RNA > 10,000 copies/ml), particularly in controllers with low but detectable viremia (HIV RNA 75-2000 copies/ml). Opsonophagocytic Ab responses correlated with plasma levels of IgG1 and IgG2 anti-HIV-1 p24 and, notably, correlated inversely with plasma HIV RNA levels in viremic HIV patients. Phagocytosis of these Abs was mediated via FcγRIIa. Isotype diversification (toward IgG2) was greatest in HIV controllers, and depletion of IgG2 from Ig preparations indicated that IgG2 Abs to HIV-1 p24 do not enhance phagocytosis, suggesting that they enhance other aspects of Ab function, such as Ag opsonization. Our findings emulate those for pDC-reactive opsonophagocytic Ab responses against coxsackie, picorna, and influenza viruses and demonstrate a previously undefined immune correlate of HIV-1 control that may be relevant to HIV vaccine development.

Reassessing immune control of hepatitis A virus

There is renewed interest in hepatitis A virus (HAV) pathogenesis and immunity after 2-3 decades of limited progress. From a public health perspective, the average age at infection has increased in developing countries, resulting in more severe hepatitis that is poorly understood mechanistically. More fundamentally, there is interest in comparing immunity to HAV and hepatitis C virus (HCV): small, positive-strand RNA viruses with very different infection outcomes. Here, we review evidence that circulating HAV virions are cloaked in membranes, with consequences for induction of innate immunity and antibody-mediated neutralization. We also consider the contribution of CD4+ helper versus CD8+ cytotoxic T cells to antiviral immunity and liver injury, and present a model of non-cytotoxic immune control of HAV infection.

Human pDCs preferentially sense enveloped hepatitis A virions

Unlike other picornaviruses, hepatitis A virus (HAV) is cloaked in host membranes when released from cells, providing protection from neutralizing antibodies and facilitating spread in the liver. Acute HAV infection is typified by minimal type I IFN responses; therefore, we questioned whether plasmacytoid dendritic cells (pDCs), which produce IFN when activated, are capable of sensing enveloped virions (eHAV). Although concentrated nonenveloped virus failed to activate freshly isolated human pDCs, these cells produced substantial amounts of IFN-α via TLR7 signaling when cocultured with infected cells. pDCs required either close contact with infected cells or exposure to concentrated culture supernatants for IFN-α production. In isopycnic and rate-zonal gradients, pDC-activating material cosedimented with eHAV but not membrane-bound acetylcholinesterase, suggesting that eHAV, and not viral RNA exosomes, is responsible for IFN-α induction. pDC activation did not require virus replication and was associated with efficient eHAV uptake, which was facilitated by phosphatidylserine receptors on pDCs. In chimpanzees, pDCs were transiently recruited to the liver early in infection, during or shortly before maximal intrahepatic IFN-stimulated gene expression, but disappeared prior to inflammation onset. Our data reveal that, while membrane envelopment protects HAV against neutralizing antibody, it also facilitates an early but limited detection of HAV infection by pDCs.

Microbial sensing by Toll-like receptors and intracellular nucleic acid sensors

Recognition of an invading pathogen is critical to elicit protective responses. Certain microbial structures and molecules, which are crucial for their survival and virulence, are recognized by different families of evolutionarily conserved pattern recognition receptors (PRRs). This recognition initiates a signaling cascade that leads to the transcription of inflammatory cytokines and chemokines to eliminate pathogens and attract immune cells, thereby perpetuating further adaptive immune responses. Considerable research on the molecular mechanisms underlying host-pathogen interactions has resulted in the discovery of multifarious PRRs. In this review, we discuss the recent developments in microbial recognition by Toll-like receptors (TLRs) and intracellular nucleic acid sensors and the signaling pathways initiated by them.

Increased IFN-α-producing plasmacytoid dendritic cells (pDCs) in human Th1-mediated type 1 diabetes: pDCs augment Th1 responses through IFN-α production

Increasing evidence suggests that type 1 IFN (IFN-αβ) is associated with pathogenesis of Th1-mediated type 1 diabetes (T1D). A major source of IFN-αβ is plasmacytoid dendritic cells (pDCs). In this study, we analyzed peripheral blood pDC numbers and functions in at-risk, new-onset, and established T1D patients and controls. We found that subjects at risk for T1D and new-onset and established T1D subjects possessed significantly increased pDCs but similar number of myeloid DCs when compared with controls. pDC numbers were not affected by age in T1D subjects but declined with increasing age in control subjects. It was demonstrated that IFN-α production by PBMCs stimulated with influenza viruses was significantly higher in T1D subjects than in controls, and IFN-α production was correlated with pDC numbers in PBMCs. Of interest, only T1D-associated Coxsackievirus serotype B4 but not B3 induced majority of T1D PBMCs to produce IFN-α, which was confirmed to be secreted by pDCs. Finally, in vitro studies demonstrated IFN-α produced by pDCs augmented Th1 responses, with significantly greater IFN-γ-producing CD4(+) T cells from T1D subjects. These findings indicate that increased pDCs and their IFN-αβ production may be associated with this Th1-mediated autoimmune disease, especially under certain viral infections linked to T1D pathogenesis.

In vivo ablation of type I interferon receptor from cardiomyocytes delays coxsackieviral clearance and accelerates myocardial disease

Acute coxsackievirus B3 (CVB3) infection is one of the most prevalent causes of acute myocarditis, a disease that frequently is identified only after the sudden death of apparently healthy individuals. CVB3 infects cardiomyocytes, but the infection is highly focal, even in the absence of a strong adaptive immune response, suggesting that virus spread within the heart may be tightly constrained by the innate immune system. Type I interferons (T1IFNs) are an obvious candidate, and T1IFN receptor (T1IFNR) knockout mice are highly susceptible to CVB3 infection, succumbing within a few days of challenge. Here, we investigated the role of T1IFNs in the heart using a mouse model in which the T1IFNR gene can be ablated in vivo, specifically in cardiomyocytes. We found that T1IFN signaling into cardiomyocytes contributed substantially to the suppression of viral replication and infectious virus yield in the heart; in the absence of such signaling, virus titers were markedly elevated by day 3 postinfection (p.i.) and remained high at day 12 p.i., a time point at which virus was absent from genetically intact littermates, suggesting that the T1IFN-unresponsive cardiomyocytes may act as a safe haven for the virus. Nevertheless, in these mice the myocardial infection remained highly focal, despite the cardiomyocytes' inability to respond to T1IFN, indicating that other factors, as yet unidentified, are sufficient to prevent the more widespread dissemination of the infection throughout the heart. The absence of T1IFN signaling into cardiomyocytes also was accompanied by a profound acceleration and exacerbation of myocarditis and by a significant increase in mortality.

IMPORTANCE

Acute coxsackievirus B3 (CVB3) infection is one of the most common causes of acute myocarditis, a serious and sometimes fatal disease. To optimize treatment, it is vital that we identify the immune factors that limit virus spread in the heart and other organs. Type I interferons play a key role in controlling many virus infections, but it has been suggested that they may not directly impact CVB3 infection within the heart. Here, using a novel line of transgenic mice, we show that these cytokines signal directly into cardiomyocytes, limiting viral replication, myocarditis, and death.

Isotype Diversification of IgG Antibodies to HIV Gag Proteins as a Therapeutic Vaccination Strategy for HIV Infection

The development of vaccines to treat and prevent human immunodeficiency virus (HIV) infection has been hampered by an incomplete understanding of "protective" immune responses against HIV. Natural control of HIV-1 infection is associated with T-cell responses against HIV-1 Gag proteins, particularly CD8⁺ T-cell responses restricted by "protective" HLA-B alleles, but other immune responses also contribute to immune control. These immune responses appear to include IgG antibodies to HIV-1 Gag proteins, interferon-a-dependant natural killer (NK) cell responses and plasmacytoid dendritic cell (pDC) responses. Here, it is proposed that isotype diversification of IgG antibodies against HIV-1 Gag proteins, to include IgG2, as well as IgG3 and IgG1 antibodies, will broaden the function of the antibody response and facilitate accessory cell responses against HIV-1 by NK cells and pDCs. We suggest that this should be investigated as a vaccination strategy for HIV-1 infection.

Nucleic acids and endosomal pattern recognition: how to tell friend from foe?

The innate immune system has evolved endosomal and cytoplasmic receptors for the detection of viral nucleic acids as sensors for virus infection. Some of these pattern recognition receptors (PRR) detect features of viral nucleic acids that are not found in the host such as long stretches of double-stranded RNA (dsRNA) and uncapped single-stranded RNA (ssRNA) in case of Toll-like receptor (TLR) 3 and RIG-I, respectively. In contrast, TLR7/8 and TLR9 are unable to distinguish between viral and self-nucleic acids on the grounds of distinct molecular patterns. The ability of these endosomal TLR to act as PRR for viral nucleic acids seems to rely solely on the mode of access to the endolysosomal compartment in which recognition takes place. The current dogma states that self-nucleic acids do not enter the TLR-sensing compartment under normal physiological conditions. However, it is still poorly understood how dendritic cells (DC) evade activation by self-nucleic acids, in particular with regard to specific DC subsets, which are specialized in taking up material from dying cells for cross-presentation of cell-associated antigens. In this review we discuss the current understanding of how the immune system distinguishes between foreign and self-nucleic acids and point out some of the key aspects that still require further research and clarification.

Enter at your own risk: how enteroviruses navigate the dangerous world of pattern recognition receptor signaling

Enteroviruses are the most common human viral pathogens worldwide. This genus of small, non-enveloped, single stranded RNA viruses includes coxsackievirus, rhinovirus, echovirus, and poliovirus species. Infection with these viruses can induce mild symptoms that resemble the common cold, but can also be associated with more severe syndromes such as poliomyelitis, neurological diseases including aseptic meningitis and encephalitis, myocarditis, and the onset of type I diabetes. In humans, polarized epithelial cells lining the respiratory and/or digestive tracts represent the initial sites of infection by enteroviruses. Control of infection in the host is initiated through the engagement of a variety of pattern recognition receptors (PRRs). PRRs act as the sentinels of the innate immune system and serve to alert the host to the presence of a viral invader. This review assembles the available data annotating the role of PRRs in the response to enteroviral infection as well as the myriad ways by which enteroviruses both interrupt and manipulate PRR signaling to enhance their own replication, thereby inducing human disease.

Differential susceptibility and response of primary human myeloid BDCA1(+) dendritic cells to infection with different Enteroviruses

Coxsackie B viruses (CVBs) and echoviruses (EVs) form the Human Enterovirus-B (HEV-B) species within the family Picornaviridae. HEV-B infections are widespread and generally cause mild disease; however, severe infections occur and HEV-B are associated with various chronic diseases such as cardiomyopathy and type 1 diabetes. Dendritic cells (DCs) are the professional antigen-presenting cells of our immune system and initiate and control immune responses to invading pathogens, yet also maintain tolerance to self-antigens. We previously reported that EVs, but not CVBs, can productively infect in vitro generated monocyte-derived DCs. The interactions between HEV-B and human myeloid DCs (mDCs) freshly isolated from blood, however, remain unknown. Here, we studied the susceptibility and responses of BDCA1(+) mDC to HEV-B species and found that these mDC are susceptible to EV, but not CVB infection. Productive EV7 infection resulted in massive, rapid cell death without DC activation. Contrary, EV1 infection, which resulted in lower virus input at the same MOI, resulted in DC activation as observed by production of type I interferon-stimulated genes (ISGs), upregulation of co-stimulatory and co-inhibitory molecules (CD80, CD86, PDL1) and production of IL-6 and TNF-α, with a relative moderate decrease in cell viability. EV1-induced ISG expression depended on virus replication. CVB infection did not affect DC viability and resulted in poor induction of ISGs and CD80 induction in part of the donors. These data show for the first time the interaction between HEV-B species and BDCA1(+) mDCs isolated freshly from blood. Our data indicate that different HEV-B species can influence DC homeostasis in various ways, possibly contributing to HEV-B associated pathology.

Isotype-switched immunoglobulin G antibodies to HIV Gag proteins may provide alternative or additional immune responses to 'protective' human leukocyte antigen-B alleles in HIV controllers

BACKGROUND

Natural control of HIV infection is associated with CD8 T-cell responses to Gag-encoded antigens of the HIV core and carriage of 'protective' human leukocyte antigen (HLA)-B alleles, but some HIV controllers do not possess these attributes. As slower HIV disease progression is associated with high levels of antibodies to HIV Gag proteins, we have examined antibodies to HIV proteins in controllers with and without 'protective' HLA-B alleles.

METHODS

Plasma from 32 HIV controllers and 21 noncontrollers was examined for immunoglobulin G1 (IgG1) and IgG2 antibodies to HIV proteins in virus lysates by western blot assay and to recombinant (r) p55 and gp140 by ELISA. Natural killer (NK) cell-activating antibodies and FcγRIIa-binding immune complexes were also assessed.

RESULTS

Plasma levels of IgG1 antibodies to HIV Gag (p18, p24, rp55) and Pol-encoded (p32, p51, p66) proteins were higher in HIV controllers. In contrast, IgG1 antibodies to Env proteins were less discriminatory, with only antigp120 levels being higher in controllers. High-level IgG2 antibodies to any Gag protein were most common in HIV controllers not carrying a 'protective' HLA-B allele, particularly HLA-B*57 (P = 0.016). HIV controllers without 'protective' HLA-B alleles also had higher plasma levels of IgG1 antip32 (P = 0.04). NK cell-activating antibodies to gp140 Env protein were higher in elite controllers but did not differentiate HIV controllers with or without 'protective' HLA-B alleles. IgG1 was increased in FcγRIIa-binding immune complexes from noncontrollers.

CONCLUSION

We hypothesize that isotype-switched (IgG2+) antibodies to HIV Gag proteins and possibly IgG1 antip32 may provide alternative or additional immune control mechanisms to HLA-restricted CD8 T-cell responses in HIV controllers.

Prevention or acceleration of type 1 diabetes by viruses

Type 1 diabetes is an autoimmune disease characterized by the destruction of insulin-producing pancreatic β-cells. Even though extensive scientific research has yielded important insights into the immune mechanisms involved in pancreatic β-cell destruction, little is known about the events that trigger the autoimmune process. Recent epidemiological and experimental data suggest that environmental factors are involved in this process. In this review, we discuss the role of viruses as an environmental factor on the development of type 1 diabetes, and the immune mechanisms by which they can trigger or protect against this pathology.

Sex-specific signaling through Toll-Like Receptors 2 and 4 contributes to survival outcome of Coxsackievirus B3 infection in C57Bl/6 mice

Background

Coxsackievirus B3 (CVB3) induces myocarditis, an inflammatory heart disease, which affects men more than women. Toll-like receptor (TLR) signaling has been shown to determine the severity of CVB3-induced myocarditis. No direct role for signaling through TLR2 had been shown in myocarditis although published studies show that cardiac myosin is an endogenous TLR2 ligand and stimulates pro-inflammatory cytokine expression by dendritic cells in vitro. The goal of this study is to determine which TLRs show differential expression in CVB3 infected mice corresponding to male susceptibility and female resistance in this disease.

Methods

Male and female C57Bl/6 mice were infected with 10² PFU CVB3 and killed on day 3 or 6 post infection. Hearts were evaluated for virus titer, myocardial inflammation, and TLR mRNA expression by PCR array and microarray analysis. Splenic lymphocytes only were evaluated by flow cytometry for the number of TLR+/CD3+, TLR+/CD4+, TLR+F4/80+ and TLR+/CD11c+ subpopulations and the mean fluorescence intensity to assess upregulation of TLR expression on these cells. Mice were additionally treated with PAM3CSK4 (TLR2 agonist) or ultrapure LPS (TLR4 agonist) on the same day as CVB3 infection or 3 days post infection to confirm their role in myocarditis susceptibility.

Results

Despite equivalent viral titers, male C57Bl/6 mice develop more severe myocarditis than females by day 6 after infection. Microarray analysis shows a differential expression of TLR2 at day 3 with female mice having higher levels of TLR2 gene expression compared to males. Disease severity correlates to greater TLR4 protein expression on splenic lymphocytes in male mice 3 days after infection while resistance in females correlates to preferential TLR2 expression, especially in spleen lymphocytes. Treating male mice with PAM reduced mortality from 55% in control CVB3 infected animals to 10%. Treating female mice with LPS increased mortality from 0% in control infected animals to 60%.

Conclusion

CVB3 infection causes an up-regulation of TLR2 in female and of TLR4 in male mice and this differential expression between the sexes contributes to disease resistance of females and susceptibility of males. While previous reports demonstrated a pathogenic role for TLR4 this is the first report that TLR2 is preferentially up-regulated in CVB3 infected female mice or that signaling through this TLR directly causes myocarditis resistance.

The role of MyD88 signaling in heterosubtypic influenza A virus infections

A mouse model of heterosubtypic influenza A virus infections was used to determine the role of MyD88 signaling in CD4+ T-cell, CD8+ T-cell, and IgG immune responses. We found that MyD88 signaling played an important role in anti-influenza A virus heterosubtypic lung and spleen CD4+ T-cell, and spleen CD8+ T-cell, immune responses. MyD88 dependent signaling was important for T-helper 1 cytokine production in anti-influenza A virus lung and spleen heterosubtypic CD4+ T-cells, but not for their frequencies. Toll-like receptor 7 dependent signaling played a partial role in anti-influenza A virus lung heterosubtypic CD4+ T-helper 1 responses and anti-influenza A virus heterosubtypic IgG2c antibody levels. Our results have important implications for the generation of effective universal influenza vaccines.

MicroRNAs regulate the pathogenesis of CVB3-induced viral myocarditis

AIMS

To evaluate the role of microRNAs (miRNAs) in the pathogenesis of Coxsackievirus B3 (CVB3)-induced viral myocarditis.

METHODS

We detected miRNA expression profiling by microarray utilizing a mouse model on day 4 after CVB3 infection. Then we validated differentially expressed miRNAs using real-time polymerase chain reaction (PCR). We predicted target genes using miRNA target prediction databases and assessed them using mRNA microarray and qualitative reverse transcription PCR measurements. By analyzing the target function of differentially expressed miRNAs, we initially explored the regulating role of miRNAs in viral myocarditis.

RESULTS

We found five differentially expressed miRNAs that are involved in regulating several important innate immune and antiviral pathways such as the Toll-like receptor signaling pathway, RIG-I-like receptor signaling pathway, NOD-like receptor signaling pathway, cytokine- cytokine receptor interaction, MAPK signaling pathway, JAK-STAT signaling pathway, and natural killer cell-mediated cytotoxicity.

CONCLUSION

miRNAs regulate the pathogenesis of viral myocarditis. This study may provide a new perspective and a deeper understanding of the pathogenesis of viral myocarditis that may help with the development of novel therapies against CVB3 infection.

Manipulating intestinal immunity and microflora: an alternative solution to viral myocarditis?

Viral myocarditis (VMC) is an important cause of heart failure and dilated cardiomyopathy with no effective clinical diagnosis and treatment, and has been commonly associated with Coxsackievirus B3 (CVB3) infection. Current evidence from CVB3 myocarditis in mice indicates that acute myocarditis is mainly mediated by the host immune responses, including Th1, Th17 and type I macrophages. Recently, innate immunity triggered by TLR3, TLR4, TLR8 and MDA5 has also been demonstrated to participate in the induction of inflammatory cytokines in response to CVB3. Apart from the heart tissue, the intestine, which is the assumed initial infection and important replication site for CVB3, needs to be investigated, where induction of innate immunity and interactions with microflora may shape the immune response involved in the pathogenesis of VMC. This review presents recent advances in research into innate and adaptive immunity to CVB3, and provides insights into developing new strategies for the future treatment for VMC.

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.

Innate and adaptive immune responses against picornaviruses and their counteractions: An overview

Picornaviruses, small positive-stranded RNA viruses, cause a wide range of diseases which is based on their differential tissue and cell type tropisms. This diversity is reflected by the immune responses, both innate and adaptive, induced after infection, and the subsequent interactions of the viruses with the immune system. The defense mechanisms of the host and the countermeasures of the virus significantly contribute to the pathogenesis of the infections. Important human pathogens are poliovirus, coxsackievirus, human rhinovirus and hepatitis A virus. These viruses are the best-studied members of the family, and in this review we want to present the major aspects of the reciprocal effects between the immune system and these viruses.

Immunology in the clinic review series; focus on type 1 diabetes and viruses: the innate immune response to enteroviruses and its possible role in regulating type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease arising as a consequence of a misdirected T cell response to the pancreatic beta cell. In recent years, there has been a growing interest in the innate immune system as a regulator of disease development. Genome-wide association studies have identified diabetes-associated polymorphisms in genes encoding proteins with functions related to the innate immune response. Moreover, enteroviruses, known to activate a strong innate immune response, have been implicated in the disease pathogenesis. In this review, we discuss the innate immune response elicited by enteroviruses and how this response may regulate T1D development.

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.

dsRNA sensors and plasmacytoid dendritic cells in host defense and autoimmunity

The innate immune system detects viruses through molecular sensors that trigger the production of type I interferons (IFN-I) and inflammatory cytokines. As viruses vary tremendously in size, structure, genomic composition, and tissue tropism, multiple sensors are required to detect their presence in various cell types and tissues. In this review, we summarize current knowledge of the diversity, specificity, and signaling pathways downstream of viral sensors and ask whether two distinct sensors that recognize the same viral component are complementary, compensatory, or simply redundant. We also discuss why viral sensors are differentially distributed in distinct cell types and whether a particular cell type dominates the IFN-I response during viral infection. Finally, we review evidence suggesting that inappropriate signaling through viral sensors may induce autoimmunity. The picture emerging from these studies is that disparate viral sensors in different cell types form a dynamic and integrated molecular network that can be exploited for improving vaccination and therapeutic strategies for infectious and autoimmune diseases.

The combination of early and rapid type I IFN, IL-1α, and IL-1β production are essential mediators of RNA-like adjuvant driven CD4+ Th1 responses

There is a growing need for novel vaccine adjuvants that can provide safe and potent T-helper type 1 (Th1) activity. RNA-like immune response modifiers (IRMs) are candidate T-cell adjuvants that skew acquired immune responses towards a Th1 phenotype. We set out to delineate the essential signaling pathways by which the RNA-like IRMs, resiquimod (R-848) and polyinosinic:polycytidylic acid (poly I:C), augment CD4+ T-helper 1 (Th1) responses. Highly purified murine conventional dendritic cells (cDCs) and conventional CD4+ T-cells were co-cultured in allogeneic and MHC congenic mixed leukocyte reactions. The activation of CD4+ Th1 cells was examined utilizing cells from mice deficient in specific RNA-sensing pattern recognition receptors and signaling mediators. R-848 and poly I:C stimulation of Type I interferon production and signaling in cDCs was essential but not sufficient for driving CD4+ Th1 responses. The early and rapid production of IL-1α and IL-1β was equally critical for the optimal activation of Th1 CD4+ T-cells. R-848 activation of Toll-like receptor 7/MyD88-dependent signaling in cDCs led to a rapid upregulation of pro-IL-1α and pro-IL-1β production compared to poly I:C activation of MyD88-independent signaling pathways. The in vitro data show that CD4+ T-cell adjuvant activity of RNA-like IRMs is mediated by a critical combination of early and rapid Type I interferon, IL-1α and IL-1β production. These results provide important insights into the key signaling pathways responsible for RNA-like IRM CD4+ Th1 activation. A better understanding of the critical signaling pathways by which RNA-like IRMs stimulate CD4+ Th1 responses is relevant to the rational design of improved vaccine adjuvants.

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.

Group B coxsackieviruses and autoimmunity: focus on Type 1 diabetes

Group B coxsackieviruses (CVB) and/or their components have been found in the blood and pancreas of patients with Type 1 diabetes (T1D). CVB infections lead to the activation of the innate and adaptive immune systems, which can result in the induction or aggravation of autoimmune processes. Persistent and/or repeated infections of pancreas islet β cells with CVB and the resulting production of IFN-α and inflammatory mediators, combined with a predisposed genetic background, may induce bystander activation of autoimmune effector T cells and an autoreactive response to islet self-antigens through molecular mimicry. Moreover, the antibody-dependent enhancement of CVB infection of monocytes, as well as infection of the thymus can intervene in the pathogenesis of T1D. In contrast with the deleterious effect of CVB, it has been shown that these viruses can protect against the development of T1D under certain experimental conditions. The role of CVB in autoimmunity is complex, and therefore a better understanding of the inducer versus protective effects of these viruses in T1D will help to design new strategies to treat and prevent the disease.

TLR7/9 versus TLR3/MDA5 signaling during virus infections and diabetes

IFN-I are pleiotropic cytokines that impact innate and adaptive immune responses. In this article, we discuss TLR7/9 versus TLR3/MDA5 signaling in antiviral responses and diabetes. pDCs are thought to have a critical role in antiviral defense because of their ability to rapidly secrete large amounts of IFN-I through TLR7/9 signaling. A recent study demonstrates that although pDCs are a source of IFN-I in vivo, their overall contribution to viral containment is limited and time-dependent, such that additional cellular sources of IFN-I are required to fully control viral infections. dsRNA sensors, such as TLR3 and MDA5, provide another important trigger for antiviral IFN-I responses, which can be exploited to enhance immune responses to vaccines. In the absence of infection, IFN-I production by pDCs or from signaling through dsRNA sensors has been implicated in the pathogenesis of autoimmune diseases such as diabetes. However, recent data demonstrate that IFN-I production via TLR3 and MDA5 is critical to counter diabetes caused by a virus with preferential tropism for pancreatic β-cells. This highlights the complexity of the host antiviral response and how multiple cellular and molecular components balance protective versus pathological responses.

2011 ESCI Award for Excellence in Basic / Translational Research: innate regulation of adaptive immunity by dendritic cells

Dendritic cells (DC) play a key role in the initiation of adaptive immunity, and the manipulation and/or targeting of DC has great potential for immune intervention. However, clinical applications are hampered by the fact that we still know relatively little about how DC become 'activated' to stimulate and direct T-cell responses. Over the last decade, much emphasis has been placed on dissecting innate signalling pathways that can trigger DC activation and promote T-cell priming. Here, we review work from our laboratory aimed at helping define 'pattern-recognition pathways' involved in DC activation by potential pathogens. One pathway for sensing infection by RNA viruses involves recognition of viral genomes or virally infected cells in endosomal compartments and utilises members of the toll-like receptor (TLRs) family, including TLR9, 7, or 3. RNA virus genomes can additionally be recognised in the cytosol by DExD/H-box helicases such as MDA5 or RIG-I, the latter of which is activated by RNAs bearing 5' tri-phosphates. Finally, a distinct pathway involves cell surface and phagosomal recognition of fungi by C-type lectins, which signal via Syk kinase. Notably, some of these pathways are involved not only in direct sensing of pathogens but also in the recognition of self-alterations that might accompany infection, such as induction of cell death. These studies help build a global picture of the receptors and signalling pathways that regulate DC activation and have applications in immunotherapy of cancer and infectious diseases.

Pattern recognition receptors and the innate immune response to viral infection

The innate immune response to viral pathogens is critical in order to mobilize protective immunity. Cells of the innate immune system detect viral infection largely through germline-encoded pattern recognition receptors (PRRs) present either on the cell surface or within distinct intracellular compartments. These include the Toll-like receptors (TLRs), the retinoic acid-inducble gene I-like receptors (RLRs), the nucleotide oligomerization domain-like receptors (NLRs, also called NACHT, LRR and PYD domain proteins) and cytosolic DNA sensors. While in certain cases viral proteins are the trigger of these receptors, the predominant viral activators are nucleic acids. The presence of viral sensing PRRs in multiple cellular compartments allows innate cells to recognize and quickly respond to a broad range of viruses, which replicate in different cellular compartments. Here, we review the role of PRRs and associated signaling pathways in detecting viral pathogens in order to evoke production of interferons and cytokines. By highlighting recent progress in these areas, we hope to convey a greater understanding of how viruses activate PRR signaling and how this interaction shapes the anti-viral immune response.

Acute hepatitis A virus infection is associated with a limited type I interferon response and persistence of intrahepatic viral RNA

Hepatitis A virus (HAV) is an hepatotropic human picornavirus that is associated only with acute infection. Its pathogenesis is not well understood because there are few studies in animal models using modern methodologies. We characterized HAV infections in three chimpanzees, quantifying viral RNA by quantitative RT-PCR and examining critical aspects of the innate immune response including intrahepatic IFN-stimulated gene expression. We compared these infection profiles with similar studies of chimpanzees infected with hepatitis C virus (HCV), an hepatotropic flavivirus that frequently causes persistent infection. Surprisingly, HAV-infected animals exhibited very limited induction of type I IFN-stimulated genes in the liver compared with chimpanzees with acute resolving HCV infection, despite similar levels of viremia and 100-fold greater quantities of viral RNA in the liver. Minimal IFN-stimulated gene 15 and IFIT1 responses peaked 1-2 wk after HAV challenge and then subsided despite continuing high hepatic viral RNA. An acute inflammatory response at 3-4 wk correlated with the appearance of virus-specific antibodies and apoptosis and proliferation of hepatocytes. Despite this, HAV RNA persisted in the liver for months, remaining present long after clearance from serum and feces and revealing dramatic differences in the kinetics of clearance in the three compartments. Viral RNA was detected in the liver for significantly longer (35 to >48 wk) than HCV RNA in animals with acute resolving HCV infection (10-20 wk). Collectively, these findings indicate that HAV is far stealthier than HCV early in the course of acute resolving infection. HAV infections represent a distinctly different paradigm in virus-host interactions within the liver.

Antibody-dependent enhancement of coxsackievirus B3 infection of primary CD19+ B lymphocytes

Coxsackievirus B3 (CVB3) is associated with several different acute and chronic forms of human disease, including myocarditis, aseptic meningitis, and pancreatitis. Moreover, CVB3 also infects immune cells like CD19+ B lymphocytes, but the viral uptake mechanism into these cells is not well understood. Therefore, primary murine and human CD19+ B cells were isolated by magnetic-activated cell separation technology and analyzed for virus receptor expression, antibody-dependent enhancement of viral infection, and different cellular surface proteins, that might be involved in mechanisms of viral uptake. Western blot analysis of these cells revealed no significant expression of the coxsackievirus-adenovirus receptor CAR. But incubation of CVB3 with serum dilutions, which exhibited binding but not neutralizing characteristics, increased viral uptake and replication significantly in a dose-dependent manner. Viral entry was reduced when Fc portions of immunoglobulins were blocked by protein A treatment. Moreover, the classical complement system rather than Fc-gamma-receptor-mediated mechanisms could be involved in viral uptake. Taken together, these data suggest an antibody-dependent enhancement of CVB3 infection of primary murine and human CD19+ B cells.

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

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

Type B coxsackieviruses and their interactions with the innate and adaptive immune systems

Coxsackieviruses are important human pathogens, and their interactions with the innate and adaptive immune systems are of particular interest. Many viruses evade some aspects of the innate response, but coxsackieviruses go a step further by actively inducing, and then exploiting, some features of the host cell response. Furthermore, while most viruses encode proteins that hinder the effector functions of adaptive immunity, coxsackieviruses and their cousins demonstrate a unique capacity to almost completely evade the attention of naive CD8(+) T cells. In this artcle, we discuss the above phenomena, describe the current status of research in the field, and present several testable hypotheses regarding possible links between virus infection, innate immune sensing and disease.

Vaccine-induced IgG2 anti-HIV p24 is associated with control of HIV in patients with a 'high-affinity' FcgammaRIIa genotype

OBJECTIVES

We have previously shown that vaccination with a recombinant fowlpox virus carrying the genes for HIV Gag-Pol and interferon-gamma (IFN-gamma) was associated with partial control of HIV replication after antiretroviral therapy (ART) was ceased but not with increased anti-HIV T-cell responses. Because IFN-gamma enhances IgG2 production, and IgG2 antibodies to HIV antigens and the 'high-affinity' polymorphism of FcgammaRIIa (the major Fc receptor for IgG2) have been associated with a favourable outcome of HIV infection, we examined the association of IgG2 antibodies to HIV p24 and 'high-affinity' polymorphisms of FcgammaRIIa with control of HIV replication in these patients.

METHODS

Plasma from weeks 0 (cessation of ART 1 week after the last vaccination), 9 and 20 was available from patients who had received the full construct vaccine, a partial construct (without IFN-gamma) or placebo. IgG2 and IgG1 anti-p24 and anti-gp41 were assayed and all patients were genotyped for the FcgammaRIIa 131 R/H polymorphism that affects IgG2 binding.

RESULTS

At week 0, IgG2 anti-p24 was present in five of nine full construct patients but none of 14 partial construct or placebo patients and was associated with a smaller increase in plasma HIV RNA over 20 weeks. Patients with IgG2 anti-p24 and the 'high-affinity' polymorphism of FcgammaRIIa exhibited lower HIV replication than other patients at week 20.

CONCLUSION

The role of IgG2 anti-HIV antibodies and FcgammaRIIa in the control of HIV replication should be investigated further. Inclusion of an IFN-gamma gene in DNA vaccine constructs might be a means of enhancing IgG2 antibody production.

Unraveling the functions of plasmacytoid dendritic cells during viral infections, autoimmunity, and tolerance

Plasmacytoid dendritic cells (pDCs) are bone marrow-derived cells that secrete large amounts of type I interferon (IFN) in response to viruses. Type I IFNs are pleiotropic cytokines with antiviral activity that also enhance innate and adaptive immune responses. Viruses trigger activation of pDCs and type I IFN responses mainly through the Toll-like receptor pathway. However, a variety of activating and inhibitory pDC receptors fine tune the amplitude of type I IFN responses. Chronic activation and secretion of type I IFN in the absence of infection can promote autoimmune diseases. Furthermore, while activated pDCs promote immunity and autoimmunity, resting or alternatively activated pDCs may be tolerogenic. The various roles of pDCs have been extensively studied in vitro and in vivo with depleting antibodies. However, depleting antibodies cross-react with other cell types that are critical for eliciting protective immunity, potentially yielding ambiguous phenotypes. Here we discuss new approaches to assess pDC functions in vivo and provide preliminary data on their potential roles during viral infections. Such approaches would also prove useful in the more specific evaluation of how pDCs mediate tolerance and autoimmunity. Finally, we discuss the emergent role of pDCs and one of their receptors, tetherin, in human immunodeficiency virus pathogenesis.

MDA5 and MAVS mediate type I interferon responses to coxsackie B virus

Coxsackie B viruses (CVB) are enteroviruses that have been associated with a variety of human diseases, including myocarditis. In the present study, we found that MDA5 and its adaptor molecule MAVS are critical for type I interferon responses to CVB, since the absence of either MAVS or MDA5 leads to deficient type I interferon production and early mortality in mice infected with CVB. Pancreatic and hepatic necrosis were observed on histopathological examination of MAVS and MDA5 knockout mice infected with CVB. Inflammatory cytokine production in response to systemic CVB infection was independent of MAVS. Surprisingly, virus titers were not elevated in MAVS-deficient mice, despite significant reductions in type I interferon levels. These data highlight the importance of type I interferon in host defense and provide insight on the mechanisms of innate immune responses following coxsackievirus infection.

Toll-like receptors and B-cell receptors synergize to induce immunoglobulin class-switch DNA recombination: relevance to microbial antibody responses

Differentiation of naïve B cells, including immunoglobulin class-switch DNA recombination, is critical for the immune response and depends on the extensive integration of signals from the B-cell receptor (BCR), tumor necrosis factor (TNF) family members, Toll-like receptors (TLRs), and cytokine receptors. TLRs and BCR synergize to induce class-switch DNA recombination in T cell-dependent and T cell-independent antibody responses to microbial pathogens. BCR triggering together with simultaneous endosomal TLR engagement leads to enhanced B-cell differentiation and antibody responses. Te requirement of both BCR and TLR engagement would ensure appropriate antigen-specific activation in an infection. Co-stimulation of TLRs and BCR likely plays a significant role in anti-microbial antibody responses to contain pathogen loads until the T cell-dependent antibody responses peak. Furthermore, the temporal sequence of different signals is also critical for optimal B cell responses, as exemplified by the activation of B cells by initial TLR engagement, leading to the up-regulation of co-stimulatory CD80 and MCH-II receptors, which result in more efficient interactions with T cells, thereby enhancing the germinal center reaction and antibody affinity maturation. Overall, BCR and TLR stimulation and the integration with signals from the pathogen or immune cells and their products determine the ensuing B-cell antibody response.