Neurotropic Enterovirus Research Articles

Therapeutic and prevention strategies against human enterovirus 71 infection.

Human enterovirus 71 (HEV71) is the cause of hand, foot and mouth disease and associated neurological complications in children under five years of age. There has been an increase in HEV71 epidemic activity throughout the Asia-Pacific region in the past decade, and it is predicted to replace poliovirus as the extant neurotropic enterovirus of highest global public health significance. To date there is no effective antiviral treatment and no vaccine is available to prevent HEV71 infection. The increase in prevalence, virulence and geographic spread of HEV71 infection over the past decade provides increasing incentive for the development of new therapeutic and prevention strategies against this emerging viral infection. The current review focuses on the potential, advantages and disadvantages of these strategies. Since the explosion of outbreaks leading to large epidemics in China, research in natural therapeutic products has identified several groups of compounds with anti-HEV71 activities. Concurrently, the search for effective synthetic antivirals has produced promising results. Other therapeutic strategies including immunotherapy and the use of oligonucleotides have also been explored. A sound prevention strategy is crucial in order to control the spread of HEV71. To this end the ultimate goal is the rapid development, regulatory approval and widespread implementation of a safe and effective vaccine. The various forms of HEV71 vaccine designs are highlighted in this review. Given the rapid progress of research in this area, eradication of the virus is likely to be achieved.

Regulation of host responses and viral replication by the mitogen-activated protein kinases in intestinal epithelial cells infected with Enterovirus 71

Enterovirus 71 (EV71) has become an important neurotropic enterovirus in children since the eradication of the poliovirus. A cytokine storm with elevated levels of proinflammatory cytokines is proposed as critical to the pathogenesis of EV71-induced brain stem encephalitis with pulmonary edema, often a fatal complication. Mitogen-activated protein kinases (MAPK) are involved in cellular processes including immune responses and apoptosis. EV71 infection can trigger MAPK activation in human cells, and extracellular signal-regulated kinase (ERK) signaling is essential for EV71 replication. In this study, we report that in EV71-infected human intestinal epithelial cells (HT-29), both ERK1/2 and Jun-N-terminal kinase 1/2 (JNK1/2) were phosphorylated and activated. While MAPK was not involved in the induction of IL-1β, the expressions of IL-6 and IL-8 were positively regulated by both ERK1/2 and JNK1/2, which were also responsible for type I IFN induction. HT-29 cells underwent apoptosis in EV71 infection, but the activation of ERK1/2 and JNK1/2 signaling appeared to suppress induction of TNF-α and FasL and inhibit extrinsic apoptosis in infected cells. Activation of NF-κB was also observed in HT-29 cells in EV71 infection, leading to increased viral replication and proinflammatory cytokine induction. However, the activation of NF-κB was inhibited by the inhibitors of ERK1/2, p38 and JNK1/2, indicating that a cross-talk may exist between MAPK and NF-κB pathways in EV71-infected intestinal epithelial cells. Understanding host responses and their underlying mechanisms may help develop effective therapeutics against EV71 and tools for control of future EV71 outbreaks.

Cell and tissue tropism of enterovirus 71 and other enteroviruses infections

Enterovirus 71 (EV71) is a member of Picornaviridae that causes mild and self-limiting hand, foot, and mouth disease (HFMD). However, EV71 infections can progress to polio-like paralysis, neurogenic pulmonary edema, and fatal encephalitis in infants and young children. Large EV71 outbreaks have been reported in Taiwan, China, Japan, Malaysia, Singapore, and Australia. This virus is considered a critical emerging public health threat. EV71 is an important crucial neurotropic enterovirus for which there is currently no effective antiviral drug or vaccine. The mechanism by which EV71 causes severe central nervous system complications remains unclear. The interaction between the virus and the host is vital for viral replication, virulence, and pathogenicity. SCARB2 or PSGL-1 receptor binding is the first step in the development of viral infections, and viral factors (e.g., 5′ UTR, VP1, 3C, 3D, 3′ UTR), host factors and environments (e.g., ITAFs, type I IFN) are also involved in viral infections. The tissue tropism and pathogenesis of viruses are determined by a combination of several factors. This review article provides a summary of host and virus factors affecting cell and tissue tropism and the pathogenesis of enteroviruses.

Enterovirus 71 2C Protein Inhibits TNF-α–Mediated Activation of NF-κB by Suppressing IκB Kinase β Phosphorylation

Enterovirus 71 (EV71), a single, positive-stranded RNA virus, has been regarded as the most important neurotropic enterovirus after the eradication of the poliovirus. EV71 infection can cause hand, foot, and mouth disease or herpangina. Cytokine storm with elevated levels of proinflammatory and inflammatory cytokines, including TNF-α, has been proposed to explain the pathogenesis of EV71-induced disease. TNF-α-mediated NF-κB signaling pathway plays a key role in inflammatory response. We hypothesized that EV71 might also moderate host inflammation by interfering with this pathway. In this study, we tested this hypothesis and identified EV71 2C protein as an antagonist of TNF-α-mediated activation of NF-κB signaling pathway. Expression of 2C protein significantly reduced TNF-α-mediated NF-κB activation in 293T cells as measured by gene reporter and gel mobility shift assays. Furthermore, overexpression of TNFR-associated factor 2-, MEK kinase 1-, IκB kinase (IKK)α-, or IKKβ-induced NF-κB activation, but not constitutively active mutant of IKKβ (IKKβ SS/EE)-induced NF-κB activation, was inhibited by 2C protein. These data together suggested that the activation of IKKβ is most likely targeted by 2C; this notion was further strengthened by immunoblot detection of IKKβ phosphorylation and IκBα phosphorylation and degradation. Coimmunoprecipitation and colocalization of 2C and IKKβ expressed in mammalian cells provided compelling evidence that 2C interacts with IKKβ. Collectively, our data indicate that EV71 2C protein inhibits IKKβ activation and thus blocks NF-κB activation.

Fatal Case of Enterovirus 71 Infection, France, 2007

A fatal case of enterovirus 71 infection with pulmonary edema and rhombencephalitis occurred in Brest, France, in April 2007. The virus was identified as subgenogroup C2. This highly neurotropic enterovirus merits specific surveillance outside the Asia-Pacific region.

Seroprevalence and molecular epidemiology of enterovirus 71 in Germany

Enterovirus 71 (EV71) has emerged as a significant pathogen with potential to cause large outbreaks. Because little is known about its seroprevalence and molecular epidemiology in Germany, data for 1997-2007 are presented. Four hundred thirty-six sera from persons aging 10 months to 75 years were tested in a neutralisation test; 63.4% of pre-school children were seronegative, whereas about 75% of adults had antibodies to EV71. Phylogenetic analysis of 28 isolates associated with neurological or cutaneous manifestations showed that isolates belonging to genogroup C1 predominated in 2000-2005, followed by a change to genogroup C2 in 2006 and 2007. This shows the importance of monitoring the diversity of one of the most relevant neurotropic enteroviruses.

Novel Antiviral Agent DTriP-22 Targets RNA-Dependent RNA Polymerase of Enterovirus 71

Enterovirus 71 (EV71) has emerged as an important virulent neurotropic enterovirus in young children. DTriP-22 (4{4-[(2-bromo-phenyl)-(3-methyl-thiophen-2-yl)-methyl]-piperazin-1-yl}-1-pheny-1H-pyrazolo[3,4-d]pyrimidine) was found to be a novel and potent inhibitor of EV71. The molecular target of this compound was identified by analyzing DTriP-22-resistant viruses. A substitution of lysine for Arg163 in EV71 3D polymerase rendered the virus drug resistant. DTriP-22 exhibited the ability to inhibit viral replication by reducing viral RNA accumulation. The compound suppressed the accumulated levels of both positive- and negative-stranded viral RNA during virus infection. An in vitro polymerase assay indicated that DTriP-22 inhibited the poly(U) elongation activity, but not the VPg uridylylation activity, of EV71 polymerase. These findings demonstrate that the nonnucleoside analogue DTriP-22 acts as a novel inhibitor of EV71 polymerase. DTriP-22 also exhibited a broad spectrum of antiviral activity against other picornaviruses, which highlights its potential in the development of antiviral agents.

Mouse and Non-human Primate Models for EV71 Disease

During the final stage of polio eradication in the Western Pacific Region (WPR) in the late 1990s, there have been large outbreaks of hand, foot and mouth disease (HFMD) due to enterovirus 71 (EV71) associated with severe neurological diseases. However, pathogenesis of EV71 infection is still poorly understood, in part, due to limited animal models to study the neurovirulence of EV71. Immediately after the identification of EV71 as one of most neurotropic enteroviruses, a non-human primate model for EV71 disease had been established in the 1970s. In response to recent EV71-aassociated HFMD outbreaks in the WPR, we have extended this non-human primate model to study the molecular basis of EV71 neurovirulence by using different genotypes of field EV71 isolates and genetically modified EV71 mutants derived from infectious molecular clones of EV71. By using the non-human primate model, we found that two major recent lineages of EV71 in the WPR, genogroups B and C, are considered to be neurovirulent, and we have analyzed the attenuation determinants and immunogenicity of EV71 for further vaccine development. The non-human primate model may provide more information on EV71 pathogenesis than previous mouse infection models. However, mouse models, using young mice and/or mouse-adapted EV71 variants, have been widely used to study the in vivo phenotypes of EV71 due to the limited availability of non-human primates. Thus, we have recently developed a novel mouse infection model using NOD/SCID mice and a mouse adapted EV71 variant, and confirmed the presence of the same attenuation determinants of EV71 both in non-human primate and mouse models. Although primate and mouse models would be still important to study the pathogenesis of EV71, identification of the cellular receptor and development of transgenic mice susceptible to EV71 may provide new insights into the molecular basis of EV71 infection in the near future.

Differential localization of neurons susceptible to enterovirus 71 and poliovirus type 1 in the central nervous system of cynomolgus monkeys after intravenous inoculation.

Poliovirus and enterovirus 71 (EV71) are both neurotropic enteroviruses that cause serious neurological diseases, such as poliomyelitis and encephalitis. The neurovirulence of EV71 in cynomolgus monkeys was demonstrated previously by intraspinal inoculation. In this study, an improved simian model of EV71 infection was established by using intravenous inoculation, which revealed clinical and neuropathological similarities between this model and human cases of encephalitis. Experimental EV71 infection induced direct neurological manifestations, such as tremor, ataxia and brain oedema, but not non-neurological complications, such as pulmonary oedema and cardiac failure. Using this model of EV71 infection, the neurotropic characteristics of the prototype strains of EV71 and poliovirus type 1 (PV1) were compared. Three monkeys were inoculated intravenously with 10(5.5) TCID50 EV71 and all developed neurological disease signs within 4-6 days of inoculation. However, after inoculation with 10(5.5) TCID50 PV1 strain OM1 (PV1-OM1), the major manifestation was flaccid paralysis, starting from the lower limbs 6-9 days post-inoculation. Histopathological and virological analyses of moribund monkeys revealed that disseminated EV71 infection was characterized by severe panencephalitis involving both the pyramidal and extrapyramidal systems. In contrast, the lesions induced by PV1-OM1 were mainly restricted to the pyramidal tract, particularly the spinal motor neurons, thalamus and motor cortex. In conclusion, neuropathological involvement in this model correlated well with the apparent differences in neurological disease induced by EV71 and PV1-OM1. Thus, intravenous inoculation with EV71 is an excellent model to study the neuropathology of EV71 and to evaluate candidate vaccines and potential antiviral agents.

Binding to decay-accelerating factor is not required for infection of human leukocyte cell lines by enterovirus 70.

Enterovirus 70 (EV70) is one of several human enteroviruses that exhibit a propensity for infecting the central nervous system (CNS). The mechanisms by which neurotropic enteroviruses gain access to and invade the CNS are poorly understood. One possibility is that circulating leukocytes become infected and carry neurotropic enteroviruses to the CNS. We examined the ability of EV70 to infect cell lines derived from lymphoid, myeloid, and monocytic lineages. Most leukocyte cell lines tested bound radiolabeled EV70 and were permissive for EV70 replication, suggesting that EV70, in contrast to other enteroviruses, has an in vitro tropism that includes lymphoid, monocytic, and myeloid cell lines. For some of the cell lines, virus binding and infection correlated with surface expression of decay-accelerating factor (DAF), an attachment protein for EV70 on HeLa cells. However, EV70 also adsorbed to and infected cell lines that expressed little or no DAF. In contrast to what was observed for HeLa cells, neither DAF-specific monoclonal antibodies nor phosphatidylinositol-specific phospholipase C treatment inhibited EV70 binding to permissive leukocyte cell lines, and antibody blockade of DAF had little or no effect on EV70 replication. We also found that neither the human coxsackievirus-adenovirus receptor nor intercellular cell adhesion molecule 1, which mediate the entry of coxsackie B viruses and coxsackievirus A21, respectively, functions as a receptor for EV70. EV70 binding to all cell lines was sensitive to sialidase treatment and to inhibition of O glycosylation by benzyl N-acetyl-alpha-D-galactosaminide. Taken together, these results suggest that a sialylated molecule(s) other than DAF serves as a receptor for EV70 on permissive human leukocyte cell lines.

Viral infection of retinal pigment epithelium: a possible role for initiation of Behçet's disease

The in vitro susceptibility of human retinal pigment epithelial cells (RPE) to representative members of different groups of human pathogenic viruses was investigated in this study. Subacute viral infection is known to change the phenotype of infected cells, thereby causing immune-mediated tissue damage. Downregulation of cell surface antigens provides a means of long-term survival of viruses and persistent infection. Therefore the aim of this study was to investigate the capacity of in vitro infection with viruses and the expression of different cell surface molecules on human RPE cells following viral infection with special emphasis on those having immune regulatory functions. Primary cultures of RPE cells were infected with various viruses. We found infection with different neurotropic viruses, respiratory viruses and enteroviruses whereas no infection was observed with lymphotropic viruses. Cytomegalovirus (CMV) downregulated MHC class I antigens on RPE, whereas coxsackie virus (CVB) and HSV did not alter MHC class I antigen expression. No induction of class II antigens was observed in RPE cells infected with CVB, HSV or CMV. Adhesion molecule ICAM-1 (CD54) was slightly increased after virus infection and the other cell surface molecules did not alter. Several common human viruses could infect RPE cells. As even animal viruses, such as pseudorabies virus, could infect these cells, it might be possible that transient infections with animal viruses could act as a trigger for 'autoimmune' retinal diseases – and under certain circumstances like genetic predisposition or immunologic disorders this could lead to Behcet's disease in the eye.

Lactoferrin inhibits enterovirus 71 infection of human embryonal rhabdomyosarcoma cells in vitro.

Enterovirus 71 (EV71), the newest member of Enterovirudae, is notable for its etiological role in epidemics of severe neurological diseases in children. It appears to be emerging as an important virulent neurotropic enterovirus in the upcoming era of poliomyelitis eradication, whereas no effective vaccine or antiviral agents are available at this moment. Human and bovine lactoferrins, iron-binding proteins belonging to the nonimmune defense system, were assayed in vitro to assess their inhibiting capacity on the cytopathic effect of EV71 on human embryonal rhabdomyosarcoma (RD) cells. Both bovine and human lactoferrins were found to be potent inhibitors of EV71 infection (mean IC(50), 10.5-24.5 microg/mL and 103.3-185.0 microg/mL, respectively). Lactoferrin probably exerts its effect at the level of viral adsorption, since the ongoing infection could not be further inhibited after the EV71 penetrated RD cells.

Mode of spread to and within the central nervous system after oral infection of neonatal mice with the DA strain of Theiler's murine encephalomyelitis virus

Theiler's murine encephalomyelitis virus is a neurotropic enterovirus known to cause biphasic neural disease after intracerebral inoculation into adult mice. The present study characterizes a neonatal mouse model with a high disease incidence for the study of the acute phase of the pathogenesis of the DA strain of Theiler's murine encephalomyelitis virus after oral infection. The route of viral spread to and within the central nervous system (CNS) was determined by examining the kinetics of viral replication in various organs and by performing histopathological analysis. Viral antigen was detected widely in the neonatal CNS, mainly in the gray matter, and it was asymmetrical and multifocal in its distribution, with considerable variation in lesion distribution from animal to animal. Necrotizing lesions appeared to expand by direct extension from infected cells to their close neighbors, with a general disregard of neuroanatomical boundaries. The diencephalon showed particular susceptibility to viral infection. Other areas of the CNS, including the cerebellum and dentate gyrus of the hippocampus, were consistently spared. Neurons with axons extending peripherally to other organs or receiving direct input from the peripheral nervous system were not preferentially affected. The kinetics of viral replication in the liver, spleen, and CNS and the histopathological findings indicate that viral entry to the CNS is via a direct hematogenous route in orally infected neonatal mice and that the disease then progresses within the CNS mainly by direct extension from initial foci.