Scavenger receptor b2 as a receptor for hand, foot, and mouth disease and severe neurological diseases

Potential of a bivalent vaccine for broad protection against enterovirus 71 and coxsackie virus 16 infections causing hand, foot, and mouth disease

Hand, foot, and mouth disease (HFMD) is a highly contagious viral infection that is mainly caused by enterovirus 71 (EV71) and coxsackievirus 16 (CVA16). As there are no specific therapeutics for HFMD, the development of a bivalent vaccine is required to cover a broad range of infections. In this study, the effectiveness of novel monovalent and bivalent vaccines targeting EV71 C4a and CVA16 was investigated for their ability to prevent viral infections in neonatal human scavenger receptor class B member 2 (hSCARB2) transgenic mice. As hSCARB2 serves as a key viral receptor for EV71, these transgenic mice are susceptible to EV71 strains and facilitate viral binding, internalization, and uncoating processes. Antisera prepared by vaccine immunization were transferred to 2-day-old hSCARB2 transgenic mice, which were then infected with EV71 C4a or CVA16 virus. The antisera generated by each monovalent or bivalent vaccine effectively protected against EV71 C4a and CVA16 infections. The examination of tissue damage and viral contents in various organs indicated that both monovalent and bivalent antisera reduced EV71 C4a viral load in the brainstem, and no significant tissue damage was observed. During CVA16 infection, the monovalent and bivalent antisera significantly reduced viral contents in both the brainstem and muscles. These results suggest that passive immunity by monovalent and bivalent antisera can effectively protect against EV71 C4a and CVA16 infections. Thus, the development of a bivalent vaccine that can provide broad protection against both CV and EV infections may be a promising strategy in preventing HFMD.

Dual blockages of a broad and potent neutralizing IgM antibody targeting GH loop of EV-As

The reported enterovirus A 71 (EVA71) vaccines and immunoglobin G (IgG) antibodies have no cross-antiviral efficacy against other enterovirus A (EV-A) which caused hand, foot and mouth disease (HFMD). Here we constructed an IgM antibody (20-IgM) based on our previous discovery to address the resistance encountered by IgG-based immunotherapy. Although binding to the same conserved neutralizing epitope within the GH loop of EV-As VP1, the antiviral breath and potency of 20-IgM are still higher than its parental 20-IgG1. The 20-IgM blocks the interaction between the EV-As and its receptors, scavenger receptor class B, member 2 (SCARB2) and Kringle-containing transmembrane protein 1(KREMEN1) of the host cell. The 20-IgM also neutralizes the EV-As at the post-attachment stages, including postattachment neutralization, uncoating and RNA release inhibition after internalization. Mechanistically, the dual blockage effect of 20-IgM is dependent on both a conserved site targeting and high affinity binding. Meanwhile, 20-IgM provides cross-antiviral efficacy in EV-As orally infected neonatal ICR mice. Collectively, 20-IgM and its property exhibit excellent antiviral activity with a dual-blockage inhibitory effect at both the pre- and post-attachment stages. The finding enhances our understanding of IgM-mediated immunity and highlights the potential of IgM subtype antibodies against enterovirus infections.

A practical method for evaluating the in vivo efficacy of EVA-71 vaccine using a hSCARB2 knock-in mouse model

Hand-foot-and-mouth disease is a contagious disease common among children under 5 years old worldwide. It is caused by strains of enterovirus, especially EV-A71, which can lead to severe disease. Vaccines are the only way to fight this disease. Accordingly, it is necessary to establish an efficient and accurate methodology to evaluate vaccine efficacy in vivo. Here, we established a practical method using a hSCARB2 knock-in mouse model, which was susceptible to EV-A71 infection at 5-6 weeks of age, to directly determine the efficacy of vaccines. Unlike traditional approaches, one-week-old hSCARB2 mice were immunized twice with a licensed vaccine, with an interval of a week. The titre of antibodies was measured after 1 week. Mice at 4 weeks of age were challenged with EV-A71 intraperitoneally and intracranially, respectively. The unimmunized hSCARB2 mice displayed systemic clinical symptoms and succumbed to the disease at a rate of approximately 50%. High viral loads were detected in the lungs, brain, and muscles, accompanied by clear pathological changes. The expression of IL-1β, IL-13, IL-17, and TNF-α was significantly upregulated. By contrast, the immunized group was practically normal and indistinguishable from the control mice. These results indicate that the hSCARB2 knock-in mouse is susceptible to infection in adulthood, and the in vivo efficacy of EV-A71 vaccine could be directly evaluated in this mouse model. The method developed here may be used in the development of new vaccines against HFMD or quality control of licensed vaccines.

Reverse Genetic Approaches for the Generation of Full Length and Subgenomic Replicon of EV71 Virus

Enterovirus 71 (EV71) is a neurotropic pathogen that causes hand, foot, and mouth disease (HFMD) and it has been consistently associated with severe neurological, cardiac, and respiratory complications. Yet there is no specific treatment for this virus and we still know little about the viral pathogenesis. In this study, we first generated an infectious cDNA clone of EV71 virus from a patient virus strain and made a full-length virus with a NanoLuc reporter gene through reverse genetic approaches. The reporter gene of this virus is genetically stable when passaging in cells and could be used for antiviral testing. In addition, we also made subgenomic replicons (SGRs) of EV71, which lacks part of the structural genes dispensable for viral replication and showed that SGR can be used for viral replication study. Overall, these reporter viral systems are useful tools for EV71 pathogenesis study and antiviral screening.

Recombinant Human SCARB2 Expressed in Escherichia coli and its Potential in Enterovirus 71 Blockage

Hand, foot and mouth disease is a common viral infectious disease caused by enteroviruses, including coxsackie A16 (CVA16) and enterovirus 71 (EV71). HFMD can cause severe symptoms in children which can be fatal. Human scavenger receptor class B member 2 (SCARB2) is a cellular receptor for EV71 and CVA16, providing a potential approach for preventing EV71 infection and transmission. In this present study, we constructed and assessed the potential of recombinant SCARB2, using E. coli expression system. To generate this construct, scarb2 gene was cloned into pET22b vector and expressed in E. coli BL21 (DE3). The expression of SCARB2 was induced by 0.1 mM IPTG and analyzed using SDS-PAGE, followed by Western blot. Expressed SCARB2 was in inclusion bodies and refolded to obtain the soluble form with recovery efficacy of 100%. This recombinant protein was then validated for binding with EV71 via indirect ELISA in two different pHs (7.4 and 5.5), which partially revealed the mechanism of virus–receptor interaction. These results envisaged potential applications for utilizing recombinant SCARB2 in preventing the virus transmission.

Severity of enterovirus A71 infection in a human SCARB2 knock-in mouse model is dependent on infectious strain and route

Enterovirus A71 (EV-A71) is a major etiological agent of human hand, foot and mouth disease, and it can cause severe neurological complications. Although several genotypes of EV-A71 strains are prevalent in different regions of the world, the genotype C4 has circulated in mainland China for more than 20 years. The pathogenicity of different EV-A71 clinical isolates varies and needs to be explored. In this study, hSCARB2 knock-in mice (N = 181) with a wide range of ages were tested for their susceptibility to two EV-A71 strains with the subgenotypes C4 and C2, and two infection routes (intracranial and venous) were compared. The clinical manifestations and pathology and their relationship to the measured viral loads in different tissues were monitored. We observed that 3 weeks is a crucial age, as mice younger than 3-week-old that were infected became extremely ill. However, mice older than 3 weeks displayed diverse clinical symptoms. Significant differences were observed in the pathogenicity of the two strains with respect to clinical signs, disease incidence, survival rate, and body weight change. We concluded that hSCARB2 knock-in mice are a sensitive model for investigating the clinical outcomes resulting from infection by different EV-A71 strains. The intracranial infection model appears to be suitable for evaluating EV-A71 neurovirulence, whereas the venous infection model is appropriate for studying the pathogenicity of EV-A71.

The binding of a monoclonal antibody to the apical region of SCARB2 blocks EV71 infection

Entero virus 71 (EV71) causes hand, foot, and mouth disease (HFMD) and occasionally leads to severe neurological complications and even death. Scavenger receptor class B member 2 (SCARB2) is a functional receptor for EV71, that mediates viral attachment, internalization, and uncoating. However, the exact binding site of EV71 on SCARB2 is unknown. In this study, we generated a monoclonal antibody (mAb) that binds to human but not mouse SCARB2. It is named JL2, and it can effectively inhibit EV71 infection of target cells. Using a set of chimeras of human and mouse SCARB2, we identified that the region containing residues 77-113 of human SCARB2 contributes significantly to JL2 binding. The structure of the SCARB2-JL2 complex revealed that JL2 binds to the apical region of SCARB2 involving α-helices 2, 5, and 14. Our results provide new insights into the potential binding sites for EV71 on SCARB2 and the molecular mechanism of EV71 entry.

Enterovirus 71 infection of motor neuron-like NSC-34 cells undergoes a non-lytic exit pathway

Enterovirus 71 (EV71) causing Hand, Foot and Mouth Disease, is regarded as the most important neurotropic virus worldwide. EV71 is believed to replicate in muscles and infect motor neurons to reach the central nervous system (CNS). To further investigate the mechanisms involved, we have employed the motor neuron cell line NSC-34. NSC-34 cells were permissive to EV71 and virus production yields were strain-dependent with differential efficacy at the entry, replication and egress steps. Furthermore, unlike all the other cell lines previously reported, EV71-infected NSC-34 cells neither displayed cytopathic effect nor underwent apoptosis. Instead, autophagy was markedly up-regulated and virus-containing autophagic vacuoles were isolated from the culture supernatant, providing the first experimental evidence that EV71 can adopt a non-lytic exit pathway. Finally, the ability of EV71 to infect productively NSC-34 cells correlated with its ability to invade the CNS in vivo, supporting the relevance of NSC-34 cells to study the intrinsic neurovirulence of EV71 strains.

A literature review and case report of hand, foot and mouth disease in an immunocompetent adult

Background

To report an uncommon case of hand, foot and mouth disease, (HFMD) in an immunocompetent adult; a highly infectious disease, characterized by the appearance of vesicles on the mouth, hands and feet, associated with coxsackieviruses and enteroviruses; including a literature review.

Case report

A 23 year Caucasian male with no medical or surgical history, no allergies, was not taking any medication and smoked ten cigarettes a day, suffering from discomfort in the oral cavity; itching, burning and pain when swallowing associated with small erythematous lesions located on the hard palate, and small ulcers in tonsillar pillars and right buccal mucosa. Mild fever of 37.8 °C and general malaise. The patient reported he had had contact with a child diagnosed with HFMD. From his background and symptoms, the patient was diagnosed with HFMD. Following symptomatic treatment, the symptoms remitted in 7 days.

Methods

A literature review in MEDLINE (PubMed). The inclusion criteria were for studies on humans over the last 5 years, using the keywords HFMD.

Results

We found 925 articles, which were subsequently reduced to 52 documents after applying the inclusion criteria. Maculopapular lesions were found on hands and feet.

Conclusions

Dentists may have a key role diagnosing the disease. A surveillance system to predict future outbreaks, encourage early diagnosis, put appropriate public health measures in place and research vaccine development is vitally important in order to control the disease.

Mannose 6-phosphate-independent Lysosomal Sorting of LIMP-2

The lysosomal integral membrane protein type 2 (LIMP-2/SCARB2) has been described as a mannose 6-phosphate (M6P)-independent trafficking receptor for β-glucocerebrosidase (GC). Recently, a putative M6P residue in a crystal structure of a recombinantly expressed LIMP-2 ectodomain has been reported. Based on surface plasmon resonance and fluorescence lifetime imaging analyses, it was suggested that the interaction of soluble LIMP-2 with the cation-independent M6P receptor (MPR) results in M6P-dependent targeting of LIMP-2 to lysosomes. As the physiological relevance of this observation was not addressed, we investigated M6P-dependent delivery of LIMP-2 to lysosomes in murine liver and mouse embryonic fibroblasts. We demonstrate that LIMP-2 and GC reach lysosomes independent of the M6P pathway. In fibroblasts lacking either MPRs or the M6P-forming N-acetylglucosamine (GlcNAc)-1-phosphotransferase, LIMP-2 still localizes to lysosomes. Immunoblot analyses also revealed comparable LIMP-2 levels within lysosomes purified from liver of wild-type (wt) and GlcNAc-1-phosphotransferase-defective mice. Heterologous expression of the luminal domain of LIMP-2 in wild-type, LIMP-2-deficient and GlcNAc-1-phosphotransferase-defective cells further established that the M6P modification is dispensable for lysosomal sorting of LIMP-2. Finally, cathepsin Z, a known GlcNAc-1-phosphotransferase substrate, but not LIMP-2, could be precipitated with M6P-specific antibodies. These data prove M6P-independent lysosomal sorting of LIMP-2 and subsequently GC in vivo.

Recent advances from studies on the role of structural proteins in enterovirus infection

Enteroviruses are a large group of small nonenveloped viruses that cause common and debilitating illnesses affecting humans and animals worldwide. The capsid composed by viral structural proteins packs the RNA genome. It is becoming apparent that structural proteins of enteroviruses play versatile roles in the virus–host interaction in the viral life cycle, more than just a shell. Furthermore, structural proteins to some extent may be associated with viral virulence and pathogenesis. Better understanding the roles of structural proteins in enterovirus infection may lead to the development of potential antiviral strategies. Here, we discuss recent advances from studies on the role of structural proteins in enterovirus infection and antiviral therapeutics targeted structural proteins.

Scavenger receptor structure and function in health and disease

Scavenger receptors (SRs) are a ‘superfamily’ of membrane-bound receptors that were initially thought to bind and internalize modified low-density lipoprotein (LDL), though it is currently known to bind to a variety of ligands including endogenous proteins and pathogens. New family of SRs and their properties have been identified in recent years, and have now been classified into 10 eukaryote families, defined as Classes A-J. These receptors are classified according to their sequences, although in each class they are further classified based in the variations of the sequence. Their ability to bind a range of ligands is reflected on the biological functions such as clearance of modified lipoproteins and pathogens. SR members regulate pathophysiological states including atherosclerosis, pathogen infections, immune surveillance, and cancer. Here, we review our current understanding of SR structure and function implicated in health and disease.

Association analysis of polymorphisms in OAS1 with susceptibility and severity of hand, foot and mouth disease

Hand, foot and mouth disease (HFMD) is a common childhood illness that mainly affects Asian children under the age of 5 years. Human enterovirus 71 (EV71) and coxsackievirus A16 (CA16) are the most common pathogens of HFMD. It is imperative that the susceptible population is screened early and that the severe illness population can be identified via genetic variation detection in children. Four single-nucleotide polymorphisms (SNP) [2'-5'-oligoadenylate synthetase1 (OAS1) rs10774671, selectin P ligand (SELPLG) rs2228315, scavenger receptor class B member 2 (SCARB2) rs41284767 and interleukin 28B (IL28B) rs12979860] were determined by Taqman assays in 333 HFMD samples and 163 control samples. The rs2228315, rs41284767 and rs12979860 polymorphisms did not differ significantly between HFMD patients and the controls, but the prevalence of the rs10774671 polymorphism was significantly different between the control children and children infected with CA16 (GG genotype vs. AA + AG genotype, P < 0.05). Children with the GG genotype were more susceptible to CA16-type HFMD. Furthermore, the rs10774671 genotype distribution was clearly different between children with severe HFMD and those with mild HFMD [P < 0.05, OR 0.240, 95% CI (0.071-0.809)]. HFMD children with the AA+AG genotype were more likely to progress to encephalitis than were those with the GG genotype. Plasma γ-interferon (IFN) expression levels among control children and the mild and severe HFMD children were detected by ELISA. Those with mild HFMD had higher γ-IFN expression levels compared with those with severe HFMD (P < 0.05). In addition, there is a significant correlation between γ-IFN levels and OAS1 rs10774671 SNP, as analysed by linear correlation assay. The GG genotype correlated with higher γ-IFN levels (P < 0.05). In short, the OAS1 rs10774671 SNP GG genotype contributed to CA16 susceptibility and was associated with the development of mild HFMD.

Histopathological features and distribution of EV71 antigens and SCARB2 in human fatal cases and a mouse model of enterovirus 71 infection

Enterovirus 71 (EV71) is a neurotropic pathogen that causes hand, foot, and mouth disease. While infection is usually self-limiting, a minority of patients infected with EV71 develop severe neurological complications. In humans, EV71 has been reported to utilize the scavenger receptor class B, member 2 (SCARB2) as a receptor for infectious cellular entry. In this study, we define the pathological features of EV71-associated disease as well as the distribution of EV71 antigen and SCARB2 in human fatal cases and a mouse model. Histopathologically, human fatal cases showed severe central nervous system (CNS) changes, mainly in the brainstems, spinal cords, and thalamus. These patient further exhibited pulmonary edema and necrotic enteritis. Immunohistochemical analysis of human fatal cases demonstrated that EV71 antigen and SCARB2 were observed mainly in neurons, microglia cells and inflammatory cells in the CNS, and epithelial cells in the intestines. However, skeletal muscle tissue was negative for EV71 antigen. In a mouse model of EV71 infection, we observed massive necrotic myositis, different degrees of viral diseases in CNS, and extensive interstitial pneumonia. In mice, EV71 exhibits strong myotropism compared to the neurotropism seen in humans. EV71 antigen was detected in the spinal cord and brainstem of mice. However, there was no clear correlation between mouse SCARB2 and EV71 antigen distribution in the mouse model, consistent with previous results that SCARB2 functions as a receptor for EV71 in humans but not mice. The EV71-induced lesions seen in the mouse model resembled the pathological changes seen in human samples. These results increase our understanding of EV71 pathogenesis and will inform further work developing a mouse model for EV71 infection.

Establishment of cell lines with increased susceptibility to EV71/CA16 by stable overexpression of SCARB2

BACKGROUND

Human enterovirus type 71 (EV71) and Coxsackievirus A group type 16 (CA16) belong to human Enterovirus species A of the family Picornaviridae. These viruses are recognized as the major pathogens responsible for epidemics of hand-foot-mouth disease (HFMD), which presents with fever and vesicular eruptions of palms, soles of the feet or mouth. Human scavenger receptor class B, member 2 (SCARB2) has been identified as the receptor for both EV71 and CA16, as overexpression of SCARB2 in cells can enhance virus replication significantly.

METHODS

In this study, we used a lentivirus packaging vector to transduce the SCARB2 gene into human embryonic kidney cells (293), human rhabdomyosarcoma cells (RD) and African green monkey kidney cells (Vero) to create stable expression lines. Expression of SCARB2 in the resulting three transgenic cell lines was confirmed by real-time RT-PCR, immunofluorescence and flow cytometry.

RESULTS

Levels of SCARB2 mRNA determined by real-time RT-PCR in 293-SCARB2 (293S) or RD-SCARB2 (RDS) transgenic cell lines were approximately 2 × 10(2) times higher than those in 293 and RD cells, respectively, and three times higher in Vero-SCARB2 (VeroS) than in Vero cells. Furthermore, EV71 and CA16 virus titers in 293S and RDS cells were 10(2)-10(3)-fold higher (detected in RD cell) than those in the parental cells, and a 10-fold higher titer of EV71 was achieved in VeroS cells compared with that in Vero cells.

CONCLUSIONS

We established for the first time three cell lines stably overexpressing SCARB2, which showed drastic increases in susceptibility to EV71/CA16 infection. These optimal cell lines may be utilized to develop inactivated vaccines for EV71/CA16 and facilitate rapid detection and isolation of HFMD pathogens or other Enterovirus serotypes. Furthermore, these stable cell lines also can serve as tools to facilitate drug screenings as well as molecular studies on virus-host interactions and pathogenesis of causative agents for HFMD.

Lysosomal membrane proteins and their central role in physiology

The lysosomal membrane was thought for a long time to primarily act as a physical barrier separating the luminal acidic milieu from the cytoplasmic environment. Meanwhile, it has been realized that unique lysosomal membranes play essential roles in a number of cellular events ranging from phagocytosis, autophagy, cell death, virus infection to membrane repair. This review provides an overview about the most interesting emerging functions of lysosomal membrane proteins and how they contribute to health and disease. Their importance is exemplified by their role in acidification, transport of metabolites and ions across the membrane, intracellular transport of hydrolases and the regulation of membrane fusion events. Studies in patient cells, non-mammalian model organisms and knockout mice contributed to our understanding of how the different lysosomal membrane proteins affect cellular homeostasis, developmental processes as well as tissue functions. Because these proteins are central for the biogenesis of this compartment they are also considered as attractive targets to modulate the lysosomal machinery in cases where impaired lysosomal degradation leads to cellular pathologies. We are only beginning to understand the complex composition and function of these proteins which are tightly linked to processes occurring throughout the endocytic and biosynthetic pathways.

Characterisation of enterovirus 71 replication kinetics in human colorectal cell line, HT29

Hand, Foot and Mouth Disease (HFMD), a contagious viral disease that commonly affects infants and children with blisters and flu like symptoms, is caused by a group of enteroviruses such as Enterovirus 71 (EV71) and coxsackievirus A16 (CA16). However some HFMD caused by EV71 may further develop into severe neurological complications such as encephalitis and meningitis. The route of transmission was postulated that the virus transmit from one person to another through direct contact of vesicular fluid or droplet from the infected or via faecal-oral route. To this end, this study utilised a human colorectal adenocarcinoma cell line (HT29) with epithelioid morphology as an in vitro model for the investigation of EV71 replication kinetics. Using qPCR, viral RNA was first detected in HT29 cells as early as 12 h post infection (hpi) while viral protein was first detected at 48 hpi. A significant change in HT29 cells’ morphology was also observed after 48 hpi. Furthermore HT29 cell viability also significantly decreased at 72 hpi. Together, data from this study demonstrated that co-culture of HT29 with EV71 is a useful in vitro model to study the pathogenesis of EV71.