Development and evaluation of indirect ELISAs for the detection of IgG, IgM and IgA1 against duck hepatitis A virus 1

Development of a cell-free toehold switch for hepatitis A virus type I on-site detection

Picornavirus hepatitis A virus (HAV) is a common cause of hepatitis worldwide. It is spread primarily through contaminated food and water or person-to-person contact. HAV I has been identified as the most common type of human HAV infection. Here, we have developed a cell-free toehold switch sensor for HAV I detection. We screened 10 suitable toehold switch sequences using NUPACK software, and the VP1 gene was used as the target gene. The optimal toehold switch sequence was selected by in vivo expression. The best toehold switch concentration was further found to be 20 nM in a cell-free system. 5 nM trigger RNA activated the toehold switch to generate visible green fluorescence. The minimum detection concentration decreased to 1 pM once combined with NASBA. HAV I trigger RNA could be detected accurately with excellent specificity. In addition, the cell-free toehold switch sensor was verified in HAV I entities. The successful construction of the cell-free toehold switch sensor provided a convenient, rapid, and accurate method for HAV I on-site detection, especially in developing countries, without the involvement of expensive facilities and additional professional operators.

A polysaccharide from Pleurotus citrinopileatus mycelia enhances the immune response in cyclophosphamide-induced immunosuppressed mice via p62/Keap1/Nrf2 signal transduction pathway

The manuscript aimed to study the immunoregulatory activity and the mechanism of the polysaccharide (CMP) from Pleurotus citrinopileatus mycelia. The mice were divided into normal group, model group, different dosage of CMP (50, 100 and 200 mg/kg, respectively) groups and levamisole hydrochloride treated group. The results showed that, compared with the model group, CMP could significantly improve the auricle swelling rate, half hemolysis value and phagocytic index in mice. The indices of immune organs were raised, and tissue damage of spleen was relieved. Splenic Th1 cells were decreased, while Th2 cells were increased, furthermore the proliferation of splenic lymphocytes and the cytotoxicity of NK cells were increased. The levels of interleukin-12 (IL-12), interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) in spleen were decreased, while interleukin-4 (IL-4) and interleukin-10 (IL-10) were increased. In serum and spleen, the levels of catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) activities were increased, while the level of malondialdehyde (MDA) was decreased. And the levels of Immunoglobulin were also increased. Western blot showed that CMP had immunoregulatory activity by activating Nrf2, Keap1, p62, HO-1, and NQO1 in the p62/Keap1/Nrf2 signaling pathway. The study proved that CMP could be used as a biological Immune regulating agent.

The DHAV-1 protein VP1 interacts with PI3KC3 to induce autophagy through the PI3KC3 complex

Duck hepatitis A virus type 1 (DHAV-1) is one of the main pathogens responsible for death in ducklings. Autophagy is a catabolic process that maintains cellular homeostasis, and the PI3KC3 protein plays an important role in the initiation of autophagy. DHAV-1 infection induces autophagy in duck embryo fibroblasts (DEFs) but the molecular mechanism between it and autophagy has not been reported. First, we determined that DHAV-1 infection induces autophagy in DEFs and that autophagy induction is dependent on the integrity of viral proteins by infecting DEFs with UV-inactivated or heat-inactivated DHAV-1. Then, in experiments using the pharmacological autophagy inducer rapamycin and the autophagy inhibitor chloroquine, autophagy inhibition was shown to reduce intracellular and extracellular DHAV-1 genome copies and viral titres. These results suggest that autophagy activated by DHAV-1 infection in DEFs affects DHAV-1 proliferation and extracellular release. Next, we screened the autophagy-inducing effects of the DHAV-1 structural proteins VP0, VP3, and VP1 and found that all DHAV-1 structural proteins could induce autophagy in DEFs but not the full autophagic flux. Finally, we found that VP1 promotes protein expression of PI3KC3 and Beclin1 by western blot experiments and that VP1 interacts with PI3KC3 by co-immunoprecipitation experiments; moreover, 3-MA-induced knockdown of PI3KC3 inhibited VP1 protein-induced autophagy in DEFs. In conclusion, the DHAV-1 structural protein VP1 regulates the PI3KC3 complex by interacting with PI3KC3 to induce autophagy in DEFs.

Development of an indirect ELISA method based on the VP4 protein for detection antibody against duck hepatitis A virus type 1

In Picornavirus, the VP4 gene is located inside the viral capsid, but the antibodies it produces have neutralizing activity. At the same time, we know that the VP4 gene is relatively conserved among the four structural proteins, which makes the usage VP4 protein-based antigen potentially meaningful. Therefore, we used purified duck hepatitis A virus type 1 (DHVA-1) recombinant VP4 protein as the coating antigen to establish an indirect method. The optimal antigen, serum and enzyme-labeled antibody dilutions were 1:400 (3.375 μg/mL), 1:80 and 1:1600, respectively. The optimal blocking buffer was 1% skimmed milk powder. The cutoff value was determined to be 0.203, and the analytical sensitivity was 1:1600. The established ELISA method has good specificity, repeatability and sensitivity. It has a high coincidence rate of 70.8 % with the DHVA-1 as the coating antigen. So, it can be used for the detection of DHAV-1 serum antibodies.

A viroporin-like 2B protein of duck hepatitis A virus 1 that induces incomplete autophagy in DEF cells

Duck hepatitis A virus 1 (DHAV-1) can cause high morbidity and fatal acute infectious hepatitis in ducklings, which seriously endangers animal husbandry. Viroporin is a small molecular weight hydrophobic transmembrane protein encoded by the virus, that has been suggested to induce autophagy in host cells by increasing the membrane permeability through disturbing the ion balance. In this study, we aimed to investigate whether the DHAV-1 2B protein can induce autophagy in DEF cells with a viroporin-like function. Bioinformatics analysis has indicated that the 2B protein is characterized by a viroporin domain, which is consistent with the type IA viroporin transmembrane protein. We experimentally confirmed that the 2B protein disturbed the Ca2+ balance of infected cells by elevating the intracellular Ca2+ concentration. Eukaryotic expression of the 2B protein upregulates the expression of microtubule-associated protein 1 light chain 3 II (LC3-II) and the number of autophagosomes in the cell. Interestingly, the Western Blot (WB) results showed that 2B protein expression induced less protein degradation of the autophagic substrate sequestosome 1 (SQSTM1/p62) than the positive control, while microscopy observations showed that the autophagosomes did not colocalize with the lysosomes. In summary, 2B protein expression induced autophagy in host cells, but the autophagic flow was incomplete. The results of this experiment are expected to provide reference scientific data for elucidating the infective and pathogenic mechanism of DHAV-1.

Development and evaluation of an indirect ELISA based on recombinant structural protein VP2 to detect antibodies against duck hepatitis A virus

An indirect enzyme-linked immunosorbent assay (I-ELISA) based on the VP2 protein of duck hepatitis A virus type 3 (DHAV-3) was established in this study. The optimal dilutions of antigen, serum and goat anti-duck IgG conjugate were 1:1600 (2.23 μg/mL), 1:160 and 1:2000, respectively. The optimal blocking buffer was 1% skim milk. The cut-off value for the method was 0.25, and the analytical sensitivity of the method was 1:5120. The results of specific evaluation showed that except for DHAV-1, DHAV-3 antisera did not cross-react with any other common duck-sensitive pathogens, indicating that this method can be used to detect DHAV-3 and DHAV-1 antibodies. The coefficients of variation (CVs) were lower than 10 %. The coincidence rate between the VP2-DHAV-3-ELISA and the neutralization test was 93.3 %. In summary, the I-ELISA method based on VP2 protein has high sensitivity, specificity, and coincidence rate compared with the neutralization test and has advantages in serum monitoring. The I-ELISA method based on VP2 protein provides a simple and rapid method for the detection of anti-DHAV antibodies and the epidemiological monitoring of DHAV.

The VP3 protein of duck hepatitis A virus mediates host cell adsorption and apoptosis

Duck hepatitis A virus (DHAV) causes an infectious disease that mainly affects 1- to 4-week-old ducklings, resulting in considerable loss to the duck industry. Although there have been many studies on DHAV in recent years, the effects on host infection and pathogenesis of DHAV-1 remain largely unknown. This study investigated the effects of the DHAV-1 structural protein VP3 on DHAV-1 virus adsorption and apoptosis to explore the role of VP3 in the viral life cycle. The effects of DHAV-1 VP3 and an antibody against the protein on virion adsorption was analyzed by qRT-PCR. The results showed that the virus copy number for the rabbit anti-VP3 IgG-treated group was significantly lower than that for the negative control group but higher than that for the rabbit anti-DHAV-1 IgG-treated group. This result indicates that VP3 mediates DHAV-1 virus adsorption but that it is not the only protein that involved in this process. In addition, a eukaryotic recombinant plasmid, pCAGGS/VP3, was transfected into duck embryo fibroblasts (DEFs), and the apoptotic rate was determined by DAPI staining, the TUNEL assay and flow cytometry. DAPI staining showed nucleus fragmentation and nuclear edge shifting. TUNEL assay results revealed yellow nuclei, and flow cytometry indicated a significant increase in the apoptotic rate. In addition, qRT-PCR revealed increased in the transcriptional levels of the apoptotic caspase-3, −8 and −9, with the largest increase for caspase-3, followed by caspase-9 and caspase-8. Enzyme activity analysis confirmed these results. Furthermore, the VP3 protein decreased the mitochondrial membrane potential, and the transcriptional levels of the proapoptotic factors Bak, Cyt c and Apaf-1 in the mitochondrial apoptotic pathway were significantly upregulated. These data suggest that expression of VP3 in DEFs induces apoptosis and may primarily activate caspase-3-induced apoptosis through mitochondrion-mediated intrinsic pathways. The findings provide scientific data to clarify DHAV-1 infection and pathogenesis.

Biochemical characterization of recombinant Avihepatovirus 3C protease and its localization

BACKGROUND

The picornaviral 3C protease mediates viral polyprotein maturation and multiple cleavages of host proteins to modulate viral translation and transcription. The 3C protease has been regarded as a valid target due to its structural similarity among different picornaviruses and minimal sequence similarity with host proteins; therefore, the development of potent inhibitors against the 3C protease as an antiviral drug is ongoing. Duck hepatitis A virus (DHAV) belongs to the Picornavidea family and is a major threat to the poultry industry. To date, little is known about the roles of the DHAV 3C protease plays during infection.

METHODS

In this study, we compared the full-length DHAV 3C protein sequence with other 3C sequences to obtain an alignment for the construction of a phylogenetic tree. Then, we expressed and purified recombinant DHAV 3C protease in the BL21 expression system using nickel-NTA affinity chromatography. The optimization of the cleavage assay conditions and the kinetic analysis for DHAV 3C protease were done by in vitro cleavage assays with a fluorogenic peptide respectively. The inhibitory activity of rupintrivir against the DHAV 3C protease was further evaluated. The localization of the 3C protease in infected and transfected cells was determined using immunofluorescence and confocal microscopy.

RESULTS

Under different expression conditions, the 3C protease was found to be highly expressed after induction with 1 mM IPTG at 16 °C for 10 h. We synthesized a fluorogenic peptide derived from the cleavage site of the DHAV polyprotein and evaluated the protease activity of the DHAV 3C protease for the first time. We used fluorimetric based kinetic analysis to determine kinetic parameters, and Vmax and Km values were determined to be 16.52 nmol/min and 50.78 μM, respectively. Rupintrivir was found to exhibit inhibitory activity against the DHAV 3C protease. Using polyclonal antibody and an indirect immunofluorescence microscopy assay (IFA), it was determined that the DHAV 3C protease was found in the nucleus during infection. In addition, the DHAV 3C protease can enter into the nucleus without the cooperation of viral proteins.

CONCLUSIONS

This is the first study to examine the activity of the DHAV 3C protease, and the activity of the DHAV 3C protease is temperature-, pH- and NaCl concentration- dependent. The DHAV 3C protease localizes throughout DHAV-infected cells and can enter into the nucleus in the absence of other viral proteins. The kinetic analysis was calculated, and the Vmax and Km values were 16.52 nmol/min and 50.78 μM, respectively, using the Lineweaver-Burk plot.

DHAV-1 Inhibits Type I Interferon Signaling to Assist Viral Adaption by Increasing the Expression of SOCS3

Duck hepatitis A virus type 1 (DHAV-1) is one of the most lethal pathogens in the duck industry. The attenuated vaccine (the CH60 strain) is cultivated through serial passage in chicken embryos and is widely used for the prevention and control of the disease. However, the specific mechanism underlying its adaptation in chicken embryos has not been fully elucidated. In this study, we first infected chicken embryo fibroblasts (CEFs) with the DHAV-1 CH60 strain. The peak of viral proliferation occurred within 36–48 h post-infection. The different DHAV-1 strains significantly induced the expression of IFNα, IFNγ, and Suppressor of cytokine signaling 3 (SOCS3) in CEFs, and we found that SOCS3 overexpression significantly promoted viral replication. Furthermore, SOCS3 overexpression significantly inhibited the expression of IFNα but promoted the expression of IFNγ. In addition, SOCS3 overexpression clearly decreased the mRNA levels of STAT1 and STAT3 in the Janus kinase (JAK)-STAT signaling pathway and inhibited the expression of the antiviral proteins MX1 and OASL. Immune-precipitation assays indicated that SOCS3 and IFNα do not physically interact. Subcellular localization of SOCS3 and IFNα revealed that SOCS3 was mainly located in the nucleus and cytoplasm, while IFNα was located only in the cytoplasm. Co-localization of these two proteins was not observed in the cytoplasm. In conclusion, the DHAV-1 CH60 strain may inhibit the expression of IFNα by increasing the SOCS3 protein and SOCS3 can in turn decrease STAT1 and STAT3 mRNA levels, thereby inhibiting the antiviral protein MX1 and ultimately promoting viral proliferation, indirectly assisting in viral adaptation in chicken embryos.

Development and evaluation of an indirect ELISA based on recombinant nonstructural protein 3A to detect antibodies to duck hepatitis A virus type 1

To develop an indirect enzyme-linked immunosorbent assay(I-ELISA) method based on 3A protein of duck hepatitis A virus type 1(DHAV-1) for detection of DHAV-1 antibody, the recombinant protein 3A of DHAV-1 was expressed in E.coli and detected by Western blotting with DHAV-1 infected duck serum. A 3A-ELISA method using the expressed 3A protein as coating antigen for the detection of antibodies to DHAV-1 was developed. The optimal antigen, serum and enzyme-labeled antibody dilutions were 1:200(6.185 μg/ml), 1:20 and 1:2000, respectively. The optimal blocking buffer was 5% BSA. The cutoff value was determined to be 0.274, and the analytical sensitivity was 1:1280. There was no cross reaction between DHAV-1 infected duck serum and other common pathogenic duck serum, indicating that I-ELISA could be used to detect DHAV-1 infected duck serum. The coefficients of variation(CVs) were lower than 10%. The concordance between the I-ELISA based on the 3A subunit of DHAV-1 and that based on the whole DHAV-1 particle was 92.7%. Taken together, the 3A-ELISA method is a highly sensitive and specific test that could be used for screening for DHAV-1 infection and monitoring DHAV-1 antibody.

Transcriptomic Characterization of a Chicken Embryo Model Infected With Duck Hepatitis A Virus Type 1

Duck hepatitis A virus type 1 (DHAV-1) is one of the most common and lethal pathogens in young ducklings. Live-attenuated DHAV vaccine (CH60 strain) developed by passaging in chicken embryos provided effective immune protection for ducklings. However, the accurate mechanism for such adaption in chicken embryos is not fully revealed. Here, we utilize RNA-sequencing to perform global transcriptional analysis of DHAV-1-innoculated embryonated livers along with histopathological and ultrastructural analysis. This study revealed that infection with DHAV-1 strain CH60 is associated with enhanced type I and II interferon responses, activated innate immune responses, elevated levels of suppressor of cytokine signaling 1 and 3 (SOCS1 and SOCS3) accompanied with abnormalities in multiple metabolic pathways. Excessive inflammatory and innate immune responses induced by the CH60 strain are related to severe liver damage. Our study presents a comprehensive characterization of the transcriptome of chicken embryos infected with DHAV-CH60 and provides insight for in-depth exploration of viral adaption and virus–host interactions.

Incompatible Translation Drives a Convergent Evolution and Viral Attenuation During the Development of Live Attenuated Vaccine

Live attenuated vaccines are widely used to protect humans or animals from pathogen infections. We have previously developed a chicken embryo-attenuated Duck Hepatitis A Virus genotype 1 (DHAV-1) vaccine (CH60 strain). This study aims to understand the mechanisms that drive a virulent strain to an attenuated virus. Here, we systematically compared five DHAV-1 chicken embryo attenuated strains and 68 virulent strains. Phylogenetic analysis indicated that duck virulent strains isolated from different geographic regions of China undergo a convergent evolution in the chicken embryos. Comparative analysis indicated that the codon usage bias of the attenuated strains were shaped by chicken codons usage bias, which essentially contributed to viral adaption in the unsuitable host driven by incompatible translation. Of note, the missense mutations in coding region and mutations in untranslated regions may also contribute to viral attenuation of DHAV-1 to some extent. Importantly, we have experimentally confirmed that the expression levels of four viral proteins (2A3pro, 2A3pro, 3C^pro, and 3D^pro) in the liver and kidney of ducks infected with an attenuated strain are significantly lower than that infected with a virulent strain, despite with similar virus load. Thus, the key mechanisms of viral attenuation revealed by this study may lead to innovative and easy approaches in designing live attenuated vaccines.

Cytokine storms are primarily responsible for the rapid death of ducklings infected with duck hepatitis A virus type 1

Duck hepatitis A virus type 1 (DHAV-1) is one of the most harmful pathogens in the duck industry. The infection of adult ducks with DHAV-1 was previously shown to result in transient cytokine storms in their kidneys. To understand how DHAV-1 infection impacts the host liver, we conducted animal experiments with the virulent CH DHAV-1 strain and the attenuated CH60 commercial vaccine strain. Visual observation and standard hematoxylin and eosin staining were performed to detect pathological damage in the liver, and viral copy numbers and cytokine expression in the liver were evaluated by quantitative PCR. The CH strain (10^8.4 copies/mg) had higher viral titers than the CH60 strain (10^4.9 copies/mg) in the liver and caused ecchymotic hemorrhaging on the liver surface. Additionally, livers from ducklings inoculated with the CH strain were significantly infiltrated by numerous red blood cells, accompanied by severe cytokine storms, but similar signs were not observed in the livers of ducklings inoculated with the CH60 strain. In conclusion, the severe cytokine storm caused by the CH strain apparently induces hemorrhagic lesions in the liver, which might be a key factor in the rapid death of ducklings.

Virologic and Immunologic Characteristics in Mature Ducks with Acute Duck Hepatitis A Virus 1 Infection

Duck hepatitis A virus 1 (DHAV-1) infection in mature ducks has previously been proposed as a small-animal model for human hepatitis A. However, basic research on the outcome of DHAV-1 infection in mature ducks is limited. Here, we examined the course of viremia, the characteristics of antibody responses, and the profiles of plasma cytokines in mature ducks infected with DHAV-1. During the course of infection, the viremia was detectable soon after infection and persisted for 196 days, however, the ducks presented as clinically asymptomatic. Specific and timely immunoglobulin G (IgG), IgM, and IgA1 responses were elicited. At the same time, extensive inhibition of viral replication was observed with increasing IgG concentration. With respect to pattern-recognition receptors, TLR-7 was mainly involved in triggering the innate defense against the DHAV-1 infection. In addition, plasma immune analytes were measured and were determined to have bidirectional roles in virus clearance. It was concluded that DHAV-1 spreads quickly in blood. The spontaneous clearance of DHAV-1 during asymptomatic infection in mature ducks depends on the cooperation of timely antibody responses and alert innate immune responses. Moreover, the delayed clearance may be associated with a weak interferon-γ-producing CD8+ T cell response. This study allows us to reveal the mechanism of clearance and persistence of DHAV-1 infection in mature ducks. We anticipate that it will provide a basis for future studies focused on defining the nature mechanisms involved in the clearance and persistence of human hepatitis virus.

Comparative analysis of virus-host interactions caused by a virulent and an attenuated duck hepatitis A virus genotype 1

Because of their better immunogenicity and the improved protection they afford, live attenuated vaccines derived from serial passaging in an abnormal host are widely used to protect humans or animals from certain pathogens. Here, we used a virulent and a chicken embryo-attenuated duck hepatitis A virus genotype 1 to compare the different regulated immune responses induced by viruses with differing virulence. In this study, the attenuated strains had lower protein expression levels than the virulent strains as identified by immunohistochemistry. This may be caused by apparent codon usage bias selected during passage. Furthermore, lower translation efficiency led to decreased viral replication, which is highly dependent on non-structural viral protein expression. Although the two strains had differing levels of virulence, both could induce strong innate immune responses and robust Tc or Th cell populations during the early stages of the immune response. However, due to fixed single nucleotide polymorphisms (SNPs) selected by passage, the virulent and attenuated strains may induce differing immune responses, with stronger Tc cell immunity induced by the attenuated strain in the spleen and thymus and stronger Tc cell immunity induced by the virulent strain in the liver, lung, bursa of Fabricius and Harderian gland. Four immune related genes (RIG-1, MDA5, IFN-β, and IL-6) were highly differentially expressed in the Harderian gland, bursa of Fabricius and thymus. This study has provided further information about differences in virus-host interactions between duck hepatitis A viruses of differing virulence.

Online Detection of Peroxidase Using 3D Printing, Active Magnetic Mixing, and Spectra Analysis

A new method for online detection of peroxidase (POD) using 3D printing, active magnetic mixing, fluidic control, and optical detection was developed and demonstrated in this study. The proposed POD detection system consisted of a 3D printing and active magnetic mixing based fluidic chip for online catalytic reaction, an optical detector with a fluidic flow cell for quantitative determination of the final catalysate, and a single-chip microcontroller based controller for automatic control of two rotating magnetic fields and four precise peristaltic pumps. Horseradish peroxidase (HRP) was used as research model and a linear relationship between the absorbance at the characteristic wavelength of 450 nm and the concentration of HRP of 1/4-1/128 μg mL-1 was obtained as A  =  0.257ln⁡(C) + 1.425 (R2  = 0.976). For the HRP spiked pork tests, the recoveries of HRP ranged from 93.5% to 110.4%, indicating that this proposed system was capable of detecting HRP in real samples. It has the potential to be extended for online detection of the activity of other enzymes and integration with ELISA method for biological and chemical analysis.