Evaluation of antigenic differences between wild and Sabin vaccine strains of poliovirus using the pseudovirus neutralization test

An efficient trans complementation system for in vivo replication of defective poliovirus mutants

The picornavirus genome encodes a large, single polyprotein that is processed by viral proteases to form an active replication complex. The replication complex is formed with the viral genome, host proteins, and viral proteins that are produced/translated directly from each of the viral genomes (viral proteins provided in cis). Efficient complementation in vivo of replication complex formation by viral proteins provided in trans, thus exogenous or ectopically expressed viral proteins, remains to be demonstrated. Here, we report an efficient trans complementation system for the replication of defective poliovirus (PV) mutants by a viral polyprotein precursor in HEK293 cells. Viral 3AB in the polyprotein, but not 2BC, was processed exclusively in cis. Replication of a defective PV replicon mutant, with a disrupted cleavage site for viral 3Cpro protease between 3Cpro and 3Dpol (3C/D[A/G] mutant) could be rescued by a viral polyprotein provided in trans. Only a defect of 3Dpol activity of the replicon could be rescued in trans; inactivating mutations in 2CATPase/hel, 3B, and 3Cpro of the replicon completely abrogated the trans-rescued replication. An intact N-terminus of the 3Cpro domain of the 3CDpro provided in trans was essential for the trans-active function. By using this trans complementation system, a high-titer defective PV pseudovirus (PVpv) (>107 infectious units per mL) could be produced with the defective mutants, whose replication was completely dependent on trans complementation. This work reveals potential roles of exogenous viral proteins in PV replication and offers insights into protein/protein interaction during picornavirus infection.

IMPORTANCE

Viral polyprotein processing is an elaborately controlled step by viral proteases encoded in the polyprotein; fully processed proteins and processing intermediates need to be correctly produced for replication, which can be detrimentally affected even by a small modification of the polyprotein. Purified/isolated viral proteins can retain their enzymatic activities required for viral replication, such as protease, helicase, polymerase, etc. However, when these proteins of picornavirus are exogenously provided (provided in trans) to the viral replication complex with a defective viral genome, replication is generally not rescued/complemented, suggesting the importance of viral proteins endogenously provided (provided in cis) to the replication complex. In this study, I discovered that only the viral polymerase activity of poliovirus (PV) (the typical member of picornavirus family) could be efficiently rescued by exogenously expressed viral proteins. The current study reveals potential roles for exogenous viral proteins in viral replication and offers insights into interactions during picornavirus infection.

Antigenic diversity of type 1 polioviruses and its implications for the efficacy of polio vaccines

Inactivated Polio Vaccines (IPV) and live Oral Polio Vaccine (OPV) were introduced in the mid-20th century, and their coordinated worldwide use led to almost complete elimination of the disease, with only one serotype of poliovirus remaining endemic in just two countries. Polio eradication will lead to discontinuation of OPV use and its replacement with IPV or other vaccines that are currently under development that will need to be tested in clinical trials. Despite decades of research, questions remain about the serological correlates of polio vaccine efficacy, specifically whether the vaccines are equally protective against immunologically different strains of the same serotype. The absence of significant morbidity does not allow use of a protection endpoint in clinical trials, so the answer could be obtained only by using surrogate markers such as immunogenicity. In this study, a panel of wild and vaccine-derived polioviruses of serotype 1 were tested in neutralization assays with sera from vaccine-immunized individuals. The results demonstrated that there was a significant difference in titers of neutralizing antibodies in human sera when measured against different strains. When measured with a homologous strain used for vaccine manufacture all subjects had detectable levels of antibodies, while neutralization tests with some heterologous strains failed to detect neutralizing antibodies in a number of subjects. Administration of a booster dose of IPV led to a significant increase in neutralizing titers against all strains. Results of the experiments using animal sera, performed to obtain more information on protectivity of neutralizing antibodies against heterologous strains, were consistent with the results obtained in the assays using human sera. These results are discussed in the context of serological biomarkers of protection against poliomyelitis, suggesting that potency of vaccines made from serologically different strains should be determined against both homologous and heterologous challenge viruses.

Pseudotyped Viruses for Enterovirus

Using a non-pathogenic pseudotyped virus as a surrogate for a wide-type virus in scientific research complies with the recent requirements for biosafety. Enterovirus (EV) contains many species of viruses, which are a type of nonenveloped virus. The preparation of its corresponding pseudotyped virus often needs customized construction compared to some enveloped viruses. This article describes the procedures and challenges in the construction of pseudotyped virus for enterovirus (pseudotyped enterovirus, EVpv) and also introduces the application of EVpv in basic virological research, serological monitoring, and the detection of neutralizing antibody (NtAb).

Pseudotyped Viruses

Pseudotyped viruses have been constructed for many viruses. They can mimic the authentic virus and have many advantages compared to authentic viruses. Thus, they have been widely used as a surrogate of authentic virus for viral function analysis, detection of neutralizing antibodies, screening viral entry inhibitors, and others. This chapter reviewed the progress in the field of pseudotyped viruses in general, including the definition and the advantages of pseudotyped viruses, their potential usage, different strategies or vectors used for the construction of pseudotyped viruses, and factors that affect the construction of pseudotyped viruses.

High-throughput analysis of anti-poliovirus neutralization antibody titre in human serum by the pseudovirus neutralization test

To monitor vulnerability of countries to poliovirus (PV) outbreaks, serosurveillance of anti-PV neutralization antibody is conducted by conventional PV neutralization test (cPNT), which uses live PV strains. We previously developed a pseudovirus PV neutralization test (pPNT) as an alternative to cPNT, which uses PV pseudovirus that expresses luciferase as a reporter in the infection without producing infectious PV. In the present study, we established a high-throughput pPNT (HTpPNT) for a large-scale serosurveillance. The HTpPNT system was evaluated with 600 human serum samples obtained from a broad range of age groups of healthy volunteers (ages of 0–89 years). HTpPNT showed high correlation with cPNT (R² for anti-type 1, 2, and 3 PV neutralization antibody titres are 0.90, 0.84, and 0.90, respectively). By using HTpPNT, we analyzed relative neutralizing antibody titre of the sera against a type 1 PV wild-type strain (Mahoney strain) to that against the type 1 Sabin strain. As a result, a correlation between the age (≥ 60 years) and the relative neutralizing antibody titre was observed (n = 15–16, P = 0.0000023–0.041), while the types of PV vaccine (i.e., oral PV vaccine and Sabin strain-based IPV) had no effect. HTpPNT would serve as a useful alternative to cPNT in a large-scale serosurveillance.

Comparison of the neutralizing activities of antibodies in clinical sera against both Sabin and wild-type polio pseudoviruses

The cost-effectiveness of the Sabin inactivated poliovirus vaccine derived from the Sabin strains (S-IPV) and its reduced biosecurity risks during its manufacture make it the vaccine of choice over the IPVs derived from wild-type polioviruses. However, it is difficult to evaluate whether S-IPVs can achieve wild-type poliovirus containment in China, making its development there less attractive. To facilitate the development and adoption of S-IPVs in China, the aim of this study was to develop an alternative neutralizing assay using either a polio pseudovirus derived from a Sabin strain (S-pNA) or one derived from a wild-type strain (w-pNA) to replace the conventional neutralizing assay which uses live polioviruses. A total of 100 sera were collected from children immunized with an oral poliovirus vaccine and their antibody titers were assessed by both the S-pNA and w-pNA. The results showed that this method was feasible for the quantification of neutralizing antibody activities in the sera of the vaccinated individuals. The Wilcoxon signed-rank sum test indicated that the neutralizing antibody titers obtained against the Sabin strains were higher than those obtained with the wild-type strains for types 1 and 3, while for type 2, the titers against the wild-type strains were higher than those against the Sabin strains (p < 0.001 for all three types). It is hoped that this assay could be used to assess whether immune sera by the S-IPV possess adequate neutralizing capacity against both attenuated and wild-type poliovirus strains.

High-Order Epistasis and Functional Coupling of Infection Steps Drive Virus Evolution toward Independence from a Host Pathway

The phosphatidylinositol-4 kinase IIIβ (PI4KB)/oxysterol-binding protein (OSBP) family I pathway serves as an essential host pathway for the formation of viral replication complex for viral plus-strand RNA synthesis; however, poliovirus (PV) could evolve toward substantial independence from this host pathway with four mutations. Recessive epistasis of the two mutations (3A-R54W and 2B-F17L) is essential for viral RNA replication. Quantitative analysis of effects of the other two mutations (2B-Q20H and 2C-M187V) on each step of infection reveals functional couplings between viral replication, growth, and spread conferred by the 2B-Q20H mutation, while no enhancing effect was conferred by the 2C-M187V mutation. The effects of the 2B-Q20H mutation occur only via another recessive epistasis between the 3A-R54W/2B-F17L mutations. These mutations confer enhanced replication in PI4KB/OSBP-independent infection concomitantly with an increased ratio of viral plus-strand RNA to the minus-strand RNA. This work reveals the essential roles of the functional coupling and high-order, multi-tiered recessive epistasis in viral evolution toward independence from an obligatory host pathway.

IMPORTANCE Each virus has a different strategy for its replication, which requires different host factors. Enterovirus, a model RNA virus, requires host factors PI4KB and OSBP, which form an obligatory functional axis to support viral replication. In an experimental evolution system in vitro, virus mutants that do not depend on these host factors could arise only with four mutations. The two mutations (3A-R54W and 2B-F17L) are required for the replication but are not sufficient to support efficient infection. Another mutation (2B-Q20H) is essential for efficient spread of the virus. The order of introduction of the mutations in the viral genome is essential (known as “epistasis”), and functional couplings of infection steps (i.e., viral replication, growth, and spread) have substantial roles to show the effects of the 2B-Q20H mutation. These observations would provide novel insights into an evolutionary pathway of the virus to require host factors for infection.