Age-dependent poliovirus replication in the mouse central nervous system is determined by internal ribosome entry site-mediated translation

A new animal model containing human SCARB2 and lacking stat-1 is highly susceptible to EV71

Enterovirus 71 (EV71) is a major threat to children worldwide. Children infected with EV71 could develop subclinical infection and hand-foot-and -mouth disease (HFMD). In severe cases, patients could develop encephalitis, paralysis, pulmonary edema, and death. A more user-friendly and robust animal model is essential to investigating EV71 pathogenesis. Here, we established a hybrid (hSCARB2(+/+)/stat-1(-/-)) mouse strain from crossbreeding SCARB2 transgenic and stat-1 KO mice, and compared the susceptibilities to EV71 infection and pathogenesis between parental and hybrid mice. Virus-encoded VP1 protein can be detected in the streaking nerve fibers in brain and spinal cord. This hybrid mouse strain at 2-week-old age can still be infected with different genotypes of EV71 at 1000-fold lower titer via an ip route. Infected hybrid mice developed earlier onset of CNS disease, paralysis, and death at a higher incidence. These advantages of this novel model meet the urgent need from the scientific community in basic and preclinical research in therapeutics and pathogenesis.

Immunodeficient mouse models with different disease profiles by in vivo infection with the same clinical isolate of enterovirus 71

Like poliovirus infection, severe infection with enterovirus 71 (EV71) can cause neuropathology. Unlike poliovirus, EV71 is often associated with hand-foot-and-mouth disease (HFMD). Here we established three mouse models for experimental infection with the same clinical isolate of EV71. The NOD/SCID mouse model is unique for the development of skin rash, an HFMD-like symptom. While the NOD/SCID mice developed limb paralysis and death at near-100% efficiency, the gamma interferon receptor knockout (ifngr KO) and stat-1 knockout mice exhibited paralysis and death rates near 78% and 30%, respectively. Productive infection with EV71 depends on the viral dose, host age, and inoculation route. Levels of infectious EV71, and levels of VP1-specific RNA and protein in muscle, brain, and spinal cord, were compared side by side between the NOD/SCID and stat-1 knockout models before, during, and after disease onset. Spleen fibrosis and muscle degeneration are common in the NOD/SCID and stat-1 knockout models. The main differences between these two models include their disease manifestations and cytokine/chemokine profiles. The pathology of the NOD/SCID model includes (i) inflammation and expression of viral VP1 antigen in muscle, (ii) increased neutrophil levels and decreased eosinophil and lymphocyte levels, and (iii) hair loss and skin rash. The characteristic pathology of the stat-1 knockout model includes (i) a strong tropism of EV71 for the central nervous system, (ii) detection of VP1 protein in the Purkinje layer of cerebellar cortex, pons, brain stem, and spinal cord, (iii) amplification of microglial cells, and (iv) dystrophy of intestinal villi. Our comparative studies on these new models with oral or intraperitoneal (i.p.) infection underscored the contribution of host immunity, including the gamma interferon receptor, to EV71 pathogenesis.

IMPORTANCE

In the past decade, enterovirus 71 (EV71) has emerged as a major threat to public health in the Asia-Pacific region. Disease manifestations include subclinical infection, common-cold-like syndromes, hand-foot-and-mouth disease (HFMD), uncomplicated brain stem encephalitis, severe dysregulation of the autonomic nerve system, fatal pulmonary edema, and cardiopulmonary collapse. To date, no effective vaccine or treatment is available. A user-friendly and widely accessible animal model for researching EV71 infection and pathogenesis is urgently needed by the global community, both in academia and in industry.

Modification of the untranslated regions of human enterovirus 71 impairs growth in a cell-specific manner

Human enterovirus 71 (HEV71) is the causative agent of hand, foot, and mouth disease and associated acute neurological disease. At present, little is known about the genetic determinants of HEV71 neurovirulence. Studies of related enteroviruses have indicated that the untranslated regions (UTRs), which control virus-directed translation and replication, also exert significant influence on neurovirulence. We used an infectious cDNA clone of a subgenogroup B3 strain to construct and characterize chimeras with 5'- and 3'-UTR modifications. Replacement of the entire HEV71 5' UTR with that of human rhinovirus 2 (HRV2) resulted in a small reduction in growth efficiency in cells of both nonneuronal (rhabdomyosarcoma) and neuronal (SH-SY5Y) origin due to reduced translational efficiency. However, the introduction of a 17-nucleotide deletion into the proximal region of the 3' UTR significantly decreased the growth of HEV71-HRV2 in SH-SY5Y cells. This observation is similar to that made with stem-loop domain Z (SLD Z)-deleted coxsackievirus B3-HRV2 5'-UTR chimeras reported previously and provides the first evidence of a potentially functional SLD Z in the 3' UTR in human enterovirus A species viruses. We further showed that the cell-specific growth impairment was caused by the synergistic effects of cis-acting UTR control elements on different stages of the virus life cycle. These chimeras will further improve our understanding of the control of HEV71 replication and its relationship to neurovirulence.

A host-specific, temperature-sensitive translation defect determines the attenuation phenotype of a human rhinovirus/poliovirus chimera, PV1(RIPO)

By using a rhinosvirus/poliovirus type 1 chimera, PV1(RIPO), with the cognate internal ribosome entry site (IRES) of human rhinovirus type 2 (HRV2), we set out to shed light on the mechanism by which this variant expresses its attenuated phenotype in poliovirus-sensitive, CD155 transgenic (tg) mice and cynomolgus monkeys. Here we report that replication of PV1(RIPO) is restricted not only in human cells of neuronal origin, as was reported previously, but also in cells of murine origin at physiological temperature. This block in replication was enhanced at 39.5°C but, remarkably, it was absent at 33°C. PV1(RIPO) variants that overcame the replication block were derived by serial passage under restrictive conditions in either mouse cells or human neuronal cells. All adapting mutations mapped to the 5'-nontranslated region of PV1(RIPO). Variants selected in mouse cells, but not in human neuronal cells, exhibited increased mouse neurovirulence in vivo. The observed strong mouse-specific defect of PV1(RIPO) at nonpermissive temperature correlated with the translational activity of the HRV2 IRES in this chimeric virus. These unexpected results must be kept in mind when poliovirus variants are tested in CD155 tg mice for their neurovirulent potential, particularly in assays of live attenuated oral poliovirus vaccine lots. Virulence may be masked by adverse species-specific conditions in mouse cells that may not allow accurate prediction of neurovirulence in the human host. Thus, novel poliovirus variants in line for possible development of human vaccines must be tested in nonhuman primates.

Enterovirus infections of the central nervous system

Enteroviruses (EV) frequently infect the central nervous system (CNS) and induce neurological diseases. Although the CNS is composed of many different cell types, the spectrum of tropism for each EV is considerable. These viruses have the ability to completely shut down host translational machinery and are considered highly cytolytic, thereby causing cytopathic effects. Hence, CNS dysfunction following EV infection of neuronal or glial cells might be expected. Perhaps unexpectedly given their cytolytic nature, EVs may establish a persistent infection within the CNS, and the lasting effects on the host might be significant with unanticipated consequences. This review will describe the clinical aspects of EV-mediated disease, mechanisms of disease, determinants of tropism, immune activation within the CNS, and potential treatment regimes.

Attenuation of rabies virus replication and virulence by picornavirus internal ribosome entry site elements

Gene expression of nonsegmented negative-strand RNA viruses is regulated at the transcriptional level and relies on the canonical 5'-end-dependent translation of capped viral mRNAs. Here, we have used internal ribosome entry sites (IRES) from picornaviruses to control the expression level of the phosphoprotein P of the neurotropic rabies virus (RV; Rhabdoviridae), which is critically required for both viral replication and escape from the host interferon response. In a dual luciferase reporter RV, the IRES elements of poliovirus (PV) and human rhinovirus type 2 (HRV2) were active in a variety of cell lines from different host species. While a generally lower activity of the HRV2 IRES was apparent compared to the PV IRES, specific deficits of the HRV2 IRES in neuronal cell lines were not observed. Recombinant RVs expressing P exclusively from a bicistronic nucleoprotein (N)-IRES-P mRNA showed IRES-specific reduction of replication in cell culture and in neurons of organotypic brain slice cultures, an increased activation of the beta interferon (IFN-beta) promoter, and increased sensitivity to IFN. Intracerebral infection revealed a complete loss of virulence of both PV- and HRV2 IRES-controlled RV for wild-type mice and for transgenic mice lacking a functional IFN-alpha receptor (IFNAR(-/-)). The virulence of HRV2 IRES-controlled RV was most severely attenuated and could be demonstrated only in newborn IFNAR(-/-) mice. Translational control of individual genes is a promising strategy to attenuate replication and virulence of live nonsegmented negative-strand RNA viruses and vectors and to study the function of IRES elements in detail.

Virology of the post-polio syndrome

The three poliovirus serotypes (PVs) cause acute paralytic poliomyelitis. Decades after being hit by polio, survivors may develop a condition known as post-polio syndrome (PPS). PPS is characterized by extreme fatigue, progressing muscular weakness and chronic pain. The pathogenesis is unclear and, thus, empirical therapies are employed. PVs are known to be able to persist in infected host cells both in vitro and in vivo. The understanding of PV genomes has made it possible to set up sensitive and specific molecular tests capable of detecting minute amounts of virus in samples from PPS patients. Current data indicate that complete PV genomes (or genomic fragments) remain present, decades after acute paralysis, in the CNS of these patients. Virus persistence is hypothesized to bring about chronic inflammation, immune-mediated injury and decreased expression of neurotrophic factors. Establishing a pathogenetic link between PV persistence and PPS would be extremely relevant to the development of an etiologic therapy aimed at virus eradication.