Internal ribosome entry site-based attenuation of a flavivirus candidate vaccine and evaluation of the effect of beta interferon coexpression on vaccine properties

Development of a Bicistronic Yellow Fever Live Attenuated Vaccine with Reduced Neurovirulence and Viscerotropism

The yellow fever (YF) live attenuated vaccine strain 17D (termed 17D) has been widely used for the prevention and control of YF disease. However, 17D retains significant neurovirulence and viscerotropism in mice, which is probably linked to the increased occurrences of serious adverse events following 17D vaccination. Thus, the development of an updated version of the YF vaccine with an improved safety profile is of high priority. Here, we generated a viable bicistronic YF virus (YFV) by incorporating the internal ribosome entry site (IRES) from Encephalomyocarditis virus into an infectious clone of YFV 17D. The resulting recombinant virus, 17D-IRES, exhibited similar replication efficiency to its parental virus (17D) in mammalian cell lines, while it was highly restricted in mosquito cells. Serial passage of 17D-IRES in BHK-21 cells showed good genetic stability. More importantly, in comparison with the parental 17D, 17D-IRES displayed significantly decreased mouse neurovirulence and viscerotropism in type I interferon (IFN)-signaling-deficient and immunocompetent mouse models. Interestingly, 17D-IRES showed enhanced sensitivity to type I IFN compared with 17D. Moreover, immunization with 17D-IRES provided solid protection for mice against a lethal challenge with YFV. These preclinical data support further development of 17D-IRES as an updated version for the approved YF vaccine. This IRES-based attenuation strategy could be also applied to the design of live attenuated vaccines against other mosquito-borne flaviviruses. IMPORTANCE Yellow fever (YF) continually spreads and causes epidemics around the world, posing a great threat to human health. The YF live attenuated vaccine 17D is considered the most efficient vaccine available and helps to successfully control disease epidemics. However, side effects may occur after vaccination, such as viscerotropic disease (YEL-AVD) and neurotropic adverse disease (YEL-AND). Thus, there is an urgent need for a safer YF vaccine. Here, an IRES strategy was employed, and a bicistronic YFV was successfully developed (named 17D-IRES). 17D-IRES showed effective replication and genetic stability in vitro and high attenuation in vivo. Importantly, 17D-IRES induced humoral and cellular immune responses and conferred full protection against lethal YFV challenge. Our study provides data suggesting that 17D-IRES, with its prominent advantages, could be a vaccine candidate against YF. Moreover, this IRES-based bicistronic technology platform represents a promising strategy for developing other live attenuated vaccines against emerging viruses.

In the era of rapid mRNA-based vaccines: Why is there no effective hepatitis C virus vaccine yet?

Hepatitis C virus (HCV) is responsible for no less than 71 million people chronically infected and is one of the most frequent indications for liver transplantation worldwide. Despite direct-acting antiviral therapies fuel optimism in controlling HCV infections, there are several obstacles regarding treatment accessibility and reinfection continues to remain a possibility. Indeed, the majority of new HCV infections in developed countries occur in people who inject drugs and are more plausible to get reinfected. To achieve global epidemic control of this virus the development of an effective prophylactic or therapeutic vaccine becomes a must. The coronavirus disease 19 (COVID-19) pandemic led to auspicious vaccine development against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) virus, which has renewed interest on fighting HCV epidemic with vaccination. The aim of this review is to highlight the current situation of HCV vaccine candidates designed to prevent and/or to reduce HCV infectious cases and their complications. We will emphasize on some of the crossroads encountered during vaccine development against this insidious virus, together with some key aspects of HCV immunology which have, so far, hampered the progress in this area. The main focus will be on nucleic acid-based as well as recombinant viral vector-based vaccine candidates as the most novel vaccine approaches, some of which have been recently and successfully employed for SARS-CoV-2 vaccines. Finally, some ideas will be presented on which methods to explore for the design of live-attenuated vaccines against HCV.

Novel Approach for Insertion of Heterologous Sequences into Full-Length ZIKV Genome Results in Superior Level of Gene Expression and Insert Stability

Zika virus (ZIKV) emerged in the Americas in 2015, presenting unique challenges to public health. Unlike other arboviruses of the Flaviviridae family, it is transmissible by sexual contact, which facilitates the spread of the virus into new geographic areas. Additionally, ZIKV can be transmitted from mother to fetus, causing microcephaly and other severe developmental abnormalities. Reliable and easy-to-work-with clones of ZIKV expressing heterologous genes will significantly facilitate studies aimed at understanding the virus pathogenesis and tissue tropism. Here, we developed and characterized two novel approaches for expression of heterologous genes of interest in the context of full-length ZIKV genome and compared them to two previously published strategies for ZIKV-mediated gene expression. We demonstrated that among the four tested viruses expressing nLuc gene, the virus constructed using a newly developed approach of partial capsid gene duplication (PCGD) attained the highest titer in Vero cells and resulted in the highest level of nLuc expression. Suitability of the PCGD approach for expression of different genes of interest was validated by replacing nLuc sequence with that of eGFP gene. The generated constructs were further characterized in cell culture. Potential applications of ZIKV clones stably expressing heterologous genes include development of detection assays, antivirals, therapeutics, live imaging systems, and vaccines.

Synergistic Internal Ribosome Entry Site/MicroRNA-Based Approach for Flavivirus Attenuation and Live Vaccine Development

The recent emergence of Zika virus underscores the need for new strategies for a rapid development of safe flavivirus vaccines. Using another flavivirus (Langat virus [LGTV]) that belongs to the group of tick-borne flaviviruses as a model, we describe a dual strategy for virus attenuation which synergistically accesses the specificity of microRNA (miRNA) genome targeting and the effectiveness of internal ribosome entry site (IRES) insertion. To increase the stability and immunogenicity of bicistronic LGTVs, we developed a novel approach in which the capsid (C) protein gene was relocated into the 3′ noncoding region (NCR) and expressed under translational control from an IRES. Engineered bicistronic LGTVs carrying multiple target sequences for brain-specific miRNAs were stable in Vero cells and induced adaptive immunity in mice. Importantly, miRNA-targeted bicistronic LGTVs were not pathogenic for either newborn mice after intracranial inoculation or adult immunocompromised mice (SCID or type I interferon receptor knockout) after intraperitoneal injection. Moreover, bicistronic LGTVs were restricted for replication in tick-derived cells, suggesting an interruption of viral transmission in nature by arthropod vectors. This approach is suitable for reliable attenuation of many flaviviruses and may enable development of live attenuated flavivirus vaccines.

The recent emergence of Zika virus underscores the need for new strategies for a rapid development of safe flavivirus vaccines. Allied separately attenuating approaches based on (i) microRNA genome targeting and (ii) internal ribosome entry site insertion are not sufficient for relievable attenuation of neurotropic flavivirus pathogenesis. Here, we describe a novel dual strategy that combines the specificity of miRNA-based and the effectiveness of IRES-based attenuating approaches, allowing us to overcome these critical limitations. This developed approach provides a robust platform for reliable attenuation of many flaviviruses and may enable development of live flavivirus vaccines.

RNA-based viral vectors

The advent of reverse genetic approaches to manipulate the genomes of both positive (+) and negative (-) sense RNA viruses allowed researchers to harness these genomes for basic research. Manipulation of positive sense RNA virus genomes occurred first largely because infectious RNA could be transcribed directly from cDNA versions of the RNA genomes. Manipulation of negative strand RNA virus genomes rapidly followed as more sophisticated approaches to provide RNA-dependent RNA polymerase complexes coupled with negative-strand RNA templates were developed. These advances have driven an explosion of RNA virus vaccine vector development. That is, development of approaches to exploit the basic replication and expression strategies of RNA viruses to produce vaccine antigens that have been engineered into their genomes. This study has led to significant preclinical testing of many RNA virus vectors against a wide range of pathogens as well as cancer targets. Multiple RNA virus vectors have advanced through preclinical testing to human clinical evaluation. This review will focus on RNA virus vectors designed to express heterologous genes that are packaged into viral particles and have progressed to clinical testing.