Development and validation of a real-time RT-PCR assay for the quantification of rabies virus as quality control of inactivated rabies vaccines

Three in one: An effective and universal vaccine expressing heterologous tandem RBD trimer by rabies virus vector protects mice against SARS-CoV-2

The rapid emergence of Severe Acute Respiratory Syndrome Coronavirus type 2 (SARS-CoV-2) variants, coupled with severe immune evasion and imprinting, has jeopardized the vaccine efficacy, necessitating urgent development of broad protective vaccines. Here, we propose a strategy employing recombinant rabies viruses (RABV) to create a universal SARS-CoV-2 vaccine expressing heterologous tandem receptor-binding domain (RBD) trimer from the SARS-CoV-2 Prototype, Delta, and Omicron strains (SRV-PDO). The results of mouse immunization indicated that SRV-PDO effectively induced cellular and humoral immune responses, and demonstrated higher immunogenicity and broader SARS-CoV-2 neutralization compared to the recombinant RABVs that only expressed RBD monomers. Moreover, SRV-PDO exhibited full protection against SARS-CoV-2 in the challenge assay. This study demonstrates that recombinant RABV expressing tandem RBD-heterotrimer as a multivalent immunogen could elicit a broad-spectrum immune response and potent protection against SARS-CoV-2, making it a promising candidate for future human or veterinary vaccines and offering a novel perspective in other vaccine design.

Evaluation of Safety and Potency of Kyasanur Forest Disease (KFD) Vaccine Inactivated with Different Concentrations of Formalin and Comparative Evaluation of In Vitro and In Vivo Methods of Virus Titration in KFD Vaccine

We evaluated the safety and potency of the Kyasanur Forest disease (KFD) vaccine inactivated with different formalin concentrations in mice, since the side effects due to higher formalin concentrations have been a major reason for vaccine refusal. Furthermore, with an objective to reduce the use of mice in vaccine testing, we performed quantification of the KFD virus by real-time PCR and compared it with in vivo titration in mice. The KFD vaccine prepared in chicken embryo fibroblast cells was inactivated with 0.04%, 0.06%, and 0.08% concentrations of formalin. The vaccine inactivated with 0.04% and 0.06% formalin failed the safety test, whereas the KFD vaccine inactivated with 0.08% formalin was safe and potent with a log protective index of 5678 in mice. This reduced formalin content may induce no/lesser side-effects of pain/swelling which may increase the vaccine acceptance. The real-time PCR on individual KFD vaccine harvests interpreted that when the CT value of each harvest is <20, the vaccine will have sufficient viral particles to pass the potency test. Comparison of the real-time PCR on tenfold dilutions of the pooled harvests with in vivo mice inoculation test revealed that the 1MLD₅₀ of the vaccine lies in the tenfold dilution that yields CT values between 31 and 34.

Canine parvovirus type 2 vaccines in Brazil: Viral load in commercial vaccine vials and phylogenetic analysis of the vaccine viruses

Canine parvovirus type 2 (CPV-2) is the etiological agent of a highly contagious and frequently fatal disease in dogs. Live attenuated vaccines (LAV) are recommended to prevent and control this disease. Commercial vaccines are typically produced with CPV-2 strains adapted to cell culture and usually non-pathogenic. The present study aimed to determine the viral load of CPV-2 vaccines commercially available in Brazil and to characterize the vaccine virus by DNA analysis of its capsid gene. The results demonstrated that all vaccine strains presented high homology of the VP2 gene and they were all closely related to the original CPV-2 strains. However, vaccine strains presented several differences in comparison with field strains currently circulating in Brazil. Seventy-one vials contained viral loads ranging from 7.4E3 to 4.9E10 DNA copies/ml. Nine vials did not contain any detectable CPV-2 DNA. In conclusion, there are genetic and antigenic differences among CPV-2 vaccines and field strains. Additionally, some vaccines have been commercialized with low titers of CPV-2. It is important to improve the quality of the vaccines to prevent or reduce the spread of CPV-2 in Brazil.

Recombinant Virus Quantification Using Single-Cell Droplet Digital PCR: A Method for Infectious Titer Quantification

The quantification of viruses is necessary for both research and clinical applications. The methods available for RNA virus quantification possess several drawbacks, including sensitivity to inhibitors and the necessity of a standard curve generation. The main purpose of this study was to develop and validate a method for the quantification of recombinant, replication-deficient Semliki Forest virus (SFV) vectors using droplet digital PCR (ddPCR). This technique demonstrated stability and reproducibility using various sets of primers that targeted inserted transgenes, as well as the nsP1 and nsP4 genes of the SFV genome. Furthermore, the genome titers in the mixture of two types of replication-deficient recombinant virus particles were successfully measured after optimizing the annealing/extension temperature and virus:virus ratios. To measure the infectious units, we developed a single-cell ddPCR, adding the whole infected cells to the droplet PCR mixture. Cell distribution in the droplets was investigated, and β-actin primers were used to normalize the quantification. As a result, the number of infected cells and the virus infectious units were quantified. Potentially, the proposed single-cell ddPCR approach could be used to quantify infected cells for clinical applications.

An inactivated recombinant rabies virus chimerically expressed RBD induces humoral and cellular immunity against SARS-CoV-2 and RABV

Many studies suggest that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can infect various animals and transmit among animals, and even to humans, posing a threat to humans and animals. There is an urgent need to develop inexpensive and efficient animal vaccines to prevent and control coronavirus disease 2019 (COVID-19) in animals. Rabies virus (RABV) is another important zoonotic pathogen that infects almost all warm-blooded animals and poses a great public health threat. The present study constructed two recombinant chimeric viruses expressing the S1 and RBD proteins of the SARS-CoV-2 Wuhan01 strain based on a reverse genetic system of the RABV SRV9 strain and evaluated their immunogenicity in mice, cats and dogs. The results showed that both inactivated recombinant viruses induced durable neutralizing antibodies against SARS-CoV-2 and RABV and a strong cellular immune response in mice. Notably, inactivated SRV-nCoV-RBD induced earlier antibody production than SRV-nCoV-S1, which was maintained at high levels for longer periods. Inactivated SRV-nCoV-RBD induced neutralizing antibodies against both SARS-CoV-2 and RABV in cats and dogs, with a relatively broad-spectrum cross-neutralization capability against the SARS-CoV-2 pseudoviruses including Alpha, Beta, Gamma, Delta, and Omicron, showing potential to be used as a safe bivalent vaccine candidate against COVID-19 and rabies in animals.

Research progress on substitution of in vivo method(s) by in vitro method(s) for human vaccine potency assays

INTRODUCTION

Potency is a critical quality attribute for controlling quality consistency and relevant biological properties of vaccines. Owing to the high demand for animals, lengthy operations and high variability of in vivo methods, in vitro alternatives for human vaccine potency assays are extensively developed.

AREAS COVERED

Herein, in vivo and in vitro methods for potency assays of previously licensed human vaccines were sorted, followed by a brief description of the background for substituting in vivo methods with in vitro alternatives. Based on the analysis of current research on the substitution of vaccine potency assays, barriers and suggestions for substituting were proposed.

EXPERT OPINION

Owing to the variability of in vivo methods, the correlation between in vivo and in vitro methods may be low. One or more in vitro method(s) that determine the vaccine antigen content and functions, should be established. Since the substitution involves with the change of critical quality attributes and specifications, the specifications of in vitro methods should be appropriately set to maintain the efficacy of vaccines. For novel vaccines in research and development, in vitro methods for monitoring the consistency and relevant biological properties, should be established based on reflecting the immunogenicity of vaccines.

Inactivated rabies vaccines: Standardization of an in vitro assay for residual viable virus detection

Human rabies, a neglected viral zoonosis, is preventable through domestic animals vaccination and post-exposure prophylaxis using inactivated rabies vaccines. During vaccine production, several mandatory in vivo quality control trials, such as potency, live virus, and safety, are responsible for the use of large numbers of laboratory animals. Over the years, global organizations encouraged the development of alternative methods to reduce, replace and refine the use of animals in the pharmaceutical industry. In this study we standardized an in vitro assay for determination of residual live virus combining viral isolation techniques with direct immunofluorescence detection and viral quantification by a molecular method. Standardization of viral recovery steps and quantification by RT-qPCR were performed and the combined method was shown to be 3 fold more sensitive than the in vivo assay. It was possible to identify viral suspensions cultures, which still had residual viable rabies virus particles, evidencing the importance to implement this method in quality control schemes of rabies vaccine production. In addition, this developed assay is more practical, inexpensive and less time consuming, producing results in just 4 days, which may allow greater agility in the internal quality control of the vaccine. The in vitro method may reduce 2/3^rd of laboratory animals numbers used for this purpose, since it can be applied in the intermediate quality control of inactivated rabies vaccine production.

Rabies is a viral disease that causes thousands of deaths worldwide every year, but can be preventable through vaccination of animals and humans. In the production of inactivated rabies vaccines, quality control assays use large amounts of laboratory animals. In order to reduce the number of animals in this practice, in vitro methods for residual live virus detection need to be developed. This study shows that combining two simple and effective techniques may be a safer and more accurate method for the detection of residual rabies virus than in vivo assays to evaluate inactivated vaccines and can be used as an intermediate quality control assay of vaccine production.