Recovery of a Far-Eastern Strain of Tick-Borne Encephalitis Virus with a Full-Length Infectious cDNA Clone

Breast milk transmission and involvement of mammary glands in tick-borne flavivirus infected mice

Tick-borne flaviviruses (TBFs) are transmitted to humans through milk and tick bites. Although a case of possible mother-to-child transmission of tick-borne encephalitis virus (TBEV) through breast milk has been reported, this route has not been confirmed in experimental models. Therefore, in this study, using type I interferon receptor-deficient A129 mice infected with Langat virus (LGTV), we aimed to demonstrate the presence of infectious virus in the milk and mammary glands of infected mice. Our results showed viral RNA of LGTV in the pup's stomach milk clots (SMCs) and blood, indicating that the virus can be transmitted from dam to pup through breast milk. In addition, we observed that LGTV infection causes tissue lesions in the mammary gland, and viral particles were present in mammary gland epithelial cells. Furthermore, we found that milk from infected mice could infect adult mice via the intragastric route, which has a milder infection process, longer infection time, and a lower rate of weight loss than other modes of infection. Specifically, we developed a nano-luciferase-LGTV reporter virus system to monitor the dynamics of different infection routes and observed dam-to-pup infection using in vivo bioluminescence imaging. This study provides comprehensive evidence to support breast milk transmission of TBF in mice and has helped provide useful data for studying TBF transmission routes.IMPORTANCETo date, no experimental models have confirmed mother-to-child transmission of tick-borne flavivirus (TBF) through breastfeeding. In this study, we used a mouse model to demonstrate the presence of infectious viruses in mouse breast milk and mammary gland epithelial cells. Our results showed that pups could become infected through the gastrointestinal route by suckling milk, and the infection dynamics could be monitored using a reporter virus system during breastfeeding in vivo. We believe our findings have provided substantial evidence to understand the underlying mechanism of breast milk transmission of TBF in mice, which has important implications for understanding and preventing TBF transmission in humans.

Accidental acquisition of a rescued Japanese encephalitis virus with unspliced introns in the viral genome when using an intron-based stabilization approach

The intron-based stabilization approach is a very useful strategy for construction of stable flavivirus infectious clones. SA₁₄-14-2 is a highly attenuated Japanese encephalitis (JE) live vaccine strain that has been widely used in China since 1989. To develop safe and effective recombinant vaccines with SA₁₄-14-2 as a backbone vector, we constructed the DNA-based infectious clone pCMW-JEV of SA₁₄-14-2 using the intron-based stabilization approach and acquired the rescued virus rDJEV, which retained the biological properties of the parental virus. Unexpectedly, a rescued virus strain with altered virulence, designated rHV-DJEV, was accidentally acquired in one of the transfection experiments. rHV-DJEV showed up to 10⁵-fold increased neurovirulence compared with the SA₁₄-14-2 parental strain. Genome sequencing showed that the inserted introns were still present in the genome of rHV-DJEV. Therefore, we think that the intron-based stabilization approach should be used with caution in vaccine development and direct iDNA immunization.

Construction and verification of an infectious cDNA clone of coxsackievirus B5

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An infectious cDNA clone of CV-B5 was constructed.

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The rescued and parental virus possessed similar biological characteristics.

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The virulence of the rescued virus was similiar to that of the parental virus.

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Viral distribution and tissue tropism of those two viruses were in agreement.