Development of a reverse genetics system for the aG strain of rabies virus in China

A ligation and restriction enzyme independent cloning technique: an alternative to conventional methods for cloning hard-to-clone gene segments in the influenza reverse genetics system

BACKGROUND

Reverse genetics is used in many laboratories around the world and enables the creation of tailor-made influenza viruses with a desired genotype or phenotype. However, the process is not flawless, and difficulties remain during cloning of influenza gene segments into reverse genetics vectors (pHW2000, pHH21, pCAGGS). Reverse genetics begins with making cDNA copies of influenza gene segments and cloning them into bi-directional (pHW2000) or uni-directional plasmids (pHH21, pCAGGS) followed by transfection of the recombinant plasmid(s) to HEK-293 T or any other suitable cells which are permissive to transfection. However, the presence of internal restriction sites in the gene segments of many field isolates of avian influenza viruses makes the cloning process difficult, if employing conventional methods. Further, the genetic instability of influenza gene-containing plasmids in bacteria (especially Polymerase Basic 2 and Polymerase Basic 1 genes; PB2 and PB1) also leads to erroneous incorporation of bacterial genomic sequences into the influenza gene of interest.

METHODS

Herein, we report an easy and efficient ligation and restriction enzyme independent (LREI) cloning method for cloning influenza gene segments into pHW2000 vector. The method involves amplification of megaprimers followed by PCR amplification of megaprimers using a bait plasmid, DpnI digestion and transformation.

RESULTS

Hard-to-clone genes: PB2 of A/chicken/Bangladesh/23527/2014 (H9N2) and PB1 of A/chicken/Bangladesh/23527/2014 (H9N2), A/chicken/Jiangxi/02.05YGYXG023-P/2015 (H5N6) and A/Chicken/Vietnam/H7F-14-BN4-315/2014 (H9N2) were cloned into pHW2000 using our LREI method and recombinant viruses were subsequently rescued.

CONCLUSION

The LREI cloning procedure represents an alternative strategy for cloning influenza gene segments which have internal restriction sites for the enzymes used in reverse genetics. Further, the problem of genetic instability in bacteria can be alleviated by growing recombinant bacterial cultures at a lower temperature. This technique can be applied to clone any influenza gene segment using universal primers, which would help in rapid generation of influenza viruses and facilitate influenza research and vaccine development.

Development of infectious clones of a wild-type Korean rabies virus and evaluation of their pathogenic potential

Most reverse genetic (RG) systems for rabies viruses (RVs) have been constructed on the genome background of laboratory-adapted strains. In this study, we developed an RG system using a Korean wild type (KGH) strain to investigate the pathogenic potential of different strains. We developed a RG system with the KGH strain for the first time. Following the complete genome sequencing of the KGH strain, pKGH infectious clones were constructed using the CMV/T7 promoter, and HamRz and HdvRz were introduced to allow self-cleavage of the synthesized RNA. We successfully recovered the rescued virus by constructing chimeric RVs in which we replaced a part of the construct with the partial gene from the fixed RC-HL strain. The rescued viruses formed clearer and countable plaques in an immunostaining plaque assay, with a distinct plaque morphology. Furthermore, compared with the chimeric RVs, the pKGH/RCinsΔ4 strain containing the KGH strain G protein exhibited a decreased efficiency of cell-to-cell spreading in BHK-21 cells and significantly reduced (100-1000 fold) replication kinetics. However, pKGH/RCinsΔ4 strain-infected mice revealed 100% morbidity at 11days post-infection, whereas other chimeric RV strains showed no mortality. Our RG system is a useful tool for studying differences in the cell-to-cell spreading efficiency and replication with respect to the different internalization patterns of street and fixed laboratory-adapted viruses.

Rabies vaccines: where do we stand, where are we heading?

Rabies being the most lethal zoonotic, vaccine-preventable viral disease with worldwide distribution of reservoir wild animals presents unique challenges for its diagnosis, management and control. Although vaccines available are highly effective, which had played the key role in controlling rabies in North America, western Europe and in a number of Asian and Latin American countries, the requirement of multiple doses along with boosters, associated cost to reduce the incidence in wild animals and prophylactic human vaccination has remained a major impediment towards achieving the same goals in poorer parts of the world such as sub-Saharan Africa and southeast Asia. Current efforts to contain rabies worldwide are directed towards the development of more safe, cheaper and efficacious vaccines along with anti-rabies antibodies for post-exposure prophylaxis. The work presented here provides an overview of the advances made towards controlling the human rabies, particularly in last 10 years, and future perspective.