Capacity of the Meq-deleted Strain Marek's Virus SC9-1 to Acquire the Meq Gene by Natural Recombination

We wished to explore the ability of the meq-deleted Marek's disease virus (MDV) vaccine strain SC9-1 to acquire the meq gene from the MDV wild strain Md5 by recombination. Chicken embryo fibroblast cells (CEFs) were co-infected with the SC9-1 vaccine virus and Md5 virus, passaged to third generation, and viral DNA was extracted from a single plaque in the cell culture. Specific pathogen-free chickens pre-immunized with the SC9-1 vaccine virus were infected with the Md5 virus. Viruses were isolated from chickens-at different time points. Then, viral DNA was extracted from a single plaque and amplification by polymerase chain reaction done to identify isolated viruses. The flip recombina-se sites (FRT) residue region was cloned and sequenced. Results showed that the isolated viruses in cultured CEFs or in chickens were the SC9-1 or Md5 virus, and recombinant viruses were not detected. Sequence analyses revealed that the homology of the FRT residue sequence between the isolated virus and parent virus was 100%. Therefore, there is little chance that SC9-1 can acquire the meq gene from Md5 by natural recombination. Also, the meq-gene knockout region had good genetic stability during serial passages in vivo and in vitro.