Oncogenic Marek's disease viruses lacking the 132 base pair repeats can still be attenuated by serial in vitro cell culture passages.

Marek's disease virus (MDV) can be attenuated by serially passing the virus in cell culture. During cell culture passage, two copies of a 132 bp repeat are expanded to over 30 copies. We deleted the two copies of the 132 bp repeat region in a pathogenic MDV and demonstrated that the virus was still pathogenic. The pattern and frequency of tumors in the parental and mutant virus were the same. Early virus replication, and the appearance of persistent neurological disease were also similar between the parental and deleted virus. Nevertheless, wild-type MDV and the deletion virus could be attenuated by serial in vitro cell culture passages. Based upon analyzing the passage 40 viruses, attenuation of the MDV lacking the 132 bp repeats appears to occur in a manner that is analogous to the process occurring wild-type MDV attenuation. Whatever process is involved in the cell culture attenuation of MDV, the mechanism does not involve the 132 bp repeat region.