Nondefective spleen necrosis virus-derived vectors define the upper size limit for packaging reticuloendotheliosis viruses

We constructed a nondefective retrovirus vector based on spleen necrosis virus (SNV), a replication-completent reticuloendotheliosis virus. We introduced different DNA sequences into this vector and studied the ability of the resulting viruses to replicate in chicken embryo fibroblasts. The replication efficiency of SNV-derived viruses decreased with increasing virus size. Viruses larger than 9.4 kilobases (kb) were rapidly overgrown by replication-competent deletion mutants. The size restriction for the efficient replication of nondefective SNV-derived viruses prevented the production of viruses larger than 10.0 kb. Analysis of the kinetics of virus particle release indicated that the size restriction occurred during virus encapsidation.