Analysis of ALV-based packaging cell lines for production of contaminant defective viruses

We have previously described avian leukosis virus-based packaging cell lines that express gag, pol, and env proteins from two transcomplementing genomes and produce helper-free stocks of retroviral vectors with different host ranges. In this report, we demonstrated that (i) despite the deletion of the psi packaging sequence, the packaging-defective transcomplementing retroviral transcripts were packaged into virions at a level that could reach 2.3% of a wild-type virus packaging level and (ii) despite deletion of the 3' LTR, these genomes were transferred along with the vector to target cells. As these genomes were also bearing a selectable gene, titers of the resulting contaminant particles could be estimated, depending on the cell line to be between 0 and 6 infectious particles/ml of supernatant.