PACKAGING OF ENDOGENOUS RETROVIRAL SEQUENCES IN RETROVIRAL VECTORS PRODUCED BY MURINE AND HUMAN PACKAGING CELLS

Interaction of retrovirus vectors and endogenous retroviruses present in packaging cell lines and target cells may result in unwanted events , such as the formation of recombinant viruses and the mobilization of therapeutic vectors. Using sensitive reverse transcriptase PCR assays , we investigated human and murine gene therapy packaging cell lines f or incorporation of endogenous retrovirus transcripts into murine leuk emia virus (MLV) vector particles and, conversely, whether vector geno mes are incorporated into human endogenous retrovirus (HERV) particles . VL30 endogenous retrovirus sequences were efficiently packaged in pa rticles produced by the murine AM12 packaging system. For every seven MLV-derived beta-galactosidase (beta-Gal) vector genomes present in th e particles, one copy of VL30 was also packaged. Although human FLY pa ckaging cells expressed several classes of HERV transcripts (HERV-K Hu RT, type C, and RTVL-H), none was detectable in the MLV vector particl es released from the cells. Nonspecific packaging of the MLV Gag-Pol e xpression vector transcripts was detected in the FLY virions at a low level (1 in 17,000 sequences). These findings indicate that human pack aging cells produce retrovirus particles far less contaminated by endo genous viral sequences than murine packaging cells. Human teratocarcin oma cells (GH cells), which produce HERV-K particles, were transduced with an MLV-derived beta-Gal vector. Although both HERV-K and RTVL-H s equences were found in association with the particles, beta-Gal transc ripts were not detected, indicating that HERV Gag proteins do not effi ciently package MLV-based vectors.