We (see section 3.3) and others (24-30) have demonstrated the potential utility of lymphokine gene 
delivery in general, and y-IFN gene delivery specifically (28-30), in murine tumor model systems. 
The data have been sufficiently encouraging to warrant clinical investigation in cancer patients. 
2.2 RETROVIRAL VECTOR PACKAGING CELL LINES 
Retroviruses are positive-strand RNA viruses that pass through a double-stranded DNA stage after 
infection (31,32). Viral DNA integrates into the host genome to form a provirus, which contains 
the viral genes encompassed on either end by viral long terminal repeat (LTR) elements which are 
required for integration. After integration, viral genes are expressed from the provirus by utilizing 
host cell machinery and transcriptional promoter and termination cis elements found within the viral 
LTRs. The full length viral RNA transcript (the next generation viral genome) is recognized via a 
specific "packaging signal" (T) found in the viral RNA genome and encapsidated into an infectious 
viral particle by the viral proteins. The retroviral life cycle is generally non-lytic for the cell, and 
allows continuous viral production over a long period of time. 
The retroviral vector used in these studies is based upon Moloney murine leukemia virus 
(MoMLV), that was engineered so that its extraneous viral coding sequences have been replaced 
with a desired gene (32-34), such as y-IFN. Unlike true viruses, the recombinant vector is 
incapable of replication since its genome lacks all of the viral structural genes. Any gene of interest 
can replace the protein coding region of the viral genome and become packaged within a packaging 
cell line (PCL) into virion particles for transduction, provided that it is contiguous with the 
essential cis elements (i.e., T, LTRs, and sequences required for priming of DNA synthesis) of 
the genome. 
The viral proteins needed for encapsidation and vector production must then be supplied in a PCL. 
The PCL is a cell line which expresses the MoMLV structural proteins, and as a result produces 
empty virion particles which lack the viral genome required for replication. Viral provector DNA 
introduced into the PCL will act as a template for the production of the RNA vector genome, which 
is then packaged into the previously empty virions, generating retroviral vector particles (33,34). 
For clinical studies, the most important safety concern with the use of retroviral vectors is the 
inherent propensity of murine retroviral PCLs to generate replication-competent retrovirus (RCRV) 
after introduction of a vector (35). This can theoretically occur by homologous recombination 
between the retroviral provector DNA and either 1) the DNA encoding the MoMLV structural 
genes present in the PCL (36), or 2) the endogenous proviruses found universally in murine cells 
(37), the species from which all the MLV-based PCLs previously used for human gene therapy 
[620] 
Recombinant DNA Research, Volume 17 
