Studies on the possible mechanism for the affect of TK positive cells on TK 
negative cells were begun by analyzing the mechanism of cell death when TK positive 
cells are exposed to GCV. Cells die by two mechanisms, necrosis and apoptosis. 
Necrotic cell death is characterized by cell swelling, cell membrane disintegration, and i 
nuclear flocculation. Apoptotic cell death is characterized by cell shrinkage, vesicle 
formation, and chromatin condensation. Cells dying by apoptosis break up into 
vesicles which can be phagocytized by nearby cells. Tumor cells have the ability to 
phagocytize apoptotic vesicles. TK negative cells may be affected by TK positive cells 
due to the ability of TK negative cells to phagocytize toxic metabolites contained in TK 
positive apoptotic vesicles. Plates 1 and 2 show light microscopy of normal and TK 
positive cells exposed GCV. The TK positive cells show the characteristic signs of 
apoptosis: cell shrinkage, vesicle formation, and chromatin condensation. 
We next analyzed the effects of TK positive cells on TK negative cells in vivo. Using 
the kbalb tumor model, mice were inoculated with varying ratios of kbalb tumor cells 
which were transduced with either the STK or LNL retroviral vector. Mice received 2 x 
10^ tumor cells subcutaneously on day 0 and were treated with GCV on day o when 
tumor diameter was approximately 2 mm (150 mg/kg I.P., b i d., x 10 doses). Groups of 
mice receiving either 50%, 90% or 100% kbalb-STK cells demonstrated tumor 
regression (figure 15). These studies were repeated with two other tumor lines, 205 
and 207. In these experiments untransduced tumor cells were mixed with STK 
transduced tumor cells in varying ratios of 0%, 1%, 10%, 50%, 90% and 100% 205- 
STK or 207- STK. All animals in groups receiving 50% or more STK transduced cells 
demonstrated tumor regression. However, some animals receiving 10% STK 
transduced cells developed tumors as did all animals in groups receiving 1% or 0% 
STK transduced cells (figure 16). 
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Recombinant DNA Research, Volume 15 
