unrelated cell lines, BC10ME and B16F10, indicated that the anti-CT26 immune response was 
specific (Fig 1 1C). 
A few animals injected with unirradiated y-IFN-expressing CT26 cells generated small tumors 
while most animals did not generate tumors (Fig 8C). The splenocytes collected from tumor-free 
vs. tumor-bearing animals were tested in vitro for anti-CT26 CTL activity. The data (Fig 1 1 D) 
indicate that the level of anti-tumor CTL activity is substantially higher in tumor-free animals than 
that observed for animals with tumors. These data suggest that the tumor cells failed to form 
tumors as a result of more potent CTL activity. This confirms the importance of CTL for inhibiting 
tumor growth. 
Substantial CTL induction was also observed after injection of Lewis lung carcinoma or the 
corresponding y-IFN-transduced tumor cells (Fig 12A). The anti-tumor response was specific, as 
determined by the inability of anti-LLT cytolytic lymphocytes to lyse the unrelated tumors, B16F10 
and CT26 (Fig 12B). 
These data show that in all three models the lack of measurable response against the unmodified 
tumor is due to insufficient CTL induction , not due to the inability of the cell to be lysed by CTL. 
These results have been confirmed by other investigators with another tumor model (30). 
3. 3. 2. 4. MURINE TUMOR VACCINATION STUDIES. To determine whether the observed 
immune responses could protect mice from challenge with unmodified tumor cells, vaccination 
studies were performed by priming mice with either unmodified, or y-IFN- transduced, irradiated 
B16F10 tumor cells. The animals were then challenged with the unmodified parental tumor cell. 
The data in Fig 13A indicate that a single pre-injection with as little as 4xl0 5 y-IFN-expressing 
tumor cells, but not unmodified tumor cells, resulted in a measurable (~ 2.5-fold) protection 
against tumor challenge. A single injection with larger numbers of cells resulted in even greater (~ 
5-fold) protective immunity (Fig 13B). Once immunity had been induced by vector-modified cells, 
the resultant immune effectors react not only against the y-IFN-expressing cell (resulting in 
decreased tumorigenicity, Fig 8) but also against the unmodified tumor cell (resulting in the 
immune protection against unmodified tumor. Fig 13). y-IFN-transduced tumor cells are thus 
capable of generating systemic, protective, anti-tumor immunity. 
3.3.3. Human Melanoma Data 
3.3.3. 1 . TRANSDUCIBILITY of HUMAN MELANOMA CELLS IN VITRO. To determine how 
effectively human melanoma cell lines could be transduced with retroviral vectors, we utilized a 
vector identical with Huy vector, except that the E. coli p-galactosidase gene is expressed instead of 
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Recombinant DNA Research, Volume 17 
