resistant to injections of unmodified CT26 tumor cells. The resistance was 
mediated primarily by CD8+ cytotoxic T lymphocytes. The anti tumor immunity 
generated by the IL-2-secreting gene-modified tumor cells was specific, 
because other non-IL-2-secreting tumor cells grew without inhibition in mice 
rejecting the IL-2-secreting tumor. Injections of unmodified tumor cells 
inactivated by high dose X-irradiation were not immunogenic and did not 
provoke anti tumor immune responses capable of protecting the animals against 
growth of non irradiated cells. Thus, secretion of IL-2 by the malignant 
cells eliminated their malignant growth properties in syngeneic mice. Animals 
rejecting the IL-2-secreting tumor cells were at least partially resistant to 
challenge with unmodified tumor cells. 
In analogous studies, Gansbacher et al . (40) transduced a weakly immunogenic 
mouse fibrosarcoma cell line with a retroviral vector carrying the gene for 
IL-2. As previously, secretion of IL-2 by the tumor cells abrogated the 
cells' tumorigenic properties and stimulated long-lasting T cell mediated 
immunity against challenge with the unmodified tumor cells. Russell et al. 
(41) obtained similar results in an analogous system. They also observed 
tumors developing in animals injected with the IL-2-secreting cells were also 
rejected. In some instances, tumors developed following injection of the 
gene-modified cells. These cells were IL-2 negative variants. Our laboratory 
has confirmed these data for IL-2-secreting B16 melanoma injected into 
syngeneic C57BL/6 mice. One hundred percent of mice injected with IL-2- 
secreting melanoma cells developed tumors. 
Rosenberg et al . (42) introduced the gene for IL-2 into a mouse sarcoma cell 
line (MCA-105) . The IL-2-secreting cells were rejected by mice syngeneic with 
MCA-205 cells. Mice subjected to sub lethal irradiation or depleted of T 
cells supported the growth of IL-2-secreting MCA-205 cells, suggesting that 
immune mechanisms were responsible for rejection of the modified cells. 
Similar results were obtained for an independently arising sarcoma cell line 
(MCA-102) . In each instance, mice rejecting the IL-2-secreting cells 
developed partial immunity to the growth of non-IL-2-secreting, unmodified 
tumor cells. IL-2 secretion by highly malignant tumor cells led to an 
increase in the cells' immunogenicity , abrogated the cells' malignant growth 
properties, and stimulated T cell-mediated anti tumor responses. 
Are the immunogenic properties of IL-2 secreting tumor cells sufficient to 
lead to the regression of established tumors? To investigate this question, 
we first established melanomas in C57BL/6 mice. Six days afterwards, we 
administered the first of three weekly injections with allogeneic IL-2- 
secreting cell-constructs expressing MAA. The results are presented in 
Manuscript 1; Appendix VII. They indicate that animals rejecting the IL-2- 
secreting cells survived significantly (P < .001) longer than untreated mice 
or mice treated according to the same protocol with X-irradiated B16 cells. 
Berd and Mastrangelo (18) investigated the immunogenic properties of 
irradiated autologous tumor in melanoma patients. They found infiltration of 
T cells in existing tumor and objective clinical regression of metastatic 
melanoma (19). Mitchell et al . induced objective clinical responses in 109 
melanoma patients by active immunotherapy with pooled allogeneic melanoma cell 
lysates administered along with an immunological adjuvant (Detox*) (20). The 
response was mediated primarily by cytotoxic T lymphocytes. Numerous other 
reports indicate the importance of T cells in the rejection of established 
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