from the spleens were capable of killing the tumor in vitro throughout the life 
span of the immunized mouse. Of note, weak cytolytic responses were generated 
by unmodified tumor cells; these were transient and disappeared three weeks 
after tumor implantation. However, mice injected with IL-2 secreting tumor cells 
generated tumor specific cytolytic activity lasting for over 75 days. 
Several of these findings have been reproduced at MSKCC in different tumor 
types such as murine melanoma B16, murine B cell lymphoma 38C13, and 
murine bladder carcinoma MBT 2. Additional experiments using irradiated IL-2 
secreting CMS5 cells demonstrated that there was no difference in their ability to 
induce immunological memory in vivo as compared to non-irradiated IL-2 
secreting tumor cells (see appendix F). Rosenberg et al (unpublished data) 
reproduced these results in the poorly immunogenic tumor MCA-102. Despite 
many attempts they were never able to generate a cellular antitumor response 
using unmodified tumor cells. Introduction of the IL-2 gene into these cells 
increased their immunogenicity to a level which generated tumor rejection and 
memory. In addition, coinjection of the IL-2 producing tumor cells with 
unmodified tumor cells produced an immune response strong enough to reject not 
only the lymphokine secreting tumor cells but the unmodified tumor cells as 
well. 
The mastocytoma cell line P815 was transfected with the murine IL-2 gene. 
Injection into mice led to tumor rejection. Analysis of the cellular immune 
response demonstrated that the frequency of CTL precursors and CTLs specific 
for P815 increased 2-4 fold as a result of IL-2 secretion by the tumor cells. 
In summary, the introduction and constitutive expression of IL-2 by tumor cells 
themselves increased the immunogenicity of a variety of histologically different 
cancer cells. It appears from these studies that the localized persistent expression 
and secretion of low levels of IL-2 may have an impact on the host antitumor 
response in the absence of any detectable toxicity. 
2.6 Gene transfer in Humans 
Most recently, Rosenberg et al have taken genetically altered autologous tumor 
infiltrating lymphocytes and injected them into patients with refractory metastatic 
tumors (20). A retroviral vector containing the neomycin resistance gene was 
introduced into autologous lymphocytes in order to monitor them when reinfused 
into patients. Of the five patients, cells from four were successfully grown in 
tissue cultures with G418, a neomycin analogue. Those cells that showed 
neomycin resistance were given intravenously to patients along with systemic IL- 
2. Peripheral blood was recovered periodically and circulating mononuclear 
cells with neomycin resistance were found. There were no additional toxicities 
seen as a result of the genetic alteration. No intact virus was recovered from any 
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