bearing these tumors. 
The increased immunogenic ity of these TNF producing tumor cells was 
demonstrated by experiments showing the immunologic nature of this tumor 
regression. TNF producing tumor cells grew well in irradiated mice but not in 
non-irradiated mice. Further, the ability of tumor cells to regress was 
abrogated by depletion of the CD8+ T cell subset (see Fig. 6). Further, 
animals that experienced regression of TNF producing tumors rejected a 
subsequent challenge of unmodified tumor indicating the state of immunity to 
the tumor that had regressed. In addition, TNF producing tumor cells could 
function in a paracrine fashion by inhibiting the growth of unmodified tumor 
cells implanted at the same site (see Fig. 7). It thus appears from these 
murine studies that tumor cells elaborating high local concentrations of TNF 
could regress in the absence of toxicity in the host and that this regression 
was immuno logically mediated. 
Similar studies were performed using TNF gene modified human tumor 
cells injected into athymic nude mice. Following the injection of 4 to 8 x 
10 6 human tumor cells into nude mice, non-transduced or neomycin transduced 
tumor cells grew progressively. However, tumors that were transduced with the 
TNF gene stopped growing after 8 to 10 days in all of the animals and complete 
tumor regression was seen in some of the mice (Fig. 8). In these experiments, 
DNA was extracted from some stable or regressing human tumors and Southern 
analysis revealed intact unrearranged proviral DNA present in all the 
regressing tumors induced by TNF-transduced cells. The proviral sequences were 
undetectable in the proliferating tumors formed by the non-transduced cells. 
The paracrine function of the TNF produced by human tumors was also seen. The 
localized elaboration of TNF by TNF transduced cells was also effective in 
suppressing tumor formation by control NeoR transduced cells injected at the 
same site. 
It thus appears that in syngeneic mice the injection of TNF modified 
tumor cells was more immunogenic than unmodified cells and could induce an 
immunity sufficient to cause the regression of these TNF gene modified tumor 
Recombinant DNA Research, Volume 15 
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