AMOUNT OF INTERLEUKIN-12 AVAILABLE AT THE TUMOR SITE IS CRITICAL FOR TUMOR-REGRESSION

The C26 colon carcinoma is resistant to systemic recombinant interleuk in 12 (rIL-12) therapy, Transduction of C26 with genes encoding the tw o subunits of murine IL-12 to release 30-80 pg/ml resulted in delayed tumor onset after injection of 5 x 10(4) cells into syngeneic BALB/c m ice and in 40% tumor regression after injection into CD4-depleted mice , Here, we analyzed the activity of rIL-12 (1 mu g/day) against C26 gr own into CD4-depleted mice, Like in mice given injections of interleuk in 12 (IL-12) gene-transduced C26 cells, depletion of CD4(+) cells led to tumor regression in 6 of 14 mice, and immunocytochemical character ization of tumorinfiltrating leukocytes showed abundant infiltration b y CD8(+) T cells and asialoGM1(+) natural killer cells, which were sca nty in tumors from nondepleted mice, On the basis of the percentage of tumor regression and leukocyte infiltration we can conclude that, in the C26 system, systemic rIL-12 (1 mu g/day) produces the same results as 30-80 pg/ml IL-12 released at the tumor site. A new polycistronic retroviral vector was then used to increase the amount of IL-12 produc ed by C26-transduced cells, C26 cells releasing 5 ng/ml IL-12, nearly 100 times more than the above-mentioned transduced cells, were tumorig enic in less than 50% of the mice given injections of 5 x 10(4) cells, In mice given injections of 5 x 10(5) cells, an initial tumor take of 100% followed by a complete tumor regression, Tumor regression was as sociated with infiltration of CD8(+) and asialoGM1(+) cells, and mice that remained tumor free were immune to a rechallenge of nontransduced C26 cells, The results indicate that the amount of IL-12 made availab le at the tumor site may determine both the type and number of infiltr ating leukocytes and the events leading to tumor regression as a ell a s it may overcame host immunosuppression.