Induction of long-lasting cytokine effect by injection of IL-12 encoding plasmid DNA

We have recently demonstrated that DNA coding for both subunits of the muri ne IL-12 heterodimer Exhibits a strong antimetastatic effect against B16-me lanoma in C57BL/6 mice and after intratumoral injection tumor regression. H ere we show that the antimetastatic effect can be detected when the DNA is injected intramuscularly 30 days before tumor cell challenge. A long-term I L-12 expression was measured for up to 50 days in the serum with a peak at day 20 amounting to about 10 ng/mL in C57BL/6 mice. CpG oligodeoxynucleotid es also induce IL-12 expression, however, only for a few hours. IL-12 DNA a dministration induces longlasting systemic IFN-gamma production, whereas IL -4 and TNF-alpha levels remained undetectable. NK cell-depleted mice showed a strong but reduced expression of murine IL-12. Expression of DNA encodin g human instead of murine IL-12 resulted in a significantly lower and trans ient expression, indicating that not plasmid-derived IL-12 production alone but the immune system of the host contributes to the long-lasting antimeta static Effect. It may be attributable to an autocrine feedback mechanism ma intaining murine IL-12 expression, whereby several cell populations includi ng NK cells are involved.