Based on the knowledge that adenovirus (Ad)-mediated expression of the muri
ne gp75 melanoma antigen (Adgp75) will effectively immunize mice against H2
-matched B16 melanoma cells, probably via cell-mediated immune mechanisms,
we hypothesized that Ad-mediated delivery of the murine interleukin-12 (IL-
12) complementary DNA heterodimer would have independent therapeutic effect
s on tumor growth, and that the combination of the two vectors would work s
ynergistically to augment the antitumor response. We evaluated the therapeu
tic effect of each vector alone and in combination for efficacy in C57BL/6
mice with preestablished (2 d) B16 melanoma-derived pulmonary metastases, u
sing the number of lung metastases as the efficacy parameter. Intraperitone
al administration of Adgp75 (10(8) PFU) reduced tumor burden to 45 +/- 7% o
f controls (P < 0.01), and AdIL12 administration (10(8) PFU, intraperitonea
lly) reduced the number of metastases to 43 +/- 7% of controls (P ( 0.01).
The combination of Adgp75 (10(8) PFU, intraperitoneally) and AdIL12 (10(8)
PFU, intraperitoneally) provided further protection (15 +/- 3%; P < 0.01 as
compared with naive control; P < 0.01 compared with Adgp75 or AdIL12 alone
). Mice receiving AdIL12 showed increased natural killer cell (NK cell) fun
ction in an in vitro cytotoxicity assay, with a dose-dependent lysis of YAC
-1 cells and, to a lesser extent, lysis of B16 cells. To assess the relativ
e contribution of major histocompatibility complex I (MHC I) Dependent and
Independent activity in combination therapy with Adgp75 plus AdIL12, we per
formed adoptive transfer experiments, using splenocytes from mice receiving
Adgp75, AdIL12, or Adgp75 + AdIL12, from among which NK cells had been sel
ectively depleted in vitro prior to adoptive transfer. Each group showed si
gnificant decreases in tumor burden resembling those with primary treatment
. Interestingly, NK-cell depletion from among cells derived from the Adgp75
- and AdIL12-treated mice significantly altered the therapeutic response (P
< 0.01 compared with the Adgp75 + AdIL12 group), suggesting a significant
role of NK-cell-mediated cytolysis in vivo, although there was still a sign
ificantly reduced tumor burden (P < 0.01 compared with that of naive contro
ls). Collectively, these data support the concept that the combination of A
dIL12 and Adgp75 provides additive effects against pulmonary metastases of
B16 melanoma by MHC-independent (NK cell) means as well as MHC-dependent cy
totoxic lymphocyte means, suggesting that this therapy may be a useful adju
vant in the treatment of metastatic melanoma.