Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

16.5 Melanoma
The remaining animals exhibited a 5-fold reduction in tumor growth compared to
controls receiving PBS, the empty vector, or the DNA vaccine lacking ubiquitin. All animals
in the three control groups grew large tumors (800±900 mm 3 ) and failed to reject
the wild-type tumor cell challenge [75].
Two lines of evidence indicated that the tumor-protective immunity induced by the
DNA vaccine was mediated by MHC class I antigen-restricted CD8 + T cells secreting
increased amounts of the pro-inflammatory cytokine IFN-g. First, adoptive transfer
of CD8 + T cells from mice successfully immunized by the DNA vaccine to syngeneic
scid/scid mice resulted in more than 60% reduction in s.c. tumor growth after challenge
with wild-type B16 melanoma cells when compared with control animals that
received CD8 + T cells from mice vaccinated with only the empty ubiquitin vector.
Second, only CD8 + T cells isolated from splenocytes of successfully vaccinated mice
induced MHC class I antigen-restricted CTL-mediated cytolytic killing of melanoma
target cells in vitro. Significantly, the presence of ubiquitin upstream of the minigene
proved to be essential for achieving this tumor-protective immunity, suggesting that
more effective antigen processing in the proteasome and presentation by antigenpresenting
cells made it possible to break peripheral T cell tolerance to tumor selfantigens
[76, 77].
Although these findings obtained with the DNA vaccine encoding melanoma minigenes
were encouraging, we attempted to improve its efficacy by combining it with
small boosts by sub-optimal doses of ch14.18±IL-2 immunocytokine. Actually, three
lines of prior evidence provided the rationale for this approach. First, we demonstrate
previously that immunocytokines targeted to the tumor microenvironment where
they actively activated and expanded CD8 + T cell. In fact, combined with CD4 + T cell
help, this approach induced effective antitumor responses and eradicated established
metastases of murine colon carcinoma [11, 12] and murine melanoma [61, 62]. Second,
we already demonstrated that the humanized huKS1/4±IL-2 immunocytokine
(huKS1/4±IL-2) elicited a long-lived cellular-mediated memory immune response
against murine colon carcinoma which, importantly, was substantially amplified by
additional boosts with non-curative, suboptimal doses of this same immunocytokine
targeted to the tumor microenvironment. Third, systemically administered IL-2 was
shown to be of key importance for boosting the efficacy of a variety of malignancies
tested in the clinic [78, 79]. In fact, modified gp100209±217 peptide-induced T cell responses
resulted in a striking objective clinical response rate of 42% among melanoma
patients only when these individuals received concurrent boosts with high systemic
doses of IL-2 [79]. Thus, IL-2 may actually be essential for breaking peripheral
T cell tolerance against melanoma self-antigens.
To prove this point, C57BL6/J mice immunized three times with the DNA vaccine
encoding ubiquitin and the gp10025±53 and TRP-2181±189 minigenes, were challenged
s.c. with wild-type murine melanoma cells, 1 week after the last immunization and
24 h later received i.v. boosts with 5 mg ch14.18±IL-2 for 4 consecutive days. This resulted
in complete tumor rejection in 75% of experimental animals with the remaining
mice revealing an almost 10-fold reduction in tumor growth when compared
with control animals. These fusion-protein boosts markedly enhanced tumor-protective
immunity when compared to the vaccine alone (P > 0.01), which resulted in tu-
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