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

276 13 Applications of CpG Motifs from Bacterial DNA in Cancer Immunotherapy
13.5.2
CpG DNA as an Adjuvant for Cancer Vaccines
Numerous studies have demonstrated the strong activity of CpG-B ODN as adjuvants
for vaccines against infectious disease in mouse models, as well as in nonhuman
primates ([88, 113±115] and reviewed in [116]). An important factor in their activity
in tumor vaccines may be that CpG-B ODN promote cross-priming with strong
cytolytic T cell and antibody responses to peptides and protein antigens independently
of T cell help [ 95, 117±122]. CpG-B ODN appear to be more effective Th1-like
adjuvants than complete Freund's [119, 123, 124] or any other adjuvant to which they
have been compared [125].
Both CpG-B and CpG-A ODN are highly effective adjuvants for tumor vaccines,
although most studies have been performed with CpG-B ODN. The first demonstration
of the efficacy of CpG DNA as an adjuvant for a tumor antigen was accidental.
In studies of Drosophila cells that were engineered to function as antigen-presenting
cells (APCs) and present tumor antigens, Sprent et al. made the surprising observation
that untransfected Drosophila cells were highly stimulatory to murine B cells,
which up-regulated co-stimulatory molecules and could provide bystander co-stimulation
for CD8 + T cells [38]. They further showed that the Drosophila DNA is unmethylated
and immune stimulatory, but loses this property after treatment with
CpG methylase. Drosophila cells transfected to express a tumor antigen in the context
of the appropriate class I MHC could induce MHC compatible spleen cells to respond
to a tumor antigen in vitro and to mediate tumor rejection in vivo [38].
We have evaluated the antitumor adjuvant properties of CpG DNA in the 38C13
murine B cell lymphoma model, in which the idiotype (Id) of the 38C13 surface
IgM serves as the tumor-associated antigen. A CpG-B ODN induced the production
of high levels of Id-specific antibody in mice immunized with the CpG together
with a conjugate of the Id to keyhole limpet hemocyanin (KLH) [123]. The CpG
ODN was highly effective when used to immunize via the intradermal or subcutaneous
routes, but was slightly less effective through the intraperitoneal route.
A dose of 25 mg of the ODN was highly effective, but a maximal response was seen
at doses of 50±100 mg. Control mice challenged with a lethal dose of tumor cells all
died within 1 month of challenge, but mice immunized together with the CpG-B
ODN had 40 % long-term survival [123]. The efficacy of the CpG-B ODN was
further improved when the Id was conjugated to GMCSF [126]. A GMCSF-Id fusion
protein, used as an immunogen without an adjuvant, was an extremely effective
immunogen for inducing anti-Id antibodies. However, almost all of the antibody
response was of the IgG1 isotype, and only 30 % of mice were long-term survivors
of a tumor challenge given 3 days after a single immunization. In contrast,
when a CpG ODN was combined with the granulocyte macrophage colony stimulating
factor (GM-CSF)±Id fusion protein, the antibody response was largely IgG2 a,
the magnitude of the antibody response was increased approximately 5-fold and the
long-term survival of the mice was improved from 30 to 70 % [126]. In a different
model system, Celis et al. showed that repeated immunization of mice with ovalbumin
(OVA) protein or peptide together with a CpG-B ODN resulted in a 10- to 100-