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

322 16 Immunocytokines: Versatile Molecules for Biotherapy of Malignant Disease
after tumor cell challenge also markedly improved the tumor-protective effect of our
CEA-based DNA vaccine in CEA transgenic mice. Thus, six of eight vaccinated mice
now completely rejected the tumor cell challenge and the remaining two animals exhibited
a marked suppression in tumor growth. These results are illustrated in
Fig. 16.2.
Taken together the results of these studies demonstrated that our orally administered,
CEA-based DNA vaccine induced effective tumor-protective immunity
mediated by MHC class I antigen-restricted CTLs. This antitumor effect correlated
with the marked up-regulation of co-stimulatory molecules on APCs, markers of activation
on T lymphocytes and increased release of pro-inflammatory cytokines.
Small, non-curative suboptimal doses of huKS1/4±IL-2 immunocytokine administered
after tumor cell challenge further increased these antitumor effects.
16.2.4
T Cell-mediated Protective Immunity against Colon Carcinoma Induced by a DNA
Vaccine encoding CEAand CD40 Ligand Trimer (CD40LT)
Since we achieved at best a partial tumor-protective response against a lethal challenge
of MC38 murine colon carcinoma cells with a CEA-based, DNA vaccine,
boosted with the huKS1/4±IL-2 immunocytokine [33], we determined whether these
results could be improved by achieving complete tumor-protection in 100% of experimental
animals. To this end, we constructed and tested a CEA-based DNA vaccine
encoding both CEA and CD40LT, and again boosted with suboptimal doses of
huKS1/4±IL-2 immunocytokine. This dual-function vaccine indeed achieved this objective
as eight out of eight CEA-transgenic mice completely rejected a lethal challenge
of MC38-CEA-KSA colon carcinoma cells following three immunization with
this DNA vaccine at 2-week intervals [39]. We attribute this success to the combined
action of the unique dual-function DNA vaccine and the huKS1/4±IL-2 fusion protein,
which accomplished the concurrent activation of both antigen-presenting dendritic
cells (DCs) and naive T cells. Possible mechanisms of action were suggested by
the up-regulated expression of several receptor/ligand pairs known to critically impact
effective activation of T cells following their interaction with DCs which present
them with MHC/peptide complexes. These included CD40/CD40LT, LFA-1/ICAM-1,
CD28/B7±1 and B7±2 and CD25/IL-2, as well as the increased secretion of pro-inflammatory
cytokines IFN-g and IL-12. Several lines of evidence pointed to effective
priming of CD8 + T cells in vivo following immunization with the pCD40LT-CEA
dual-function DNA vaccine and challenge with colon carcinoma cells. First, a
marked activation of T cells and CD11c + dendritic-like cells was indicated by the decisive
up-regulation in expression of T cell integrins LFA-1 and ICAM-1, which are
known to synergize in the binding of lymphocytes to APCs [34]. In fact, this transient
binding of naive T cells to APCs is crucial in providing time for these cells to
sample large numbers of MHC molecules on the surface of each APC for the presence
of specific peptides. This mechanism may increase the chance of a naive T cell
recognizing its specific MHC/peptide ligand, followed by signaling through the TCR
and induction of a conformational change in LFA-1. This, in turn, will greatly in-