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

326 16 Immunocytokines: Versatile Molecules for Biotherapy of Malignant Disease
Fig. 16.3 Prevention of pulmonary Lewis
lung metastases by an oral, CEAbased
DNA vaccine boosted by huKS1/
4±IL-2 immunocytokine C57BL/6J mice
transgenic for CEA (n = 8) were immunized
3 times at 2-week intervals by oral
gavage with 100 ml PBS containing 10 8
attenuated S. typhimurium harboring
either pW-CEA or the control vector
pER-CEA. Additional controls were naive
mice that received only PBS. Experimental
pulmonary metastases were induced
2 weeks after the last immunization by
i.v. injection of 5 × 10 5 LLC-CEA-KSA
cells suspended in 100 ml PBS into the
lateral tail vein. Mice were sacrificed 4
weeks after i.v. tumor cell challenge. Tumor
burden was determined as the percentage
of lung surface covered by tumor
nodules.
to APCs by providing sufficient time for T cells to sample large numbers of MHC
molecules on the surface of APCs for the presence of specific peptides. This prolonged
sampling increases the chances of a naive T cell recognizing its specific peptide/MHC
ligand, followed by signaling through the TCR, and induction of a conformational
change in LFA-2. In turn, the increased affinity of LFA-2 for CD48 stabilizes
the association between the antigen-specific T cell and the APC [51].
We also observed a marked increase in expression of CD28 on T cells and of the costimulatory
CD80 molecule on DCs, both of which are of considerable importance
in providing the two independent signals required for effective activation of naive
T cells: (1) antigen recognition and binding of the peptide/MHC complex to the TCR
providing the first signal, and (2) ligation of CD28 with CD80 and/or CD86 to initiate
T cell responses and production of armed effector cells providing the second signal
[52, 53].
A pronounced increase in expression of the high-affinity IL-2 receptor a chain CD25
over controls indicated that T cell activation took place in secondary lymphoid tissues
after vaccination and tumor cell challenge. The notion that such lymphoid tissues
from mice exhibiting tumor-protective immunity contained tumor antigen-specific
CD8 + T cells was further supported by the finding that splenocytes isolated from