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

318 16 Immunocytokines: Versatile Molecules for Biotherapy of Malignant Disease
these SCID mice were challenged with CT26-KSA tumor cells at the 6-week time
point and then received a boost with huKS1/4±IL-2 immunocytokine 4 days later,
the putative memory remaining in these mice again recognized the tumor antigen
as indicated by a markedly increased generation of CD8 + T cells. Importantly, only
those CD8 + T cells specifically lysed CT26-KSA tumor target cells in vitro. Further
evidence for strong priming of these very same CD8 + T cells was indicated by their
strong expression of CD25, a marker for the high-affinity IL-2 receptor a chain, and
CD69, an early T cell activation marker. Further evidence for effective priming of
these CD8 + T cells was provided by their substantial release of IL-2 and of pro-inflammatory
cytokines IL-12 and GM-CSF measured by a sandwich ELISA [15].
The data obtained in our colorectal carcinoma model demonstrated independence of
the immunologic response from the target antigen used for directing IL-2 into the
tumor microenvironment. Specifically, the direction of IL-2 into the tumor microenvironment
was followed by the induction of a CD8 + T cell response in conjunction
with antigen peptides presented via MHC class I antigens on the surface of tumor
cells and via MHC class II antigens by antigen-presenting cells (APC), which are
both well-established requirements for T cell priming [16]. The localization of these
events occurring in our tumor model was suggested to be in the draining lymph
nodes. In fact, activation of T cells was indicated by FACS analysis of single-cell suspensions
obtained from draining lymph nodes at the termination of immunocytokine
treatment. In this case, expression of both the early T cell activation marker
CD69 and the CD25 marker indicating the high-affinity IL-2 receptor a chain were
markedly elevated [15]. There is also evidence from the literature that the actual presentation
of tumor-derived peptides via MHC class II antigens on APCs occurs in
the draining lymph nodes rather than in the tumor microenvironment. Thus, Maass
et al. [17] reported that IL-2 transduced tumor cells are eliminated from the inoculation
site by macrophages and granulocytes followed by an invasion of APCs at the tumor
cell inoculation site, which travel to the locoregional lymph nodes, where T cell
activation then occurs. This contention was based on the presence of T cell activation
markers only in the draining lymph node system, but not at the site of vaccination.
Once effective priming of a CD8 + T cell response was induced in our colon carcinoma
model system, further proliferation and expansion of such T cells was required
for an optimal antitumor effect. This was facilitated in the presence of both the T cell
growth factor IL-2 and the peptide antigen(s) presented via MHC class I antigens on
tumor cells, which provided an effective growth stimulus via IL-2 and T cell receptors,
respectively. The expansion of CD8 + effector T cells induced effective eradication
of micrometastases which was accomplished 5 days after the end of treatment
with the IL-2 immunocytokine. After tumor cell eradication was achieved, most of
these effector T cells did undergo apoptotic elimination and only a small number
persisted as memory cells that, once properly reactivated, elicited the long-lived tumor-protective
immune responses observed in our colon carcinoma model [15].