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

332 16 Immunocytokines: Versatile Molecules for Biotherapy of Malignant Disease
against a lethal challenge with wild-type tumor cells. Importantly, immunocytokinetreated
animals exhibited an inflammatory response indicated by heavy infiltrates of
lymphocytes intermixed with a few granulocytes and macrophages. Strong staining
of CD8 + T cells, and to a lesser extent CD4 + T cells, infiltrating the tumor microenvironment
was observed while occasional NK cells were located primarily in the tumor
periphery [62]. More rigorous proof for a T cell-mediated mechanism was obtained
in C57BL/6 scid/scid mice that lack mature T and B cells, and in C57BL/6 beige/beige
devoid of functional NK cells. Significantly, an absence of NK cells did not hinder
the successful treatment of established pulmonary melanoma metastases; however,
a lack of T lymphocytes abrogated this effect. This was further demonstrated by selective
in vivo depletion of distinctive T cell subpopulations and in vitro cytotoxicity
assays indicating that CD8 + T cells are mediators of the ch14.18±IL-2-induced, MHC
class I antigen-restricted immune response in this tumor model [61, 62]. Interestingly,
the depletion of CD4 + T cells partially diminished the efficacy of the immunocytokine-mediated
immune response. In fact, the absence of cytolytic activity in the
CD4 + T cell compartment against melanoma target cells in vitro suggested a helper
function of CD4 + T cells for the most optimal activation of CD8 + T cell response. The
mechanism of help provided by CD4 + T cells was mediated by CD40/CD40L interactions
and not by the release of IL-2 from this T cell subpopulation. This conclusion
was based on the observation that ch14.18±IL-2-mediated anti-melanoma activity
was partially abrogated in CD40L knockout (KO) mice, but not in IL-2 KO mice in
which the immunocytokine was completely effective. Partial abrogation of the antitumor
effect was induced with anti-CD40L antibody to the same extent as with CD4 +
T cell depletion. Also, a complete antitumor response induced by hu14.18±IL-2 could
be reconstituted in CD40L KO mice by simultaneous stimulation with anti-CD40
antibody [65].
Successful therapy with the ch14.18±IL-2 immunocytokine also resulted in longlived,
transferable tumor immunity. Consequently, mice cured of established s.c.
melanoma or pulmonary metastases by the immunocytokine completely rejected a
subsequent lethal i.v. challenge with melanoma cells in at least 50 % of all animals
up to 4 months after the initial treatment [67]. Significantly, challenges with an unrelated
syngeneic thymoma cell line (EL4), expressing the GD2 docking site, induced
fulminant metastases in the same mice that could, however, be fully protected
against challenges with murine melanoma cells. These findings suggested that as
yet undefined tumor antigens recognized by T cells are required to induce a tumorprotective
immunity. Importantly, this is completely independent of the GD2 docking
site that simply serves to deliver IL-2 to the tumor microenvironment [67].
Taken together, the findings obtained in the melanoma model provided proof of concept
for an immunocytokine to direct IL-2 to the tumor microenvironment, eradicate
established metastases and induce a T cell-mediated memory immune response,
suggesting that this approach may be used as a non individualized tumor vaccine.