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

262 12 Principles and Strategies Employing Heat Shock Proteins for Immunotherapy of Cancers
alone. Similar immunization enhanced the cytotoxic activity of mouse splenic lymphocytes
against untreated and heat-treated colon-26 cells in an in vitro assay. Clark
and Menoret have found that pretreatment of tumor cells with heat shock led to increased
immunogenicity [82]. Surprisingly, immunization with heat-shocked cells,
but not with untreated cells, induced TNF-a-producing cells in the spleen in an antigen-independent
manner. Recently, it was found that heat-shocked apoptotic tumor
cells are effective tumor vaccines [83]. It is unclear from all the above studies, however,
how much of the changes are due to the result of increased HSP production
versus a ªnon-specificº effect of heat alone.
Studies of naturally arising tumor variants from a rat colon carcinoma clone suggested
that tumor regression is associated with their ability to synthesis the strictly
heat inducible HSP70 [84]. Furthermore, inducible HSP70, induced during nonapoptotic
cell death increases tumor immunogenicity in vivo [85]. Increasing HSP expression
in tumor cells alone by transfection of HSP70 genes under different promoters
indicated that (i) HSP70 augments antigen presentation through the MHC
class I pathway as revealed by increased lysis of the engineered cells by the cognate
CTL clone [86], and (ii) induces an infiltrate of T cells, macrophages and predominantly
DCs into the tumors as well as an intratumoral profile of Th1 cytokine expression
(interferon, TNF and IL-12) [87].
Thus whole cells with increasing level of HSPs, particularly the inducible HSP70,
should be examined clinically for the immunotherapy of human malignancies.
12.2.3.2 Modulation of the site of HSP expression for cancer immunotherapy
The appreciation of immunological properties of HSPs has inspired an effort in exposing
endogenous HSPs extracellularly to stimulate effective anti-tumor immunity.
This is best represented by gp96. gp96 is predominantly a lumenal protein of the ER,
although both secretion and surface expression of gp96 have been observed (see
above). Since the cell biological basis of ªectopicº trafficking of gp96 is unknown, two
groups have adopted a genetic approach to understand the immunological significance
of extracellular gp96. Gp96 contains an ER retention motif, Lys±Asp±Glu±Leu
(KDEL), at its C-terminus. By deleting KDEL and replacing it with the hinge, CH2 and
CH3 domains of murine IgG1,Yamazaki et al. have created a secretory form of gp96,
gp96±Ig [88]. Transfection of tumor cells with gp96±Ig led to active secretion of it without
evidence of intracellular accumulation. Gp96-secreting tumor cells are able to
prime tumor-specific CD8 + Tcell immunity in a CD4 + Tcell-independent manner.
Similarly, Zheng et al. [111] have replaced the C-terminal KDEL motif of gp96 with a
transmembrane domain of murine platelet-derived growth factor receptor. High
levels of cell surface expression of gp96 in type I orientation was achieved consistently.
By using the gp96 surface-expressing target cells, two questions are examined.
First, can tumor cells, by virtue of its surface expression, stimulate DCs to mature?
Secondly, can gp96 surface-expressing tumor cells prime T cells more effectively? It
was found that direct access of gp96 to DCs, by cell surface expression, induces DC
maturation resulting in secretion of proinflammatory cytokines IL-1˜, IL-12, chemokine
monocyte chemoattractant protein (MCP)-1, and up-regulation of the expression
of MHC I, MHC II, CD80, CD86 and CD40. Furthermore, surface expression of