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

110 6 Immune Cells in the Tumor Microenvironment
6.5.1
The CD95±CD95 Ligand (CD95L) Pathway
The CD95±CD95L pathway has been implicated in tumor-mediated apoptosis of immune
cells, contributing to immune privilege of tumors [82]. CD95L ([Fas ligand
(FasL)] is expressed on the surface of a wide variety of tumor cells [83±88], and
mRNA for FasL has been detected in cultured tumor cells and in tumor tissues by in
situ hybridization (reviewed in [89]). A controversy that developed in respect to FasL
expression in tumor cells can perhaps be explained based on its well-documented
cleavage into soluble fragments by membrane associated matrix metalloproteinases
(MMPs), resulting in its removal from the cell surface of tumors [90, 91]. Also,
mRNA expression of FasL may be weak in some tumors, perhaps because of its very
rapid turnover, which implies that many cycles of amplification may be necessary for
its detection by reverse transcription-polymerase chain reaction (RT-PCR). As these
mechanisms are likely to differ in magnitude in various tumor cells, depending on
the cellular MMP profile, surface expression of FasL is a highly variable trait in these
cells. Nevertheless, in situ studies indicate that a substantial proportion of T cells undergo
apoptosis at the tumor site [26]. Furthermore, we observed that CD8 + T cells
present in the tumor are particularly sensitive to apoptosis (see Fig. 6.2B). Functional
in vitro studies as well as immunohistology in situ indicate that FasL expressed in tumors
is able to induce apoptosis of lymphocytes [83, 85]. The presence and numbers
of apoptotic TIL in the tumor appear to correlate with FasL expression on the tumor
cells [83, 85]. A number of recent studies have correlated the level of FasL expression
on tumor cells to poor prognosis and survival in patients with cancer [92±94]. In addition
to the ability of such tumor cells to induce immune cell apoptosis, expression
of FasL might also provide them with an advantage in metastasizing and colonizing
distant organs, for example, liver [95]. In vitro, co-culture of FasL + tumor cells with
activated Fas + T lymphocytes was shown to induce lymphocyte apoptosis, which
could be blocked in the presence of anti-FasL antibodies or Fas±Fc constructs as well
as anti-Fas antibodies and caspase inhibitors [85]. Tumor cell lines stably transduced
with the FasL gene, so that they not only expressed but also secreted FasL and their
supernatants rapidly induced apoptosis of Fas + T cells [85, 96]. These co-culture experiments
were criticized as inconclusive, because of the possibility that apoptosis of
T cells was not induced by the tumor, but by fratricidal death of the T cells activated
in the presence of the tumor [97]. However, using a variety of anti-FasL antibodies
and amplification strategies, we do not detect FasL expression on T cells co-incubated
with the tumor or on the surface of T lymphocytes freshly isolated from the circulation
of patients with cancer [85, 98]. We, therefore, attribute the observed apoptosis
to the effects mediated by the tumor and refer to it as tumor-induced cell death
(TICD). It is possible that lymphocytes are also dying in consequence of their activation
in the tumor microenvironment by activation-induced cell death (AICD). In any
event, it is the presence of the tumor that brings about the demise of immune cells.
It is also important to remember that not only T cells but also DC and macrophages
are susceptible to apoptosis in the tumor microenvironment, which is distinctly antiinflammatory
[59].