My cancer presentation delved into
discovering the role of CD95/CD95L in tumor counterattack and the
validity of such an escape mechanism. While researching this
mechanism I came across an older paper that suggested future studies
in this area may be able to apply CD95/CD95L to increasing the
stressfulness of tissue grafts. So I got onto google and searched to
see if this mechanism had been applied to tissue grafts yet, and sure
enough there is an ongoing effort to utilize cancer's trickery to aid
the medical community. Here are the results of my research, but
first, some background information on CD95/CD95L, allogeneic immune
response and the current therapies used to treat side effects of
tissue grafts.
Background:
CD95/CD95L is a ligand and receptor
set typically used by T-cells to induce apoptosis in pathogens or
non-self cells. Here is a simplified version of how this system
works.
First the T-cell receptor (TCR) recognizes the major histocompatability complex (MHC) on the cell (in this case a tumor cell) and engages the CD95L to the CD95 receptors on the tumor cell. This induces a signal cascade that results in apoptosis of the cell (tumor). For more details on this process visit our wiki.
An allogeneic immune response occurs
when dendritic cells (DCs) from both the host and donor present MHC
molecules. Typically this immune response comes in the form of host
T-cells searching and then destroying (inducing apoptosis) of the
non-self cells (there is also and element of donor T-cells seeking
host cells to destroy.) The common name for this response is graft
vs host disease or GVHD.
Until now several strategies have been
developed to prevent or treat GVHD. The first is the ex vivo removal
of donor T-cells prior to infusion. Three other methods have been
used to modify T-cells by triggering clonal anergy of effector
T-cells, controlling the expansion of T-cells and blocking T-cells
adhesion molecules. The goal of this experiment is to selectively
kill alloreactive T-cells.
New Therapy:
This new study is landmark because it
is trying to take the guess work and generality out of current
therapies. The idea of this experiment is to develop a sort of
'buffer' that would work between host and donor tissues to prevent
both host and donor immune response by selectively inducing apoptosis
in alloreactive T-cells.
To create the buffer DC cells from
mice were taken and exposed to the cDNA of CD95L.
When the 'killer' DCs were implemented
between donor and host cells of a syngeneic nature (nearly compatible
tissues) the DCs were capable of suppressing the immune response.
However, when the same buffer was implemented between tissues of
allogeneic nature, only about 70% of T-cells were destroyed. This
led to researches theorizing that a hybrid of donor and host DCs were
necessary to suppress the immune response in allogeneic conditions.
Through a complex mechanism (see the paper for details on the
method), researches were able to develop the killer hybrid DCs and
continue testing into allogeneic conditions.
Positive Results In Vivo:
The first test was to see how well
killer hybrid DCs could inhibit the immune response compared to
control hybrid DCs (DCs without the CD95L cDNA mix). The results
from the study are seen in the figure below.
Here BALB/c (recipient) and A/J
(donor) are two allogeneic mice types. The '1st donor' is
an injection to sensitize the immune system to the non-self cells.
PBS is just a saline solution. We see that if the recipient is
sensitized with A/J, and then treated with saline and control DCs,
there were similar levels of swelling. However, mice sensitized and
then treated with killer DCs had similar levels of swelling to those
mice that were not sensitized first. The immune response is not as
strong with unsensitized mice, so for the level of swelling to be
similar in this case shows that the killer DCs have a measurable
effect on the immune response.
The next test run in vivo measured
what sort of benefit killer hybrid DCs lent to staving off the
symptoms of GVHD. 3 million spleen cells from allogeneic donors were
grafted onto the spleen of host mice and then injected with either
saline (open circles) or killer hybrid DCs (closed circles) on days
0, 3, 5, and 7. A Kaplan-Meier plot is seen below illustrating the
results.
Clearly, despite both sets of mice
dying in the end, the mice treated with killer hybrid DCs did
markedly better, keeping the mice alive an additional 12 days.
Negative Results In Vivo:
Despite
the positive results
seen in the tests above, there still appears to be some pretty severe
limitations on this therapy. In the test below the recipient was
first sensitized and then on days 7, 14 and 60 given tissue grafts of
allogeneic cells. They were either treated with killer hybrid DCs or
saline.
A
second limitation of this therapy is the size of the tissue graft
that can be adequately treated. Below is a Kaplan-Meier plot showing
a similar test to 'A,' but this time the size of tissue sample is
changed in each graph.
Notice
that as the number of cells/animal increase, the difference between
the two treatments decreases. By the time 100 million cells are
grafted onto the spleen on the mice, there is almost no difference in
additional days of survival. Clearly there is a limit to what size
of graft can be aided by this current treatment.
Conclusion:
In
conclusion, research so far has yielded beneficial results; however,
the current level of understanding is not enough to guarantee this
treatment a place in future therapies. This treatment has proven to
'wear off' before 60 days in addition to being unable to treat a
large (>30 million cells/animal) tissue graft. Perhaps with
further testing and development this mechanism will prove to be
useful in a therapeutic sense. I conclude that the real benefit of
this study was to provide frameworks for clinical applications of the
killer hybrid DCs technology in addition to furthering understanding
of CD95/CD95L pathways. Hopefully with further research into this
treatment, further understanding of tumor counterattack will emerge. Possibly by studying how the immune system is capable of adapting to killer hybrid DCs treatment, researchers can apply this to boost the immune system to stop tumor counterattack.
Resources:
http://bloodjournal.hematologylibrary.org/content/98/12/3465.full.html
Strand, S. (1998). Immune evasion by tumors: involvement of the cd95 system and its clinical implications. Molecular Medicine Today.
