THURSDAY, June 12, 2003
Session 1: Stem Cells and Regenerative Medicine:
Overcoming Immune Rejection
Silviu Itescu, M.D., Assistant Professor,
Division of Surgical Science,
Columbia University College of Physicians and Surgeons;
Director, Transplantation Immunology, Department of Surgery, New York-Presbyterian Hospital
PROF. ITESCU: Dr. Kass and Council members,
I want to thank you very much for having me at this session.
I would again like to apologize for the technical nature of the
paper, but please I would really appreciate if anybody has any questions
as we go through even from my slide presentation, just feel free
to interrupt and ask questions freely, and I'll be delighted
to explain in more detail.
The objectives, I think, are twofold. Firstly, what I'd like
to do is to give you an overview of the current state of knowledge
and clinical practice in terms of the basic immunobiology of organ
transplantation and methods by which we currently immunosuppress
patients and prevent organ transplant rejection.
And secondly, I think the objective is to gain an understanding
of those issues as they may relate to the subsequent use of stem
cells for organ regeneration or for tissue regeneration, and so
that you understand the fundamental issues that will be faced by
physicians trying to manipulate or to use stem cells for those type
But I think of even more interest, towards the end of the presentation
by understanding the obstacles to accepting a foreign organ, there
are some interesting new concepts and data that have arisen regarding
the use of stem cells to alleviate some of these issues that I would
like to really open up for discussion at the end of the presentation.
I'm not sure how to switch the projector on actually. It
may require a — okay.
What this slide shows is that despite the fact that the number
of patients on our transplant waiting lists continue to exponentially
grow year by year by year, what you can see is that there continues
to be a major limitation in the supply of organs.
In yellow you see the living related donors have remained minimally
pretty much the same over the years and, more important, the cadaveric
donors have remained the same. So we have a real problem in terms
of shortage of donors.
And it is the same for the previous levels for renal transplantation,
and this slide shows the same for heart transplantation. Significantly
increased numbers of patients were on the transplant waiting lists.
And what this results in is a significant increase or significant
rate of mortality of patients who otherwise could be saved if they
had an appropriate organ donor.
And what you see for kidney transplant patients who are on the
waiting list, the mortality now is fairly low, five percent, despite
a very long waiting time, and that is because we have dialysis as
a modality to keep patients alive.
In contrast, until the last couple of years, you can see that
for the heart transplant patients on that waiting list, mortality
was as high as 30 to 31 percent, and that is because we didn't
have until very recently any way of keeping patients alive who were
waiting for a heart transplant.
More recently, over the past couple of years, there's been
quite a significant new development, and that is the development
of left ventricular resist devices. You can see these type of artificial
devices that are essentially the equivalent for a patient on a heart
transplant waiting list to what is dialysis for a kidney transplant
This is an artificial pump that takes over the function of the
left ventricle and keeps the patient alive while we find an organ
donor for the patient. And this has now brought the mortality rate
on the transplant waiting list back down to almost zero, quite a
Nevertheless, there are few ways by which we can increase the
ability to have donors. One is the use of live donor organs. The
pros of using live donor organs are that they are superior in terms
of outcome to cadaveric. The costs are much less than cadaveric,
and there are psychological benefits in terms of donor/recipient
The risks. There are some small risks to the donor in terms of
morbidity and mortality, inconvenience, and obviously the overall
decision is a voluntary one for the donor.
The short and long-term risks of a living, related or unrelated
and living donor donation for a kidney, there's a very small
risk of death primarily related to pulmonary emboli. There are
some major complications that could be seen primarily related to
the surgical procedure. Approximately one to two percent of donors
will have some degree of clinical complication.
The long-term risk is pretty low to the donor. However, the survival
of a living related organ transplant is significantly superior to
a cadaveric organ donor. You can see that in a kidney transplant,
the living related organ significantly survives for a significantly
greater period than cadaveric organ.
And interestingly, even a living unrelated donor, in blue, has
a better outcome than a cadaveric donor, and what I wanted to emphasize
here is that a living unrelated donor is typically a spouse, where
genetically completely disparate between the husband and wife.
Nevertheless, the outcome is almost the same as a partially matched
sibling related graft donor.
The reason for this probably is the fact that you can prepare
the spouse well in advance. The whole procedure is done in a convenient
way, timed appropriately, with minimal risk to the donor. But the
outcome is significantly superior to cadaveric. That's really
the point of this.
And what you can see, there's a change in distribution in
the U.S. in terms of living donors, particularly in terms of spouses
who are now providing organs to their spouse in need, as well as
actually unrelated donors who are now donating organs to unrelated
In contrast to that, you can see that family related donation
has pretty much remained unchanged of these.
Okay. I'd like to move on to some of the immunologic issues
that primarily a cadaveric or unrelated donor recipient pair will
undergo when an organ is transplanted. The primary difference between
individuals is at the level of a certain complex called the HLA
complex or the MHC complex. We're talking about genes that
incurred a protein on the surface of all of our cells that are called
HLA genes or HLA molecules, two types, Class I and Class II, and
these Class I and Class II molecules you can see in blue are two-chained
molecules that serve to present a foreign protein or antigen to
our own T cells so that they can be recognized as foreign and rejected
or removed from the circulation.
This is the fundamental basis of how our immune system will recognize
bacteria or viruses and eliminate them from our circulation, and
typically what happens is that if this a virus — consider
this to be a virus — taken up by these cells which are called
antigen presenting cells, but otherwise known as macrophages, the
virus is taken up in a vesicle. It's then broken down into
little pieces or little peptides, and the peptide in yellow is then
assembled in the cytoplasm of the cell together with an HLA molecule
in blue, and the complex of a piece of the virus plus the HLA molecule
is then shuttled up to the cell's surface, and as a complex
is presented to this structure over here, which is called the T
cell receptor on the surface of the T cell.
Typically this is a CD-4 helper T cell, but the key issue is that
this T cell receptor structure recognizes a three dimensional structure
between the peptide antigen and the HLA molecule, and the whole
thing activates a whole pathway in the T cell that then results
in removal of the whole invading antigen or invading virus or bacterium.
And this is a typical immune recognition reaction that occurs
throughout life, throughout our blood systems and, you know, our
immune organs. You can imagine, therefore, if this now becomes
a foreign organ, the HLA molecule on the surface of this foreign
cell will be viewed as totally different from all of the HLA molecules
of a given individual by our own T cell receptor. It will no longer.
It will no longer be seen as a non-molecule. It will be seen as
And the fact that the HLA molecules on a foreign cell, this structure
itself is different from the HLA molecules of a given individual
always will trigger a reaction as though this whole structure was
a foreign structure.
What I have just outlined to you is actually a very complex immunological
phenomenon. So if anyone has any questions, I'm sure this is
not a straightforward issue to understand, but please, just feel
free to ask.
Can we just focus that a little bit?
Okay. So with the notion that if this is now the — consider
this to be the foreign cell from the foreign organ that has been
transplanted. Understanding that the MHC or the HLA molecule on
the surface of this cell is totally foreign to the recipient, it
can trigger two types of an immune response. It can trigger what's
called a direct immune response and an indirect immune response.
The direct immune response is when the foreign HLA molecule as
a total structure is seen as foreign by the T cell receptor of the
recipient T cell. So under normal circumstances, as I've mentioned
to you, the T cell receptor will recognize a little peptide that
should be here, not the HLA, just the peptide.
In this scenario, the T cell receptor recognizes the whole HLA
as a foreign piece, and so this is called direct recognition, and
it allows the immune system to be activated and attack this cell
A second process of immune recognition, the indirect process,
is one where this HLA molecule is internalized in the cell of origin,
and a little piece is secreted, and you see there is the little
piece of the foreign HLA molecule, and it's just this little
piece that then comes up.
And this cell now is the recipient cell, if you can imagine.
It's the recipient antigen presenting cell. And so this little
yellow piece, which is a component of the original foreign HLA,
is now presented as an antigen by our own HLA molecule.
And so this recognition is the more classic form of immune recognition
the same as you would recognize a virus or a bacterium. You not
only recognize a little piece of this entire structure, and this
is called an indirect allorecognition. But it's two different
types of recognition directed at the foreign HLA molecule of the
donor. Both of them drive the immune response against the donor
This is the basis actually of an immune assay called tissue typing
or mixed lymphocyte reaction, which allows us to test how different
the HLA molecules of one donor are in terms of recognition by the
HLA molecules of the recipient.
And so what we typically do is take cells from a potential, let's
say, kidney allograft donor, and this really is done in terms of
selecting a potential living related donor. We want to know really
how different is the potential donor to the potential recipient
in terms of those HLA molecules.
So we'll take cells, blood cells, from a potential donor.
We'll know the typical type of HLA molecules on the surface.
We mix them with the HLA molecules or with the cells of the potential
recipient, and we look for the type of reaction. We look to see
whether the recipient cells proliferate and become activated or
don't proliferate and don't become activated.
And essentially the degree of proliferation and activation is
a measure of the difference between the HLA types on the surface
of the donor and on the surface of the recipient, and what you don't
want to be doing is transplanting a potential donor whose cells
stimulate very vigorously a proliferative response in the recipient
Okay. Now, in terms of once you have selected your donor for
living related, for example, based on as good a matching as you
can get at the HLA locus or for a cadaveric kidney transplant we
also do HLA typing. We try to minimize the differences in HLA molecules
between donor recipient.
For cardiac transplant actually we don't have the time to
do that. In fact, for cardiac transplant recipients there is no
HLA matching. We take whatever comes because of the severe shortage
But essentially once you've actually bitten the bullet and
you've accepted a particular donor, there's a variety of
types of rejections that can occur, and those are defined based
on both the immunobiology of the type of reaction and the time
taken to induce the rejection process.
And there are at least four types of rejection processes that
I'd like to talk about. One is called hyperacute. One is accelerated.
One is acute, and one is chronic, and as indicated, hyperacute happens
within minutes to hours, and the fundamental issue here is that
unbeknownst hopefully to the transplant physician, the recipient
had what we call pre-formed anti-donor antibodies in his circulation,
and as soon as the organ gets put in these, if you've got this
type of a serum, you will reject the organ immediately, within minutes.
It is a horrendous outcome.
So we always try to exclude the possibility that a given recipient
has antibodies that might destroy the organ, and we do what's
called a donor specific antibody cross-match prior to any organ
being transplanted. This is both for kidneys, hearts, and lungs.
Assuming that this doesn't occur, and I'll show you an
example of this in a minute, but assuming that this doesn't
occur because you've been diligent and have excluded this risk,
the typical type of rejection that would occur in the absence of
immunosuppression is an accelerated or an acute process which happens
within the first six to seven days primarily.
And this is a result of either reactivation of T cells that have
previously been activated by some type of similar HLA, such as blood
transfusion or primary activation of T cells, that the natural process
by how the T cells will recognize the foreign HLA, and it will
take them about seven days to recognize them, and you will get an
acute rejection process.
And we understand that, and this is what we have tried always
to prevent by treating with a number of immunosuppressive drugs
at the same time.
In addition to this, and while the patient always remains at risk
of recurrent episodes of acute rejection, there is another process
called chronic rejection of the graft which happens within months
to years, and we'll talk about that in more detail, that causes
a very complex and unclear, but this really is the major limitation
of long-term survival of the graft at this point in time.
Just an example of what I was talking about in terms of the hyper
acute form of rejection. If an individual has pre-formed antibodies
against the HLA of that particular donor, as soon as the blood comes
into contact with the foreign allograft or the foreign organ, those
antibodies will bind to the surface of the organ. They'll recognize
the foreign HLA immediately and activate a whole complex that results
in a clot formation in the blood vessel and occlusion of the blood
vessels to the organ.
This is what it looks like. This is a classic hyperacute rejection
of the heart, where you can see swelling of the heart, hemorrhage,
and then within minutes the heart goes black and you've lost
the organ and the patient really is in extremis, and often dies
So in this day and age, it's very rare to see that type of
hyperacute rejection, and we try to minimize the risk of that by
screening and doing a variety of assays.
Assuming that you've gone through that first couple of days,
what happens is through those two processes I talked about, indirect;
so direct recognition of the foreign HLA and the indirect of the
HLA, the CD4 T cell, helper T cell is the orchestrator of the immune
response, is activated by the foreign HLA molecules. The CD4 cells
start to divide and proliferate, and they then help these cells
called CD8, or cytotoxic T cells, to become activated. These are
the effectors of the rejection process.
They also help B cells make antibodies, and the antibodies provide
a second barrage, second attack against the graft. So these are
the two effector arms that cause the rejection process, particular
CD8 cytotoxic cells. But the CD4 cells are the primary cells that
recognize the foreign HLA and orchestrate the whole process.
And you might recall that CD4 cells, which are the primary orchestrators
of all immune responses, are also the targets of the HIV virus.
This slide is a little complex, but just to show you that the
cytotoxic CD8 cells that I mentioned ultimately destroy the graft
by secreting a variety of factors that punch holes in the graft
and cause it to leak and explode pretty much.
We'll just keep going.
Okay. So that complexity in terms of cellular activation has
defined multiple targets for immunosuppression. You can inhibit
the ability of the CD4 T cell to recognize the HLA molecule directly.
You can inhibit T cell activation and secretion of cytokines, and
we'll talk about that in a minute.
The most important cytokine here is IL-2 because IL-2 activates
the whole downstream pathway of CD8 cells and other T cells that
are important in the rejection process. You can then try to suppress
activation of other T cells and B cells, and then we have drugs
at all of these points that synergistically will inhibit the immune
The axioms of immunosuppression, three major things to think about.
You want to have an immunosuppressive effect of the drug that you're
giving. You want to minimize the immunodeficient complications
obviously, and you want to minimize the non-immune toxicity.
And the best way to do these, to maximize the immunosuppressive
effect and minimize these two other issues is by combining drugs
that work at different sites of the immune response.
And so you see, for example, if we only treat with one agent,
and cyclosporin is our example here, we have a fairly minimal, specific
immunomodulatory effect, and a fairly high range of nonspecific
or immunosuppressive effects.
But if we now add two drugs working at two different sites of
action, we maximize or optimize our immunosuppressive effect at
a selective level, while minimizing our undesirable side effects,
and that's the rationale for combining immunosuppressive therapies.
In addition to that, you want to use drugs that act on different
arms of the immune response and at different time points in a synergistic
fashion. So all patients are routinely treated with glucocorticosteroids,
which are essentially nonspecific inhibitors of most arms of the
immune response, and they act very early to switch everything off.
Now, what you don't want is to continue to have to be using
high doses of steroids because they're associated with lots
of side effects, most notably global immunosuppression. So you
want to use steroids early on and be able to switch the steroid
We also now use cyclosporin or FK506. These are related agents.
These have made probably the biggest difference in the transplant
survival over the past 20 years by acting directly on T cell activation
to prevent IL2 and other cytokine production.
And then there's a bunch of other drugs that alone or in combination
are used to prevent the second stage of immune activation, other
cells, the recruitment of other cells, the differentiation of other
cells, and the prototype of this is azathioprine. Now the most
commonly used drug in this group is MMF, or mycophenolate mofetil.
And then there's a bunch of other experimental approaches to
prevent other aspects of the immune response.
What this slide shows you is the impact of cyclosporin on allograft
survival here in kidney transplant. You can see that prior to the
early '80s — actually cyclosporine was introduced, I think,
around 1978, and you can see the difference in survival of kidney
allografts. This is graft survival as opposed to patient.
Patient survival is not the issue. If the graft is rejected,
it dies, and the patient goes back on dialysis. So what you see
here is in the late '70s, early '80s, our one-year graft
survival was as low as about 50 percent for kidney allograft. It's
now much higher than this actually. It's about 95 percent one
year survival, and it's the same for cardiac allografts.
So this drug, when this drug came in, it revolutionized the treatment
of solid organ allografts, and the way it works, let me just go
Cyclosporine binds a calcium activation factor in the cytoplasm
called calcineurin and turns on a whole cascade of events that in
the DNA and in the nucleus of the activated T cell inhibits the
ability to activate various genes of cytokine production, such as
IL-2 and interferon gamma, which regulate immune cell divisions.
If you don't make these cytokines, you do not get immune cell
division. That seems to be a critical component of preventing rejection.
However, despite the fact that we understand this process of immune
activation and were able to prophylactically treat patients and
prevent acute episodes of rejection, we clearly have improved short-term
Despite all of that, there is a second process that kicks in some
time during the course of the recipient's life span, and what
you see is that, for example, with cardiac transplants over time,
over a five-year course, there's a cumulative loss of as much
as 40 percent of the allografts in heart transplant patients.
And with kidney transplants, this curve is pushed forward, but
you still have about a 50 percent loss of allografts at ten years.
So the question is: if we're able to reduce the incidence
of acute rejections, why are we still getting a limited long-term
survival of the graft itself?
And the reason is this pathologic, unusual lesion that happens
in actually just about every graft that's put in whether the
heart or the kidney or the lungs. You get a lesion like this in
the blood vessels of the allograft. It's very unusual. It's
proliferating the lesion where you can see that the lumen of the
blood vessel is almost occluded by these proliferating cells.
And this is in the heart. This is called accelerative transplant
related vasculopathy. In the kidney, this is, again, a chronic
And the cause of this is not clear, but it probably is due to
— much of that, as I suggested, is due to an ongoing immune
response against the foreign HLA antigens, which are expressed here
on the luminal side of the blood vessel.
And even though the patient is not experiencing major acute episodes
of cellular rejection, there is an ongoing subclinical level of
rejection process going on that damages and causes these vessels
to close off.
The causes of this process really is multi-factorial, but HLA,
attack against the HLA of the donor continues to be the number one
cause of this.
And you can see that chronic rejection — this is in our
cardiac transplant population at Columbia — chronic rejection
accompanies those patients who had many more episodes of acute rejection.
So for every hit that you have, for every acute rejection episode
that you have, you'll destroy a little bit more of your heart,
and you're more likely to go on to get chronic rejection as
opposed to those individuals in blue who have less episodes of acute
So fulminant episodes of acute rejection, even though you survive
them and your graft continues to work well and we can reverse them
will predispose to chronic rejection and even in the absence of
fulminant episodes of acute rejection, ongoing immune reactivity
against the donor HLA will also predispose you to chronic rejection.
As you can see in this slide, these are patients who actually
have not had episodes of acute rejection over a number of years,
but make antibodies against the foreign HLA. In yellow are the
kidney patients or heart patients. Antibodies against the HLA antibodies,
in yellow the HLA of the donor, have a poorer outcome in terms of
developing graft failure.
So even in the absence of acute episodes of rejection, an immune
response that results in anti-HLA reactivity causes graft loss,
and this shows the same.
So what that means is that we need to be vigilant. We need to
monitor the patients very closely. We can't just expect that
a combination immunosuppressive regimen that is used initially is
going to keep all patients in check and all patients quiet.
We have an active immunologic monitoring of particularly our cardiac
transplant recipients who are much more prone to having rejection
episodes, and we actively on a weekly basis measure antibodies,
T cell function. We identify patients who are at high risk of cellular
rejection. We have a whole algorithm that allows us to study these
patients on a weekly basis, and you can see that this algorithm
allows us to be very flexible and change the type of therapies that
we use so that if at a particular time point certain test number
one plus test number two in terms of immune activation become positive,
we know that we need to change our immunotherapy.
In contrast, if these assays remain negative or perhaps revert
from a positive to a negative, we can reduce the type of biopsies
that we do. Instead of doing a biopsy every 30 to 60 days, we may
reduce our biopsy frequency to every 90 days.
And this is the type of dynamic approach that we use to monitor
our patients and to modify the type of immunosuppression that's
required. It's not a static process.
And I think what I'm trying to emphasize is that the recipient's
immune response continues to be a major obstacle in terms of accepting
and long-term survival of the graft. And so since these issues
are so complex and so difficult, the Holy Grail really of transplant
immunobiologists has always been to try to induce a state of transplantation
tolerance, in other words, a permanent acceptance of the graft that
can allow the recipient ideally actually to not require any type
And the definition or criteria of transplantation tolerance are
outlined here: specific immunologic unresponsiveness to alloantigens,
otherwise known as HLA antigens, of the donor, of the graft, in
the absence, total absence of continuous immunosuppression; prevention
of acute rejection and promulgation of graft survival; acceptance
of a second test graft on the same original donor strain —
this is for experimental tolerance in animals — and the specificity
is then confirmed because the recipient will reject a third party
In other words, tolerance is defined as being absolutely tolerant
only to the organ that you've received or that you've seen,
but not tolerant to an unrelated donor. This type of tolerance
has been extremely difficult, if at all possible, to detect or to
attain in humans, although many studies have suggested that perhaps
a term called microchimerism or chimerism may actually reflect a
state of tolerance in organ transplant recipients.
And what chimerism relates to is the presence in your circulating
blood of at least five percent of your circulating cells being of
donor origin. Whether that, in fact, really does induce tolerance
is really not clear, but the concept is that if five to ten percent
of your circulating blood pool comes from the donor, essentially
what's happened is that the passenger cells from the graft that
was placed into the recipient, the passenger cells left the graft,
circulated to the lymphoid organs, particularly the bone marrow
and the thymus, engrafted in the bone marrow and the thymus, and
then have maintained their own essentially self-renewal capability,
and continue to be shed and secreted into the circulation to essentially
make the recipient think that cells of that HLA type are the same
Again, in experimental models, you need to attain at least five
to ten percent of your circulating cells being of donor origin in
order to have a so-called chimeric state that may reflect tolerance.
And so what are the mechanisms by which tolerance might be attained?
And again, all of this we're talking about now has not been
defined in humans, but is based on experimental models in both small
animals and larger primates.
The mechanism includes clonal deletion, clonal anergy, and perhaps
development of regulatory cells that change the type of cytokines
that are produced.
Clonal deletion refers to the ability of the thymus to regulate
the type of T cells that we produce that reject the organ. In other
words, if we know that CD4 T cells in our blood recognize the HLA
of the donor, why are they not deleted or why are they not removed
by mechanisms in our body that can actually do that to our own T
cells that attack our own grafts?
And so the function of the thymus is to actually remove self-reactive
T cells that we all have at various time points, and if our thymus
can remove our own autoreactive T cells, could we perhaps induce
our thymus to remove those T cells that are reactive to the graft?
And so this process of clonal deletion or how do you induce clonal
deletion is one mechanism by which tolerance might be attained.
Clonal anergy does not involve the thymus. Clonal anergy is the
same sort of concept: can you remove those deleterious clones,
but this happens in the periphery, in lymph nodes, similar mechanisms
but in a different location.
Regulatory cells that change your cytokine profile — let
me just go to the next slide — it's well known that the
type of cytokines that induce an acute rejection process, Interleukin
2 and interferon gamma, and I've already mentioned that cyclosporin
primarily inhibits the production of these cytokines, but these
cytokines are made by certain types of T cells called TH-1 cells.
It has been shown that if you can prevent the development of these
TH-1 cells and instead skew the immune reactivity towards what's
called a TH-2 type of T cell that produces a totally different repertoire
of cytokines, this type of T cell preponderance and this type of
cytokine preponderance appears to switch off the immune response
and allow engraftment and allow a tolerogenic state.
And this switching between a TH-1 and a TH-2 phenotype of cells
can be attained, again, by a variety of experimental approaches,
but if we can drive to this preponderance of cells, we might be
able to prevent allograft rejection.
Other novel experimental approaches include inhibition of co-stimulatory
molecules on the surface of the graft. We know that HLA was —
well, HLA is the primary antigen that drives the T cell immune response.
There are other molecules on the graft that help deactivation of
the T cell, and these are called CD28 and CD40 ligand at least.
There are many others as well.
If you don't have molecules on the surface of the graft, an
HLA, foreign HLA molecule by itself may not necessarily activate
the T cell. So there are ways of perhaps altering the graft so
that it doesn't express these co-stimulatory molecules, that
may induce a tolerogenic state.
Other approaches include the use of MHC or HLA peptides that can
mimic the foreign HLA and bind to the T cell and switch it off,
and perhaps also induction of death, artificial induction of death
of the T cell that mediates rejection.
Another molecule that's important in this pathway is called
Fas ligand that could be over expressed, again, by genetically altering
the organ, over expression of Fas ligand to kill the T cell that
recognizes Fas ligand instead of allowing it to become activated.
These are all experimental approaches, none of which have yet
been proven in men.
Let me just keep going.
This is one approach that I would like to just touch on very briefly.
Amongst the co-stimulatory molecules is the IL-2 receptor, IL-2
to IL-2 receptor, and you can see that if you don't express
a functional IL-1 receptor on your T cell, the T cell is not able
to then proliferate and become activated.
So we said that cyclosporin is a very active drug by virtue of
the fact that IL-2 is inhibited. However, how about if we can inhibit
the IL-2 receptor rather than inhibiting IL-2?
And that has led to a strategy from a number of the large pharmaceutical
companies that have developed anti-IL-2 receptor monoclonal antibodies,
and here's an example of one where in conjunction with other
immunosuppressive agents you can see that it delays the onset of
kidney allograft rejection.
However, what's interesting here is that as soon as the antibody
treatment is stopped at around day 180, you can see that the rejection
process starts to come together again.
In other words, what this tells you is that this type of an approach
of an antibody to block a surface receptor that may be considered
important for the immune response does not lead to a tolerogenic
state. It simply leads to an inhibition of the immune system.
Because when you stop that treatment, you should continue to have
an inability to reject the organ. Because these rejection rates
come together, it means you have an induced tolerance.
And so we get back to what I was just talking about, which is
can we artificially allow the thymus to delete cells that would
otherwise be alloreactive. And the way we can do this experimentally
is to use a drug called cyclophosphamide. It is yet another immunosuppressive
drug, but what this essentially does is it activates a pathway in
the thymus that causes T cells that would reject the organ to explode
or die through a process called apoptosis.
And what you see here is that if we actually treat — this
is now in patients — if we treat patients with cyclophosphamide
to induce apoptosis in the thymus of alloreactive T cells, we inhibit
the rejection process very significantly relative to patients who
are treated with another combination of drugs.
And so this is a major clue to the fact that the thymus can be
manipulated to enable a tolerogenic state to occur.
And that leads to this. This is really my last slide. This leads
to understanding a phenomenon that was described quite a number
of years ago in kidney transplant recipients where blood transfusion
that preceded the kidney allograft, and this is blood transfusion
of the same donor type infused peripherally prior to actually putting
the kidney transplant in, significantly prevented the rejection
of the organ.
And this phenomenon has now been known for many years, although
it has been poorly explained. Many variations of this have now
taken place in clinical practice, trying to define the cells that
most likely prevent allograft rejection.
And we have moved from using whole blood because it's very
difficult to know exactly how many cells. If you actually give
too many cells, you can actually induce an immune response. You
can induce an accelerated rejection process.
It seems to be very critically linked to how many blood cells
are actually transfused, which means that if we don't understand
which cells are doing this effect, you really haven't got a
handle on the process.
So now people are looking at other subpopulations, and since the
whole field of stem cell transplants and bone marrow transplants
have moved along pretty dramatically over the past few years, people
have tried to now look at whether cells in the adult bone marrow,
particularly stem cells in the adult bone marrow, may have this
effect when transfused prior to an organ transplant.
And what I'd like to just quickly do is to sort of shift —
that is my last slide — but I'd like to just quickly shift
into some aspects of my paper, which I think addressed the current
state of knowledge in terms of how cells from the bone marrow or
from other sources might actually do this sort of thing, induce
a tolerogenic state and prevent organ rejection.
In order to — if we could just switch off the - thank you.
In order to understand the type of tolerance that we're talking
about, we need to understand a little bit about the characteristics,
particularly the immunogenic characteristics of stem cells, both
embryonic and adult stem cells.
Embryonic stem cells have been known for a number of years in
murine models and recently in human cell lines, but adult stem cells,
a population of adult stem cells has recently been described to
be present in the adult bone marrow that has many features and many
characteristics that seem to be shared with embryonic stem cells.
So I'd like to sort of combine the discussion in terms of
the characteristic of both these cell types because they're
Both embryonic and adult mesenchymal type stem cells do not express
HLA Class I and Class II molecules and demonstrate reduced surface
expression of co-stimulatory molecules required for T cell activation.
When one transplants either embryonic stem cells or mesenchymal
stem cells from the adult, one finds long-term graft survival of
the cells despite the fact that these cells to acquire HLA Class
II antigens after in vivo differentiation.
A striking recent observation of the mesenchymal stem cell population
has recently been noted that they broadly inhibit T cell proliferation
and activation by various types of antigenic stimuli, including
those from HLA of foreign donors. Mesenchymal stem cells have been
shown to inhibit both naive and memory T cell responses, to affect
cell proliferation, and to reduce the number of interferon gamma
And we actually know more about mesenchymal stem cells, their
ability to escape immune surveillance, than we do about embryonic
stem cells at this point in time, but nevertheless, there's
some information that both when they're transplanted are able
to escape immune surveillance.
Extending the observations of donor derived blood transfusions
to induce a tolerogenic state. Several groups have tried to reproduce
this type of an approach using embryonic derived stem cells or mesenchymal
The two underlying mechanisms by which creation of a mixed chimeric
host results in tolerance as I've mentioned are, one, thymic
deletion of potentially donor specific alloreactive T cells and,
two, non-thymic peripheral mechanisms as we've mentioned.
The theoretical advantages of either of these cell types is that
because they don't express HLA Class I or Class II, they might
be able to migrate to the thymus, might be able to reeducate the
thymus so that the thymus then thinks that these cells are part
of its own normal repertoire, and the thymus will then eliminate
potentially those type of T cells that could reject these type of
And, in fact, in experiments where either embryonic or mesenchymal
stem cells from the adult have been injected intravenously, we know
that long-term acceptance of these cells has been accompanied by
the presence of large numbers of these cells in the recipient thymus.
And in a particularly interesting study using rat embryonic stem
cells recently, it was demonstrated that intravenous injection of
rat embryonic stem cells in the absence of any type of immunosuppression
resulted in long-term engraftment, as well as the thymus, and resulted
secondarily in the recipient rat being able to accept a cardiac
allograft of the exact same HLA type as the embryonic stem cells,
but not a cardiac allograft of an unrelated donor, which meets all
of the criteria of tolerance induction.
In the only study to date using mesenchymal adult derived stem
cells, what we have been able to see is that a similar type of long-term
engraftment in the bone marrow in the thymus can be achieved with
adult mesenchymal stem cells, and I think it's reasonable to
anticipate that a similar experiment would likely also demonstrate,
although obviously it still has to be done, that the engraftment
in the thymus might lead to long-term tolerance and acceptance of
a graft of the same HLA type.
So I think those type of experiments are very exciting and raise
the possibility that stem cells might have, by virtue of the fact
that they do not express high levels of HLA, and even when they
are induced to express HLA molecules, might have very special characteristics
that allow them to evade immune surveillance and, even more importantly,
might actually allow them to re-educate the host to accept a different
type of donor tissue.
And perhaps the hope would be the induction of tolerance might
actually obviate the need for all of the stuff that I just told
you about this morning, all of those complexities in terms of immunosuppression.
And I think I'll stop there.
CHAIRMAN KASS: Thank you very much.
Since my guess is that for at least the non-medical, non-scientific
members of the house, this was complicated. Let me see if I could
try to formulate something of the nub of this and ask you to elaborate.
I think the large part, the preliminary part of the talk indicates
the enormous complexity of the immune system and the difficulties
of getting especially long-term graft survival and also the need
for long-term immunosuppressants, which have risks and harms of
And the strategies for inducing tolerance have up until this point
tried to attack various parts of the host response, immunological
But if I understand the most exciting part of this, the last part
of this discussion, the suggestion seems to be that the use of stem
cells, mesenchymal or embryonic - this is now the concept and not
the data — that such cells, first of all, to begin with lack
the HLA provocative antigens so that they are not themselves immediately
Second, that they can apparently take up residence in the thymus
and, if given from the same donor, if given from the individual
who is then to serve as the donor of a particular organ, that increases
the survival of a solid tissue donation from that person.
Is that correct so far?
PROF. ITESCU: That would be one potential use
of such cells, yes.
CHAIRMAN KASS: And let me draw out an implication
for regenerative medicine using stem cells or their derivatives.
Again, just the concept now, not the evidence.
The concept would be that because these are immunologically unprovocative
materials, original stem cells introduced into a patient which presumably
would take up residence in the thymus and perhaps, as you suggest,
re-educate the immune response even after the HLA antigens appear
in those cells; re-educate the immune response to no longer regard
those cells as foreign; might make the host now receptive to even
the introduction of differentiated cells derived from those stem
cell lines. One could put in heart cells taken from these stem
cell lines and have them not seen as foreign.
Am I understanding the concept correctly?
PROF. ITESCU: Yes, I think that's actually
correct. In fact, it's almost a beneficial aspect. The stem
cell that is initially infused does not express HLA, takes residence
in the thymus in the absence of HLA expression, and then is induced
to express HLA at that site because it's the actual expression
of the HLA molecule that allows a re-education process and tolerance
induction to that HLA.
So that if it didn't express HLA at all, at any time point,
you would not develop tolerance to that HLA molecule. So it doesn't
get rejected in the first instance because it doesn't express
HLA. It enables engraftment in the thymus long enough to up-regulate
its HLA molecule, and then it re-educates the thymus to accept that
So you can then come along with a differentiated tissue or organ
which now does express that same HLA and it will be de-rejected
it from the outset. That's right.
DR. KRAUTHAMMER: So it essentially causes a
change in the immunological identity of the recipient.
PROF. ITESCU: And that's the concept of
CHAIRMAN KASS: Right. So it's a magic bullet
if it works. Would be.
PROF. ITESCU: It would be, yeah.
CHAIRMAN KASS: Paul McHugh.
DR. McHUGH: That was a splendid presentation
with very exciting prospects in the future, and of course, the most
exciting prospect to any of us who have followed the transplantation
business from the time when — I was an intern at the Brigham
in the 1950s. So I was there in the beginning - is the possibility
and the prayer of xenotransplantation.
But you know all about that. Will this bring to bear the possibility
that we'll be able to use animal tissues ultimately for transplantation
of these vital organs and the use of animal stem cells and, therefore,
animal organs will spare us not only this problem you have here
of people in need and the lack of donors, but also much of the ethical
problems related to stem cells?
PROF. ITESCU: Well, my laboratory actually was
very heavily involved in trying to understand the rejection processes
of disparate xenografts, and we have tried to develop a variety
of immunologic strategies to try to overcome that.
At this point in time I actually am much more hopeful that regenerative
medicine using adult stem cells is much closer to reality than xenotransplantation
for a number of reasons.
Most importantly, I think, is the fact that the differences between
animals and humans is a wide array of antigens. HLA is just one
antigen, and the fact that we're all so closely related as humans
is simply the fact that the only thing that differentiates us is
the HLA structure from individual to individual.
However, between species not only is HLA different, but there
are many other structural genes, and I, frankly, am fairly pessimistic
actually that those differences are likely to be overcome between
So I would emphasize actually the human stem cell regenerative
possibilities rather than xenotransplantation.
CHAIRMAN KASS: Rebecca Dresser.
PROF. DRESSER: Two questions. I was wondering
how long it takes for this re-education process of the immune system
after the first inducing of the stem cells.
And the other question was exactly where is this in terms of laboratory
data theory, human data, animal data. How much of this is hope
and how much of this is demonstrated?
PROF. ITESCU: Sure. The chimerism concept all
comes from human data. In other words, for many years people have
looked at whether donor cells continue to circulate in the recipient's
blood stream or organs, months, years, many years after a transplant.
And so this concept that the recipients who do better are those
who have the higher percentage of donor cells circulating has been
around for many years, and it is data from humans.
In terms of proving that that has anything to do with tolerance,
you have to go back now to the animal models. The experiments that
I've outlined to you are actually recent experiments in the
last 12 months primarily using embryonic derived stem cells where
those embryonic cells were able to be engrafted from one rat to
another rat type, induced chimerism, induced tolerance to those
cells, and subsequently were able to induce a state of nonresponsiveness
to a heart transplant of the same donor.
Now, the experiments with adult mesenchymal stem cells are also
within the past 12 to 24 months, and what we know about adult mesenchymal
stem cells is that if they're injected at a time of in utero
- the experiments that I was referring to where these cells were
injected in utero in developing fetuses — and those
cells can then engraft and survive for at least one to two years
after birth. This is human cells actually in an animal model, and
those cells survive, are not rejected. They're found in multiple
organ types, including the thymus.
What we also have learned in the last 12 months is that these
same adult mesenchymal stem cells which do engraft in vitro
are able to actually inhibit immune responses from other human T
cells against themselves and also inhibit the immune responses of
other T cells against other antigens, including HLA antigens.
So we have a lot of information in vitro that mesenchymal
adult stem cells are functional. We know in vivo they're
able to engraft in a similar way to the way that the embryonic stem
cells can engraft and not get rejected.
And so the only experiment that's missing is that same other
experiment that has been done with the fetal cells, and that is
can actually induce a tolerogenic state to allow a solid organ graft
to be put in.
PROF. DRESSER: And how long does it take to
induce that state?
PROF. ITESCU: In small animal models, we are
talking about probably weeks. To translate that to man, you know,
is a guess, but I would think weeks to months would be the objective.
CHAIRMAN KASS: Bill May.
DR. MAY: You said that adult stem cells from
the bone marrow may significantly reduce organ rejection, and later
you said adult stem cells have many characteristics shared with
embryonic stem cells.
I'm interested in the policy implications of this. Do you
feel that we should simply follow out, play out the line of exploration
with adult stem cells and delay explorations with embryonic stem
cells, or should we be following both tracks at the same time?
PROF. ITESCU: I think what we know at this point
in time about both cell populations, and again, what one calls a
stem cell in the bone marrow differs from experimental group to
experimental group. There are many types of stem cells in the adult
But there are a number of well defined stem cells, and I'd
talk about mesenchymal stem cells. There's another group of
perhaps an even earlier progenitor cell type to the mesenchymal
stem cell, but what I think most people will agree on is that the
stem cells defined in the adult bone marrow have many features that
are similar to true embryonic stem cells in terms of surface markers
and in terms of the way they behave, but there are some differences
So, in particular, what we know is that at this point in time
I think the adult stem cells probably do not have the same range
of differentiation capability that an embryonic stem cell does in
terms of their ability to become almost any organ in terms of differentiation.
Nonetheless, the differentiation capability of adult stem cells
at this point is quite remarkable.
Secondly, the self-regenerating capacity, meaning how many times
can the cell continue to divide before it stops dividing, and that's
one of the fundamental features of any stem cell. The more division
it can undergo, the more likely it is to then differentiate and
become a useful clinical entity.
The self-renewal capacity of most of the adult stem cells that
have been defined to date probably it's fair to say are not
quite to the same degree as great as the self-renewal capability
of an embryonic stem cell.
I would say at this point in time it remains an open question
as to whether these differences are at a clinical level, and I think
I would support continued research in both cell types to understand
(a) whether these differences are relevant; (b) whether the diverse
functional capability and range of differentiation exhibited by
the adult stem cell is sufficient in many cases.
In the work that we're doing with respect to the cardiovascular
system and the heart, we have found that so far there are certain
stem cells that are terrific in terms of their ability to improve
So I would view these fields as overlapping and say that at this
point it's far too early to decide which field is going to be
the right way to go for every type of regeneration strategy that
one is looking for.
But I think that there's enough hope in the adult stem cell
area that it may be sufficient to go down that path. I would at
this point continue research in both areas.
CHAIRMAN KASS: Dan Foster.
DR. FOSTER: I have two questions in regard to possible downstream
effects that might be difficult, and the first question, I presume
that the injection of the stem cell of one sort or the other to
protect against the donor's solid organ transplantation would
be that the immune inhibition there would be, the tolerance there
would be restricted to the donor. It would not, in other words,
in your view interfere with immune responses to infectious agents
or anything else. I presume that's correct, right?
PROF. ITESCU: The hope would be twofold. To
call this tolerance, it would have to be a tolerogenic state induced
only against the inducing cell and antigen and a continued responsiveness
to any other antigen. That would be the hope of the whole process.
Otherwise you would have global immunosuppression. That's right.
DR. FOSTER: The second question is probably unimportant to somebody
who needs a heart or something of that sort, but if you're look
in simple terms at the immune system, it does two things. It fights
off invaders, as you say, viruses and bacteria and so forth, and
it surveys for cancer. I mean, so that in one sense anybody who
gets a cancer has had in some way a failure of the immune system
to see the oncogenic antigens and as a consequence not to delete
I mean, a lot of people think that all of us are forming cancers
all the time, and that the failure is the immune system.
I presume that there is at least a theoretical possibility that
since even donor tissues or other tissues might get a malignancy
that that might be impaired with the tolerant state that was there
so that you might be at risk for malignancy, even early malignancy.
I mean in some ways tumor suppressor genes, you may get very early
things. I think if I needed a heart I wouldn't worry about
that, but just in theory that might be something that we would have
to worry about even if there was not a defect against the one wing
of the immune system that fights off against infection, but might
make us vulnerable to the other thing that's devastating.
PROF. ITESCU: No, I think that's actually
an excellent point, and that's right. So in other words, if
a cell is so primordial and so early that it confers some type of
protection against itself being rejected, it's also that same
cell type that is so plastic that it has the ability to differentiate
into so many different lineages that it also has a high rate of
Typically the embryonic stem cells obviously have the high risk
potential for teratoma formation. Again, if you then think about
the adult mesenchymal stem cell versus the embryonic stem cell,
there's a tradeoff between these two. So the embryonic stem
cell is the most pluripotent, has the highest rate of proliferation
and cell divisions. The mesenchymal stem cell has a slightly more
differentiated, let's say, than the embryonic stem cell. So
it has a less likelihood of — it has less cell divisions left
and maybe a little bit more of a restricted differentiation pattern.
But if you then take these two cell types and if they are both
able to induce a state of immune nonresponsiveness, the one that's
less likely to induce a neoplastic transformation, I think, would
be the adult mesenchymal stem cell.
So from that point of view there would be a preference to that
versus the former.
CHAIRMAN KASS: Could I ask just a technical
question? This so-called re-education process that stem cells might
induce in the host thymus, could that process persist even if the
mesenchymal cells disappear?
In other words, if the adult cells are not self-perpetuating and,
therefore, die out as a population, could a short term residence
in the thymus be sufficient to confer a long-term tolerance for
a subsequent graft?
PROF. ITESCU: Yes, I think so. And examples
of that are many experiments in transplant immunobiology where people
have directly injected HLA molecules into the thymus using specific
HLA molecules of the subsequent donor organ to induce a state of
The organ is then transplanted and long-term tolerance has been
achieved. Presumably the injection of the HLA molecules resulted
only in a transient expression in the thymus.
CHAIRMAN KASS: Thank you.
Janet and then Michael Gazzaniga.
DR. ROWLEY: I wanted to ask several questions.
Going back to the mesenchymal stem cells, I assume that they are
a related type to those that we heard about earlier from Catherine
Verfaillie. They may not be quite as primitive as the ones she's
been able to identify, but it is the same lineage.
PROF. ITESCU: That's right.
DR. ROWLEY: Now, going on into the real world,
and let's say it's a kidney transplant, you would then get
bone marrow mesenchymal cells from the donor. So it would be cultured
to identify the stem cells. Those would be injected into the potential
recipient, and then at some point later you would take the kidney
from the donor and hope that you had induced tolerance.
PROF. ITESCU: Yes. I think that's about
right. Now, to be fair, you know, to try to do that in the setting
of an acute process, such as, for example, when we have a cardiac
transplant donor, we are talking about hours from the moment of
identification to actual transplantation.
To be fair, I think in that type of scenario it would be extremely
difficult to then isolate bone marrow stem cells, purify, inject,
and hope to achieve tolerance all in the same space of time.
I think this type of a strategy and approach is much more amenable
to living related transplants and would really increase the survival
and success of those type of transplants.
DR. ROWLEY: Part of the reason for pursuing
this is to show that this may be potentially useful in a subset
of patients, but as you just said, for cardiac transplants this
approach is limited and maybe not even be feasible.
PROF. ITESCU: Well, let me extend the thought.
If you needed to use — at this point in time, I think, with
known technology of how to immunoselect and how to get hold of these
cells, I think it would be a little bit impractical in the situation
of a cardiac transplant patient.
I think there is the possibility that you could potentially use
mesenchymal stem cells of an unrelated donor that could potentially
enhance the ability of the thymus to be nonresponsive at the time
that a graft was implanted and induce tolerance to that graft through
mechanisms other than simply the HLA molecules that the stem cell
Do you see what I'm saying?
DR. ROWLEY: Okay. What occurs to me is why
don't you then get hyperactive response to those rather than
PROF. ITESCU: Well, for reasons that are unclear
at this point, stem cells do not seem to induce an active immune
response to themselves. They seem to down-regulate immune responses
DR. ROWLEY: In general.
PROF. ITESCU: In general.
DR. ROWLEY: Right.
PROF. ITESCU: So there may be a possibility
of the potential of using unrelated stem cells plus a third party
graft and you induce tolerance to that particular graft, but not
to another graft because the immune system has seen only that graft
in conjunction with the stem cells that were infused.
DR. ROWLEY: Okay. Now, but following on with
this, I guess in terms of the use of embryonic stem cells, unless
it's in this same context of a neutral or an unrelated stem
cell affecting, in a sense, general immune unresponsiveness, you're
not going to take an organ from the embryonic stem cells and transplant
it into a donor as you would with the adult stem cell scenario that
we just pursued.
So I guess I'm a little bit confused as to how you foresee
embryonic stem cells in the sense of organ transplantation or is
it in the example you just gave?
PROF. ITESCU: Yes. The ideal scenario would
be to have mesenchymal stem cells obtained from the same donor where
the organ is coming from. So I think in that type of combination,
the embryonic stem cells would have no role. I think a second scenario
would be where stem cells of whatever source you want could potentially
be used as a local immunosuppressive agent for that particular organ
that's used at that time point.
And, again, we're talking about a tolerogenic induction to
the HLA of that organ.
A third possible use of stem cells in this way might be, again,
irrespective of whether you're talking about adult stem cells
or embryonic stem cells, to induce a state of tolerance by injection
of the cells followed by a more differentiated set of cells for
So as a strategy to prevent rejection of stem cell derived tissue
regeneration. So, for example, if we wanted to improve cardiac
function using adult stem cells and we would take, let's say,
adult mesenchymal stem cells, differentiate them in vitro
into cardiomyocytes, we would be concerned that if we took those
cardiomyocytes now and injected them directly into the heart they
might get rejected.
So what we might want to do then is take our source of mesenchymal
stem cells from a given individual with heart failure, let's
say; take those mesenchymal stem cells, set aside a number of them
and try to differentiate into cardiomyocytes, take the first batch
of mesenchymal stem cells from the same patient, infuse them back,
allow that patient to develop a tolerogenic state, and we're
talking about mesenchymal cells from a different donor, not from
obviously the same, where we're able to have a larger bank of
cells to provide back to the first recipient.
DR. ROWLEY: Okay, and my final question is:
do you have to worry in this situation about graft versus host disease?
PROF. ITESCU: That's a very good point.
In fact, that's, again, an advantage of using stem cells for
this process. Whereas whole bone marrow transplantation or whole
bone marrow used to try to induce tolerance runs a risk of graft
versus host disease.
As you probably know, bone marrow transplants, I think, in general,
allogeneic transplants keep something like 20, 25 percent incidence
of graft versus host disease. The animal treatments with to date
only embryonic stem cells that has been published has not resulted
in any type of GVHD, as you would anticipate that you wouldn't
get if these cells, in fact, inhibit rather than activate immune
DR. FOSTER: But in fairness, in the bone marrow transplantation,
a low level of graft versus host disease proves to be advantageous,
DR. ROWLEY: Well, that's true if you're
doing it with leukemia because then you get a graft versus leukemia.
DR. FOSTER: I'm talking specifically leukemia, yes.
PROF. ITESCU: But we don't want it to happen
if we're trying to induce tolerance.
CHAIRMAN KASS: Michael Gazzaniga.
DR. GAZZANIGA: I should remember the answer to this, but is there
an interesting variation in tolerance to grafts? In other words,
there's a subpopulation that seems to just take it and all of
the tricks in the medical bag don't seem to be that necessary?
PROF. ITESCU: There's no question there
are some patients who do wonderfully well with minimal immunosuppression
for many, many, many years, and absolutely I would be the first
to say that we have no idea why some people accept the graft so
So that clearly a mechanism of tolerance induction in some individuals
works beautifully. I would suspect that it's the same type
of mechanisms we're talking about today that take for whatever
reason in some individuals better than others, but it will be the
same mechanism. It won't be different mechanisms, I think.
DR. GAZZANIGA: Is there any way that you could predict that?
PROF. ITESCU: Yeah, and what I was alluding
to earlier, the type of chimerism approaches. What people do is
using genetic probes or genetic markers, you can look for the amount
of donor cells or donor tissue in the blood or in the ingrafted
in various organisms, the lymph nodes or bone marrow.
You can do these kind of fancy tests, and you can certainly predict
that if there's a high percentage of donor cells that continue
to be present two, three years out, actually two or three years
— if the patient has already gone two or three years, you
know it has done well, but let's say in the first three to six
months which is the highest risk of rejection.
If you see a high rate of persistence of donor cells, you can
predict that this patient is going to do better.
DR. GAZZANIGA: So could you use that as information in maybe
jumping the patient ahead on the transplant list because you think
there's going to be a —
PROF. ITESCU: Well, you don't know that
until the patient has already been transplanted.
DR. GAZZANIGA: I was asking if there were predictors.
PROF. ITESCU: No, there are no predictors prior
to a transplant as to who's going to accept the particular donor
at a given point in time. There are no global — what I was
trying to emphasize is that the exquisite response, the exquisite
specificity of the immune system here dictates that there's
going to be a very, very close and tight response between the donor's
immune system and the particular genetics of the host.
And those two are so specific that they cannot be predicted globally,
and that forms the basis for why we actually monitor each one of
our patients very closely and we tailor our therapy on an individual
basis. You just cannot make global decisions like that.
DR. GAZZANIGA: And one final thing. Maybe it was on your first
graph, but I didn't catch it. If you were to say what the survival
rate, transplantation survival rate in 2003 versus ten years ago
was, how much has it changed with all of the new technologies?
PROF. ITESCU: The most dramatic leap was probably
about 20 years ago, as I mentioned, with cyclosporin coming in from
50 percent one year survival to about 80 percent. We've now
gone from about 80 percent ten years ago to I would say 95 to 100
percent one year survival for kidneys and for hearts.
You can't get any better than that at this point in time.
The biggest problem right now still is that five and ten-year survival
rates, and those are the issues that I was getting at. I think
still donor-recipient immune activity is the problem, that we cannot
CHAIRMAN KASS: Janet.
DR. ROWLEY: I'm not sure whether people
have questions along this line because mine is a different question.
So coming back to the first slide where you show the great disparity
between the number of people who need kidney transplants and the
number of potential donors and cadaveric donors, and your data on
cadaveric donors would suggest that that's not necessarily an
avenue to pursue actively because the results are so much poorer
in terms of response.
The question I have, and that we've had minimal discussion
on this, is whether we should change the strategy of obtaining donors,
and I'm interested in your view on whether there should be a
program for paying donors for their organs.
It has been pointed out to us that everybody in the system makes
money except for the donor, and the donor is the essential individual
in this whole chain of events, and I'm curious as to your perspective
PROF. ITESCU: Okay. I think that it's
clear that we're not increasing the donor pool. I would disagree.
I think outcome with cadaveric transplants is excellent. My only
point, that was with living related is even better, but there's
nothing wrong with our current outcomes with cadaveric donors.
And I wish that we would be able to increase our cadaveric donor
pool. So if question number one is should the families of cadaveric
donors be appropriately looked after, I think the answer should
be yes. I think that the families need to be involved in this whole
In terms of the living related and whether there should be issues
with reimbursement, I would strongly oppose that.
CHAIRMAN KASS: We've got time just for a
couple of questions. Michael Sandel, Bill Hurlbut.
PROF. SANDEL: Why do the cadaveric organs not
work as well as living ones?
PROF. ITESCU: For a number of reasons. First
of all, because the living ones you've got more time to select
on the basis of how well the donor-recipient are matched, number
one. I mean, that's the objective. You find the best match,
and that's the one that's going to be the best in your family
And you're already starting out with related individuals who
are at least going to be 50 percent identical, you know, because
you inherit 50 percent from your mother and 50 percent from your
father. So you will share at least 50 percent of your genes with
siblings, et cetera.
So we're starting out with a closer pool, and then you're
looking for a perfect match. That's the number one reason.
But interestingly, as I mentioned, spousal related grafts which
are HLA completely unrelated generally also do better than cadaveric.
So the answer is more complex than that.
It's also probably because if you can perfectly plan and coordinate
the surgery so that everything goes according to schedule, you've
minimized all of the potential risk factors, et cetera. That obviously
impacts on the outcome. You can, you know, organize your timing
and the spouse will be there exactly on time and provide the organ,
CHAIRMAN KASS: There's no chimerism there.
PROF. ITESCU: You know, that's a very interesting
question. It may very well be some other interesting immunologic
issues that may also explain this.
CHAIRMAN KASS: Yeah. Bill Hurlbut.
DR. HURLBUT: Two parts to my question. First,
can you make some comment about the effect of mixing of peoples
from diverse geographic regions and how it may be making more complex
the search for a compatible donor and increasing the reshuffling
of HLA types?
I assume that in a specific population there would have been a
greater match. Whether that's making it more difficult, but
softening the immune rejection because people are mushing towards
the middle, if you will or if it's diversifying a complex mismatch
problem. That's the first question.
Second, if, in fact, it's possible for the marrow cells, mesenchymal
stem cells or the ES cells, to induce a kind of generalized inhibitory
response or lack of response that makes tolerance of a completely
different third party organ donor, then would it be reasonable to
say that it might turn out that you only needed one or a few lines
of these, let's say, ES cells for the moment, these ES cells
or would you need many that would be a better match for the situation?
Do you get what I'm getting at?
PROF. ITESCU: Yes, I know.
DR. HURLBUT: Because the argument will be made
that the few stem cell lines that are currently available either
might be adequate to that task or that we need many more.
PROF. ITESCU: No. So let me address the question
number one first. I think clearly that as you're getting mixing
of populations, it's making local identification of appropriate
donors much harder. Actually there's much more diversity.
It's not going to make inhibition of rejection easier because
the type of exposure to antigens, that takes generations. What
you're talking about essentially from a practical point of view,
it makes selection and identification of a matched donor much harder.
More homogeneous populations have a much easier time of finding
DR. HURLBUT: So we're heading for a worse
problem with rejection.
PROF. ITESCU: Absolutely, yeah.
And in terms of your second question, if we're talking about
an unrelated stem cell inducing tolerance to an organ transplant,
it's essentially, I think, pretty much irrelevant what the HLA
type of the stem cell is.
And so rather than needing a wider pool of stem cells, you probably
can get away with a more narrow stem cell population because you're
not specifically trying to induce tolerance to HLA antigens that
that stem cell expresses.
It's the ability of that stem cell to switch off the immune
response to HLA antigens on the organ that is seen at the same time
as the stem cell is present. So you potentially would not need
many cell lines, whether we're talking about embryonic or whether
we're talking about adult stem cells.
DR. HURLBUT: So just to clarify, if, in fact,
there are stem cell lines not grown on mouse feeder cells, even
just a few of them, that might turn out to be the central resource
to effect these ends.
PROF. ITESCU: Potentially.
CHAIRMAN KASS: Before we close, I want to be
somewhat flatfooted here so that as we hear the presentations in
July I know what I'm supposed to think about the immunological
aspects or prospects of stem cells for regenerative medicine.
You seem to be suggesting the following, and this was, in fact,
the reason for the invitation, to learn what we could from experience
with immunological problems in the transplantation of solid organs
to anticipate the possible obstacles to the uses of stem cells in
We've heard something surprising here in a way, that stem
cells, both embryonic and adult might actually hold some promise
for making the immunological obstacles to the solid organ transplantation
less than they now are. That would be the first point.
And that second, if this matter of education of recipient T cells
by some kind of chimerism works, there may not be as large a problem
in the use of stem cells for regenerative medicine as has been anticipated,
and in fact, just to make a footnote, it might not be necessary
as some people have argued that only by cloning can one get immunologically
compatible stem cells for therapy. Is that correct?
PROF. ITESCU: I think that would be the natural
extension of the argument, although I think at this point in time
far too few data are available to be able to make those kind of
conclusions, but I think that's right. The surprising evidence
to date, I think, is that even when stem cells of either source
are enabled to differentiate sufficiently to up-regulate their HLA
molecules, they still seem to be able to engraft, and they still
seem to be able to regulate immune responses in a downward fashion.
So potentially what that does suggest is that these cells may
be less immunogenic than other cells in our body and might produce
to requirement for cloning even when used therapeutically, yeah.
CHAIRMAN KASS: Thank you very much.
DR. ROWLEY: Can I just interject though? You
said in your presentation that the data all come from experimental
animals, and so this last exchange, in fact, we don't know whether
the data from the animals are applicable to the human system.
PROF. ITESCU: Absolutely. I agree entirely
with that, too.
CHAIRMAN KASS: Thank you very much, Dr. Itescu,
for an interesting and enlightening presentation and discussion.
We'll take a break for 15 minutes. Come back at ten minutes
(Whereupon, the foregoing matter went off the record
at 10:36 a.m. and went back on the record at 10:54 a.m.)