While work to combat immune rejection of animal organs is progressing,
concerns remain as to the risks of disease transmission.
There are more than five times as many people on the waiting list for organ
transplants as will actually receive one. Although the number of transplants
increased by about 30% during the 1990s, the number of candidates doubled.
This acute shortage of human organs has prompted significant research and
development into alternatives, specifically xenotransplantation, defined as
the transplantation of organs from other animal species into humans.
This approach is under investigation for a wide range of conditions, including
replacement of the heart, lungs, liver, and kidneys. Xenotransplantation is
also being developed for non−whole organ scenarios such as diabetes,
neurodegenerative disorders, chronic pain control, and ex vivo perfusion events.
Research has focused in large part on pig organs and tissues because their
biochemical profile is similar to that of human organs. However, associated
with all types of xenotransplantation are some significant problems, including
hyperacute rejection, delayed rejection, and cell-based immune rejection.
Additionally, novel infections may be transmitted from the donor to the recipient
and spread to the human population. There is considerable debate over the
pros and cons of xenotransplantation, and the US and the UK have already taken
significant regulatory steps to foster this debate and to control clinical
trials. Several companies are pursuing xenotransplantation in step with the
regulatory frameworks, as the potential market for this procedure is believed
to be significant.
Historical perspective The first attempts to use animal organs in humans were reported in the
early 1960s, involving the transplantation of chimpanzee kidneys. The results
were significant: one patient survived for nine months with normal kidney
function before dying from the effects of immunosuppression, while another
had a rejection episode that was relieved by steroid therapy1,
2.
These results showed that xenograft survival and function was possible and
could last for considerable periods of time, despite the relatively poor immunosuppression
protocols then available.
Following these efforts, baboon-to-human heart, liver, and kidney transplants
were attempted, none of which achieved one-year survival of either the graft
or the patient3,
4. From then on, interest in xenotransplantation
diminished, in part because rejection based on preformed antibodies became
an insurmountable problem. In addition, the development of hemodialysis, coupled
with greater human organ availability as a result of the public's acceptance
of the notion of brain death, created a false sense of security about the
real need for organ transplants. However, the increase in transplant candidates
in this decade, and the ensuing quantitative crunch from an outstripped supply,
has ultimately led to a resurgence of interest in xenotransplants as a source
of organs, especially for children.
Current state Current interest in xenotransplantation is fueled by promising results
and significant demand. In addition to academic clinical work, there is considerable
corporate interest in the field. Table 1 shows a selection
of biotechnology and pharmaceutical companies with xenotransplantation programs.
Table 1. Selected companies with xenotransplantation programs
There are also several strategic alliances in this area. In 1997, BioTransplant
and Novartis Pharma expanded their two-year-old xenotransplantation collaboration
to include BioTransplant's technology, which creates transplant tolerance
by a mixed bone marrow chimerism approach. The technology involves transplanting
bone marrow from the donor to reeducate the recipient's immune system
to accept the donor organ as self.
Under the agreement, Novartis obtained a worldwide license to the technology
in exchange for license payments, research funding, and milestone payments
to BioTransplant totaling $36 million. In addition, BioTransplant will receive
royalties on eventual sales, and retains the right to copromote products resulting
from the collaboration in North America under certain circumstances. This
agreement extends an existing agreement between the two companies covering
a gene therapy approach to create tolerance in xenotransplantation.
Novartis entered into an additional xenotransplantation agreement in 1997,
this time with T-Cell Sciences (now Avant). This is a $25 million option and
license agreement for the development of TP10, T-Cell Sciences' lead
complement inhibitor5, for use in xenotransplantation and allotransplantation
(human to human). Avant will receive annual option fees and supplies of TP10
for clinical trials—the combination of which is valued at up to approximately
$5 million—in return for granting Novartis a two-year option to license
TP10 with exclusive worldwide marketing rights (except for Japan), in the
fields of xenotransplantation and allotransplantation. Should Novartis exercise
its option to license TP10 and continue development, it will provide an equity
investment, licensing fees, and milestone payments based upon attainment of
certain development and regulatory goals.
Industry challenges Xenotransplantation can be thought of as an extreme but necessary solution
to a very difficult problem, namely irreversible and terminal tissue and organ
failure. It must deal with two considerable technical problems, one of which
also has significant societal implications. The first has to do with rejection
of the organ and cells, and the other is the risk of introducing novel infections
into the human population.
There are essentially three types of xenograft rejection, which vary even
further depending on whether the transplant is of a whole organ or just cells:
hyperacute rejection, with almost immediate onset, mediated by preformed and
new antibodies against foreign antigens; delayed xenograft/acute vascular
rejection, with a longer time profile and a mechanism that is not fully understood,
but which is likely to involve host antibody and immune cell binding to the
vascular endothelium of the xenograft, leading to its destruction; and finally,
cellular immune response through pathways that are similar to the rejection
pathways of allografts and are mediated by histocompatibility determinants
and other cell surface components.
There is marked progress in resolving hyperacute rejection against transplanted
cells, mediated by antibodies against the Gal carbohydrate epitope
displayed on the surface of pig cells, for example. Although preformed and
new antibodies are created against this antigen, research suggests that this
humoral response can be overwhelmed simply by transplanting larger numbers
of donor cells6.
Delayed rejection is targeted against the vascular endothelium of the transplanted
organ. Although its mechanism is not well understood, it is believed that
induced antibody responses are critical. Therefore, attenuation of the B-cell
response, which is central to new antibody production, is being evaluated
as a way to overcome this immunological barrier, with promising results in
animal models7.
Cell-based rejection is particularly strong in xenotransplants, mimicking
that in transplantation in general; it involves helper and killer T cells
and others. There is a consensus that the best way to overcome it is not by
general immunosuppression, which is insufficient, but by inducing tolerance
to specific xenoantigens before transplantation, for example, by the mixed
chimerism bone marrow approach described earlier.
Even if all of the immunological barriers just described were to be overcome,
the final challenge to the development of xenotransplantation as a viable
therapy is the prospect of novel infections, particularly viral ones, being
introduced through the recipient into the human population. The debate about
the societal risk posed remains unresolved. However, there is a renewed effort
to develop methods of evaluating the risk, to establish surveillance, to improve
virus screening and diagnostic capabilities, as well as to introduce rigorous
clinical trial guidelines, to encourage communication among all involved,
and to establish a permanent oversight body to regulate the procedure, similar
to the Recombinant Advisory Committee of the National Institutes of Health,
which regulates genetic engineering. As an example of this debate, some are
of the opinion that xenotransplantation poses a lesser risk of transmitting
known infections to humans than does allotransplantation8, whereas
others focus on the unknown risks posed by the multiple animal retroviruses,
lentiviruses, herpesviruses, and other harmful agents that may be found in
animal donor organs and cells9.
Clinical relevance Despite the technical challenges and debate over xenotransplantation's
acceptability, efforts continue in the clinic to address these issues. Increasing
attention is directed to ex vivo perfusion through animal livers10.
The promising results with this significant application suggest that this
avenue will continue to be explored.
Efforts have also been directed at xenografting cells as opposed to whole
organs in certain conditions. For instance, clinicians have been attempting
to implant fetal pig islet cells into human diabetic patients. Although one
effort by Swedish researchers resulted in the survival of the porcine cells
in at least one patient and the production of pig C-peptide, demonstrating
basic functional "take," these patients had no significant reduction
in insulin requirement11. Other studies have used fetal pig
neural cells in patients with Huntington's or Parkinson's disease.
Although the pig tissue has survived, early benefits (i.e., improvements in
quality of life) for these patients are still unclear12. Despite
these results, another study reports on the effect of microencapsulated bovine
chromaffin cells on monkey models of Parkinson's disease13.
The cells were encapsulated in alginate-polylysine-alginate membranes and
resulted in a reduction of certain Parkinsonian symptoms for up to nine months
compared with controls, suggesting that this approach merits continued clinical
interest. Finally, an AIDS patient has received baboon bone marrow cells to
boost T cell levels but no evidence of graft survival was obtained14.
In addition, a major issue for the clinical practice of xenotransplantation
is survival and longevity of the xenografts. Although there is increasing
interest in the use of pig organs for this purpose, there is a lack of information
regarding their longevity and that of their primate hosts beyond the first
month after transplantation. A recent report describes how pig kidneys transplanted
into immunosupressed monkeys that had their kidneys removed had normal renal
function and enabled the animals to live for more than two months15.
Results such as these are important for the future clinical application of
porcine organs as xenografts.
The future Although the debate over the risk posed by the infection potential of xenotransplantation
is ongoing, the factors driving its development approaches are more pressing
than ever. The need for human organs is simply increasing faster than their
availability. Validation for the approach will come with continued improvements
in the battle against rejection of xenografts. An interesting approach is
that of using retroviral gene therapy to inhibit the production of xenoreactive
antibodies, which are involved in the hyperacute and delayed types of rejection.
One study that holds significant promise reported the absence of such antibodies
in an animal model when bone marrow was genetically modified to produce the
enzyme that actually makes the Gal epitope. Production of this epitope
thus rendered the animal tolerant16.
In the future, cells, instead of whole organs, will be used for a variety
of major organ diseases. A recent report describes the xenotransplantation
of immortalized human hepatocytes in rats suffering from experimental acute
liver failure17, and these approaches will continue to be explored
in the future. Complementing these approaches will be continued advances in
encapsulation technology itself, including agarose/polystyrene sulfonic acid
constructs18 and others, which will broaden the range of cells
that can be used for these purposes.
On another front, researchers are trying to modify genetically the pig
organ donors themselves, so that they do not present the critical epitopes
identified and linked with rejection phenomena. It is believed that a combination
of genetic engineering of xenograft tissue to underexpress or eliminate the
expression of such antigens, coupled with tolerance conditioning of the recipient
by chimeric or genetically engineered bone marrow, will help overcome these
difficulties19. The future will continue to see innovations
such as modifying xenograft organs by gene therapy approaches to improve the
immune characteristics, efficiency and therefore longevity of these organs
and their hosts20.
Finally, both the US and the UK are proceeding with the establishment of
oversight groups and guidelines to monitor and regulate clinical trials, as
well as to continue and increase the public debate over the risks posed by
the procedure.
Conclusions Xenotransplantation, a field with a 30-year history, has received renewed
attention recently as a result of promising clinical efforts. It addresses
an acute shortage of organs for transplantation, but has a long way to go
before it is an accepted and even routine clinical therapy. What makes it
attractive is that it potentially obviates the agonizing and uncertain wait
for organs, and also that it offers potential treatments for currently intractable
degenerative disorders. The concept has generated a great deal of healthy
skepticism with regard to the potential risks for novel infections. Efforts
are increasing to address the challenges posed by the various forms of rejection
of xenotransplants, and the early results are deepening our understanding
of the underlying mechanisms, as well as helping to define the exact nature
of the risks involved. Society's need, in conjunction with clinical efforts
and corporate interest in this area, promise to keep xenotransplantation in
the limelight as a technology to watch.
Gal carbohydrate epitope
displayed on the surface of pig cells, for example. Although preformed and
new antibodies are created against this antigen, research suggests that this
humoral response can be overwhelmed simply by transplanting larger numbers
of donor cells
