Research and Development of Xenotransplantation using Animals

Kerong Pang

1,† a

, Tianzheng Wan

2,† b

and Xiang Yuan

3,† c

Qingdao, Shandong, China

College of Life Sciences, Northwest A&F University, Yangling, Shaanxi, China

Lifescience Gateway, McMaster University, Guangdong, Shenzhen, China

†

These authors contributed equally

Keywords: Xenotransplantation, Immune Rejection, Immunosuppression.

Abstract: Xenotransplantation is the transplantation of a tissue or organ from one species into another. It is expected to

be used in the clinical treatment of end-organ failure in the future. In recent years, xenotransplantation is being

widely studied because of its great potential. Because the homotransplantation donors are mainly derived from

human origin and are in clinical shortage in quantity, while the xenotransplantation donors are from pigs,

there is no apparent difference in organ quality between human and pig sources. xenotransplantation donors

also have adequate quantity. This paper first introduces the general situation of xenotransplantation. Then, it

discusses the technology and process of xenotransplantation in detail. What is more, this study also provides

the prospects of xenotransplantation. It is hoped this article can provide new ideas to the research in

xenotransplantation.

1 INTRODUCTION

Xenotransplantation is a new technique in the field of

medicine. Xenotransplantation refers to the

transplantation of tissue from one species into another.

For example, from pigs to people. Different from

homotransplantation, there are great differences

between different species, which makes the

possibility of successful transplantation very small.

Animal organs are used as substitutes to reduce the

shortage of organs. This technology is in great

demand. Xenotransplantation is the only effective

way to treat end-stage organs.

However, the shortage of organs continues to

increase. The organs needed for transplantation

mainly come from donors and criminals. Every year,

more than 1 million people in the world need organ

transplantation because of end-stage organ problems

or accidents, but only 10% of them receive organ

transplantation. Many people don’t get treatment.

Compared with human organ transplantation,

xenotransplantation has enough donor sources and

can be mass-produced, so its price should be lower

https://orcid.org/0000-0002-3261-749X

https://orcid.org/0000-0002-8235-0885

https://orcid.org/0000-0002-7853-4817

than human organ transplantation (in some countries,

the price of heart transplantation is about 450,000

yuan). The research of xenotransplantation began in

1667. In recent years, there are more and more

researches on xenotransplantation. For example, 75%

of heart valve surgery patients choose biological

valves, i.e. pig valves or bovine pericardium, as tissue

materials. The survival rate was over 90%. If the

technology works. This will be great progress in the

field of medicine, accelerating human civilization and

prolonging human life.

There are abounding technology to achieve the

goal that is applying xenotransplantation in daily life.

The traditional technique implemented the

transformation of heterologous organs in patients by

a humanized modification for gene knockout of donor

organs from transgenic animals and using

immunosuppressants to promote transplantation.

With the development of gene editing, CRISPR/Cas

technology was also applied in xenotransplantation,

which offers changes based on repairing broken DNA

strands. Even though such technology is successfully

used in xenotransplantation, some barriers also exist

during application. For example, animals without

Pang, K., Wan, T. and Yuan, X.

Research and Development of Xenotransplantation using Animals.

DOI: 10.5220/0011216500003443

In Proceedings of the 4th International Conference on Biomedical Engineering and Bioinformatics (ICBEB 2022), pages 461-466

ISBN: 978-989-758-595-1

461

correct gene editing cannot be converted into mature

organ donors. Before implementing

xenotransplantation, the heterologous organ required

the inhibition of cellular immune responses,

modifying the major histocompatibility complex,

eliminating the cross-species transmission risk of

virus from donor animals, and so on.

This article systematically introduces the general

situation of xenotransplantation in this paper,

including its development history, technical route,

and the test results in recent years. Especially narrates

the difficulties of current xenotransplantation

technology. In addition, this paper also reasonably

analyzes the relevant literature and predicts its bright

future.

2 BASIC INTRODUCTION OF

XENOTRANSPLANTATION

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2.1 Concept of Xenotransplantation

Xenotransplantation can provide an unlimited source

of donors to humans through animals. Therefore, It is

the dream of humans for many years. More and more

people are researching xenotransplantation. In theory,

it is the best choice to choose the closest primate (such

as a monkey) as the source of graft. But the primate

number is very rare. As a result, pigs, which are

similar to human organs in size and function, become

the best choice for researchers. In theory, for

example, pig islets could be transplanted into humans.

Because pig insulin works for humans. And pigs can

reproduce rapidly, which makes it easy for them to be

genetically modified.

The biggest problem of xenotransplantation is the

rejection of transplanted organs. This is mainly due to

the common rejection of cells and immune cells in

organs. Lead to adverse reactions. It can even kill

people. At the same time, there is a potential risk of

xenotransplantation, that is, the risk of infection

source transmission. For example, viruses in pigs

spread from pigs to people. To solve these problems,

scientists need further research.

2.2 The Status of Xenotransplantation

The effects of xenotransplantation on organs have

been improved (Cowan 2017). It improves the

research efficiency of xenotransplantation.

Xenotransplantation is the best way to treat organ

failure. Therefore, there is a huge demand for this

technology. Many people worldwide are waiting for

the success of xenotransplantation and integration

into people's lives. However, there are many concerns

about this technology. Scientists have proved that

xenotransplantation is a feasible alternative to

homotransplantation. However, the current situation

of this technology is not mature.

However, with the continuous research of

scientists, people have found a large number of

obstacles to xenotransplantation, and designed many

potential solutions. Scientists have made models of

xenogeneic corneal transplantation. An anterior

lamellar keratoplasty model of non-human primates

in pigs was established. This model can predict the

effect of transgenic pigs on xenogeneic corneal

transplantation (Vabres 2020). At the same time, the

experimental model of transplanting pig liver into

baboon has also been completed (Navarro-Alvarez

2020). The experiment of islet transplantation from

human to rodent has also been completed (Iuamoto

2017). These experiments and models have made a

great step forward in human xenotransplantation.

2.3 Results of Xenotransplantation and

Its Effect on People

In recent years, there is a growing interest in

xenotransplantation. The effect of

xenotransplantation on many organs has been

significantly improved. It improves the research

efficiency of xenotransplantation. The PERV virus

has subsided. However, the prevention of xenograft

rejection is still a great challenge (Cowan 2017).

In 2016, Swiss researchers made a breakthrough

in this field. With the administrative and security

parts of clinical preliminaries entering the program

and using polygenic pigs. The endurance pace of pig

xenotransplantation in non-human primate models

has been altogether improved (Yung 2017). To make

xenotransplantation a clinical reality, it is necessary

to work on the resistant procedure. The most

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interesting possibility soon is the utilization of

homozygotes α-Organs of galactose quality knockout

pigs.

Variety of organic pathways in the all-out range

of xenograft dismissal (Hoerbelt 2004). With the

continuous advancement of human progress and the

efforts of scientists, there will be more and more

breakthroughs in xenotransplantation. Then the

solution and the technology are officially recognized

and put into use. So that the choice of patients whose

organs have reached the end of their life is no longer

limited to allogeneic transplantation.

Xenotransplantation will become the best choice for

them.

3 BARRIERS OF

XENOTRANSPLANTATION

AND THE TECHNOLOGY TO

BREAKTHROUGH

3.1 Obstacles and Strategies to

Overcome in Xenotransplantation

When human organ donor is in a shortage, it is well

known that the application of xenotransplantation can

acquire more benefit than homotransplantation

technology. However, before a huge amount of organ

supplementation from animals, some obstacles need

to be overcome, for example, the immunological

rejection of heterologous organ in the human body

prevents the goal to utilize animals as an organ donor

(figure 1).

Figure 1: The main obstacle in xenotransplantation lies in how to eliminate the immune rejection after the operation. For

patients, immunosuppressant adjuvant therapy is needed to reduce cellular immune response and optimize treatment for

transplant-induced inflammation and coagulopathy. For donor pigs, editing and modification at the gene level should be

carried out in advance, which mainly includes the modification of MHC, the inhibition of macrophage function, and the

elimination of endogenous viruses that can be transmitted across species.

In recent years, the development of genetic

technology and research of immunosuppression

medications had prepared the basics for the effective

regulation of these flaws. As an example, the porcine

gene had been optimized to enable human

immunoreaction decline, also, it decreased liveness

rates of porcine endogenous retroviruses (PERVs),

prevented coagulation disorders and other reactions.

Meanwhile, researchers need to ensure the organ is

functioning normally and living for a long period

after xenotransplantation, for instance, operate

corresponding immunosuppression strategy to

transfer the pancreas from transgenic pig to human

and make certain the organ working, as usual, to

return blood sugar and insulin level to normal. Deeper

research is necessary, applying the

immunosuppression strategy still has a limitation for

the health of the patient. Moreover, another strategy,

immunotherapy, can build a mouse model (gene and

tissue of mice are substituted by human genes and

cells), simulating the growing environment of

xenograft organs and tissue under the human immune

system. This model is one of the latest advances in

immunosuppression strategies.

Research and Development of Xenotransplantation using Animals

463

3.2 Suppressive Regulation of Immune

Response

Many experiments had been done in the study of

xenotransplantation, especially in the control of the

immune response. For example, the application of

TEVMP with good mechanical and physiological

characteristics as a biological artificial artery in pigs

to simulate the physiological blood flow system of the

human body, and the study of xenograft rejection in

blood vessels (Kim 2021). In addition, many

transplantation experiments have been performed to

challenge xenoimmunological rejection, such as the

implantation of human cancer cells in humanized

mice (which have a human immune system). The

mice were also tested for tumor treatment results

which have replaced the original human patients (Jin

2021).

As a deeper study, CD47 is a ligand for

macrophage inhibitory receptors, if introducing

human inhibitory regulators of macrophage into the

pig, it could effectively inhibit the rejection reaction

of macrophage in the human body after

xenotransplantation. By the technique of chromatin

transfer, researchers successfully obtained ideal

hCD47 Knock-in Pigs, after two rounds of clone.

Moreover, to test the results' effectiveness, the

researchers transplanted porcine progenitor cells into

mice and found that several alleles in the mice were

able to bind with hCD47. This result showed that the

expression of hCD47 had a significant protective

effect on the transplantation and persistence of

porcine cells in this model, possibly by regulating

phagocytosis of macrophages (Tena 2014).

3.3 Second Sectionxenotransplantation

Experiment Practice

Interestingly, there have also been practices involving

xenografts, for instance, using pig islet grafts as

substitutes for allografts. Researchers evaluated

clinically available drug immunosuppression

regimens, such as belimumab for maintenance and

adalimumab for control of inflammatory responses.

The experimental subjects were non-human primates.

After transplantation of pig islets, blood glucose was

normal, and there were no organ failures and no

serious adverse reactions. The survival days of the

transplanted organs in all three recipients were more

than 100 days (Kim 2021).

To ensure long-term insulin independence, in

another paper, researchers used several strategies.

The implantation of embryonic porcine pancreas

tissue into diabetic animals is a key aspect of

xenotransplantation, to be more precise, the location

of the islet implantation is also critical. In previous

experiments, it was placed in the liver, where low

oxygen and inflammatory reactions could hinder the

survival of islets. However, when implanted into the

renal capsule, the islet was well protected, but the site

was prone to ischemic injury. Finally, when the islet

was transplanted into the gastric submucosal-space,

its direct contact with the blood flow is delayed, but

the arterial blood can continue to maintain. As a

result, transporting oxygen and nutrients can enable

the graft to survive. Another advantage is that

drainage of venous blood allows insulin to work

directly in the liver (Marigliano 2011).

4 REDUCE IMMUNE

REJECTION AND

INFLAMMATION

There are still many problems to be overcome in

xenogeneic heart transplantation. Such problems

mainly focus on postoperative immune rejection,

inflammatory reaction, and infection. At the same

time, there are also some ethical and religious issues

(Garcia 2021). At present, various research teams

have proved several possible methods to reduce

immune rejection and inflammation.

4.1 Gene Editing Pig

First of all, it is necessary to cultivate donor pigs with

gene editing in routine xenotransplantation

experiments, knock out or modify some genes that

can cause immune rejection, and porcine cell virus

genes, to reduce the potential risk of immune

rejection (Tomasi 2021). Due to the different genes of

patients, there will be slight differences between gene

editing pigs and different recipients. These

differences may cause a serious inflammatory

reaction after transplantation (Reichart 2021). Li et al.

designed a new method to quickly detect the

xenogeneic immune response of humans to pigs,

which can express special endothelial cell markers,

verify the stimulation strength of inflammatory

cytokines, and detect xenogeneic immune response

by the culture and transformation of immortalized

primary porcine living derived cell (Li 2021).

In addition, how to improve the production

efficiency of gene editing pigs is also one of the hot

issues. Cho et al. introduce the human endothelial

protein C receptor (hEPCR) and human

thrombomodulin (hTM) genes into porcine neonatal

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ear fibroblasts. It can be used as donor cells for

reclining to increase production efficiency. The

cloned fetal kidney cells also have the same function

(Cho 2022).

The survival time of organs after the operation can

be improved, and immune rejection can be reduced

by improving the preservation method of the donor's

heart before operation (Goerlich 2021). Corbin et al.

Found that compared with traditional preservation

methods, low-temperature standing preservation was

carried out on the ice, and used XVIVO© Heart

solution (XHS) based cardioplegia can improve the

survival rate and function of grafts.

4.2 Proinflammatory Factor and

Cardiomyocyte

There are two additional ways to reduce the

occurrence of immune rejection and inflammation

after surgery. One is to continuously apply anti-CD40

antibodies and other immunosuppressants and control

the expression of pro-inflammatory factor hTNF

(Hara 2021, Mohiuddin 2016). When human blood

contacts pig hearts, it induces the activation and

proliferation of cytotoxic T cells and NK cells. hTNF

is one of the important factors in the process of

activation and proliferation.

The other is by inhibiting the harmful

proliferation of donor heart cells (Längin 2018).

Research shows excessive proliferation and

hypertrophy of cardiomyocytes will lead to

multifocal myocardial necroses and thrombosis and

cause secondary liver failure. Matthias et al. tested

with baboons. They weaned the recipient baboons at

an early stage, applied hypertension treatment, and

took additional tacrolimus, which finally reduced the

excessive growth of the heart.

5 CONCLUSIONS

In summary, this paper analyzes the basic

introduction of xenotransplantation, the current

situation of xenotransplantation, the results of

xenotransplantation, and its impact on human beings.

Some effective strategies for suppressing the immune

system can enhance the survival rate and durability of

transplanted organs. However, some methods still

need a deeper study that researchers have not

eliminated the threat of inflammatory reaction and

virus infection carried by allogeneic organs

completely. The next research will focus on Gene

editing pigs. Those donor pigs in use today can

survive as long as six months with just a few genes

knocked out, and it is not hard to imagine that these

xenoorgans could survive even longer when more

genes are modified. Researchers can also find more

suitable immunosuppressants with better effects and

fewer side effects. In the next study, they can

gradually start clinical research under international

recognition and supervision.

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