**4. A shift in species**

A hard truth about human organ donation is that even with exponential increases in donor numbers, it is unlikely that the organ shortage would be relieved. The diversity of the human species, paired with the efficiency of the immune system, significantly reduces the chances that a given organ will be compatible with the patients in greatest need. Although immunosuppressive drugs can enhance length of survival, chronic rejection remains a risk the greater the mismatch between organ and patient. Furthermore, the donor geographic proximity, organ size and timing of availability of a compatible organ with a matching patient may be limiting. Thus, even as human organ donation continues to be optimized, there remains an immense need for additional organs above and beyond the availability of human donors.

In order to address the above concerns and provide sufficient numbers of compatible organs, a number of approaches, both biological and mechanical, are being actively explored. Use of animal organs provides solutions to the challenges of availability and function. Multiple mammalian species possess organs which may substitute effectively for their human analogs and, in the case of agricultural species, can be rapidly bred in sufficient numbers to overcome organ shortages. Through use of controlled facilities, production of animals can be regulated and disease exposure eliminated. Additionally, careful breeding schedules can provide organs of appropriate size for any given patient on a predictable schedule for optimal timing of surgery. Finally, recent advancements in DNA sequencing and assembly and genome engineering technologies, paired with the advanced understanding of the cellular and molecular immunology responses in transplant rejection, allow the creation of animals which could pro-

Examples of xenotransplantation can be found recorded as early as the seventeenth century, in which the transfusion of blood from animals into human patients was described [1]. In the eighteenth century, more complex tissues such as skin were tested as grafts in human patients [2]. In 1905, Princeteau transferred rabbit kidney sections into a child with immediate positive results, however, after 16 days the child died of pulmonary complications [3]. Soon thereafter, two kidney xenotransplants were attempted, with one patient receiving an organ from goat, the other from pig. Unlike Princeteau's experiment, neither organs functioned and both apparently failed due to thrombosis [4]. Similarly, an attempt by Unger in 1910 to transplant kidneys from a chimpanzee into humans led to failure due to thrombosis in about a day [5]. In 1923, Neuhof transplanted a kidney from a lamb into a human patient, allowing the patient

In the early twentieth century, an odd offshoot of xenotransplantation was created due to interest in "rejuvenation" via transplant of animal testis in human males, as demonstrated by Voronoff in Russia [7] and Brinkley in the US [8] using chimpanzee or goat testis, respectively. So popular was the use of goat testis in the US, an entire radio empire was built around advertising the services, with many patients claiming enhanced fertility and sexual function [8].

The field of immunology developed in parallel with surgical approaches to xenotransplantation. As the mechanisms of immune rejection were better defined, the enormity of the challenges facing transplant of organs between members of the same species were recognized.

vide an unlimited supply of rejection-free organs.

332 Organ Donation and Transplantation - Current Status and Future Challenges

**2. Early beginnings**

to survive 9 days [6].

Although the close evolutionary relationship between non-human primates and humans would suggest an advantage in using chimpanzee or baboon organs for xenotransplantation, clinical, practical and ethical considerations prevent them from being a viable option. Non-human primate organs do indeed function almost identically to human organs, but are subject to a variety of diseases which are readily transmissible to humans [16]. Given the relatively fragile state of patients receiving multiple immunosuppressive drugs, the risk of primate zoonoses is too great. In addition, chimpanzees, baboons and many other non-human primates are impractical for large scale breeding. The low numbers of progeny of non-human primates limits the production of large numbers of animals by natural breeding or *in vitro* fertilization compared with agricultural species. Finally, use of non-human primates as organ donors faces insurmountable ethical barriers.

A much more viable approach is the use of pig organs for xenotransplantation. Porcine organs are structurally and physiologically close to humans, and therefore can functionally substitute for analogous human organ functions. Unlike non-human primates, pigs are more evolutionarily distant from humans and thus have a greatly reduced risk of transmission of diseases to human patients, which can be essentially eliminated through genetic manipulation [17]. Husbandry techniques for pigs are extremely well-understood, with large litter sizes and rapid cycle times, allowing production of populations that could overcome organ shortages in a much shorter timeframe than possible with non-human primates. Furthermore, a suite of genome engineering technologies is available for use in pigs to make critical changes to enhance survival and function of the pig organ, while avoiding the human immune rejection response. In fact, the complex engineering approaches now available may actually provide organs with advantages over even closely-matched human organs.

characterized as distinct stages based on histological and clinical data, each is due to highly interconnected pathways and mechanisms that are challenging to separate. In fact, these responses were defined as pathologic observations prior to development of more detailed

Xenotransplantation

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http://dx.doi.org/10.5772/intechopen.76502

HAR is primarily due to immediate binding of pre-existing host natural antibodies specific for xenoantigens expressed by the donor tissues. Antibody binding can activate the endothelial cells, causing the release of immune activators, as well as inducing the complementmediated destruction of the endothelial layer, reducing the barrier function and allowing host cells to infiltrate the organ. Cell debris released by the damage to the endothelium and products of the complement cascade also stimulate coagulation and the innate inflammatory response. These pathways synergize during rejection to create stronger responses that are

AVR/AHXR, like HAR, is also mediated by host antibodies. However, instead of pre-existing natural antibodies, AVR/AHXR is often the result of humoral responses which lead to production of antigen-specific antibodies. The AVR/AHXR is delayed due to the time it takes to induce

**Figure 1.** Overview of the xeno-rejection response. The human immune response to xeno-organs initiates within minutes to hours with hyperacute rejection (HAR, upper right), in which pre-existing antibodies (Ab) in human serum bind to xenoantigens (xeno Ag) on the surface of the pig cells. This results in cell destruction and presentation of porcine antigens to human helper (TH) and cytolytic (TC) T cells, as well as groups of pro-inflammatory and pro-immunity cytokines and other soluble mediators. Activation of human helper T cells (TH) stimulates human B cells (B) over the course of days to weeks, resulting in production of induced antibodies (induced Ab) as part of the acute vascular rejection/acute humoral xenograft rejection (AVR/AHXR, lower right) response. These secondary antibodies are often more specific and higher affinity than the pre-existing human serum antibodies, and also cause cellular destruction of the xenograft. in parallel with AVR/AHXR, the acute cellular rejection response (ACR, lower left) is carried out by human NK cells (NK), macrophages (M) and the xenograft-specific cytolytic T cells (TC) recruited to the xeno-organ within days to weeks. The activated cells express a variety of molecules to attack the porcine cells, as well as secrete additional cytokines to recruit more human immune cells. After weeks to months, the human immune response may be again induced to react to the xeno-organ during chronic rejection (CR, upper left), leading to specific antibody (Ab) responses from B cells (B) or cytolytic T cell (TC) destruction. A large collection of cytokines, chemokines, complement and coagulation factors

(center) play a key role in regulating the complex set of reactions occurring in every aspect of rejection.

analyses of the molecular immunology mechanisms.

more pathogenic and can be less amenable to control.
