Protection Against Hyperacute Xenograft Rejection of Transgenic Rat Hearts Expressing Human Decay Accelerating Factor (DAF) Transplanted into Primates

Advances in Genome Editing and Application to the Generation of Genetically Modified Rat Models

The rat has been extensively used as a small animal model. Many genetically engineered rat models have emerged in the last two decades, and the advent of gene-specific nucleases has accelerated their generation in recent years. This review covers the techniques and advances used to generate genetically engineered rat lines and their application to the development of rat models more broadly, such as conditional knockouts and reporter gene strains. In addition, genome-editing techniques that remain to be explored in the rat are discussed. The review also focuses more particularly on two areas in which extensive work has been done: human genetic diseases and immune system analysis. Models are thoroughly described in these two areas and highlight the competitive advantages of rat models over available corresponding mouse versions. The objective of this review is to provide a comprehensive description of the advantages and potential of rat models for addressing specific scientific questions and to characterize the best genome-engineering tools for developing new projects.

Study of the microcirculation in hDAF transgenic rat livers xenoperfused with human blood

BACKGROUND

The microcirculation was assessed in the livers of human decay accelerating factors (hDAF) and wild-type transgenic rats by fluorescent intravital microscopy, histology and histomorphology to determine the benefits of hDAF expression for the microcirculation of a rat liver xenograft perfused with human blood.

METHODS

Male hDAF transgenic rats (group A; n = 20) and wild-type Sprague-Dawley rats (group B; n = 20) were xenoperfused with human blood, while other male wild-type Sprague-Dawley rats (group C; n = 10) were perfused with allogeneic blood. Following plasma and leukocyte staining with fluorescein sodium, and platelet staining with rhodamine, the right lobe of the liver was assessed by intravital microscopy, counting the numbers of perfused sinusoids and leukocytes adhering to the endothelium per mm(2), and calculating the acinar perfusion index (Pi). The liver underwent histological assessment at the end of each experiment. Mean +/- SEM values were calculated and the Mann-Whitney U-test was used for statistical analysis.

RESULTS

The number of perfused sinusoids was higher in the group of hDAF rat livers (group A) and controls (group C) than in the group of non-transgenic rat livers perfused with human blood (group B) (P < 0.05), although only group C still had a significantly more perfused sinusoids than the other groups after 90 min of perfusion (P < 0.05). The acinar perfusion index was higher in groups A and C than in group B (P < 0.05); here again, only group C still had a significantly higher Pi than group B after 90 min of perfusion (P < 0.05). There was a massive accumulation of leukocytes that peaked after 5 min and persisted throughout the perfusion in all three groups. Histology showed portal and subendothelial hepatic vein hemorrhage, necrosis and inflammatory reaction, which were particularly evident in group B.

CONCLUSION

In our study, rat livers transgenic for hDAF were better protected against early tissue damage by perfusion with human blood, but this did not result in a longer xenograft survival.

Transgenic modifications of the rat genome

The laboratory rat (R. norvegicus) is a very important experimental animal in several fields of biomedical research. This review describes the various techniques that have been used to generate transgenic rats: classical DNA microinjection and more recently described techniques such as lentiviral vector-mediated DNA transfer into early embryos, sperm-mediated transgenesis, embryo cloning by nuclear transfer and germline mutagenesis. It will also cover techniques associated to transgenesis such as sperm cryopreservation, embryo freezing and determination of zygosity. The availability of several technologies allowing genetic manipulation in the rat coupled to genomic data will allow biomedical research to fully benefit from the rat as an experimental animal.

Long term phenobarbital administration does not promote the multiplication of hepatocytes replicating after single cyproterone acetate administration

Cyproterone acetate (CPA) is a synthetic antiandrogenic compound which is widely used in clinic but suspected to be hepatocarcinogenic. CPA is also mitogenic in rat liver. Using genetic labeling of dividing cells, we examined whether hepatocytes dividing in response to acute CPA administration could give rise to preneoplastic foci after administration of a tumor promoter: phenobarbital. CPA was administered orally to rats and dividing hepatocytes were genetically labeled using retroviral vectors carrying the beta-galactosidase gene. After labeling rats were given phenobarbital for 10 months and sacrificed. The presence of beta-galactosidase labeled hepatocytes as well as preneoplastic hepatocytes was assessed by immunohistochemistry. Genetic labeling of hepatocytes was obtained in all animals. At the end of phenobarbital administration, no hepatic tumors were observed. Preneoplastic foci were not increased in treated animals as compared to control rats. Moreover beta-galactosidase positive hepatocytes were never detected in preneoplastic foci. Finally, the size of the beta-galactosidase positive clusters was smaller in treated animals as compared to control rats. We conclude that acute CPA administration is not carcinogenic in rat liver and does not initiate preneoplastic hepatocytes capable to give rise to foci after phenobarbital promotion. Therefore the mitogenic property of CPA is distinct from its putative carcinogenic activity. Finally, analysis of the size of beta-galactosidase positive cells clusters demonstrate that phenobarbital does not induce hepatocyte proliferation in rats.

Hyperacute Lung Rejection in the Pig-to-Human Model 4: Evidence for Complement and Antibody Independent Mechanisms

BACKGROUND

We assessed whether the combination of complement regulation and depletion of xenoreactive antibodies improves the outcome of pulmonary xenografts compared with either strategy alone.

METHODS

Lungs from pigs heterozygous (hDAF(+/-)) or homozygous (hDAF(+/+)) for the human decay accelerating factor transgene (hDAF) or their nontransgenic litter mates (hDAF(-/-)) were perfused with heparinized whole human blood. In additional groups, xenoreactive natural antibodies (XNA) were depleted by pig lung perfusion (hDAF(-/-)/AbAbs, hDAF(+/-)/AbAbs) before the experiment. This combined approach was augmented by adding soluble complement receptor 1 (sCR1) to the perfusate in one further group (hDAF(+/-)/AbAbs/sCR1).

RESULTS

HDAF(-/-) lungs perfused with unmodified human blood were rejected after 32.5 min (interquartile range, IQR 5 to 210). HDAF(+/-) lungs survived for 90 min (IQR 10 to 161, P = 0.54). Both groups showed a rapid rise in pulmonary vascular resistance (PVR), which is a characteristic feature of hyperacute rejection (HAR). This phenomenon was blunted in the hDAF(+/+) group, although survival (48 min, IQR 14 to 111) was not further prolonged. Antibody depletion (AbAbs) led to a significant increase in survival time (hDAF(-/-)/AbAbs: 315 min, IQR 230 to 427; hDAF(+/-)/AbAbs: 375 min, IQR 154 to 575), reduced PVR and less complement production. Addition of sCR1 reduced complement elaboration but did not further improve survival (200 min, IQR 128 to 580) and surprisingly tended to increase PVR.

CONCLUSIONS

Depletion of xenoreactive antibodies is more effective than membrane-bound complement regulation to blunt hyperacute rejection of pulmonary xenografts, but even the combined approach including soluble-phase complement inhibition is not sufficient to reliably prevent organ failure within hours. It therefore seems likely that other factors independent of antibody and complement contribute to HAR in this model.

CHARACTERIZATION OF HUMAN CD55 AND CD59 TRANSGENIC PIGS AND KIDNEY XENOTRANSPLANTATION IN THE PIG-TO-BABOON COMBINATION1

New transgenic pigs expressing combinations of regulators of complement activation and other molecules are needed to resist xenograft hyperacute rejection (HAR) and to further analyze and treat xenograft rejection. Double transgenic pigs for human CD55 (hCD55) and human CD59 (hCD59) using the promoter of the human elongation factor 1 alpha gene were generated, and their kidneys were transplanted into nonimmunosuppressed baboons. hCD55 and hCD59 were mainly expressed by the endothelial cells, and these cells showed increased resistance to complement-mediated lysis. Baboons receiving kidneys from hCD55hCD59 pigs survived for 5 and 6 days, and displayed alterations in coagulation. Thrombocytopenia and platelet microthrombi were present within the kidneys. Nontransgenic kidneys showed HAR in less than 2 days. Kidneys from pigs expressing hCD55hCD59 displayed protection against HAR in the absence of immunosuppression. Rejection was associated with coagulopathy leukocyte infiltration and a rebound of anti-alpha Gal antibodies.

Generation of heme oxygenase-1-transgenic rats

Heme oxygenase-1 (HO-1) expression protects cells from a variety of cellular insults and inhibits inflammation. However, its role in the regulation of immune responses has not yet been clearly established. We generated HO-1 transgenic rats to directly test the impact of HO-1 on the different immune mechanisms. To temporally control the expression of HO-1, we used a one-plasmid tetracycline (tet)-inducible system. This plasmid contains the H-2K(b) promoter, which transcribes the tet transactivator (tTA) and expression of a human HO-1 cDNA is obtained in the absence of tetracycline. The DNA construct was microinjected into one-cell rat embryos and mothers and pups were maintained with tetracycline. Eight transgenic founders were obtained. Analysis of transgene expression in the absence of tet showed that 2 lines (12.4 and 12.6) expressed HO-1 mRNA in several organs (as detected by reverse transcription polymerase chain reaction) and at the protein level only in the thymus. Expression levels of transgene-derived HO-1 increased after withdrawal of tet compared with transgenic rats maintained with tet, as detected by analysis of mRNA levels by quantitative real-time reverse transcription polymerase chain reaction. Gross examination and histopathological analysis of several organs in both lines showed no anomalies. Thymocytes and splenocytes of both lines showed normal cell subpopulations and allogeneic proliferation compared with controls. Systemic immune responses against cognate antigens were normal in both lines, as evaluated by the proliferation of lymph node cells and the production of antibodies against keyhole limpet hemocyanin after immunization. Animals from line 12.6 rejected transplanted allogeneic hearts with the same kinetics as controls. In conclusion, short-term induction of HO-1 overexpression did not modify immune responses compared to those of control non-transgenic animals.

Hyperacute lung rejection in the pig-to-human model. 2. Synergy between soluble and membrane complement inhibition

BACKGROUND

The role of complement in hyperacute lung xenograft rejection has not been elucidated. The present study evaluates the effect of complement (C) C3/C5 convertase inhibition on hyperacute rejection of pig lung by human blood.

METHODS

In an established ex-vivo model, lungs from pigs heterozygous for human decay accelerating factor (hDAF), non-transgenic littermate control pigs, or farm-bred pigs were perfused with fresh human blood that was either unmodified or treated with soluble complement receptor type 1 (sCR1: TP10, 100 microg/ml).

RESULTS

Non-transgenic lungs from littermate controls had a median survival time of 35 min (range 5 to 210; P = 0.25 vs. farm-bred piglets: median 5 min, range 5 to 10). Lungs expressing hDAF survived for a median of 90 min (range 10 to 161; P = 0.5 and 0.01 vs. littermate and farm-bred controls, respectively), with sCR1, whereas hDAF (-) lungs failed by 35 min (range 6 to 307), hDAF (+) lungs survived for 330 min (range 39 to 577) [P = 0.002 vs. farm-bred; P = 0.08 vs. hDAF (-); P = 0.17 vs. sCR1/hDAF (-)]. The rise in pulmonary vascular resistance (PVR) at 5 min was blunted only by hDAF (+) with sCR1 (0.26 +/- 0.2 vs. 0.5 to 0.7 mmHg/ml/min for other groups). Plasma C3a and sC5b-9 and tissue deposition of C5b-9 were dramatically diminished using sCR1, and further decreased in association with hDAF. Histamine and thromboxane were produced rapidly in all groups.

CONCLUSION

Complement plays an important role in lung HAR. However, even potent inhibition of C3/C5 convertase, both membrane bound in lung and by a soluble-phase inhibitor in the blood, does not prevent activation of inflammatory responses known to be particularly injurious to the lung. Our findings implicate a role for innate immune pathways resistant to efficient complement regulation. The role of anti-species antibody, coagulation pathway dysregulation, and additional environmental or genetic influences remain to be defined.

A nonionic amphiphile agent promotes gene delivery in vivo to skeletal and cardiac muscles

Direct injection of naked DNA into skeletal or cardiac muscle induces detectable gene expression. Although this provides a practical system for transgene expression, the reported efficacy is too low to confer a therapeutic benefit. By following a rational strategy based on the supramolecular structures adopted by active complexes, we have discovered a novel nonionic amphiphile synthetic agent [poly(ethyleneoxide)(13)-poly(propyleneoxide)(30)-poly(ethyleneoxide)(13) block copolymer; PE6400] that enables gene expression in up to 35% of muscle fibers from mouse tibial cranial muscle. PE6400 abolishes the ceiling effect on transgene expression of increasing amounts of naked DNA and permits long-term expression of the beta-galactosidase reporter gene in immunologically tolerant transgenic rats. This improvement in gene expression over naked DNA was observed irrespective of the reporter gene, ranging from 0.7 to 3.4 kb, and of the animal model used. In skeletal muscle, the PE6400 formulation led to a level of transfection efficiency similar to that obtained by electrotransfer. PE6400 also promotes high transgene expression in cardiac muscle. In contrast, PE6400-DNA formulations were inefficient in vitro in established cell lines and in isolated cardiomyocytes. When microinjected into the cell cytoplasm, PE6400 promotes DNA trafficking into the nucleus and induces gene expression. PE6400 provides a simple gene delivery system for skeletal and myocardial gene transfer. We propose that the PE6400 formulation could serve for the treatment of diseases primarily affecting muscle or for the expression of therapeutic proteins for local or systemic benefit.

lacZ transgenic rats tolerant for beta-galactosidase: recipients for gene transfer studies using lacZ as a reporter gene

Gene transfer of reporter genes may trigger immune responses against the heterologous protein resulting in shortening of gene expression and inflammation. We generated transgenic rats expressing the lacZ gene under the control of the human immunodeficiency virus type 1 (HIV-1) long-terminal repeat (LTR) (HIV-lacZ) to obtain rats with undetectable transgene expression using histologic methods, thus avoiding interference with beta-galactosidase (beta-gal) expression from gene transfer, and displaying immune tolerance toward beta-gal. LacZ transgenic mice with tolerance toward beta-gal have already been used for gene transfer but rats constitute unique animal models with several advantages compared to mice. Two transgenic lines displayed low levels of beta-gal mRNA in most organs tested, as detected only by reverse transcription-polymerase chain reaction (RT-PCR). The protein was undetectable by immunohistology and was only detected in the thymus and spleen using a sensitive enzyme-linked immunosorbent assay (ELISA). HIV-lacZ transgenic rats displayed immune tolerance to beta-gal because immunization with beta-gal resulted in markedly lower cellular and antibody responses compared to wild-type controls, whereas immunization with a nonrelated antigen, keyhole limpet hemocyanin (KLH), resulted in comparable immune responses. The usefulness of this model in gene transfer was tested using a retroviral vector, which does not elicit destructive immune responses against transduced cells. Retroviral-mediated nlslacZ gene transfer in the liver resulted in nuclear beta-gal expression for longer than 12 months in HIV-lacZ transgenic rats, whereas wild-type controls showed nuclear beta-gal expression for less than 1 month. After gene transfer of nlslacZ to the liver, antibodies, cytotoxic T lymphocytes (CTLs), and proliferation against beta-gal were detected in wild-type controls but not in HIV-lacZ transgenic rats. In conclusion, HIV-lacZ transgenic rats displaying low beta-gal expression and immune tolerance toward beta-gal are a useful tool to analyze the spatial and temporal expression of the beta-gal protein in gene transfer experiments using lacZ as a reporter gene.

Cytotoxic immune response blunts long-term transgene expression after efficient retroviral-mediated hepatic gene transfer in rat

Vectors derived from oncoretroviruses can transduce a small proportion of hepatocytes when injected in the regenerating liver. Transgene expression may be sustained for months without immune response. In striking contrast, we observed a rapid extinction when the intravenous injection of a high input of nuclear beta-galactosidase (beta-gal) expression vector, one day after partial hepatectomy, led to a significant proportion of transduced cells in the liver. Extinction was associated with liver inflammation on tissue sections and appearance of antibodies against the transgene product, while vector genomes became undetectable in liver tissue by PCR. These observations suggested the elimination of transduced cells by an immune response. Transgenic rats tolerant for cytoplasmic beta-gal, or normal rats depleted in CD8 T lymphocytes, steadily expressed the beta-gal vector. In the spleen of normal rats, we detected cytotoxic cells directed against cells expressing beta-gal after the injection of the beta-gal vector. In jaundiced Gunn rats deficient in bilirubin glucuronosyl transferase (BGT1) and treated with a human BGT1 cDNA expression vector, we observed the same kinetics of extinction as well as the appearance of anti-BGT1 antibodies. This study demonstrates that retrovirus-mediated gene transfer may induce cytotoxic T lymphocytes specifically directed against transgene-expressing cells.

CD97-decay-accelerating factor interaction is not involved in leukocyte adhesion to endothelial cells

BACKGROUND

Effective improvement in xenograft survival is achieved using transplants from transgenic pigs expressing human complement (C) regulatory proteins, including decay-accelerating factor (DAF), CD59, and CD46 on endothelial cells (ECs). The aim of this study was to investigate whether human DAF expression in porcine ECs, as well as regulating C activation, can modify intercellular events through its interaction with its receptor, CD97, on human leukocytes.

METHODS

Cellular interactions between human leukocytes and porcine ECs were investigated in vitro using ECs from either wild-type or DAF-transgenic pigs. Static leukocyte adhesion and T cell activation assays were performed using porcine ECs as target or effector cells, respectively. The role of the DAF-CD97 interaction was investigated using specific blocking monoclonal antibodies (mAbs) against human DAF and its receptor, CD97, in adhesion assays.

RESULTS

Adhesion of U937 or Jurkat T cells, both expressing human DAF and CD97, was quantitatively similar for wild-type and transgenic-DAF-expressing pig ECs. Furthermore, blocking the CD97-DAF interaction did not inhibit xenogeneic leukocyte-endothelium adhesion, whereas blocking the very late antigen 4-vascular cell adhesion molecule-1 pathway reduced this adhesion by 50-80%. Furthermore, DAF and CD97 expression was not up-regulated during tumor necrosis factor-alpha- or lipopolysaccharide-mediated EC activation, unlike the adhesion molecules E-selectin, vascular cell adhesion molecule-1, and intracellular adhesion molecule (ICAM)-1.

CONCLUSION

We found that high levels of human DAF expressed on ECs abrogates C-mediated cell damage but did not affect the in vitro adhesive properties or antigen-presenting cell function of genetically modified porcine ECs.

Rapid and accurate determination of zygosity in transgenic animals by real-time quantitative PCR

Successful identification of homozygous and heterozygous transgenic animals with currently available techniques demands tedious and time-consuming procedures with a high proportion of ambiguous results. Real-time PCR is a quantitative and extremely precise method with high throughput that could be applied to the analysis of large numbers of animals differing only by a factor of two in the amount of target sequences. We defined the technical conditions of real-time PCR to co-amplify a transgene and a reference gene using two fluorogenic probes and the comparative cycle threshold method. We applied these conditions to the analysis of zygosity in a line of transgenic rats. Real-time PCR allowed clear-cut identification of all transgenic animals analysed (n = 45) as homozygous or heterozygous. Southern blot analysis of these animals using an internal quantitative control and PhosphorImager quantification showed ambiguous results in six of them and was concordant with real-time PCR in the rest. Mating of homozygous and heterozygous animals, as defined by real-time PCR, showed transgene transmission to the offspring following expected Mendelian laws. Real-time PCR allows rapid, precise, non-ambiguous and high throughput identification of zygosity in transgenic animals. This technique could be helpful in the establishment of breeding programs for transgenic colonies and in experiments in which gene dosage effects could have a functional impact.

In vivo cell lineage analysis in cyproterone acetate-treated rat liver using genetic labeling of hepatocytes

Genetic labeling using recombinant retroviruses is a powerful strategy for the study of cell lineage in the liver. However, this type of vector is only able to infect dividing cells. The synthetic steroid cyproterone acetate (CPA) is mitogenic and carcinogenic in the adult rat liver. In this study, we used retroviral vectors carrying the nuclear targeted beta-galactosidase gene to selectively label and follow the fate of hepatocytes dividing on administration of CPA. Labeled cells as well as those in mitosis were preferentially located around the portal tract, whereas apoptotic bodies were predominant in the pericentral area. Labeled hepatocytes did not disappear after apoptosis, suggesting a preferential elimination of nontransduced cells. The presence of labeled binucleated hepatocytes showed the persistence of a binucleation process. Finally, we performed long-term analysis of labeled cells in transgenic animals tolerant for beta-galactosidase and treated with 2-acetylaminofluorene (2-AAF) to promote the growth of CPA-initiated hepatocytes. The presence of beta-galactosidase-positive hepatocyte clones showed that hepatocytes divided during treatment with 2-AAF. Only 3% of beta-galactosidase clones were positive for the placental form of glutathione S-transferase (GSTp), indicating the absence of a preferential appearance of preneoplastic foci in the population of beta-galactosidase-labeled hepatocytes. In conclusion, our results show that the mitogenic and tumor-initiating activities of CPA are directed toward different hepatocyte populations.

Contrast in the efficacy of hDAF mouse hearts between ex vivo perfusion and transplantation into primates

In recent experiments, in which we compared hDAF transgenic rat hearts perfused with 15% human serum in the Langendorff device and hDAF rat hearts transplanted into cynomolgus monkeys, we demonstrated that in the ex vivo heart perfusion model both homozygous and heterozygous hDAF hearts survived longer as nontransgenic controls. Surprisingly, we found that only homozygous hDAF hearts were protected against hyperacute rejection in vivo. The first aim of this study was to determine whether perfusion of mouse hearts with higher human serum concentrations or human blood might explain some of the differences found in survival time of the recently performed experiments with rat heart xenografts. Secondly, we investigated whether the observed differences in survival times of rat xenografts between in vivo and ex vivo transplantation would also hold for mouse hearts transgenic for hDAF. An ex vivo model was used to perfuse hDAF mouse hearts and controls with human serum or blood, and hDAF transgenic hearts and controls were transplanted into cynomolgus monkeys. hDAF transgenic mouse hearts survived significantly longer than their controls when perfused with 15% human serum, but no difference was found when 30% human serum was used, or when these hearts were transplanted into cynomolgus monkeys. However, in both the in vivo and ex vivo models the amount of PMNs adhering to the vascular endothelium was significantly lower in hDAF transgenes as compared with their controls. In conclusion, in the ex vivo situation, the efficacy of hDAF transgenesis in preventing HAR is limited by serum complement concentration.

Antigraft antibody-mediated expression of metalloproteinases on endothelial cells. Differential expression of TIMP-1 and ADAM-10 depends on antibody specificity and isotype

Endothelial cell (EC) interaction with antigraft antibodies (Abs) mediates EC injury and activation involved in vascular graft rejection. The aim of this study was to identify EC genes regulated in response to antigraft Ab binding that contribute to the endothelium alterations implicated in graft rejection or survival. By means of RNA differential display, 13 cDNA fragments corresponding to genes differentially expressed in ECs incubated with antigraft Abs were identified. Among these cDNAs were found the tissue inhibitor of metalloproteinase-1 (TIMP-1) and a desintegrin and metalloproteinase (ADAM-10). We demonstrated that TIMP-1 and ADAM-10 mRNA and protein expression was rapidly upregulated in ECs in response to antigraft Ab binding. Our data showed that TIMP-1 was upregulated in response to human IgG but not IgM and anti-galactosyl (Gal) alpha1-3Gal human xenogeneic Abs. In contrast, upregulation of ADAM-10 in ECs was shown to be mostly mediated by anti-Galalpha1-3Gal IgM Abs. Specific effects of human IgG and IgM xenogeneic Abs on endothelial transcripts indicate that different isotypes and specificities of Abs may mediate different EC changes. Our results suggest that interaction of ECs with antigraft Abs, according to their specificity, selectively induces synthesis and release of metalloproteinases and inhibitors, controlling proteolytic processes and immunological events that respectively contribute to graft rejection or survival.

Evidence for distinct complement regulatory and measles virus binding sites on CD46 SCR2

Human CD46, or membrane cofactor protein, is a regulator of complement activation and is used as a cellular receptor by measles virus. Using a series of 13 single point mutants, the region of short consensus repeat (SCR) 2 domain involved in the regulation of complement activation was mapped to residues E84, N94, Y98, E102, E103, I104 and E108. Molecular modelling localized all residues, with the exception of E84, close to each other on the external lateral face of the molecule, away from the residues important for the binding of measles virus, which are localized on the top of the molecule. The E84 residues is localized in the SCR1-2 hinge and the deleterious effect of its substitution by an alanine residue could affect the relative orientation and / or tilt of SCR1 on SCR2. Taken together, the results suggest that the measles virus binding and cofactor activity of CD46 map to distinct areas on the SCR2 module.

Animal models in xenotransplantation

The severe shortage of donor organs has provided a strong impetus to push the investigation into the use of animal organs for humans. Xenotransplantation will not only benefit patients, but also represents a unique and potentially profitable business opportunity. However, there are many barriers to successful clinical xenotransplantation, including immunological barriers, physiological incompatibility, zoonosis and ethical concerns. This overview will focus on currently available animal models used in attempts to break through the immunological barriers to xenotransplantation. There are many advantages to using small animal, namely rodent, models in xenotransplantation research. For example, the use of the mouse model allows the use of knockout mice and careful dissection of rejection mechanisms at the molecular level. The following models can be used to study hyperacute rejection (HAR): guinea-pig-to-rat, mouse-to-rabbit, guinea-pig-to-mouse, rat-to-presensitised mouse and rat-to-alpha-Gal knockout mouse. The hamster-to-rat, mouse-to-rat and rat-to-mouse models are commonly used to study acute vascular rejection. Large animal models are complex and expensive, but they are more relevant to clinical xenotransplantation. Based on experiments using transgenic pig-to-primate models, HAR can be overcome. However, acute vascular rejection remains a major barrier at the present time. A pig cartilage-to-monkey model has been developed to study chronic rejection. Other novel models such as pig venous segment-to-monkey model and rat-to-primate model may represent viable options to study immunological barriers following xenotransplantation. Like many other medical breakthroughs, animal research will continue to make enormous contributions towards the eventual success of xenotransplantation.