Manipulation of xenogeneic skin and/or renal graft survival in the rat-mouse concordant combination by portal vein pretransplant transfusion

Small animal models of xenotransplantation

Organ transplantation has become a successful and acceptable treatment for end-stage organ failure. Such success has allowed transplant patients to resume a normal lifestyle. The demands for transplantation have been steadily increasing, as more patients and new diseases are being deemed eligible for treatment via transplantation. However, it is clear that human organs will never meet the increasing demand of transplantation. Therefore, scientists must continue to pursue alternative therapies and explore new treatments to meet the growing demand for the limited number of organs available. Transplanting organs from animals into humans (xenotransplantation) is one such therapy. The observed enthusiasm for xenotransplantation, irrespective of the severe shortage of human organs and tissues available for transplantation, can be said to stem from at least two factors. First, there is the possibility that animal organs and tissues might be less susceptible than those of humans to the recurrence of disease processes. Second, a xenograft might be used as a vehicle for introducing novel genes or biochemical processes which could be of therapeutic value for the transplant recipient.To date, millions of lives have been saved by organ transplantation. These remarkable achievements would have been impossible without experimental transplantation research in animal models. Presently, more than 95% of organ transplantation research projects are carried out using rodents, such as rats and mice. The key factor to ensure the success of these experiments lies in state-of-the art experimental surgery. Small animal models offer unique advantages for the mechanistic study of xenotransplantation rejection. Currently, multiple models have been developed for investigating the different stages of immunological barriers in xenotransplantation. In this chapter, we describe six valuable small animal models that have been used in xenotransplantation research. The methodology for the small animal model establishment includes animal selection, preoperative care, anesthesia, postoperative care, and detailed procedures.

Intrasplenic preconditioning: a model for the study of xenostimuli accommodation

BACKGROUND

Discordant xenotransplantation, the grafting of organs from one phylogenic species to another, results in hyper-acute rejection (HAR). HAR is associated with the deposition of recipient preformed xenoreactive natural antibodies and complement on the endothelium of the donor organ, leading to activation and apoptosis of the endothelium, an event associated with xenograft rejection. Endothelial resistance to HAR, termed "accommodation," an active protection of graft endothelium, may be achieved by previous stimulation of endothelial cells by discordant xenoantibodies.

MATERIALS AND METHODS

Forty-eight male Wistar rats were used to evaluate HAR induction in an isolated, dually perfused in-situ rat liver transfused with human blood. This ex-vivo model served to mimic rat-to-human liver xenotransplantation. Preconditioning of the liver endothelium was induced by rat intrasplenic injection of human blood (n=8) or effluent of previously xenotransfused rat liver (n=8), i.e., high versus low xenoantibody solution, each undertaken 1d before liver xenotransfusion. Two other groups were not preconditioned. Preconditioned and non-preconditioned rats were perfused directly with human blood, and eight rats were used as controls (non-preconditioned Krebs-perfused). Eight rats were perfused directly with human blood, and eight rats were used as controls. The effluent that exited these first-line livers was used to perfuse the second-line livers.

RESULTS

Portal and hepatic artery perfusion pressures, resistances, rates of oxygen extraction, lactic acid and pH, and wet-to-dry weight ratio values were significantly increased in livers xenotransfused with blood indicating HAR, compared with unchanged values in livers perfused with Krebs solution. Portal pressure and resistance were best protected from HAR by the blood preconditioning in the blood perfused group, while the hepatic artery perfusion system was better protected by the perfusate precondition-blood perfused group. The physiologic effects of HAR were attenuated in most second-line livers.

CONCLUSIONS

Attenuation of HAR in rats' livers is achieved by preconditioning with xenoantibodies and/or by "filtering out" xenoantibodies present in the circulation, and is suggestive of accommodation. This novel method may be useful in future studies aimed at refining methods for accommodating xenotransplantation.

Cell death recognition model for the immune system

It is essential for the immune system to recognize markers or understand rules required for discriminating antigens that should be actively responded to from those be tolerated. Although the classic self-nonself theory over the past five decades has been challenged by "danger" model and "infectious nonself" model, etc., no theories could fit for all. Cell death is important not only for its role in homeostasis, but also for its decisive effects on the immune responses. Different ways of cell death, apoptosis or necrosis, transmit fundamentally opposite driving forces for the immune system, inducing tolerance or initiating adaptive immune responses. The progress in understanding phagocytosis and process of apoptotic and necrotic cells leads the author to propose "cell death" recognition model for the immune system. Four principles are important in this model. First, only antigens shedding from apoptotic or necrotic cells rather than those from healthy cells, can be presented to naïve T cells. Second, either apoptotic cells or necrotic cells, but not healthy cells, can attract phagocytes, namely dendritic cells (DC) or macrophages that are also antigen presenting cells (APC), to scavenge dead cells. Third, macrophages or DC residing in non-lymphoid tissues phagocytose dying/dead cells, migrate to lymphoid tissues and present antigens to naïve T cells there. Fourth, tolerance or adaptive responses are not dependent on whether the antigens are self or nonself, but on the ways of cell death during antigen presentation. Importantly, tolerance and adaptive immunity are all dominant responses and the impact of cell death on immune responses is a dynamic balance between them. "Cell death" recognition model could more easily explain various immune phenomena, including infection, self tolerance and autoimmunity, tumor immunity as well as transplant rejection. Investigation into the roles and mechanisms of cell death mediated immune responses and finding out key modulators will prompt better understanding the ways of immune recognition and provide novel strategies for the management of autoimmunity, tumors, infections as well as transplantation.

Rat genotype influences quantitative and qualitative aspects of xenogeneic immune response to pig blood mononuclear cells

BACKGROUND

Xenotransplantation seeks to have cells with discordant genotypes co-exist. The hypothesis that host genotype modulates xenogeneic immune response (IR) was tested.

METHODS

Two inbred rat strains [Dark Agouti (DA) and Lewis (LEW)] representing diverse IR phenotypes were immunized with porcine blood mononuclear cells (BMC). Delayed-type hypersensitivity (DTH), immunoglobulin (Ig), antibody (Ab) and isotype bias of Ab response were evaluated.

RESULTS

DTH to pig BMC was greater in DA than in LEW rats. Natural Ab was qualitatively different between strains (IgM and IgA predominated in DA, IgM and IgG(2a) predominated in LEW). Twice as much IgG was elicited from DA than LEW rats and DA utilized all isotypes whereas LEW did not use IgG(2a) or IgG(2c). IR bias was diametrically opposed; type 1 in DA but type 2 in LEW. Strains even differed in Ig profiles and dermal responses to saline injections and mitogen. The DA rats were the higher responders to pig BMC.

CONCLUSIONS

Recipient genotype had significant and broad effects on IR to porcine BMC and may influence xenograft rejection and xenotolerance induction. Moreover, this study suggests that caution should prevail when interpreting data derived from a single inbred strain, particularly given that humans, the target species, are genetically diverse.

In Vivo Recognition by the Host Adaptive Immune System of Microencapsulated Xenogeneic Cells

BACKGROUND

Microencapsulation is under consideration as a means of enabling pancreatic islet transplantation. To understand better the ongoing destructive host response, we examined whether the adaptive immune system of the recipient recognized polymer-encapsulated xenogeneic cells implanted intraperitoneally.

METHODS

Balb/c mice were implanted with xenogeneic Chinese hamster ovary cells, inside and outside poly(hydroxyethyl methacrylate-methyl methacrylate) microcapsules, and responses were compared with xenografted Chinese hamster skin (positive control). Capsules were localized within an agarose rod. Splenocyte proliferation upon rechallenge in vitro, antibody titer in serum, and Th1/2 polarization (assessed by interleukin-4 and interferon-gamma in supernatants of antigen-challenged splenocytes and immunoglobulin [Ig]G1 and IgG2a antibody isotypes in serum) were measured.

RESULTS

Encapsulation did not prevent a strong recipient antibody response. Splenocyte proliferation in vitro did not differ after priming by implanted cells, inside or outside capsules. Thus, the capsule membrane did not prevent indirect recognition of shed antigens. However, after 10 days of implantation, proliferation was lower than that induced by skin grafts, although this difference disappeared by 2 months. This transient T-cell suppression was unexpected because encapsulated cell viability was already compromised by 10 days. The influence of Th1/2 bias did not explain the observed suppression. Cells inside capsules elicited a consistent Th2 response, whereas cells outside capsules elicited a mixed response, and skin xenografts showed an initial Th2 response that became mixed by 2 months.

CONCLUSIONS

Encapsulation does not prevent host immune responses, but the inflammatory response to the implanted biomaterials or xenogeneic cells may be responsible both for encapsulated cell death and transient T-cell suppression.

Apoptosis: the quiet death silences the immune system

Apoptosis plays an essential role in maintaining cellular homeostasis during development, differentiation, and pathophysiological processes. In the immune system, recent investigations reveal that during the course of T-cell development in the thymus, negative selection of autoreactive immature T-cells is a typical apoptotic process. In addition, apoptosis is also involved in cytotoxic killing of target cells and the regulation of lymphocyte homeostasis during immune responses. Interestingly, recent evidence has suggested that cells dying by apoptosis are actively involved in immunosuppression in various circumstances. We have shown that apoptotic cells could inhibit the expression of CD69 during T-cell activation. Furthermore, apoptotic cells phagocytosed by macrophages and/or dendritic cells are immunosuppressive, a process likely mediated by the production of transforming growth factor-beta1. Since apoptosis is a common mechanism by which excessive cells in many tissues and organs are eliminated in various pathophysiological processes, we believe that further investigation into the mechanisms by which apoptotic cells affect the immune system will not only lead to a better understanding of the significance of apoptosis during immune responses, but will also provide novel strategies for the management of autoimmune diseases and transplantation.

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.

Immunomodulatory effects of pretransplant donor blood transfusion on T-cell-independent xenoreactive immunity

BACKGROUND

Pretransplant blood transfusions have beneficial effects on both clinical and experimental allograft survival. In the present study, we examined whether pretransplant hamster blood transfusions (pHBT) alone or together with peritransfusion immunosuppressive strategies designed to target B cells and/or natural killer (NK) cells, could modulate T cell-independent (T-I) xenoreactivity in athymic nude rats.

METHODS

Hamster or mouse hearts were heterotopically xenotransplanted into untreated or treated athymic nude rats receiving either pHBT, anti-B cell or anti-NK cell therapy alone or their combinations. Xenoreactive antibodies (xAbs) and the percentage of NK cells were analyzed by FACScan analysis. NK cytotoxicity was measured by a standard 4 hr 51Cr release assay. Xenografts (Xgs) were examined by hematoxylin-eosin (H&E), by light microscopic method with Masson's trichrome and orcein staining, by immunofluorescent staining for immunoglobulin M and C3 deposition, and by immunohistochemical staining for infiltration of NK cells and macrophages (Mphis).

RESULTS

In 1 of 6 rats given pHBT alone 2 weeks before receiving hamster xenografts, Xg survival was prolonged to 55 days compared with 3.0+/-1.2 days in the other 5 animals and with 3.0+/-0.6 days in untreated animals. In the 55 days, surviving Xg infiltration of Mphis and NK cells was seen together with severe signs of chronic rejection, such as fibrosis and obliterative vasculopathy. The addition of the anti-B cell immunosuppressant MNA715 (malononitriloamide x920715, 20 mg/kg/day) from day -14 to day +14 or of 100 microL of rabbit anti-asialo GM1 serum ([anti-ASGM1] an NK cell depleting antibody) on day -14 resulted in a significant and species-specific prolongation of the survival of hamster Xgs, respectively 59.8+/-9.6 days and 58.2+/-14.7 days (P<0.001 vs. control group), but not of mouse heart Xgs that were rejected in a normal tempo. All prolonged hamster Xgs were infiltrated with Mphis and NK cells and developed severe lesions of chronic rejection, such as fibrosis and obliterative vasculopathy. In contrast, MNA715 or anti-ASGM1 alone had no effect on Xg survival (4.8+/-1.7 days and 2.7+/-0.6 days, respectively). Combined MNA715/anti-ASGM1 treatment only moderately promoted Xg survival (10+/-5.0 days; P<0.001). A simultaneous administration of pHBT, MNA715, and anti-ASGM1 induced indefinite and species-specific Xg survival in all recipients. In vivo and in vitro studies demonstrated that both T-I B cell and NK cell species-specific xenotolerance were achieved.

CONCLUSIONS

Pretransplant blood transfusion may have a species-specific immunomodulatory effect on T-I xenoreactivity. This effect is further enhanced by a temporary co-administration of MNA715 or by a single injection of anti-ASGM1. A combination of pHBT, MNA715, and anti-ASGM1 induces species-specific T-I xenotolerance.

Alterations in chemokine mRNA expression in animals receiving portal vein immunization and renal allo- or xenotransplantation precede altered cytokine production

We have analyzed chemokine mRNA expression in graft tissue of C3H/HEJ mice receiving allogeneic (C57BL/6) or xenogeneic [Lewis (LEW) rat donors] kidney grafts and correlated this with graft survival. Since donor-specific portal vein (pv) immunization is known to increase allo- and xenograft survival, in some cases recipients also received pretransplant pv or intravenous (iv) immunization; other animals received the antioxidant N-acetylcysteine (NAc) to examine the role of ischemia/reperfusion injury in the changes observed. Graft tissue and lymph nodes draining the respective grafts were obtained at various times posttransplantation and used for quantitative polymerase chain reaction analysis of mRNAs for different chemokines. In addition, lymphocytes were restimulated in culture with donor antigen and supernatants assayed for different cytokines. We observed that early increases in mRNA for MCP-1 preceded a polarization to type 2 cytokine production. Infusion of NAc twice daily for 4 days following transplantation further altered chemokine mRNA expression (increased MCP-1 and RANTES; decreased CINC); led to more enhanced type 2 cytokine production relative to control animals; and further increased xenograft survival. By use of heteroantibodies to different chemokines, anti-MCP-1 alone, but not antibodies to MIP-1alpha or RANTES, abolished this early polarization in cytokine production, implying a causal link between MCP-1 production and polarization in cytokine production. We conclude that manipulation of chemokine production early after transplantation might indirectly modify graft outcome by modifying cytokine production.

An Immunoadhesin Incorporating the Molecule OX-2 Is a Potent Immunosuppressant That Prolongs Allo- and Xenograft Survival

We have established that, in mice receiving donor-specific immunization by the portal vein, the increased graft survival seen is associated with the increased expression of a molecule (OX-2) on a subpopulation of dendritic cells (DC), and polarization of cytokine production to type 2 cytokines on Ag-specific restimulation of cells from these mice. Furthermore, infusion of a mAb to OX-2 blocks both the increased graft survival and the altered cytokine production seen. We have constructed an immunoadhesin in which the extracellular domain of OX-2 is linked to the murine IgG2a Fc region, and we have expressed this molecule (OX-2:Fc) in a eukaryotic (baculovirus) expression system. Incubation of lymphocytes with 50 ng/ml OX-2:Fc inhibits a primary mixed lymphocyte reaction in vitro, as assayed by proliferation and induction of cytotoxic T cells, and also alters cytokine production with decreased IL-2 (IFN-γ) production and increased IL-4 (IL-10) production. Similarly, in vivo infusion of OX-2:Fc promotes increased allo- and xenograft (both skin and renal grafts) survival and decreases the Ab response to sheep erythrocytes. Our data suggest this molecule might have clinical importance in allo- and xenotransplantation.

Down-modulation of host reactivity by anti-CD44 in skin transplantation

BACKGROUND

A major goal in transplantation medicine is to achieve donor-specific tolerance while sustaining unaltered immunoreactivity toward donor-independent stimuli. Pretransplant immunization and concomitant blockade of costimulatory molecules may be one way to achieve this goal. We investigated whether transplant acceptance could be achieved by sensitization with semiallogeneic blood and blockade of CD44s (standard isoform) or CD44v6 (variant exon 6), since the adhesion molecule CD44 is known to function as a costimulatory molecule in T-cell activation.

METHODS

Immunoregulatory regimens were examined in BDX rats that had received full-thickness (DA x BDX)F1 skin grafts by controlling graft acceptance and immunoreactivity.

RESULTS

When BDX rats received full-thickness (DA x BDX)F1 skin grafts together with either anti-CD44s or anti-CD44v6, graft rejection was delayed, but none of the animals accepted the graft. An analysis of immunoreactivity revealed reduced numbers of infiltrating lymphocytes in anti-CD44s- as well as anti-CD44v6-treated rats. Expansion of donor-specific helper and cytotoxic T cells was particularly impaired in anti-CD44v6-treated rats. The effect of anti-CD44s could not be intensified by presensitization with donor-derived blood. However, when rats received anti-CD44v6 concomitantly with presensitization, 75% permanently accepted the graft and 50% accepted a second graft provided they were continuously treated with anti-CD44v6 and received a low dose of cyclosporine (CsA) during the first weeks after grafting. The frequency of graft-reactive helper T cells was reduced to less than 10% of the level in controls, and cytotoxic T cells could hardly be detected.

CONCLUSION

According to the in vivo and the vitro analyses of the graft and the draining lymph nodes, anti-CD44s blocked homing of activated lymphocytes into the graft, while anti-CD44v6 inhibited clonal expansion of donor-specific T cells. Suppression by anti-CD44v6 apparently functioned distinctly to cyclosporine and was most effective in combination with presensitization. Since expression of CD44v6 on lymphocytes is restricted to a short period during lymphocyte activation, anti-CD44v6 treatment could lead to a quite specific immunosuppression during a limited time period.

Mechanism of concordant corneal xenograft rejection in mice: synergistic effects of anti-leukocyte function-associated antigen-1 monoclonal antibody and FK506

BACKGROUND

The mechanisms of corneal xenogeneic immunoreaction, as well as the potential role of immunosuppressive therapy in the suppression of corneal xenograft rejection, have not been thoroughly explored.

METHODS

BALB/c mice who received orthotopic corneal transplants (Lewis rats donors) were administered intraperitoneally anti-leukocyte function associated antigen-1 (LFA-1) monoclonal antibody (mAb) or FK506 (3 mg/kg/day) or both of these immunosuppressants during a 12-day postoperative period. Histological (hematoxylin-eosin stain) and immunohistochemical evaluations of enucleated eyes were performed. Humoral immune response and delayed-type hypersensitivity (ear-swelling assay) were evaluated.

RESULTS

The mean (+/-SD) graft survival time in the untreated control, FK506-treated, anti-LFA-1 mAb-treated, and combined-treatment groups was 5.8+/-0.8, 9.4+/-4.0, 8.7+/-5.0, and 67.7+/-16.4 days, respectively. In the untreated control group, mouse IgG, IgM, and C3 were expressed on the rat corneal grafts during the early postoperative phase. Flow cytometry studies revealed high titers of xenoreactive IgG and IgM antibodies. T helper 1 cytokines were expressed on xenografted corneal beds, and delayed-type hypersensitivity was induced. However, local expression of IgM, C3 and T helper 1 cytokines, serum antibodies of IgG and IgM, and delayed-type hypersensitivity were suppressed in the anti-LFA-1 mAb- plus FK506-treated group.

CONCLUSIONS

Both humoral and cell-mediated immune reaction play an important role in the initial rejection in rat-to-mouse corneal xenotransplantation. The treatment with anti-LFA-1 mAb in combination with FK506 synergistically suppresses concordant corneal xenogeneic reaction.