Application of cyclophosphamide-induced tolerance in α1,3-galactosyltransferase knockout mice presensitized with Galα1-3Galβ-4-GlcNAc antigens

PURPOSE: Hyperacute rejection (HAR) mediated by the natural antibody (nAb) against Galalpha1-3Galbeta-4-GlcNAc (alphaGal) is the major obstacle in xenogeneic organ transplantation. Previously, we reported the acceptance of donor heart grafts in anti-alphaGal nAb-producing galactosyltransferase knockout (GalT KO) mice after cyclophosphamide (CP)-induced tolerance conditioning. In the present study, we applied our tolerance induction conditioning in presensitized recipient mice. METHODS: GalT KO (alphaGal(-/-), H-2(b/d)) recipient mice were presensitized with alphaGal(+) rabbit red blood cells (RRBCs). Presensitized or nonsensitized recipient mice were treated with CP-induced tolerance conditioning, consisting of AKR (alphaGal(+/+), H-2(k)) spleen cells (SC), CP, busulfan (BU), and AKR bone marrow cells (BMC). We assessed the survival of donor hearts and skin grafts and analyzed the production of anti-alphaGal Abs by flow cytometry. RESULTS: Donor mixed chimerism was achieved in the presensitized GalT KO mice treated with CP-induced tolerance conditioning. In parallel with the disappearance of anti-alphaGal Abs, permanent acceptance of donor heart grafts and skin grafts was observed in presensitized and GalT KO mice treated with CP-induced tolerance conditioning. CONCLUSIONS: Both B-cell and T-cell tolerance was achieved in the presence of a higher titer of anti-alphaGal Abs after treatment with CP-induced tolerance conditioning.     

Fetal porcine thymus engraftment,survival and CD4 reconstitution in alphaGal-KO mice is impaired in the presence of high levels of antibodies against alphaGal

Xenospecific T-cell tolerance can be induced among murine and human T-cells by porcine thymic grafting. However, anti-alpha 1,3-galactosyltranserase (alphaGal) (Galalpha1-3Galbeta1-4GlcNAc-R) natural antibodies (NAbs) pose a major barrier to porcine xenografts in humans. We used alphaGal knockout (KO) and muchain KO mice to explore the effect of natural anti-alphaGal and other xenoantibodies on porcine thymic engraftment and to examine the potential of thymic tissue to tolerize anti-alphaGal antibody-producing cells. Thymectomized [adult thymectomy (ATX)] non-immunized and rabbit red blood cell (RRBC) pre-transplant immunized alphaGal-KO (knockout), wild-type (WT) and mu chain KO B6 mice were treated with 3Gy total body irradiation (TBI), and T and natural killer (NK) cell depleting monoclonal antibodies (mAbs). These conditioned mice were grafted with fetal porcine thymus and liver (FP THY/LIV) tissue under the kidney capsule. Flow cytometric analysis was performed to follow CD4 reconstitution as a measure of FP THY engraftment and function. Only mice with >10% CD4+ peripheral blood lymphocytes (PBL) were considered successfully engrafted. Enzyme-linked immunosorbent assay (ELISA) was used to assess the kinetics of immunoglobulin M (IgM) and IgG anti-alphaGal antibodies. Anti-pig antibodies were monitored by flow cytometry (FCM). FP THY engrafted successfully in most of the immunoglobulin deficient mice (11 out of 12, 92%) and the outcome was similar in WT B6 controls (8 out of 12, 67%). Non-immunized alphaGal-KO mice grafted with FP THY had a similar success rate (7 out of 11) to that observed in non-immunized alphaGal-WT controls (2 out of 4). In contrast, alphaGal-KO mice immunized pre-transplant with RRBC, then grafted with FP THY/LIV, showed a significant reduction in the success of thymic grafting (2 out of 9, 22%) compared with pre-transplant immunized WT controls (4 out of 7; 57%) and non-immunized alphaGal-KO mice (7 out of 11, 64%). Anti-Gal and anti-pig antibody levels were not markedly augmented by porcine thymus grafts in mice with successful thymus grafts. FP THY engraftment is impaired in the presence of high levels of anti-alphaGal xenoantibodies. However, low levels of anti-alphaGal antibodies and other mouse anti-pig NAbs appear not to play a major role in the rejection of FP THY. Although grafting FP THY expressing the alphaGal epitope did not tolerize B cells producing anti-alphaGal antibodies in a T-cell independent manner, it prevented T-cell dependent sensitization by inducing T-cell tolerance to porcine antigens.     

Rejection of Cardiac Xenografts Transplanted from α1,3‐Galactosyltransferase Gene‐Knockout (GalT‐KO) Pigs to Baboons

The use of α1,3‐galactosyltransferase gene‐knockout (GalT‐KO) swine donors in discordant xenotransplantation has extended the survival of cardiac xenografts in baboons following transplantation. Eight baboons received heterotopic cardiac xenografts from GalT‐KO swine and were treated with a chronic immunosuppressive regimen. The pathologic features of acute humoral xenograft rejection (AHXR), acute cellular xenograft rejection (ACXR) and chronic rejection were assessed in the grafts. No hyperacute rejection developed and one graft survived up to 6 months after transplantation. However, all GalT‐KO heart grafts underwent graft failure with AHXR, ACXR and/or chronic rejection. AHXR was characterized by interstitial hemorrhage and multiple thrombi in vessels of various sizes. ACXR was characterized by TUNEL⁺ graft cell injury with the infiltration of T cells (including CD3 and TIA‐1⁺ cytotoxic T cells), CD4⁺ cells, CD8⁺ cells, macrophages and a small number of B and NK cells. Chronic xenograft vasculopathy, a manifestation of chronic rejection, was characterized by arterial intimal thickening with TUNEL⁺ dead cells, antibody and complement deposition, and/or cytotoxic T‐cell infiltration. In conclusion, despite the absence of the Gal epitope, acute and chronic antibody and cell‐mediated rejection developed in grafts, maintained by chronic immunosupression, presumably due to de novo responses to non‐Gal antigens.     

Anti-Gal antibodies in humans and 1, 3alpha-galactosyltransferase knock-out mice

BACKGROUND: Due to the absence of alphaGAL epitopes, humans and galactosyltransferase knock-out (GALT/ KO) mice express high levels of anti-Gal antibodies. We describe the properties of mouse anti-GAL antibodies. METHODS: Anti-GAL IgG antibodies were quantified by affinity purification. Antibody affinities and avidities were determined in direct binding and competition assays. Antibody-mediated rejection was investigated using hyperimmunized GALT/KO mice as recipients of GAL+ heart allografts. RESULTS: In young GALT/KO mice the levels of anti-GAL antibodies were low. Immunization of GALT/KO mice resulted in increased anti-GAL antibody expression. In mouse serum 0.6% of IgG was specific for alphaGAL compared to 0.5% in human serum. The avidity of purified mouse and human anti-GAL IgG was 30 and 6 nM, the affinity 15 and 50 microM, respectively. The isotype distribution in mouse and human anti-GAL IgG appeared to be similar to the isotype distribution in normal sera. The affinity of mouse and human anti-GAL IgM was 150 and 750 microM, respectively. Immunized GALT/KO recipients of GAL+ heart transplants rejected their grafts within 2 hr although nonimmunized GALT/KO mice retained their grafts for up to 6 days. Immunohistological examination of the rejected GAL+ hearts revealed massive deposition of IgM and IgG on endothelial cells of the graft with a concomitant deposition of complement. CONCLUSIONS: Our studies demonstrate that anti-GAL antibodies from immunized GALT/KO mice bind alphaGAL with an avidity/affinity similar to human anti-GAL antibodies and are able to induce hyperacute rejection of GAL+ heart allografts.     

Structural characterization of α1,3-galactosyltransferase knockout pig heart and kidney glycolipids and their reactivity with human and baboon antibodies

Diswall M, Ångström J, Karlsson H, Phelps CJ, Ayares D, Teneberg S, Breimer ME. Structural characterization of α1,3‐galactosyltransferase knockout pig heart and kidney glycolipids and their reactivity with human and baboon antibodies. Xenotransplantation 2010; 17: 48–60. © 2010 John Wiley & Sons A/S. Abstract: Background: α1,3‐galactosyltranferase knockout (GalT‐KO) pigs have been established to avoid hyperacute rejection in GalT‐KO pig‐to‐human xenotransplantation. GalT‐KO pig heart and kidney glycolipids were studied focusing on elimination of Gal‐antigens and whether novel antigens would appear. Non‐human primates are used as pre‐clinical transplantation experimental models. Therefore, sera from baboons transplanted with GalT‐KO hearts were compared with human serum regarding reactivity with pig glycolipids. Methods: Neutral and acidic glycolipids were isolated from GalT‐KO and WT pig hearts and kidneys. Glycolipid immune reactivity was tested on TLC plates using human affinity‐purified anti‐Gal Ig, anti‐blood group monoclonal antibodies, lectins, and human serum as well as baboon serum collected before and after GalT‐KO pig heart transplantations. Selected glycolipid fractions, isolated by HPLC, were structurally characterized by mass spectrometry and proton NMR spectroscopy. Results: GalT‐KO heart and kidney lacked α3Gal‐terminated glycolipids completely. Levels of uncapped N‐acetyllactosamine precursor compounds, blood group H type 2 core chain compounds, the P1 antigen and the x₂ antigen were increased. Human serum antibodies reacted with Gal‐antigens and N‐glycolylneuraminic acid (NeuGc) in WT organs of which only the NeuGc reactivity remained in the GalT‐KO tissues. A clear difference in reactivity between baboon and human antibodies with pig glycolipids was found. This was most pronounced for acidic, not yet identified, compounds in GalT‐KO organs which were less abundant or lacking in the corresponding WT tissues. Conclusions: GalT‐KO pig heart and kidney completely lacked Gal glycolipid antigens whilst glycolipids synthesized by competing pathways were increased. Baboon and human serum antibodies showed a different reactivity pattern to pig glycolipid antigens indicating that non‐human primates have limitations as a human pre‐clinical model for immune rejection studies.     

Complement regulation in the GalT KO era

Miyagawa S, Yamamoto A, Matsunami K, Wang D, Takama Y, Ueno T, Okabe M, Nagashima H, Fukuzawa M. Complement regulation in the GalT KO era.
Xenotransplantation 2010; 17: 11–25. © 2010 John Wiley & Sons A/S. Abstract: A number of institutes have reported on the successful production of α‐galactosyltransferase knockout (GalT‐KO) pigs. After producing such pigs, hyperacute rejection appeared to no longer be a problem. However, acute vascular rejection (AVR)/acute humoral xenograft rejection (AHXR) is defined as a rejection that begins within 24 h after transplantation and gradually destroys the graft. The origin of AVR/AHXR continues to be a controversial topic, but is generally thought to be initiated by xeno‐reactive antibodies, including non‐Gal antibodies and subsequent activation of the graft endothelium, the complement and the coagulation systems. The complement is activated via the classical pathway by non‐Gal antigens and ischemia‐reperfusion injury, via the alternative pathway, especially on islets, and via the lectin pathway. Therefore the complement system is still an important recognition and effector mechanism of AVR/AHXR. In addition, quite recently, based on the relationship between complement and coagulation systems, a new pathway has been proposed. All complement regulatory proteins (CRPs) have the ability to regulate complement activation in different ways. Therefore, to effectively protect xenografts against AVR/AHXR, it appears reasonable to employ not only one but several CRPs including anti‐complement drugs. Non‐Gal antigens, such as the Hanganutziu‐Deicher antigen, is still present on GalT‐KO grafts. The further assessment of antigens continues to be an important issue in the area of clinical xenotransplantation. The above conclusions suggest that the expression of human CRPs on GalT‐KO grafts is necessary. Moreover, multilateral inhibition of complement activation is required in conjunction with the regulation of the coagulation system.     

T cell and B cell tolerance to GALalpha1,3GAL-expressing heart xenografts is achieved in alpha1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism.

BACKGROUND: We have previously demonstrated that mixed xenogeneic chimerism and donor-specific T-cell tolerance can be induced in the rat-to-mouse species combination by using a relatively nontoxic, nonmyeloablative conditioning regimen. However, natural antibodies (NAbs) against Galalpha1,3Gal (Gal) pose an additional major barrier to pig-to-human vascularized xenograft acceptance. METHODS: To determine whether the mixed chimerism approach could also overcome this humoral barrier, T cell-depleted rat (GalT+/+) bone marrow cells (BMC) were transplanted to alpha1,3-galactosyltransferase deficient (GalT-/-) mice conditioned with a nonmyeloablative regimen, consisting of transient T cell and natural killer (NK) cell depletion, 3 Gy whole body irradiation, and 7 Gy thymic irradiation. RESULTS: By giving a high dose (180x106) of rat BMC, persistent mixed chimerism could be induced in GalT-/- mice, although the level of donor-type hematopoietic repopulation declined over time. Induction of mixed chimerism was associated with a rapid disappearance of anti-Gal and anti-rat NAb in the sera. Both anti-Gal Ab-producing cells and B cells with receptors recognizing Gal were undetectable in mixed chimeras, even when the chimerism levels declined, suggesting that a very low level of chimerism could effectively maintain B-cell tolerance to Gal, probably by clonal deletion and/or receptor editing. Mixed chimeras accepted subsequently transplanted donor-type rat hearts (>100 days) without immunosuppressive therapy, whereas delayed vascular and even hyperacute rejection of rat hearts occurred in conditioned control GalT-/- mice. Cellular rejection occurred by 5-6 days in conditioned control wild-type mice. CONCLUSIONS: These findings demonstrate that induction of mixed chimerism with a nonmyeloablative regimen can prevent vascularized xenograft rejection by cellular and anti-Gal Ab-dependent pathways in GalT+/+-to-GalT-/- species combinations.     

Fate of alphaGal +/+ pancreatic islet grafts after transplantation into alphaGal knockout mice

BACKGROUND: Important phylogenetic differences between pig and human tissues prevent xenotransplantation from becoming a clinically feasible option. Humans lack the galactose-alpha1,3-galactose (alphaGal) epitope on endothelial cell surfaces and therefore have preformed anti-alphaGal antibodies. The role of these antibodies in rejection of non-vascular xenografts remains controversial. This study investigated the role of anti-alphaGal antibodies in rejection of non-vascularized alphaGal+/+ grafts in alphaGal -/- mice. METHODS: alphaGal +/+ and alphaGal -/- pancreatic islets were transplanted under the renal capsule of streptozotocin-induced diabetic (1) alphaGal -/- mice and (2) alphaGal +/+ mice. alphaGal -/- recepients were immunized with rabbit red blood cell membranes (RRBCs) to produce elevated anti-alphaGal antibody levels. RESULTS: Six of the 18 alphaGal -/- mice rejected the alphaGal +/+ grafts within 68 days whereas indefinite graft survival was achieved in the control groups. Animals with surviving islet grafts were challenged with alphaGal +/+ skin grafts. Although all alphaGal +/+ skin grafts were rejected within 58 days, the islet grafts remained intact. This observation correlated with the level of alphaGal expression (which was very low on islets compared to skin) rather than the actual titre of anti-alphaGal antibody. DISCUSSION: The results suggest that the level of alphaGal expression plays an important role in graft survival. Therefore, its removal is important in the development of a pig islet donor for future clinical therapy.     

Organ-specific differences in the function of MCP-1 and CXCR3 during cardiac and skin allograft rejection

BACKGROUND: Chemokines are well-established to function in the recruitment of leukocytes into allografts in the course of rejection. Moreover, some studies have indicated that there are organ-specific differences in chemokine function, but the mechanism accounting for this difference is not known. METHODS: Fully major histocompatibility complex-mismatched vascularized cardiac transplants or skin transplants were performed using BALB/c (H-2d), C57BL/6 (H-2b), MCP-1-/- (H-2b) and CXCR3-/- (H-2b) mice as donors or recipients. Also, skin grafts (H-2b) were placed onto SCID mice (H-2d) that received BALB/c splenocytes (H-2d) by adoptive transfer either at the time of transplantation, or after a period of 28 days. RESULTS: Cardiac allografts in MCP-1-/- recipients survived significantly longer (P<0.0005) than wild-type (WT) controls. However, there was no prolongation of survival when MCP-1-/- grafts were used a donors in WT mice. In contrast, the absence of donor but not recipient MCP-1 prolonged skin allograft survival. WT donor cardiac grafts in CXCR3-/- recipients had a modest prolongation of survival (P<0.0005), whereas CXCR3-/- donor cardiac grafts in WT recipients were rejected similar to controls. Also, while recipient CXCR3 had no effect on the rejection of skin, CXCR3-/- donor skin grafts survived significantly longer than WT controls. This survival advantage was lost when vascularized CXCR3-/- skin grafts were used as donors in the SCID model of rejection. CONCLUSION: Recipient derived MCP-1 and CXCR3 are functional in the rejection of vascularized, but not nonvascularized, allografts. In contrast, donor-derived MCP-1 and CXCR3 are functional in nonvascularized, but not vascularized grafts.     

Xenogeneic thymokidney and thymic tissue transplantation in a pig-to-baboon model: I. Evidence for pig-specific T-cell unresponsiveness

BACKGROUND: The potential of xenotransplantation for clinical application will require overcoming barriers of humoral and cellular rejection, through strategies using immune suppression or tolerance induction. This laboratory has previously reported the induction of tolerance in the discordant xenogeneic model of pig-to-rodent thymic transplantation. We also have described a miniature swine model of fully mismatched allogeneic composite vascularized thymokidney transplantation that induced transplantation tolerance. We tested a combination of these approaches in a clinically relevant pig-to-primate model of xenotransplantation. METHODS: Composite thymokidney grafts were prepared 40 to 80 days before transplantation by the autologous implantation of thymic tissue under the renal capsule of human decay-accelerating factor transgenic swine. Baboons received xenotransplants of both human decay-accelerating factor composite thymokidneys and omental implants of thymic tissue. Recipients were treated with an immunosuppressive-conditioning regimen including thymectomy or thymic irradiation, extracorporeal immunoadsorption of anti-alphaGal antibodies and T-cell depletion. Recipients were followed for indicators of xenograft rejection, T-cell depletion and reconstitution, anti-alphaGal antibody levels, and mixed lymphocyte responses. Immunologic responses were studied in those animals that survived for more than 3 weeks. RESULTS: Thymokidney xenografts survived for up to 30 days, with evidence of viable thymic epithelium and Hassall's corpuscles under the renal capsule and in the omental implants, and with evidence of few host lymphocytes. Three animals demonstrated donor-specific unresponsiveness, while maintaining normal alloresponses, in mixed-lymphocyte-response assays performed after immunosuppression had been stopped. Rejected grafts demonstrated humoral damage without evidence of cellular infiltrates. After graftectomy, one animal maintained donor-specific cellular unresponsiveness and stable anti-alphaGal antibody levels for more than 2 months. CONCLUSIONS: We concluded that composite thymokidney and thymic-tissue xenotransplantation from swine to baboons can induce donor-specific cellular unresponsiveness and stable anti-alphaGal antibody levels, suggesting avoidance of sensitization after xenotransplantation. The presence of viable donor-swine thymic epithelium could have a role in the development of donor-specific T-cell tolerance. Further strategies to address humoral rejection could prolong graft survival and result in long-term tolerance to xenografts.     

Prolonged survival of GalT‐KO swine skin on baboons

Weiner J, Yamada K, Ishikawa Y, Moran S, Etter J, Shimizu A, Smith RN, Sachs DH. Prolonged survival of GalT‐KO swine skin on baboons. Xenotransplantation 2010; 17: 147–152. © 2010 John Wiley & Sons A/S. Abstract: Background: Allogeneic skin is currently the best alternative to autologous skin as a temporary treatment for severe burns, but it has several drawbacks. As a potential alternative, we have evaluated GalT‐KO swine skin, which lacks expression of the Gal epitope, to investigate the effect of eliminating this epitope on survival of pig‐to‐baboon skin grafts. Methods: Two adult baboons that had fully recovered from previous T cell depletion received simultaneous skin grafts from: (i) GalT‐KO swine, (ii) Gal‐positive swine, (iii) a third‐party baboon, and (iv) self (control skin). Recipients were treated with cyclosporin for 12 days and the survival, gross appearance, and histology of the grafts were compared. Results: In both baboons, the GalT‐KO skin survived longer than either the Gal‐positive swine skin or the allogeneic skin. Early rejection of the Gal‐positive skin appeared to be mediated by cytotoxic preformed anti‐Gal IgM antibodies, while the rejection of GalT‐KO skin appeared to result from cellular mechanisms. Conclusions: GalT‐KO skin may have potential clinical benefits as an alternative to allogeneic skin as a temporary treatment for severe skin injuries.     

Hyperexpression of Foxp3 and IDO during acute rejection of islet allografts

BACKGROUND: We investigated the hypothesis that Foxp3+ cells are an integral component of antiallograft immunity but are dominated by pathogenic effectors. METHODS: Wild-type H-2b C57BL/6 (B6) mice or B6 mice with a targeted disruption of c-Rel gene (c-Rel-/-) were used as recipients of islet grafts from allogeneic DBA/2 (H-2d) mice or syngeneic B6 mice. We developed kinetic quantitative polymerase chain reaction assays and measured intragraft expression of mRNA for Foxp3, IDO, cytolytic molecules, proinflammatory cytokines, and chemokines/receptors. RESULTS: Intraislet levels of mRNA for Foxp3, IDO, CD3, CD25, tumor necrosis factor-alpha, RANTES, IP-10, and CXCR3 were highest in DBA/2 islet allografts from WT B6 recipients compared to DBA/2 islet allografts from c-Rel-/- B6 recipients or syngeneic B6 islet grafts from WT B6 mice. The ratio of granzyme B or IFN-gamma to Foxp3 was higher with the DBA/2 islet allografts from the WT B6 recipients compared to DBA/2 islet allografts from c-Rel-/- B6 recipients or B6 islet grafts from WT B6 recipients. CONCLUSIONS: Foxp3+ cells are an integral component of acute rejection of allografts but may be dominated by pathogenic effectors.     

Survival and function of CD47‐deficient thymic grafts in mice

Wang Y, Wang H, Wang S, Fu Y, Yang Y‐G. Survival and function of CD47‐deficient thymic grafts in mice. Xenotransplataion 2010; 17: 160–165. © 2010 John Wiley & Sons A/S. Abstract: Background: We have previously shown that the interspecies incompatibility of CD47 plays an important role in triggering rejection of xenogeneic hematopoietic cells by macrophages. However, it remains unknown whether CD47 incompatibility also contributes to the rejection of non‐hematopoietic xenografts. Aims: Here, we investigated the role of CD47 in preventing macrophage‐mediated rejection of thymic epithelial cells in a mouse model of thymic transplantation across the CD47 barrier. Methods: Wild‐type (WT) and CD47 KO mice were thymectomized and treated with T cell‐depleting mAbs, and implanted with fetal thymus from syngeneic WT or CD47 KO donors. Results: Transplantation of CD47 KO mouse thymus led to T cell recovery in thymectomized, T cell‐depleted WT mice. Similar to the control WT mouse thymic grafts, CD47 KO mouse thymic grafts showed a normal distribution of thymocyte subsets, and almost all of the thymocytes were recipient origin. Furthermore, histological analysis confirmed long‐term survival of CD47 KO mouse thymic epithelial cells in WT mouse recipients. Conclusions: These results demonstrate that, unlike hematopoietic cells, CD47 KO mouse thymus can survive and function in WT mice. Furthermore, our data implicate that the role of CD47 in xenograft rejection may differ for different types of xenografts, and that CD47 incompatibility is unlikely to impede thymic xenotransplantation, a potential approach to inducing xenotolerance, by triggering macrophage‐mediated rejection.     

Up to 9-day survival and control of thrombocytopenia following alpha1,3-galactosyl transferase knockout swine liver xenotransplantation in baboons

Kim K, Schuetz C, Elias N, Veillette GR, Wamala I, Varma M, Smith RN, Robson SC, Cosimi AB, Sachs DH, Hertl M. Up to 9‐day survival and control of thrombocytopenia following GalT‐KO swine liver xenotransplantation in baboons. Xenotransplantation 2012; 19: 256–264.. © 2012 John Wiley & Sons A/S. Abstract: Background:  With standard miniature swine donors, survivals of only 3 days have been achieved in primate liver‐transplant recipients. The recent production of alpha1,3‐galactosyl transferase knockout (GalT‐KO) miniature swine has made it possible to evaluate xenotransplantation of pig organs in clinically relevant pig‐to‐non‐human primate models in the absence of the effects of natural anti‐Gal antibodies. We are reporting our results using GalT‐KO liver grafts. Methods:  We performed GalT‐KO liver transplants in baboons using an immunosuppressive regimen previously used by our group in xeno heart and kidney transplantation. Post‐operative liver function was assessed by laboratory function tests, coagulation parameters and histology. Results:  In two hepatectomized recipients of GalT‐KO grafts, post‐transplant liver function returned rapidly to normal. Over the first few days, the synthetic products of the donor swine graft appeared to replace those of the baboon. The first recipient survived for 6 days and showed no histopathological evidence of rejection at the time of death from uncontrolled bleeding, probably caused by transfusion‐refractory thrombocytopenia. Amicar treatment of the second and third recipients led to maintenance of platelet counts of over 40 000 per μl throughout their 9‐ and 8‐day survivals, which represents the longest reported survival of pig‐to‐primate liver transplants to date. Both of the last two animals nevertheless succumbed to bleeding and enterococcal infection, without evidence of rejection. Conclusions:  These observations suggest that thrombocytopenia after liver xenotransplantation may be overcome by Amicar therapy. The coagulopathy and sepsis that nevertheless occurred suggest that additional causes of coagulation disturbance must be addressed, along with better prevention of infection, to achieve long‐term survival.     

Beta7 integrins contribute to skin graft rejection

BACKGROUND: Because integrins alpha4beta7 and alphaEbeta7 contribute to epidermotropism of T-cells during skin inflammation, we sought to study their role in skin allograft rejection. METHODS: Wild-type (WT) (beta7+/+) and beta7 gene knockout (beta7-/-) C57BL/6 (H-2(b)) mice and SJL/J (H-2(s)) mice served as donors and recipients of allogeneic skin grafts. An anti-integrin beta7 subunit mAb (FIB504.64) was used to treat WT beta7+/+ C57BL/6 recipients of skin grafts from SJL/J mice. RESULTS: WT C57BL/6 recipients acutely rejected skin from SJL/J mice in 13 days. In contrast, the survival of SJL/J skin on either beta7-/- gene knockout or WT C57BL/6 recipients treated with anti-beta7 subunit mAb, was prolonged by 6 to 7 additional days (P<0.01). The survival of skin allografts from either beta7-/- or beta7+/+ C57BL/6 mice received by SJL/J recipients was not prolonged (P >0.05). CONCLUSIONS: Beta7 integrins contribute to skin graft rejection, in accord with their role in mediating the epidermotropism of T-cells during skin inflammation.     

Results of Gal‐Knockout Porcine Thymokidney Xenografts

Clinical transplantation for the treatment of end‐stage organ disease is limited by a shortage of donor organs. Successful xenotransplantation could immediately overcome this limitation. The development of homozygous α1,3‐galactosyltransferase knockout (GalT‐KO) pigs removed hyperacute rejection as the major immunologic hurdle to xenotransplantation. Nevertheless, GalT‐KO organs stimulate robust immunologic responses that are not prevented by immunosuppressive drugs. Murine studies show that recipient thymopoiesis in thymic xenografts induces xenotolerance. We transplanted life‐supporting composite thymokidneys (composite thymus and kidneys) prepared in GalT‐KO miniature swine to baboons in an attempt to induce tolerance in a preclinical xenotransplant model. Here, we report the results of seven xenogenic thymokidney transplants using a steroid‐free immunosuppressive regimen that eliminated whole‐body irradiation in all but one recipient. The regimen resulted in average recipient survival of over 50 days. This was associated with donor‐specific unresponsiveness in vitro and early baboon thymopoiesis in the porcine thymus tissue of these grafts, suggesting the development of T‐cell tolerance. The kidney grafts had no signs of cellular infiltration or deposition of IgG, and no grafts were lost due to rejection. These results show that xenogeneic thymus transplantation can support early primate thymopoiesis, which in turn may induce T‐cell tolerance to solid organ xenografts.     

IgG, but not IgM, mediates hyperacute rejection in hepatic xenografting

We reported previously that no classical features of hyperacute rejection (HAR) could be found in liver grafts in the guinea-pig (GP)-to-rat model and that recipients died shortly after transplantation of non-immunologic causes. Thus, the GP-to-rat model is not suitable for studying the mechanisms of discordant liver xenograft rejection. In the hamster to rat model, long-term survival of a liver graft is possible, but extremely low levels of xenoreactive natural antibodies are present. To mimic a discordant situation with pre-formed IgM and IgG antibodies, we sensitized rats 1 or 5 weeks before grafting. Specific anti-hamster IgM antibodies were found in recipients sensitized at week –1 but not week –5. Anti-hamster IgG was present in all recipients, albeit considerably higher in animals sensitized 5 weeks before grafting. In these two models, we examined the mechanism of HAR of liver grafts and compared this with heart xenografts. Control heart and liver grafts were rejected 4 and 7 days after transplantation respectively. Liver grafts in recipients sensitized at week –5 showed venous congestion and bleeding after reperfusion, indicating HAR, however this was not observed after sensitization at week –1. This surprising finding was confirmed by histology. Massive extravasation, edema, and acute liver cell degradation were noticed in grafts subjected to HAR. Liver grafts of recipients sensitized at week –1 showed only minimal changes. Heart grafts were rejected hyperacutely in both sensitization models. IgG antibodies could be detected on liver grafts in the group sensitized at week –5 but not in the group sensitized at week –1. Minimal IgM depositions were found on liver grafts of animals sensitized 1 week before grafting. Rejected heart grafts from similar sensitization groups showed identical antibody depositions; only IgM depositions were massive. Complement depositions were found in all groups. These results indicate that IgG, but not IgM, mediates HAR in hepatic xenografting. Such a predominance of IgG over IgM does not exist for heart grafts.     

Hearts from transgenic pigs constructed with CD59/DAF genomic clones demonstrate improved survival in primates

We have previously created transgenic pigs bearing the human complement regulatory proteins CD59 and decay‐accelerating factor (DAF) by either the intercellular transfer or the cDNA transgenic method. To achieve more physiologic protein expression, we constructed a new line of transgenic pigs with CD59 and DAF human genomic clones. We transplanted these CD59/DAF transgenic pig hearts into baboons immunosuppressed with cyclosporine, methylprednisone or leflunomide/mofetil mycophenolate. The four wild‐type hearts survived for 20–80 min, whereas the four CD59/DAF hearts functioned for 85–130 h. Immunohistochemical staining showed levels of CD59 and DAF protein expression similar to that in human hearts. Wild‐type and transgenic hearts demonstrate a similar level of IgM deposition, although transgenic hearts suffered less hyperacute rejection and thus less membrane attack complex deposition. The histology of the transgenic grafts after explant was consistent with acute vascular rejection, with a high level of IgG deposit compared with wild‐type control. We conclude that this new line of CD59/DAF transgenic pigs express high levels of the transgene products, which conferred longer survival because of better protection from hyperacute rejection. Similar to previous transgenic pigs, however, these animals suffered from delayed xenograft rejection.