Effect of αGal on corneal xenotransplantation in a mouse model

Choi HJ, Kim MK, Lee HJ, Jeong SH, Kang HJ, Park C‐S, Park C‐G, Kim SJ, Wee WR. Effect of αGal on corneal xenotransplantation in a mouse model. Xenotransplantation 2011; 18: 176–182. © 2011 John Wiley & Sons A/S. Abstract: Background: It has been reported that hyperacute rejection (HAR) does not occur after pig‐to‐nonhuman corneal xenotransplantation. However, considering that immune privilege is already disrupted in most human corneal recipients, and the expression of αGal can be gradually reduced after pig‐to‐rat corneal transplantation, the long‐term survival of corneal grafts from wild‐type pigs cannot be guaranteed. Accordingly, we aimed to investigate the effect of αGal on the change in anti‐Gal antibodies, using sensitized α1,3‐galactosyltransferase gene‐knockout (GTKO) mice recipients. Methods: C57BL/6 (B6) and GTKO mice were divided into 5 groups and underwent orthotopically full thickness cormeal transplantation as follows (n=4 for each group): (1) group 1: B6 to B6; (2) group 2: fresh porcine posterior corneal lamella to B6; (3) fresh porcine posterior corneal lamella to GTKO; (4) group 4: decellularized porcine posterior corneal lamella to GTKO, and (5) group 5: B6 to GTKO. Before transplantation, all GTKO recipients were sensitized using intraperitoneal injections of rabbit blood cells. Median survival times (MST) for the corneal grafts of the different groups were compared and plasma concentrations of IgG/IgM anti‐Gal antibodies were evaluated at 1 week, 2 weeks and 3 weeks post‐transplantation. Results: There were no differences in MSTs between groups. Although there was no HAR of fresh porcine posterior corneal grafts even in sensitized GTKO recipients, αGal expression was induced in the transplanted fresh porcine corneal grafts and plasma concentration of IgG anti‐Gal antibody was gradually increased in fresh porcine cornea‐grafted GTKO recipients. On the contrary, αGal expression did not increase in the grafts and plasma concentration of anti‐Gal antibodies did not change after transplantation using decellularized porcine corneas. Conclusions: Our findings suggest that αGal may affect the long‐term survival of porcine corneal xenografts via antibody‐mediated rejection, although αGal does not have an effect on acute rejection and decellularized porcine corneas may enable the long‐term survival of porcine corneal xenografts.     

Acute cell-mediated rejection in orthotopic pig-to-mouse corneal xenotransplantation

Abstract: Background:  To investigate the role of T cells and natural killer (NK) cells in mediating corneal xenograft rejection in a pig-to-mouse model.
Methods:  Pig corneas were orthotopically transplanted into BALB/c, C57BL/6, nude, severe combined immunodeficiency (SCID), and NOD/SCID/γc^null (NOG) mice. Graft survival was clinically assessed by slit-lamp biomicroscopy and median survival times (MST) were calculated. The rejected grafts were histologically evaluated using antibodies against CD4, CD8, NK1.1, and F4/80.
Results:  The pig corneal xenografts were acutely rejected by BALB/c and C57BL/6 mice (MST 9.0 days), while nude, SCID and NOG mice rejected pig corneas in a more delayed fashion (MST 16.0, 16.4, and 16.9 days, respectively). The majority of infiltrating cells found in rejected grafts in C57BL/6 mice were macrophages and CD4⁺ T cells, while CD8⁺ T cells and NK cells were rarely found. The grafts in nude mice had markedly decreased inflammatory infiltration with small numbers of macrophages and CD4⁺ T cells. Infiltration was even more modest in grafts in SCID and NOG mice.
Conclusions:  T cells play an important role in acute rejection of pig corneal xenografts in mice, although acute rejection is not solely the result of T-cell-mediated immunity. NK cells are less likely to be involved in the rejection process.     

清除补体避免猪到猕猴异种心脏移植超急性排斥反应的实验研究

目的 观察纯化的眼镜蛇毒因子(CVF)对猪到狱猴异种心脏移植超急性排斥反应的影响。方法 以幼猪为供者，施行猪到狱猴腹腔内异位心脏移植，实验组(n＝4)使用CVF完全清除受者体内补体，对照组(n＝5)不使用CVF，两个组术后均采用环抱素A、甲泼尼龙和环磷酰胺抑制排斥反应，通过检测血清C3、C4水平及总补体活性验证CVF的效果，移植心停跳时切取移植心进行病理检查。结果 在使用CVF后，实验组血清C3降为0，总补体活性CH50值也几乎为0，末发现明显毒副反应，移植猪心存活时间平均为lld，最长达13d，病理学提示均发生了延迟性异种排斥反应；对照组3个移植心在移植后60min内发生超急性排斥反应，另2个分别存活22h及6d。结论 纯化的CVF有良好的清除补体的作用，且末见明显副作用；使用CVF可克服猪到狱猴异种心脏移植超急性排斥反应的发生。     

The immunobiology of pig‐to‐nonhuman primate islet xenotransplantation: insights,innovation, and impact

Previous studies of pig‐to‐non‐human primate (NHP) islet xenotransplantation have provided important insights into the immune recognition and effector pathways operative in this relevant preclinical model. The specifics of the xenograft product, microenvironment at the implantation site, and the immunosuppressive regimen significantly influence the mechanisms underlying the rejection of xenogeneic islets. Our current understanding of the immunological barriers to survival of pig islets in NHPs is largely based on studies on intraportal islet xenografts and on comparisons with islet allografts. The demonstration of cell‐mediated rejection of intraportal porcine islet xenografts at about 1 month posttransplant in monkeys immunosuppressed with the same protocols that prevent monkey islet allograft rejection indicates that islet xenograft rejection involves cellular mechanisms that are not present in acute islet allograft rejection. While these mechanisms remain poorly defined the demonstration of long‐term diabetes reversal after intraportal islet xenotransplantation in non‐human primates immunosuppressed with anti‐CD40L but not with anti‐CD40 antibody‐based protocols suggests that the therapeutic efficacy of anti‐CD40L in this transplantation setting likely involves the depletion of donor‐reactive, activated T cells besides CD40:CD40L costimulation blockade. Rejection of intraportal islet xenografts in NHPs immunosuppressed with CTLA4‐Ig and rapamycin was mediated largely by IL‐15‐primed, CXCR3+CD8+ memory T cells recruited by IP‐10 (CXCL10) positive pig islets and macrophages that showed staining for IL‐12 and iNOS. Adding basiliximab induction and tacrolimus maintenance therapy to this protocol prevented rejection in 24 of 26 recipients followed for up to 275 days. Comparison of both groups suggests, though by no means conclusive, that prolongation of graft survival in this large cohort was associated with reduced direct T cell responses to xenoantigens, reduced proportion of intrahepatic (intragraft) B cells and IFN‐γ+ and IL‐17+ CD4 and CD8 T cells, and increased local production of immunoregulatory molecules linked with Tregs, including TGF‐β, Foxp3, HO‐1, and IL‐10. Anti‐pig non‐Gal IgG antibody elicitation was suppressed in both groups. We are currently exploring the concept of negative vaccination to markedly minimize the need for immunosuppression in islet xenotransplantation. Peritransplant administration of donor apoptotic cells extended pig‐to‐mouse islet xenograft survival to >250 days when combined with peritransplant B cell‐depletion and rapamycin. This costimulation blockade‐sparing, antigen‐specific immunotherapy is expected to cause rapid pretransplant clonal deletion of indirect and anergy of direct xenospecific T cells while inducing regulatory T cells. As anti‐CD40L antibodies, B cell depleting antibodies are expected to interfere with indirect antigen presentation, costimulation, and cytokine production required for optimal T cell proliferation, memory formation, and intragraft CD8+ effector function. It is conceivable that additional strategies must be employed in NHPs and eventually in diabetic patients to achieve – as previously with anti‐CD40L antibodies – more complete, yet selective depletion of donor‐reactive, activated T‐cells for the purpose of stable xenograft acceptance.     

Role of antibody-independent complement activation in rejection of porcine bone marrow cells in mice

BACKGROUND: Although complement activation has been shown to be important in the rejection of solid organs in some xenogeneic species combinations, its role in the rejection of xenogeneic marrow engraftment is unknown. METHODS: The effect of complement depletion with cobra venom factor on porcine bone marrow cell (BMC) engraftment was examined in 3 Gy-irradiated C.B-17 severe combined immunodeficiency mice receiving 10(8) pig BMC. RESULTS: At 26 days after transplantation, the percentages of swine class I+, myeloid, and CD2+ cells in marrow, spleen, and peripheral blood, and the numbers of porcine myeloid progenitor cells in marrow, were increased in cobra venom factor-treated recipients compared with simultaneous control recipients. Consistent with the in vivo results, preheating serum (56 degrees C for 30 min) reduced the inhibitory effect of severe combined immunodeficiency mouse serum on the proliferation of pig BMC in vitro. CONCLUSION: Murine complement is capable of resisting xenogeneic hematopoietic engraftment through an antibody-independent mechanism.     

Selective rejection of porcine islet xenografts by macrophages

Abstract:  Background: Our previous study has shown that porcine antigen‐primed and CD4+ T cell‐activated macrophages are capable of recognition and rejection of porcine xenografts after adoptive transfer. However, whether this is an absolute xenograft specific rejection remains to be confirmed. Methods:  Mouse islet allografts and neonatal porcine islet cell cluster (NICC) xenografts were admixed and transplanted under the left kidney capsule, and NICC xenografts alone were transplanted under the right kidney capsule of strepotozotocin‐induced diabetic NOD‐SCID mice. After achievement of normoglycemia, the NOD‐SCID recipients were transferred with macrophages purified from NICC transplant NOD‐SCID mice reconstituted with CD4+ T cells. Five weeks after macrophage transfer the left kidney with the admixed grafts were removed. Graft survival and function following macrophage transfer was assessed by blood glucose measurement and immunohistochemistry. Results and conclusions:  Adoptive transfer with activated macrophages did not affect the normalized blood glucose levels in NOD‐SCID recipients of admixed grafts until left nephrectomy 5 weeks post‐macrophage transfer. Insulin‐positive and porcine C‐peptide‐negative mouse islets were detected in the admixed grafts. The surviving mouse islets in the admixed grafts were surrounded but not infiltrated by macrophages. The nephrectomized recipients demonstrated sustained hyperglycemia and completely destroyed NICC xenografts in their remaining right kidneys 8 weeks after macrophage transfer. Taken together, these data provide direct evidence of porcine islet xenograft specific rejection by activated macrophages.     

Pig islet xenograft rejection in a mouse model with an established human immune system

Abstract: Background: Xenotransplantation from pigs provides a potential solution to the severe shortage of human pancreata, but strong immunological rejection prevents its clinical application. A better understanding of the human immune response to pig islets would help develop effective strategies for preventing graft rejection. Methods: We assessed pig islet rejection by human immune cells in humanized mice with a functional human immune system. Humanized mice were prepared by transplantation of human fetal thymus/liver tissues and CD34⁺ fetal liver cells into immunodeficient mice. Islet xenograft survival/rejection was determined by histological analysis of the grafts and measurement of porcine C‐peptide in the sera of the recipients. Results: In untreated humanized mice, adult pig islets were completely rejected by 4 weeks. These mice showed no detectable porcine C‐peptide in the sera, and severe intra‐graft infiltration by human T cells, macrophages, and B cells, as well as deposition of human antibodies. Pig islet rejection was prevented by human T‐cell depletion prior to islet xenotransplantation. Islet xenografts harvested from T‐cell‐depleted humanized mice were functional, and showed no human cell infiltration or antibody deposition. Conclusions: Pig islet rejection in humanized mice is largely T‐cell‐dependent, which is consistent with previous observations in non‐human primates. These humanized mice provide a useful model for the study of human xenoimmune responses in vivo.     

Increased human tumor necrosis factor-α levels induce procoagulant change in porcine endothelial cells in vitro

Lee KG, Lee H, Ha JM, Lee YK, Kang HJ, Park C‐G, Kim SJ. Increased human tumor necrosis factor‐α levels induce procoagulant change in porcine endothelial cells in vitro. Xenotransplantation 2012; 19: 186–195. © 2012 John Wiley & Sons A/S. Abstract: Background: Intravascular thrombosis and systemic coagulation abnormalities are major hurdles to successful xenotransplantation and are signs of acute humoral rejection. Increased expression of tissue factor (TF) is associated with the development of microvascular thrombosis in xenografts. To develop an effective strategy to prevent accelerated coagulation in xenografts, we investigated the mechanism by which porcine endothelial cells (PECs) become procoagulant after contact with human blood. Methods: The changes in TF mRNA levels and activity in PECs after incubation with 20% human serum or human bioactive molecules, including C5a, tumor necrosis factor‐α (TNFα) and interleukin (IL)‐1α, were evaluated using real‐time PCR and the factor Xa chromogenic assay, respectively. The procoagulant changes in PECs by these agonists were evaluated by measuring the coagulation time of human citrated plasma suspended with PECs pretreated with each agonist. TF expression and coagulation times were also assessed in PECs transfected with short interfering RNA (siRNA) designed to knock down porcine TF. We also examined the production of proinflammatory cytokines in human whole‐blood or plasma after contact with PECs, which were screened using the cytometric bead array system. TNFα levels were measured using ELISA in whole‐blood after contact with PECs, with or without the addition of xenoreactive antibodies or C1 esterase inhibitor. Results: Porcine TF mRNA and activity in PECs were up‐regulated in response to human TNFα and IL‐1α but were not affected by C5a or 20% human serum. Up‐regulation of TF expression by human TNFα or IL‐1α shortened PEC‐induced coagulation time, while siRNA‐mediated knockdown of TF expression prolonged coagulation time. The incubation of PECs with human whole‐blood led to a significant increase in human TNFα levels in the blood, which was promoted by the addition of xenoreactive antibodies and prevented by C1 esterase inhibitor. Conclusions: Human TNFα level increases in human blood after contact with PECs, which is attributed to xenoreactive antibody binding and subsequent complement activation. Human TNFα induces procoagulant changes in PECs with increased TF expression. This study suggests that human TNFα may be one of the mediators linking complement activation with procoagulant changes in the xenoendothelium.     

Preconditioning with the prostacyclin analog epoprostenol and cobra venom factor prevents reperfusion injury and hyperacute rejection in discordant liver xenotransplantation

Abstract: Liver xenografts transplanted from guinea pig to rat suffer from inadequate organ reperfusion and initial dysfunction, despite sufficient complement depletion using cobra venom factor (CVF). Reperfusion injury is prevented when complement depleted donors are treated with the prostacyclin analog epoprostenol. Histological analysis suggests that epoprostenol preconditioning prevents post-reperfusion spasms of the intrahepatic branches of the portal vein and strongly reduces appearance of hepatocyte apoptosis shortly after transplantation. Cobra-venom-treated rats show breakdown of glucose metabolism and die in acute hypoglycaemia, whereas the additional application of epoprostenol restores gluconeogenesis. Consequently, recipient survival after epoprostenol and CVF treatment is significantly improved compared with animals receiving CVF only (5.1 ± 2.6 h vs. 17.9 ± 5.1 h). These data demonstrate that initial dysfunction of discordant liver grafts in the guinea-pig-to-rat species combination, can be overcome by the application of epoprostenol combined with CVF. Using this pharmacologic regimen, the discordant guinea-pig-to-rat model appears useful to study further questions concerning functional and immunological compatibility of a discordant liver xenograft.     

Prolonged Cardiac Allograft Survival in Mouse Model After Complement Depletion with Yunnan Cobra Venom Factor

Background

Activation of the complement system is the leading mechanism that causes antibody-mediated acute rejection and hyperacute rejection after xenotransplantation. The major cause of acute rejection in allogeneic transplantation is the T cell–mediated specific immune response. We studied the effects of complement on acute rejection after cardiac allotransplantation using complement depletion with cobra venom factor (CVF) in the mouse.

Materials and Methods

The Balb/c-C57 mouse model of heterotopic cardiac allograft was used. The mice were divided into 2 groups, a control group and a CVF-treated group. After intravenous injection of CVF, the experimental group was observed for allograft survival time. Twelve mice from the control and experimental groups were sacrificed on days 3, 5, and 7 after the operation. The pathologic grade of acute rejection, deposition of C3 in tissue, extent of infiltration by CD4+ and CD8+ T cells, and expression of MHC-II, B7-1, and B7-2 were compared between the 2 groups.

Results

In the CVF-treated group, mean (SD) survival of the cardiac allograft was 26.2 (1.7) days, and in the control group was 8.4 (0.4) days (P < .01). Pathologic examination and immunohistochemistry demonstrated that the grade of acute rejection, deposition of C3 in tissue, extent of infiltration of CD4+ and CD8+ T cells, and expression of MHC-II, B7-1, and B7-2 were significantly decreased in the CVF-treated group.

Conclusion

Depletion of complement in the serum with CVF inhibits acute cardiac allograft rejection in the mouse.     

Comparison of immunogenicity and porcine-to-rhesus lamellar corneal xenografts survival between fresh preserved and dehydrated porcine corneas

Li A, Pan Z, Jie Y, Sun Y, Luo F, Wang L. Comparison of immunogenicity and porcine‐to‐rhesus lamellar corneal xenografts survival between fresh preserved and dehydrated porcine corneas. Xenotransplantation 2011; 18: 46–55. © 2011 John Wiley & Sons A/S. Abstract: Background: To compare the immunogenicity of fresh preserved and dehydrated lamellar porcine corneas in porcine‐to‐mouse heterotopic transplantation and to investigate the survival of preserved porcine corneas as xenografts in porcine‐to‐rhesus lamellar corneal transplantation. Methods: Dehydrated and fresh preserved, endothelium deprived, porcine corneas were cut into fragments and grafted beneath the kidney capsule of BALB/c mice. Porcine‐specific delayed type hypersensitivity (DTH) and antibody (IgM, IgG) immune responses of the recipient mice were assessed. In addition, fresh preserved and dehydrated porcine corneas were used in porcine‐to‐rhesus lamellar corneal xenotransplantation. The rhesus recipients were divided into three groups. Dehydrated corneas were applied to Group 1 and 3, and fresh preserved corneas were applied to Group 2. Only Group 3 received subconjunctival injections with triamcinolone acetonide for 1 month. All xenografts were evaluated by slit‐lamp microscopy for 6 months. Two recipients in each group were examined by in vivo confocal microscopy and then killed for corneal histopathological staining 3 months after surgery. Results: Neither fresh preserved nor dehydrated corneal fragments evoked any measurable change in mice recipient humoral immune status, but both could induce porcine‐specific DTH at 1, 2, and 4 week after being grafted. However, the intensity of the DTH responses evoked by dehydrated corneas was lower than that evoked by fresh preserved corneas. In porcine‐to‐rhesus lamellar keratoplasty, with the exception one graft in Group 2 that developed characteristics of rejection, all the xenografts remained transparent or translucent up to 6 month after surgery. Histopathological examination of Group 1 showed that the infratemporal xenografts were much thicker and displayed some inflammatory cell infiltration in the peripheral portion of the graft and bed interface. However, in Group 3, which was treated with triamcinolone acetonide, there was an easily identified scar at the lamellar interface with no inflammatory cells present. In the rejected graft in Group 2, infiltrating cells included a few eosinophils and massive lymphocytes. Confocal microscopy examination showed that activated keratocytes localized in the anterior stroma and highly reflective tissue at the interface of the graft and bed. Conclusions: Porcine corneas might be an ideal source for clinical lamellar corneal xenotransplantation. In cases of tectonic lamellar transplantation, the possibility to use dehydrated pig material may become an option in the future.     

Complement Inhibition Enables Renal Allograft Accommodation and Long‐Term Engraftment in Presensitized Nonhuman Primates

Protection against humoral injury mediated by donor‐specific antibodies (DSA), also known as accommodation, may allow for long‐term allograft survival in presensitized recipients. In the present study, we determined the role of complement in renal allograft accommodation in donor skin‐presensitized nonhuman primates under conventional immunosuppression. Donor skin allografts were transplanted to presensitized recipients 14 days prior to renal transplantation. Renal allografts not receiving any immunosuppressive treatment developed accelerated rejection with predominantly humoral injury, which was not prevented using conventional cyclosporine (CsA) triple therapy. Inhibition of complement activation with the Yunnan‐cobra venom factor (Y‐CVF) successfully prevented accelerated antibody‐mediated rejection and resulted in successful accommodation and long‐term renal allograft survival in most presensitized recipients. Accommodation in this model was associated with the prevention of the early antibody responses induced against donor antigens by complement inhibition. Some antiapoptotic proteins and complement regulatory proteins, including Bcl‐2, CD59, CD46 and clusterin, were upregulated in the surviving renal allografts. These results suggest that the complement inhibition‐based strategy may be valuable alternative in future clinical cross‐match positive or ABO‐incompatible transplantation.     

Rejection of porcine islet xenografts mediated by CD4+ T cells activated through the indirect antigen recognition pathway

We have previously demonstrated that human T cells responding to porcine islets are primarily CD4+ and recognized porcine major histocompatibility complex class I molecules through the indirect pathway of antigen presentation. To determine whether this mechanism is responsible for rejection of adult porcine islets xenografts, porcine islets from adult pigs were transplanted under the kidney capsule of streptozotocin-treated CD4-knockout (KO), CD8-KO, Ig-KO and normal C57BL/6 mice. Islet xenografts were acutely rejected with similar kinetics when transplanted into normal C57BL/6 (MST=17.6 +/- 3.5 days) and Ig-KO (MST=19.0 +/- 1.7 days) mice. Interestingly, islet xenografts were rejected significantly earlier when transplanted into CD8-KO mice as compared with normal C57BL/6 (MST=7.0 +/- 0.01 days, P=2 x 10-4). Histopathological analysis revealed classical acute cellular rejection with severe diffuse interstitial cellular infiltrates in all rejected islet xenografts. In contrast, islet xenografts were not rejected when transplanted into CD4-KO mice (MST >/= 100 days, P=1 x 10-9). Histopathological analysis revealed no cellular infiltrates and intact islet xenografts. CD4+ T cells from both normal C57BL/6 and CD8-KO xenograft recipients showed detectable proliferative responses to porcine islets in the presence but not in the absence of syngeneic antigen-presenting cells. In addition, the anti-islet proliferative responses observed in normal C57BL/6 mice were significantly lower than those observed in CD8-KO mice. IgG anti-porcine antibodies were readily detected in C57BL/6 and CD8-KO xenograft recipients but not in Ig-KO or CD4-KO recipients. These results indicate that indirectly activated CD4+ T cells mediate acute rejection of adult porcine islet xenografts and that xenoreactive CD8+ T cells and antibodies are not necessary in this process.     

延长非协调性异种心脏移植存活时间的研究

目的 寻找延长非协调性异种心脏移植存活时间的方法。方法 实验分Ａ、Ｂ、Ｃ、Ｄ４组,将异种心脏（豚鼠－大鼠）移植于颈部。Ａ组：移植前受体用眼镜蛇蛇毒因子（ＣＶＦ）１５０ｕｇ．ｋｇ－１．ｄ＾－１,腹膜腔内注射（ｉ．ｐ）,每天分２次,相隔３ｄ,共４次;Ｂ组：在Ａ组的基础上,移植前１ｈ加用己酮可可碱（ＰＴＸ）５０ｍｇ／ｋｇ（ｉ．ｐ）,然后每隔６ｈ按２５ｍｇ／ｋｇ（ｉ．ｐ）至发生排斥反应为止;Ｃ组：在Ａ组的     

Manipulation of the anti-αGal antibody-αGal epitope system in experimental discordant xenotransplantation

Abstract: In 1991, evidence was provided to indicate that hyperacute rejection of pig organs by humans and baboons was initiated by the binding of anti-agalactosyl (αGal) antibodies to the vascular endothelium. A search for suitable donor species that do not express aGal epitopes has demonstrated their absence in birds, including ratites (e.g., ostrich, emu) and a reptile (alligator), and very weak expression in a large rodent (the capybara). Studies on ratites would suggest that they are unlikely to be suitable organ donors for humans. In 1991, it was also proposed that anti-aGal antibody-antigen binding could be prevented either by depletion of antibody using an immunoaffinity column of the specific aGal oligosaccharide, or by the continuous intravenous infusion of the oligosaccharide, leading to binding (and thus inactivation) of the antibody to the aGal hapten. Synthetic aGal oligosaccharides have recently become available in sufficient quantities to allow experience to be gained with immunoaffinity columns, which have demonstrated complete, but temporary, elimination of baboon serum cytotoxicity to pig PK15 cells in vivo. A search for a natural and inexpensive source of aGal has revealed a subfraction of porcine stomach mucin that demonstrates high potency in inhibiting anti-aGal antibodies both in vitro and in vivo. As an alternative therapeutic option, anti-idiotypic antibodies have been produced in mice that are specific for human and baboon anti-aGal antibodies. Their intravenous administration to baboons results in immediate reduction in serum cytotoxicity for 24 hr. Their use in delivering a toxin, such as ricin, to the B lymphocytes that produce the antibody (and express the same idiotype) is suggested. Studies on complement depletion by the administration of purified cobra venom factor (CVF) to baboons have demonstrated that, even in the absence of measurable levels of CH50, both Clq and properdin are deposited on the vascular endothelium, and that IgM, IgG, and IgA bind to the endothelium of a transplanted pig heart. The prior identification of anti-αGal IgM, IgG, and IgA by our group suggests that these antibodies might be playing a role in both classical (by IgM and IgG) and alternative (by IgA) complement pathway activation in this experimental model and in pig-to-human xenotransplantation. CVF contains a terminal aGal structure, and its administration to an otherwise unmodified baboon leads to a massive increase in anti-αGal level, suggesting that anti-CVF antibodies may, at least in part, comprise anti-aGal. The important role of anti-aGal antibodies in xenograft rejection has now been clearly established, and techniques for preventing expression of aGal epitopes in donor animals, either by genetic engineering or by gene therapy, are discussed.     

The pig analogue of CD59 protects transgenic mouse hearts from injury by human complement

BACKGROUND: It has been proposed that hyperacute rejection (HAR) of pig-to-primate vascularized xenografts is due in large part to ineffective regulation of recipient complement by pig complement regulatory proteins (CRPs), and indeed transgenic expression of human CRPs in pigs can prevent hyperacute rejection. However, at least one pig CRP (CD59) efficiently regulates human complement in vitro, suggesting that it is the level of expression of a particular CRP(s) rather than cross-species incompatibility that explains the HAR of porcine xenografts. We investigated the relative effectiveness of transgenically expressed pig and human CD59 in providing protection of mouse hearts from human complement in an ex vivo setting. METHODS: Transgenic mice expressing pig CD59 or human CD59 under the control of the human ICAM-2 promoter, which restricts expression in tissues to vascular endothelium, were used. Hearts from mice expressing similar levels of pig CD59 or human CD59 were perfused ex vivo with 10% human plasma and heart function was monitored for 60 min. Sections of perfused hearts were examined for deposition of the membrane attack complex (MAC). RESULTS: Control nontransgenic hearts (n=5) were rapidly affected by the addition of human plasma, with mean function falling to less than 10% of the initial level within 15 min. In contrast, hearts expressing either pig CD59 (n=6) or human CD59 (n=8) were protected from plasma-induced injury, maintaining 31 and 35% function, respectively, after 60 min of perfusion. MAC deposition was markedly reduced in both pig CD59 and human CD59 transgenic hearts compared to nontransgenic control hearts. CONCLUSIONS: When highly expressed on endothelium in transgenic mice, pig CD59 provided equivalent protection to human CD59 in a model of human complement-mediated xenograft rejection. Thus supranormal expression of endogenous porcine CRPs may be a feasible alternative to the expression of human CRPs in preventing HAR of pig-to-primate xenografts.     

Differential Roles of CD8+ and CD8− T Lymphocytes in Corneal Allograft Rejection in ''High-Risk'' Hosts

We examined the role of perforin and FasL in corneal allograft rejection mediated by CD8+ and CD8 T cells. BALB/c corneas were transplanted orthotopically into vascularized, 'high-risk' graft beds in C57BL/6 mice, perforin knockout mice and FasL-defective gld/gld mice. CD8+ and CD8 T cells were collected following graft rejection and adoptively transferred to SCID mice, which were then challenged with BALB/c corneal allografts. In every case, CD8 T cells could mediate graft rejection when adoptively transferred to SCID mice that received BALB/c corneal allografts. Although CD8+ T cells also mediated graft rejection, the tempo was slower. Moreover, CD8+ T cells collected FasL-defective donors that had rejected corneal allografts, mediated corneal allograft rejection in only 50% of the SCID mice that received the adoptively transferred cells. In some cases, CD8+ T-cell-mediated rejection occurred in the absence of delayed-type hypersensitivity and cytotoxic T-lymphocyte activity, but was associated with CD8+ T-cell-mediated apoptosis of BALB/c corneal cells in vitro. The results demonstrate the redundancy in immune mechanisms of corneal allograft rejection. Either CD8+ or CD8 T cells can produce corneal allograft rejection, however functional FasL is necessary for optimal rejection, even in a high-risk setting.     

中华眼镜蛇蛇毒因子在豚鼠—大鼠异种心脏移植中的作用

目的 应用中眼镜蛇蛇毒因子（CVF）消耗补体,观察豚鼠心脏在移植入Wistar大鼠腹腔内后对超急性排斥反应的变化。方法 按0．2μg/g体重CVF大鼠腹腔内分两次间隔6小时注射,18小时后进行心脏移植。脾切除及腹腔内注射环磷酰胺（Cy）均在移植前一天进行。设计分为四组：A组为对照组,不用任何药物;B组仅用CVF;C组应用CVF＋Cy＋脾切除;D组Cy＋脾切除。Cy的用量为60mg/kg体重腹腔内注射。检测各组受体的供心存活时间,并在供心停跳后取出行光镜、电镜检查。结果 A、B、C、D各组供心存活时间分别为15－3120分钟。供心存活时间的统计学分析：A组与B、C两组比较P值＜0．01,A组与D组比较,B组与C组比较P值＞0．05,B组与D组比较,C组与D组比较P值＜0．01。光镜、电镜结果提示：B、C组与A、D组有明显不同。结论 CVF能明显抑制补体活性,减轻或延缓超急性排斥反应的发生,使供体器官存活时间延长。CVF具有异种器官移植的基础研究和临床开发意义。     

Complement Independent Antibody‐Mediated Endarteritis and Transplant Arteriopathy in Mice

Complement fixation, as evidenced by C4d in the microvasculature, is a widely accepted criterion of antibody‐mediated rejection. Complement fixation has been shown to be essential in acute antibody‐mediated rejection, but its role in chronic rejection has not been addressed. Previous studies showed that passive transfer of complement fixing monoclonal IgG2a anti‐H‐2K^k into B6.RAG1^?/? KO recipients of B10.BR hearts led to progressive chronic transplant arteriopathy (CTA) over 14–28 days, accompanied by C4d deposition. The present studies were designed to test whether complement was required for these lesions. We report that a noncomplement fixing donor‐specific alloantibody (DSA, monoclonal IgG1 anti‐H‐2K^k) injected into B6.RAG1^‐/‐ KO recipients of B10.BR hearts also promotes CTA, without C4d deposition. Furthermore, a passive transfer of DSA (monoclonal IgG2a anti‐H‐2K^k) initiated endarteritis followed by CTA in B6.RAG1^?/‐ mice genetically deficient in the third component of complement (RAG1^?/?C3^?/?). These studies indicate that antibody to class I MHC antigens can trigger chronic arterial lesions in vivo without complement participation, in contrast to acute antibody‐mediated rejection. This pathway may be relevant to C4d‐negative chronic rejection sometimes observed in patients with DSA, and argues that lack of C4d deposition does not exclude antibody‐mediated chronic rejection.     

Porcine endogenous retrovirus encodes xenoantigens involved in porcine cellular xenograft rejection by mice

BACKGROUND: Identification of the antigens that stimulate transplant rejection can help develop graft-specific antirejection strategies. The xenoantigens recognized during rejection of porcine cellular xenografts have not been clearly defined, but it has been assumed that major histocompatibility complex (MHC) xenoantigens are involved. METHODS: The role of porcine endogenous retrovirus (PERV) as a source of xenoantigens was examined. The authors used morphometry to compare the kinetics of swine leukocyte antigen (SLA) pig thyroid xenograft rejection in control mice and mice immunized with PERV PK15 cells (porcine kidney epithelial cells), PERV SLA pig peripheral blood lymphocytes (PBL), PERV virions purified from PK15 cells, and PERV or PERV A pseudotypes produced from infected human 293 cells. The tempo of rejection for cellular xenografts of PERV A pseudotype-producing human 293 cells, uninfected human 293 cells, and PK15 cells in PERV-preimmunized and control mice was also compared. RESULTS: Mice immunized with PK15 cells rejected pig thyroid xenografts significantly faster at day 5 than control mice and mice immunized with pig PBL. This correlated with the amount of PERV RNA and virions produced, but not with the amount of SLA class I MHC expressed by PK15 cells. Immunization of mice with PERV virions purified from porcine PK15 cells and with PERV virions or PERV A pseudotypes produced by human 293 cells also induced accelerated xenograft rejection by 5 days. Accelerated rejection induced by virus pretreatment was CD4 T-cell dependent and restricted to PERV-expressing cellular xenografts of porcine or nonporcine origin. CONCLUSIONS: PERV acts as a significant source of xenoantigens that target porcine cellular xenografts for rejection.