The role of monocytes and macrophages in delayed xenograft rejection

Highly disparate xenogeneic skin graft tolerance induction by fetal pig thymus in thymectomized mice: Conditioning requirements and the role of coimplantation of fetal pig liver

BACKGROUND

Highly disparate xenogeneic pig skin graft tolerance and efficient repopulation of mouse CD4+ T cells are achieved in thymectomized (ATX) B6 mice that receive T cell and natural killer (NK) cell depletion by injection of a mixture of monoclonal antibodies (mAbs) (GK1.5, 2.43, 30-H12, and PK136) on days -6, -1, +7, and +14 and 3 Gy total body irradiation (TBI) followed by implantation of fetal pig thymus/liver (FP THY/LIV) grafts on day 0. The requirements for each treatment in this model to achieve pig skin graft tolerance have not previously been defined. Therefore, we performed a series of experiments to address the role of each treatment in achieving maximal skin graft tolerance.

METHODS

Peripheral mouse CD4+ T-cell repopulation and pig skin graft survival were followed in this pig-to-mouse model in which recipient B6 mice were treated with modified regimens that omitted thymectomy, 3 Gy TBI, anti-Thy1.2, and anti-NK1.1 mAbs, injection of a mixture of mAbs on day +14, or coimplantation of FP LIV, respectively.

RESULTS

Prolongation but not permanent survival of donor MHC-matched pig skin grafts was observed in euthymic B6 mice that received T and NK cell depletion, 3 Gy TBI, and 7 Gy thymic irradiation and FP THY/LIV in the mediastinum, suggesting that full xenogeneic tolerance was not achieved in euthymic mice. However, after grafting FP THY alone to ATX B6 mice treated either with the "standard" regimen, or with a conditioning regimen that omitted all components of the conditioning regimen except treatment with anti-CD4 and anti-CD8 mAbs, efficient peripheral repopulation of mouse CD4+ T cells and long-term donor MHC-matched pig skin graft acceptance were observed.

CONCLUSIONS

Highly disparate xenogeneic pig skin graft tolerance can be achieved by grafting FP THY alone in anti-CD4 and anti-CD8 mAb-treated ATX B6 mice, but not in euthymic B6 mice. Additional treatment of ATX recipient mice with anti-Thy1.2 and NK1.1 mAbs and 3 Gy TBI is not essential for donor pig skin graft tolerance induction.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

Adherence of human monocytes and NK cells to human TNF-alpha-stimulated porcine endothelial cells

Discordant xenograft models undergoing delayed rejection response are characterized by xenograft infiltration with host monocytes and NK cells, associated with the release of large quantities of pro-inflammatory cytokines, such as TNF-alpha. In the present study, human monocytes (PBMo)/NK cells (PBNK) isolated from peripheral blood and cultured porcine aortic endothelial cells (PAEC) treated with recombinant human TNF-alpha (rhTNF-alpha) were used to investigate their adhesive interactions and mAbs against porcine E-selectin, human CD11a and CD49d were used to test their relative contributions to such intercellular adhesions. The PBMo exhibited significantly greater adherence to resting (unstimulated) PAEC than PBNK. The rhTNF-alpha upregulated E-selectin and vascular cell adhesion molecule-1 (VCAM-1) expression on PAEC and augmented the adhesiveness of PAEC for PBMo and PBNK in a time- and dose-dependent manner. In mAb blocking assays, anti-E-selectin, anti-CD11a and anti-CD49d mAbs did not inhibit PBMo adherence to rhTNF-alpha-stimulated PAEC when used singly, but resulted in a maximal inhibitory effect when used in combination. Regarding PBNK, anti-E-selectin mAb had no marked influence on PBNK adherence. The combined use of anti-CD11a and anti-CD49d mAbs produced additive reduction in the PBNK binding to rhTNF-alpha-stimulated PAEC, even to far below baseline (unstimulated) levels. Therefore, it is concluded that human TNF-alpha promotes the adhesiveness of PAEC for human monocytes and NK cells and that the mechanism underlying the increased adherence differs for PBMo and PBNK.

Synthetic peptides which inhibit the interaction between C1q and immunoglobulin and prolong xenograft survival

BACKGROUND

Acute vascular xenograft rejection (AVXR), also termed delayed xenograft rejection (DXR), occurs when hyperacute rejection (HAR) is prevented by strategies directed at xenoreactive natural antibodies and/or complement activation. We have hypothesized that AVXR/DXR is initiated in part by early components of the complement cascade, notably C1q. We have developed synthetic peptides (termed CBP2 and WY) that interfere with the interaction between C1q and antibody.

METHODS

CBP2 and the WY-conjugates were used as inhibitors of immunoglobulin aggregate binding to solid phase C1q. Inhibition of complement activation by the peptides of the classical system was determined using lysis assays with sensitized sheep red blood cells or porcine aortic endothelial cells as targets and of the alternate complement pathway using guinea pig red blood cells as targets. Two transplant models were used to study the effects of administering peptides to recipients: rat heart transplant to presensitized mouse, and guinea heart transplant to PVG C6-deficient rats.

RESULTS

CBP2 and WY-conjugates inhibited immunoglobulin aggregate binding to C1q. The peptides also inhibited human complement-mediated lysis of sensitized sheep red blood cells and porcine aortic endothelial cells in a dose-dependent manner and the WY-conjugates prevented activation of the alternate complement pathway as shown by inhibition of guinea pig red blood cells lysis with human serum. In addition, the use of the peptides and conjugates resulted in significant prolongation of xenograft survival.

CONCLUSIONS

The CBP2 and WY peptides exhibit the functional activity of inhibition of complement activation. These peptides also prolong xenograft survival and thus provide reagents for the study of the importance of C1q and other complement components in transplant rejection mechanisms.

Acute humoral xenograft rejection: destruction of the microvascular capillary endothelium in pig-to-nonhuman primate renal grafts

The major cause of xenograft loss beyond hyperacute rejection is a form of injury, traditionally termed delayed xenograft rejection (DXR), whose pathogenesis is unknown. Here we analyze the immunologic and morphologic features of DXR that develops in pig kidney xenografts transplanted into nonhuman primates. Kidneys from miniature swine were transplanted into cynomolgus monkeys (n = 14) or baboons (n = 11) that received regimens aimed to induce mixed chimerism and tolerance. No kidney was rejected hyperacutely. Morphologic and immunohistochemical studies were performed on serial biopsies, and an effort was made to quantify the pathologic features seen. The early phase of DXR (Days 0-12) was characterized by focal deposition of IgM, IgG, C3, and scanty neutrophil and macrophage infiltrates. The first abnormality recognized was glomerular and peritubular capillary endothelial cell death as defined by in situ DNA nick-end labeling (TUNEL). Damaged endothelial cells underwent apoptosis and, later, frank necrosis. The progressive phase developed around Day 6 and was characterized by progressive deposition of IgM, IgG, C3, and prominent infiltration of cytotoxic T cells and macrophages, with a small number of NK cells. Thrombotic microangiopathy developed in the glomeruli and peritubular capillaries with TUNEL+ endothelial cells, platelet aggregation, and destruction of the capillary network. Only rare damaged arterial endothelial cells and tubular epithelial cells were observed, with rare endothelialitis and tubulitis. In the advanced phase of DXR, interstitial hemorrhage and infarction occurred. During the development of DXR, the number of TUNEL+ cells increased, and this correlated with progressive deposition of antibody. The degree of platelet aggregation correlated with the number of TUNEL+ damaged endothelial cells. We conclude that peritubular and glomerular capillary endothelia are the primary targets of renal DXR rather than tubular epithelial cells or arterial endothelium and that the earliest detectable change is endothelial cell death. DXR was characterized by progressive destruction of the microvasculature (glomeruli and peritubular capillaries) and formation of fibrin-platelet thrombi. Both cytotoxic cells and antibodies potentially mediate the endothelial damage in DXR; however, in this model, DXR is largely humorally mediated and is better termed "acute humoral xenograft rejection."

The critical role of mouse CD4+ cells in the rejection of highly disparate xenogeneic pig thymus grafts

Long-term survival of fetal pig thymus (FP THY) grafts and efficient repopulation of mouse CD4+ T cells is achieved in thymectomized (ATX) B6 mice that receive T and NK cell depletion by injection of a cocktail of mAbs (GK1.5, 2.43, 30-H12, and PK136) and fetal pig thymus/liver (FP THY/LIV) grafts. The requirement for each mAb in this conditioning regimen in order to avoid the rejection of FP THY grafts has not yet been defined. In our present studies, CD4 cell-depleted ATX B6 mice and euthymic MHC class II-deficient (IIKO) mice were employed to investigate the role of mouse CD4+ cells in the rejection of FP THY grafts in vivo. After grafting FP THY/LIV to CD4+ cell-depleted ATX B6 mice, efficient repopulation of mouse CD4+ T cells was observed in the periphery. However, only two of four mice had remaining FP THY grafts by 17 weeks post-implantation, and these were of poor quality, whereas four of four T and NK cell-depleted ATX B6 mice had well-developed FP THY grafts. Furthermore, three of four FP THY/LIV-grafted, CD4+ cell-depleted ATX B6 mice rejected donor MHC-matched pig skin grafts. In contrast, three of three FP THY/LIV grafted, T and NK cell-depleted, ATX B6 mice accepted donor MHC-matched pig skin grafts, suggesting that optimal tolerance to xenogeneic pig antigens was not achieved in mice conditioned only with anti-CD4 mAb. ATX B6 mice treated with only anti-CD8 mAb rejected FP THY completely by 6 weeks post-grafting, a time when CD4+ cell-depleted ATX B6 mice had well-vascularized FP THY grafts. In addition, when euthymic IIKO mice were pre-treated with the standard conditioning regimen that includes four different mAbs, FP THY grafts survived and supported the repopulation of mouse CD4+ T cells in the periphery, while high levels of mouse CD8+ T cells developed in host thymi. These studies suggest that mouse CD4+ T cells play a critical role in the acute rejection of xenogeneic FP THY grafts. Without help from CD4+ cells, mouse CD8+ cells, NK, NK/T, and TCR(gamma/delta)+ T cells do not mediate acute rejection of FP THY grafts. Furthermore, our results suggest that other cell subsets besides CD4+ T cells play a role in the delayed rejection of highly disparate xenogeneic FP THY grafts.

The effect of macrophage depletion on delayed xenograft rejection: studies in the guinea pig-to-C6-deficient rat heart transplantation model

The purpose of this study was to investigate the effect of macrophage depletion, using liposome-encapsulated dichloromethylene diphosphonate (Lip-Cl2MDP), on delayed xenograft rejection (DXR) in the guinea pig-to-C6-deficient rat heart transplantation model. In this model, hyperacute rejection does not occur, but, in untreated recipients, xenografts are still destroyed by DXR within 1-2 days. Graft survival was 68 +/- 8.4 h in splenectomized control rats, 77 +/- 16.3 h with Lip-Cl2MDP alone, 99 +/- 10.4 h with deoxysperguarlin (DSG; P < 0.01 vs. controls), and 127 +/- 19.4 h with Lip-Cl2MDP plus DSG (P < 0.01 vs. DSG alone). Treatment with DSG alone or in combination with Lip-Cl2MDP resulted in significant reductions in serum IgM levels at rejection. Immunohistological studies showed that Lip-Cl2MDP alone or in combination with DSG reduced infiltration of grafts by both EDI + and ED2 + macrophages. These experiments support the concept that macrophages play an important role in DXR and suggest that strategies targeting macrophages may be useful in controlling DXR.

T cell response in xenorecognition and xenografts: a review

Xenotransplantation has recently become a subject of interest for the transplantation community due to the current organ shortage, which could be partially or even totally solved by the development of this strategy. The humoral response, which arises as a result of species disparities, is the major obstacle to the success of xenotransplantation. However, if the use of different strategies such as plasmapheresis, immunoadsorption, the utilization of organs from transgenic pigs for complement regulatory molecules and new immunosuppressive drugs, may allow to overcome or reduce the early antibody mediated rejections (hyperacute or acute vascular rejection), delayed responses based on cellular activations will still occur. In this review, despite the fact that different cell populations have been shown to be implicated in these phenomena (NK, granulocytes, macrophages), we will focus on recent published information concerning T cell response only, in xenorecognition.

IgY antiporcine endothelial cell antibodies effectively block human antiporcine xenoantibody binding

Avian IgY antibodies are structurally different from mammalian IgGs and do not fix mammalian complement components or bind human Fc receptors. As these antibody-mediated interactions are believed to play significant roles in both hyperacute rejection (HAR) and acute vascular xenograft rejection (AVXR), IgY antibodies to xenoantigen target epitopes may inhibit these rejection processes. In this report, we show that chicken IgY antibodies to alpha-Gal antigen epitopes and to other porcine aortic endothelial cell (PAEC) antigens block human xenoreactive natural antibody binding to both porcine and rat cardiac tissues and porcine kidney tissues. Chicken IgY antibodies blocked complement-mediated lysis of PAECs by human serum, and inhibited antibody-dependent cell-mediated lysis of PAECs by heat-inactivated human serum plus peripheral blood leukocytes. Binding of IgY to porcine endothelial cells did not affect cell morphology nor expression of E-selectin. These results suggest that avian IgYs could be of potential use in inhibiting pig-to-human xenograft rejection.

Anti-Gal antibody-mediated allograft rejection in alpha1,3-galactosyltransferase gene knockout mice: a model of delayed xenograft rejection

BACKGROUND

The key role of anti-galactose alpha1,3-galactose (anti-alphaGal) xenoantibodies in initiating hyperacute xenograft rejection has been clearly demonstrated using a variety of in vitro and in vivo approaches. However, the role of anti-alphaGal antibodies in mediating post-hyperacute rejection mechanisms, such as antibody-dependent cellular cytoxicity, remains to be determined, primarily because of the lack of a small animal model with which to study this phenomena.

METHODS

Hearts from wild-type mice were transplanted heterotopically into alpha1,3-galactosyltransferase knockout (Gal KO) mice, which like humans develop antibodies to the disaccharide galactose alpha1,3-galactose (Gal). At the time of rejection, hearts were examined histologically to determine the mechanism of rejection.

RESULTS

Hearts from wild-type mice transplanted into high-titer anti-alphaGal recipients were rejected in 8-13 days. Histological examination demonstrated a cellular infiltrate consisting of macrophages (80-90%), natural killer cells (5-10%), and T cells (1-5%). In contrast, wild-type hearts transplanted into low anti-Gal titer recipients demonstrated prolonged (>90 day) survival. However, a significant proportion (30-40%) of these underwent a minor rejection episode between 10 and 13 days, but then recovered ("accommodated").

CONCLUSIONS

The results of this study suggest that the Gal KO mouse is a useful small animal vascularized allograft model, in which the role of anti-alphaGal antibody in graft rejection can be studied in isolation from other rejection mechanisms. The titer of anti-alphaGal antibody was found to be the critical determinant of rejection. The histopathological features of rejection in this model are very similar to other models of delayed xenograft rejection, in both the timing and composition of the cellular infiltrate. The Gal KO mouse therefore provides a new rodent model, which will aid in the identification of the distinct components involved in the pathogenesis of delayed xenograft rejection.

Contribution of activated macrophages to the process of delayed xenograft rejection

BACKGROUND

When hyperacute rejection, involving natural xenoreactive antibodies (XAb) and/or complement (C), can be prevented, xenografts (Xgs) undergo delayed xenograft rejection associated with a progressive mononuclear cell infiltration. We have previously shown that XAb formation can be totally suppressed in leflunomide (LF)-treated, T-deficient nude rats receiving hamster hearts. Hence, this model was well-suited to study a role played by other factors, e.g., natural killer (NK) cells and macrophages (Mphi). The relative contribution of Mphi to delayed xenograft rejection was investigated.

METHODS

In addition to LF (20 mg/kg/24 hr p.o.), anti-asialoGM-1 serum (1 mg/48 hr i.v.) and N omega-nitro-L-arginine methyl ester (L-NAME; 100 mg/kg/24 hr i.v.) were given. Graft-infiltrating cells, deposition of cytokines (interferon-gamma [IFN-gamma] and tumor necrosis factor-alpha [TNF-alpha]), IgM and C, and expression of endothelial cell (EC) P- and E-selectins were investigated by immunohistochemistry. In some cases, rat rTNF-alpha or anti-TNF-alpha antibodies were injected intravenously.

RESULTS

Xgs rejected after 3 days by LF-treated rats showed an absence of IgM, C, and T cells, but the infiltration of NK cells and Mphi, together with the presence of IFN-gamma and TNF-alpha. Addition of NK cell depletion resulted in a significantly prolonged survival of Xgs (6 days; P<0.001) in which NK cells and IFN-gamma had disappeared, but Mphi were still prominent. Additional blockade of Mphi nitric oxide (NO) with L-NAME further prolonged Xg survival (11 days; P<0.001). In these rejected Xgs, Mphi, TNF-alpha, and EC expression of P- and E-selectins was still found, together with platelet thrombi, neutrophil-EC adhesion, and vessel intima lesions. The role of TNF-alpha in initiating this Xg rejection was further demonstrated by the acceleration of Xg rejection after injection of rTNF-alpha and by a synergism between L-NAME and anti-TNF-alpha antibodies in hampering the acceleration of Xg rejection seen after transfer of sensitized Mphi.

CONCLUSION

In the absence of XAb, T cells, and NK cells, Mphi can still reject Xgs. Both NO-dependent and NO-independent mechanisms are involved. In the latter case, Mphi-derived, TNF-alpha-associated EC activation may play a role.