Alpha-galactosyl-mediated activation of porcine endothelial cells: studies on CD31 and VE-cadherin in adhesion and signaling

Immunometabolism of Liver Xenotransplantation and Prospective Solutions

End-stage liver diseases, such as hepatocellular carcinoma or acute liver failure, critically necessitate liver transplantation. However, the shortage of available organ donors fails to meet the rapidly growing transplantation demand. Due to the high similarity of liver tissue structure and metabolism between miniature pigs and humans, xenotransplantation of pig livers is considered as a potentially viable solution to organ scarcity. In the 2024, teams from China first time have successfully transplanted a genetically modified Bama miniature pig liver into a clinically brain-dead man lasting for 10 days. This milestone in human xenotransplantation research not only confirms the feasibility of clinical application of xenotransplantation, but also underscores the daunting and protracted nature of this pathway. Despite advanced gene-editing technologies theoretically circumventing the occurrence of most transplant rejection reactions, patients still face challenges such as chronic immune rejection, coagulation disorders, and thrombotic microangiopathy after receiving xenografts. Moreover, prolonged use of immunosuppressive drugs may induce irreversible immune dysfunction, leading to opportunistic infections and metabolic disorders. This article compares the similarities and differences in livers between humans and pigs, summarizes the immunometabolism of xenotransplantation based on current findings, and provides research perspectives on pre-transplantation and post-transplantation strategies for prolonging the survival time of xenografts.

Cytotoxic Responses Mediated by NK Cells and Cytotoxic T Lymphocytes in Xenotransplantation

Xenotransplantation represents a potential solution to the shortage of organs for transplantation. The recent advancements in porcine genetic modification have addressed hyperacute and acute vascular rejection; however, challenges persist with regard to delayed xenograft rejection. Porcine endothelial cells (pECs) represent a crucial target in the context of xenograft rejection, which is mediated by cytotoxic lymphocytes. It is crucial to comprehend the manner in which human natural killer (NK) cells and cytotoxic CD8+ T lymphocytes (CTL) recognize and target pECs in order to develop efficacious prophylactic strategies against rejection. The objective of the present review is to synthesize the existing knowledge regarding the mechanisms and techniques employed to modulate xenogeneic responses mediated by human NK cells and CTL. We will elucidate recent methodological advancements, debate potential novel strategies, and emphasize the imperative necessity for further research and innovative approaches to enhance graft survival.

The Innate Cellular Immune Response in Xenotransplantation

Xenotransplantation is very attractive strategy for addressing the shortage of donors. While hyper acute rejection (HAR) caused by natural antibodies and complement has been well defined, this is not the case for innate cellular xenogeneic rejection. An increasing body of evidence suggests that innate cellular immune responses contribute to xenogeneic rejection. Various molecular incompatibilities between receptors and their ligands across different species typically have an impact on graft outcome. NK cells are activated by direct interaction as well as by antigen dependent cellular cytotoxicity (ADCC) mechanisms. Macrophages are activated through various mechanisms in xenogeneic conditions. Macrophages recognize CD47 as a "marker of self" through binding to SIRPα. A number of studies have shown that incompatibility of porcine CD47 against human SIRPα contributes to the rejection of xenogeneic target cells by macrophages. Neutrophils are an early responder cell that infiltrates xenogeneic grafts. It has also been reported that neutrophil extracellular traps (NETs) activate macrophages as damage-associated pattern molecules (DAMPs). In this review, we summarize recent insights into innate cellular xenogeneic rejection.

The Regulation of Neutrophil Extracellular Trap-induced Tissue Damage by Human CD177

Background

Neutrophil-induced tissue damage contributes to the rejection in xenotransplantation. Therefore, suppressing neutrophil function could be effective in suppressing xenogeneic rejection. In a previous study, we demonstrated that the ectopic expression of human cluster of differentiation 31 (CD31) on porcine endothelial cells (PEC) significantly suppressed neutrophil-mediated cytotoxicity through the homophilic binding of CD31. Cluster of differentiation 177 (CD177) was recently reported to be a high-affinity heterophilic binding partner for CD31 on endothelial cells. Thus, we hypothesized that human CD177 on PEC might induce a stronger suppression in neutrophil-mediated cytotoxicity compared with CD31. In this study, the inhibitory function of human CD177 on PEC in neutrophil-mediated cytotoxicity was investigated.

Methods

PEC were transfected with a cloning plasmid containing cDNA inserts that encoded for hCD177 and hCD31 genes. Neutrophil-induced cytotoxicity was evaluated by flow cytometry after coculturing with PEC or PEC/CD177 in the presence of phorbol 12-myristate 13-acetate. To elucidate the mechanisms responsible for hCD177-induced suppression, the phosphorylation of src homology region 2 domain containing phosphatase 1 was measured by immunoblot analysis.

Results

Human CD177 on PEC induced a significant reduction in neutrophil-induced cytotoxicity. In addition, CD177 on PEC induced a significant increase in the phosphorylation of src homology region 2 domain-containing phosphatase 1 in neutrophils and suppressed NETosis.

Conclusions

These findings suggest that human CD177 suppresses neutrophil-mediated cytotoxicity through the inhibition of NETosis.

Human CD31 on porcine cells suppress xenogeneic neutrophil-mediated cytotoxicity via the inhibition of NETosis

BACKGROUND

Xenotransplantation is one of the promising strategies for overcoming the shortage of organs available for transplant. However, many immunological obstructions need to be overcome for practical use. Increasing evidence suggests that neutrophils contribute to xenogeneic cellular rejection. Neutrophils are regulated by activation and inhibitory signals to induce appropriate immune reactions and to avoid unnecessary immune reactivity. Therefore, we hypothesized that the development of neutrophil-targeted therapies may have the potential for increased graft survival in xenotransplantation.

METHODS

A plasmid containing a cDNA insert encoding the human CD31 gene was transfected into swine endothelial cells (SEC). HL-60 cells were differentiated into neutrophil-like cells by culturing them in the presence of 1.3% dimethyl sulfoxide for 48 hours. The cytotoxicity of the differentiated HL-60 cells (dHL-60) and peripheral blood-derived neutrophils was evaluated by WST-8 assays. To investigate the mechanism responsible for hCD31-induced immunosuppression, citrullinated histone 3 (cit-H3) and phosphorylation of SHP-1 were detected by a cit-H3 enzyme-linked immunosorbent assay (ELISA) and Western blotting, respectively.

RESULTS

A significant decrease in dHL-60 and neutrophil-mediated cytotoxicity in SEC/hCD31 compared with SEC was seen, as evidenced by a cytotoxicity assay. Furthermore, the suppression of NETosis and the induction of SHP-1 phosphorylation in neutrophils that had been co-cultured with SEC/CD31 were confirmed by cit-H3 ELISA and Western blotting with an anti-phosphorylated SHP-1.

CONCLUSION

These data suggest that human CD31 suppresses neutrophil-mediated xenogenic cytotoxicity via the inhibition of NETosis. As CD31 is widely expressed in a variety of inflammatory cells, human CD31-induced suppression may cover the entire xenogeneic cellular rejection, thus making the generation of human CD31 transgenic pigs very attractive for use in xenografts.

The Role of NK Cells in Pig-to-Human Xenotransplantation

Recruitment of human NK cells to porcine tissues has been demonstrated in pig organs perfused ex vivo with human blood in the early 1990s. Subsequently, the molecular mechanisms leading to adhesion and cytotoxicity in human NK cell-porcine endothelial cell (pEC) interactions have been elucidated in vitro to identify targets for therapeutic interventions. Specific molecular strategies to overcome human anti-pig NK cell responses include (1) blocking of the molecular events leading to recruitment (chemotaxis, adhesion, and transmigration), (2) expression of human MHC class I molecules on pECs that inhibit NK cells, and (3) elimination or blocking of pig ligands for activating human NK receptors. The potential of cell-based strategies including tolerogenic dendritic cells (DC) and regulatory T cells (Treg) and the latest progress using transgenic pigs genetically modified to reduce xenogeneic NK cell responses are discussed. Finally, we present the status of phenotypic and functional characterization of nonhuman primate (NHP) NK cells, essential for studying their role in xenograft rejection using preclinical pig-to-NHP models, and summarize key advances and important perspectives for future research.

β1 Integrin as a Xenoantigen in Fetal Porcine Mesencephalic Cells Transplanted into the Rat Brain

Xenografts of fetal porcine mesencephalic cells implanted into the rat striatum are generally rejected within several weeks. The fetal donor mesencephalon predominantly consists of neurons, but also contains microglial and endothelial cells, which are more immunogenic. In the present work, we investigated the occurrence of donor endothelial cells in grafts of porcine mesencephalic cells implanted into the rat striatum. Pig endothelial cells were monitored by immunochemical methods, using a monoclonal antibody (mAb) that recognizes a peptidic epitope of the porcine β1 integrin, and isolectin IB4, for the staining of the Galα1,3Gal epitope. The analysis also involved the detection of the pig hyaluronate receptor CD44, and the cell adhesion molecule CD31. The anti-β1 integrin mAb revealed endothelial-like cells in grafts of porcine mesencephalic cells as soon as 1 week after implantation. A similar staining pattern was obtained with the IB4 lectin. Unlike aortic endothelial cells, these pig brain-derived endothelial-like cells were not recognized by the anti-CD44 antibody. They also failed to express the CD31 adhesion molecule, a fact which suggests that they remained poorly mature, even in grafts maintained during 45 days in immunosuppressed rats. Interestingly, a strong expression of β1 integrin immunoreactivity was noticed in a large proportion (80%) of the cells freshly dissociated from the fetal pig mesencephalic tissue. The immunoreactivity decreased progressively after transplantation of the cells into the rat brain. This observation suggests that dissociated neuroblasts are capable of a temporary expression of β1 integrin. This molecule is known to participate in the process of cell sorting and migration in the developing brain. Hence, its expression could be the hallmark of a rescue mechanism triggered by the disruption of the cell/matrix interactions during the dissociation of the fetal mesencephalon. This disruption might account for part of the dramatic cell death process that occurs during the manipulation of the donor tissue.

Human leukocytes regulate ganglioside expression in cultured micro-pig aortic endothelial cells

Gangliosides are ubiquitous components of the membranes of mammalian cells that are thought to play important roles in various cell functions such as cell-cell interaction, cell adhesion, cell differentiation, growth control, and signaling. However, the role that gangliosides play in the immune rejection response after xenotransplantation is not yet clearly understood. In this study, the regulatory effects of human leukocytes on ganglioside expression in primary cultured micro-pig aortic endothelial cells (PAECs) were investigated. To determine the impact of human leukocytes on the expression of gangliosides in PAECs, we performed high-performance thin layer chromatography (HPTLC) in PAECs incubated with FBS, FBS containing human leukocytes, human serum containing human leukocytes, and FBS containing TNF-α. Both HPTLC and immunohistochemistry analyses revealed that PAECs incubated with FBS predominantly express the gangliosides GM3, GM1, and GD3. However, the expression of GM1 significantly decreased in PAECs incubated for 5 h with TNF-α (10 ng/mL), 10% human serum containing human leukocytes, and 10% FBS containing human leukocytes. Taken together, these results suggest that human leukocytes induced changes in the expression profile of ganglioside GM1 similar to those seen upon treatment of PAECs with TNF-α. This finding may be relevant for designing future therapeutic strategies intended to prolong xenograft survival.

Human leukocyte transmigration across Galalpha(1,3)Gal-negative porcine endothelium is regulated by human CD18 and CD99

BACKGROUND

In pig-to-human xenotransplantation cross-species receptor interactions mediate cellular infiltration and rejection of porcine grafts. However, the mechanisms responsible for recruitment of human leukocyte subsets across porcine endothelial cells (EC) remain largely unknown. Here, we investigated the role of CD99, CD18, and Galalpha(1,3)Gal (Gal) in this process.

METHODS

Adhesion and transmigration of human peripheral blood mononuclear cell (PBMC) subsets on Gal and Gal porcine EC (pEC) and on human EC was analyzed using a two-compartment system separated by a permeable membrane. The mechanisms of human PBMC recruitment to pEC were investigated by blocking cell surface receptors and by differentially measuring adhesion and transendothelial migration (TEM).

RESULTS

Blocking of CD18, but not CD99, decreased human PBMC adhesion on pEC, whereas blocking of CD18 or CD99 strongly reduced the subsequent human PBMC TEM across pEC. The inhibitory effect of CD99 blockade was slightly stronger across pEC as compared with human EC. A critical role for Gal in TEM of human monocytes, B, natural killer (NK), NK/T, and T cells was excluded by evaluating TEM across pEC derived from Gal and Gal pigs.

CONCLUSIONS

CD99 and CD18, but not Gal, play a critical role in human monocyte and lymphocyte TEM across pEC, and their respective porcine ligands may serve as targets to specifically inhibit human leukocyte recruitment in pig-to-human xenotransplantation.

No role of alpha-Gal in human monocyte-endothelial cell interactions in vitro

Vascularized organ xenografts undergoing acute vascular rejection (AVR) are infiltrated by innate immune cells such as monocytes/macrophages. Herein, human monocyte static and dynamic adhesion to, and migration across, human and porcine aortic endothelial cells (HAEC and PAEC) were investigated. To elucidate the role of Gal alpha1,3Gal (alpha-Gal) epitopes in these processes in the absence of anti-Gal antibodies (Ab), this determinant was aberrantly expressed in HAEC. HAEC were transduced with a lentiviral vector encoding the porcine alpha1,3 galactosyltransferase to express alpha-Gal at high frequencies (75-95%). Alpha-Gal expression on HAEC did not increase their ability to support monocyte transendothelial migration or adhesion under either static or flow conditions. Porcine and human endothelium supported static adhesion and migration of monocytes equally well. However, human monocytes adhered less to PAEC than to HAEC (P = 0.03) under flow following human, but not porcine, tumour necrosis factor-alpha stimulation. In the absence of anti-Gal Ab, the alpha-Gal epitope does not contribute to increased monocyte adhesion to, or migration across, endothelium. Thus, inhibiting adhesion receptor-ligand interactions essential for the adhesion of human monocytes to porcine endothelium may be more important than carbohydrate remodelling of donor pigs to prevent adhesion/infiltration of monocytes into organ xenografts during AVR.

Alpha Gal ligation of pig endothelial cells induces protection from complement and apoptosis independently of NF-kappa B and inflammatory changes

Cytoprotection of endothelial cells (EC) is important in EC biology and pathophysiology, including graft rejection. Using porcine aortic EC and human complement as an in vitro model of xenotransplantation, we have reported that ligation of EC Gal alpha (1-3)Gal epitopes (alpha Gal) with antibodies or lectins BS-I and IB4 induces EC resistance to injury by complement. However, before the protective response is observed, alpha Gal ligation induces an early, proinflammatory response. Using a similar model, we now investigated whether the early inflammatory response, as well as NF-kappa B activation, is required for induction of cytoprotection. Despite up-regulation of EC mRNA for many inflammatory cytokines rapidly after BS-I stimulation, recombinant cytokines or conditioned media from EC incubated with BS-I failed to induce protection when used to stimulate EC. While the lectin-induced inflammatory response was markedly reduced by inhibition of NF-kappa B, the protection from complement and apoptosis was unaffected. The lectins caused up-regulation of mRNA for protective genes A20, porcine inhibitor of apoptosis protein and hemoxygenase-1, which was not modified by NF-kappa B inhibition. These findings suggest that induction of cytoprotection in porcine EC by alpha Gal ligation results from activation of pathways that are largely independent of those that elicit NF-kappaB activation and the inflammatory response.

Role of poly(ADP-ribose) polymerase activation in diabetic neuropathy

Oxidative and nitrosative stress play a key role in the pathogenesis of diabetic neuropathy, but the mechanisms remain unidentified. Here we provide evidence that poly(ADP-ribose) polymerase (PARP) activation, a downstream effector of oxidant-induced DNA damage, is an obligatory step in functional and metabolic changes in the diabetic nerve. PARP-deficient (PARP(-/-)) mice were protected from both diabetic and galactose-induced motor and sensory nerve conduction slowing and nerve energy failure that were clearly manifest in the wild-type (PARP(+/+)) diabetic or galactose-fed mice. Two structurally unrelated PARP inhibitors, 3-aminobenzamide and 1,5-isoquinolinediol, reversed established nerve blood flow and conduction deficits and energy failure in streptozotocin-induced diabetic rats. Sciatic nerve immunohistochemistry revealed enhanced poly(ADP-ribosyl)ation in all experimental groups manifesting neuropathic changes. Poly(ADP-ribose) accumulation was localized in both endothelial and Schwann cells. Thus, the current work identifies PARP activation as an important mechanism in diabetic neuropathy and provides the first evidence for the potential therapeutic value of PARP inhibitors in this devastating complication of diabetes.

Are anti-endothelial cell antibodies a pre-requisite for the acute vascular rejection of xenografts?

BACKGROUND

Vascular rejection occurring within the first few weeks after transplantation is still the major immunological barrier to the long term survival of xenografts. Currently there is no consensus about what to call this type of rejection (acute vascular rejection, delayed xenograft rejection or acute humoral xenograft rejection), nor about how to prevent or treat it.

METHODS

A review of published evidence to define the heterogeneity of this phase of rejection and examine the role of antibodies, complement and graft-infiltrating inflammatory cells.

RESULTS

i) antibodies are always involved in acute vascular rejection; ii) this antibody-mediated rejection may be complement-dependent or -independent; iii) inflammatory cells may mediate an antibody- and complement-independent phase of rejection in some small animal models (which, in its pure form cannot be called 'vascular rejection') iv) there remain significant questions about the relevance of 'accommodation' and the importance of coagulation abnormalities.

CONCLUSIONS

Without doubt, future research would be helped by distinguishing between these different forms of delayed xenograft rejection, using terminology to reflect the involvement of specific pathophysiological mechanisms. An updated classification of the stages of xenograft rejection is proposed here.

Rolling adhesion of human NK cells to porcine endothelial cells mainly relies on CD49d-CD106 interactions

BACKGROUND

Acute vascular rejection in pig-to-primate xenotransplantation involves recognition and damage of porcine (po) endothelial cells (EC) by human (hu) leukocytes, probably including natural killer (NK) cells. To study such interactions we analyzed rolling and static adhesion of hu NK cells to po EC.

METHODS

The effects of blocking hu and po adhesion molecules on the adhesion hu NK cells to po EC monolayers was analyzed under shear stress (10 min, 37 degrees C, 0.7 dynes/cm2) or under static conditions (10 min, 37 degrees C). All used cell populations were phenotypically characterized by flow cytometry.

RESULTS

Blocking of CD106 on po EC or its ligand CD49d on hu NK cells decreased rolling adhesion of both fresh and activated hu NK cells by more than 75%. Masking of CD62L on fresh but not activated hu NK resulted in a 44% decrease in rolling adhesion, in line with the diminished cell surface expression of CD62L upon activation. Antibodies to CD31, CD54, CD62E, and CD62P on EC or CD11a, CD18, and CD162 on NK cells had only minor effects on rolling adhesion. The adhesion of the FcgammaRIII- hu NK cell line NK92 to po EC was inhibited by 95% after masking po CD106 whereas antibodies to po CD31, CD54, CD62E, or CD62P had no effect, thereby excluding effects of Fc-receptor-dependent binding of hu NK cells to po EC. Static adhesion of activated NK cells was reduced by approximately 60% by blocking either CD49d or CD106, by 47% by blocking CD11a, and by 82% upon simultaneous blocking of CD11a and CD49d.

CONCLUSIONS

Interactions between hu CD49d and po CD106 are crucial for both rolling and firm adhesion of hu NK cells to po EC and thus represent attractive targets for specific therapeutic interventions to prevent NK cell-mediated responses against po xenografts.

Influence of humoral immunoreaction on hepatic nonparenchymal cells in ex situ xenoperfused rat livers

BACKGROUND

The influence of xenogeneic humoral immunoreaction on hepatic nonparenchymal cells (NPCs) was evaluated ex situ in xenoperfused rat livers.

METHODS

Isolated rat livers were perfused via the portal vein (PV) for 240 min. The perfusates consisted of fresh rat blood (group 1), fresh human blood (group 2), and fresh human blood containing 5 microg/mL soluble complement receptor type 1 (Group 3).

RESULTS

Deposition of human IgM and C(5b-9) complement was observed in group 2, although only human IgM deposition was detected in group 3. Portal vein pressure in group 2 rose drastically during the first 10 min. Creatine kinase BB component gradually increased in all groups, followed by an elevation in alanine aminotransferase and both parameters were significantly higher in group 2 than in groups 1 and 3. In group 2, platelet thrombi in the peripheral PVs and periportal hemorrhage were observed after 10 min, and massive necrosis around the central veins after 240 min; these changes were not observed in group 1 or 3. Production of tumor necrosis factor alpha and alpha interferon and expression of intercellular adhesion molecule 1 (ICAM-1) were lower in group 2 than in groups 1 and 3. In group 2, there were negative areas for ICAM-1 and tumor necrosis factor alpha staining around the central veins after 240 min, which were consistent with necrotic areas.

CONCLUSIONS

In xenoperfused rat livers, humoral mediators initially caused the disturbance of microcirculation, which would induce long ischemia in the pericentral areas, resulting in massive necrosis. NPC necrosis may be responsible for less production of cytokines and adhesion molecules in the xenoperfused livers.

Adhesive interactions between human NK cells and porcine endothelial cells

Human natural killer (NK) cells are able to adhere to xenogeneic porcine endothelial cells (EC) and evidence from in vitro studies as well as animal models suggests a potential role for NK cells in the cellular recognition and damage of porcine xenogeneic tissues. One possible explanation for the observed NK cell-mediated xenogeneic cytotoxicity against porcine EC is the molecular incompatibility between porcine major histocompatibility complex (MHC) class I molecules and MHC-specific inhibitory receptors on human NK cells. In this review we attempt to summarize the current knowledge concerning adhesive interactions between human NK cells and porcine EC under special considerations of the cross-species receptor-ligand interactions. Methodological differences in assessing adhesion between various studies are reviewed and comparisons to the syngeneic/allogeneic adhesion mechanisms are made. Finally, the therapeutic potential of blocking antibodies and transgenic HLA expression in preventing NK-cell adhesion and xenogeneic cytotoxicity is discussed.

Signaling through CD31 protects endothelial cells from apoptosis

Endothelial damage has been implicated in the pathogenesis of chronic rejection. Conversely, expression of protective genes [including A20, A1, bcl-xl, and hemoxygenase-1 (HO-1)] in the endothelium has been associated with long-term graft survival. Overexpression of protective genes in cultured endothelial cells confers protection from apoptosis and prevents expression of inflammatory molecules through inactivation of NF-kappaB. CD31 (PECAM-1) expressed at endothelial cell junctions is ligated by leukocytes during transendothelial migration. Our laboratory has recently shown that cross-linking CD31 using a monoclonal antibody (LCI-4) triggers signaling events in endothelial cells. In this study, we demonstrate that treatment with LCI-4 protected serum-starved endothelial cells from apoptosis. CD31 cross-linking also led to elevation of A20 and A1 mRNA levels and activation of the transcription factor Sp-1. In summary, signaling through CD31 on endothelial cells leads to protection from apoptosis in association with up-regulation of two protective molecules, A20 and A1.

Professional and non-professional antigen-presenting cells in the porcine small intestine

We have previously presented evidence of a highly organized and compartmentalized structure of the small intestinal lamina propria of the pig. In this work, we have identified at least two major populations of cells in this site expressing high levels of major histocompatibility complex (MHC) class II antigens. One is CD45 positive and is a potent initiator of a primary immune response, this is a function usually associated with dendritic cells. These cells have characteristic dendritic morphology, but show evidence of phagocytosis as well as other phenotypic markers of immature dendritic cells. Some cells show evidence of ongoing immune maturation. We have also isolated CD45 negative endothelial cells bearing significant amounts of MHC class II, which do not trigger a mixed lymphocyte reaction. These findings have implications for the functional role of healthy gut lamina propria and clearly implicate this site as capable of differential antigen presentation by a heterogeneous population of antigen-presenting cells.