Activation of the lectin pathway of complement in pig-to-human xenotransplantation models

Mechanism and regulation of the complement activity in kidney xenotransplantation

Xenotransplantation is emerging as one of several potential solutions for addressing organ donor shortages, with significant progress bringing it closer to clinical application. However, challenges remain, particularly concerning complement system dysregulation caused by species differences, as well as xenoantigens and coagulopathy. Complement regulatory proteins expressed on endothelial cells of donor xenografts are less compatible with complement components in recipients. These difficulties contribute to hyperacute rejection, characterized by antibody-mediated complement activation that destroys the graft within 24 h. Moreover, because molecules are incompatible across different species, ischemia-reperfusion injury or infection can easily elicit complement activity via all three pathways, resulting in xenograft loss via complement-mediated vascular injury. Complement activity also stimulate innate and adaptive immune cells. To address this issue, genetic modifications in donor pigs and the development of novel medicines have been tested in preclinical models with promising results. Pigs modified to express human complement-regulating molecules such as CD46, CD55, and CD59 have shown longer kidney xenograft survivals over years in preclinical models with nonhuman primates, paving the way for clinical trials. Anti-complement component agents such as C1 esterase and C5 inhibitors have also been shown to increase xenograft survivals. This review examines the role of the complement system in kidney xenotransplantation, emphasizing new research and clinical trial advancements.

Alteration of γδ T cell subsets in non-human primates transplanted with GGTA1 gene-deficient porcine blood vessels

BACKGROUND

αGal-deficient xenografts are protected from hyperacute rejection during xenotransplantation but are still rejected more rapidly than allografts. Despite studies showing the roles of non-Gal antibodies and αβ T cells in xenograft rejection, the involvement of γδ T cells in xenograft rejection has been limitedly investigated.

METHODS

Six male cynomolgus monkeys were transplanted with porcine vessel xenografts from wild-type (n = 3) or GGTA1 knockout (n = 3) pigs. We measured the proportions and T cell receptor (TCR) repertoires of blood γδ T cells before and after xenotransplant. Grafted porcine vessel-infiltrating immune cells were visualized at the end of experiments.

RESULTS

Blood γδ T cells expanded and infiltrated into the graft vessel adventitia following xenotransplantation of α-Gal-deficient pig blood vessels. Pre- and post-transplant analysis of γδ TCR repertoire revealed a transition in δ chain usage post-transplantation, with the expansion of several clonotypes of δ1, δ3, or δ7 chains. Furthermore, the distinctions between pre- and post-transplant δ chain usages were more prominent than those observed for γ chain usages.

CONCLUSION

γδ TCR repertoire was significantly altered by xenotransplantation, suggesting the role of γδ T cells in sustained xenoreactive immune responses.

Minimizing Ischemia Reperfusion Injury in Xenotransplantation

The recent dramatic advances in preventing "initial xenograft dysfunction" in pig-to-non-human primate heart transplantation achieved by minimizing ischemia suggests that ischemia reperfusion injury (IRI) plays an important role in cardiac xenotransplantation. Here we review the molecular, cellular, and immune mechanisms that characterize IRI and associated "primary graft dysfunction" in allotransplantation and consider how they correspond with "xeno-associated" injury mechanisms. Based on this analysis, we describe potential genetic modifications as well as novel technical strategies that may minimize IRI for heart and other organ xenografts and which could facilitate safe and effective clinical xenotransplantation.

Recent advances into the role of pattern recognition receptors in transplantation

Pattern recognition receptors (PRRs) are germline-encoded sensors best characterized for their critical role in host defense. However, there is accumulating evidence that organ transplantation induces the release or display of molecular patterns of cellular injury and death that trigger PRR-mediated inflammatory responses. There are also new insights that indicate PRRs are able to distinguish between self and non-self, suggesting the existence of non-clonal mechanisms of allorecognition. Collectively, these reports have spurred considerable interest into whether PRRs or their ligands can be targeted to promote transplant survival. This review examines the mounting evidence that PRRs play in transplant-mediated inflammation. Given the large number of PRRs, we will focus on members from four families: the complement system, toll-like receptors, the formylated peptide receptor, and scavenger receptors through examining reports of their activity in experimental models of cellular and solid organ transplantation as well as in the clinical setting.

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.

Release of pig leukocytes and reduced human NK cell recruitment during ex vivo perfusion of HLA-E/human CD46 double-transgenic pig limbs with human blood

BACKGROUND

In pig-to-human xenotransplantation, interactions between human natural killer (NK) cells and porcine endothelial cells (pEC) are characterized by recruitment and cytotoxicity. Protection from xenogeneic NK cytotoxicity can be achieved in vitro by the expression of the non-classical human leukocyte antigen-E (HLA-E) on pEC. Thus, the aim of this study was to analyze NK cell responses to vascularized xenografts using an ex vivo perfusion system of pig limbs with human blood.

METHODS

Six pig forelimbs per group, respectively, stemming from either wild-type (wt) or HLA-E/hCD46 double-transgenic (tg) animals, were perfused ex vivo with heparinized human blood for 12 hours. Blood samples were collected at defined time intervals, cell numbers counted, and peripheral blood mononuclear cells analyzed for phenotype by flow cytometry. Muscle biopsies were analyzed for NK cell infiltration. In vitro NK cytotoxicity assays were performed using pEC derived from wt and tg animals as target cells.

RESULTS

Ex vivo, a strong reduction in circulating human CD45 leukocytes was observed after 60 minutes of xenoperfusion in both wt and tg limb groups. NK cell numbers dropped significantly. Within the first 10 minutes, the decrease in NK cells was more significant in the wt limb perfusions as compared to tg limbs. Immunohistology of biopsies taken after 12 hours showed less NK cell tissue infiltration in the tg limbs. In vitro, NK cytotoxicity against hCD46 single tg pEC and wt pEC was similar, while lysis of double tg HLA-E/hCD46 pEC was significantly reduced. Finally, circulating cells of pig origin were observed during the ex vivo xenoperfusions. These cells expressed phenotypes mainly of monocytes, B and T lymphocytes, NK cells, as well as some activated endothelial cells.

CONCLUSIONS

Ex vivo perfusion of pig forelimbs using whole human blood represents a powerful tool to study humoral and early cell-mediated rejection mechanisms of vascularized pig-to-human xenotransplantation, although there are several limitations of the model. Here, we show that (i) transgenic expression of HLA-E/hCD46 in pig limbs provides partial protection from human NK cell-mediated xeno responses and (ii) the emergence of a pig cell population during xenoperfusions with implications for the immunogenicity of xenografts.

Surface modification of pig endothelial cells with a branched heparin conjugate improves their compatibility with human blood

Corline Heparin Conjugate (CHC), a compound of multiple unfractionated heparin chains, coats cells with a glycocalyx-like layer and may inhibit (xeno)transplant-associated activation of the plasma cascade systems. Here, we investigated the use of CHC to protect WT and genetically modified (GTKO.hCD46.hTBM) pig aortic endothelial cells (PAEC) in two pig-to-human in vitro xenotransplantation settings. Model 1: incubation of untreated or hTNFα-treated PAEC with 10% human plasma induced complement C3b/c and C5b-9 deposition, cellular activation and coagulation activation in WT and GTKO.hCD46.hTBM PAEC. Coating of untreated or hTNFα-treated PAEC with CHC (100 µg/ml) protected against human plasma-induced endothelial activation and damage. Model 2: PAEC were grown on microcarrier beads, coated with CHC, and incubated with non-anticoagulated whole human blood. Genetically modified PAEC significantly prolonged clotting time of human blood (115.0 ± 16.1 min, p < 0.001) compared to WT PAEC (34.0 ± 8.2 min). Surface CHC significantly improved the human blood compatibility of PAEC, as shown by increased clotting time (WT: 84.3 ± 11.3 min, p < 0.001; GTKO.hCD46.hTBM: 146.2 ± 20.4 min, p < 0.05) and reduced platelet adhesion, complement activation, coagulation activation and inhibition of fibrinolysis. The combination of CHC coating and genetic modification provided the greatest compatibility with human blood, suggesting that pre-transplant perfusion of genetically modified porcine organs with CHC may benefit post-transplant xenograft function.

Complement inhibition in a xenogeneic model of interactions between human whole blood and porcine endothelium

Xenotransplantation (xeno-Tx) is considered as an alternative solution to overcome the shortage of human donor organs. However, the success of xeno-Tx is hindered by immune reactions against xenogeneic cells (e. g. of porcine origin). More specifically, activation of innate immune mechanisms such as complement and triggering of the coagulation cascade occur shortly after xeno-Tx, and adhesion of human leukocytes to porcine endothelium is another early critical step mediating the immune attack. To investigate the therapeutic potential of complement inhibition in the context of xenogeneic interactions, we have employed a whole-blood model in the present study. Incubation of human blood with porcine endothelial cells (PAECs) led to activation of complement and coagulation as well as to increased leukocyte adhesion. The observed responses can be attributed to the pig-to-human xenogeneicity, since the presence of human endothelium induced a minor cellular and plasmatic inflammatory response. Importantly, complement inhibition using a potent complement C3 inhibitor, compstatin analogue Cp40, abrogated the adhesion of leukocytes and, more specifically, the attachment of neutrophils to porcine endothelium. Moreover, Cp40 inhibited the activation of PAECs and leukocytes, since the levels of the adhesion molecules E-selectin, ICAM-1, ICAM-2, and VCAM-1 on PAECs and the surface expression of integrin CD11b on neutrophils were significantly decreased. Along the same line, inhibition of CD11b resulted in decreased leukocyte adhesion. Taken together, our findings provide a better understanding of the mechanisms regulating the acute innate immune complications in the context of xeno-Tx and could pave the way for complement-targeting therapeutic interventions.

The porcine innate immune system: an update

Over the last few years, we have seen an increasing interest and demand for pigs in biomedical research. Domestic pigs (Sus scrofa domesticus) are closely related to humans in terms of their anatomy, genetics, and physiology, and often are the model of choice for the assessment of novel vaccines and therapeutics in a preclinical stage. However, the pig as a model has much more to offer, and can serve as a model for many biomedical applications including aging research, medical imaging, and pharmaceutical studies to name a few. In this review, we will provide an overview of the innate immune system in pigs, describe its anatomical and physiological key features, and discuss the key players involved. In particular, we compare the porcine innate immune system to that of humans, and emphasize on the importance of the pig as model for human disease.

CD55 limits sensitivity to complement-dependent cytolysis triggered by heterologous expression of α-gal xenoantigen in colon tumor cells

Engineering cancer cells to express heterologous antigen α-gal and induce the destruction of tumor cells depending on the complement cascade may be a promising strategy of tumor therapy. However, the feasibility and effect of using α-gal to induce colorectal adenocarcinoma cell line cytolysis is not yet known. In this study, we evaluated α-gal expression's ability to sensitize human colorectal adenocarcinoma cell lines to complement attack in cell lines LoVo, SW620, and Ls-174T. Nearly all α-gal-expressing LoVo and SW620 cells were killed by normal human serum (NHS), but α-gal-expressing Ls-174T cells showed no significant lysis. We analyzed the expression levels of membrane-bound complement regulatory proteins (mCRPs) on the three cell lines, and their protective role in α-gal-mediated activation of the complement. LoVo showed no expression of any of the three proteins. CD59 was strongly expressed by SW620 and Ls-174T. CD46 and CD55 varied between the two cell lines. CD46 on SW620 was only half the intensity of CD46 on Ls-174T. Ls-174T showed a notable expression of CD55, while expression of CD55 on SW620 was not detected. The sensitivity of Ls-174T expressing α-gal to NHS greatly increased following the downregulation of CD46 and CD55 with short hairpin RNA (shRNA). However, there is no increase in cell killing when CD59 expression was diminished. Our findings suggest that the use of α-gal as antigen to induce tumor cell killing may be a potential therapeutic strategy in colon cancer and that CD55 plays a primary role in conferring resistance to lysis.

Complement dependent early immunological responses during ex vivo xenoperfusion of hCD46/HLA-E double transgenic pig forelimbs with human blood

BACKGROUND

Besides α1,3-galactosyltransferase gene (GGTA1) knockout, several transgene combinations to prevent pig-to-human xenograft rejection are currently being investigated. In this study, the potential of combined overexpression of human CD46 and HLA-E to prevent complement- and NK-cell-mediated xenograft rejection was tested in an ex vivo pig-to-human xenoperfusion model.

METHODS

α1,3-Galactosyltransferase knockout heterozygous, hCD46/HLA-E double transgenic (transgenic) as well as wild-type pig forelimbs were ex vivo perfused with whole, heparinized human and autologous pig blood, respectively. Blood samples were analyzed for the production of porcine and/or human inflammatory cytokines as well as complement activation products. Biopsy samples were examined for deposition of human and porcine C3b/c, C4b/c, and C6 as well as CD62E (E-selectin) and CD106 (VCAM-1) expression. Apoptosis was measured in the porcine muscle tissue using TUNEL assays. Finally, the formation of thrombin-antithrombin (TAT) complexes was measured in EDTA plasma samples.

RESULTS

No hyperacute rejection was seen in this model. Extremity perfusions lasted for up to 12 h without increase in vascular resistance and were terminated due to continuous small blood losses. Plasma levels of porcine cytokines IL1β, IL-6, IL-8, IL-10, TNF-α, and MCP-1 as well as human complement activation markers C3a (P = 0.0002), C5a (P = 0.004), and soluble C5b-9 (P = 0.03) were lower in blood perfused through transgenic as compared to wild-type limbs. Human C3b/c, C4b/c, and C6 as well as CD62E and CD106 were deposited in tissue of wild-type limbs, but significantly lower levels (P < 0.0001) of C3b/c, C4b/c, and C6 deposition as well as CD62E and CD106 expression were detected in transgenic limbs perfused with human blood. Transgenic porcine tissue was protected from xenoperfusion-induced apoptosis (P < 0.0001). Finally, TAT levels were significantly lower (P < 0.0001) in transgenic limb as compared to wild-type limb xenoperfusions.

CONCLUSION

Transgenic hCD46/HLA-E expression clearly reduced humoral xenoresponses since all, the terminal pathway of complement activation, endothelial cell activation, muscle cell apoptosis, inflammatory cytokine production, as well as coagulation activation, were all downregulated. Overall, this model represents a useful tool to study early immunological responses during pig-to-human vascularized xenotransplantation in the absence of hyperacute rejection.