Human-porcine receptor-ligand compatibility within the immune system: relevance for xenotransplantation

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.

Transplanting organs from pigs to humans

The success of organ transplantation is limited by the complications of immunosuppression, by chronic rejection, and by the insufficient organ supply, and thousands of patients die every year while waiting for a transplant. With recent progress in xenotransplantation permitting porcine organ graft survival of months or even years in nonhuman primates, there is renewed interest in its potential to alleviate the organ shortage. Many of these advances are the result of our heightened capacity to modify pigs genetically, particularly with the development of CRISPR-Cas9-based gene editing methodologies. Although this approach allows the engineering of pig organs that are less prone to rejection, the clinical application of xenotransplantation will require the ability to avoid the ravages of a multifaceted attack on the immune system while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. In this review, we will discuss the potential and limitations of these modifications and how the engineering of the graft can be leveraged to alter the host immune response so that all types of immune attack are avoided.

IXA Honorary Member Lecture, 2017: The long and winding road to tolerance

The last 15 years or so have seen exciting progress in xenotransplantation, with porcine organ grafts surviving months or even years in non-human primates. These advances reflect the application of new scientific knowledge, improved immunosuppressive agents, and genetic engineering. The field has recently enjoyed a renaissance of interest and hope, largely due to the exponential increase in our capacity to genetically engineer porcine source animals. However, immune responses to xenografts are very powerful and widespread clinical application of xenotransplantation will depend on the ability to suppress these immune responses while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. Our work over the last three decades has aimed to engineer the immune system of the recipient in a manner that achieves specific tolerance to the xenogeneic donor while preserving otherwise normal immune function. Important proofs of principle have been obtained, first in rodents, and later in human immune systems in "humanized mice" and finally in non-human primates, demonstrating the capacity and potential synergy of mixed xenogeneic chimerism and xenogeneic thymic transplantation in tolerizing multiple arms of the immune system. Considering the fact that clinical tolerance has recently been achieved for allografts and the even greater importance of avoiding excessive immunosuppression for xenografts, it is my belief that it is both possible and imperative that we likewise achieve xenograft tolerance. I expect this to be accomplished through the availability of targeted approaches to recipient immune conditioning, understanding of immunological mechanisms of tolerance, advanced knowledge of physiological incompatibilities, and the availability of inbred miniature swine with optimized use of genetic engineering.

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.

Interleukin-8 mediates neutrophil-endothelial interactions in pig-to-human xenogeneic models

BACKGROUND

Human neutrophils are sequestered by pig lung xenografts within minutes during ex vivo perfusion. This phenomenon is not prevented by pig genetic modifications that remove xeno-antigens or added human regulatory molecules intended to down-regulate activation of complement and coagulation pathways. This study investigated whether recipient and donor interleukin-8 (IL-8), a chemokine known to attract and activate neutrophils during inflammation, is elaborated in the context of xenogeneic injury, and whether human or pig IL-8 promote the adhesion of human neutrophils in in vitro xenograft models.

METHODS

Plasma levels of pig, human or non-human primate (NHP) IL-8 from ex vivo pig lung perfusion experiments (n = 10) and in vivo pig-to-baboon lung transplantation in baboons (n = 22) were analysed by ELISA or Luminex. Human neutrophils stimulated with human or pig IL-8 were analysed for CD11b expression, CD18 activation, oxidative burst and adhesion to resting or TNF-activated endothelial cells (EC) evaluated under static and flow (Bioflux) conditions. For some experiments, human neutrophils were incubated with Reparixin (IL-8/CXCL8 receptor blocker) and then analysed as in the in vitro experiments mentioned above.

RESULTS

Plasma levels of pig IL-8 (~6113 pg/mL) increased more than human (~1235 pg/mL) between one and four hours after initiation of ex vivo lung perfusion. However, pig IL-8 levels remained consistently low (<60 pg/mL) and NHP IL-8 plasma levels increased by ~2000 pg/mL after four hours in a pig-to-baboon lung xenotransplantation. In vitro, human neutrophils' CD11b expression, CD18 activation and oxidative burst all increased in a dose-dependent manner following exposure to either pig or human IL-8, which also were associated with increased adhesion to EC in both static and flow conditions. Reparixin inhibited human neutrophil activation by both pig and human IL-8 in a dose-dependent fashion. At 0.1 mg/mL, Reparixin inhibited the adhesion of IL-8-activated human neutrophils to pAECs by 84 ± 2.5%.

CONCLUSIONS

Pig IL-8 increased in an ex vivo model of pig-to-human lung xenotransplantation but is not detected in vivo, whereas human or NHP IL-8 is elevated to a similar degree in both models. Both pig and human IL-8 activate human neutrophils and increase their adhesion to pig aortic ECs, a process significantly inhibited by the addition of Reparixin to human neutrophils. This work implicates IL-8, whether of pig or human origin, as a possible factor mediating in lung xenograft inflammation and injury and supports the evaluation of therapeutic targeting of this pathway in the context of xenotransplantation.

Optimization of intrabone delivery of hematopoietic progenitor cells in a swine model using cell radiolabeling with [89]zirconium

Intrabone (IB) hematopoietic cell transplantation (HCT) of umbilical cord blood in humans remains experimental and the technique has not been optimized. It is unknown whether hematopoietic progenitor cells (HPCs) injected IB are initially retained in the marrow or rapidly enter into the venous circulation before homing to the marrow. To develop an IB-injection technique that maximizes HPC marrow-retention, we tracked radiolabeled human HPCs following IB-injection into swine. We developed a method to radionuclide-label HPCs using a long-lived positron emitter (89) Zr and protamine sulfate that resulted in cellular-retention of low-dose radioactivity. This approach achieved radioactivity levels sufficient for detection by positron emission tomography with both high sensitivity and spatial resolution when fused with computed tomography. We found that conditions utilized in pilot IB-HCT clinical trials conducted by others led to both rapid drainage into the central venous circulation and cellular extravasation into surrounding muscle and soft tissues. By optimizing the needle design, using continuous real-time intra-marrow pressure monitoring, and by reducing the infusion-volume and infusion-rate, we overcame this limitation and achieved high retention of HPCs in the marrow. This method of IB cellular delivery is readily applicable in the clinic and could be utilized in future investigational IB-HCT trials aimed at maximizing marrow retention of HPCs.

Xenotransplantation: immunological hurdles and progress toward tolerance

The discrepancy between organ need and organ availability represents one of the major limitations in the field of transplantation. One possible solution to this problem is xenotransplantation. Research in this field has identified several obstacles that have so far prevented the successful development of clinical xenotransplantation protocols. The main immunologic barriers include strong T-cell and B-cell responses to solid organ and cellular xenografts. In addition, components of the innate immune system can mediate xenograft rejection. Here, we review these immunologic and physiologic barriers and describe some of the strategies that we and others have developed to overcome them. We also describe the development of two strategies to induce tolerance across the xenogeneic barrier, namely thymus transplantation and mixed chimerism, from their inception in rodent models through their current progress in preclinical large animal models. We believe that the addition of further beneficial transgenes to Gal knockout swine, combined with new therapies such as Treg administration, will allow for successful clinical application of xenotransplantation.

Human cytokines activate JAK-STAT signaling pathway in porcine ocular tissue

Background

The JAK/STAT (Janus Tyrosine Kinase, Signal Transducers and Activators of Transcription) pathway is associated with cytokine or growth factor receptors and it is critical for growth control, developmental regulation and homeostasis. The use of porcine ocular cells as putative xenotransplants appears theoretically possible. The aim of this study was to investigate the response of various porcine ocular cells in vitro to human cytokines in regard to the activation of JAK-STAT signaling pathways.

Methods

Porcine lens epithelial cells, pigmented iris epithelial cells and pigmented ciliary body cells were used in this study. These cells were isolated from freshly enucleated porcine eyes by enzymatic digestion. Cultured cells between passages 3–8 were used in all experiments. Electromobility shift assay (EMSA), proliferation assay, immunofluorescence staining and flow cytometry were used to evaluate the JAK-STAT signaling pathway in these cells.

Results

JAK/STAT signaling pathways could be activated in porcine pigmented epithelial ciliary body cells, in pigmented iris epithelial cells and in lens epithelial cells in response to porcine and human interferons and cytokines. All cells showed very strong STAT1 activation upon stimulation with porcine interferon-gamma. Porcine ocular cells also respond to human cytokines; IFN-alpha induced strong activation of STAT1 in EMSA, flow cytometry and immunofluorescence experiments whereas activation of STAT3 was less strong in EMSA, but strong in flow cytometry and immunofluorescence. Human recombinant IL-6 activated STAT3 and human IL-4 activated STAT6. With the help of immunofluorescence assay and flow cytometry we observed nuclear localization of STAT proteins after activation of porcine ocular cells with cytokines and interferons. Human IFN-α had an inhibitory effect on porcine ocular cells in proliferation assays.

Conclusion

Our study demonstrated that some types of human cytokines and interferon activate intracellular JAK-STAT signaling pathways in porcine ocular cells. We hypothesize that direct stimulation of the JAK-STAT pathway in porcine cells in response to human cytokines will lead to complications or failure, if pig-to-human ocular tissue xenotransplantation were to be carried out. For successful xenotransplantation among other obstacles there must be new approaches developed to regulate signaling pathways.

Both CD45RA+ and CD45RO+ human CD4+ T cells drive direct xenogeneic T-cell responses against porcine aortic endothelial cells

BACKGROUND

Xenogeneic cellular immune responses are mediated by either direct or indirect pathways depending on the participation of donor or host antigen presenting cells, respectively. The contribution of direct response of human T cells, especially memory T cells, to porcine antigen presenting cells is currently unknown. Here, we sought to determine whether human peripheral blood memory/activated phenotype T cells are directly responsive to porcine endothelial cells.

METHODS

Porcine aortic endothelial cells (PAECs) were prepared from Yorkshire or miniature pigs. Highly purified human T cells, including naïve and memory/activated phenotype cells, were incubated with PAECs with or without the addition of exogenous cytokines. T-cell proliferation and T-cell receptor (TCR) Vbeta usage in response to PAECs were analyzed.

RESULTS

Both CD8(+) and CD4(+) T cells responded directly to PAECs and exhibited exclusive responsiveness to SLA class I and class II molecules, respectively. Naïve and memory/activated phenotype CD4(+) T cells responded against PAECs, whereas only naïve phenotype CD8(+) T cells contributed to such a response. In addition, both populations of xenogeneic human CD4(+) T cells exhibited similar and diverse V beta usage.

CONCLUSION

Due to the considerable contribution of human CD45RO(+)CD4(+) T cells to the xenoreactivity against PAECs, effective control of xenogeneic memory/activated T-cell responses would significantly affect long-term survival of transplanted grafts.

Porcine DC-SIGN: molecular cloning, gene structure, tissue distribution and binding characteristics

DC-SIGN, a human C-type lectin, is involved in the transmission of many enveloped viruses. Here we report the cloning and characterization of the cDNA and gene encoding porcine DC-SIGN (pDC-SIGN). The full-length pDC-SIGN cDNA encodes a type II transmembrane protein of 240 amino acids. Phylogenetic analysis revealed that pDC-SIGN, together with bovine, canis and equine DC-SIGN, are more closely related to mouse SIGNR7 and SIGNR8 than to human DC-SIGN. pDC-SIGN has the same gene structure as bovine, canis DC-SIGN and mouse SIGNR8 with eight exons. pDC-SIGN mRNA expression was detected in pig spleen, thymus, lymph node, lung, bone marrow and muscles. pDC-SIGN protein was found to express on the surface of monocyte-derived macrophages and dendritic cells, alveolar macrophages, lymph node sinusoidal macrophage-like, dendritic-like and endothelial cells but not of monocytes, peripheral blood lymphocytes or lymph node lymphocytes. A BHK cell line stably expressing pDC-SIGN binds to human ICAM-3 and ICAM-2 immunoadhesins in a calcium-dependent manner, and enhances the transmission of porcine reproductive and respiratory syndrome virus (PRRSV) to target cells in trans. The results will help better understand the biological role(s) of DC-SIGN family in innate immunity during the evolutionary process.

Porcine PD-L1: cloning, characterization, and implications during xenotransplantation

BACKGROUND

Effective intervention achieved by manipulating cell-mediated xenogeneic immune responses would critically increase the clinical feasibility of xenotransplantation as immediate hyperacute rejections become controllable through genetic modulations of donor organs. Endogenous negative regulatory signals like the programmed death 1 (PD-1)-programmed death ligand 1 (PD-L1) system are candidate targets for the control of cell-mediated xenogeneic immune response.

METHODS

A porcine PD-L1 molecule was cloned using RACE (rapid amplification of cDNA ends) technology based on the human PD-L1 sequence. The functional effects of cloned porcine PD-L1 were tested on human CD4(+) T cell activation using porcine PD-L1-transfected bystander cells. Cellular proliferation was monitored by [3H] thymidine incorporation, and human T cell apoptosis was measured by flow cytometry.

RESULTS

Porcine PD-L1 (GenBank accession number AY837780) was found to have 73.8% sequence homology with human PD-L1 and to contain two immunoglobulin domains in its extracellular region. Moreover, porcine PD-L1 expressed on Chinese hamster ovary (CHO) cells inhibited human CD4(+) T cell proliferation stimulated with anti-CD3 only or anti-CD3 plus anti-CD28. Percentages of apoptotic activated human T cells increased by over 30% in the presence of porcine PD-L1/CHO cells, and the addition of recombinant human PD-1-Fc fusion proteins during human T cell activation reversed the inhibitory effects of porcine PD-L1.

CONCLUSIONS

Cloned porcine PD-L1 showed high sequence homology with human PD-L1 and a similar molecular structure. Moreover, porcine PD-L1 inhibited human CD4(+) T cell activation in human PD-1-dependent manner, and this involved activated T cell apoptosis. The authors suggest that PD-1-PD-L1 might play an important endogenous immune regulatory role during xenogeneic transplantation, and that the effective application of this system would improve transplanted xenogeneic organ survival.

Compatibility of porcine and human interleukin 2: implications for xenotransplantation

BACKGROUND

Xenotransplantation provides a possible solution to the severe shortage of allogeneic organ donors. The pig, which shares many physiological similarities with humans, makes it an optimal species for preclinical experimentation and clinical applications. Interleukin 2 (IL2) is a potent growth factor secreted primarily by T helper lymphocytes and it is vital to the cellular expansion required for a productive immune response and the development and peripheral expansion of CD4+CD25+ regulatory T cells. Therefore, it is essential to understand of the compatibility of IL2 between pigs and humans.

METHODS

We first compared the cDNA and protein sequences and the crystal structures of human and porcine IL2 and IL2 receptors, respectively. The effect of IL2 to induce T cell proliferation was determined by 3H-thymidine incorporation and cell cycle detection.

RESULTS

Porcine IL2 induced very limited proliferation of human lymphocytes while it functioned well on porcine lymphocytes. Human IL2 had remarkably reduced effects on porcine lymphocytes whereas it worked well on human lymphocytes.

CONCLUSION

Our present study showed that the interaction of IL2 and IL2R across species might have defects. Together with the wide physiological functions of IL2, our data indicated that physiological disorders could be caused by the poor function of xenogeneic donor IL2 on host cells in full hematopoietic chimera. Our data suggested an additional potential advantage for the mixed xenogeneic chimeras.

Significant Inhibition of Human CD8+ Cytotoxic T Lymphocyte-Mediated Xenocytotoxicity by Overexpression of the Human Decoy Fas Antigen

BACKGROUND

Human CD8(+) CTL-mediated killing may be important for xenograft rejection. The purpose of this study was to explore the preventing methods for CTL-mediated xenocytotoxicity by overexpression of human decoy Fas, which lacks a death domain in its cytoplasmic region, by binding competition with endogenous pig Fas. Moreover, the cytoprotective effect of this CTL-killing of membrane-bound human FasL, which is resistant to metalloproteolytic cleavage, was also assessed.

METHODS

Human CTL were generated by the stimulation of human PBMC with swine endothelial cells (SEC) and human IL-2, subsequently a CD8(+) population were selected by magnetic beads and employed as the effector cells. Stable SEC transfectants expressing either decoy Fas or membrane-bound FasL were established. Double-transfectants were also created. The amelioration of cytotoxicity to these transfectants was examined with Cr release assay. RESULTS.: Human CD8(+) CTL were highly detrimental against parental SEC. This CTL-killing was strongly inhibited by anti-FasL mAb treatment, however partial suppression was observed by Concanamycin A treatment. The overexpression of either decoy Fas or membrane-bound FasL in SEC markedly inhibited CTL-xenocytotoxicity. The double expressions of these molecules also significantly reduced this xenocytotoxicity despite the low levels of expression of either decoy Fas or membrane-bound FasL.

CONCLUSION

These findings indicate that the strong xenocytotoxicity of human CD8(+) CTL is mediated mainly by the Fas/FasL pathway. The overexpression of either decoy Fas or membrane-bound FasL were quite effective in preventing CTL-killing. Furthermore, the combined expression of both molecules in pig cells may create a window of opportunity for prolonging xenograft survival.

The neutrophil: the unnoticed threat in xenotransplantation?

BACKGROUND

Xenotransplantation offers one way to circumvent the widening gap between the demand for and supply of human organs for transplantation, and the pig is widely regarded as the donor animal most likely to prove appropriate. Most attention has focused on the adaptive immune response to xenogeneic tissue. However, there is optimism that it may soon be possible to overcome that hurdle. In this paper, we consider the possibility of the direct recognition of xenogeneic tissue by neutrophils.

METHODS

We studied in vitro the interaction of human neutrophils with cultured porcine endothelial cells in assays of adhesion (both static and flow), activation on the basis of chemiluminescence, and diapedesis and chemotaxis using split-well chambers.

RESULTS

Human neutrophils showed increased adhesiveness to porcine endothelium in both static and flow adhesion systems. While this did not activate the neutrophils at rest, in the presence of suboptimal concentrations of a parallel stimulus, phorbol myristate acetate, the interaction of human neutrophils with porcine endothelium caused a much greater respiratory burst than their interaction with controls. In addition, they showed greater diapedesis through porcine endothelium. Of greatest interest is the observation that porcine endothelium secretes a molecule that is chemotactic for human neutrophils.

CONCLUSIONS

On the basis of these observations, we should consider the potential for neutrophil-mediated low-grade damage to xenografts emerging as a significant problem when others have been circumvented.

A novel monoclonal antibody inhibits the immune response of human cells against porcine cells: identification of a porcine antigen homologous to CD58

BACKGROUND

Human CD58 is an adhesion molecule that interacts with CD2 on lymphocytes. We describe here an antibody that blocks responses of human peripheral blood mononuclear cells (PBMCs) to porcine cells and reacts with a porcine protein with homology to CD58.

METHODS

Antibodies were isolated with a screen for inhibition of the human antiporcine response. One of these antibodies was used for immunoaffinity purification of a protein that was identified by molecular weight determination, endoglycosidase sensitivity, and microsequencing analysis as a porcine homologue of CD58.

RESULTS

The antigen recognized by this antibody was a cell surface protein of relative molecular mass (Mr)=45,000 containing N-linked carbohydrate chains. Immunoaffinity purification of this protein and microsequencing revealed homology to sheep CD58 as well as sequences that were common to this protein and both sheep and human CD58. The protein was widely distributed on porcine cells, including lymphocytes, endothelial cells, muscle cells, and neuronal cells. This antibody efficiently inhibited lysis of porcine targets by human PBMCs in addition to preventing proliferation of the human PBMCs in response to the porcine cells.

CONCLUSIONS

The CD2 interaction with porcine cells is important for the efficient recognition of porcine tissue, and inhibition of the human antiporcine immune response with the antibody is likely to be caused by the disruption of the human CD2 interaction with this porcine homologue of CD58. The antibody may prove to be useful for the blocking of this interaction without interfering with other functions of T cells.

Characterization of pig intercellular adhesion molecule-2 and its interaction with human LFA-1

Understanding molecular interactions between human leukocytes and porcine endothelium is important for the future success of pig-to-human xenotransplantation. Here we describe the analysis of pig intercellular adhesion molecule-2 (ICAM-2). A 1020-basepair ICAM-2 cDNA generated from pig lung RNA contained an open reading frame (ORF) encoding a 277-amino-acid protein with six potential N-linked glycosylation sites. The mature protein sequence was 55% identical to human ICAM-2, with conservation of five out of six residues critical for binding of the human protein to its ligand LFA-1. Northern blot analysis identified ICAM-2 transcripts of 4.0 and 1.4 kb in cultured pig endothelial cells and mRNA was detected in pig lung, spleen, kidney, liver and heart by RT-PCR. The gene structure and endothelial expression of pig ICAM-2 were strikingly similar to those of its human and mouse counterparts. However, unlike human ICAM-2, expression of pig ICAM-2 on cultured endothelial cells was not down-regulated by treatment with the inflammatory cytokines TNF-alpha and IL-1beta. Pig ICAM-2 expressed on stable transfectants supported firm adhesion of cells expressing human LFA-1. This conservation of function across the species barrier suggests that pig ICAM-2 plays a role in the cellular interactions associated with xenograft rejection.

Induction of human T-cell tolerance to porcine xenoantigens through mixed hematopoietic chimerism

Xenotransplantation from pigs could provide a potential solution to the severe shortage of allogeneic donor organs. Because xenogeneic tissues are subject to vigorous immune rejection, tolerance induction is likely to be essential to the success of clinical xenotransplantation. Here we explore the possibility of inducing human T-cell tolerance to porcine xenografts through mixed chimerism. We previously showed that NOD/SCID-Tg mice expressing porcine cytokine transgenes permit the induction of durable porcine hematopoietic chimerism. In this study we achieved human T-cell development in these mice by engrafting human fetal thymus/liver tissues. In porcine hematopoietic chimeras, human thymus grafts were populated with porcine class II(high) cells in addition to human cells, and human T cells were tolerant of the porcine hematopoietic donor as measured by mixed lymphocyte reaction assay and skin grafting. This study proves the principle that porcine chimerism induces tolerance of xenoreactive human T cells.

The degree of phylogenetic disparity of islet grafts dictates the reliance on indirect CD4 T-cell antigen recognition for rejection

Cellular xenograft rejection involves a pronounced contribution of CD4 T-cells recognizing antigens in association with recipient MHC class II molecules. However, the requirement for such "indirect" antigen recognition for acute islet xenograft is not clear, especially as a function of the phylogenetic disparity between the donor and recipient species. In vitro studies show that C57BL/6 (B6) mouse T-cells respond directly to either allogeneic BALB/c or phylogenetically related xenogeneic WF rat stimulator cells while having undetectable responses to phylogenetically disparate porcine stimulator cells. Although all types of grafts rejected acutely in wild-type mice, this response demonstrated markedly differing dependence on host MHC class II antigen presentation, depending on the donor species. While BALB/c islet allografts were acutely rejected in B6 MHC class II-deficient (C2D) recipients, WF rat xenografts demonstrated marked prolongation in C2D hosts relative to wild-type recipients. Interestingly, neonatal porcine islet (NPI) xenografts uniformly survived long term (>100 days) in untreated C2D hosts despite transfer of wild-type CD4 T-cells, demonstrating that survival in C2D recipients was not secondary to a lack of CD4 T-cells seen in such mice. Taken together, these results show a marked hierarchy in the requirement for host MHC class II-restricted indirect pathway in the rejection of pancreatic islet grafts. Thus, while cellular rejection of porcine xenografts is generally quite vigorous, this pathway is relatively finite, displaying a major reliance on host MHC class II-dependent antigen presentation for acute rejection.

Human natural killer cell activity against porcine targets: modulation by control of the oxidation-reduction environment and role of adhesion molecule interactions

Xenotransplantation, especially using porcine sources, has been proposed as a means to alleviate the shortage of human organs for transplantation. NK cells appear to be important mediators of the xenogeneic immune responses, including the human anti-pig response. Having previously established the redox regulation of NK cell activity against tumor target cells, we now report that the interaction of human NK cells with porcine target cells is also regulated by redox. Thiol-deprivation strongly diminished the capacity of IL-2-activated human NK cells to kill porcine endothelial cells. This inhibition correlated with reduced proliferation and interferon (IFN)-gamma production by IL-2-activated NK cells. For fresh NK cells, pretreatment with diethyl maleate (DEM), which was used to deplete intracellular thiols, reduced lysis of porcine and human targets. Because many adhesion molecules exhibit interspecies recognition, we further investigated whether changes in expression of adhesion molecules might explain our observations. DEM treatment reduced the expression of CD11b and CD29 on fresh NK cells. Monoclonal antibody blocking studies showed that the combination of mAb to CD11b and CD18 reduced lytic activity against both PAEC as well as K562, although other qualitative differences were observed between the porcine and human target cells. These findings suggest that the oxidative stress-induced downregulation of CD18 may be important in modulating cytotoxic activity of fresh NK cells against PAEC and K562 targets through reduced formation of the CD11b/CD18 heterodimer. Thus, the appropriate manipulation of redox status may provide a means to enhance survival of non-human animal tissues in humans through modulation of adhesion molecule expression/interactions.

Mixed chimerism induces donor-specific T-cell tolerance across a highly disparate xenogeneic barrier

Induction of tolerance is likely to be essential for successful xenotransplantation because immune responses across xenogeneic barriers are vigorous. Although mixed hematopoietic chimerism leads to stable donor-specific tolerance in allogeneic and closely related xenogeneic (eg, rat-to-mouse) combinations, the ability of this approach to induce tolerance across a highly disparate xenogeneic barrier has not yet been demonstrated. In this study, we investigated the immune responses of murine T cells that developed in mice with pre-established porcine hematopoietic chimerism. Our results show for the first time that induction of porcine hematopoietic chimerism can eliminate the development of antiporcine donor responses in a highly disparate xenogeneic species. Porcine hematopoietic chimeras showed donor-specific nonresponsiveness in the mixed lymphocyte reaction, lack of antidonor IgG antibody production, and acceptance of donor skin grafts. Thus, mixed chimerism is capable of inducing tolerance in a highly disparate xenogeneic combination and may have clinical potential to prevent xenograft rejection. (Blood. 2002;99:3823-3829)

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.

Human CD154 induces activation of porcine endothelial cells and up-regulation of MHC class II expression

BACKGROUND

CD40 is expressed on a number of antigen-presenting cells and also on vascular endothelium. It has been shown that engagement of CD40 on vascular endothelium by CD154 on platelets and CD154-bearing cell lines leads to the induction of adhesion molecule expression. Having cloned porcine CD40, and shown that it is capable of binding human CD154, we investigate whether human CD154 can activate porcine endothelial cells (EC) through CD40 ligation.

METHODS

Human Jurkat clone D1.1 (CD154+), or clone E6.1 (CD154-), were co-cultured with EC from pig aorta and human aorta and umbilical vein for various times in the presence or absence of blocking antibody to CD154.

RESULTS

Human and pig EC were shown to express CD40 by flow cytometry by using soluble human CD154 (CD154Ckappa). Co-culture of pig EC with CD154-expressing Jurkat D1.1 cells led to the induction of E-selectin by 6 hr (peak 24 hr) and vascular cell adhesion molecule-1 (VCAM-1) by 6 hr (peak 48 hr). Similar results were also observed with human EC. Porcine EC were induced to up-regulate major histocompatibility complex class II at 24 hr by co-culture with Jurkat D1.1 cells through a CD40-dependent mechanism. In contrast, no up-regulation was observed on human EC.

CONCLUSIONS

A number of cells can express CD154, including T cells, natural killer cells, and platelets, and these could signal graft EC through the CD40 pathway. These results demonstrate a possible role for the CD40 pathway in the activation of vascular endothelium in the rejection of porcine xenografts.

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.

Human T-cell-porcine endothelial cell interactions induce human Th1 cytokines and porcine activation markers

BACKGROUND

Since the future of clinical transplantation will undoubtedly include xenotransplantation, there is a need to examine human anti-pig cellular reactions. The objective of this study is to use human anti-porcine mixed lymphocyte endothelial cell culture (MLEC) to investigate cell interactions, cross-species molecular compatibilities, and the induction of human cytokines and porcine activation markers.

METHODS

Human peripheral blood mononuclear cells or enriched CD4+ T cells depleted of professional antigen-presenting cells were cultured with resting pig aortic endothelial cells in the absence of exogenous cytokines. T-cell proliferative responses were measured and PAEC were monitored for cell surface markers by flow cytometry. Culture supernatants were assayed for human TNF-alpha and IFN-gamma by ELISA.

RESULTS

Human T cells proliferated strongly in response to PAEC (median stimulation index = 75), even in serum-free cultures. High levels of the human Th1 cytokines TNF-alpha (20-350 pg/ml) and IFN-gamma (200-3800 pg/ml) were detected only in cultures containing PAEC, with levels peaking on Day 4. CD4+ T-cell-enriched, APC-depleted responders maintained proliferative anti-PAEC responses and cytokine release. By Day 3, MHC Class II and VCAM expression was induced in 92-96% PAEC: mean fluorescence intensity (MFI) increased from 5 to 83 +/- 12 and 166 +/- 74, respectively, and MHC Class I was increased from MFI 31 to 965 +/- 269.

CONCLUSIONS

These results indicate that MLEC is an excellent in vitro model in which to study human anti-porcine cellular responses. Human T cells are activated in response to direct antigen presentation by PAEC, which are also activated in this system. Specific cytokines, receptors, and adhesion molecules appear to cross the xenograft barrier and play a critical role in T-cell - PAEC interactions. Such interactions are likely to affect VEC activation and immune responses to porcine xenografts in vivo.

Effect of redox modulation on xenogeneic target cells: the combination of nitric oxide and thiol deprivation protects porcine endothelial cells from lysis by IL-2-activated human NK cells

Evidence suggests that NK cells contribute to the pathogenesis of delayed rejection of vascularized xenografts, and NK cells have been suggested to participate in hyperacute xenograft rejection. Endothelial cells have been shown to be the primary target of the recipient's immune responses that mediate both hyperacute and delayed xenograft rejection. Under conditions of oxidative stress induced by thiol deprivation, but not under normal conditions, pretreatment of porcine aortic endothelial cells (PAECs) with the NO donor, S-nitroso-N-acetyl-penicillamine, dramatically inhibited killing of PAEC target cells by IL-2-activated human NK cells. This same combined treatment reduced both surface expression and mRNA levels of E-selectin. Moreover, anti-E-selectin mAb, but not Ab to VCAM-1, protected PAEC from lysis by human IL-2-activated NK cells in a dose-dependent manner. These findings suggest that expression of porcine E-selectin is important for the cytotoxicity of PAEC mediated by activated human NK cells and may be involved in the redox-mediated modulation of that cytotoxicity. It is known that NF-kappa B activation is required for transcription of E-selectin, and the current data show that the suppression of E-selectin expression by S-nitroso-N-acetyl-penicillamine pretreatment and thiol deprivation was associated with reduced NF-kappa B DNA-binding activity in PAEC. These data suggest that the regulation of porcine E-selectin may be important for modulating delayed xenograft rejection and that manipulation of cellular redox systems may provide a means to protect xenogeneic endothelial cells from NK cell-mediated cytotoxicity.