Identification, detection, and in vitro characterization of cynomolgus monkey natural killer cells in delayed xenograft rejection of hDAF transgenic porcine renal xenografts

Porcine UL-16 Binding Protein 1 Is Not a Functional Ligand for the Human Natural Killer Cell Activating Receptor NKG2D

Natural killer (NK) cells play a vital role in xenotransplantation rejection. One approach to induce NK cell immune tolerance is to prevent the NK cell-mediated direct killing of porcine cells by targeting the interaction of the activating receptor NKG2D and its ligands. However, the identity of porcine ligands for the human NKG2D receptor has remained elusive. Previous studies on porcine UL-16 binding protein 1 (pULBP-1) as a ligand for human NKG2D have yielded contradictory results. The goal of the present study was to clarify the role of pULBP-1 in the immune response and its interaction with human NKG2D receptor. To accomplish this, the CRISPR/Cas9 gene editing tool was employed to disrupt the porcine ULBP-1 gene in a 5-gene knockout porcine endothelial cell line (GGTA1, CMAH, β4galNT2, SLA-I α chain, and β-2 microglobulin, 5GKO). A colony with two allele mutations in pULBP-1 was established as a 6-gene knockout pig cell line (6GKO). We found that pULBP-1-deficient pig cells exhibited a reduced binding capacity to human NKG2D-Fc, a recombinant chimera protein. However, the removal of ULBP-1 from porcine endothelial cells did not significantly impact human NK cell degranulation or cytotoxicity upon stimulation with the pig cells. These findings conclusively demonstrate that pULBP-1 is not a crucial ligand for initiating xenogeneic human NK cell activation.

Immune response after pig-to-human kidney xenotransplantation: a multimodal phenotyping study

BACKGROUND

Cross-species immunological incompatibilities have hampered pig-to-human xenotransplantation, but porcine genome engineering recently enabled the first successful experiments. However, little is known about the immune response after the transplantation of pig kidneys to human recipients. We aimed to precisely characterise the early immune responses to the xenotransplantation using a multimodal deep phenotyping approach.

METHODS

We did a complete phenotyping of two pig kidney xenografts transplanted to decedent humans. We used a multimodal strategy combining morphological evaluation, immunophenotyping (IgM, IgG, C4d, CD68, CD15, NKp46, CD3, CD20, and von Willebrand factor), gene expression profiling, and whole-transcriptome digital spatial profiling and cell deconvolution. Xenografts before implantation, wild-type pig kidney autografts, as well as wild-type, non-transplanted pig kidneys with and without ischaemia-reperfusion were used as controls.

FINDINGS

The data collected from xenografts suggested early signs of antibody-mediated rejection, characterised by microvascular inflammation with immune deposits, endothelial cell activation, and positive xenoreactive crossmatches. Capillary inflammation was mainly composed of intravascular CD68+ and CD15+ innate immune cells, as well as NKp46+ cells. Both xenografts showed increased expression of genes biologically related to a humoral response, including monocyte and macrophage activation, natural killer cell burden, endothelial activation, complement activation, and T-cell development. Whole-transcriptome digital spatial profiling showed that antibody-mediated injury was mainly located in the glomeruli of the xenografts, with significant enrichment of transcripts associated with monocytes, macrophages, neutrophils, and natural killer cells. This phenotype was not observed in control pig kidney autografts or in ischaemia-reperfusion models.

INTERPRETATION

Despite favourable short-term outcomes and absence of hyperacute injuries, our findings suggest that antibody-mediated rejection in pig-to-human kidney xenografts might be occurring. Our results suggest specific therapeutic targets towards the humoral arm of rejection to improve xenotransplantation results.

FUNDING

OrganX and MSD Avenir.

Co-expression of HLA-E and HLA-G on genetically modified porcine endothelial cells attenuates human NK cell-mediated degranulation

Natural killer (NK) cells play an important role in immune rejection in solid organ transplantation. To mitigate human NK cell activation in xenotransplantation, introducing inhibitory ligands on xenografts via genetic engineering of pigs may protect the graft from human NK cell-mediated cytotoxicity and ultimately improve xenograft survival. In this study, non-classical HLA class I molecules HLA-E and HLA-G were introduced in an immortalized porcine liver endothelial cell line with disruption of five genes (GGTA1, CMAH, β4galNT2, SLA-I α chain, and β-2 microglobulin) encoding three major carbohydrate xenoantigens (αGal, Neu5Gc, and Sda) and swine leukocyte antigen class I (SLA-I) molecules. Expression of HLA-E and/or HLA-G on pig cells were confirmed by flow cytometry. Endogenous HLA-G molecules as well as exogenous HLA-G VL9 peptide could dramatically enhance HLA-E expression on transfected pig cells. We found that co-expression of HLA-E and HLA-G on porcine cells led to a significant reduction in human NK cell activation compared to the cells expressing HLA-E or HLA-G alone and the parental cell line. NK cell activation was assessed by analysis of CD107a expression in CD3-CD56+ population gated from human peripheral blood mononuclear cells. CD107a is a sensitive marker of NK cell activation and correlates with NK cell degranulation and cytotoxicity. HLA-E and/or HLA-G on pig cells did not show reactivity to human sera IgG and IgM antibodies. This in vitro study demonstrated that co-expression of HLA-E and HLA-G on genetically modified porcine endothelial cells provided a superior inhibition in human xenoreactive NK cells, which may guide further genetic engineering of pigs to prevent human NK cell mediated rejection.

IL-18BP Improves Early Graft Function and Survival in Lewis-Brown Norway Rat Orthotopic Liver Transplantation Model

Interleukin-18 (IL-18) can effectively activate natural killer (NK) cells and induce large concentrations of interferon-γ (IFN-γ). In healthy humans, IL-18 binding protein (IL-18BP) can inhibit the binding of IL-18 to IL-18R and counteract the biological action of IL-18 due to its high concentration and high affinity, thus preventing the production of IFN-γ and inhibiting NK-cell activation. Through previous studies and the phenomena observed by our group in pig-non-human primates (NHPs) liver transplantation experiments, we proposed that the imbalance in IL-18/IL-18BP expression upon transplantation encourages the activation, proliferation, and cytotoxic effects of NK cells, ultimately causing acute vascular rejection of the graft. In this research, we used Lewis-Brown Norway rat orthotopic liver transplantation (OLTx) as a model of acute vascular rejection. AAV8-Il18bp viral vectors as gene delivery vehicles were constructed for gene therapy to overexpress IL-18BP and alleviate NK-cell rejection of the graft after transplantation. The results showed that livers overexpressing IL-18BP had reduced damage and could function longer after transplantation, effectively improving the survival time of the recipients.

Genetic engineering of porcine endothelial cell lines for evaluation of human-to-pig xenoreactive immune responses

Xenotransplantation (cross-species transplantation) using genetically-engineered pig organs offers a potential solution to address persistent organ shortage. Current evaluation of porcine genetic modifications is to monitor the nonhuman primate immune response and survival after pig organ xenotransplantation. This measure is an essential step before clinical xenotransplantation trials, but it is time-consuming, costly, and inefficient with many variables. We developed an efficient approach to quickly examine human-to-pig xeno-immune responses in vitro. A porcine endothelial cell was characterized and immortalized for genetic modification. Five genes including GGTA1, CMAH, β4galNT2, SLA-I α chain, and β2-microglobulin that are responsible for the production of major xenoantigens (αGal, Neu5Gc, Sda, and SLA-I) were sequentially disrupted in immortalized porcine endothelial cells using CRISPR/Cas9 technology. The elimination of αGal, Neu5Gc, Sda, and SLA-I dramatically reduced the antigenicity of the porcine cells, though the cells still retained their ability to provoke human natural killer cell activation. In summary, evaluation of human immune responses to genetically modified porcine cells in vitro provides an efficient method to identify ideal combinations of genetic modifications for improving pig-to-human compatibility, which should accelerate the application of xenotransplantation to humans.

Pseudorabies Virus Infection Causes Downregulation of Ligands for the Activating NK Cell Receptor NKG2D

Herpesviruses display a complex and carefully balanced interaction with important players in the antiviral immune response of immunocompetent natural hosts, including natural killer (NK) cells. With regard to NK cells, this delicate balance is illustrated on the one hand by severe herpesvirus disease reported in individuals with NK cell deficiencies and on the other hand by several NK cell evasion strategies described for herpesviruses. In the current study, we report that porcine cells infected with the porcine alphaherpesvirus pseudorabies virus (PRV) display a rapid and progressive downregulation of ligands for the major activating NK cell receptor NKG2D. This downregulation consists both of a downregulation of NKG2D ligands that are already expressed on the cell surface of an infected cell and an inhibition of cell surface expression of newly expressed NKG2D ligands. Flow cytometry and RT-qPCR assays showed that PRV infection results in downregulation of the porcine NKG2D ligand pULBP1 from the cell surface and a very substantial suppression of mRNA expression of pULBP1 and of another potential NKG2D ligand, pMIC2. Furthermore, PRV-induced NKG2D ligand downregulation was found to be independent of late viral gene expression. In conclusion, we report that PRV infection of host cells results in a very pronounced downregulation of ligands for the activating NK cell receptor NKG2D, representing an additional NK evasion strategy of PRV.

Xenotransplantation: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

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.

Stimulatory and inhibitory receptor interactions in xenotransplantation

PURPOSE OF REVIEW

Cellular human antipig immune responses are increasingly recognized as an important barrier to successful clinical xenotransplantation. This review addresses the role of monocytes/macrophages, natural killer (NK) cells, and T cells in xenograft rejection. We focus on the receptor-ligand interactions that regulate the responses of these cells to porcine tissues and thus could be targets for immunomodulation.

RECENT FINDINGS

Activation of human monocytes by pig cells is partly due to the incapacity of porcine ligands to bind to inhibitory receptors such as signal regulatory protein alpha. Porcine UL16-binding protein 1 can functionally interact with human NK group 2D protein, thereby contributing to human NK cell activity. Transgenic pigs overexpressing human leukocyte antigen class E were generated. Cells from these pigs induced diminished NK-cell lysis, suggesting that human leukocyte antigen class E expression compensates for the inability of porcine ligands to bind to the inhibitory CD94/NK group 2A receptor on human NK cells. A new concept for the modulation of antipig T-cell reactivity may result from the finding that porcine antigen-presenting cells that overexpress human negative costimulatory PD ligands also induce diminished responses of human T cells.

SUMMARY

Disruption of stimulatory receptor-ligand interactions (e.g. by blocking antibodies or 'knockout/down' technologies) combined with transgenic overexpression of inhibitory ligands in porcine cells and tissues could be an effective approach to downregulate human antipig cellular immune responses.

Xenotransplantation: role of natural immunity

Hyperacute rejection, mediated by natural anti-Galalpha1,3Galbeta1,4GlcNAc (alphaGal) antibodies and the classically activated complement pathway, was identified as the first major barrier to the survival of porcine organs in humans. Subsequently, discordant pig-to-nonhuman primate and concordant rodent models revealed key roles for T and B lymphocytes in the second form of rejection, acute vascular rejection (AVR) or delayed xenograft rejection (DXR). As significant progress was made in strategies to circumvent or suppress xenoreactivity of the adaptive immune system, it became clear that, apart from natural antibodies, other innate immune system elements actively participate in AVR/DXR and represent a barrier to xenograft acceptance that may be particularly difficult to overcome. Observations in pig-to-primate and semi-discordant and concordant rodent models indicate that Natural Killer (NK) cells play a more prominent role in xenograft than in allograft rejection. Several mechanisms through which human NK cells recognize porcine endothelial cells have been elucidated and these appear to be more diverse than those involved in NK cell alloreactivity. Further, it has been demonstrated that human macrophages and neutrophils can directly recognize pig derived cells and can mediate direct xenograft damage. Here, we review the recent progress in the understanding of the xenoreactivity of the natural immune system, focussing on preclinical pig-to-(non)human primate systems, and discuss the proposed strategies to overcome these barriers.

Play it in E or G: utilization of HLA-E and -G in xenotransplantation

Human NK cell-mediated graft rejection is likely to be one of several biological obstacles to routine pig-to-human xenotransplantation. Abrogating NK cell activation by either elimination of activating ligands on porcine cells or expression of molecules serving as ligands for NK cell inhibitory receptors, or both, could overcome this hurdle. HLA-E and -G exhibit very limited polymorphism and are ligands for NK cell inhibitory receptors. This review summarizes successes and limitations of their use in xenotransplantation as inferred from ex vivo analyses of NK cell activity, highlights potential effects they may have on T-cell responses, and considers prospects of preclinical trials and potential outcomes.

A Study of Soluble HLA-G1 Protecting Porcine Endothelial Cells Against Human Natural Killer Cell-Mediated Cytotoxicity

Human natural killer (NK) cells, which can mediate direct lysis of porcine endothelial cells, play an important role in xenograft rejection. HLA-G, which is a critical molecule in maintaining maternal immune tolerance of semi-allogenic fetus, is able to protect susceptible target cells from lysis induced by NK cells. In this study, we investigated whether soluble HLA-G1 (sHLA-G1) protected porcine xenogeneic cells against human NK cell-mediated lysis.

METHODS

The human sHLA-G1 genomic DNA (pcDNA3-sHLA-G1) was transfected into a B lymphoblastoid cell line 721.221 (LCL721.221) by nucleofector. The sHLA-G1 expression of the transfected LCL721.221 cells was identified by RT-PCR and Dot-ELISA. The sHLA-G1 protein was purified by affinity chromatography on anti-HLA-ImAb W6/32 coupled to cyanogen-bromide-activated Sepharose 4B from culture supernates of transfectants. Various concentrations of sHLA-G(1) protein (0, 2, 4, 6, or 8 microg/mL) were added to a NK cell-mediated xenogenic cell lysis system with either NK92 cells or fresh human peripheral blood mononuclear cells (PBMCs) cocultured with the porcine endothelial cells line. A LDH release assay was used to evaluate NK cell-mediated cytotoxicity.

RESULTS

sHLA-G1 provided significant protection of porcine endothelial cells against human NK-mediated cytotoxicity in a dose-dependent manner. The rates of NK92 cell-mediated cytotoxicity were reduced to 83.4 +/- 5.7% (2 microg/mL), 56.6 +/- 9.3% (4 microg/mL), 39.3 +/- 10.2% (6 microg/mL), and 31.2 +/- 4.9% (8 microg/mL) versus 96.9 +/- 3.0% in the control group (P < .01). Similarly, adding 6 microg/mL sHLA-G1 reduced the mean rate of PBMC-mediated cytotoxicity (n = 4) to 5.8 +/- 1.6% from 23.9 +/- 1.3% in the control group (P < .01).

CONCLUSIONS

These results indicated that sHLA-G1 protected xenogeneic porcine endothelial cells against attack by human NK cells, thus providing a new approach to overcome NK-mediated immunity to xenografts.

HLA-E expression on porcine cells: protection from human NK cytotoxicity depends on peptide loading

Human NK cells lyse porcine cells and may play an important role in the cell-mediated rejection of pig-to-human xenografts. Lysis is probably a consequence of the failure of human MHC-specific killer inhibitory receptors to recognize porcine MHC class I molecules. A majority of activated human NK cells express the HLA-E-specific inhibitory receptor CD94/NKG2A. The aim of this study was therefore to test the hypothesis that stable surface expression of HLA-E on porcine cells protects against xenogeneic NK-mediated cytotoxicity. Porcine lymphoblastoid (13 271) and endothelial (pEC) cell lines were transfected with constructs coding for HLA-E together with the leader sequence of HLA-B7 or -A2. HLA-E was correctly expressed on 13 271 cells while pEC required peptide-pulsing and/or IFN-gamma stimulation to express the HLA-E complex on the cell surface. HLA-E-expressing porcine cells were partially protected from lysis mediated by human polyclonal NK populations and completely protected from killing by NKG2Abright NK clones. In conclusion, the capability of different porcine cell types to express HLA-E on the cell surface can differ considerably depending decisively on the availability of peptides. These findings are important for the applicability of transgenic HLA-E expression as an approach to protect porcine tissues from human NK cytotoxicity.

Identification of NKG2A and NKp80 as specific natural killer cell markers in rhesus and pigtailed monkeys

Investigations of natural killer (NK) cells in simian models of disease have been hampered by a lack of appropriate phenotypic markers and by an inadequate understanding of the regulation of NK cell activities. In the present study, a panel of monoclonal antibodies (mAbs) specific for various human NK receptors was screened for cross-reactivity with NK cells from rhesus macaques and pigtailed macaques. Flow cytometric analyses using anti-human NKG2A and anti-human NKp80 mAbs individually, and particularly in combination with anti-CD16 mAb, allowed for the identification of the entire NK cell population in both species. NK cells in monkeys were generally identified by negative selection of peripheral blood mononuclear cells (PBMCs) for the absence of T-cell, B-cell, and monocyte markers. mAb-mediated ligation of NKp80 induced NK cell cytotoxicity, while in the case of NKG2A it displayed a clear capability to inhibit the lysis of target cells by NK cells from macaques, as well as from humans. This new phenotypic and functional characterization of NKG2A and NKp80 in rhesus and pigtailed macaque NK cells provides a new approach in the analysis of their innate immune system.

An HLA-E single chain trimer inhibits human NK cell reactivity towards porcine cells

HLA-E, when expressed by pig cells, could alleviate human natural killer (NK) cell-mediated rejection of porcine xenografts by providing a potent inhibitory ligand for human NK cells expressing CD94/NKG2A. Yet cell-surface expression of HLA-E on porcine epithelial (LLC-PK1) cells was not observed after transfection with an expression vector harboring HLA-E alone or in combination with an expression vector containing human beta2m. A single chain trimer (SCT) of HLA-E consisting of, in the following order (from N- to C-terminus), the leader peptide of human beta2m, VMAPRTLIL (an HLA-E-binding peptide), a 15 amino acid linker, mature human beta2m, a 20 amino acid linker, and mature HLA-E heavy chain was engineered. Cell-surface expression and correct folding of HLA-E SCT was shown by FACS analyses of stably transfected LLC-PK1 cells. Untransfected LLC-PK1 cells were readily lysed by the NK cell lines NKL and NK-92, while LLC-PK1 cells expressing HLA-E SCT were almost completely protected. In addition, the HLA-E SCT recapitulates the peptide dependent properties of normal HLA-E trimeric complexes in that an HLA-E SCT with an hsp60 derived peptide, though expressed at the cell-surface, did not inhibit NK cell-mediated lysis. The HLA-E SCT, which conferred protection against NK cell-mediated killing, also inhibited NK cell IFN-gamma secretion elicited by co-culture of NKL cells with LLC-PK1 cells. Thus, HLA-E SCT, in which all three components of a normal HLA-E protein complex are in one polypeptide chain, is immunologically functional as it is able to modulate NK cell cytotoxicity and cytokine secretion.

Expression of human ecto-5'-nucleotidase in pig endothelium increases adenosine production and protects from NK cell-mediated lysis

Ecto-5'-nucleotidase (E5'N) is an endothelial surface enzyme that controls conversion of extracellular nucleotides into immunosuppressive adenosine. We evaluated whether expression of human E5'N on pig endothelial cells (EC) attenuates human NK cell-mediated cytotoxicity. A pig EC line was stably transfected with human E5'N and human NK cell adhesion and cytotoxicity toward pig EC cultures was measured by flow cytometry and intracellular enzyme release. E5'N activity in pig EC lysates increased from 0.68 +/- 0.07 to 1013 +/- 293 nmol/min/mg protein, whilst the rate of AMP to adenosine metabolism by intact cells increased from 0.37 +/- 0.05 to >300 nmol/min/mg protein in non-transfected and transfected cells, respectively. The rate of adenosine production in transfected cells increased also with ATP as the extracellular substrate. Cytotoxicity of human NK cells was reduced from 10.7 +/- 0.4% and 11.1 +/- 1.1% with non-transfected pig EC to 5.2 +/- 0.2% and 5.0 +/- 0.2% in transfected cells with 50 microM and 250 microM AMP, respectively. Reduction of cytotoxicity in E5'N-transfected EC was abolished by the E5'N inhibitor and was mimicked in non-transfected EC by the addition of adenosine, demonstrating the key role of adenosine produced by E5'N in inhibiting NK cell cytotoxicity. We suggest that overexpression of E5'N in EC of transgenic pigs is a possible strategy to ameliorate rejection after xenotransplantation.

Endothelial Cells Derived from Pigs Lacking Gal??(1,3)Gal: No Reduction of Human Leukocyte Adhesion and Natural Killer Cell Cytotoxicity

BACKGROUND

The expression of galactose-alpha(1,3)galactose (Gal) on porcine cells represents a major barrier to xenotransplantation. The generation of Gal-/- pigs to overcome this barrier redirected the focus of research to other rejection mechanisms, including cellular immunity. The present in vitro study investigated (1) the adhesive interactions between human leukocyte subsets and primary endothelial cells derived from inbred Gal-/- and Gal+/+ pigs, and (2) the susceptibility of such Gal-/- porcine endothelial cells to human natural killer (NK) cell cytotoxicity.

METHODS

Primary porcine aortic endothelial cells (PAEC) were isolated from Gal-/- (PAEC-Gal-/-) and Gal (PAEC-Gal+/+) pigs. Human peripheral blood mononuclear cells (PBMC), polymorphonuclear neutrophils (PMN), and NK cells were isolated from healthy volunteers and tested in functional adhesion and cytotoxicity assays.

RESULTS

Adhesion of human PBMC, PMN, or purified NK cells on PAEC-Gal-/- cells was not different from that on PAEC-Gal+/+ cells. Comparing the different leukocyte subsets of PBMC, a preferential adhesion of NK and B cells on both PAEC-Gal-/- and PAEC-Gal+/+ was detected. Tumor-necrosis factor-alpha stimulation of PAEC-Gal-/- and PAEC-Gal+/+ induced an increase of CD62E and CD106 expression and increased cellular adhesion, in particular, of PMN. The lack of Gal-/- expression on PAEC-Gal cells did not prevent xenogeneic human NK-cell cytotoxicity mediated by freshly isolated or interleukin-2-activated NK cells.

CONCLUSIONS

Neither human leukocyte adhesion nor xenogeneic NK-cell cytotoxicity against PAEC are impaired by the lack of Gal, indicating that Gal is not a dominant target of cellular rejection.

Lack of Galactose-α-1,3-Galactose Expression on Porcine Endothelial Cells Prevents Complement-Induced Lysis but Not Direct Xenogeneic NK Cytotoxicity

The galactose-alpha-1,3-galactose (alphaGal) carbohydrate epitope is expressed on porcine, but not human cells, and therefore represents a major target for preformed human anti-pig natural Abs (NAb). Based on results from pig-to-primate animal models, NAb binding to porcine endothelial cells will likely induce complement activation, lysis, and hyperacute rejection in pig-to-human xenotransplantation. Human NK cells may also contribute to innate immune responses against xenografts, either by direct recognition of activating molecules on target cells or by FcgammaRIII-mediated xenogeneic Ab-dependent cellular cytotoxicity (ADCC). The present study addressed the question as to whether the lack of alphaGal protects porcine endothelial cells from NAb/complement-induced lysis, direct xenogeneic NK lysis, NAb-dependent ADCC, and adhesion of human NK cells under shear stress. Homologous recombination, panning, and limiting dilution cloning were used to generate an alphaGal-negative porcine endothelial cell line, PED2*3.51. NAb/complement-induced xenogeneic lysis of PED2*3.51 was reduced by an average of 86% compared with the alphaGal-positive phenotype. PED2*3.51 resisted NK cell-mediated ADCC with a reduction of lysis ranging from 30 to 70%. However, direct xenogeneic lysis of PED2*3.51, mediated either by freshly isolated or IL-2-activated human NK cells or the NK cell line NK92, was not reduced. Furthermore, adhesion of IL-2-activated human NK cells did not rely on alphaGal expression. In conclusion, removal of alphaGal leads to a clear reduction in complement-induced lysis and ADCC, but does not resolve adhesion of NK cells and direct anti-porcine NK cytotoxicity, indicating that alphaGal is not a dominant target for direct human NK cytotoxicity against porcine cells.

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.

Isolation and characterization of immortalized porcine aortic endothelial cell lines

Primary porcine endothelial cells have a limited life span in culture. After four to five passages, they tend to de-differentiate and eventually reach senescence. The aim of this work was to establish immortalized porcine aortic endothelial cell lines (AOCs) to facilitate in vitro studies of different pathological process involving the endothelium. Primary porcine aortic endothelial cells (PAECs) were transfected with a plasmid containing the SV40 genome and selected on the basis of morphological and phenotypical features. Flow cytometry analysis demonstrated uptake of acetylated low density lipoproteins (Ac-LDL) and constitutive expression of SLA class I, CD29, CD31, CD41/61, CD80/86, CD46, SWC3, and LAMP-1 antigens by all analyzed lines and showed little differences to primary cells. The functional similarity between primary and immortalized endothelial cells was demonstrated in a cytotoxicity assay using a human natural killer cell line (NKL) as effector. The AOCs cell lines should be valuable tools for in vitro study of the human immune response against pig endothelial cells. In addition, they would be very useful to gain insight in the pathogenesis of some viral haemorrhagic diseases of pig such as African swine fever (ASF) or classical swine fever (CSF).

Xenogeneic human NK cytotoxicity against porcine endothelial cells is perforin/granzyme B dependent and not inhibited by Bcl-2 overexpression

Because of organ shortages in clinical allotransplantation, the potential of pig-to-human xenotransplantation is currently being explored showing a possible critical role for natural killer (NK) cells in the immune response against xenografts. Therefore, we analyzed the cytotoxic pathways utilized by human natural killer cells (hNK) against porcine endothelial cells (pEC). Transmission electron microscopy of pEC cocultured with hNK cells showed both apoptotic and necrotic cell death, whereas soluble factors such as Fas ligand or TNFalpha did not induce apoptosis in pEC. NK lysis of pEC was abrogated by concanamycin A and ammonium chloride, reagents inhibiting the perforin/granzyme B (grB) pathway, but only partially blocked by caspase inhibition with z-VAD-fmk. Overexpression of bcl-2 protected pEC against apoptosis induced by staurosporine or actinomycin D, but failed to prevent hNK cell-mediated lysis. In conclusion, pEC are lysed in vitro by hNK cells via the perforin/grB pathway and are not protected from NK lysis by overexpression of bcl-2.

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.

HLA-G inhibits rolling adhesion of activated human NK cells on porcine endothelial cells

Human NK cells adhere to and lyse porcine endothelial cells (pEC) and therefore may contribute to the cell-mediated rejection of vascularized pig-to-human xenografts. Since MHC class I molecules inhibit the cytotoxic activity of NK cells, the expression of HLA genes in pEC has been proposed as a potential solution to overcome NK cell-mediated xenogeneic cytotoxicity. HLA-G, a minimally polymorphic HLA class I molecule that can inhibit a wide range of NK cells, is an especially attractive candidate for this purpose. In this study we tested whether the expression of HLA-G on pEC inhibits the molecular mechanisms that lead to adhesion of human NK cells to pEC and subsequent xenogeneic NK cytotoxicity. To this end two immortalized pEC lines (2A2 and PED) were stably transfected with HLA-G1. Rolling adhesion of activated human NK cells to pEC monolayers and xenogeneic cytotoxicity against pEC mediated by polyclonal human NK lines as well as NK clones were inhibited by the expression of HLA-G. The adhesion was partially reversed by masking HLA-G on pEC with anti-HLA mAbs or by masking the HLA-G-specific inhibitory receptor ILT-2 on NK cells with the mAb HP-F1. The inhibition of NK cytotoxicity by HLA-G was only partially mediated by ILT-2, indicating a role for other unknown NK receptors. In conclusion, transgenic expression of HLA-G may be useful to prevent human NK cell responses to porcine xenografts, but is probably not sufficient on its own. Moreover, the blocking of rolling adhesion by HLA-G provides evidence for a novel biological function of HLA molecules.

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.