Immortalized bone-marrow derived pig endothelial cells

Evolutionary-Related High- and Low-Virulent Classical Swine Fever Virus Isolates Reveal Viral Determinants of Virulence

Classical swine fever (CSF) has been eradicated from Western and Central Europe but remains endemic in parts of Central and South America, Asia, and the Caribbean. CSF virus (CSFV) has been endemic in Cuba since 1993, most likely following an escape of the highly virulent Margarita/1958 strain. In recent years, chronic and persistent infections with low-virulent CSFV have been observed. Amino acid substitutions located in immunodominant epitopes of the envelope glycoprotein E2 of the attenuated isolates were attributed to positive selection due to suboptimal vaccination and control. To obtain a complete picture of the mutations involved in attenuation, we applied forward and reverse genetics using the evolutionary-related low-virulent CSFV/Pinar del Rio (CSF1058)/2010 (PdR) and highly virulent Margarita/1958 isolates. Sequence comparison of the two viruses recovered from experimental infections in pigs revealed 40 amino acid differences. Interestingly, the amino acid substitutions clustered in E2 and the NS5A and NS5B proteins. A long poly-uridine sequence was identified previously in the 3' untranslated region (UTR) of PdR. We constructed functional cDNA clones of the PdR and Margarita strains and generated eight recombinant viruses by introducing single or multiple gene fragments from Margarita into the PdR backbone. All chimeric viruses had comparable replication characteristics in porcine monocyte-derived macrophages. Recombinant PdR viruses carrying either E2 or NS5A/NS5B of Margarita, with 36 or 5 uridines in the 3'UTR, remained low virulent in 3-month-old pigs. The combination of these elements recovered the high-virulent Margarita phenotype. These results show that CSFV evolution towards attenuated variants in the field involved mutations in both structural and non-structural proteins and the UTRs, which act synergistically to determine virulence.

Removal of the Erns RNase Activity and of the 3' Untranslated Region Polyuridine Insertion in a Low-Virulence Classical Swine Fever Virus Triggers a Cytokine Storm and Lethal Disease

In this study, we assessed the potential synergistic effect of the E^rns RNase activity and the poly-U insertion in the 3′ untranslated region (UTR) of the low-virulence classical swine fever virus (CSFV) isolate Pinar de Rio (PdR) in innate and adaptive immunity regulation and its relationship with classical swine fever (CSF) pathogenesis in pigs. We knocked out the E^rns RNase activity of PdR and replaced the long polyuridine sequence of the 3′ UTR with 5 uridines found typically at this position, resulting in a double mutant, vPdR-H₃₀K-5U. This mutant induced severe CSF in 5-day-old piglets and 3-week-old pigs, with higher lethality in the newborn (89.5%) than in the older (33.3%) pigs. However, the viremia and viral excretion were surprisingly low, while the virus load was high in the tonsils. Only alpha interferon (IFN-α) and interleukin 12 (IL-12) were highly and consistently elevated in the two groups. Additionally, high IL-8 levels were found in the newborn but not in the older pigs. This points toward a role of these cytokines in the CSF outcome, with age-related differences. The disproportional activation of innate immunity might limit systemic viral spread from the tonsils and increase virus clearance, inducing strong cytokine-mediated symptoms. Infection with vPdR-H₃₀K-5U resulted in poor neutralizing antibody responses compared with results obtained previously with the parent and RNase knockout PdR. This study shows for the first time the synergistic effect of the 3′ UTR and the E^rns RNase function in regulating innate immunity against CSFV, favoring virus replication in target tissue and thus contributing to disease severity.

IMPORTANCE CSF is one of the most relevant viral epizootic diseases of swine, with high economic and sanitary impact. Systematic stamping out of infected herds with and without vaccination has permitted regional virus eradication. However, the causative agent, CSFV, persists in certain areas of the world, leading to disease reemergence. Nowadays, low- and moderate-virulence strains that could induce unapparent CSF forms are prevalent, posing a challenge for disease eradication. Here, we show for the first time the synergistic role of lacking the E^rns RNase activity and the 3′ UTR polyuridine insertion from a low-virulence CSFV isolate in innate immunity disproportional activation. This might limit systemic viral spread to the tonsils and increase virus clearance, inducing strong cytokine-mediated symptoms, thus contributing to disease severity. These results highlight the role played by the E^rns RNase activity and the 3′ UTR in CSFV pathogenesis, providing new perspectives for novel diagnostic tools and vaccine strategies.

TNF-Mediated Inhibition of Classical Swine Fever Virus Replication Is IRF1-, NF-κB- and JAK/STAT Signaling-Dependent

The sera from pigs infected with virulent classical swine fever virus (CSFV) contain substantial amounts of tumor necrosis factor (TNF), a prototype proinflammatory cytokine with pleiotropic activities. TNF limits the replication of CSFV in cell culture. In order to investigate the signaling involved in the antiviral activity of TNF, we employed small-molecule inhibitors to interfere specifically with JAK/STAT and NF-κB signaling pathways in near-to-primary endothelial PEDSV.15 cells. In addition, we knocked out selected factors of the interferon (IFN) induction and signaling pathways using CRISPR/Cas9. We found that the anti-CSFV effect of TNF was sensitive to JAK/STAT inhibitors, suggesting that TNF induces IFN signaling. Accordingly, we observed that the antiviral effect of TNF was dependent on intact type I IFN signaling as PEDSV.15 cells with the disrupted type I IFN receptor lost their capacity to limit the replication of CSFV after TNF treatment. Consequently, we examined whether TNF activates the type I IFN induction pathway. With genetically modified PEDSV.15 cells deficient in functional interferon regulatory factor 1 or 3 (IRF1 or IRF3), we observed that the anti-CSFV activity exhibited by TNF was dependent on IRF1, whereas IRF3 was dispensable. This was distinct from the lipopolysaccharide (LPS)-driven antiviral effect that relied on both IRF1 and IRF3. In agreement with the requirement of IRF1 to induce TNF- and LPS-mediated antiviral effects, intact IRF1 was also essential for TNF- and LPS-mediated induction of IFN-β mRNA, while the activation of NF-κB was not dependent on IRF1. Nevertheless, NF-κB activation was essential for the TNF-mediated antiviral effect. Finally, we observed that CSFV failed to counteract the TNF-mediated induction of the IFN-β mRNA in PEDSV.15 cells, suggesting that CSFV does not interfere with IRF1-dependent signaling. In summary, we report that the proinflammatory cytokine TNF limits the replication of CSFV in PEDSV.15 cells by specific induction of an IRF1-dependent antiviral type I IFN response.

Abrogation of the RNase activity of Erns in a low virulence classical swine fever virus enhances the humoral immune response and reduces virulence, transmissibility, and persistence in pigs

The prevalence of low virulence classical swine fever virus (CSFV) strains makes viral eradication difficult in endemic countries. However, the determinants for natural CSFV attenuation and persistence in the field remain unidentified. The aim of the present study was to assess the role of the RNase activity of CSFV E^rns in pathogenesis, immune response, persistent infection, and viral transmission in pigs. To this end, a functional cDNA clone pPdR-H₃₀K-36U with an E^rns lacking RNase activity was constructed based on the low virulence CSFV field isolate Pinar de Rio (PdR). Eighteen 5-day-old piglets were infected with vPdR-H₃₀K-36U. Nine piglets were introduced as contacts. The vPdR-H₃₀K-36U virus was attenuated in piglets compared to the parental vPdR-36U. Only RNA traces were detected in sera and body secretions and no virus was isolated from tonsils, showing that RNase inactivation may reduce CSFV persistence and transmissibility. The vPdR-H₃₀K-36U mutant strongly activated the interferon-α (IFN-α) production in plasmacytoid dendritic cells, while in vivo, the IFN-α response was variable, from moderate to undetectable depending on the animal. This suggests a role of the CSFV E^rns RNase activity in the regulation of innate immune responses. Infection with vPdR-H₃₀K-36U resulted in higher antibody levels against the E2 and E^rns glycoproteins and in enhanced neutralizing antibody responses when compared with vPdR-36U. These results pave the way toward a better understanding of viral attenuation mechanisms of CSFV in pigs. In addition, they provide novel insights relevant for the development of DIVA vaccines in combination with diagnostic assays for efficient CSF control.

A Polyuridine Insertion in the 3' Untranslated Region of Classical Swine Fever Virus Activates Immunity and Reduces Viral Virulence in Piglets

Classical swine fever (CSF), a highly contagious viral disease of pigs, is still endemic in some countries of Asia and Central and South America. Considering that the 3′ untranslated region (3′ UTR) plays an important role in flavivirus replication, the present study showed for the first time that a long polyuridine sequence acquired in the 3′ UTR by an endemic CSFV isolate can activate immunity, control viral replication, and modulate disease in piglets. Our findings provide new avenues for the development of novel vaccines against infections with CSF virus and other flaviviruses. Knowledge of molecular virulence determinants is also relevant for future development of rapid and efficient diagnostic tools for the prediction of the virulence of field isolates and for efficient CSF control.

Low-virulence classical swine fever virus (CSFV) strains make CSF eradication particularly difficult. Few data are available on the molecular determinants of CSFV virulence. The aim of the present study was to assess a possible role for CSFV virulence of a unique, uninterrupted 36-uridine (poly-U) sequence found in the 3′ untranslated region (3′ UTR) of the low-virulence CSFV isolate Pinar de Rio (PdR). To this end, a pair of cDNA-derived viruses based on the PdR backbone were generated, one carrying the long poly-U insertion in the 3′ UTR (vPdR-36U) and the other harboring the standard 5 uridines at this position (vPdR-5U). Two groups of 20 5-day-old piglets were infected with vPdR-36U and vPdR-5U. Ten contact piglets were added to each group. Disease progression, virus replication, and immune responses were monitored for 5 weeks. The vPdR-5U virus was significantly more virulent than the vPdR-36U virus, with more severe disease, higher mortality, and significantly higher viral loads in serum and body secretions, despite similar replication characteristics in cell culture. The two viruses were transmitted to all contact piglets. Ninety percent of the piglets infected with vPdR-36U seroconverted, while only one vPdR-5U-infected piglet developed antibodies. The vPdR-5U-infected piglets showed only transient alpha interferon (IFN-α) responses in serum after 1 week of infection, while the vPdR-36U-infected piglets showed sustained IFN-α levels during the first 2 weeks. Taken together, these data show that the 3′ UTR poly-U insertion acquired by the PdR isolate reduces viral virulence and activates the innate and humoral immune responses without affecting viral transmission.

IMPORTANCE Classical swine fever (CSF), a highly contagious viral disease of pigs, is still endemic in some countries of Asia and Central and South America. Considering that the 3′ untranslated region (3′ UTR) plays an important role in flavivirus replication, the present study showed for the first time that a long polyuridine sequence acquired in the 3′ UTR by an endemic CSFV isolate can activate immunity, control viral replication, and modulate disease in piglets. Our findings provide new avenues for the development of novel vaccines against infections with CSF virus and other flaviviruses. Knowledge of molecular virulence determinants is also relevant for future development of rapid and efficient diagnostic tools for the prediction of the virulence of field isolates and for efficient CSF control.

Importance of the Major Histocompatibility Complex (Swine Leukocyte Antigen) in Swine Health and Biomedical Research

In pigs, the major histocompatibility complex (MHC), or swine leukocyte antigen (SLA) complex, maps to Sus scrofa chromosome 7. It consists of three regions, the class I and class III regions mapping to 7p1.1 and the class II region mapping to 7q1.1. The swine MHC is divided by the centromere, which is unique among mammals studied to date. The SLA complexspans between 2.4 and 2.7 Mb, depending on haplotype, and encodes approximately 150 loci, with at least 120 genes predicted to be functional. Here we update the whole SLA complex based on the Sscrofa11.1 build and annotate the organization for all recognized SLA genes and their allelic sequences. We present SLA nomenclature and typing methods and discuss the expression of SLA proteins, as well as their role in antigen presentation and immune, disease, and vaccine responses. Finally, we explore the role of SLA genes in transplantation and xenotransplantation and their importance in swine biomedical models.

Small-Molecule Immunosuppressive Drugs and Therapeutic Immunoglobulins Differentially Inhibit NK Cell Effector Functions in vitro

Small-molecule immunosuppressive drugs (ISD) prevent graft rejection mainly by inhibiting T lymphocytes. Therapeutic immunoglobulins (IVIg) are used for substitution, antibody-mediated rejection (AbMR) and HLA-sensitized recipients by targeting distinct cell types. Since the effect of ISD and IVIg on natural killer (NK) cells remains somewhat controversial in the current literature, the aim of this comparative study was to investigate healthy donor's human NK cell functions after exposure to ISD and IVIg, and to comprehensively review the current literature. NK cells were incubated overnight with IL2/IL12 and different doses and combinations of ISD and IVIg. Proliferation was evaluated by ³[H]-thymidine incorporation; phenotype, degranulation and interferon gamma (IFNγ) production by flow cytometry and ELISA; direct NK cytotoxicity by standard ⁵¹[Cr]-release and non-radioactive DELFIA assays using K562 as stimulator and target cells; porcine endothelial cells coated with human anti-pig antibodies were used as targets in antibody-dependent cellular cytotoxicity (ADCC) assays. We found that CD69, CD25, CD54, and NKG2D were downregulated by ISD. Proliferation was inhibited by methylprednisolone (MePRD), mycophenolic acid (MPA), and everolimus (EVE). MePRD and MPA reduced degranulation, MPA only of CD56^bright NK cells. MePRD and IVIg inhibited direct cytotoxicity and ADCC. Combinations of ISD demonstrated cumulative inhibitory effects. IFNγ production was inhibited by MePRD and ISD combinations, but not by IVIg. In conclusion, IVIg, ISD and combinations thereof differentially inhibit NK cell functions. The most potent drug with an effect on all NK functions was MePRD. The fact that MePRD and IVIg significantly block NK cytotoxicity, especially ADCC, has major implications for AbMR as well as therapeutic strategies targeting cancer and immune cells with monoclonal antibodies.

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.

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.

Human anti-pig NK cell and CD8+ T-cell responses in the presence of regulatory dendritic cells

BACKGROUND

Dendritic cells (DC) play a major role in natural killer (NK) cell and cytotoxic T lymphocyte (CTL) activation leading to cell-mediated xenogeneic responses. In contrast, the use of in vitro differentiated regulatory DC may represent an attractive approach to protect porcine endothelial cells (pEC) from human cell-mediated immune responses. In this study, we evaluated the potential of human regulatory DC to reduce xenogeneic NK cell and CTL responses to pEC.

METHODS

Human monocytes were differentiated into DC with GM-CSF and IL-4 in the absence or presence of rapamycin or IL-10. The effect of regulatory DC on xenogeneic NK cell and CTL responses was evaluated by analyzing phenotype, IFNγ production, degranulation, and cytotoxicity by flow cytometry and cytotoxicity assays.

RESULTS

Upon maturation with LPS, Rapa-DC and IL-10-DC displayed different phenotypes and cytokine production profiles. In contrast to untreated DC, both Rapa-DC and IL-10-DC induced significantly less IFNγ production and NK cell degranulation in response to pEC, but did not affect NK cell-mediated pEC lysis. Low production of IL-18 by Rapa-DC, and of IL-12 by IL-10-DC were linked to the deficient IFNγ production by NK cells as shown by partial reversion of IFNγ production upon cytokine reconstitution. In contrast to untreated DC efficiently generating xenoantigen-specific CTL, priming of CTL in the presence of IL-10-DC was impaired as shown by lower IFNγ production and cytotoxicity of CTL in response to pEC.

CONCLUSION

Both Rapa-DC and IL-10-DC controlled human anti-porcine NK cell responses, in particular IFNγ production, whereas IL-10-DC presented stronger regulatory properties of anti-porcine CTL responses. These in vitro findings indicate that regulatory DC could be a useful tool to promote xenograft tolerance in vivo.

Presence of atypical porcine pestivirus (APPV) genomes in newborn piglets correlates with congenital tremor

Pestiviruses are highly variable RNA viruses belonging to the continuously growing family Flaviviridae. A genetically very distinct pestivirus was recently discovered in the USA, designated atypical porcine pestivirus (APPV). Here, a screening of 369 sera from apparently healthy adult pigs demonstrated the existence of APPV in Germany with an estimated individual prevalence of 2.4% and ~10% at farm level. Additionally, APPV genomes were detected in newborn piglets affected by congenital tremor (CT), but genomes were absent in unaffected piglets. High loads of genomes were identified in glandular epithelial cells, follicular centers of lymphoid organs, the inner granular cell layer of the cerebellum, as well as in the trigeminal and spinal ganglia. Retrospective analysis of cerebellum samples from 2007 demonstrated that APPV can be found in piglets with CT of unsolved aetiology. Determination of the first European APPV complete polyprotein coding sequence revealed 88.2% nucleotide identity to the APPV sequence from the USA. APPV sequences derived from different regions in Germany demonstrated to be highly variable. Taken together, the results of this study strongly suggest that the presence of APPV genomes in newborn piglets correlates with CT, while no association with clinical disease could be observed in viremic adult pigs.

Entry of human cytomegalovirus into porcine endothelial cells depends on both the cellular vascular origin and the viral strain

BACKGROUND

Primary infection and reactivation of human cytomegalovirus (HCMV) is associated with allograft rejection. Pig-to-human xenotransplantation is regarded as an alternative to circumvent donor organ shortage and inevitably, porcine endothelial cells (pEC) will be exposed to human pathogens, among them HCMV. Infection of pEC with HCMV induces apoptosis and entry is sufficient to induce phenotypic alterations, which have the potential to result in rejection and vasculopathy. We investigated the mechanisms used by HCMV to enter pEC from different anatomical origins and compared them with the entry mechanisms used to enter human endothelial cells (hEC).

METHODS

Immortalized porcine aortic (PEDSV.15) and porcine microvascular bone marrow derived EC (2A2) as well as primary human aortic (HAEC) and microvascular EC (HMVEC) were inoculated with the endotheliotropic (TB40/E) or the fibroblast propagated (TB40/F) HCMV strains at multiplicity of infection (MOI) ranging from 0.3 to 5. EC were analyzed for receptor expression and their involvement in HCMV entry. The role of endocytosis was evaluated by treating EC with specific inhibitors, and the involvement of the endolysosomal pathway was investigated by confocal microscopy.

RESULTS

Silencing of platelet-derived growth factor receptor alpha resulted in a reduced expression of viral immediate early (IE) antigen only in pEC infected with either TB40/E or TB40/F whereas silencing of β1 integrins reduced expression of IE proteins in all EC except for TB40/F-infected microvascular pEC. TB40/E enters hEC and pEC by a similar mechanism dependent on dynamin-2, lipid rafts, actin and pH, whereas entry of TB40/F in pEC occurs mainly by a dynamin-2-dependent, clathrin-, lipid rafts-independent mechanism and in a pH-dispensable manner. When actin polymerization was prevented, TB40/F could enter pEC in an actin-independent fashion. Disturbance of the microtubule cytoskeleton resulted in an inhibition of infection of TB40/E-infected EC, whereas infection of TB40/F-infected pEC was not modified. Finally, viral particles located in vesicles of the endolysosomal pathway, suggesting that HCMV uses this pathway for intracellular trafficking following entry.

CONCLUSIONS

Our findings demonstrate that HCMV uses a variety of entry mechanisms that are dependent on the strain and on the vascular origin of the cells. Given the profound effect of pEC infection with HCMV, prevention of such an infection will be crucial for clinical application of xenotransplantation. A potential avenue is to render porcine grafts resistant to HCMV infection by blocking viral entry and propagation.

Regulatory sequences of the porcine THBD gene facilitate endothelial-specific expression of bioactive human thrombomodulin in single- and multitransgenic pigs

BACKGROUND

Among other mismatches between human and pig, incompatibilities in the blood coagulation systems hamper the xenotransplantation of vascularized organs. The provision of the porcine endothelium with human thrombomodulin (hTM) is hypothesized to overcome the impaired activation of protein C by a heterodimer consisting of human thrombin and porcine TM.

METHODS

We evaluated regulatory regions of the THBD gene, optimized vectors for transgene expression, and generated hTM expressing pigs by somatic cell nuclear transfer. Genetically modified pigs were characterized at the molecular, cellular, histological, and physiological levels.

RESULTS

A 7.6-kb fragment containing the entire upstream region of the porcine THBD gene was found to drive a high expression in a porcine endothelial cell line and was therefore used to control hTM expression in transgenic pigs. The abundance of hTM was restricted to the endothelium, according to the predicted pattern, and the transgene expression of hTM was stably inherited to the offspring. When endothelial cells from pigs carrying the hTM transgene--either alone or in combination with an aGalTKO and a transgene encoding the human CD46-were tested in a coagulation assay with human whole blood, the clotting time was increased three- to four-fold (P<0.001) compared to wild-type and aGalTKO/CD46 transgenic endothelial cells. This, for the first time, demonstrated the anticoagulant properties of hTM on porcine endothelial cells in a human whole blood assay.

CONCLUSIONS

The biological efficacy of hTM suggests that the (multi-)transgenic donor pigs described here have the potential to overcome coagulation incompatibilities in pig-to-primate xenotransplantation.

Identification of the tetraspanin CD82 as a new barrier to xenotransplantation

Significant immunological obstacles are to be negotiated before xenotransplantation becomes a clinical reality. An initial rejection of transplanted vascularized xenograft is attributed to Galα1,3Galβ1,4GlcNAc-R (Galα1,3-Gal)-dependent and -independent mechanisms. Hitherto, no receptor molecule has been identified that could account for Galα1,3-Gal-independent rejection. In this study, we identify the tetraspanin CD82 as a receptor molecule for the Galα1,3-Gal-independent mechanism. We demonstrate that, in contrast to human undifferentiated myeloid cell lines, differentiated cell lines are capable of recognizing xenogeneic porcine aortic endothelial cells in a calcium-dependent manner. Transcriptome-wide analysis to identify the differentially expressed transcripts in these cells revealed that the most likely candidate of the Galα1,3-Gal-independent recognition moiety is the tetraspanin CD82. Abs to CD82 inhibited the calcium response and the subsequent activation invoked by xenogeneic encounter. Our data identify CD82 on innate immune cells as a major "xenogenicity sensor" and open new avenues of intervention to making xenotransplantation a clinical reality.

Efficient sensing of infected cells in absence of virus particles by plasmacytoid dendritic cells is blocked by the viral ribonuclease E(rns.)

Plasmacytoid dendritic cells (pDC) have been shown to efficiently sense HCV- or HIV-infected cells, using a virion-free pathway. Here, we demonstrate for classical swine fever virus, a member of the Flaviviridae, that this process is much more efficient in terms of interferon-alpha induction when compared to direct stimulation by virus particles. By employment of virus replicon particles or infectious RNA which can replicate but not form de novo virions, we exclude a transfer of virus from the donor cell to the pDC. pDC activation by infected cells was mediated by a contact-dependent RNA transfer to pDC, which was sensitive to a TLR7 inhibitor. This was inhibited by drugs affecting the cytoskeleton and membrane cholesterol. We further demonstrate that a unique viral protein with ribonuclease activity, the viral E^rns protein of pestiviruses, efficiently prevented this process. This required intact ribonuclease function in intracellular compartments. We propose that this pathway of activation could be of particular importance for viruses which tend to be mostly cell-associated, cause persistent infection, and are non-cytopathogenic.

Plasmacytoid dendritic cells (pDC) represent the most potent producers of interferon type I and are therefore of major importance in antiviral defences. A TLR7-dependent induction of interferon-α in pDC by infected cells in the absence of virions has been demonstrated for hepatitis C virus. Here, we show that this pathway is also very efficient for classical swine fever virus, a pestivirus that is also a member of the Flaviviridae. Our data indicate a transfer of RNA from the donor cell to pDC in a cell-contact-dependent manner requiring intact lipid rafts and cytoskeleton of the donor cell. Importantly, we demonstrate that the enigmatic viral E^rns protein unique to pestiviruses efficiently prevents this pathway of pDC activation. This novel function of E^rns is dependent on its RNase activity within intracellular compartments. The present study underlines the importance of pDC activation by infected cells and identifies a novel pathway of virus escaping the interferon system. Considering that E^rns is required for pestiviruses to establish persistent infection of foetuses after transplacental virus transmission resulting in the development of immunotolerant animals, this report also points on a possible role of pDC in preventing immunotolerance after viral infection of foetuses.

Mycoplasma suis infection results endothelial cell damage and activation: new insight into the cell tropism and pathogenicity of hemotrophic mycoplasma

Hemotrophic mycoplasmas (HM) are highly specialized red blood cell parasites that cause infectious anemia in a variety of mammals, including humans. To date, no in vitro cultivation systems for HM have been available, resulting in relatively little information about the pathogenesis of HM infection. In pigs, Mycoplasma suis-induced infectious anemia is associated with hemorrhagic diathesis, and coagulation dysfunction. However, intravasal coagulation and subsequent consumption coagulopathy can only partly explain the sequence of events leading to hemorrhagic diathesis manifesting as cyanosis, petechial bleeding, and ecchymosis, and to disseminated coagulation. The involvement of endothelial activation and damage in M. suis-associated pathogenesis was investigated using light and electron microscopy, immunohistochemistry, and cell sorting. M. suis interacted directly with endothelial cells in vitro and in vivo. Endothelial activation, widespread endothelial damage, and adherence of red blood cells to the endothelium were evident in M. suis-infected pigs. These alterations of the endothelium were accompanied by hemorrhage, intravascular coagulation, vascular occlusion, and massive morphological changes within the parenchyma. M. suis biofilm-like microcolonies formed on the surface of endothelial cells, and may represent a putative persistence mechanism of M. suis. In vitro analysis demonstrated that M. suis interacted with the endothelial cytoskeletal protein actin, and induced actin condensation and activation of endothelial cells, as determined by the up-regulation of ICAM, PECAM, E-selectin, and P-selectin. These findings demonstrate an additional cell tropism of HM for endothelial cells and suggest that M. suis interferes with the protective function of the endothelium, resulting in hemorrhagic diathesis.

Transgenic expression of human heme oxygenase-1 in pigs confers resistance against xenograft rejection during ex vivo perfusion of porcine kidneys

BACKGROUND

The major immunological hurdle to successful porcine-to-human xenotransplantation is the acute vascular rejection (AVR), characterized by endothelial cell (EC) activation and perturbation of coagulation. Heme oxygenase-1 (HO-1) and its derivatives have anti-apoptotic, anti-inflammatory effects and protect against reactive oxygen species, rendering HO-1 a promising molecule to control AVR. Here, we report the production and characterization of pigs transgenic for human heme oxygenase-1 (hHO-1) and demonstrate significant protection in porcine kidneys against xenograft rejection in ex vivo perfusion with human blood and transgenic porcine aortic endothelial cells (PAEC) in a TNF-α-mediated apoptosis assay.

METHODS

Transgenic and non-transgenic PAEC were tested in a TNF-α-mediated apoptosis assay. Expression of adhesion molecules (ICAM-1, VCAM-1, and E-selectin) was measured by real-time PCR. hHO-1 transgenic porcine kidneys were perfused with pooled and diluted human AB blood in an ex vivo perfusion circuit. MHC class-II up-regulation after induction with IFN-γ was compared between wild-type and hHO-1 transgenic PAEC.

RESULTS

Cloned hHO-1 transgenic pigs expressed hHO-1 in heart, kidney, liver, and in cultured ECs and fibroblasts. hHO-1 transgenic PAEC were protected against TNF-α-mediated apoptosis. Real-time PCR revealed reduced expression of adhesion molecules like ICAM-1, VCAM-1, and E-selectin. These effects could be abrogated by the incubation of transgenic PAECs with the specific HO-1 inhibitor zinc protoporphorine IX (Zn(II)PPIX, 20 μm). IFN-γ induced up-regulation of MHC class-II molecules was significantly reduced in PAECs from hHO-1 transgenic pigs. hHO-1 transgenic porcine kidneys could successfully be perfused with diluted human AB-pooled blood for a maximum of 240 min (with and without C1 inh), while in wild-type kidneys, blood flow ceased after ∼60 min. Elevated levels of d-Dimer and TAT were detected, but no significant consumption of fibrinogen and antithrombin was determined. Microthrombi could not be detected histologically.

CONCLUSIONS

These results are encouraging and warrant further studies on the biological function of heme oxygenase-I expression in hHO-1 transgenic pigs in the context of xenotransplantation.

Protection of porcine endothelial cells against apoptosis with interleukin-4

BACKGROUND

Apoptosis is crucial for tissue development and homeostasis, and insufficient apoptosis is pivotal in cancer pathogenesis. Apoptosis may also be important in tissue injury and in this case, it is of interest to induce protection against apoptosis. In organ transplantation, apoptosis has been implicated in acute vascular rejection (AVR); in xenotransplantation, the inducers of apoptosis of relevance in AVR, such as tumor necrosis factor-α (TNF-α), also cause endothelial cell (EC) activation. We have previously shown that interleukin (IL)-4 and IL-13 induced protection in porcine ECs against activation and apoptosis triggered by TNF-α. Now we define signaling processes activated by IL-4 in porcine ECs and mechanisms required for IL-4-induced protection against apoptosis.

METHODS

Porcine aortic ECs were used as primary cultures or as virus-induced immortalized cells derived from galactosyl transferase-deficient (Gal(-/-) ) or wild-type pigs. ECs were stimulated with porcine IL-4, either extrinsically or transduced with recombinant adenovirus (adeno) IL-4, and analyzed using immunoblotting. Apoptosis was induced with TNF-α plus cycloheximide and assessed using neutral red uptake or flow cytometry. The role of various signaling proteins in IL-4-induced protection was established using pharmacologic inhibitors and siRNA downregulation of protein expression.

RESULTS

IL-4 induced similar degrees of phosphorylation of STAT6 in all 3 types of ECs, and STAT6 was phosphorylated through Jak3. IL-4 induced phosphorylation of Bad through Jak3. Stimulation of ECs with IL-4 caused protection of ECs against apoptosis with an absolute requirement of Jak3/STAT6 activation and major participation of mammalian target of rapamycin complex 2 (mTORC2), Akt, and extracellular signal-regulated kinase 1/2. IL-4 caused no increase in EC levels of protective proteins hemoxygenase-1, inhibitor of apoptosis protein, heat shock protein 70, Bcl-2, and Bcl-xL. ECs transduced with adenoIL-4 exhibited strong and durable protection from apoptosis. Gal(-/-) ECs were as susceptible to induction of protection with IL-4 as wild-type ECs.

CONCLUSIONS

IL-4 induces activation of Jak3/STAT6 and phosphorylation of Bad in porcine ECs, ultimately resulting in effective protection of the ECs from apoptosis. Delineation of downstream signals activated by IL-4 that are required for induction of protection suggests possible sites of intervention to design effective therapeutic agents. This is of interest because substances such as IL-4 have pleiotropic effects and cannot be used directly due to potential deleterious effects. Inducing resistance to apoptosis in porcine vascular endothelium may be important to facilitate xenograft survival and accommodation.

Propagation of classical swine fever virus in vitro circumventing heparan sulfate-adaptation

Amplification of natural virus isolates in permanent cell lines can result in adaptation, in particular enhanced binding to heparan sulfate (HS)-containing glycosaminoglycans present on most vertebrate cells. This has been reported for several viruses, including the pestivirus classical swine fever virus (CSFV), the causative agent of a highly contagious hemorrhagic disease in pigs. Propagation of CSFV in cell culture is essential in virus diagnostics and research. Adaptation of CSFV to HS-binding has been related to amino acid changes in the viral E(rns) glycoprotein, resulting in viruses with altered replication characteristics in vitro and in vivo. Consequently, a compound blocking the HS-containing structures on cell surfaces was employed to monitor conversion from HS-independency to HS-dependency. It was shown that the porcine PEDSV.15 cell line permitted propagation of CSFV within a limited number of passages without adaptation to HS-binding. The selection of HS-dependent CSFV mutants was also prevented by propagation of the virus in the presence of DSTP 27. The importance of these findings can be seen from the altered ratio of cell-associated to secreted virus upon acquisition of enhanced HS-binding affinity, a phenotype proposed previously to be related to virulence in the natural host.

Efficiency of porcine endothelial cell infection with human cytomegalovirus depends on both virus tropism and endothelial cell vascular origin

BACKGROUND

Human cytomegalovirus (HCMV) infection or reactivation has been linked to allograft rejection resulting from endothelial injury and immune activation. In pig-to-human xenotransplantation, currently investigated to circumvent the shortage of human organs in transplantation medicine, the porcine endothelium will inevitably be exposed to human pathogens such as HCMV. We investigated the susceptibility of porcine endothelial cells (pEC) to HCMV infection.

METHODS

Immortalized porcine aortic (PEDSV15) and porcine microvascular bone-marrow derived EC (2A2) as well as a panel of primary pEC originated from different vascular beds were inoculated with the endotheliotropic (TB40/E) and the fibroblast propagated (TB40/F) HCMV strains at multiplicity of infection (MOI) ranging from 0.1 to 5. Viral replication kinetics, development of cytopathology and release of viral progeny were analyzed.

RESULTS

All viral strains infected pEC with differences in both infection efficiency and kinetics of cytopathology. Moreover, differences in susceptibility of pEC derived from distinct vascular beds were observed. HCMV underwent a complete replication cycle in about 5% of the infected pEC. Comparing the permissiveness of pEC to human aortic EC (HAEC) revealed differences in strain susceptibility and lower rates of late antigen expression in pEC. Finally, HCMV-infected pEC released viral particles but with a lower efficiency than infected HAEC.

CONCLUSIONS

Our data demonstrate that HCMV productively infects pEC, therefore finding strategies to render pEC resistant to HCMV infection will be of interest to reduce the potential risk carried by HCMV reactivation in xenotransplantation.

HLA-Cw4 expression on porcine endothelial cells reduces cytotoxicity and adhesion mediated by CD158a+ human NK cells

BACKGROUND

Human natural killer (NK) cell-mediated cytotoxicity represents a hurdle in pig-to-human xenotransplantation. It was previously reported that the expression of human major histocompatibility complex class I molecules, including HLA-B27, -Cw3, -E, and -G, partially protects porcine endothelial cells (pEC) from human NK-mediated cytotoxicity and that HLA-G inhibits NK adhesion to pEC. Here, we tested if HLA-Cw4 expression on pEC alone, or concurrently with HLA-Cw3, prevents human NK adhesion and cytotoxicity against pEC via recognition of the killer-cell immunoglobulin-like receptors (KIR) CD158a (KIR2DL1) and CD158b (KIR2DL2/3), respectively.

METHODS

Two pEC lines (2A2 and PEDSV.15) were transfected with HLA-Cw3 and HLA-Cw4. HLA and KIR expression on porcine and human cells were analyzed by flow cytometry. The effect of HLA expression on pEC on human NK-mediated cytotoxicity and adhesion was tested by (51)Cr-release and dynamic adhesion assays, respectively.

RESULTS

HLA-Cw4 expression on pEC reduced cytotoxicity mediated by CD158a(+) polyclonal human NK cells by an average of 58%, and by CD158a(bright) NK cell clones by 68%, but not by NK cells expressing low levels of CD158. Co-expression of HLA-Cw3 and HLA-Cw4 on pEC did not mediate further protection against NK cytotoxicity. The expression of HLA-Cw4 reduced the adhesion of human NK cells on pEC by a mean of 53%.

CONCLUSIONS

While transgenic expression of HLA-Cw4 on pEC reduces NK cell adhesion and cytotoxicity, co-expression with HLA-Cw3 is not sufficient to completely overcome human NK-mediated cytotoxicity in vitro.

Molecular genetics of the swine major histocompatibility complex, the SLA complex

The swine major histocompatibility complex (MHC) or swine leukocyte antigen (SLA) complex is one of the most gene-dense regions in the swine genome. It consists of three major gene clusters, the SLA class I, class III and class II regions, that span approximately 1.1, 0.7 and 0.5Mb, respectively, making the swine MHC the smallest among mammalian MHC so far examined and the only one known to span the centromere. This review summarizes recent updates to the Immuno Polymorphism Database-MHC (IPD-MHC) website (http://www.ebi.ac.uk/ipd/mhc/sla/) which serves as the repository for maintaining a list of all SLA recognized genes and their allelic sequences. It reviews the expression of SLA proteins on cell subsets and their role in antigen presentation and regulating immune responses. It concludes by discussing the role of SLA genes in swine models of transplantation, xenotransplantation, cancer and allergy and in swine production traits and responses to infectious disease and vaccines.

HLA-E/human beta2-microglobulin transgenic pigs: protection against xenogeneic human anti-pig natural killer cell cytotoxicity

BACKGROUND

Natural killer (NK) cells participate in pig-to-primate xenograft rejection both by antibody-dependent and -independent mechanisms. A majority of human NK cells express the inhibitory receptor CD94/NKG2A, which binds specifically to human leukocyte antigen (HLA)-E, a trimeric complex consisting of the HLA-E heavy chain, beta2-microglobulin (beta2m), and a peptide derived from the leader sequence of some major histocompatibility complex class I molecules.

METHODS

To use this mechanism for protection of pig tissues against human NK cell-mediated cytotoxicity, we generated transgenic pigs by pronuclear microinjection of genomic fragments of HLA-E with an HLA-B7 signal sequence and of human beta2-microglobulin (hubeta2m) into zygotes.

RESULTS

Three transgenic founder pigs were generated. Northern blot analysis of RNA from peripheral blood mononuclear cells revealed the presence of the expected transcript sizes for both transgenes in two of the three founders. The founder with the highest expression and his offspring were characterized in detail. Fluorescence-activated cell sorting (FACS) and Western blot analyses demonstrated consistent expression of HLA-E and hubeta2m in peripheral blood mononuclear cells. Immunohistochemistry revealed the presence of HLA-E and hubeta2m on endothelial cells of many organs, including heart and kidney. In vitro studies showed that lymphoblasts and endothelial cells derived from HLA-E/hubeta2m transgenic pigs are effectively protected against human NK cell-mediated cytotoxicity, depending on the level of CD94/NKG2A expression on the NK cells. Further, HLA-E/hubeta2m expression on porcine endothelial cells inhibited the secretion of interferon (IFN)-gamma by co-cultured human NK cells.

CONCLUSIONS

This novel approach against cell-mediated xenogeneic responses has important implications for the generation of multitransgenic pigs as organ donors for clinical xenotransplantation.

Characterization of porcine UL16-binding protein 1 endothelial cell surface expression

BACKGROUND

Natural killer (NK) cells participate in the immune response against solid organ allo- and xenografts and are tightly regulated through signals mediated by inhibiting and activating receptors expressed on their cell surface. Human NK cytotoxicity against porcine endothelial cells (pEC) is mediated by the interaction of the activating human NK receptor hNKG2D and its corresponding ligand on pEC, porcine UL-16 binding protein 1 (pULBP1). The aim of the present study was to characterize the regulation of pULBP1 cell-surface expression on primary porcine aortic endothelial cells (PAEC).

METHODS

A monoclonal antibody (mAb; aE5-63) directed against pULBP1 was generated by immunizing C57BL/6 mice with the pEC line PEDSV.15, and used in cellular ELISA to determine pULBP1 cell surface expression. PAEC were either left untreated or stimulated with human or porcine cytokines [interferon gamma (IFN-gamma), tumor necrosis factor alpha (TNF-alpha)], human serum, cultured under hypoxic conditions, or infected with human or porcine cytomegalovirus (CMV).

RESULTS

Neither human nor porcine IFN-gamma stimulation changed pULBP1 expression, whereas both human and porcine TNF-alpha stimulation as well as human and porcine CMV infection significantly decreased pULBP1 expression on PAEC. Coculture of PAEC with human serum strongly increased pULBP1 expression depending on the binding of human anti-pig antibodies. Exposure of PAEC to hypoxia only slightly increased pULBP1 expression.

CONCLUSIONS

In conclusion, (i) the novel anti-pULBP1 IgM mAb aE5-63 represents a useful tool to study pULBP1/hNKG2D-mediated responses in xenotransplantation, and (ii) the expression of pULBP1, a human-pig cross-species functional hNKG2D ligand, on the surface of PAEC is modulated by various stimuli associated with transplantation.

Cytokine secretion depends on Galalpha(1,3)Gal expression in a pig-to-human whole blood model

Transplants from alpha1,3-galactosyltransferase (Gal) gene-knockout pigs to nonhuman primates are largely protected from hyperacute but not acute humoral xenograft rejection. The present study investigates the role of Gal in cytokine responses using a novel pig-to-human whole blood in vitro model, developed for species-specific analysis of porcine and human cytokines. Porcine (n = 7) and human (n = 27) cytokines were measured using ELISA or multiplex technology, respectively. Porcine aortic endothelial cells from control (Gal(+/+)) and Gal-deficient (Gal(-/-)) pigs were incubated with human lepirudin anticoagulated whole blood from healthy donors. E-selectin expression was measured by flow cytometry. The C3 inhibitor compstatin and a C5aR antagonist were used to study the role of complement. Cytokine species specificity was documented, enabling detection of 2 of 7 porcine cytokines and 13 of 27 human cytokines in one single sample. Gal(+/+) porcine aortic endothelial cells incubated with human whole blood showed a marked complement C5b-9 dependent up-regulation of E-selectin and secretion of porcine IL-6 and IL-8. In contrast, Gal(-/-) cells responded with E-selectin and cytokine expression which was so weak that the role of complement could not be determined. Human IL-6, IL-8, IFN-gamma, MIP-1alpha, MIP-1beta, eotaxin, and RANTES were detected in the Gal(+/+) system, but virtually no responses were seen in the Gal(-/-) system (p = 0.03). The increase in human cytokine release was largely complement dependent and, in contrast to the porcine response, mediated through C5a. Species-specific analysis of cytokine release revealed a marked, complement-dependent response when Gal(+/+) pig cells were incubated with human whole blood, compared with Gal(-/-) cells which induced virtually no cytokine release.

The effects of tolerance on allograft damage caused by the innate immune system

BACKGROUND

It is not known whether tolerance can be induced in a strong proinflammatory milieu or whether the induction of tolerance can prevent interferon (IFN)-gamma-associated graft injury. To address these questions, we studied the effects of rIFN-gamma infusion on porcine cardiac allograft survival.

METHODS

Recombinant interferon (rIFN)-gamma was continuously infused into the left anterior descending artery of hearts transplanted into major histocompatibility complex-inbred miniature swine treated with a 12-day course of cyclosporine A. Group 1 recipients received a nearly syngeneic heart, group 2 recipients received a class I disparate heart, and group 3 recipients were cotransplanted with a class I-disparate heart and kidney, a procedure demonstrated to induce tolerance to both grafts. A fourth group of animals were not transplanted but received intracoronary rIFN-gamma infusion into the native heart.

RESULTS

rIFN-gamma perfusion not only accelerated the acute rejection of class I-disparate hearts (mean survival time, 19+/-7.21 vs. 38+/-8.19; P=0.025) but caused near-syngeneic heart transplants, which otherwise survived indefinitely, to reject within 35 days. In contrast, rIFN-gamma perfusion had no demonstrable effects on hearts grafts in tolerant recipients or on autologous hearts.

CONCLUSIONS

These results suggest that tolerance induction can occur in the presence of IFN-gamma-mediated inflammation, and that tolerance induction can prevent the tissue injury caused by the overproduction of IFN-gamma. This suggests that the beneficial effects of tolerance may include protection from nonspecific inflammatory responses, such as those produced by ischemia-reperfusion injury and brain death.

Bone marrow cells contribute to neointimal formation after stent implantation in swine

BACKGROUND

Recent studies suggest that neointimal cells in atherosclerotic lesions are partly derived from bone marrow (BM) cells. However, studies with large animal models have not yet clarified how BM cells contribute to neointimal formation in restenotic lesions. We examined the expression of CD34, a hematopoietic stem cell marker, in the neointima after coronary stent implantation in porcine.

METHODS

Pigs underwent balloon injury in the coronary arteries followed by stent implantation. The arteries were harvested at 3, 7, and 28 days after the stenting. The samples were used for immunohistochemistry for CD34, smooth muscle embryo (SMemb), alpha-smooth muscle cell actin (alpha-SMA), macrophage, c-kit, and AC133 antibodies. In morphometric analysis, each layer of vascular wall was calculated in the sections.

RESULTS

At 3 days, the expressions of CD34 and SMemb were minimal, and many macrophages were seen around the stent struts. At 7 days, co-expression of CD34 and SMemb was observed around the struts, and 11.5% of the neointimal cells were stained by CD34. In addition, c-kit positive cells and AC133 positive cells are detected in neointimal area. At 28 days, the neointima had thickened and expressed alpha-SMA rather than SMemb, and a few CD34-positive cells were detected. In morphometric analysis, luminal area/total vascular area was significantly smaller and intimal area/total vascular area was significantly bigger in 7 and 28 days than in the day of implantation.

CONCLUSION

BM cells of possibly hematopoietic origin partially contributed to neointimal formation after coronary stent implantation in a large animal model.

Characterization of natural human anti-non-gal antibodies and their effect on activation of porcine gal-deficient endothelial cells

BACKGROUND

The generation of Galalpha1-3Gal (Gal) transferase deficient pigs has increased the interest in non-Gal antigens potentially representing important targets for xenoreactive antibody binding leading to xenograft rejection. The present study addressed the levels and immunoglobulin isotypes of preformed human anti-non-Gal antibodies and their potential to activate porcine endothelial cells.

METHODS

Porcine endothelial cells lacking the Gal epitope (Gal-/-) were used to measure immunoglobulin (Ig) M and IgG subclass anti-non-Gal antibodies, using sera from 80 blood donors and pooled human AB serum. Antibodies specific for the non-Gal Hanganutziu-Deicher (HD) xenoantigen were measured by enzyme-linked immunosorbent assay. Activation of Gal-/- and Gal+/+ endothelial cells by human serum was measured, in the presence or absence of complement inhibitors, by E-selectin cell-surface expression using flow cytometry.

RESULTS

Anti-non-Gal antibody levels varied considerably among individual sera and comprised approximately 10% of total anti-porcine antibodies without sex or age differences. Among the IgG subclasses only IgG1 and IgG2 were detected. Human serum-induced E-selectin expression on Gal-/- cells was less than 20% compared with Gal+/+ cells, correlated with anti-HD IgM and IgG antibody levels (P=0.027 and 0.032, respectively), and was largely complement-independent in accordance with the lack of IgG3 anti-non-Gal antibodies. In contrast, E-selectin upregulation on Gal+/+ cells was reduced in complement blocking experiments.

CONCLUSION

Preformed anti-non-Gal antibodies, in particular anti-HD antibodies, were present in all human sera samples, activated porcine endothelial cells, and may therefore play a role in xenograft rejection using organs from GalT-/- pigs.

Reactivity of human natural antibodies to endothelial cells from Galalpha(1,3)Gal-deficient pigs

BACKGROUND

Xenoreactive human natural antibodies (NAb) are predominantly directed against galactose-alpha(1,3)galactose (Gal). Binding of immunoglobulin (Ig) G and IgM NAb activates porcine endothelial cells (pEC) and triggers complement lysis responsible for hyperacute xenograft rejection. In vitro, IgG NAb induce human natural killer (NK) cell-mediated lysis of pEC by antibody-dependent cell-mediated cytotoxicity (ADCC). The present study examined the levels of anti-porcine NAb in a large number of individuals and addressed the functional role of non-Gal anti-porcine NAb.

METHODS

Sera from 120 healthy human blood donors were analyzed for the presence of anti-porcine NAb by flow cytometry using porcine red blood cells (pRBC), lymphoblastoid cells (pLCL), and pEC derived from control or Gal-deficient pigs. Xenogeneic complement lysis was measured by flow cytometry using human serum and rabbit complement. ADCC was analyzed by chromium-release assays using human serum and freshly isolated NK cells.

RESULTS

Human IgM binding to pRBC was found in 93% and IgG binding in 86% of all samples. Non-Gal NAb comprised 13% of total IgM and 36% of total IgG binding to pEC. NAb/complement-induced lysis and ADCC of Gal-deficient compared to Gal-positive pEC were 21% and 29%, respectively. The majority of anti-Gal and non-Gal IgG NAb were of the IgG2 subclass.

CONCLUSIONS

The generation of Gal-deficient pigs has overcome hyperacute anti-Gal-mediated xenograft rejection in nonhuman primates. Non-Gal anti-porcine NAb represent a potentially relevant immunological hurdle in a subgroup of individuals by inducing endothelial damage in xenografts.

Transgenic expression of HLA-E single chain trimer protects porcine endothelial cells against human natural killer cell-mediated cytotoxicity

BACKGROUND

The susceptibility of porcine endothelial cells (pEC) to human natural killer (NK) cells is related to the failure of human major histocompatibility complex (MHC)-specific killer inhibitory receptors to recognize porcine MHC class I molecules. The aims of this study were (i) to assess the protection of pEC against xenogeneic NK-mediated cytotoxicity afforded by the stable expression of HLA-E single chain trimers (SCT) composed of a canonical HLA-E binding peptide antigen, VMAPRTLIL, the mature human beta2-microglobulin, and the mature HLA-E heavy chain, and (ii) to test whether HLA-E expression on pEC and porcine lymphoblastoid cells affects the adhesion of human NK cells.

METHODS

Porcine EC lines expressing different levels of HLA-E SCT were generated by Ca(2)PO(4)-transfection followed by limiting dilution cloning. Surface expression of HLA-E was measured by flow cytometry. Susceptibility of transfected pEC lines against human NK cells was tested in (51)Cr-release cytotoxicity assays. Interactions between human NK cells and HLA-E positive pEC or porcine lymphoblastoid cells were further addressed in adhesion and conjugation assays.

RESULTS

The level of protection of pEC from human NK-mediated cytotoxicity correlated with the intensity of surface HLA-E expression. Furthermore, the HLA-E SCT-mediated protection was specifically reversed by blocking the HLA-E specific NK inhibitory receptor CD94/NKG2A. HLA-E expression does neither affect the adhesion of human NK cells to pEC nor the heteroconjugate formation between human NK and porcine 13271.10 cells.

CONCLUSIONS

Stable surface expression of HLA-E on pEC was achieved in the absence of extrinsic peptide pulsing and provided partial protection from human NK cytotoxicity. Though insufficient to inhibit xenogeneic NK cell reactivity completely, transgenic HLA-E expression on pig organs might contribute to a successful application of clinical xenotransplantation in combination with other protective strategies.

Porcine UL16-binding protein 1 expressed on the surface of endothelial cells triggers human NK cytotoxicity through NKG2D

Cellular rejection mechanisms, including NK cells, remain a hurdle for successful pig-to-human xenotransplantation. Human anti-pig NK cytotoxicity depends on the activating receptor NKG2D. Porcine UL16-binding protein 1 (pULBP1) and porcine MHC class I chain-related protein 2 (pMIC2) are homologues of the human NKG2D ligands ULBP 1-4 and MICA and B, respectively. Although transcribed in porcine endothelial cells (pEC), it is not known whether pULBP1 and pMIC2 act as functional ligands for human NKG2D. In this study, surface protein expression of pULBP1 was demonstrated by flow cytometry using a novel pULBP1-specific polyclonal Ab and by cellular ELISA using NKG2D-Fc fusion protein. Reciprocally, pULBP1-Fc bound to primary human NK cells, whereas pMIC2-Fc did not. Transient and stable down-regulation of pULBP1 mRNA in pEC using short-interfering RNA oligonucleotide duplexes and short hairpin RNA, respectively, resulted in a partial inhibition of xenogeneic NK cytotoxicity through NKG2D in (51)Cr release assays. In contrast, down-regulation of pMIC2 mRNA did not inhibit NK cytotoxicity. Human NK cytotoxicity against pEC mediated by freshly isolated or IL-2-activated NK cells through NKG2D was completely blocked using anti-pULBP1 polyclonal Ab. In conclusion, this study suggests that pULBP1 is the predominant, if not only, functional porcine ligand for human NKG2D. Thus, the elimination of pULBP1 on porcine tissues represents an attractive target to protect porcine xenografts from human NK cytotoxicity.

Human NK cells can lyse porcine endothelial cells independent of their expression of Galalpha(1,3)-Gal and killing is enhanced by activation of either effector or target cells

BACKGROUND

Xenotransplantation of pig organs may provide an approach to alleviate the severe shortage of human organs. Natural antibodies against Galalpha(1,3)-Gal (alphaGal) epitopes cause hyperacute rejection of pig organs in primates. However, evidence for the role of alphaGal in the natural killer (NK) cell-mediated xenoresponse has been contradictory.

METHODS

We investigated the recognition of alphaGal by human NK cells using endo-beta-galactosidase C, an enzyme that cleaves alphaGal, and endothelial cells (EC) from alpha1,3-galactosyltransferase null pigs that do not synthesize alphaGal. Endo-beta-galactosidase C treatment variably reduced the susceptibility of porcine EC to lysis by fresh human NK cells.

RESULTS

Removal of alphaGal from porcine EC using endo-beta-galactosidase C, produced variable results, i.e. cytotoxicity was decreased in half of the human NK cell donors tested. The two EC strains from alphaGal-/- pigs were marginally, and not significantly, less susceptible to lysis by naïve human NK cells compared with alphaGal-expressing cells obtained from animals from the same herd, but these differences were not statistically significant (P > 0.10). Treatment of porcine EC with recombinant human tumor necrosis factor (TNF)-alpha, which is known to activate porcine EC, enhanced the susceptibility of all target cells to lysis by fresh human NK cells. Surface expression of MHC or adhesion molecules on alphaGal-/- cells, compared with wild type cells, showed no consistent difference in either MHC or adhesion molecules CD106 (VCAM-1), CD31 (PECAM) or CD62E (E-selectin), either with or without TNF-alpha stimulation, that could explain the differential susceptibility to lysis. Strikingly, all alphaGal-/- and wild type EC exhibited similar susceptibility to human NK cells that had been cultured for 5 days with or without interleukin-2.

CONCLUSIONS

These findings demonstrate that human NK cells can kill porcine targets in the absence of alphaGal, and donor variability plays a major role in whether alphaGal has a role in determining susceptibility of porcine EC to lysis. Moreover, susceptibility to lysis of alphaGal null EC is enhanced to the level of wild type EC by activation of either effector or target cells. Elimination of alphaGal alone from source pigs will be insufficient to circumvent the NK cell mediated destruction of porcine EC.

Adherence of Streptococcus suis to Porcine Endothelial Cells

Streptococcus suis can cause invasive diseases in pigs and humans, such as meningitis or arthritis. Adherence to and invasion of endothelial cells might represent important steps in survival and spread of S. suis within the host. We tested in vitro adherence and invasion of S. suis strains using a porcine brain microvascular and aortal endothelial cell line. Four S. suis strains were tested with and without prior treatment with porcine serum containing anti-S. suis antibodies. Strains included a capsular serotype 2 strain and its non-encapsulated isogenic mutant strain, as well as two non-typeable (NT) strains, which expressed no capsule under our experimental conditions. Strains adhered to both cell lines to different extents depending on encapsulation and pre-treatment with porcine immune serum. The serotype 2 strain showed almost no adherence, whereas the non-encapsulated mutant strain adhered strongly. Similarly, both NT strains adhered substantially better than the serotype 2 strain. Pre-treatment of bacteria with porcine serum increased adherence of the encapsulated serotype 2 strain and decreased adherence of the non-encapsulated strains. None of the strains was able to efficiently invade either of the two cell lines, except for one NT strain, which showed a very low extend of invasion. Our results suggest that S. suis can adhere to but not invade porcine endothelial cells, and that this interaction may involve different bacterial surface structures, such as capsular polysaccharides and/or binding sites for serum components.

Human NK Cytotoxicity against Porcine Cells Is Triggered by NKp44 and NKG2D

Pig-to-human xenotransplantation has been proposed as a means to alleviate the shortage of human organs for transplantation, but cellular rejection remains a hurdle for successful xenograft survival. NK cells have been implicated in xenograft rejection and are tightly regulated by activating and inhibitory receptors recognizing ligands on potential target cells. The aim of the present study was to analyze the role of activating NK receptors including NKp30, NKp44, NKp46, and NKG2D in human xenogeneic NK cytotoxicity against porcine endothelial cells (pEC). (51)Cr release and Ab blocking assays were performed using freshly isolated, IL-2-activated polyclonal NK cell populations as well as a panel of NK clones. Freshly isolated NK cells are NKp44 negative and lysed pEC exclusively in an NKG2D-dependent fashion. In contrast, the lysis of pEC mediated by activated human NK cells depended on both NKp44 and NKG2D, since a complete protection of pEC was achieved only by simultaneous blocking of these activating NK receptors. Using a panel of NK clones, a highly significant correlation between anti-pig NK cytotoxicity and NKp44 expression levels was revealed. Other triggering receptors such as NKp30 and NKp46 were not involved in xenogeneic NK cytotoxicity. Finally, Ab-dependent cell-mediated cytotoxicity of pEC mediated by human NK cells in the presence of xenoreactive Ab was not affected by blocking of activating NK receptors. In conclusion, strategies aimed to inhibit interactions between NKp44 and NKG2D on human NK cells and so far unknown ligands on pEC may prevent direct NK responses against xenografts but not xenogeneic Ab-dependent cell-mediated cytotoxicity.

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.

Human polymorphonuclear neutrophils are recruited by porcine chemokines acting on CXC chemokine receptor 2, and platelet-activating factor

BACKGROUND

Pig-to-human xenotransplantation is hampered by strong humoral and cellular immune responses, including acute vascular rejection (AVR). Infiltration of vascular xenografts by recipient polymorphonuclear neutrophils (PMN) is an early feature of AVR. Since little is known about the initiation of PMN recruitment, the present study investigated whether activated porcine endothelial cells (EC) release factors that induce human PMN recruitment.

METHODS

Primary and immortalized porcine aortic EC cultures were stimulated with phorbol-myristate acetate/ionomycin, lipopolysaccharide, tumor-necrosis factor-alpha, or interferon-gamma. The interleukin (IL)-8 concentration of porcine EC supernatants was tested by ELISA. Human and porcine PMN were isolated from peripheral blood by Ficoll sedimentation and centrifugation, characterized by morphology and flow cytometry, and analyzed for chemotaxis using Boyden chambers or Transwells. PMN chemokine receptor desensitization was determined by intracellular calcium-flux measurements.

RESULTS

Porcine EC supernatants contained significant amounts of porcine IL-8 and triggered chemotaxis in both human and porcine PMN. Chemotaxis of porcine, but not human, PMN was inhibited by anti-porcine IL-8 antibodies and recombinant porcine IL-8 induced strong chemotaxis only in porcine PMN. Porcine EC supernatants desensitized human PMN CXC-chemokine receptor (CXCR) 2, but not CXCR1, a receptor for human IL-8. Human PMN chemotaxis induced by porcine EC supernatants was significantly inhibited by blocking CXCR2 and platelet-activating factor (PAF).

CONCLUSIONS

Both chemokines acting via CXCR2 and PAF are released by porcine EC inducing efficient chemotaxis of human PMN. These mechanisms responsible for the recruitment of human PMN to porcine endothelium during cell-mediated rejection of xenografts represent potential targets for preventive strategies.

Immortalization of Bone Marrow Mesenchymal Stem Cells from Inbred Pig for Regenerative Medicine

The aim of this experiment is to investigate whether MSCs from Banna Minipig Inbred Line (BMI) could be immortalized by introducing SV40 large T antigen gene. MSCs were isolated from BMI and transfected with pSV3neo plasmid. Normal BMI-MSCs would apoptosis and senescence during proliferation while its population doubling (PD) number exceeded 20. However, SV40-transfected cells were immortal. As same as that of normal BMI-MSCs, transfected MSCs were positive for stem cell markers and negative for differentiated osteoblast specific marker. After cultured in osteogenesis supplement media, cbfa1 and calcium deposit on stimulated cells were enhanced obviously. There were no data to prove the tumorigenicity potential of the immortalized cells. Furthermore, histological analysis demonstrated that bone formation was initiated in the pores of HA/TCP implants loaded immortalized BMI-MSCs 7 weeks postimplantation. BMI-MSCs were immortalized by introducing SV40 large T antigen into the cells and still kept the stem cell characters and might be used as seeding cells for tissue engineering as well as stable test cells for biocompatibility of bone biomaterials.

Anti-non-Gal porcine endothelial cell antibodies in acute humoral xenograft rejection of hDAF-transgenic porcine hearts in cynomolgus monkeys

BACKGROUND

Anti-Gal alpha 1-3Gal (Gal) antibodies play a major role in hyperacute rejection and acute humoral xenograft rejection (AHXR) in porcine-to-nonhuman primate transplantation. The role of anti-non-Gal antibodies in AHXR is less well defined.

METHODS

Eleven cynomolgus monkeys received a heterotopic heart transplant from a human decay-accelerating factor transgenic pig, and maintenance immunosuppression with cyclosporin A or tacrolimus, steroids, mycophenolate sodium or mycophenolate mofetil, and in 10 animals the Gal-containing soluble glycoconjugate GAS914. Six ended with AHXR (6 to 78 day survival) and five did not show AHXR (9 to 36 day survival). Anti-Gal antibodies were depleted in vivo with GAS914, or in vitro with Gal-coated Sepharose beads. IgM- and IgG-class anti-non-Gal antibodies in serum depleted of anti-Gal antibodies were measured by flow cytometry using porcine endothelial target cells.

RESULTS

Compared with pre-transplant values, all six recipients with AHXR showed a substantially higher level of anti-non-Gal IgM antibodies at rejection; in five animals there was also an increase in IgG-class antibodies. There was no relevant change in recipients without AHXR. AHXR at time of cessation of heart contraction could be preceeded by a steady increase in antibody level starting 2 to 3 weeks earlier.

CONCLUSIONS

AHXR is invariably associated with increased circulating anti-non-Gal antibodies. These antibodies are not observed in recipients without AHXR, and five of six recipients with AHXR were adequately depleted of anti-Gal antibodies by maintenance GAS914. This indicates that anti-non-Gal antibodies play a significant role in the pathogenesis of AHXR. Also, the assessment of these antibodies could be used as an early monitor of AHXR.

Hyperacute rejection of hDAF-transgenic pig organ xenografts in cynomolgus monkeys: influence of pre-existing anti-pig antibodies and prevention by the alphaGAL glycoconjugate GAS914

BACKGROUND

Our introductory pig-to-cynomolgus monkey heart or kidney transplantation using organs from pigs transgenic for human decay-accelerating factor (hDAF), showed a high incidence of hyperacute rejection (HAR), which was ascribed to extraordinary high levels of anti-pig antibodies. We evaluated the efficacy of GAS914, a Gal alpha 1-3Gal trisaccharide linked to a poly-l-lysine backbone, in inhibition of HAR.

METHODS

hDAF transgenic heterotopic heart (n = 15) or life-supporting kidney (n = 8) transplantation included induction with cyclophosphamide or anti-thymocyte globulin, and maintenance with cyclosporine or tacrolimus, steroids and mycophenolate sodium/mofetil. Four doses of GAS914 were given before transplantation. Rejection was confirmed by graft histology, and anti-pig antibody levels were determined in various assays.

RESULTS

Four of six heart transplants without GAS914 treatment showed HAR. Nine subsequent transplants with GAS914 pre-treatment, did not show HAR (chi-square, P < 0.05). Two of four kidney transplants without GAS914 treatment ended with HAR. Four subsequent transplants with GAS914 did not show HAR. Animals with HAR showed extremely high antibody levels. Samples just before transplantation showed significantly higher antibody levels in recipients presenting with HAR. In all assays antibody levels were significantly lowered by GAS914 pre-treatment.

CONCLUSIONS

HAR of hDAF solid organs could be ascribed to high levels of anti-pig antibodies. It is hypothesized that the hDAF transgene shows a threshold in efficacy, above which an overwhelming attack by antibodies and complement activation cannot be modulated to prevent HAR. HAR does not occur when animals with lower levels are used, or when antibodies are effectively depleted from the circulation by GAS914 treatment.

Anti-pig antibody levels in non-human primates of various origin

BACKGROUND

Natural anti-porcine antibodies play a major role in hyperacute solid organ xenograft rejection in the pig-to-non-human primate model. Work from other groups and our experience in transplantation experiments has shown that antibody levels are highly variable between non-human primate species, and that extremely high levels can mediate hyperacute rejection even if organs from animals transgenic for human decay-accelerating factor are used.

METHODS

Sera were obtained from cynomolgus monkeys wild-caught in Mauritius, captive-bred in the Philippines, captive-bred in Indonesia (Indonesia-Ind), and originating from Indonesia but colony-bred in USA (Indonesia-USA), from baboons wild-caught in Kenya, and from rhesus monkeys originating from India but colony-bred in USA (10 animals in each group). Antibody levels were determined using assays for haemolytic antibody (APA), IgM and IgG class anti-Galalpha1-3Gal antibody, and IgM and IgG class anti-endothelial cell antibody.

RESULTS

Cynomolgus monkeys from the Philippines and Indonesia-USA and rhesus monkeys showed median APA and IgM antibody levels in the same range as a pooled human serum standard, and median IgG levels well below the level in this standard. Cynomolgus monkeys from Mauritius and Indonesia-Ind showed extremely high APA levels (median seven to 10 times the human serum standard): IgM class antibodies were also higher, while IgG class antibodies were in the range of the level in the human serum standard. Antibody levels in baboons were in between these two categories. The results of the APA assay showed a highly statistically significant correlation with the assays of IgM antibody, and this was also the case for the IgM antibody assays, indicative of the assessment of the same antibodies in these assays. The same was observed for the assays for IgG antibody. Taking body weight as an indicator for age, there was no relationship between body weight and levels of antibodies.

CONCLUSIONS

Natural antibody levels show a significant variation between various groups of non-human primates, with levels in some groups well above those in a human serum standard.

Porcine endogenous retrovirus transmission characteristics of galactose alpha1-3 galactose-deficient pig cells

Galactose alpha1-3 galactose (Gal) trisaccharides are present on the surface of wild-type pig cells, as well as on viruses particles produced from such cells. The recognition of Gal sugars by natural anti-Gal antibodies (NAb) in human and Old World primate serum can cause the lysis of the particles via complement-dependent mechanisms and has therefore been proposed as an important antiviral mechanism. Recently, pigs have been generated that possess disrupted galactosyl-transferase (GGTA1) genes. The cells of these pigs do not express Gal sugars on their surface, i.e., are Gal null. Concerns have been raised that the risk of virus transmission from such pigs may be increased due to the absence of the Gal sugars. We investigated the sensitivity of porcine endogenous retrovirus (PERV) produced from Gal-null and Gal-positive pig cells to inactivation by purified NAb and human serum. PERV produced in Gal-null pig cells was resistant to inactivation by either NAb or human serum. In contrast, although Gal-positive PERV particles were sensitive to inactivation by NAb and human serum, they required markedly higher concentrations of NAb for inactivation compared to the Gal-positive cells from which they were produced. Complete inactivation of Gal-positive PERV particles was not achievable despite the use of high levels of NAb, indicating that NAb-mediated inactivation of cell-free PERV particles is an inefficient process.

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 Fas-ligand expression on porcine endothelial cells does not protect against xenogeneic natural killer cytotoxicity*

Several human leukocyte subsets including natural killer (NK) cells, cytotoxic T lymphocytes (CTL), and polymorphonuclear neutrophils (PMN) participate in cellular immune responses directed against vascularized pig-to-human xenografts. As these leukocytes express the death receptor Fas either constitutively (PMN) or upon activation (NK, CTL), we explored in vitro whether the transgenic expression of Fas ligand (FasL) on porcine endothelial cells (EC) is a valuable strategy to protect porcine xenografts. The porcine EC line 2A2 was stably transfected with human FasL (2A2-FasL) and interactions of 2A2-FasL with human leukocytes were analyzed using functional assays for apoptosis, cytotoxicity, chemotaxis, adhesion under shear stress, and transmigration. FasL expressed on porcine EC induced apoptosis in human NK and T cells, but did not protect porcine EC against killing mediated by human NK cells. 2A2-FasL released soluble FasL, which induced strong chemotaxis in human PMN. Adhesion under shear stress of PMN on 2A2-FasL cells was increased whereas transendothelial migration was decreased. In contrast, FasL had no effect on the adhesion of NK cells but increased their transmigration through porcine EC. Although FasL expression on porcine EC is able to induce apoptosis in human effector cells, it did not provide protection against xenogeneic cytotoxicity. The observed impact of FasL on adhesion and transendothelial migration provides evidence for novel biological functions of FasL.

Dextran sulfate acts as an endothelial cell protectant and inhibits human complement and natural killer cell-mediated cytotoxicity against porcine cells

BACKGROUND

The innate immune system, including complement and natural killer (NK) cells, plays a critical role in activation and damage of endothelial cells (ECs) during xenograft rejection. The semisynthetic proteoglycan analog dextran sulfate (DXS, molecular weight 5,000) is known to inhibit the complement and coagulation cascades. We hypothesized that DXS may act as an "EC-protectant" preventing complement and NK lysis by functionally replacing heparan sulfate proteoglycans that are shed from the EC surface on activation of the endothelium.

METHODS

Binding of DXS to ECs, deposition of human complement, cytotoxicity, and heparan sulfate expression after exposure to normal human serum were analyzed by flow cytometry. The efficacy of DXS to protect ECs from xenogeneic NK cell-mediated cytotoxicity was tested in standard 51Cr-release assays.

RESULTS

DXS dose-dependently inhibited all three pathways of complement activation. Binding of DXS to porcine cells increased on treatment with human serum or heparinase I and correlated positively with the inhibition of human complement deposition. This cytoprotective effect of DXS was still present when the challenge with normal human serum was performed up to 48 hr after DXS treatment of the cells. DXS incubation of porcine ECs with and without prior tumor necrosis factor-alpha stimulation reduced xenogeneic cytotoxicity mediated by human NK cells by 47.3% and 25.3%, respectively.

CONCLUSIONS

DXS binds to porcine cells and protects them from complement- and NK cell-mediated injury in vitro. It might therefore be used as a novel therapeutic strategy to prevent xenograft rejection and has potential for clinical application as an "EC protectant."

Ultra-sensitive and specific detection of porcine endogenous retrovirus (PERV) using a sequence-capture real-time PCR approach

Use of porcine xenografts presents as a possible solution to the current shortage of human allografts limiting transplantation procedures. While no definitive observation of in vivo porcine endogenous retrovirus (PERV) transmission in humans has been reported, the in vitro ability of PERV to infect human cells and the observation of PERV transmission to immunodeficient mice suggest a need for ultra-sensitive techniques to monitor porcine xenograft recipients and contacts for possible PERV transmission. In an effort to enhance current PCR-based PERV detection, the feasibility of combining nucleic acid sequence-capture with use of a quantitative real-time 5' nuclease assay was examined. Sequence-capture by means of oligonucleotide hybridization to a conserved PERV gag sequence and attachment to magnetic beads was used to extract and concentrate PERV A, B and C DNA from sample material containing high levels of background human DNA. Sequence-capture oligonucleotide design incorporated selective substitution of dUTP for dTTP in order to facilitate eventual oligonucleotide destruction. In addition, sequence-capture oligonucleotides were located outside of the amplified region in order to minimize the effects of possible PCR carry-over. Quantitative PCR was then undertaken using a real-time 5' nuclease assay incorporating primers and probe also specific for a conserved PERV gag region. Sequence-capture real-time PCR assessment of PERV levels demonstrated a dynamic range of at least five orders of magnitude, a sensitivity between 0.005 and 0.028 PERV copies per microg background human DNA and a specificity between 98.2 and 100% (95% CI). In contrast, while real-time PERV PCR in the absence of a sequence-capture step demonstrated a similar specificity between 98.4 and 100% (95% CI), the sensitivity of this conventional approach was between 0.2 and 1.0 PERV copies per microg background human DNA. In conclusion, the increased sensitivity of PERV detection obtained by the combined use of PERV-specific sequence-capture and quantitative real-time PERV PCR suggest that this approach should enhance the effectiveness and reliability of monitoring procedures currently applied to porcine xenograft recipients and contacts.

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).

MHC Alloantigens Elicit Secondary, But Not Primary, Indirect In Vitro Proliferative Responses

The relative contributions of direct and indirect pathways of allorecognition to graft rejection remain controversial. Recent reports suggest that the indirect pathway may play a prominent role in both acute and chronic allograft rejection. Such studies suggest that MHC-derived allopeptides are more immunogenic than those derived from minor histocompatibility or other nominal Ags. The aim of this study was to characterize the immunogenicity of MHC alloantigens in MHC-defined miniature swine via primary and secondary MLR culture assays. APCs were selectively depleted from either responder or stimulator cell populations to specifically analyze direct and indirect proliferative responses, respectively. Radio-resistant cytokine secretion and subsequent backstimulation of responder cells was eliminated by using stimulators that were either lysed or unresponsive to the responder MHC haplotypes. When the effect of backstimulation was eliminated from MLR culture assays, indirect proliferative responses were not observed among naive responders. Only after in vivo priming of responder animals could indirect proliferation be detected. These data do not refute the potential importance of indirect allorecognition in graft rejection. However, they suggest that MHC-derived alloantigens behave similarly in vitro to minor histocompatibility Ags, with comparable immunogenicity. These data also suggest that the MLR culture assay does not accurately reflect the importance of indirect mechanisms that have previously been reported in experimental models of graft rejection. A greater understanding of the indirect pathway and the associated immunogenicity of MHC allopeptides has the potential benefit of enabling the development of therapeutic interventions to prevent or halt allograft rejection.

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.