Can regulatory T cells improve outcomes of sensitised patients after HLA-Ab incompatible renal transplantation: study protocol for the Phase IIa GAMECHANgER-1 trial

BACKGROUND

Kidney transplantation is the gold-standard treatment for patients with kidney failure. However, one-third of patients awaiting a kidney transplant are highly sensitized to human leukocyte antigens (HLA), resulting in an increased waiting time for a suitable kidney, more acute and chronic rejection, and a shorter graft survival compared to non-highly sensitised patients. Current standard immunosuppression protocols do not adequately suppress memory responses, and so alternative strategies are needed. Autologous polyclonally expanded regulatory T cells (Tregs) have been demonstrated to be safe in transplant settings and could be a potential alternative to modulate memory immune alloresponses.

METHODS

The aim of this trial is to determine whether adoptive transfer of autologous Tregs into HLA sensitised patients can suppress memory T and B cell responses against specific HLA antigens. This is a two-part, multi-centre, prospective clinical trial, comprising an observational phase (Part 1) aiming to identify patients with unregulated cellular memory responses to HLA (Pure HLA Proteins) followed by an interventional phase (Part 2). The first 9 patients identified as being eligible in Part 1 will undergo baseline immune monitoring for 2 months to inform statistical analysis of the primary endpoint. Part 2 is an adaptive, open labelled trial based on Simon's two-stage design, with 21 patients receiving Good Manufacturing Practice (GMP)-grade polyclonally expanded Tregs to a dose of 5-10 × 106 cells/kg body weight. The primary EP is suppression of in vitro memory responses for 2 months post-infusion. 12 patients will receive treatment in stage 1 of Part 2, and 9 patients will receive treatment in stage 2 of Part 2 if ≥ 50% patients pass the primary EP in stage 1.

DISCUSSION

This is a prospective study aiming to identify patients with unregulated cellular memory responses to Pure HLA Proteins and determine baseline variation in these patterns of response. Part 2 will be an adaptive phase IIa clinical trial with 21 patients receiving a single infusion of GMP-grade polyclonally expanded Tregs in two stages. It remains to be demonstrated that modulating memory alloresponses clinically using Treg therapy is achievable.

TRIAL REGISTRATION

EudraCT Number: 2021-001,664-23. REC Number: 21/SC/0253. Trial registration number ISRCTN14582152.

Thrombalexins: Cell-Localized Inhibition of Thrombin and Its Effects in a Model of High-Risk Renal Transplantation

Allograft transplantation into sensitized recipients with antidonor antibodies results in accelerated antibody-mediated rejection (AMR), complement activation, and graft thrombosis. We have developed a membrane-localizing technology of wide applicability that enables therapeutic agents, including anticoagulants, to bind to cell surfaces and protect the donor endothelium. We describe here how this technology has been applied to thrombin inhibitors to generate a novel class of drugs termed thrombalexins (TLNs). Using a rat model of hyperacute rejection, we investigated the potential of one such inhibitor (thrombalexin-1 [TLN-1]) to prevent acute antibody-mediated thrombosis in the donor organ. TLN-1 alone was able to reduce intragraft thrombosis and significantly delay rejection. The results confirm a pivotal role for thrombin in AMR in vivo. This approach targets donor organs rather than the recipient and is intended to be directly translatable to clinical use.

Factor VII promotes hepatocellular carcinoma progression through ERK-TSC signaling

We previously demonstrated PAR2 starts upstreamed with tissue factor (TF) and factor VII (FVII), inhibited autophagy via mTOR signaling in HCC. However, the mechanism underlying for merging functions of PAR2 with the coagulation system in HCC progression remained unclear. The present study aimed to investigate the role of TF, FVII and PAR2 in tumor progression of HCC. The expressions of TF, FVII and PAR2 from HCC specimens were evaluated by immunohistochemical stains and western blotting. We found that the expression of FVII, but not TF and PAR2, directly related to the vascular invasion and the clinical staging. Importantly, a lower level of FVII expression was significantly associated with the longer disease-free survival. The addition of FVII but not TF induced the expression of PAR2 and phosphorylation of ERK1/2, whereas knockdown of FVII decreased PAR2 expression and ERK1/2 phosphorylation in HCC cell lines. Furthermore, levels of phosphor-TSC2 (Ser664) were increased after treatment with FVII and PAR2 agonist whereas these were significantly abolished in the presence of a potent and specific MEK/ERK inhibitor U0126. Moreover, mTOR knockdown highly reduced Hep3B migration, which could be reverted by FVII but not TF and PAR2. These results indicated that FVII/PAR2 signaling through MEK/ERK and TSC2 axis for mTOR activation has potent effects on the migration of HCC cells. In addition, FVII/PAR2 signaling elicits an mTOR-independent signaling, which promotes hepatoma cell migration in consistent with the clinical observations. Our study indicates that levels of FVII, but not TF, are associated with tumor migration and invasiveness in HCC, and provides clues that evaluation of FVII expression in HCC may be useful as a prognostic indicator in patients with HCC and may form an alternative target for further therapy.

Fibrocytes mediate intimal hyperplasia post-vascular injury and are regulated by two tissue factor-dependent mechanisms

BACKGROUND

CD34(+) α-smooth muscle actin (SMA)(+) cells mediate intimal hyperplasia (IH) after mechanical endoluminal injury. We previously found that IH is tissue factor (TF) dependent. The precise phenotype of the CD34(+) cells mediating IH is unknown and the mechanisms of TF are also unknown.

OBJECTIVE

To define the phenotype of cells mediating IH and compare the effects of inhibiting TF on different subsets of CD34(+) cells.

METHODS

Endoluminal injury was induced in C57BL/6 and two strains of mice expressing a human tissue factor pathway inhibitor (hTFPI) fusion protein on different subsets of CD34(+) cells. Confocal microscopy, immunocytofluorescence and real-time PCR were used to determine phenotype.

RESULTS

Neointimal cells in C57BL/6 mice were defined as a subset of fibrocytes (CD34(+) CD45(+) collagen-1(+) ) expressing SMA, CD31, TIE-2, CXCR4 and CXCL12. Similar cells circulated post-injury and were also found in mice expressing hTFPI on CD34(+) CD31(+) cells, though in these mice, hTFPI inhibited CD31(+) fibrocyte hyperplasia, so no IH developed. Mice with hTFPI on all CD34(+) α-SMA(+) cells repaired arteries back to a pre-injured state. No CD31(+) fibrocytes were found in these mice unless an anti-hTFPI antibody was administered. Similar findings in protease activated receptor (PAR)-1-deficient mice suggested hTFPI prevented thrombin signaling through PAR-1. In vitro, thrombin increased the number of CD31(+) fibrocytes.

CONCLUSIONS

Inhibition of TF on CD31(+) fibrocytes inhibits IH whereas inhibition on all CD34(+) α-SMA(+) cells (or PAR-1 deficiency) inhibits the appearance of CD31(+) fibrocytes and promotes repair. These data enhance our understanding of IH and suggest novel ways to promote regenerative repair.

Renal allograft recipients fail to increase interferon-γ during invasive fungal diseases

Invasive fungal diseases are a major cause of death in renal allograft recipients. We previously reported that adjunctive recombinant human interferon-γ therapy has clinical utility for invasive fungal diseases after renal transplantation. We have now developed a rapid peripheral blood-based quantitative real-time PCR assay that enables accurate profiling of cytokine imbalances. Our preliminary studies in renal transplant patients with invasive fungal diseases suggest that they fail to mount an adequate interferon-γ response to the fungal infection. In addition, they have reduced IL-10 and increased TNF-α when compared to stable renal transplant patients. These preliminary cytokine profiling-based observations provide a possible explanation for the therapeutic benefit of adjunctive human interferon-γ therapy in renal allograft recipients with invasive fungal diseases.

Transplant accommodation--are the lessons learned from xenotransplantation pertinent for clinical allotransplantation?

"Accommodation" refers to a vascularized transplant that has acquired resistance to antibody-mediated rejection (AMR). The term was coined in 1990, but the phenomenon was first described after clinical ABO-incompatible (ABOi) renal transplantation in the 1980s and is recognized as a common outcome in this context today. Because of the absence, until recently of reliable animal models of allograft accommodation, it has been studied extensively by investigators in the xenotransplantation field. With recent advances in the ability to recognize and diagnose AMR in human organs, the growth of desensitization programmes for transplantation into sensitized recipients and the availability of therapies that have the potential to promote accommodation, it is timely to review the literature in this area, identifying lessons that may inform preclinical and clinical studies in the future.

Outcome of patients with preformed donor-specific antibodies following alemtuzumab induction and tacrolimus monotherapy

It has been shown that low-level preformed donor-specific antibodies (DSAbs) detected by luminex beads in the setting of a negative CDC and flow cytometry crossmatch (CDC/FCXM) are associated with inferior allograft outcomes. The relevance of preformed DSAbs in patients receiving alemtuzumab induction and tacrolimus monotherapy has not been studied. Four hundred and eighty renal transplant recipients with a negative CDC/FCXM had their pretransplant sera retrospectively screened for DSAbs. 45/480 (9.4%) of patients were found to have preformed DSAbs. Females and patients receiving regrafts were more likely to have a DSAb (p = 0.008 and p < 0.0001, respectively). Patients with DSAbs had inferior allograft survival (p = 0.047), increased incidence of antibody-mediated rejection (p < 0.0001) and inferior allograft function at 6 months posttransplant (p = 0.017). Patients with HLA class I DSAb (alone or in combination with a Class II DSAb) with high mean fluorescence intensities (MFIs) were at highest risk. We conclude that patients with preformed DSAb are at high risk of adverse outcomes when receiving a minimal immunosuppressive regime incorporating alemtuzumab induction. Patients found to have a preformed DSAb despite a negative crossmatch might benefit from augmented immunosuppression.

Atorvastatin or transgenic expression of TFPI inhibits coagulation initiated by anti-nonGal IgG binding to porcine aortic endothelial cells

BACKGROUND

Intravascular thrombosis remains a barrier to successful xenotransplantation. Tissue factor (TF) expression on porcine aortic endothelial cells (PAECs), which results from their activation by xenoreactive antibodies (Abs) to Galα1,3Gal (Gal) and subsequent complement activation, plays an important role.

OBJECTIVES

The present study aimed to clarify the role of Abs directed against nonGal antigens in the activation of PAECs to express functional TF and to investigate selected methods of inhibiting TF activity.

METHODS

PAECs from wild-type (WT), α1,3-galactosyltransferase gene-knockout (GT-KO) pigs, or pigs transgenic for CD46 or tissue factor pathway inhibitor (TFPI), were incubated with naïve baboon serum (BS) or sensitized BS (with high anti-nonGal Ab levels). TF activity of PAECs was assessed.

RESULTS

Only fresh, but not heat-inactivated (HI), naïve BS activated WT PAECs to express functional TF. Similarly, PAECs from CD46 pigs were resistant to activation by naïve BS, but not to activation by fresh or HI sensitized BS. HI sensitized BS also activated GT-KO PAECs to induce TF activity. TF expression on PAECs induced by anti-nonGal Abs was inhibited if serum was pretreated with (i) an anti-IgG Fab Ab or (ii) atorvastatin, or (iii) when PAECs were transgenic for TFPI.

CONCLUSIONS

Anti-nonGal IgG Abs activated PAECs to induce TF activity through a complement-independent pathway. This implies that GT-KO pigs expressing a complement-regulatory protein may be insufficient to prevent the activation of PAECs. Genetic modification with an 'anticoagulant' gene (e.g. TFPI) or a therapeutic approach (e.g. atorvastatin) will be required to prevent coagulation dysregulation after pig-to-primate organ transplantation.

Atorvastatin or transgenic expression of TFPI inhibits coagulation initiated by anti-nonGal IgG binding to porcine aortic endothelial cells

BACKGROUND

Intravascular thrombosis remains a barrier to successful xenotransplantation. Tissue factor (TF) expression on porcine aortic endothelial cells (PAECs), which results from their activation by xenoreactive antibodies (Abs) to Galα1,3Gal (Gal) and subsequent complement activation, plays an important role.

OBJECTIVES

The present study aimed to clarify the role of Abs directed against nonGal antigens in the activation of PAECs to express functional TF and to investigate selected methods of inhibiting TF activity.

METHODS

PAECs from wild-type (WT), α1,3-galactosyltransferase gene-knockout (GT-KO) pigs, or pigs transgenic for CD46 or tissue factor pathway inhibitor (TFPI), were incubated with naïve baboon serum (BS) or sensitized BS (with high anti-nonGal Ab levels). TF activity of PAECs was assessed.

RESULTS

Only fresh, but not heat-inactivated (HI), naïve BS activated WT PAECs to express functional TF. Similarly, PAECs from CD46 pigs were resistant to activation by naïve BS, but not to activation by fresh or HI sensitized BS. HI sensitized BS also activated GT-KO PAECs to induce TF activity. TF expression on PAECs induced by anti-nonGal Abs was inhibited if serum was pretreated with (i) an anti-IgG Fab Ab or (ii) atorvastatin, or (iii) when PAECs were transgenic for TFPI.

CONCLUSIONS

Anti-nonGal IgG Abs activated PAECs to induce TF activity through a complement-independent pathway. This implies that GT-KO pigs expressing a complement-regulatory protein may be insufficient to prevent the activation of PAECs. Genetic modification with an 'anticoagulant' gene (e.g. TFPI) or a therapeutic approach (e.g. atorvastatin) will be required to prevent coagulation dysregulation after pig-to-primate organ transplantation.

Exogenous interferon-gamma immunotherapy for invasive fungal infections in kidney transplant patients

The incidence of invasive fungal infections (IFIs) in nonneutropenic solid organ transplant patients is increasing. We report our clinical experience with the use of interferon-gamma (IFN-gamma) immunotherapy in seven renal transplant patients who developed life threatening, disseminated IFIs refractory to conventional antifungal drug therapy. The infections were all microbiologically and histologically proven. The rapid cure of these disseminated infections with exogenous IFN-gamma injections was not associated with impaired kidney allograft function despite the use of liposomal amphotericin B in all cases. No clinical toxicity from the IFN-gamma immunotherapy was seen and no IFI relapsed during long-term follow-up. Our experience is both uncontrolled and in patients with unpredictable fungal infection-related outcomes. However, compared to standard approaches, the accelerated cure of life threatening, disseminated IFIs with 6 weeks of combination antifungal drug therapy and IFN-gamma immunotherapy saved lives, retained allograft function and led to substantial cost savings in this small patient group.

Critical roles for thrombin in acute and chronic inflammation

Thrombin can amplify inflammation induced by other stimuli, either through ischemia (consequent upon thrombosis), indirectly through generation of downstream mediators such as activated protein C, or directly via signals through protease activated receptors (PAR). This paper will summarize recent data from our laboratory indicating that thrombin is required to initiate CCR2-dependent leukocyte recruitment and that it is the principal determinant of the outcome after vascular injury, via PAR-1 activation of a distinct subset of smooth muscle cell progenitors. In both, tissue factor (TF) initiates thrombin generation and the thrombin acts locally, exemplifying that the initiation phase can generate autocrine or paracrine signalling molecules. Thrombin is an important constituent of innate immunity, able to amplify and modify responses to invading pathogens or tissue damage. With novel anti-thrombin therapeutics and agents to target PAR, a new understanding of the importance of thrombin may allow the development of innovative anti-inflammatory strategies.

Progenitor cells and vascular disease

Vascular progenitor cells have been the focus of much attention in recent years; both from the point of view of their pathophysiological roles and their potential as therapeutic agents. However, there is as yet no definitive description of either endothelial or vascular smooth muscle progenitor cells. Cells with the ability to differentiate into mature endothelial and vascular smooth muscle reportedly reside within a number of different tissues, including bone marrow, spleen, cardiac muscle, skeletal muscle and adipose tissue. Within these niches, vascular progenitor cells remain quiescent, until mobilized in response to injury or disease. Once mobilized, these progenitor cells enter the circulation and migrate to sites of damage, where they contribute to the remodelling process. It is generally perceived that endothelial progenitors are reparative, acting to restore vascular homeostasis, while smooth muscle progenitors contribute to pathological changes. Indeed, the number of circulating endothelial progenitor cells inversely correlates with exposure to cardiovascular risk factors and numbers of animal models and human studies have demonstrated therapeutic roles for endothelial progenitor cells, which can be enhanced by manipulating them to overexpress vasculo-protective genes. It remains to be determined whether smooth muscle progenitor cells, which are less well studied than their endothelial counterparts, can likewise be manipulated to achieve therapeutic benefit. This review outlines our current understanding of endothelial and smooth muscle progenitor cell biology, their roles in vascular disease and their potential as therapeutic agents.

The interface between coagulation and immunity

Coagulation proteases are involved in generating fibrin after vascular injury (hemostasis) but they also have multiple other effects, many of which are mediated independently of fibrin generation, via interactions with specific cell membrane-expressed "protease activated receptors". In inflammation, this family of proteins has a complex influence, the facets of which are still incompletely understood, though a common feature in different models appears to be amplification of innate signals that are initially generated by pathogenic elements or, in the context of transplantation, ischemia or anti-graft antibodies, for instance. There is increasing evidence that these proteases may also have specific effects on cells involved in adaptive immunity and on cells that mediate chronic inflammation and fibrosis. Understanding whether these effects are relevant in the responses generated against transplanted organs is important, as it could lead ultimately to the development of novel ways to promote long-term graft survival.

Extraanatomic stents for transplant ureteric stenosis

Surgical and standard endourology options are limited in transplant patients with severe ureteric stenosis, particularly when access to the transplant renal pelvis is limited. The use of a silicone-polytetrafluoroethelene (PTFE)-bonded extraanatomic urinary tract stent for urinary tract drainage is described in two patients. This technique of ureteric reconstruction in renal transplantation may be considered when standard approaches have failed. It appears to be safe when performed by radiologists and urologists with expertise in percutaneous renal access.

Postinjury vascular intimal hyperplasia in mice is completely inhibited by CD34+ bone marrow-derived progenitor cells expressing membrane-tethered anticoagulant fusion proteins

BACKGROUND

Coagulation proteins promote neointimal hyperplasia and vascular remodelling after vessel injury, but the precise mechanisms by which they act in vivo remain undetermined.

OBJECTIVES

This study, using an injury model in which the neointima is derived from bone marrow (BM)-derived cells, compared inhibition of tissue factor or thrombin on either BM-derived or existing vascular smooth muscle cells.

METHODS

Two transgenic (Tg) mouse strains expressing membrane-tethered tissue factor pathway inhibitor (TFPI) or hirudin (Hir) fusion proteins driven by an alpha smooth muscle actin (SMA) promoter were generated (alpha-TFPI-Tg and alpha-Hir-Tg) and the phenotype after wire-induced endovascular injury was compared with that in wild-type (WT) controls.

RESULTS

WT mice developed progressive neointimal expansion, whereas injury in either Tg was followed by repair back to a preinjured state. This was also seen when WT mice were reconstituted with BM from Tg mice but not when Tgs were reconstituted with WT BM, in which injury was followed by slowly progressive neointimal expansion. Injection of CD34+ cells from Tg mice into injured WT mice resulted in the accumulation of fusion protein-expressing cells from day 3 onwards and an absence of neointimal hyperplasia in those areas.

CONCLUSIONS

Neointimal development after wire-induced endovascular injury in mice was completely inhibited when BM-derived cells infiltrating the damaged artery expressed membrane tethered anticoagulant fusion proteins under an alpha-SMA promoter. These findings enhance our understanding of the pathological role that coagulation proteins play in vascular inflammation.

Cure of disseminated cryptococcal infection in a renal allograft recipient after addition of gamma-interferon to anti-fungal therapy

The immunosuppressive regimens that are used in solid organ transplantation are potent inhibitors of Th0 as well as Th1 and Th2 cell-mediated immune responses. This predisposes patients to disseminated cryptococcal infections. Mortality in such patients remains very high despite advances in anti-fungal chemotherapy. We describe a case of disseminated cryptococcal disease in a renal allograft recipient that failed to respond to prolonged treatment with several anti-fungal drugs. However, addition of the immuno-modulator, interferon-gamma, resulted in the formation of granulomas and the resolution of his disease within 4-6 weeks. As we cannot find a similar example of combination therapy for disseminated cryptococcal disease in the solid organ transplant literature, we propose that interferon-gamma could be used in synergy with anti-fungal drugs to cure disseminated cryptococcal infections in solid organ transplant patients.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

"Accomodated" pig endothelial cells promote nitric oxide-dependent Th-2 cytokine responses from human T cells

BACKGROUND

Cardiac and renal allo- and xenografts can become naturally resistant to vascular rejection. Understanding this process of "accommodation" would enhance our understanding of vascular inflammatory responses and have implications for immune manipulation and tolerance induction. A feature of these grafts is infiltration by leukocytes secreting a Th-2 pattern of cytokines.

METHODS

HLA-DR-1-transfected, immortalized porcine endothelial cells (IPEC) were incubated with polyclonal human immunoglobulin G (IgG) for 6 days before incubation with purified human CD4+ T cells.

RESULTS

IgG-incubated IPEC stimulated a normal proliferative response from alloreactive T cells. However, interferon (IFN)-gamma levels were significantly reduced, whereas interleukin (IL)-5 and IL-10 were maintained at levels equivalent to those stimulated by control IPEC. Cognate interaction between T cells and IPEC was not required for this effect, because IgG-incubated, MHC-class II-negative IPEC caused reduced IFN-gamma secretion during a response to human Epstein-Barr virus-transformed B cells. Experiments with the nitric oxide (NO) donor, (z)-1-2-[2-Aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO), and the NO synthase inhibitor, NG-monomethyl-L-arginine.monoacetate (L-NMMA), showed that NO released by the IgG-incubated IPEC was actively involved in the development of this phenotype.

CONCLUSIONS

These data suggest a novel, IgG-mediated, NO-dependent mechanism by which endothelial cells (EC) influence T cell responsiveness and that the Th-2 cytokine skewing seen in "accommodated" grafts may be a secondary phenomenon, resulting from the T-EC interactions.

Human thrombin and FXa mediate porcine endothelial cell activation; modulation by expression of TFPI-CD4 and hirudin-CD4 fusion proteins

Aside from their critical role in thrombosis, activated coagulation factors also have inflammatory properties and these may be important during delayed xenograft rejection (DXR). This study assessed whether porcine EC could be activated by factor Xa (FXa) and thrombin (FIIa) and whether expression of tissue factor pathway inhibitor (TFPI)-CD4 and hirudin-CD4 fusion proteins could prevent such activation. Incubation of porcine EC with human FXa and FIIa induced cell surface expression of E-selectin, VCAM and tissue factor (TF) in a time-dependent and concentration-dependent manner. In contrast, porcine EC transfected with a human TFPI-CD4 fusion protein were selectively resistant to these pro-inflammatory effects of FXa but not FIIa. Likewise, the transfectants expressing the hirudin-CD4 fusion protein were selectively resistant to the pro-inflammatory effects of FIIa but not those of FXa. When combined, the FXa and FIIa had an additive effect on the activation of control EC. In contrast, coexpression of both hirudin-CD4 and TFPI-CD4 fusion proteins completely inhibited the upregulation of VCAM with the FXa/FIIa mix. These results indicate that expression of novel anticoagulant fusion proteins on the surface of porcine EC can protect against EC activation induced by human coagulation factors FXa and FIIa. In vivo, we anticipate that expression of these fusion proteins on the endothelium of transplanted xenografts, besides preventing intravascular thrombosis, will also protect against EC activation induced by trace amounts of FIIa and FXa, thereby further protecting the grafts from DXR.

Transplant accommodation in highly sensitized patients: a potential role for Bcl-xL and alloantibody

Transplantation of renal allografts into recipients with circulating anti-HLA antibodies results in hyperacute rejection. In some cases, however, antibodies return without causing harm; this phenomenon has been termed 'accommodation'. We have investigated this process in human allotransplantation. We removed anti-HLA antibodies by immunoadsorption in seven highly sensitized dialysis patients who subsequently underwent renal transplantation. Immunohistochemistry of renal biopsies for IgG and antiapoptotic proteins was performed. We also developed a model of 'accommodation' using anti-HLA antibodies eluted from sensitized patients and incubated with human umbilical vein endothelial cells (HUVECs) at different concentrations. Their effect on HUVEC phenotype was then analysed. Anti-donor antibody returned in 4/7 patients, without evidence of hyperacute rejection. Three out of four of these 'accommodated' grafts showed specific endothelial up-regulation of Bcl-xL and 2/2 tested positive for endothelial IgG deposition. HUVECs incubated with subsaturating concentrations of anti-HLA antibody showed increased expression of Bcl-xL, were rendered refractory to endothelial cell activation and became resistant to complement-mediated lysis. In contrast, HUVECs incubated with saturating concentrations underwent activation and expressed low levels of Bcl-xL. In conclusion, endothelial Bcl-xL expression defines the accommodation process in human allografts and this phenotype may be initiated by exposure of endothelium to low concentrations of anti-donor HLA antibodies.

Nitric oxide-mediated expression of Bcl-2 and Bcl-xl and protection from tumor necrosis factor-alpha-mediated apoptosis in porcine endothelial cells after exposure to low concentrations of xenoreactive natural antibody

BACKGROUND

Cardiac and renal allo- and xenografts can acquire a natural resistance to vascular rejection. This "accommodation" involves endothelial cell (EC) expression of "survival genes" such as Bcl family members and hemoxygenase 1. Understanding what initiates this protective process would have profound implications; our hypothesis is that low concentrations of antigraft antibodies may mediate these changes.

METHODS

In vitro cultured primary and immortalized porcine EC were incubated with polyclonal human IgG for 6 days and then examined for phenotype changes.

RESULTS

The cells acquired resistance to tumor necrosis factor-alpha-mediated apoptosis (50-100% reduction at 6 hr) and up-regulated expression of Bcl-2 and Bcl-xl; sustained expression was accompanied by inducible nitric oxide (NO) synthase expression and by enhanced production of NO by EC. Two observations suggested that NO was actively involved in the process of Bcl-2 and Bcl-xl induction. First, (z)-1-2-[2-aminoethyl)-N- (2-ammonioethyl)amino]diazen-1-ium-1,2-diolate, an NO donor, was able to induce similar changes in porcine EC to those induced by anti-pig antibodies. Second, an NO synthase inhibitor NG-monomethyl-L-arginine.monoacetate was able to specifically inhibit the anti-pig antibody-mediated expression of Bcl-2 or Bcl-xl.

CONCLUSIONS

These data strongly support the hypothesis that Bcl-2 and Bcl-xl expression and protection from apoptosis in EC may result from antibody-mediated NO production through the neoexpression of inducible NO synthase.

Porcine CTLA4-Ig lacks a MYPPPY motif, binds inefficiently to human B7 and specifically suppresses human CD4+ T cell responses costimulated by pig but not human B7

The CTLA4 receptor (CD152) on activated T lymphocytes binds B7 molecules (CD80 and CD86) on APC and delivers a signal that inhibits T cell proliferation. Several regions involved in binding to B7 are known, but the relative importance of these is not clear. We have cloned porcine CTLA4 (pCTLA4). Although highly homologous to human CTLA4 (hCTLA4), the predicted protein sequence contains a leucine for methionine substitution at position 97 in the MYPPPY sequence. A fusion protein constructed from the extracellular regions of pCTLA4 and the constant regions of human IgG1 (pCTLA4-Ig) bound porcine CD86 with equivalent affinity to that of hCTLA4-Ig. However, pCTLA4-Ig bound poorly to human CD80 and CD86 expressed on transfectants and EBV-transformed human B cells. In functional assays with MHC class II-expressing porcine endothelial cells and human B cells, pCTLA4-Ig blocked human CD4+ T cell responses to pig but not human cells, whereas control hCTLA4-Ig inhibited responses to both. Comparison between mouse, human, and porcine CTLA4-Ig suggests that the selective binding of pCTLA4-Ig to porcine CD86 molecules is due to the L for M substitution at position 97. Our results indicate that pCTLA4-Ig may be a useful reagent to define the precise nature of the interaction between B7 and CTLA4. By failing to inhibit the delivery of costimulatory signals provided by human B7, it may also prove to be a relatively specific inhibitor of the direct human T cell response to immunogenic pig tissue.

Cloning of porcine intercellular adhesion molecule-1 and characterization of its induction on endothelial cells by cytokines

BACKGROUND

The transplantation of pig organs into humans requires a detailed knowledge of similarities and differences between the two species in the molecular physiology of host defense mechanisms. We therefore set out to identify porcine intercellular adhesion molecule (ICAM)-1 and to characterize its expression by endothelial cells.

METHODS

Porcine ICAM-1 cDNA was isolated from an endothelial cell cDNA library. An anti-pig ICAM-1 monoclonal antibody was generated and used to investigate the regulation by cytokines of ICAM-1 expression by porcine aortic endothelial cells (PAEC), using flow cytometry.

RESULTS

We found that porcine ICAM-1 was similar in primary structure to human ICAM-1, with five Ig-like domains. COS-7 cells transfected with porcine ICAM-1 supported beta2 but not alpha4 integrin-dependent adhesion of human T lymphoblasts. There was a low-level surface expression of ICAM-1 on unstimulated PAEC and increased expression after stimulation with tumor necrosis factor (TNF)-alpha. However expression of ICAM-1 seemed to be significantly lower than that of vascular cell adhesion molecule-1, both on unstimulated and TNF-alpha-activated PAEC. Recombinant porcine interferon-gamma weakly stimulated ICAM-1 expression when incubated alone with PAEC but had an inhibitory effect on the increase in ICAM-1 due to TNF-alpha, both at 8 and 24 hr.

CONCLUSIONS

Our observations confirm the existence of ICAM-1 in the pig and provide novel insights into how porcine and human endothelial cells differ in terms of adhesion molecule expression and cytokine responsiveness. Such differences are potentially important in interpreting models of inflammation in the pig and also in understanding the process of rejection of porcine xenografts.

Costimulatory blockade by the induction of an endogenous xenospecific antibody response

Xenogeneic tissues induce vigorous T cell immunity, reflecting the ability of costimulatory molecules to function across species barriers. We describe a strategy to inhibit costimulation that exploits species differences using the model of porcine pancreatic islet transplantation into mice. Mice were immunized with chimeric peptides that contained a known T cell epitope and selected sequences of the porcine costimulatory molecule CD86. This resulted in anti-peptide antibody responses that recognized intact porcine CD86, blocked costimulation by porcine CD86 but not murine CD86 in vitro, and prolonged the survival of porcine islet grafts in vivo. This strategy of inducing endogenous donor-specific costimulatory blockade has potential clinical applicability.

HLA-G inhibits the transendothelial migration of human NK cells

The expression of the non-classical MHC class I molecule HLA-G is normally restricted to the placenta during pregnancy, where it is found on fetal endothelial cells and on invasive cytotrophoblast cells, specifically those at the maternal / fetal interface. Its precise physiological role has yet to be defined. HLA-G may have nonimmune functions relating to angiogenesis and placentation, but most evidence suggests that it protects fetal cells from lysis by maternal uterine NK cells, which are found in large numbers around invading trophoblast cells. This effect is due to specific interaction with inhibitory receptors expressed on NK cells. We have examined the hypothesis that another function of HLA-G is to inhibit NK cell migration. Using an in vitro transmigration assay system, we present data to support this hypothesis. NK cell migration across porcine endothelial cells transfected with HLA-G1 was specifically inhibited compared to migration across HLA-A2-transfected monolayers. HLA- G1 had no influence on the migration of a control T lymphocyte line. These results support the idea that in vivo, HLA-G may inhibit NK cell traffic across the placenta.

Regulated inhibition of coagulation by porcine endothelial cells expressing P-selectin-tagged hirudin and tissue factor pathway inhibitor fusion proteins

BACKGROUND

Thrombotic vascular occlusion resulting in infarction occurs during hyperacute rejection of allografts transplanted into sensitized patients and remains a major problem in experimental xenotransplantation. A similar process is also found in disorders of diverse etiology including atherosclerosis, vasculitis, and disseminated intravascular coagulation.

METHODS

We have previously constructed two membrane-tethered anticoagulant fusion proteins based on human tissue factor pathway inhibitor and the leech anticoagulant hirudin and demonstrated their functional efficacy in vitro. These constructs have now been modified by the addition of a P-selectin sequence to the cytoplasmic tail to localize them in Weibel-Palade bodies. They have been transfected into Weibel-Palade body-positive endothelial cells isolated from the inferior vena cava of normal pigs.

RESULTS

In resting endothelial cells, fusion protein expression colocalized with P-selectin and was confined to Weibel-Palade bodies. These cells had a procoagulant phenotype in recalcified human plasma. However, after activation with phorbol ester the anticoagulant proteins were rapidly relocated to the cell surface where they specifically inhibited the clotting of human plasma.

CONCLUSIONS

Novel anticoagulant molecules may prove useful therapeutic agents for gene therapy in thrombotic disease and postangioplasty or for transgenic expression in animals whose organs may be used for clinical xenotransplantation. Expression in vascular endothelial cells may be regulated by inclusion of P-selectin cytoplasmic sequence, to restrict cell surface expression to activated endothelium.

Inhibition of tissue factor-dependent and -independent coagulation by cell surface expression of novel anticoagulant fusion proteins

BACKGROUND

Thrombotic vascular occlusion occurs in disorders of diverse etiology, including atherosclerosis, vasculitis, and disseminated intravascular coagulation. The same process results in hyperacute rejection of renal allografts transplanted into sensitized patients and remains a major problem in experimental xenotransplantation.

METHODS

We have previously described the design and expression of several genetic constructs encoding novel fusion proteins with anticoagulant properties. They are based on two naturally occurring soluble anticoagulant proteins, human tissue factor pathway inhibitor (hTFPI) and the leech protein hirudin, which act early and late in the clotting cascade, respectively. We report the expression of human hTFPI-CD4 on the surface of immortalized porcine endothelial cells (IPEC), and show that it functions across the species divide as evidenced by the binding of membrane-expressed porcine tissue factor (pTF)-human factor VIIa complexes.

RESULTS

Using a human plasma recalcification clotting assay, we distinguished between pTF-dependent and pTF-independent fibrin generation, and we have demonstrated that expression of hTFPI-CD4 on IPEC effectively prevented pTF-dependent clotting. Moreover, we show that when hTFPI-CD4 was co-expressed with the hirudin construct, the procoagulant properties of in vitro cultured, activated IPEC were almost completely abolished.

CONCLUSIONS

These results suggest that these novel anticoagulant molecules may prove useful therapeutic agents for gene therapy or for transgenic expression in animals whose organs may be used for cliniCal xenotransplantation.

Phenotypic characterization of histiocytes infiltrating a leiomyofibrosarcoma

We described previously a unique cutaneous tumour in a young pig, which was characterized by several criteria as a histiocytic leiomyofibrosarcoma. The lipid-laden macrophages (histiocytes) which permeated the tumour were CD2+/CD18+/CD49d+ but MAC387 (L1 antigen) and CD15 negative. The present study compared the phenotypes of histiocytes in tumour metastases in the liver with resident liver macrophages, revealing differential expression of certain macrophage activation markers. After repeated subcutaneous passage of the tumour in athymic (nu/nu) mice, flow cytometry demonstrated a rapid loss of porcine MHC Class II, but a more prolonged expression of porcine MHC Class I, consistent with our immunohistological observations. Mouse macrophages (CD2+/F4.80+) infiltrated the later-passage tumours, suggesting that the histiocytes were not of neoplastic origin.

Expression of hirudin fusion proteins in mammalian cells: a strategy for prevention of intravascular thrombosis

BACKGROUND

Intravascular thrombosis occurs in disorders of diverse pathogeneses, including allograft and xenograft rejection. In this in vitro study, we describe an approach for tethering the specific thrombin inhibitor hirudin to plasma membranes as part of a genetic strategy for regulating intravascular coagulation.

METHODS AND RESULTS

An HLA class I leader sequence was fused with hirudin linked to domains 3 and 4 of human CD4 and intracytoplasmic sequence from either CD4 or human P-selectin. The constructs were transfected into mouse fibroblasts, Chinese hamster ovary (CHO)-K1 cells, immortalized porcine endothelial cells (IPECs), and a pituitary secretory cell line (D16/16). Thrombin binding to the hirudin fusion proteins expressed on fibroblasts and CHO-K1 cells could be blocked by an anti-hirudin monoclonal antibody and by pretreatment of thrombin with either the synthetic tripeptide thrombin inhibitor PPACK or native hirudin. Hirudin expression significantly modified the procoagulant phenotype of IPECs in human plasma, leading to prolongation of clotting times. Hirudin-CD4-P-selectin fusion proteins accumulated in adrenocorticotropic hormone-containing granules in D16/16 cells, with no cell surface expression except on activation with phorbol ester, when hirudin relocated to the outer membrane.

CONCLUSIONS

Hirudin fusion proteins were expressed on mammalian cells, where they reduced local thrombin levels and inhibited fibrin formation. Regulated expression was achieved on activated cells by use of the cytoplasmic sequence from P-selectin. In vivo, these fusion proteins may prove useful transgenic or gene therapy agents for preventing intravascular thrombosis.

T cell-mediated xenograft rejection: specific tolerance is probably required for long term xenograft survival

T cell-mediated mechanisms of xenograft rejection appear resistant to standard immunosuppression protocols used to prevent allograft rejection and, consequently, higher doses of immunosuppressive drugs are required to promote xenograft compared to allograft survival. Evidence from recent studies suggests that porcine xenografts may be especially immunogenic in humans because of a prominent and vigorous indirect xenoresponse and because of the ability of porcine endothelium to activate human T cells. This has led to an anxiety that systemic immunosuppressives, used as the mainstay of therapy for clinical xenotransplantation, may not allow the long-term survival of porcine organs transplanted into human recipients. This article will review the biology of T cell xenoresponses, present the case for the development of novel graft-specific immunosuppressive regimes in clinical xenotransplantation, and review recent experimental progress in this area.

Xenotransplantation: steps towards a clinical reality

It was clear that significant research progress has been made in the two years since the last congress, although, as the chairman J.P. Soulillou (Nantes) concluded, there were no singular advances announced during the week; rather, it was a congress for assimilation of information. It was also apparent that there is now an attitude of realism in the xenotransplantation community. The prevailing mood is one of cautious optimism, to contrast with the unguarded optimism of the previous meeting. The future for clinical xenotransplantation now appears more secure than it has been previously.

In vitro accommodation of porcine endothelial cells by low dose human anti-pig antibody: reduced binding of human lymphocytes by accommodated cells associated with increased nitric oxide production

Transplanted xenografts, protected from rejection by depletion of xenoreactive natural antibodies (XNA) and complement, can sometimes survive when complement levels and titres of anti-graft antibodies return to baseline; this phenomenon is called accommodation. We have previously reported that low concentrations of human IgG induce a change in the phenotype of immortalised porcine endothelial cells (IPEC) consistent with the development of accommodation. The cells acquired a resistance to lysis by human complement and showed a reduced expression of VCAM. In this study, we extend these findings by showing that VCAM expression falls on several IPEC clones and on primary porcine endothelial cells. Moreover, we show that these accommodated cells bind fewer human lymphocytes compared to controls, implying that leukocyte traffic through accommodated endothelium may be altered compared to that through normal endothelium. Finally we show that during the induction of accommodation, porcine endothelial cells produce greater amounts of nitric oxide than controls, due to the expression of inducible nitric oxide synthase (iNOS). We speculate that nitric oxide may be an important mediator in accommodation.

Glaxo/MRS Young Investigator Prize. Xenotransplantation: immune barriers beyond hyperacute rejection

1. The use of organs from animal donors (xenotransplantation) is a potential solution to the chronic shortage of allogeneic organs and currently the pig is thought to be the most suitable donor for man. However, porcine organs are rejected rapidly by a vascular process called hyperacute rejection which has so far prevented clinical xenotransplantation. Although it is likely that this barrier will be overcome in the near future by the application of novel strategies, probably involving the use of organs from transgenic pigs, data from animal models indicate that multiple other immune mechanisms will contribute to the rejection of xenografts. 2. We have described two aspects of these immune mechanisms. First, the phenomenon of 'accommodation', whereby xenografts acquire in vivo resistance to vascular rejection, has been explored in an in vitro model utilizing immortalized porcine endothelial cells. The results indicate that human anti-pig antibodies induce a concentration-dependent and time-dependent change in porcine endothelial cells compatible with the development of accommodation. 3. Secondly, the in vitro human anti-porcine T-cell response has been documented in detail, with particular emphasis on quantitative and qualitative comparisons with the in vitro T-cell alloresponse. The results of this work, which indicate that the response to porcine xenografts is likely to be significantly stronger than that against allografts, have important implications for the level of conventional immunosuppression that may be necessary to prevent xenograft rejection, and provide an important basis for the development of strategies to promote xenograft-specific immunosuppression and tolerance.

Human tissue factor pathway inhibitor fused to CD4 binds both FXa and TF/FVIIa at the cell surface

Tissue factor pathway inhibitor (TFPI) is one of the main regulators of the tissue factor (TF) pathway of coagulation. To tether human TFPI to the cell surface, full length or truncated TFPI lacking the third Kunitz domain were fused with domains three and four and the carboxy-terminal sequence of human CD4. Constructs were transfected into a mouse fibroblast cell line and individual clones were checked for expression using monoclonal antibodies directed against the first two TFPI Kunitz domains and against CD4. Specific human FXa binding was detected by flow cytometry using an anti-FX polyclonal antibody, and inhibition of FXa proteolytic activity was verified by chromogenic substrate assay using S-2765. In addition, TFPI-CD4-expressing cells, preincubated with FXa, specifically bound human TF-FVIIa complexes as revealed with an anti-human TF polyclonal antibody. No functional difference was observed between full length or truncated TFPI-CD4. These results demonstrate that functionally intact TFPI can be tethered to the cell surface. Genetic manipulation of, for example, endothelial cells leading to the stable expression of TFPI may inhibit the development of coronary artery heart disease following cardiac allotransplantation, and may inhibit thrombosis in the context of xenotransplantation.

Clinical xenotransplantation of solid organs

A possible solution to the chronic shortage of allografts is xenotransplantation, the use of tissue from an animal donor. Most experts believe that the pig will provide the most suitable solid organs for use in human beings. Although porcine organs are rapidly rejected by a process called hyperacute rejection (HAR), there is hope that several novel therapeutic strategies, already tested in animal models, will overcome this hurdle in patients. Successful clinical trials of these strategies, expected within the next few years, may herald the era of clinical xenotransplantation. However, there is increasing evidence that other barriers, both immune and non-immune, might exist to limit the survival of xenografts beyond the HAR phase. New strategies to overcome these barriers will be needed if long-term xenograft survival equivalent to, or better than, that of allografts is ever to be achieved.

In vitro accommodation of immortalized porcine endothelial cells: resistance to complement mediated lysis and down-regulation of VCAM expression induced by low concentrations of polyclonal human IgG antipig antibodies

The capacity of vascularized xenografts to survive in the face of normal levels of circulating antigraft antibodies and complement has been ascribed to a phenomenon referred to as "endothelial cell accommodation." The mechanisms whereby accommodation might occur have remained obscure. We have investigated this phenomenon in an in vitro system. A preparation of polyclonal immunoglobulin, human normal globulin (HNG), induced a change in the phenotype of immortalized porcine endothelial cells (IPEC) suggestive of accommodation; the cells became resistant to complement mediated lysis and displayed a reduced expression of surface VCAM and MHC class I. The accommodated phenotype only manifested after 72 hr incubation with HNG and was optimal after 120 hr. In an analysis of all the experiments performed, the development of resistance to complement mediated lysis appeared independent of the inducing dose of HNG. However, down-regulation of VCAM was only manifest when subsaturating doses were used. Our results suggest that IgG xenoreactive antibodies can mediate changes in porcine endothelial cell phenotype consistent with accommodation. The dependence on both time and dose of antibody applied might explain why accommodation has been difficult to achieve consistently in in vivo models of discordant xenotransplantation. By demonstrating a functional interaction between human VLA-4 and porcine VCAM, we speculate that the down-regulation in expression of VCAM on accommodated endothelium may have an important regulatory effect on traffic of inflammatory cells into xenografts. Our results have important implications for the development of strategies to promote accommodation of xenografts.

Detection of primary direct and indirect human anti-porcine T cell responses using a porcine dendritic cell population

The pathways of human anti-pig T cell xenorecognition have been investigated. Freshly isolated porcine alveolar lavage (AL) cells induced primary proliferative responses by human peripheral and cord blood mononuclear cells which were inhibited by anti-HLA-DR antibody (indirect xenorecognition). Following over-night culture, the AL cells acquired the capacity to stimulate proliferation by purified human T cells which was inhibited by anti-SLA-DR antibody (direct xenorecognition). The marked increase in immunogenicity in the porcine AL cells was accompanied by a phenotypic change consistent with dendritic cell maturation. Limiting dilution assays indicate that the total anti-pig T cell response, in particular that mediated by indirect xenorecognition, is stronger than comparable alloresponses.

Major histocompatibility complex class II-expressing endothelial cells induce allospecific nonresponsiveness in naive T cells

The role of endothelial cells (EC) in initiating a primary T cell response is of importance in clinical transplantation and autoimmunity since EC are the first allogeneic target encountered by the recipient's immune system and may display tissue-specific autoantigens in the context of an inflammatory response. In this study, we have investigated the antigen-presenting cell function of human umbilical vein-derived EC (HUVEC), depleted of constitutively major histocompatibility complex class II+ cells and induced to express class II molecules by interferon-gamma. The results show that HUVEC do not express B7 but can support proliferation by antigen-specific T cell clones. In contrast, they were unable to initiate a primary alloresponse using three independent HUVEC cultures and MHC class II-mismatched CD4+ T cells from eight donors. The response to HUVEC was reconstituted by trans-costimulation provided by DAP.3 transfectants expressing human B7.1. Coculture of peripheral blood T cells with EC expressing allogeneic DR molecules had markedly different effects on CD45RO+ and RA+ subsets. Subsequent reactivity of the RO+ T cells was unaffected by exposure to EC, indicating a neutral encounter. In contrast, culture with DR+ EC induced allospecific nonresponsiveness in RA+ T cells.

Prospects for xenografting

New insights into the mechanisms of hyperacute rejection, endothelial cell activation and accommodation are clarifying the processes that determine the rejection of discordant xenografts. Recent advances include the identification of endothelial cell antigens targeted by human anti-pig antibodies and an increasing understanding of the mechanisms underlying endothelial cell activation. Novel ways to prevent hyperacute rejection, including new therapeutic agents and the use of organs from transgenic animals, are promising to significantly improve the early survival of pig organs transplanted into man. Clinical xenotransplantation has become a realizable goal.