Neonatal porcine islet cells induce human CD4+, but not CD8+, lymphocyte proliferation and resist cell-mediated cytolytic injury in vitro

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

Differentiation of human pluripotent stem cells into β-cells: Potential and challenges

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) hold great potential as the basis for cell-based therapies of degenerative diseases, including diabetes. Current insulin-based therapies for diabetes do not prevent hyperglycaemia or the associated long-term organ damage. While transplantation of pancreatic islets can achieve insulin independence and improved glycemic control, it is limited by donor tissue scarcity, challenges of purifying islets from the pancreas, and the need for immunosuppression to prevent rejection of transplants. Large-scale production of β-cells from stem cells is a promising alternative. Recent years have seen considerable progress in the optimization of in vitro differentiation protocols to direct hESCs/iPSCs into mature insulin-secreting β-cells and clinical trials are now under way to test the safety and efficiency of hESC-derived pancreatic progenitor cells in patients with type 1 diabetes. Here, we discuss key milestones leading up to these trials in addition to recent developments and challenges for hESC/iPSC-based diabetes therapies and disease modeling.

Normalization of glucose post-transplantation into diabetic rats of pig pancreatic primordia preserved in vitro

Embryonic day (E) 28 (E28) pig pancreatic primordia transplanted into the mesentery of non-immunosuppresed steptozotocin (STZ)-diabetic Lewis rats normalize levels of circulating glucose within 2-4 weeks. Exocrine tissue does not differentiate after transplantation of pancreatic primordia. Rather individual endocrine (beta) cells engraft within the mesentery.To determine whether transplanted pig pancreatic primordia engraft, differentiate and function in rat hosts after preservation in vitro, we implanted pig pancreatic primordia into STZ-diabetic rats either directly or after 24 hours of suspension in ice-cold University of Wisconsin (UW) preservation solution with added growth factors. Here we show engraftment in mesentery and mesenteric lymph nodes and normalization of glucose levels in STZ-diabetic rat hosts following transplantation of preserved E28 pig pancreatic primordia comparable to glucose normalization after transplantation of non-preserved E28 pancreatic primordia.

Organogenesis of kidney and endocrine pancreas: the window opens

Growing new organs in situ by implanting developing animal organ primordia (organogenesis) represents a novel solution to the problem of limited supply for human donor organs that offers advantages relative to transplanting embryonic stem (ES) cells or xenotransplantation of developed organs. Successful transplantation of organ primordia depends on obtaining them at defined windows during embryonic development within which the risk of teratogenicity is eliminated, growth potential is maximized, and immunogenicity is reduced. We and others have shown that renal primordia transplanted into the mesentery undergo differentiation and growth, become vascularized by blood vessels of host origin, exhibit excretory function and support life in otherwise anephric hosts. Renal primordia can be transplanted across isogeneic, allogeneic or xenogeneic barriers. Pancreatic primordia can be transplanted across the same barriers undergo growth, and differentiation of endocrine components only and secrete insulin in a physiological manner following mesenteric placement. Insulin-secreting cells originating from embryonic day (E) 28 (E28) pig pancreatic primordia transplanted into the mesentery of streptozotocin-diabetic (type 1) Lewis rats or ZDF diabetic (type 2) rats or STZ-diabetic rhesus macaques engraft without the need for host immune-suppression. Our findings in diabetic macaques represent the first steps in the opening of a window for a novel treatment of diabetes in humans.

Islet xenotransplantation using gal-deficient neonatal donors improves engraftment and function

Significant deficiencies in understanding of xenospecific immunity have impeded the success of preclinical trials in xenoislet transplantation. Although galactose-α1,3-galactose, the gal epitope, has emerged as the principal target of rejection in pig-to-primate models of solid organ transplant, the importance of gal-specific immunity in islet xenotransplant models has yet to be clearly demonstrated. Here, we directly compare the immunogenicity, survival and function of neonatal porcine islets (NPIs) from gal-expressing wild-type (WT) or gal-deficient galactosyl transferase knockout (GTKO) donors. Paired diabetic rhesus macaques were transplanted with either WT (n = 5) or GTKO (n = 5) NPIs. Recipient blood glucose, transaminase and serum xenoantibody levels were used to monitor response to transplant. Four of five GTKO versus one of five WT recipients achieved insulin-independent normoglycemia; transplantation of WT islets resulted in significantly greater transaminitis. The WT NPIs were more susceptible to antibody and complement binding and destruction in vitro. Our results confirm that gal is an important variable in xenoislet transplantation. The GTKO NPI recipients have improved rates of normoglycemia, likely due to decreased susceptibility of xenografts to innate immunity mediated by complement and preformed xenoantibody. Therefore, the use of GTKO donors is an important step toward improved consistency and interpretability of results in future xenoislet studies.

Acceleration of functional maturation and differentiation of neonatal porcine islet cell monolayers shortly in vitro cocultured with microencapsulated sertoli cells

The limited availability of cadaveric human donor pancreata as well as the incomplete success of the Edmonton protocol for human islet allografts fasten search for new sources of insulin the producing cells for substitution cell therapy of insulin-dependent diabetes mellitus (T1DM). Starting from isolated neonatal porcine pancreatic islets (NPIs), we have obtained cell monolayers that were exposed to microencapsulated monolayered Sertoli cells (ESCs) for different time periods (7, 14, 21 days). To assess the development of the cocultured cell monolayers, we have studied either endocrine cell phenotype differentiation markers or c-kit, a hematopoietic stem cell marker, has recently been involved with growth and differentiation of β-cell subpopulations in human as well as rodent animal models. ESC which were found to either accelerate maturation and differentiation of the NPIs β-cell phenotype or identify an islet cell subpopulation that was marked positively for c-kit. The insulin/c-kit positive cells might represent a new, still unknown functionally immature β-cell like element in the porcine pancreas. Acceleration of maturation and differentiation of our NPI cell monolayers might generate a potential new opportunity to develop insulin-producing cells that may suite experimental trials for cell therapy of T1DM.

Xenotransplantation of pancreatic and kidney primordia-where do we stand?

Lack of donor availability limits the number of human donor organs. The need for host immunosuppression complicates transplantation procedures. It is possible to 'grow' new pancreatic tissue or kidneys in situ via xenotransplantation of organ primordia from animal embryos (organogenesis of the endocrine pancreas or kidney). The developing organ attracts its blood supply from the host, enabling the transplantation of pancreas or kidney in 'cellular' form obviating humoral rejection. In the case of pancreas, selective development of endocrine tissue takes place in post-transplantation. In the case of kidney, an anatomically-correct functional organ differentiates in situ. Glucose intolerance can be corrected in formerly diabetic rats and ameliorated in rhesus macaques on the basis of porcine insulin secreted in a glucose-dependent manner by beta cells originating from transplants. Primordia engraft and function after being stored in vitro prior to implantation. If obtained within a 'window' early during embryonic pancreas development, pig pancreatic primordia engraft in non immune suppressed diabetic rats or rhesus macaques. Engraftment of pig renal primordia transplanted directly into rats requires host immune suppression. However, embryonic rat kidneys into which human mesenchymal cells are incorporated into nephronic elements can be transplanted into non-immune suppressed rat hosts. Here we review recent findings germane to xenotransplantation of pancreatic or renal primordia as a novel organ replacement strategy.

Innate immunity and heat shock response in islet transplantation

Islet transplantation is an extremely effective therapy for patients with type I diabetes, providing tight control of blood glucose and persistent insulin release. Islet grafts struggle with various stress responses and immunity attacks, which contribute to loss of islet grafts in the long term. In this review we focus upon the innate immunity and heat shock responses, which are closely relevant to the outcome of islet grafts. Potential strategies provided by more comprehensive interventions to control innate immunity and by selective induction of heat shock proteins may ameliorate the outcome of islet transplantation.

Acute exposure to streptozotocin but not human proinflammatory cytokines impairs neonatal porcine islet insulin secretion in vitro but not in vivo

BACKGROUND

Neonatal porcine islets (NPI) are a potentially useful source of beta cells for transplantation to treat type 1 diabetes mellitus. However, cytokine exposure following xenotransplantation is likely to prevent successful NPI xenograft survival. In this study, we examined the effects of human proinflammatory cytokines (IL-1 beta, IFN gamma, TNFalpha) on NPI function and cell death. These cytokines have been shown to be cytotoxic to beta cells, in part through the generation of nitric oxide. Therefore, we also examined NPI function after acute oxidative stress caused by streptozotocin (STZ), a nitric oxide-generating beta cell cytotoxin.

METHODS

Cultured NPI were exposed to human IL-1 beta, TNFalpha and IFN gamma for 48 h or STZ for 30 min in vitro. Cytokine exposed islets were transplanted into diabetic mice and assessed for function. Mice transplanted with control NPI were injected with STZ and also assessed metabolically.

RESULTS

In vitro exposure to STZ, but not cytokines, significantly reduced NPI glucose stimulated insulin secretion (1.1 +/- 0.1 vs. 4.3 +/- 1.3-fold stimulation index in STZ vs. control, P < 0.05) in addition to cellular DNA recovery (57.6 +/- 4.4%, P < 0.05). Total cellular insulin content was significantly reduced in NPI exposed to either cytokines (56.6 +/- 8.1%) or STZ (45.7 +/- 1.6%) compared to controls (P < 0.05). Interestingly, both STZ and cytokines did not appear to negatively affect NPI function post-transplant.

CONCLUSIONS

The potent nitric oxide generating cytotoxin STZ is able to impair in vitro NPI beta cell insulin release whereas human cytokines (IL-1 beta, TNFalpha, IFN gamma) do not affect the secretory response nor are they cytotoxic in vitro. These results may have implications for the development of anti-rejection protocols to be used in clinical NPI xenotransplants.

Long-term engraftment following transplantation of pig pancreatic primordia into non-immunosuppressed diabetic rhesus macaques

BACKGROUND

Transplantation therapy for human diabetes is limited by a shortage of donor organs, and transplant function diminished over time by cell death and limited potential for expansion of beta cells in pancreas or islets. Outcomes are complicated by immunosuppression. A way to overcome supply and expansion problems is to xenotransplant embryonic tissue. Previously, we have shown that beta cells originating from embryonic day (E) 28 (E28) pig pancreatic primordia transplanted into the mesentery of streptozotocin (STZ)-diabetic (type 1) Lewis rats or Zucker Diabetic Fatty (ZDF) diabetic (type 2) rats engraft and normalize glucose tolerance without the need for host immune-suppression.

METHODS

In this study, we transplant E28 pig pancreatic primordia in the mesentery of STZ-diabetic rhesus macaques.

RESULTS

Long-term engraftment of pig beta cells within liver, pancreas and mesenteric lymph nodes post-transplantation of E28 pig pancreatic primordia into STZ-diabetic rhesus macaques is demonstrated by electron microscopy, positive immune-histochemistry for insulin, and positive RT-PCR and in situ hybridization for porcine proinsulin mRNA. Insulin requirements were reduced in one macaque followed over 22 months post-transplantation and porcine insulin detected in plasma using sequential affinity chromatography, HPLC and mass spectrometry. Of potential importance for application of this transplantation technology to treatment of diabetes in humans and confirmatory of our previous findings in Lewis and ZDF rats, no host immunosuppression is required.

CONCLUSIONS

Under selected circumstances, pancreatic primordia elicit a muted immune response relative to more differentiated tissue, such that engraftment occurs in non-immunosuppressed hosts. Our findings that pig pancreatic primordia engraft long-term in non-immunosuppressed STZ-diabetic rhesus macaques establishes the potential for their use in human diabetics.

Delayed functional maturation of neonatal porcine islets in recipients under strict glycemic control

BACKGROUND

The aim of this study was to compare the functional maturation of neonatal porcine islet (NPI) grafts exposed to long-term hyperglycemia with those implanted under euglycemic conditions.

METHODS

mice Neonatal porcine islets were transplanted under the left renal capsule of diabetic SCID mice (group H), or in diabetic SCID mice who were also implanted with 500 BALB/c islets under the right renal capsule (group N). On day 42, the right kidneys were removed in both groups.

RESULTS

No animals in group H achieved euglycemia within 3 weeks after transplantation. Thus, these mice were exposed to long-term hyperglycemia. Mice in group N became euglycemic immediately after transplantation, however after removal of BALB/c grafts on day 42 they exhibited significantly higher blood glucose levels than in group H and showed glucose intolerance after glucose administration. Cellular insulin content of NPI grafts harvested on day 58 or 72 was significantly lower in group N mice compared to group H.

CONCLUSIONS

These results suggest that tight control of glycemia reduces the functional maturation of NPI grafts.

Efficacy and Limitations of Natural Killer Cell Depletion in Cyclophosphamide-Induced Tolerance

PURPOSE

We previously developed a cyclophosphamide (CP)-induced tolerance protocol, consisting of an intravenous injection of 1 x 10(8) donor spleen cells (SC) given on day 0 and an intraperitoneal injection of 200 mg/kg CP given on day 2. In the present study, we modified this protocol with natural killer cell (NK) depletion in recipient mice, and evaluated the efficacy of tolerance induction.

METHODS

We used B10.D2 (H-2d; IE+) and B10 (H-2b; IE-) mice as both donors and recipients. The recipient mice were treated with donor SC, CP, and donor bone marrow cells (BMCs) with or without NK depletion.

RESULTS

A higher level of mixed chimerism was achieved in the NK-depleted recipients. Survival of both the skin and heart donor grafts was significantly prolonged in the NK-depleted recipients. Donor reactive Vbeta11+ T cells were found at the same level as in untreated control mice. Pretreatment with recipient NK cell depletion was effective in inducing higher levels of donor mixed chimerism; however, permanent engraftment of donor bone marrow was not achieved.

CONCLUSION

Survival of donor grafts was remarkably prolonged in the NK cell-depleted group, but transplantation tolerance could not be induced. Our results suggest that NK cell depletion in CP-induced tolerance conditioning has some effect on the induction of donor-specific tolerance.

Phenotypic and functional characterization of human T cell clones indirectly activated against adult pig islet cells

BACKGROUND

Xenotransplanted patients produce xenospecific IgG1 antibodies directed against epitopes other than Galalpha1,3Gal. IgG1 antibody production is believed to be dependent upon T cell help. Therefore, as a natural continuation of our work aimed at characterizing the xenoimmune antibody response against pig islet cells, we have also examined the T cell response. T cell reactivity against islet cells is believed to result from indirect antigen presentation, and our in vitro study was designed to mimic the response in vivo. The main purpose of this study was to characterize the phenotype, the immunological specificity and the functional capacity of indirectly activated T cell clones, reactive against pig islet cell antigens.

MATERIALS AND METHODS

Human T cell clones, activated against pig islet cells in the presence of autologous antigen-presenting cells, were produced from limiting dilutions of bulk cultures. Clonality was investigated by T cell receptor Vbeta (TcRVbeta) expression analysis. Clonal specificity was studied in proliferation assays using different pig cells as stimulators. ELISpot experiments were performed to detect cytokine production patterns. The cytotoxic capacity of the clones was assessed using standard cell-mediated lysis tests and different porcine and human target cells. Several long-term bulk cultures of human lymphocytes, indirectly activated against pig islet cells, maintained for up to 60 days, were used as a control for possible bias in the selection of the clones.

RESULTS

Nineteen CD4+ TcRValphabeta+ T cell clones were recovered. No activation of natural killer T cells or gammadelta-T cells was recorded. There was no bias in the TcRVbeta-usage. The immunological specificity differed between clones; some were specifically reactive against pig islet cell antigens, while others were reactive with antigens present on a variety of pig cells. All clones produced a broad spectrum of cytokines, e.g. interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, interleukin (IL)5, IL10 and IL13, with no evidence of bias for a particular phenotype. None of the T cell clones were cytotoxic against pig islet cells, but two clones were cytotoxic against pig phytohemagglutinin (PHA)-blasts.

CONCLUSION

The analysis of several, indirectly activated, human CD4+ T cell clones shows that the response against pig islet cells is heterogeneous both with regard to immunological specificity and functional characteristics. This heterogeneity was further confirmed by analysis of the long-term bulk cultures of human lymphocytes, indirectly activated against pig islet cells.

T-cell responses during pig-to-primate xenotransplantation

Xenotransplantation using porcine organs may resolve a chronic shortage of donor organs for clinical transplantation if significant immunological barriers can be overcome. To determine the potential role of T lymphocytes in Xenograft (Xg) rejection, we transplanted transgenic hCD46 porcine hearts heterotopically into baboon recipients.

METHODS

Recipients were treated to deplete anti-Gal antibody with a non-antigenic alpha-Gal polyethylene glycol polymer (TPC) (n = 2), TPC plus rituximab (anti-CD20) (n = 1) or were untreated (n = 1). None of the recipients received T-cell immunosuppression.

RESULTS

All Xgs failed within 7 days and showed evidence of a mixed humoral and cellular rejection process. Cellular infiltration consisting primarily of CD4+ T cells and few CD8+ T cells. Proliferation and cytotoxicity assays showed sensitization of CD4+ and CD8+ T cells that reacted with porcine IFN-gamma (pIFN-gamma)-stimulated porcine aortic endothelial cells (PAEC). The CD4+ lymphocytes displayed greater cytotoxicity than CD8+ cells. An increased frequency of PAEC-specific interleukin (IL) 2 and IFN-gamma-secreting T cells was observed, suggesting a Th1 cytokine bias. An increase in the percentage of circulating CD4+CD28- cells was observed at the time of rejection and over 50% of the CD4+ cells recovered from residual pig tissue at necropsy lacked CD28 expression.

CONCLUSIONS

These findings show that lymphocytes are efficiently stimulated by PAEC antigens and can mediate direct tissue destruction. These studies (1) provide an insight into the potential of cellular-mediated cardiac Xg rejection, (2) show for the first time the induction of cytotoxic pig-specific CD4+CD28- lymphocytes and (3) provide a rational basis for determining different modes of immunosuppression to treat Xg rejection.

Effect of long-term culture on the expression of antigens and adhesion molecule in single porcine pancreatic endocrine cells

BACKGROUND

Changes in the expression of galactose (Gal) alpha1-3Gal, swine lymphocyte antigen (SLA) class II and intracellular adhesion molecule (ICAM)-1 of single porcine pancreatic endocrine (PE) cells during the culture period were investigated.

METHODS

Cultured porcine PE-cells were fixed in 10% buffered formalin for histological evaluation. At 1, 3, 6, 9 and 12 weeks of culture, mRNA was obtained from porcine PE-cells so that the expression of SLA class II and ICAM-1 genes could be examined by reverse transcriptase-polymerase chain reaction.

RESULTS

The rates of Galalpha1-3Gal and SLA class II-positive cells did not decrease during the culture period, but the rates of Galalpha1-3Gal and SLA class II strongly positive cells significantly decreased. ICAM-1-positive cells were scarcely observed during the culture period. SLA class II and ICAM-1 mRNAs were detected at 1 and 3 weeks of culture, but were not detected after 6 weeks of culture.

CONCLUSIONS

These results suggest that partial reduction in the expression of these antigens could be obtained by a long-term culture.

Reversal of diabetes in pancreatectomized pigs after transplantation of neonatal porcine islets

Neonatal porcine islets (NPIs) are able to grow and to reverse hyperglycemia after transplantation in immunoincompetent mice. The aim of this study was to demonstrate the feasibility of allogeneic NPI grafts to achieve normoglycemia in a pancreatectomized diabetic pig. NPIs were isolated from pancreases of 1- to 3-day-old pigs, cultured, and then transplanted via the portal vein into the liver of totally pancreatectomized pigs (mean body weight, 20.8 kg). Each pig received NPIs consisting of 3.1 +/- 0.3 x 10(6) beta-cells/kg (12,476 +/- 1,146 islet equivalent/kg). The six pigs that were given cyclosporine and sirolimus achieved normoglycemia by day 14 without insulin therapy. Three pigs died of surgical complications shortly after transplantation, whereas the other three remained insulin independent up to day 69. Of seven nonimmunosuppressed recipients, four pigs became normoglycemic by day 14 without insulin treatment, with two of the animals remaining normoglycemic long term. Well-preserved insulin-positive cells were found in the graft at the end of follow-up with a significant increase in insulin content in long-term survivors of both groups. This study demonstrates for the first time that allogeneic NPIs can reverse hyperglycemia in totally pancreatectomized diabetic pigs.

The efficacy of CD40 ligand blockade in discordant pig-to-rat islet xenotransplantation is correlated with an immunosuppressive effect of immunoglobulin

BACKGROUND

The authors' aim was to evaluate the efficacy of immunosuppression with monoclonal anti-CD40 ligand antibodies (aCD40L) or nonspecific polyclonal intravenous immunoglobulin (IVIG) in the pig-to-rat islet xenotransplantation model.

METHODS

Fetal porcine islet-like cell clusters were transplanted under the kidney capsule of nondiabetic rats. All antibodies were administered alone or in combination with cyclosporine A (CsA). In addition, some animals were administered antibodies plus tacrolimus (TAC) or sirolimus (SIR). Twelve days after transplantation, islet xenograft survival and rejection were evaluated using immunohistochemistry.

RESULTS

aCD40L plus CsA had a pronounced inhibitory effect on islet xenograft rejection for up to 12 days after transplantation. Unexpectedly, treatment with a monoclonal control antibody (anti-keyhole limpet hemocyanin [aKLH]) plus CsA had a similar inhibitory effect. Furthermore, a similar inhibition of islet xenograft rejection was observed also in animals administered IVIG plus CsA. Monotherapy with aCD40L, aKLH, IVIG, or CsA had no effect on the rejection process. Also, when aCD40L or aKLH was administered together with TAC, islet xenograft rejection was inhibited. There was no marked difference compared with rats treated with aCD40L or aKLH and CsA. Immunosuppression with aCD40L or aKLH in combination with SIR also inhibited pig-to-rat islet xenograft rejection, but the protective effect was not as pronounced.

CONCLUSIONS

Immunosuppression with high doses of antibodies, monoclonal or polyclonal, in combination with CsA or TAC inhibits pig-to-rat islet xenograft rejection. No specific effect of co-stimulatory blockade with aCD40L could be observed. Instead, the results indicate a nonspecific immunosuppressive effect of high doses of antibodies in this model.

Porcine endogenous retroviral nucleic acid in peripheral tissues is associated with migration of porcine cells post islet transplant

Porcine islets represent an alternative source of insulin-producing tissue, however, porcine endogenous retrovirus (PERV) remains a concern. In this study, SCID mice were transplanted with nonencapsulated (non-EC), microencapsulated (EC) or macroencapsulated (in a TheraCyte trade mark device) neonatal porcine islets (NPIs), and peripheral tissues were screened for presence of viral DNA and mRNA. To understand the role of an intact immune system in PERV incidence, mice with established NPI grafts were reconstituted with splenocytes. Peripheral tissues were screened for PERV and porcine DNA using PCR. Tissues with positive DNA were analyzed for PERV mRNA using RT-PCR. No significant difference was observed between non-EC and EC transplants regarding presence of PERV or porcine-specific DNA or mRNA. In reconstituted animals, little PERV or porcine DNA, and no PERV mRNA was detected. No PERV or porcine-specific DNA was observed in animals implanted with a TheraCyte trade mark device. In conclusion, an intact immune system significantly lowered the presence of PERV. Microencapsulation of islets did not alter PERV presence, however, macroencapsulation in the TheraCyte device did. Lower PERV incidence coincided with lower porcine DNA in peripheral tissues, linking the presence of PERV to migration of porcine cells.

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

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

The immunological properties of adult hippocampal progenitor cells

Adult hippocampal progenitor cells (AHPCs) derived from mature rats were studied in mixed co-cultures and shown not to elicit a proliferative response from human peripheral blood mononuclear cells (PBMCs) or allogeneic spleen cells. FACS analysis revealed low class I and no detectable class II (Ia) MHC expression by these cells. RT-PCR showed that AHPCs express the anti-inflammatory cytokine TGF-beta1. AHPCs did not, however, significantly impede the proliferation of OKT3- or PHA-stimulated PBMCs. Taken together, these results indicate that AHPCs are non-immunogenic in vitro. This is consistent with their pattern of MHC expression and does not require an active immunosuppressive mechanism.

Rejection of porcine islet xenografts mediated by CD4+ T cells activated through the indirect antigen recognition pathway

We have previously demonstrated that human T cells responding to porcine islets are primarily CD4+ and recognized porcine major histocompatibility complex class I molecules through the indirect pathway of antigen presentation. To determine whether this mechanism is responsible for rejection of adult porcine islets xenografts, porcine islets from adult pigs were transplanted under the kidney capsule of streptozotocin-treated CD4-knockout (KO), CD8-KO, Ig-KO and normal C57BL/6 mice. Islet xenografts were acutely rejected with similar kinetics when transplanted into normal C57BL/6 (MST=17.6 +/- 3.5 days) and Ig-KO (MST=19.0 +/- 1.7 days) mice. Interestingly, islet xenografts were rejected significantly earlier when transplanted into CD8-KO mice as compared with normal C57BL/6 (MST=7.0 +/- 0.01 days, P=2 x 10-4). Histopathological analysis revealed classical acute cellular rejection with severe diffuse interstitial cellular infiltrates in all rejected islet xenografts. In contrast, islet xenografts were not rejected when transplanted into CD4-KO mice (MST >/= 100 days, P=1 x 10-9). Histopathological analysis revealed no cellular infiltrates and intact islet xenografts. CD4+ T cells from both normal C57BL/6 and CD8-KO xenograft recipients showed detectable proliferative responses to porcine islets in the presence but not in the absence of syngeneic antigen-presenting cells. In addition, the anti-islet proliferative responses observed in normal C57BL/6 mice were significantly lower than those observed in CD8-KO mice. IgG anti-porcine antibodies were readily detected in C57BL/6 and CD8-KO xenograft recipients but not in Ig-KO or CD4-KO recipients. These results indicate that indirectly activated CD4+ T cells mediate acute rejection of adult porcine islet xenografts and that xenoreactive CD8+ T cells and antibodies are not necessary in this process.

Cloning and potential utility of porcine Fas ligand: overexpression in porcine endothelial cells protects them from attack by human cytolytic cells

Endothelial cells (EC) are primary targets of the recipient's immune response to transplanted organs and constitutively express Fas (CD95) ligand (FasL) on their surface. We investigated the role of porcine FasL in the generation of the human anti-pig response in vitro. Porcine aortic endothelial cells (PAEC) lysed a Fas+ human T-cell line, Jurkat. Anti-human Fas monoclonal antibody (mAb) specifically inhibited this killing in a dose-dependent manner, suggesting that porcine FasL recognizes and binds human Fas to induce apoptosis of human Fas+ cells. We next cloned porcine FasL, identifying an open reading frame of 849 base pairs predicting a protein of 282 amino acids. The predicted amino acid sequence was 85, 76, and 75% homologous to the predicted amino acid sequences of human, mouse, and rat, respectively, and found that PAEC expressed both FasL mRNA and protein. Transient transfection was used to increase or induce porcine FasL expression in PAEC or COS-7 cells. Transfection of PAEC with a plasmid encoding porcine FasL increased their ability to induce apoptosis in Jurkat cells, fresh human T cells activated with IL-2 and anti-CD3, and fresh IL-2-activated human (natural killer) NK cells. Moreover, porcine Fas L-transfected COS-7 cells induced significant apoptosis in Jurkat cells compared with that induced by mock-transfected COS-7 cells. Finally, the overexpression of porcine FasL in PAEC reduced their susceptibility as target cells to lysis by activated human NK or T cells. These findings suggest that porcine FasL overexpression in EC of vascularized xenografts may provide protection from cellular xenograft rejection.

In vitro recognition and impairment of pig islet cells by baboon immune cells: similarity to human cellular reactions

BACKGROUND

Grafting pig islets into patients with type 1 diabetes requires control of the strong cellular xenogeneic rejection. This in vitro study compared the cellular reaction of baboons and humans to pig islet cells (PICs) to confirm the validity of using these animals for further in vivo preclinical trials.

METHODS

Baboon or human peripheral blood mononuclear cells (PBMCs) or subsets were co-incubated with PICs from specific pathogen-free adult pigs for 7 days to determine the mechanisms and intensity of PBMC proliferation. Interleukin (IL) 10 and interferon (IFN) gamma secretion were assessed by enzyme-linked immunosorbent assay. Because proliferation was not indicative of aggression, a test based on perifusion analysis of the alteration of basal and stimulated insulin releases from PIC incubated with different baboon and human cells was developed.

RESULTS

Baboon PBMCs strongly proliferated in response to PICs (stimulation index [SI]=24.8+/-6.9 [n=8] vs. 23.9+/-3.4 [n=34] for human PBMCs), showing considerable variation in intensity among animals (2.3<SI<63) and humans (1.8<SI<97). PBMC proliferation was inhibited in baboons and humans by anti-CD4 (% inhibition of SI: 71+/-10% and 75+/-7%, respectively) and anti-DR (75+/-35% and 80+/-6%) monoclonal antibodies (MoAbs) or by depletion of MHC class II+ cells (99+/-1% and 90+/-6%). Blocking by anti-CD8 or anti-CD16 MoAbs was weaker and variable among both animals and humans. IL-10 production by baboon and human PBMCs in response to PICs increased more than IFN-gamma production after 2 days of coculture, but the IL-10/IFN-gamma ratio was inverted after 5 days of coculture. After 7 days (and even after only 2 days) of coculture with baboon (n=8) or human (n=18) PBMCs, basal and glucose-stimulated insulin secretions from PICs were almost completely abolished (P<0.0001). The drop in insulin release could have mainly resulted from lysis of PICs, because the number of PICs decreased by 78% after 7 days of co-incubation with PBMCs. A decrease of insulin release by PBMCs was reproduced with plastic-adherent cells and was abolished by depletion of MHC class II+ cells or by addition of 100 microg/ml gadolinium (which inhibits macrophages), but not by cyclosporine. In baboons, as in humans, insulin release was also decreased after coculture of PICs with enriched T lymphocytes remixed with antigen-presenting cells (APCs).

CONCLUSIONS

This study provides the first data on in vitro comparison of baboon and human cell-mediated recognition and impairment of PICs. Proliferation of PBMCs against PICs involves mainly CD4 T cells, with indirect recognition mediated by baboon or human MHC class II+ APCs. The Th2/Th1 profile of cytokines secreted in response to PICs was similar in baboon and human PBMCs. The model based on alteration of insulin secretion indicates that PIC impairment by whole mononuclear cells was strong and rapid and that a crucial role was played by MHC class II+ and plastic-adherent cells. Two mechanisms appear to be responsible for the role of these cells: (1) early and strong direct effect, which is potentially involved in vivo in primary nonfunction of islets aggressed by monocytes and macrophages; and (2) presentation of PIC xenoantigens, which leads to impairment by T lymphocytes possibly involved in in vivo-specific cellular rejection. The mechanisms and intensity of baboon cellular reactions to PICs in vitro were similar to those observed in humans, which suggests that the baboon is a suitable model for the study of cellular mechanisms during preclinical trials of pig islet xenografts.

In vitro co-incubation of pig islet cells with xenogeneic human blood mononuclear cells causes loss of insulin release during perifusion: involvement of non-T-cell- and T-cell-mediated mechanisms

Because the different steps of the human cellular immune rejection of pig islets are still poorly understood, our previous work concerned the intensity and mechanisms of the proliferation of human peripheral blood mononuclear cells (PBMC) to adult pig islet cells (PIC). As lymphocyte proliferation is not indicative of alteration of PIC, the present in vitro study evaluated cell-mediated immune effectors possibly involved in impairment of adult PIC. A test was thus developed, based on perifusion analysis of the alteration of insulin release from PIC incubated with different human cells. Compared to PIC incubation alone or with autologous pig splenocytes, seven-day co-incubation with whole human peripheral blood mononuclear cells (PBMC) (n = 18) led to almost complete abolition of basal and stimulated insulin releases (p < 0.0001). This effect could not be reversed by extensive sequential washes before perifusion of PIC, and the number of PIC was decreased by 78% after seven-day co-incubation with PBMC. PBMC are a complex mixture of cells involved in different xenogeneic mechanisms, and two components of this PIC impairment were then detected separately. First, the effect of PBMC against PIC was decreased (p < 0.0001) after removal of either MHC class II+ or CD14+ cells from PBMC. On the contrary, decreasing effect (p < 0.001) on insulin secretion was observed when only plastic-adherent or CD14+ cells were co-incubated with PIC. Additionally, alteration of insulin release from PIC cultured with PBMC or plastic-adherent cells was abolished dose-dependently (p < 0.0001 and p < 0.04, respectively) by gadolinium chloride (which inhibits macrophages), but not modified by cyclosporin A or mycophenolate mofetil which did not alter insulin release from PIC but blocked the proliferation of PBMC against PIC. A second mechanism was also detected, since co-incubation of PIC with purified human T cells remixed with antigen-presenting cells led to a decrease (p < 0.0001) of insulin release. This model based on the alteration of dynamic basal and stimulated insulin secretion provides detailed account of in vitro human cell-mediated impairment of PIC. It shows that the xenogeneic effect of whole mononuclear cells was strong and rapid. A crucial role was played by MHC class II+, CD14+, and plastic-adherent cells. Two mechanisms appear to be responsible for the role of these cells: 1) early direct effect, potentially involved in vivo in primary nonfunction of islets aggressed by monocytes/macrophages; and 2) the presentation of PIC xenoantigens leading to impairment by T lymphocytes, which may be involved in in vivo specific cellular rejection.

Bovine islets are less susceptible than human islets to damage by human cytokines

BACKGROUND

The potential benefits of islet xenografting in type 1 diabetes include the intriguing, but still unanswered, possibility that the grafted xenoislets may be less subjected to human autoimmune attack. Cytokines may play a major role in the pathogenesis of autoimmune diabetes by causing impairment of insulin release and pancreatic islet cell toxicity.

METHODS

We compared insulin secretion, islet cell death and survival, inducible nitric oxide synthase (iNOS) mRNA expression, nitrite production, and Bcl-2 and Bax mRNA expression in isolated human and large mammal (bovine) islets exposed to 50 U/ml recombinant human interleukin-1, 1,000 U/ml recombinant human tumor necrosis factor-alpha and 1,000 U/ml recombinant human interferon-gamma.

RESULTS

After 24-hr exposure, a marked decrease of glucose-stimulated insulin secretion was observed with human, but not with bovine islets. After 48-hr exposure, human, but not bovine, pancreatic islets showed a significantly higher percentage of apoptotic cells compared to controls. Treatment of human islets with human cytokines induced up-regulation of iNOS mRNA, increased levels of nitrites, and down-regulation of Bcl-2 mRNA, with unchanged levels of Bax mRNA. These parameters were not affected by cytokines in bovine islets.

CONCLUSIONS

Bovine islets are less susceptible than human islets to the effects of human cytokines, which may be a potential advantage of xenotransplantation.