T cell response in xenorecognition and xenografts: a review

Biology and Diseases of Swine

Swine are used in biomedical research as models for biomedical research and for teaching. This chapter covers normative biology and behavior along with common and emerging swine diseases. Xenotransplantation is discussed along with similarities and differences of swine immunology.

Development of a consensus protocol to quantify primate anti-non-Gal xenoreactive antibodies using pig aortic endothelial cells

BACKGROUND

Scientists working in the field of xenotransplantation do not employ a uniform method to measure and report natural and induced antibody responses to non-Galα(1,3)Gal (non-Gal) epitopes. Such humoral responses are thought to be particularly pathogenic after transplantation of vascularized GalTKO pig organs and having a more uniform assay and reporting format would greatly facilitate comparisons between laboratories.

METHODS

Flow cytometry allows examination of antibody reactivity to intact antigens in their natural location and conformation on cell membranes. We have established a simple and reproducible flow cytometric assay to detect antibodies specific for non-Gal pig antigens using primary porcine aortic endothelial cells (pAECs) and cell culture-adapted pAEC cell lines generated from wild type and α1,3galactosyl transferase knockout (GalTKO) swine.

RESULTS

The consensus protocol we propose here is based on procedures routinely used in four xenotransplantation centers and was independently evaluated at three sites using shared cells and serum samples. Our observation support use of the cell culture-adapted GalTKO pAEC KO:15502 cells as a routine method to determine the reactivity of anti-non-Gal antibodies in human and baboon serum.

CONCLUSIONS

We have developed an assay that allows the detection of natural and induced non-Gal xenoreactive antibodies present in human or baboon serum in a reliable and consistent manner. This consensus assay and format for reporting the data should be accessible to laboratories and will be useful for assessing experimental results between multiple research centers. Adopting this assay and format for reporting the data should facilitate the detection, monitoring, and detailed characterization of non-Gal antibody responses.

Immunogenicity in xenogeneic scaffold generation: antigen removal vs. decellularization

Decades of research have been undertaken towards the goal of tissue engineering using xenogeneic scaffolds. The primary advantages associated with use of xenogeneic tissue-derived scaffolds for in vitro development of replacement tissues and organs stem from the inherent extracellular matrix (ECM) composition and architecture. Native ECM possesses appropriate mechanical properties for physiological function of the biomaterial and signals for cell binding, growth and differentiation. Additionally, xenogeneic tissue is readily available. However, translation of xenogeneic scaffold-derived engineered tissues or organs into clinical therapies requires xenoantigenicity of the material to be adequately addressed prior to implantation. Failure to achieve this goal will result in a graft-specific host immune rejection response, jeopardizing in vivo survival of the resultant scaffold, tissue or organ. This review explores (i) the appropriateness of scaffold acellularity as an outcome measure for assessing reduction of the immunological barriers to the use of xenogeneic scaffolds for tissue engineering applications and (ii) the need for tissue engineers to strive for antigen removal during xenogeneic scaffold generation.

New concepts of immune modulation in xenotransplantation

The shortage of human organs for transplantation has focused research on the possibility of transplanting pig organs into humans. Many factors contribute to the failure of a pig organ graft in a primate. A rapid innate immune response (natural anti-pig antibody, complement activation, and an innate cellular response; e.g., neutrophils, monocytes, macrophages, and natural killer cells) is followed by an adaptive immune response, although T-cell infiltration of the graft has rarely been reported. Other factors (e.g., coagulation dysregulation and inflammation) appear to play a significantly greater role than in allotransplantation. The immune responses to a pig xenograft cannot therefore be controlled simply by suppression of T-cell activity. Before xenotransplantation can be introduced successfully into the clinic, the problems of the innate, coagulopathic, and inflammatory responses will have to be overcome, most likely by the transplantation of organs from genetically engineered pigs. Many of the genetic manipulations aimed at protecting against these responses also reduce the adaptive response. The T-cell and elicited antibody responses can be prevented by the biological and/or pharmacologic agents currently available, in particular, by costimulation blockade-based regimens. The exogenous immunosuppressive regimen may be significantly reduced by the presence of a graft from a pig transgenic for a mutant (human) class II transactivator gene, resulting in down-regulation of swine leukocyte antigen class II expression, or from a pig with "local" vascular endothelial cell expression of an immunosuppressive gene (e.g., CTLA4-Ig). The immunomodulatory efficacy of regulatory T cells or mesenchymal stromal cells has been demonstrated in vitro but not yet in vivo.

Human dominant-negative class II transactivator transgenic pigs - effect on the human anti-pig T-cell immune response and immune status

Swine leucocyte antigen (SLA) class II molecules on porcine (p) cells play a crucial role in xenotransplantation as activators of recipient human CD4(+) T cells. A human dominant-negative mutant class II transactivator (CIITA-DN) transgene under a CAG promoter with an endothelium-specific Tie2 enhancer was constructed. CIITA-DN transgenic pigs were produced by nuclear transfer/embryo transfer. CIITA-DN pig cells were evaluated for expression of SLA class II with/without activation, and the human CD4(+) T-cell response to cells from CIITA-DN and wild-type (WT) pigs was compared. Lymphocyte subset numbers and T-cell function in CIITA-DN pigs were compared with those in WT pigs. The expression of SLA class II on antigen-presenting cells from CIITA-DN pigs was significantly reduced (40-50% reduction compared with WT; P < 0·01), and was completely suppressed on aortic endothelial cells (AECs) even after activation (100% suppression; P < 0·01). The human CD4(+) T-cell response to CIITA-DN pAECs was significantly weaker than to WT pAECs (60-80% suppression; P < 0·01). Although there was a significantly lower frequency of CD4(+) cells in the PBMCs from CIITA-DN (20%) than from WT (30%) pigs (P < 0·01), T-cell proliferation was similar, suggesting no significant immunological compromise. Organs and cells from CIITA-DN pigs should be partially protected from the human cellular immune response.

A humanized mouse model to study human immune response in xenotransplantation

BACKGROUND

A major barrier to the clinical application of xenotransplantation as a treatment option for patients is T cell-mediated rejection. Studies based on experimental rodent models of xenograft tolerance or rejection in vivo have provided useful information about the role of T cell immune response in xenotransplantation. However not all observations seen in rodents faithfully recapitulate the human situation. This study aimed to establish a humanized mouse model of xenotransplantation, which mimics xenograft rejection in the context of the human immune system.

METHODS

NOD-SCID IL2rgamma-/- mice were transplanted with neonatal porcine islet cell clusters (NICC) followed by reconstitution of human peripheral blood mononuclear cells (PBMC). Human leukocyte engraftment and islet xenograft rejection were confirmed by flow cytometric and histological analyses.

RESULTS

In the absence of human PBMC, porcine NICC transplanted into NOD-SCID IL2rgamma-/- mice revealed excellent graft integrity and endocrine function. Human PBMC demonstrated a high level of engraftment in NOD-SCID IL2rgamma-/- mice. Reconstitution of NICC recipient NOD-SCID IL2rgamma-/- mice with human PBMC led to the rapid destruction of NICC xenografts in a PBMC number-dependent manner.

CONCLUSIONS

Human PBMC-reconstituted NOD-SCID IL2rgamma-/- mice provide an ideal model to study human immune responses in xenotransplantation. Studies based on this humanized mouse model will provide insight for improving the outcomes of clinical xenotransplantation.

Relative efficiency of porcine and human cytotoxic T-lymphocyte antigen 4 immunoglobulin in inhibiting human CD4+ T-cell responses co-stimulated by porcine and human B7 molecules

α1,3-Galactosyltransferase gene-knockout pigs transgenic for porcine cytotoxic T-lymphocyte antigen 4 immunoglobulin (pCTLA4-Ig) have been produced to reduce T-cell-mediated rejection following xenotransplantation. The level of soluble pCTLA4-Ig in their blood was greatly in excess of the therapeutic level in patients, rendering the pigs immune-incompetent. Soluble pCTLA4-Ig produced by these transgenic pigs was evaluated for binding to porcine and human (h) B7 molecules, and for its inhibitory effect on allogeneic and xenogeneic human T-cell responses. Porcine CTLA4-Ig-expressing peripheral blood mononuclear cells (PBMCs) and aortic endothelial cells (AECs) were evaluated for their direct inhibitory effect on hCD4+ T-cell responses. Soluble pCTLA4-Ig and purified hCTLA4-Ig showed similar binding to pB7 molecules, but pCTLA4-Ig showed significantly less binding to hB7 molecules. The pCTLA4-Ig and hCTLA4-Ig inhibited the response of hCD4+ T cells to pAECs equally, but pCTLA4-Ig was less successful in inhibiting the human allogeneic response. The hCD4+ T-cell response to PBMCs from pCTLA4-Ig pigs was significantly lower than that of non-pCTLA4-Ig pigs. Although pCTLA4-Ig was detected in the cytoplasm of pCTLA4-Ig-expressing pAECs, only a minimal level of soluble pCTLA4-Ig was detected in the supernatant during culture, and pCTLA4-Ig-expressing pAECs did not inhibit the xenogeneic direct human T-cell response. High-level tissue-specific production of pCTLA4-Ig may be required for sufficient immunosuppression for organ or cell (e.g., islets) transplantation.

Determination of the precursor frequency and the reaction intensity of xenoreactive human T lymphocytes

BACKGROUND

It is acknowledged that the response of human T cells to xenogeneic targets is more potent than that to allogeneic targets. However, it is not clear whether the more vigorous T cell response to xenoantigens than to alloantigens is attributable to a higher frequency or stronger reaction of xenoreactive T cells.

METHODS

We determined the precursor frequencies (PFs) and stimulation indexes (SIs) of xenoreactive human T cells by performing a mixed lymphocyte reaction (MLR) assay using a carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeling technique. Irradiated porcine or human peripheral blood mononuclear cells (PBMCs)used as stimulator cells--were cultured with CFSE-labeled human PBMCs--used as responder cells.

RESULTS

The SIs of the xenoreactive CD4(+) T cells were significantly higher than those of the alloreactive CD4(+) T cells, whereas the PFs of the alloreactive and xenoreactive CD4(+) T cell precursors were almost identical, suggesting a stronger reaction by a single xenoreactive CD4(+) T cell. In contrast, the SIs of the xenoreactive CD8(+) T cells did not differ from those of the alloreactive CD4(+) T cells, and the PFs of the allo- and xenoreactive CD8(+) T cell precursors were also identical. Addition of a soluble human CD47-Fc fusion protein in the porcine-to-human MLR assay caused a statistically significant reduction of the SIs of the xenoreactive CD4(+) T cells. Such an alteration was abrogated by further addition of blocking antibodies (Abs) against either human CD47 or signal regulatory protein-alpha in the porcine-to-human MLR assay. Addition of human CD47-Fc after the depletion of non-T cells from the population of human responder PBMCs in this MLR assay did not influence the SIs of the xenoreactive CD4(+) T cells.

CONCLUSIONS

The more vigorous T cell response to xenoantigens than to alloantigens is possibly attributable to a stronger reaction of xenoreactive T cells; the interspecies incompatibility of CD47 may contribute to such xenoreactive CD4(+) T cell responses via an indirect pathway.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

In vitro suppression of xenoimmune-mediated macrophage activation by human CD4+CD25+ regulatory T cells

BACKGROUND

Macrophages are important effector cells in T cell-mediated xenograft rejection. The aim of this study was to determine whether CD4+CD25+ regulatory T cells (Tregs) were capable of suppressing macrophage activation in vitro.

METHODS

Porcine cell or xenoantigen-primed human peripheral blood mononuclear cells, CD4+ T cell-depleted peripheral blood mononuclear cells, or CD14+ macrophages plus autologous CD4+CD25- T cells were cultured with or without expanded autologous Tregs. Transwell cultures were used to separate the various components to determine the need for cell-cell contact.

RESULTS

Pig cell primed CD14+ macrophages required the presence of CD4+CD25- T cells for activation and increased expression of CD40, interleukin-12, and tumor necrosis factor-alpha. This up-regulated expression of macrophage activation markers was reduced substantially in the presence of autologous Tregs. Coculture with Tregs did not alter macrophage viability but reduced the capacity of macrophages to stimulate proliferation of responder T cells. Tregs required direct contact with CD4+CD25- T cells to inhibit macrophage activation but activated macrophage phenotype was not altered by separating the stimulated human peripheral blood mononuclear cells or CD14+ macrophages from Tregs in a transwell system. Macrophages did not require direct cell contact with porcine stimulator cells for full activation by CD4+CD25- T cells.

CONCLUSIONS

Human Tregs were able to suppress xenoantigen-primed and CD4+ T-cell-mediated macrophage activation and antigen-presenting cell function. However, Tregs had no direct effect on macrophages in vitro.

The role of cyclosporine and mycophenolate in an orthotopic porcine-to-rat corneal xenotransplantation

We performed this study to investigate the feature of rejection in porcine-to-rat corneal orthotopic transplantation and to evaluate the effect of cyclosporine and mycophenolate on the xeno-rejection. Orthotopic corneal transplantation was done at 91 Sprague-Dawley rats, and they were divided into 10 groups based on the combination of immunosuppressants including dexamethasone, cyclosporine, and mycophenolate mofetil. Graft survival was analyzed and grafted eyes were examined with Hematoxylin & Eosin and CD4 or CD8 staining. Enzyme-linked immunosorbent assays were done for interleukin-2 (IL-2), IL-4, IL-5, IL-10, and interferon (IFN)-gamma in cornea, lacrimal gland, and cervical lymph nodes. The longest median survival of the immune suppressant group was 11.00+/-1.96 days, which showed no statistical differences compared with that of control (8.00+/-1.52 days). The neutrophils were prominent in the early phase but soon gave way to the monocytes. The number of CD8+ cells was higher than that of CD4+ cells. IL-2 and IFN-gamma markedly increased at 10 to13 days in cornea, lacrimal glands, and cervical lymph nodes, which showed a decrease with immunosuppressants except in the cornea. In conclusion, cyclosporine and mycophenolate could not prevent the rejection in porcine to rat orthotopic corneal xenograft associated with infiltration of CD8+ and innate immune cells.

Characterization and expansion of baboon CD4+CD25+ Treg cells for potential use in a non-human primate xenotransplantation model

BACKGROUND

It is well established that CD4(+)CD25(+) regulatory T (Treg) cells can modulate allogeneic immune responses. Xenotransplantation, proposed as a means to address the critical shortage of human organs, may also benefit from similar approaches to avert rejection. Baboons are a preferred preclinical animal model for xenogeneic organ transplantation experiments, and the characterization of baboon Treg cells will be beneficial to future tolerance studies in this animal model.

METHODS

We analyzed CD4(+)CD25(+) T cells from baboon lymph nodes, spleens, and blood by flow cytometry, then purified and expanded porcine antigen-specific baboon CD4(+)CD25(high) cells in vitro to evaluate their regulatory activity in the baboon anti-pig xenogeneic responses.

RESULTS

CD4(+)CD25(high) T cells were 1.7%, 3.1%, and 1.9% of baboon splenic, lymph node, and blood T cells, respectively. The CD4(+)CD25(high) T cells expressed the Treg cell-associated transcription factor, FoxP3. Proliferation/suppression assays using irradiated pig peripheral blood mononuclear cells as stimulators showed that Treg cells suppressed the vigorous baboon CD4(+)CD25(-) T-cell anti-pig proliferation response and cytokine secretion. Expanded baboon Treg cells suppressed baboon anti-pig CD4(+)CD25(-) T-cell proliferation approximately 4- to 10-fold more than freshly isolated Treg cells. Expanded Treg cells suppressed proliferation to primary cells from the same pig used for expansion more effectively than proliferation to stimulators from a different strain of pig, suggesting a level of antigen specificity.

CONCLUSION

We demonstrate that baboon Treg cells suppress immune responses to xenogeneic stimulation. These studies suggest that adoptive transfer of expanded Treg cells into transplant recipients may provide an approach to prevent cell-mediated rejection of grafts and potentially induce tolerance in the pig to baboon xenotransplantation preclinical model.

Corneal rat-to-mouse xenotransplantation and the effects of anti-CD4 or anti-CD8 treatment on cytokine and nitric oxide production

Corneal xenotransplantation may be an alternative approach to overcome shortage of allografts for clinical transplantation. Orthotopic corneal rat-to-mouse xenotransplantation and syngeneic transplantation was performed and the effects of anti-CD4 and anti-CD8 treatments on corneal xenograft survival and production of cytokines, interleukin (IL)-2, IL-4, IL-10, gamma-interferon (IFN-gamma) and nitric oxide (NO) were evaluated. RT-PCR was used to determine the expression of genes for cytokines and inducible nitric oxide synthase (iNOS) in the grafts. The presence of iNOS protein in grafts was detected by immunofluorescent staining. We found that corneal xenotransplantation was associated with a strong upregulation of genes for both Th1 and Th2 cytokines and with NO production in the graft. Treatment of xenograft recipients with mAb anti-CD4, but not anti-CD8, resulted in a profound inhibition of IL-2, IL-4 and IL-10 production, and in a significant prolongation of corneal xenograft survival. The results show that upregulation of Th2 cytokines after corneal xenotransplantation does not correlate with xenograft rejection. Rather, corneal graft rejection is associated with the expression of genes for IFN-gamma and iNOS and with NO production.

Human CD4+CD25+ regulatory T cells suppress anti-porcine xenogeneic responses

Due to the shortage of human organs, xenotransplantation is being explored as an alternative to allotransplantation, but immune rejection remains a major hurdle to its implementation. We tested the ability of human CD4+CD25+ T cells (Treg cells) to suppress CD4+ T cell-mediated anti-porcine xenoresponses usingin vitroassays. Human Treg cells were hyporesponsive to porcine cell stimulation and suppressed the proliferative response of CD4+CD25- T cells in a dose-dependent manner, and comparison of the allo- and xenoresponses indicated that more Treg cells might be required to suppress the xenogeneic response than the allogeneic response. Stimulation of CD4+CD25- T cells with porcine cells resulted in secretion of IFN-gamma, TNF-alpha, IL-10, IL-6 and IL-2, and Treg cells suppressed the secretion of these cytokines, as well as the CD4+CD25- T-cell cytolytic response against porcine cells. These results suggest a potential role for Treg cells in promoting xenograft survival.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The neutrophil: the unnoticed threat in xenotransplantation?

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

In Vivo Recognition by the Host Adaptive Immune System of Microencapsulated Xenogeneic Cells

BACKGROUND

Microencapsulation is under consideration as a means of enabling pancreatic islet transplantation. To understand better the ongoing destructive host response, we examined whether the adaptive immune system of the recipient recognized polymer-encapsulated xenogeneic cells implanted intraperitoneally.

METHODS

Balb/c mice were implanted with xenogeneic Chinese hamster ovary cells, inside and outside poly(hydroxyethyl methacrylate-methyl methacrylate) microcapsules, and responses were compared with xenografted Chinese hamster skin (positive control). Capsules were localized within an agarose rod. Splenocyte proliferation upon rechallenge in vitro, antibody titer in serum, and Th1/2 polarization (assessed by interleukin-4 and interferon-gamma in supernatants of antigen-challenged splenocytes and immunoglobulin [Ig]G1 and IgG2a antibody isotypes in serum) were measured.

RESULTS

Encapsulation did not prevent a strong recipient antibody response. Splenocyte proliferation in vitro did not differ after priming by implanted cells, inside or outside capsules. Thus, the capsule membrane did not prevent indirect recognition of shed antigens. However, after 10 days of implantation, proliferation was lower than that induced by skin grafts, although this difference disappeared by 2 months. This transient T-cell suppression was unexpected because encapsulated cell viability was already compromised by 10 days. The influence of Th1/2 bias did not explain the observed suppression. Cells inside capsules elicited a consistent Th2 response, whereas cells outside capsules elicited a mixed response, and skin xenografts showed an initial Th2 response that became mixed by 2 months.

CONCLUSIONS

Encapsulation does not prevent host immune responses, but the inflammatory response to the implanted biomaterials or xenogeneic cells may be responsible both for encapsulated cell death and transient T-cell suppression.

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

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

Absence of humoral and cellular alloreactivity in baboons sensitized to pig antigens

AIM

to study whether sensitization to pig antigens results in humoral and/or cellular sensitization to alloantigens in baboons, and thus increases the risks of organ allotransplantation after xenotransplantation. Serum from baboons that were naive (n = 4), sensitized to Gal alpha 1,3Gal (Gal) antigens (n = 2), or sensitized to Gal + non-Gal pig antigens (n = 2) were tested by flow cytometry for the presence of immunoglobulin G (IgG) and IgM antibodies that bind to pig or baboon peripheral blood mononuclear cells (PBMC). Two allosensitized baboons were used as positive controls. The same 10 sera were tested in a complement-mediated cytotoxicity assay to detect cytotoxic antibodies against pig, allo and self-PBMC. The T-cell responses of the same baboons to allogeneic and pig PBMC stimulators in mixed lymphocyte reaction (MLR) were studied. All baboon sera contained cytotoxic antibodies that bound to pig PBMC. Binding and cytotoxicity were higher in xenosensitized baboons, particularly in those sensitized to Gal + non-Gal antigens (P < 0.001). None of the naive or xenosensitized baboon sera bound to baboon PBMC. Serum from allosensitized baboons showed anti-baboon IgG and IgM binding, but there was no increase in binding to pig PBMC or in cytotoxicity to pig cells. The MLR response to pig stimulators in baboons sensitized to non-Gal pig antigens was greater than that of naive or Gal-sensitized baboons (P < 0.001), but there was no increase in the response to baboon cells. In baboons, no in vitro evidence that a previous pig xenograft might endanger the outcome of a subsequent allograft was documented.

Comparison of in vivo lymphocyte proliferation between allogeneic and xenogeneic heart transplantation in mice

There are controversial in vitro data comparing the strength of the cellular immune response between allogeneic and xenogeneic stimulator/responder combinations. The present study therefore compares in vivo lymphocyte proliferation using heart transplantation (HTx) models in mice. Heterotopic HTx into BALB/c mice was performed using donor organs from mice (BALB/c and C57BL/6) or Lewis rats. Intraperitoneally given bromodeoxyuridine (BrdU) was incorporated into the DNA and was subsequently analyzed by flow cytometry. On postoperative days 3 and 5, proliferation of splenocytes, CD4(+) T-lymphocytes, and CD19(+) B-lymphocytes was significantly higher after xenogeneic than after allogeneic and isogeneic HTx. No significant difference was observed when proliferation of CD8(+) lymphocytes was determined. The increased in vivo proliferation after xenotransplantation may reflect an earlier and probably stronger cellular immune response compared to allogeneic transplantation. The higher CD4(+) lymphocyte proliferation underscores the importance of this cell population in xenograft rejection.

The effect of mitoxantrone on anti-pig immunization in baboons

Besides virological and physiological concerns, the success of xenotransplantation (Xt) is still dependent on the prevention of delayed xenograft rejection (DXR). Although multifactorial, DXR is mainly due to xenonatural antibody (Ab) recognizing their xenogenic antigen (Ag) followed by complement activation. Despite the use of intensive treatments capable of inhibiting the humoral response, DXR can still not be avoided and always occurs within weeks following transplantation. Moreover, these latter treatments currently used in Xt could not be used clinically in humans because of their high risk of over-immunosuppressing the patients. Mitoxantrone (Mx) is a drug well known for its antiproliferative properties and is used clinically in oncology and in the treatment of relapsing multiple sclerosis. In models of arthritis in rats, it has been shown to be 10 to 20 times more powerful than cyclophosphamide (CyP) at blocking both inflammatory and B-cell responses. Because of its B-cell inhibitory capacity and considering the implication of the humoral response in xenograft rejection, we have compared Mx with CyP for its ability to block in vivo anti-pig immunization induced via subcutaneous injections of pig red blood cells into baboons. Neither drug was able to inhibit the anti-pig responses following the first and second immunizations, emphasizing the particularity of preformed Ab responses. However, the rise in Ab in the Mx treated animals was significantly delayed as compared with the non-treated as well as the CyP treated animals and was mainly because of a profound depletion of circulating B-cells. Mx displays an interesting antihumoral effect that we now intend to test in a pig kidney to baboon Xt model, with anticipated administration of the drug allowing an early B-cell depletion.

Cellular participation in delayed xenograft rejection of hCD55 transgenic pig hearts by baboons

Delayed xenograft rejection (DXR) of pig organs by baboons currently represents the major obstacle to successful xenotransplantation. Although antibodies (Abs) are believed to play a fundamental role in this form of rejection, so far little is known concerning the potential cellular component. Biopsies taken during DXR of human CD55 transgenic pig hearts by non-treated (n = 2), alpha-Gal immunoadsorbed (n = 2), or immunosuppressed (n = 9) baboons were studied. The cellular element was explored by determining not only its phenotype by classical immunohistochemical techniques but also its activity in terms of cytokines, cytolytic enzymes and other mediators using quantitative reverse transcription polymerase chain reaction. All porcine xenografts underwent DXR; within 5 days in non-treated and immunoadsorbed animals but significantly delayed (6 to 29 days) in immunosuppressed animals. Cellular infiltration in non-immunosuppressed grafts consisted predominantly of monocytes/macrophages, CD8 cells and a few CD4 T-cells. The predominant baboon transcripts detectable were the proinflammatory cytokines interleukin1-alpha and tumor necrosis factor-alpha, the lymphokine interferon-gamma and the cytotoxic enzyme granzyme B. However, these cellular components were lacking in the immunosuppressed animals. Despite these differences, strong immunoglobulin M (IgM) and C5b-9 complement deposition was observed in all animals at rejection. Together our findings suggest that although the humoral response plays a predominant role in DXR through IgM Abs and complement activation, there is a clear cellular infiltrate in DXR in this model that is likely to contribute to rejection through a strong pro-inflammatory and cytotoxic environment, necessitating substantial immunosuppression for a prolonged graft survival.

Xenotransplantation

The continued and growing success of lung allotransplantation has intensified the worldwide shortage of donor organs. Yet, xenotransplantation remains a daunting challenge. Additional molecular incompatibilities and unforeseen complications will continue to be discovered. Progress has been made, notably on the generation of alpha-Gal double knockout pigs. Progressive increases in organ survival times have been seen for most organs after significant investments of time and money. The lung continues to be an organ with the lowest supply of cadaveric donors and the least potential for expanded living donation or mechanical alternatives. As such, the impetus for xenotransplantation is strong. The lung appears to be exquisitely sensitive to xenograft rejection and resistant to strategies that have been moderately successful in other organs. A complex program involving genetically modified donor organs, recipient preparation for antibody removal or tolerance promotion, and multitargeted drug therapy will likely be required for successful clinical application.

Analysis of human CD4 T lymphocyte proliferation induced by porcine lymphoblastoid B cell lines

BACKGROUND

This study was undertaken to characterize the two porcine lymphoblastoid cell lines L23 and L35, derived from a pig inoculated by the retrovirus Tsukuba-1, and to determine how they induce a strong human lymphocyte proliferation.

METHODS

Phenotypic characterization was performed by flow cytometry and reverse transcriptase-polymerase chain reaction analyses. Xenogeneic mixed lymphocyte reactions (XMLR) were performed using unfractionated human peripheral blood mononuclear cells (huPBMC) and purified CD4+ T lymphocytes as responding cells, in the presence of blocking antibodies and fusion proteins.

RESULTS

The immunoglobulin genes were demonstrated to be rearranged in L23 and L35 cell lines, in agreement with the expression of a B cell phenotype. Both induced a similar proliferation of huPBMCs and purified human CD4+ lymphocytes from adult or cord blood (naïve cells). Proliferation of CD4+ T lymphocytes was completely blocked by anti-SLA-DR plus anti-SLA-DQ mAbs, excluding human lymphocyte transformation by porcine viruses. The frequency of proliferative precursors was inconsistent with that induced by a retroviral superantigen but similar to classical direct xenoantigen presentation as observed with other porcine antigen-presenting cells. Extensive analysis of costimulatory signals led to the identification of the CD28 pathway, in agreement with membrane expression of B7 molecules on L23 and L35 cells, and of the CD2 pathway in L35 cells.

CONCLUSION

These two porcine lymphoblastoid cell lines have been further characterized and clearly identified as belonging to the B cell lineage. By expressing major histocompatibility complex class II antigens and costimulatory molecules, they induce a vigorous proliferative response of human CD4+ lymphocytes through a direct presentation pathway.

CD4+ T cells initiate pancreatic islet xenograft rejection via an interferon-gamma-dependent recruitment of macrophages and natural killer cells

BACKGROUND

In this study, the mechanisms by which CD4+ T cells interact with the innate immune system in xenograft rejection were investigated.

METHODS

Fetal pig pancreas (FPP) grafts were transplanted into female SCID mice. The FPP recipient SCID mice were reconstituted with exogenous leukocytes obtained from male BALB/c mice.

RESULTS

Although nonreconstituted SCID recipients or recipients reconstituted with CD4+ T cell-depleted leukocytes showed indefinite FPP graft survival with very few macrophages infiltrating their grafts, reconstitution of SCID recipients with as few as 2x10(5) CD4+ T cells was sufficient to induce rapid xenograft rejection. CD4+ T cells secreted interferon-gamma but not interleukin-4 and initiated the activation and accumulation of macrophages and natural killer cells, that were responsible for the rapid graft destruction. Suppression of interferon-gamma prolonged graft survival and suppressed the macrophages and natural killer cell accumulation and activation.

CONCLUSIONS

These results demonstrate that CD4+ T cell-dependent cellular xenograft rejection was a result of macrophage and natural killer cell accumulation and activation, but was not mediated by eosinophils. Consistent with this was the finding that interferon-gamma but not interleukin-4 was in part responsible for mediating this effect.

CD97-decay-accelerating factor interaction is not involved in leukocyte adhesion to endothelial cells

BACKGROUND

Effective improvement in xenograft survival is achieved using transplants from transgenic pigs expressing human complement (C) regulatory proteins, including decay-accelerating factor (DAF), CD59, and CD46 on endothelial cells (ECs). The aim of this study was to investigate whether human DAF expression in porcine ECs, as well as regulating C activation, can modify intercellular events through its interaction with its receptor, CD97, on human leukocytes.

METHODS

Cellular interactions between human leukocytes and porcine ECs were investigated in vitro using ECs from either wild-type or DAF-transgenic pigs. Static leukocyte adhesion and T cell activation assays were performed using porcine ECs as target or effector cells, respectively. The role of the DAF-CD97 interaction was investigated using specific blocking monoclonal antibodies (mAbs) against human DAF and its receptor, CD97, in adhesion assays.

RESULTS

Adhesion of U937 or Jurkat T cells, both expressing human DAF and CD97, was quantitatively similar for wild-type and transgenic-DAF-expressing pig ECs. Furthermore, blocking the CD97-DAF interaction did not inhibit xenogeneic leukocyte-endothelium adhesion, whereas blocking the very late antigen 4-vascular cell adhesion molecule-1 pathway reduced this adhesion by 50-80%. Furthermore, DAF and CD97 expression was not up-regulated during tumor necrosis factor-alpha- or lipopolysaccharide-mediated EC activation, unlike the adhesion molecules E-selectin, vascular cell adhesion molecule-1, and intracellular adhesion molecule (ICAM)-1.

CONCLUSION

We found that high levels of human DAF expressed on ECs abrogates C-mediated cell damage but did not affect the in vitro adhesive properties or antigen-presenting cell function of genetically modified porcine ECs.

Aromatic quinolinecarboxamides as selective, orally active antibody production inhibitors for prevention of acute xenograft rejection

The prevention of xenograft rejection is substantially dependent on inhibiting antibodies (Ab) produced by B-cells independently of T-cell signals (TI-1). Due to their ubiquitous biochemical mechanisms of action, the immunosuppressants currently employed not only fail to discriminate between B- and T-cells but also have a narrow therapeutic window and, thus, their prolonged use in complex immunosuppressive regimens is problematic. By capitalizing on the target enzyme-bound (DHODH) structure 1b of one of these compounds, leflunomide, and modulating part of its multiple mechanisms of action to gain selectivity, the quinoline-8-carboxamide 3 was designed as a potentially weak enzyme inhibitor but effective immunosuppressant. Compound 3 fulfilled the mechanistic criteria set and had 10-fold B-cell over T-cell selectivity. Its pyridyl analogue 4 was found to be a highly potent and selective B-cell immunosuppressant with a 75-fold selectivity for B- over T-cells (as judged by the MLR data) and no general cytotoxicity at concentrations up to 160-fold higher than those required to inhibit B-cells. In the mouse, 4 effectively blocked TI-1 Ab production and suppressed Ab-mediated xenograft rejection in a xenotransplantation model under a once-daily dosing regimen, with efficacy down to 0.3 mg/kg/day po. These are the first data demonstrating the feasibility of the development of drugs specific for impeding Ab production.

Targeting of lymphotoxin-alpha to the tumor elicits an efficient immune response associated with induction of peripheral lymphoid-like tissue

A recombinant antibody-lymphotoxin-alpha fusion protein induced an adaptive immune response protecting mice from melanoma. Importantly, this fusion protein elicited the formation of a lymphoid-like tissue in the tumor microenvironment containing L-selectin+ T cells and MHC class II+ antigen-presenting cells, as well as B and T cell aggregates. Furthermore, PNAd+/TCA4+ high endothelial venules were observed within the tumor, suggesting entry channels for naive T cell infiltrates. Over the course of therapy, a marked clonal expansion of certain TCR specificities occurred among tumor-infiltrating lymphocytes that displayed reactivity against melanoma cells and the TRP-2(180-188) peptide. Consequently, naive T cells may have been recruited to as well as primed and expanded in the lymphoid-like tissue induced by the lymphotoxin-alpha fusion protein at the tumor site.

Rat xenograft survival in mice treated with donor-specific transfusion and anti-CD154 antibody is enhanced by elimination of host CD4+ cells

BACKGROUND

Treatment with a donor-specific transfusion (DST) and a brief course of anti-mouse CD154 (anti-CD40-ligand) monoclonal antibody (mAb) prolongs the survival of both allografts and rat xenografts in mice. The mechanism by which allograft survival is prolonged is incompletely understood, but depends in part on the presence of CD4+ cells and the deletion of alloreactive CD8+ T cells. Less is known about the mechanism by which this protocol prolongs xenograft survival.

METHODS

We measured rat islet and skin xenograft survival in euthymic and thymectomized mice treated with combinations of DST, anti-CD154 mAb, anti-CD4 mAb, and anti-CD8 mAb. Recipients included C57BL/6, C57BL/6-scid, C57BL/6-CD4null, and C57BL/6-CD8null mice.

RESULTS

Pretreatment with a depleting anti-CD4 mAb markedly prolonged the survival of both skin and islet xenografts in mice given DST plus anti-CD154 mAb. Comparable prolongation of xenograft survival was obtained in C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb. In contrast, anti-CD8 mAb did not prolong the survival of either islet or skin xenografts in mice treated with DST and anti-CD154 mAb. Thymectomy did not influence xenograft survival in any treatment group. Adoptive transfer of splenocytes from C57BL/6-CD4null recipients treated with DST and anti-CD154 mAb and bearing long-term skin xenografts revealed the presence of residual xenoreactive cells.

CONCLUSIONS

These data suggest that treatment with DST and anti-CD154 mAb induces a state of "functional" transplantation tolerance. They also support the hypothesis that both the induction and maintenance of graft survival based on this protocol depend on different cellular mechanisms in allogeneic and xenogeneic model systems.