Total synthesis of the natural antigen involved in the hyperacute rejection response to xenotransplants

The major glycosphingolipid in pig vascular endothelium is the ceramide pentasaccharide Gal alpha(1 --> 3)Gal beta(1 --> 4)GlcNAc beta(1 --> 3)Gal beta(1 --> 4)Glc beta(1 --> 0)Cer (1), which binds specifically to human anti-Gal antibody and is involved in the hyperacute rejection response in xenotransplantation from pig to man. The synthesis of 1 and its methyl glycoside 2 is described.

Neural tissue xenografting

Neural transplantation may become an important treatment alternative for focal brain disorders. To date, the most successful grafts have been obtained in patients with Parkinson's disease. Completely normalized dopamine production and reduction of Parkinsonian symptoms have been demonstrated 10 years after grafting. However, the allogeneic donor tissue has to be obtained from induced abortions, and there are logistical difficulties, risks of infection, and ethical constraints limiting a wider clinical use. Xenografting is an alternative that could bridge these limitations if immunological rejection could be prevented. Pig embryonic neural tissue has been grafted to patients with Parkinson's disease, but no functional benefits have clinically been proven so far. The immune reactions to neural xenografts were incompletely characterized at the time of these early clinical trials, and it is likely that the treatments used were insufficient and that the grafts were rejected. In this article we will review new experiments addressing the immune responses against porcine neural tissue grafted to the adult brain, including the role of antibodies, complement, natural killer (NK) cells, lymphocytes, as well as the effects of immunosuppressive drugs and donor tissue modifications.

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.

Natural antibodies and the host immune responses to xenografts

Natural antibodies are present in the serum of individuals in the absence of known antigenic stimulation. These antibodies are primarily IgM, polyreactive, and encoded by immunoglobulin V genes in germline configuration. Natural antibodies are produced by B-1 lymphocytes, cells that form the primary cell of the fetal and newborn B cell repertoire and may represent the basic foundation upon which the adult repertoire of B cell antibodies is based. Natural antibodies react with a variety of endogenous and exogenous antigens, including xenoantigens expressed by tissues between unrelated species. These antibodies are capable of causing the immediate rejection of grafts exchanged across species barriers. One of the central issues related to our understanding of the immunopathologic mechanisms responsible for rejection of xenografts is whether pre-formed natural antibodies and new antibodies induced following xenotransplantation are produced by the same pathways of B cell antibody production. We have established in studies conducted in rodents and humans that the initial phases of antibody production xenogeneic tissues involves the use of a restricted population of Ig germline genes to encode xenoantibody binding. As the humoral xenoantibody response matures, the same closely-related groups of Ig V genes are used to encode antibody binding and there is evidence for an isotype switch to IgG antibody production and the appearance of somatic mutations consistent with antigen-driven affinity maturation. Our findings in both rodent and human studies form the basis for our proposal that the xenograft response reflects the use of B cell natural antibody repertoires originally intended to provide protection against infection. The host humoral response is inadvertently recruited to mount antibody responses against foreign grafts because they display carbohydrate antigens that are shared by common environmental microbes. This model of xenoantibody responses is being tested in our laboratory through the analysis of the binding of xenoantibodies in their original non-mutated configuration, and the examination of the effect of specific point mutations and gene shuffling have on xenoantibody binding activity. Establishment of the relationships between Ig structural changes and subsequent changes in binding affinity should provide important insights into the role that, natural antibodies and the cells that produce them play in the evolution of the host's humoral responses to xenografts.

Induction of anti-Forssman antibodies in the hamster-to-rat xenotransplantation model

BACKGROUND

In the hamster-to-rat heart xenotransplantation model, the serum response of the host contributes to determine whether the xenograft is accommodated or rejected.

METHODS

To further characterize the serum response in this model, we compared anti-hamster antibodies found in naive LEW-1A rats, or in LEW-1A rats rejecting or accommodating a hamster heart, using a combination of cobra venom factor (CVF) and cyclosporin A (CsA) given for 10 days, and then CsA alone.

RESULTS

Hamster hearts grafted into rat recipients contained IgG and IgA deposits to the same extent whether the xenograft was rejected or accommodated. Only immunoglobulins of the IgM isotype were found to be more abundant in recipients rejecting their graft. A significant part of this IgM response was directed toward the Forssman antigen, a sphingolipid present in the hamster but not in the rat. However, although anti-Forssman antibodies bind in situ to hamster tissues, this binding was not able to induce hyperacute rejection after antibody transfer. Furthermore, depletion of anti-Forssman antibodies from a rejecting serum did not modify its rejection properties.

CONCLUSION

Unlike the pig-to-primate discordant xenotransplantation model, in which preexisting anti-carbohydrate antibodies are directly responsible for hyperacute rejection, in the concordant hamster-to-rat situation, the evoked IgM anti-Forssman carbohydrate antibodies do not appear to be the main cause of the vascular rejection.

Guinea pig C3 specific rabbit single chain Fv antibodies from bone marrow, spleen and blood derived phage libraries

We constructed combinatorial immunoglobulin libraries from the whole rabbit antibody repertoire of bone marrow, spleen and peripheral blood of a rabbit immunized with guinea pig complement protein C3. By means of the phage display technology we selected guinea pig C3 specific single chain Fv (scFv) antibodies from each of the libraries. None of the scFv antibodies cross reacted with guinea pig C3a, human C3 or rat C3. The frequency of bone marrow derived C3 positive clones was much higher as compared to blood or spleen derived clones. Additionally bone marrow and spleen derived clones show higher diversity than clones, obtained from blood, as determined by fingerprint analysis with the restriction enzyme AluI. Dissociation rate constants for all scFvs were similar, indicating that the source of the scFvs had no influence on affinities. The antibody fragments were used to analyze complement activation during xenotransplantation. Several blood or bone marrow derived scFvs bound to C3 located on rat liver endothelium after hyperacute rejection of a heterotopically transplanted rat liver into guinea pig. These data demonstrate that monoclonal rabbit scFvs can be easily generated from recombinant phage display libraries, constructed from spleen, blood or bone marrow. The selected guinea pig C3 specific scFvs appear to be useful to detect complement activation during xenotransplantation in guinea pigs.

The influence of naturally occurring heterophilic anti-immunoglobulin antibodies on direct measurement of serum proteins using sandwich ELISAs

Sandwich ELISAs have become a widely used method for the quantitative detection of serum proteins. However, they can be biased by a variety of interfering substances. As reported recently, we observed false-positive levels of interferon (IFN)-alpha and -beta in up to 27% of sera from healthy blood donors using commercial ELISAs. We now demonstrate that two different groups of naturally occurring heterophilic antibodies (IgG-type) are responsible for these titers. Group I (representing 85% of positive samples) binds to the Fab region of IgG from goat, mouse, rat, horse, and bovidae (but not rabbit). Group II (15%) recognizes an epitope in the Fc region of mouse, horse, bovine, and rabbit (but not goat or rat) immunoglobulins. The antibodies did not crossreact with human IgG subclasses but contributed to false-positive IgG rheumatoid factor levels obtained using a commercially available ELISA. To investigate the susceptibility of assays to these artifacts, various combinations of capture and detection antibodies have been tested. On this basis, we defined the relative risks that standard ELISAs might be influenced by heterophilic anti-immunoglobulin antibodies. In general, assays that use monoclonal antibodies for both capture and detection are less susceptible than others which include at least one polyclonal antiserum. However, only systems utilizing rabbit F(ab')(2) fragments have been found to be immune to this interference.

Xenotransplantation: in vitro analysis of synthetic alpha-galactosyl inhibitors of human anti-Galalpha1-->3Gal IgM and IgG antibodies

Pig-to-human xenotransplantation might be an option to overcome the increasing shortage of human donor organs. However, naturally occurring antibodies in human blood against the Galalpha1-->3Gal antigen on pig endothelial cells lead to hyperacute or, if prevented, acute or delayed vascular rejection of the pig graft. The purpose of this study was therefore to evaluate synthetic oligosaccharides with terminal Galalpha1-->3Gal to inhibit antigen-binding and cytotoxicity of anti-alphaGal antibodies against pig cells. Different oligosaccharides were synthesized chemically and by a combined chemico-enzymatic approach. These included monomeric di-, tri-, and pentasaccharides, a polyacrylamide-conjugate (PAA-Bdi), as well as di-, tetra-, and octamers of Galalpha1-->3Gal. All were tested for inhibitory activity by anti-alphaGal ELISA and complement-dependent cytotoxicity tests. PAA-Bdi was the best inhibitor of binding as well as cytotoxicity of anti-alphaGal antibodies. Monomeric oligosaccharides efficiently prevented binding of anti-alphaGal IgG, but less well that of anti-alphaGal IgM, with tri- and pentasaccharides showing a better efficacy than the disaccharide. The two trisaccharides Galalpha1-->3Galbeta1-->4GlcNAc and Galalpha1-->3Galbeta1-->3GlcNAc were equally effective. Oligomers of Galalpha1-->3Gal were more effective than monomers in blocking the binding of anti-alphaGal IgG. However, they could not block IgM binding, nor could they match the efficacy of PAA-Bdi. We conclude that oligosaccharides with terminal Galalpha1-->3Gal, most effectively as PAA-conjugates, can prevent binding and cytotoxicity of human anti-alphaGal in vitro. The PAA-Bdi conjugate might be most suited for use as a Sepharose-bound immunoabsorption material.

Transient nature of interference effects from heterophile antibodies: examples of interference with cardiac marker measurements

Two-site immunoassay methods have become the standard technique for measurement of a wide variety of drugs, hormones, and cell proteins. One limitation of these methods is their susceptibility to interference from heterophilic antibodies present in the sera of some patients. Human anti-murine antibodies represent a common heterophile antibody that can bind to mouse immunoglobulin and as well as to immunoglobulin from other species. While the mechanism of human anti-murine antibody interference has been well characterized, the time course over which this interference occurs and the susceptibility of different immunoassay procedures to human anti-murine antibody interference from patients with human anti-murine antibody have not been as well described. We report on the time course of interference in assays for cardiac markers for two patients with human anti-murine antibodies. We measured creatine kinase MB isoenzyme (CKMB) and troponins I and T using three different vendors' immunoassay procedures. Our results demonstrate that assay interference due to human anti-murine antibody interference is a transient phenomenon. In one of our patients, human anti-murine antibody interference appeared suddenly, peaked approximately 9 days following its appearance, and gradually resolved over the next 3 weeks. In addition, we found that immunoassay methods from different vendors can show highly variable interference effects when human anti-murine antibody-containing specimens are analyzed.

Characterization of a pig-to-goat orthotopic lung xenotransplantation model to study beyond hyperacute rejection

BACKGROUND

A pig-to-goat orthotopic lung xenograft model was developed to test whether depletion of goat xenoreactive antibodies against pig red blood cells would prolong pig lung xenograft survival.

METHODS

Adult goats with anti-pig xenoreactive antibodies underwent left pneumonectomy followed by orthotopic transplantation of pig left lung (group 1) or immunodepletion of their xenoreactive antibodies by extracorporeal right pig lung perfusion before transplantation without (group 2) or with (group 3) complete clampage of the right pulmonary artery. In group 4, goat left lungs were orthotopically transplanted into pigs and served as negative controls (pig serum does not have anti-goat xenoreactive antibodies). Each study group included 5 animals. Immunosuppression in surviving recipients included cyclosporine and azathioprine.

RESULTS

Group 1 recipients died 7 +/- 3 hours after xenograft reimplantation of severe pulmonary hypertension and dysfunction and vasogenic shock, with little evidence of histologic xenograft injury. Group 2 xenografts had a stable circulatory and respiratory function on reperfusion and survived 9 +/- 4 days. Group 3 animals also tolerated complete occlusion of the right pulmonary artery, and xenografts assured the total respiratory support for 4 +/- 1 days. After immunodepletion, goat serum showed no detectable titers of xenoreactive antibodies, which began to reappear by postoperative day 2, where xenografts showed histologic stigmata of acute (humoral and cellular-mediated) rejection that evolved to a complete xenograft necrose at death. Group 4 xenografts showed scattered features of acute rejection 5 +/- 1 days after the operation.

CONCLUSIONS

Pig left lung xenografts can provide prolonged and complete respiratory support after depletion of goat xenoreactive antibodies, but they ultimately necrose once recipient xenoreactive antibodies return to pretransplantation values.

Heterophile anti-mouse immunoglobulin antibodies may interfere with cytokine measurements in patients with HLA alleles protective for type 1A diabetes

Wilson and coworkers (Wilson SB, Kent SC, Patton KT, Orban T, Jackson RA, Exley M, Porcelli S, Schatz DA, Atkinson MA, Balk SP, Strominger JL, Hafler DA: Extreme Th1 bias of invariant V alpha24J alpha Q T-cells in type 1 diabetes. Nature 391:177-181, 1998) have recently reported raised serum levels of interleukin-4 (IL-4) in anti-islet autoantibody-positive first-degree relatives of patients with type 1A diabetes who did not progress to diabetes. Protection from diabetes has been noted for several human lymphocyte antigen (HLA) alleles, such as HLA DR2-DQA1*0102-DQB1*0602. We, therefore, wanted to determine whether this cytokine phenotype was associated with HLA genes protective for type 1A diabetes. We used a two-site fluoroimmunoassay with the same monoclonal antibodies as those reported by Wilson et al. Using this assay, we have found evidence for human heterophile antibodies mimicking serum IL-4: all serum IL-4 reactivity was lost if mouse serum or mouse immunoglobulin were added to the assay; serum IL-4 activity was bound and then eluted by protein A/G chromatography; and levels of anti-mouse antibodies correlated with apparent serum IL-4. This pseudo-IL-4 activity was found in a subset of control subjects, patients with type 1A diabetes, and their relatives and was primarily associated with specific HLA alleles protective for type 1A diabetes (e.g., DQB1*0602). After adjustment for HLA, positive levels of heterophile antibodies were not associated with protection from diabetes. The confounding effect of protective HLA alleles associated with heterophile antibodies could explain the previously reported association between raised serum IL-4 and protection from type 1A diabetes. The mechanism by which specific DQ alleles protect from diabetes and are associated with increased heterophile antibodies is currently unknown.

Xenotransplantation: the importance of the Galalpha1,3Gal epitope in hyperacute vascular rejection

The transplantation of organs from other species into humans is considered to be a potential solution to the shortage of human donor organs. Organ transplantation from pig to human, however, results in hyperacute rejection, initiated by the binding of human natural antidonor antibody and complement. The major target antigen of this natural antibody is the terminal disaccharide Galalphal,3Gal, which is synthesized by Galbeta1,4GlcNAc alpha1,3-galactosyltransferase. Here we review our current knowledge of this key enzyme. A better understanding of structure, enzyme properties, and expression pattern of alpha1,3-galactosyltransferase has opened up several novel therapeutic approaches to prevent hyperacute vascular rejection. Cloning, and expression in vitro of the corresponding cDNA, has allowed to develop strategies to induce immune tolerance, and deplete or neutralize the natural xenoreactive antibody. Elucidation of the genomic structure has led to the production of transgenic animals that are lacking alpha1,3-galactosyltransferase activity. A detailed knowledge of the enzyme properties has formed the basis of approaches to modify donor organ glycosylation by intracellular competition. Study of the expression pattern of alpha1,3-galactosyltransferase has helped to understand the mechanism of hyperacute rejection in discordant xenotransplantation, and that of complement-mediated, natural immunity against interspecies transmission of retroviruses.

[Mechanisms of xenotransplant rejection]

The recent shortage of allogeneic human organs in transplantation medicine has led to a renewed interest in evaluating the feasibility of xenotransplantation, particularly of porcine origin. Discordant xenotransplants between phylogenetically distant species such as pig and primates are challenged by a series of strong rejection reactions. Hyperacute rejection is dominantly mediated by humoral responses of the immune system (natural antibodies, complement cascade) and the activation of coagulation factors, whereas delayed xenograft rejection and T-cell mediated responses are also mediated by elements of the cellular immune system. The mechanisms of chronic xenograft rejection remain to be elucidated. The present review is focused on the mechanisms of rejection and the immunobiology of discordant, vascularized xenotransplants. We attempt to provide a brief description of the dynamics, the humoral and cellular mediators, and the pathogenesis of xenograft rejection. Finally, an overview on potentially therapeutic strategies to overcome xenograft rejection is presented.

[Transplantation of porcine Langerhans islets for therapy of type I diabetes. The way to clinical application]

Transplantation of isolated pancreatic islets provides an interesting alternative to the present cure for diabetes: insulin injections and pumps. These are characterized by an insufficient glucose haemostasis and in the long run can induce kidney failure, blindness, heart failure, and amputations. Up to now more than 293 allogeneic islet transplantations have been performed in diabetics with chronical kidney failure. Despite some success, no real breakthrough has been yet achieved, though great efforts are being made to improve the various methodological steps on the way to clinical transplantation. The use of animal (xenogeneic) organs could be a solution to overcome the shortage of allogeneic donors. The current experimental and clinical research focuses on the use of pigs as organ donors, which have a number of advantages over the immunologically more compatible primates. This article reports on success and open questions concerning the efforts to isolate porcine islets for future clinical transplantation: the search for a suitable pig breed, the various isolation steps, purification and in vitro culture, transplantation models using-small and large animals, first clinical trials, and immunological reactions against the xenogeneic tissue. In addition, strategies to circumvent tissue rejection and future perspectives are discussed.

Donor selection for xenotransplantation: detection of Galalpha1-3Gal on different porcine organs

The expression of the major porcine xenoantigens (Galalpha1-3Gal) in different tissues varies between species. The selection of suitable donors and the interpretation of studies which attempt to prevent hyperacute rejection are dependent on donor expression of Galalpha1-3Gal. Screening of large number of animals to find potential Galalpha1-3Gal negative donors requires a robust, tissue-based and practical method of assessing Galalpha1-3Gal expression. In this study, we have assessed the expression of Galalpha1-3Gal in a variety of pig organs using anti Galalpha1-3Gal antibody. Biopsies of heart, kidney, ear and tail were obtained from 20 outbred pigs. Biopsies were fixed in formalin and stained with a human anti Galalpha1-3Gal antibody obtained from pooled human AB serum passed down a Galalpha1-3Gal immunoadsorbent column. Tissue from all 4 organs from all 20 pigs expressed Galalpha13Gal. This study shows that detection of Galalpha1-3Gal on an ear or tail biopsy is a simple but very reliable method for assessing Galalpha1-3Gal expression on the heart and kidney and facilitates donor selection for xenotransplantation.

Heterophile antibodies to bovine and caprine proteins causing false-positive human immunodeficiency virus type 1 and other enzyme-linked immunosorbent assay results

Heterophile antibodies are a well-recognized cause of erroneous results in immunoassays. We describe here a 22-month-old child with heterophile antibodies reactive with bovine serum albumin and caprine proteins causing false-positive results to human immunodeficiency virus type 1 and other infectious serology testing.

IgG, but not IgM, mediates hyperacute rejection in hepatic xenografting

We reported previously that no classical features of hyperacute rejection (HAR) could be found in liver grafts in the guinea-pig (GP)-to-rat model and that recipients died shortly after transplantation of non-immunologic causes. Thus, the GP-to-rat model is not suitable for studying the mechanisms of discordant liver xenograft rejection. In the hamster to rat model, long-term survival of a liver graft is possible, but extremely low levels of xenoreactive natural antibodies are present. To mimic a discordant situation with pre-formed IgM and IgG antibodies, we sensitized rats 1 or 5 weeks before grafting. Specific anti-hamster IgM antibodies were found in recipients sensitized at week -1 but not week -5. Anti-hamster IgG was present in all recipients, albeit considerably higher in animals sensitized 5 weeks before grafting. In these two models, we examined the mechanism of HAR of liver grafts and compared this with heart xenografts. Control heart and liver grafts were rejected 4 and 7 days after transplantation respectively. Liver grafts in recipients sensitized at week -5 showed venous congestion and bleeding after reperfusion, indicating HAR, however this was not observed after sensitization at week -1. This surprising finding was confirmed by histology. Massive extravasation, edema, and acute liver cell degradation were noticed in grafts subjected to HAR. Liver grafts of recipients sensitized at week -1 showed only minimal changes. Heart grafts were rejected hyperacutely in both sensitization models. IgG antibodies could be detected on liver grafts in the group sensitized at week -5 but not in the group sensitized at week -1. Minimal IgM depositions were found on liver grafts of animals sensitized 1 week before grafting. Rejected heart grafts from similar sensitization groups showed identical antibody depositions; only IgM depositions were massive. Complement depositions were found in all groups. These results indicate that IgG, but not IgM, mediates HAR in hepatic xenografting. Such a predominance of IgG over IgM does not exist for heart grafts.

Cross-species reactivity of a panel of antibodies with monkey and porcine tissue

The continuing shortage of organs available for transplantation limits the number of patients able to benefit from this highly successful form of therapy. Interest in alternative sources of organs has now turned towards the pig because of its physiological similarity to human. There is a requirement therefore for reagents not only for research purposes but possibly for studying xenotransplants in the clinical situation in the future. In this study, we have concentrated on determining the cross-species reactivity of a large panel of antibodies directed against human leukocyte markers, testing peripheral blood leukocytes and also including renal tissue to determine non-leukocyte cross-reactivity. A total of 63 out of 127 antibodies cross-reacted with cynomolgus monkey cells. Twenty of these antibodies stained similar populations of leukocytes to human, whereas the remaining 43 reacted with clearly different populations. The majority of antibodies (108/127) were unreactive with porcine leukocytes, reflecting the evolutionary differences between pig and man. Of the 19 antibodies cross-reactive with porcine cells, seven reacted with similar proportions of leukocytes to human, whereas the remaining 12 antibodies stained entirely different populations. The most interesting, and potentially most useful, antibodies were four that reacted with human, cynomolgus monkey and porcine tissue in a similar manner, suggesting that the epitopes recognized are present on similar molecules. These antibodies were directed against CD29 (MEM1O1A, K20) and CD18 (BU87, 7E4), the common beta1- and beta2-integrin subunits respectively. This study demonstrates that there are antigens common to cynomolgus monkey, pig and man that react with currently available antibodies. Nevertheless, when determining cross-species reactivity of human antibodies, it is important to consider the possibility that there may be additional non-leukocyte reactivity in other tissues.