Two novel assays of alloantibody-secreting cells demonstrating resistance to desensitization with IVIG and rATG

Donor-specific alloantibody presents a major barrier to the successful transplantation of kidneys and hearts. However, the study of alloantibody production has been hampered by both an inadequate source of antibody-secreting cells (ASCs) and a paucity of assays to determine their function. We describe two new assays that allow for the determination of the frequency and specificities of allo-ASCs in humans using purified HLA as targets. These assays demonstrated allo-ASCs in the CD138(+) fraction of the bone marrow, but not in peripheral blood. Alloantibody specificities in these assays correlated well with those detected in the serum suggesting that bone marrow-derived ASCs are indeed a major source of alloantibody in vivo. However, ASCs for a specific HLA antigen were rare with an estimated frequency of only 1/2 x 10(6) marrow cells. Pretransplant treatment in vivo with multiple plasmaphereses and low-dose IVIG alone or in combination with rATG had no effect on ASC number or alloantibody production. These techniques allow for the study of allospecific ASCs and provide a method to test the potential efficacy of agents on alloantibody production in vivo.

Banff '05 Meeting Report: differential diagnosis of chronic allograft injury and elimination of chronic allograft nephropathy ('CAN')

The 8th Banff Conference on Allograft Pathology was held in Edmonton, Canada, 15-21 July 2005. Major outcomes included the elimination of the non-specific term "chronic allograft nephropathy" (CAN) from the Banff classification for kidney allograft pathology, and the recognition of the entity of chronic antibody-mediated rejection. Participation of B cells in allograft rejection and genomics markers of rejection were also major subjects addressed by the conference.

The future of organ replacement: needs, potential applications, and obstacles to application

The supply of organs available for transplantation is already far smaller than the demand, and the demand may grow substantially in the near future. For this reason, it is timely to consider how organ function might be replaced in the future. In this article, we consider new technologies that might be used to replace organ function, the obstacles to applying new technologies, and how those obstacles might be overcome in the development of new strategies for organ replacement.

Cryptic speciation in the cosmopolitan and clonal human pathogenic fungus Aspergillus fumigatus

Microbes and other organisms smaller than one to a few millimeters in size are hypothesized to have global populations, in contrast to the geographically restricted ranges of larger organisms. However, fungi, which routinely have reproductive propagules no larger than 10 micrometers, challenge the generality of this hypothesis because recent studies have shown that globally distributed morphological species embrace two or more geographically restricted phylogenetic species. We used the concordance of gene genealogies to recognize phylogenetic species in the globally distributed opportunistic human pathogenic fungus, Aspergillus fumigatus. Based on DNA sequence data of five loci for each of 63 individuals collected from five continents, we have delineated two phylogenetic species in this single morphological species. Unlike all other fungi examined to date, both genetically isolated groups showed a global distribution with no evidence of a correlation between genotype and geographic location. Sexual reproduction has never been observed in A. fumigatus, but when the same data were used to explore the association of alleles at the five loci for one of the phylogenetic species, evidence was found to support recombination. The discovery of a cryptic species is medically relevant because different species are likely to differ in virulence or drug resistance. The discovery of a globally distributed A. fumigatus species clade highlights the need for ecological studies of the fungus to either document global dispersal or propose alternative mechanisms by which it persists as single, global phylogenetic population.

New approaches to replacing failing organs

The foremost cause of death and disability is organ failure. Failure of the heart, lungs, kidney, and liver is typically addressed by organ transplantation; however, the number of human organs available for this purpose is quite limited. For this reason, some new technologies are being advanced for the treatment of organ failure. This communication summarizes potential limitations of these technologies.

B cell-dependent T cell development

T and B cells are thought to develop independently. While it is widely recognized that T cells help B cells in the production of antibodies to protein antigens, less well understood is whether or how B cells contribute to T cell development and function. Defects in cell-mediated immunity in individuals with B cell deficiency and in B cell-deficient mice suggest that B cells contribute to T cell function. The question of whether T cell development is B cell dependent was revisited using two novel mouse strains: mice with monoclonal T cells (MT) and mice with monoclonal compartments of both B and T cells (MBT). It was found that T cell development and thymocyte selection is modified by the presence of B cells. These results suggest that B cells, or B cell products, contribute to thymocyte selection and T cell development.

The possibility of B-cell-dependent T-cell development

The development of T cells is thought to be independent of B cells. However, defects in cell-mediated immunity in individuals with B-cell deficiency suggest the contrary. To test whether B cells affect T-lymphocyte development, we constructed mice with a monoclonal T-cell compartment (MT) and monoclonal B- and T-cell compartments (MBTs). In these mice, the T cells expressed a DO 11.10 transgenic (DO-T) cell receptor restricted to major histocompatibility complex (MHC) class IId. While CD4+ DO-T lymphocytes are rare in transgenic H-2b MT mice, we found that in H-2b MBT mice under the influence of B cells, DO-T lymphocytes mature into large numbers of CD4+ peripheral T cells. H-2b MBT mice have more CD4+ thymocytes than H-2b MT mice. These data are consistent with the view that B cells play some role in thymocyte development.

Genetic modification of xenografts

For nearly a century, xenotransplantation has been seen as a potential approach to replacing organs and tissues damaged by disease. Until recently, however, the application of xenotransplantation has seemed only a remote possibility. What has changed this perspective is the advent of genetic engineering of large animals; that is, the ability to add genes to and remove genes from lines of animals that could provide an enduring source of tissues and organs for clinical application. Genetic engineering could address the immunologic, physiologic and infectious barriers to xenotransplantation, and could allow xenotransplantation to provide a source of cells with defined and even controlled expression of exogenous genes. This communication will consider one perspective on the application of genetic engineering in xenotransplantation.

Deficiency of 5-lipoxygenase accelerates renal allograft rejection in mice

Acute renal allograft rejection is associated with alterations in renal arachidonic acid metabolism, including enhanced synthesis of leukotrienes (LTs). LTs, the products of the 5-lipoxygenase (5-LO) pathway, are potent lipid mediators with a broad range of biologic activities. Previous studies, using pharmacological agents to inhibit LT synthesis or activity, have implicated these eicosanoids in transplant rejection. To further investigate the role of LTs in acute graft rejection, we transplanted kidneys from CByD2F1 mice into fully allogeneic 129 mice that carry a targeted mutation in the 5lo gene. Unexpectedly, allograft rejection was significantly accelerated in 5-LO-deficient mice compared with wild-type animals. Despite the marked reduction in graft survival, the 5lo mutation had no effect on the hemodynamics or morphology of the allografts. Although LTB4 levels were reduced, renal thromboxane B2 production and cytokine expression were not altered in 5-LO-deficient allograft recipients. These findings suggest that, along with their proinflammatory actions, metabolites of 5-LO can act to enhance allograft survival.

Xenotransplantation and other means of organ replacement

Exciting new technologies, such as cellular transplantation, organogenesis and xenotransplantation, are thought to be promising approaches for the treatment of human disease. The feasibility of applying these technologies, however, might be limited by biological and immunological hurdles. Here, we consider whether, and how, xenotransplantation and various other technologies might be applied in future efforts to replace or supplement the function of human organs and tissues.

The immunological hurdles to cardiac xenotransplantation

The main hurdle to clinical application of cardiac xenotransplantation is the immune response of the recipient against the graft. Although all xenografts arouse an intense immune response, the effect of that response depends very much on whether the graft consists of isolated cells or an intact organ, such as the heart. Intact organs, which are transplanted by primary vascular anastomosis, are subject to severe vascular injury owing to the reaction of immune elements with the endothelial lining of donor blood vessels. Vascular injury leads to hyperacute rejection, acute vascular rejection, and chronic rejection. The immunological basis for these types of rejection and potential therapies, which might be used to avert them, are discussed.

Xenotransplantation and tolerance

The application of xenotransplantation faces daunting immunological hurdles, some of which might be overcome with the induction of tolerance. Porcine organs transplanted into primates are subject to several types of rejection responses. Hyperacute rejection mediated by naturally occurring xenoreactive antibodies and complement can be overcome without tolerance. Acute vascular rejection and cellular rejection, however, may present important opportunities for immunological tolerance, and humoral rejection might be approached by various mechanisms including (i) clonal deletion, (ii) anergy, (iii) immune deviation, (iv) induction of immunoregulatory or suppressor cells, or (v) veto cells. B-cell tolerance, useful for preventing humoral rejection, might be approached through clonal anergy. It remains to be determined, however, whether tolerance induction is required for xenotransplantation and by which means the various mechanisms of tolerance can be applied in the setting of xenotransplantation. Regardless, the study of tolerance will surely expand understanding of the physiology and pathophysiology of the immune system.

The pathology of cardiac xenografts

The pathology of cardiac xenografts has yielded critical insights into the mechanisms of xenograft rejection and the therapeutic procedures that might be applied to preventing or treating it. The conditions seen in rejecting cardiac xenografts include hyperacute rejection, acute vascular rejection, and cellular rejection. Hyperacute and acute vascular rejection of cardiac xenografts have features typical of humoral injury. Less is known about cellular rejection and only speculation can be offered about chronic rejection. Still, these features allow critical testing of pathogenetic mechanisms and therapies.

Platelet-mediated activation of endothelial cells: implications for the pathogenesis of transplant rejection

BACKGROUND

Platelets exert their normal functions at sites of endothelial disruption by plugging discontinuities in blood vessels and secreting products that promote thrombosis, inflammation, and the healing of wounds. Whether platelets might induce these changes in xenograft blood vessels, leading to development of acute vascular rejection, has been uncertain.

METHODS

To examine the role of human platelets in modulation of xenograft endothelium, pig endothelial cells were treated with human platelets.

RESULTS

Treatment of quiescent porcine endothelial cells with human platelets modulated the endothelial cells. Whereas resting human platelets caused little change in normal porcine endothelial cells, platelets activated with small amounts of thrombin induced striking changes in the endothelial cells, including the induction of tissue factor activity, the expression of E-selectin, and the secretion of endothelin-1. These changes were induced, at least in part, by interleukin-1 (IL-1) associated with the platelet surface and were modified by the secretion of transforming growth factor-beta (TGF-beta).

CONCLUSION

These findings may explain how the activation of platelets at an early point in the rejection of vascularized organ xenografts or in chronic diseases might contribute to thrombotic, ischemic, and inflammatory changes characteristic of an organ xenograft undergoing rejection.

Immunobiology of xenotransplantation

The transplantation of organs between disparate species is hindered by severe immune responses of the recipient against the graft. These immune responses gives rise to hyperacute and acute vascular rejection and to cellular rejection. Research during the past decade has shed light on the elements of the immune system responsible for the rejection of xenografts and has provided novel and incisive therapies which might be applied to these problems.

Unexpected anti-alpha GalNAc antibodies in alpha-galactosyl transferase-deficient mice: complex relationship between genotype and the natural antibody repertoire

Mice lacking the alpha-galactosyl transferase gene (GalT(-/-) mice) have been used extensively as a model for xenotransplantation. Unlike wild type (WT) mice, GalT(-/-) mice do not produce Gal alpha 1-3Gal and are known to produce natural IgM specific for Gal alpha 1-3Gal, as do humans and higher primates. In addition to natural anti-Gal alpha 1-3Gal IgM in GalT(-/-) mice, we identified natural IgM which bound alpha-N-acetylgalactosamine (alpha GalNAc) but not Gal alpha 1-3Gal or blood group A. Although unexpected, these antibodies were expressed at 10-fold greater concentrations in GalT(-/-) mice than in WT mice. One explanation for this unexpected observation is that the production of natural antibodies is affected by self-antigen(s) that are similar but not identical to targets recognized by the natural antibody. Thus, the natural humoral immune system may be unresponsive to "near-self" antigens even though the individual is not tolerant to those antigens. Another explanation for the unexpected results is that there may be unanticipated and uncharacterized differences between GalT(-/-) mice and WT mice. These studies underscore the need to extensively characterize phenotypes in KO mice and indicate that the relationship between genotype and the natural immune repertoire can be complex.

Human xenospecific T suppressor cells inhibit T helper cell proliferation to porcine aortic endothelial cells, and NF-kappaB activity in porcine APC

Human T suppressor cells (Ts), capable of preventing autologous T helper cells (Th) from reacting against xenogeneic pig endothelial cells and pig APC can be generated in vitro. Ts derive from a population of CD3(+)CD8(+)CD28(-) T lymphocytes and specifically recognize the MHC class I antigens of the APC used for in vitro immunization. To study the mechanism that underlies suppression, we investigated whether Ts inhibit the expression of costimulatory molecules in xenogeneic professional and semiprofessional APC. We found that Ts down-regulate Th-induced expression of CD86 in pig APC, and that this effect occurs at the level of transcription, as indicated by nuclear run-on and Northern blot assays. EMSA results revealed that inhibition of CD86 expression is mediated by inactivation of transcription factor NF-kappaB. Furthermore, transfection of pig APC with a vector expressing NF-kappaB p65 partially rescued Th-induced expression of the CD86 molecule. These results strongly support the concept that xenospecific Ts inhibit the APC function of xenogeneic cells by preventing activation of NF-kappaB.

Prospects for xenotransplantation of the liver

A severe shortage of human livers for allotransplantation has sparked interest in the potential use of animals in lieu of humans as a source of livers, that is xenotransplantation. Xenotransplantation might also provide a means by which recurrence of hepatitis might be averted. Among the types of xenografts that might be undertaken are extracorporeal "xenoperfusion" or perfusion of devices containing xenogeneic hepatocytes, auxiliary liver transplants, bridge liver transplants, and hepatocyte transplants. The hurdles to xenotransplantation of the liver include the immune response of the recipient against the graft, incompatibility of the graft with complex physiologic and biochemical systems of the recipient, and the possibility of transferring infectious agent from the graft to the recipient. Recent progress in characterizing and overcoming these hurdles has encouraged some optimism regarding the ultimate application of xenotransplantation for the treatment of human disease.

Biophysical characteristics of anti-Gal(alpha)1-3Gal IgM binding to cell surfaces: implications for xenotransplantation

BACKGROUND

Natural antibodies directed against cell surface carbohydrates are thought to be vital to host defense and to initiate the rejection of xenografts and ABO-incompatible allografts. The biophysical properties underlying the association and dissociation of these antibodies from cell surfaces is incompletely understood. We investigated those properties for the binding of Galalpha1-3Gal antibodies to porcine endothelial cell surfaces, because such interactions might be relevant to the clinical application of xenotransplantation.

RESULTS AND CONCLUSIONS

The initial rate of binding of anti-Galalpha1-3Gal antibodies to endothelial cells was found to depend on antibody concentration, antibody diffusion, and antigen concentration. The presence of an intact glycocalyx had a greater impact on antibody binding than mobility of antigen in cell membranes. Disruption of glycocalyx increased the amount of antibody bound at equilibrium by more than 50%. Although the binding of anti-Galalpha1-3Gal antibodies to cell surfaces could be inhibited by soluble Galalpha1-3Gal, once bound, some anti-Galalpha1-3Gal could not be dissociated by competitive inhibitors of binding or by denaturation of the bound Ig with chaotropic reagents, but could be dissociated by reduction of disulfide bonds, suggesting that attachment to cell surfaces was, at least in part, by means other than specific reaction with the epitope.

Immune complex formation after xenotransplantation : evidence of type III as well as type II immune reactions provide clues to pathophysiology

Rejection of renal and cardiac xenografts is initiated when natural antibodies of the recipient bind to donor endothelium, activating complement on the surface of endothelial cells. Pulmonary xenotransplants, however, reveal less evidence of antibody binding and complement activation and, in contrast to other xenografts, fare worse when the complement of the graft recipient is depleted. Accordingly, we asked whether distinct immunochemical reactions might occur after xenotransplantation of the lung and what implications such reactions might have for pulmonary pathophysiology. Analysis of serum from baboons after transplantation with porcine lungs revealed complexes containing baboon IgM and porcine von Willebrand factor. The baboon IgM in these complexes was specific for Galalpha1-3Gal. Immune complexes were also seen, albeit to a lesser extent, in the serum of kidney and heart xenotransplant recipients. Deposits of porcine von Willebrand factor and baboon C3 were detected in livers and spleens of transplanted baboons. These results indicate pulmonary xenotransplantation eventuates in formation of immune complexes and in the deposition of those complexes at distant sites. Immune complex formation could explain the peculiar fate of xenoreactive antibodies after pulmonary xenotransplantation and might contribute to the pathophysiology of the lung and systemic changes not previously considered a complication of xenotransplantation.

Antigen expression in xenotransplantation: how low must it go?

BACKGROUND

Acute vascular rejection (AVR) is an important immunological barrier to xenotransplantation. Thought to be initiated by xenoreactive antibodies, acute vascular rejection might, in principle, be avoided by engineering animals to express low levels of antigen. The extent to which antigen expression would have to be decreased to achieve such a goal is unknown.

METHODS

We estimated the decrease in expression of a xenogeneic antigen, Galalpha1-3Gal, which might be needed to avert acute vascular rejection of xenotransplants based on the decrease in antibody binding to endothelium that would prevent tissue damage.

RESULTS

The level of decrease needed in Galalpha1-3Gal expression needed to avoid acute vascular rejection was estimated to exceed 96% of baseline. The extent of the decrease needed reflected, in part, a substantial "excess" of Galalpha1-3Gal on porcine endothelial cell surfaces.

CONCLUSIONS

Although the change in antigen expression required to avoid acute vascular rejection might be conditioned by various factors, the very large magnitude of this change necessitates application of highly efficient approaches to antigen modification.

A human CD46 transgenic pig model system for the study of discordant xenotransplantation

BACKGROUND

The chronic shortage in the supply of human organs available for allotransplantation has turned attention toward the use of animals as potential donors, with pigs as the most likely species under consideration. Hyperacute rejection, the initial and immediate barrier to a pig-to-primate xenograft, has been addressed by generation of transgenic pigs that express the human membrane-bound complement-regulatory proteins CD59 and/or CD55. Difficulty has been encountered in generation of transgenic animals that express a third membrane-bound complement-regulatory protein, CD46.

METHODS

We have generated transgenic animals by using a large genomic construct that encompasses the entire human CD46 gene.

RESULTS

We report the first description of transgenic mice and pigs that express high levels of human CD46 in a cell and tissue type-specific manner, resembling patterns of endogenous CD46 expression observed in human tissues. Furthermore, when human CD46 transgenic porcine hearts were transplanted into baboons, the grafts did not succumb to hyperacute rejection, and survival extended for up to 23 days. Under the same conditions, nontransgenic grafts underwent hyperacute rejection within 90 min.

CONCLUSIONS

This is the first report to describe generation of transgenic pigs that express human CD46, and the first in vivo demonstration of the ability of human CD46 expressed on pig organs to regulate complement activation and overcome hyperacute rejection upon transplantation of a vascularized organ into nonhuman primates.

The role of anti-Galalpha1-3Gal antibodies in acute vascular rejection and accommodation of xenografts

BACKGROUND

A major impediment to the transplanting of porcine organs into humans is the susceptibility of porcine organs to acute vascular rejection, which can destroy a vascularized xenograft over a period of hours to days. Acute vascular rejection of porcine-to-primate xenografts is thought to be triggered by binding of xenoreactive antibodies to the graft. We tested whether antibodies, binding to Galalpha1-3Gal epitopes in porcine tissue, initiate this phenomenon.

METHODS AND RESULTS

Specific depletion of anti-Galalpha1-3Gal antibodies from the blood of baboons, using extracorporeal perfusion of separated plasma through columns of Sepharose beads covalently linked to the antigenic trisaccharide, Galalpha1-3Galbeta1-4GlcAc, averted the development of acute vascular rejection in porcine organs transgenic for human decay-accelerating factor and CD59. More importantly, after immunodepletion was stopped and Gala1-3Gal antibodies were allowed to return, these same organs continued to function and remained pathologically normal and thus seemed to achieve a state of accommodation.

CONCLUSION

These results demonstrate that anti-Galalpha1-3Gal antibodies cause acute vascular rejection and suggest that depletion of these antibodies leads to accommodation of the donor cardiac xenograft and could supply an important model for additional study.

Natural anti-carbohydrate IgM in mice: dependence on age and strain

Natural anti-carbohydrate antibodies in humans play a key role in natural immunity and in recognition of allogeneic and xenogeneic antigens. Presumably, natural anti-carbohydrate antibodies in mice have similar functions; but these antibodies have not been extensively characterized. An assay was developed and used to screen for anti-carbohydrate IgM in the serum of BDF-1 mice. Among the natural anti-carbohydrate IgM identified, anti-betaGlcNAc IgM were the most abundant. Anti-betaGlcNAc IgG was not detected. Levels of anti-betaGlcNAc IgM were very low in 3-week-old BDF-1 mice and increased until 5 to 7 months of age. Levels of serum anti-betaGlcNAc IgM similar to those in BDF-1 mice were found in the serum of some strains related to the BDF-1 strain (DBA and C57BL/6) and in BDF mice lacking the galactosyl transferase gene. However, in two strains unrelated to the BDF-1 strain (FVB and SJL), levels of anti-betaGlcNAc IgM were less than one-tenth of those found in BDF-1 mice. These results provide considerable insight into the effect of age on the production of natural anti-carbohydrate antibodies in mice and indicate that production of those antibodies is strongly dependent on the strain of mouse. These studies will help in future development of murine models for studying the biological and medical roles of natural anti-carbohydrate antibodies.

Phenotypic and functional maturation of dendritic cells mediated by heparan sulfate

Primary immune responses are thought to be induced by dendritic cells. To promote such responses, dendritic cells must be activated by exogenous agonists, such as LPS, or by products of activated leukocytes, such as TNF-alpha and IL-1. How dendritic cells might be activated in the absence of exogenous stimuli, or without the immediate presence of activated leukocytes, as might occur in immunity to tumor cells or transplants, is unknown. We postulated that heparan sulfate, an acidic, biologically active polysaccharide associated with cell membranes and extracellular matrices, which is rapidly released under conditions of inflammation and tissue damage, might provide such a stimulus. Incubation of immature murine dendritic cells with heparan sulfate induced phenotypic maturation evidenced by up-regulation of I-A, CD40, CD54 (ICAM-1), CD80 (B7-1), and CD86 (B7-2). Dendritic cells exposed to heparan sulfate exhibited a markedly lowered rate of Ag uptake and increased allostimulatory capacity. Stimulation of dendritic cells with heparan sulfate induced release of TNF-alpha, IL-1beta, and IL-6, although the maturation of dendritic cells was independent of these cytokines. These results suggest that soluble heparan sulfate chains, as products of the degradation of heparan sulfate proteoglycan, might induce maturation of dendritic cells without exogenous stimuli, thus contributing to the generation and maintenance of primary immune responses.

Xenotransplantation of the liver

Xenotransplantation of the liver, in its broadest conception, might involve the transplantation of an intact organ or xenogeneic hepatocytes, or the use of an intact xenogeneic liver or cells as an ex vivo "device." The indications for xenotransplantation include not only hepatic failure but also, potentially, the treatment of metabolic diseases. The hurdles to xenotransplantation include immune, physiologic, and infectious complications. New information and progress in experimental systems are bringing xenotransplantation closer to clinical application.