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

The five dimensions of B cell tolerance

B cell tolerance has been generally understood to be an acquired property of the immune system that governs antibody specificity in ways that avoid auto-toxicity. As useful as this understanding has proved, it fails to fully explain the existence of auto-reactive specificities in healthy individuals and contribution these may have to health. Mechanisms underlying B cell tolerance are considered to select a clonal repertoire that generates a collection of antibodies that do not bind self, ie tolerance operates more or less in three dimensions that largely spare autologous cells and antigens. Yet, most B lymphocytes in humans and probably in other vertebrates are auto-reactive and absence of these auto-reactive B cells is associated with disease. We suggest that auto-reactivity can be embodied by extending the concept of tolerance by two further dimensions, one of time and circumstance and one that allows healthy cells to actively resist injury. In this novel concept, macromolecular recognition by the B cell receptor leading to deletion, anergy, receptor editing or B cell activation is extended by taking account of the time of development of normal immune responses (4th dimension) and the accommodation (or tolerance) of normal cells to bound antibody, activation of complement, and interaction with inflammatory cells (fifth dimension). We discuss how these dimensions contribute to understanding B cell biology in health or disease.

Cryptic B cell response to renal transplantation

Transplantation reliably evokes allo-specific B cell and T cell responses in mice. Yet, human recipients of kidney transplants with normal function usually exhibit little or no antibody specific for the transplant donor during the early weeks and months after transplantation. Indeed, the absence of antidonor antibodies is taken to reflect effective immunosuppressive therapy and to predict a favorable outcome. Whether the absence of donor-specific antibodies reflects absence of a B cell response to the donor, tolerance to the donor or immunity masked by binding of donor-specific antibodies to the graft is not known. To distinguish between these possibilities, we devised a novel ELISPOT, using cultured donor, recipient and third-party fibroblasts as targets. We enumerated donor-specific antibody-secreting cells in the blood of nine renal allograft recipients with normal kidney function before and after transplantation. Although none of the nine subjects had detectable donor-specific antibodies before or after transplantation, all exhibited increases in the frequency of donor-specific antibody-secreting cells eight weeks after transplantation. The responses were directed against the donor HLA-class I antigens. The increase in frequency of donor-specific antibody-secreting cells after renal transplantation indicates that B cells respond specifically to the transplant donor more often than previously thought.

Novel functions of B cells in transplantation

PURPOSE OF REVIEW

This manuscript reviews current knowledge and recent findings regarding antibody-independent functions of B cells in transplantation.

RECENT FINDINGS

Until recently the functions of B cells in transplantation have been attributed almost entirely to the antibodies they produce. However, the results of recent trials of B-cell-depleting agents for treatment of antibody-mediated rejection as well as auto-immune disease raised awareness that B cells mediate functions independent of antibody synthesis.

SUMMARY

These 'nonclassical' functions place B cells at the center of immune regulation with the power to enhance or inhibit immunity.

Altered functionality of anti-bacterial antibodies in patients with chronic hepatitis C virus infection

Background

Using comparative glycoproteomics, we have previously identified a glycoprotein that is altered in both amount and glycosylation as a function of liver cirrhosis. The altered glycoprotein is an agalactosylated (G0) immunoglobulin G molecule (IgG) that recognizes the heterophilic alpha-gal epitope. Since the alpha gal epitope is found on gut enterobacteria, it has been hypothesized that anti-gal antibodies are generated as a result of increased bacterial exposure in patients with liver disease.

Methods

The N-linked glycosylation of anti-gal IgG molecules from patients with fibrosis and cirrhosis was determined and the effector function of anti-bacterial antibodies from over 100 patients examined. In addition, markers of microbial exposure were determined.

Results

Surprisingly, the subset of agalactosylated anti-gal antibodies described here, was impaired in their ability to mediate complement mediated lysis and inhibited the complement-mediated destruction of common gut bacteria. In an analysis of serum from more than 100 patients with liver disease, we have shown that those with increased levels of this modified anti-gal antibody had increased levels of markers of bacterial exposure.

Conclusions

Anti-gal antibodies in patients with liver cirrhosis were reduced in their ability to mediate complement mediated lysis of target cells. As bacterial infection is a major complication in patients with cirrhosis and bacterial products such as LPS are thought to play a major role in the development and progression of liver fibrosis, this finding has many clinical implications in the etiology, prognosis and treatment of liver disease.

Macrophage phenotype and remodeling outcomes in response to biologic scaffolds with and without a cellular component

Recently, macrophages have been characterized as having an M1 or M2 phenotype based on receptor expression, cytokine and effector molecule production, and function. The effects of macrophage phenotype upon tissue remodeling following the implantation of a biomaterial are largely unknown. The objectives of this study were to determine the effects of a cellular component within an implanted extracellular matrix (ECM) scaffold upon macrophage phenotype, and to determine the relationship between macrophage phenotype and tissue remodeling. Partial-thickness defects in the abdominal wall musculature of Sprague-Dawley rats were repaired with autologous body wall tissue, acellular allogeneic rat body wall ECM, xenogeneic pig urinary bladder tissue, or acellular xenogeneic pig urinary bladder ECM. At 3, 7, 14, and 28 days the host tissue response was characterized using histologic, immunohistochemical, and RT-PCR methods. The acellular test articles were shown to elicit a predominantly M2 type response and resulted in constructive remodeling, while those containing a cellular component, even an autologous cellular component, elicited a predominantly M1 type response and resulted in deposition of dense connective tissue and/or scarring. We conclude that the presence of cellular material within an ECM scaffold modulates the phenotype of the macrophages participating in the host response following implantation, and that the phenotype of the macrophages participating in the host response appears to be related to tissue remodeling outcome.

Noncognate function of B cells in transplantation

How B cells affect the outcome of transplants is a question of enduring interest. Initial efforts to answer that question suggested, wrongly, that B cells have no impact on transplantation. Now, however, B cells are known to influence not only the outcome of vascularized grafts through the production of anti-donor antibodies but also the competence of cellular immunity through a number of physiologic functions. In this study, we explain why the importance of B cells was overlooked in the past and consider the range of noncognate functions of B cells that may determine the outcome of transplants.

Accommodation of grafts: implications for health and disease

Accommodation refers to the acquired resistance of a graft to immune-mediated injury. It is typically observed after antibodies that would cause rejection of a graft are removed from a recipient and then later return. In addition to being induced in this manner, accommodation can occur spontaneously, without depleting antibodies. Indeed, we postulate spontaneous accommodation may be the most common outcome of clinical organ transplantation. The paper reviews the current understanding of accommodation, emphasizing recent advances and important questions. Among the recent advances are the discoveries of potentially broader relevance of accommodation for biology and immunology and pathways by which accommodation may be achieved. To investigate these pathways and to understand how accommodation begins and how it evolves, clinical organ transplants might offer a useful and incisive model.

Allosensitization does not increase the risk of xenoreactivity to alpha1,3-galactosyltransferase gene-knockout miniature swine in patients on transplantation waiting lists

BACKGROUND

The recent availability of alpha1,3-galactosyltransferase knockout (GalT-KO) miniature swine has eliminated anti-Gal antibodies as the major barrier to xenotransplantation, potentially bringing this modality closer to clinical application. Highly-allosensitized patients, who have poor prospects of receiving a suitable cross-match negative human organ, might be the first patients to benefit from xenotransplantation of porcine organs. However, concerns exist regarding cross-reactivity of alloreactive anti-human leukocyte antigen (HLA) antibodies against xenogeneic swine leukocyte antigen (SLA) antigens. We have investigated this question using sera from such patients on GalT-KO target cells.

METHODS

Using flow cytometry and complement-dependent cytotoxicity (CDC) assays, we have tested a panel of 88 human serum samples from patients awaiting cadaveric renal allotransplantation for reactivity against: 1) human; 2) standard miniature swine; and 3) GalT-KO peripheral blood lymphocytes (PBL) and cultured endothelial cells.

RESULTS

Anti-swine IgM and IgG antibody binding, as well as CDC, were significantly attenuated on GalT-KO versus standard swine. No correlation was found between the degree of anti-human panel reactive antibodies (PRA) and xenoreactivity against either standard or GalT-KO miniature swine. Treatment of sera with dithiothreitol (DTT) showed that the majority of remaining lymphocytotoxicity against GalT-KO swine was mediated by preformed IgM antibodies. Patients with high alloreactivity but low anti-GalT-KO xenoreactivity were readily identified.

CONCLUSIONS

Highly allosensitized patients awaiting renal transplants appear to be at no increased risk of xenosensitization over their non-sensitized cohorts, and could therefore be candidates for xenotransplantation using GalT-KO swine donors.

Sensitization with Xenogeneic Tissues Alters the Heavy Chain Repertoire of Human Anti-Galα1–3Gal Antibodies

BACKGROUND

Antigen sensitization alters the use of genes encoding the variable and constant regions of immunoglobulin, changing avidity, and function. Alterations in variable region genes induced by carbohydrate antigens have been studied extensively in animals but are incompletely characterized in humans. We asked how sensitization with the carbohydrate Galalpha1-3Gal modifies antibody heavy chain use.

METHODS

To overcome limited access to B cells, we analyzed anti-Galalpha1-3Gal antibodies from the serum of naïve and sensitized human subjects with anti-sera specific for VH families.

RESULTS

We find that in preimmune subjects, heavy chains of IgM anti-Galalpha1-3Gal derived primarily from VH3 family members, whereas the heavy chains of IgG are from diverse VH families. After sensitization, heavy chains of IgM and IgG antibodies both derived from diverse VH families.

CONCLUSIONS

The preimmune repertoire of IgM antibodies to Galalpha1-3Gal is thus more restricted than the antibody repertoire after sensitization, suggesting an antigen-induced shift in the repertoire.

Fate of alphaGal +/+ pancreatic islet grafts after transplantation into alphaGal knockout mice

BACKGROUND

Important phylogenetic differences between pig and human tissues prevent xenotransplantation from becoming a clinically feasible option. Humans lack the galactose-alpha1,3-galactose (alphaGal) epitope on endothelial cell surfaces and therefore have preformed anti-alphaGal antibodies. The role of these antibodies in rejection of non-vascular xenografts remains controversial. This study investigated the role of anti-alphaGal antibodies in rejection of non-vascularized alphaGal+/+ grafts in alphaGal -/- mice.

METHODS

alphaGal +/+ and alphaGal -/- pancreatic islets were transplanted under the renal capsule of streptozotocin-induced diabetic (1) alphaGal -/- mice and (2) alphaGal +/+ mice. alphaGal -/- recepients were immunized with rabbit red blood cell membranes (RRBCs) to produce elevated anti-alphaGal antibody levels.

RESULTS

Six of the 18 alphaGal -/- mice rejected the alphaGal +/+ grafts within 68 days whereas indefinite graft survival was achieved in the control groups. Animals with surviving islet grafts were challenged with alphaGal +/+ skin grafts. Although all alphaGal +/+ skin grafts were rejected within 58 days, the islet grafts remained intact. This observation correlated with the level of alphaGal expression (which was very low on islets compared to skin) rather than the actual titre of anti-alphaGal antibody.

DISCUSSION

The results suggest that the level of alphaGal expression plays an important role in graft survival. Therefore, its removal is important in the development of a pig islet donor for future clinical therapy.

The footprint of antibody bound to pig cells: evidence of complex surface topology

The disaccharide Gal(alpha)1-3Gal is found on more than 45 different molecules on the endothelium of porcine cells and has recently attracted considerable interest, being the major target recognized by xenoreactive antibodies. In this study, the distribution and topology of Gal(alpha)1-3Gal on porcine endothelial cells was examined to access whether some Gal(alpha)1-3Gal-containing molecules might be preferentially recognized by antibodies binding to Gal(alpha)1-3Gal. Thirteen percent of the Gal(alpha)1-3Gal was found on glycolipid and 87% on glycoproteins. Of all the glycoproteins and glycolipids containing Gal(alpha)1-3Gal, two molecules, fibronectin and the integrin beta1 subunit, were most intensely labeled by galactose oxidase, suggesting that these molecules may be preferentially exposed on the apical surface of the endothelium. Binding of anti-Gal(alpha)1-3Gal antibodies to endothelial cell surfaces significantly diminished labeling of fibronectin and the integrin beta1 subunit by galactose oxidase, indicating that these glycoproteins are targets for the antibodies when binding to intact porcine cells.

Mechanisms of Gal(alpha)1-3Gal(beta)1-4GlcNAc-R (alphaGal) expression on porcine valve endothelial cells

OBJECTIVE

We have previously reported that porcine valve endothelium does not express immunodetectable levels of the carbohydrate Gal(alpha)1-3Galbeta1-4GlcNAc-R (known as alphaGal), suggesting that fresh porcine valve may be immunoprivileged. In this study, we further investigated the mechanisms of alphaGal expression on porcine valve endothelial cells.

METHODS

Primary cultures of porcine valvular endothelial cells were established and compared with porcine aortic endothelial cells and human vein endothelial cells. Immunoblotting, reverse transcriptase-polymerase chain reaction, and flow cytometry were used to compare the expression of alphaGal at both the protein and messenger RNA levels.

RESULTS

Porcine valvular endothelial cells grew rapidly on a gelatin substrate. Similar to our previous in vivo results, valve endothelial cells expressed alphaGal much less intensely than did aortic endothelial cells. Porcine aortic endothelial cells expressed an isolectin B4 (isolectin B4 lectin Bandeiraea simplicifolia) immunodetectable band at 135 kd that was not visible on porcine valve endothelial cells or on human vein endothelial cells. Reverse transcriptase-polymerase chain reaction documented three transcripts of the alphaGal gene that were identically expressed on porcine valve and aortic endothelial cells. Furthermore, flow cytometry showed an almost identical surface profile between porcine aortic and valve endothelial cells, in contrast with human vein endothelial cells.

CONCLUSIONS

Cultures of primary valve endothelial cells were established and exhibited similar phenotypic patterns in vitro to those we have previously documented in vivo. RNA and flow cytometric analyses documented no difference between the RNA expression and surface protein profile for alphaGal, although whole-cell extracts demonstrated an immunodetectable band on Western blotting that was present on aortic endothelial cells but not on valve endothelial cells. These findings clarify the mechanism of expression of alpha1,3galactosyltransferase gene expression in valve endothelial cells, suggesting that delayed rejection of fresh porcine cardiac valves may occur.