Anti-Gal antibody-mediated skin graft rejection requires a threshold level of Gal expression

Correlations between the alpha-Gal antigen, antibody response and calcification of cardiac valve bioprostheses: experimental evidence obtained using an alpha-Gal knockout mouse animal model

Introduction

Preformed antibodies against αGal in the human and the presence of αGal antigens on the tissue constituting the commercial bioprosthetic heart valves (BHVs, mainly bovine or porcine pericardium), lead to opsonization of the implanted BHV, leading to deterioration and calcification. Murine subcutaneous implantation of BHVs leaflets has been widely used for testing the efficacy of anti-calcification treatments. Unfortunately, commercial BHVs leaflets implanted into a murine model will not be able to elicit an αGal immune response because such antigen is expressed in the recipient and therefore immunologically tolerated.

Methods

This study evaluates the calcium deposition on commercial BHV using a new humanized murine αGal knockout (KO) animal model. Furtherly, the anti-calcification efficacy of a polyphenol-based treatment was deeply investigated. By using CRISPR/Cas9 approach an αGal KO mouse was created and adopted for the evaluation of the calcific propensity of original and polyphenols treated BHV by subcutaneous implantation. The calcium quantification was carried out by plasma analysis; the immune response evaluation was performed by histology and immunological assays. Anti-αGal antibodies level in KO mice increases at least double after 2 months of implantation of original commercial BHV compared to WT mice, conversely, the polyphenols-based treatment seems to effectively mask the antigen to the KO mice's immune system.

Results

Commercial leaflets explanted after 1 month from KO mice showed a four-time increased calcium deposition than what was observed on that explanted from WT. Polyphenol treatment prevents calcium deposition by over 99% in both KO and WT animals. The implantation of commercial BHV leaflets significantly stimulates the KO mouse immune system resulting in massive production of anti-Gal antibodies and the exacerbation of the αGal-related calcific effect if compared with the WT mouse.

Discussion

The polyphenol-based treatment applied in this investigation showed an unexpected ability to inhibit the recognition of BHV xenoantigens by circulating antibodies almost completely preventing calcific depositions compared to the untreated counterpart.

Attaining threshold antibody cytotoxicity for selective tumor cell destruction: an opinion article

We propose a novel immunotherapeutic paradigm that justifies application of several antibodies to various membrane-associated antigens to achieve a critical threshold density of immune complexes on the surface of cancer cells sufficient for triggering downstream cytolytic pathways. Indeed, some cancer-associated antigens (such as cancer/testis antigens) were found to be expressed on many cancer (but not normal) cells, with their baseline membrane expression levels being originally quite low for some of them, or even further down-regulated due to immune-driven cell selection. To achieve the mandatory threshold density of membrane-associated immune complexes on malignant cells, the concept stipulates combined application of antibodies specific for a cancer-associated antigen along with antibodies against an antigen expressed not only on tumor, but also on normal cells. In the proposed scenario it is of vital importance that the latter antibodies should be applied in suboptimal dosage to exclude the destruction of normal cells devoid of a cancer-associated antigen. Malignant cells often co-express antigens not present concurrently on normal cells at high levels. In such cases, suboptimal dosages of antibodies specific for those antigens could also be applied to achieve cumulative effect leading to selective destruction of tumour cells. Hence, the described immunotherapeutic technology could be used metaphorically speaking as a kind of ‘immunological knife’, which is capable of highly selective destruction of cancer cells without destroying normal cells.

Rat-to-Chinese tree shrew heart transplantation is a novel small animal model to study non-Gal-mediated discordant xenograft humoral rejection

Since α-1,3-galactosyltransferase knockout (GalT-KO) pigs became available, there has been an increasing interest in non-Gal natural antibody (nAb)-mediated xenograft rejection. To better understand mechanisms of non-Gal nAb-mediated rejection, a simple small animal model without gene manipulation would be extremely valuable. Here, we tested whether the Chinese tree shrew (CTS), which is a small-sized mammal that is phylogenetically close to primates, could serve as a model for discordant xenograft rejection.

METHODS

Study 1: Expression of α-Gal antigens in hearts and kidneys of CTSs and rats was assessed by IB4 lectin binding. Presence of anti-Gal and anti-non-Gal IgM and IgG nAb in CTS sera was tested by FACS using Gal+ and GalTKO PBMC as well as BSA-ELISA. Study 2: Rat hearts were transplanted into CTS recipients (group 1, n = 7), and CTS hearts were transplanted in rats [n = 10; seven received no immunosuppression (group 2) and three received FK506 + leflunomide (group 3)].

RESULTS

Study 1: Both CTSs and rats had α-Gal expression in hearts and kidneys. ELISA showed CTSs do not have anti-Gal nAb, and flow cytometry indicated CTSs have anti-non-Gal IgM and IgG nAb in serum. Study 2: Rat hearts in CTSs were uniformly rejected within 35 mins, while CTS hearts in rats continued beating until day 5 without immunosuppression, and up to day 8 with immunosuppression.

CONCLUSION

Rat-to-CTS heart transplantation is a discordant xenotransplant model, CTS-to-Rat heart transplantation is a concordant xenotransplant model. CTSs are valuable small animals to study mechanisms and strategies to avoid non-Gal nAb-mediated xenograft rejection.

Altered glycosylation in donor mice causes rejection of strain-matched skin and heart grafts

Differential protein glycosylation in the donor and recipient can have profound consequences for transplanted organs, as evident in ABO-incompatible transplantation and xenotransplantation. In this study, we investigated the impact of altered fucosylation on graft acceptance by using donor mice overexpressing human α1,2-fucosyltransferase (HTF). Skin and heart grafts from HTF transgenic mice were rapidly rejected by otherwise completely matched recipients (median survival times 16 and 14 days, respectively). HTF skin transplanted onto mice lacking T and B cells induced an natural killer cell-mediated innate rejection crisis that affected 50-95% of the graft at 10-20 days. However, in the absence of adaptive immunity, the residual graft recovered and survived long-term (>100 days). Experiments using "parked" grafts or MHC class II-deficient recipients suggested that indirect rather than direct antigen presentation plays a role in HTF skin graft rejection, although the putative antigen(s) was not identified. We conclude that altered glycosylation patterns on donor tissue can trigger a powerful rejection response comprising both innate and adaptive components. This has potential implications for allotransplantation, in light of increasing recognition of the variability of the human glycome, and for xenotransplantation, where carbohydrate remodeling has been a lynchpin of donor genetic modification.

Gal/non-Gal antigens in pig tissues and human non-Gal antibodies in the GalT-KO era

Our knowledge regarding Gal and non-Gal antigens in GalT-KO pig tissues can be summarized as α3Galactosyl-tranferase gene knock out eliminates the Galα3Galβ4GlcNAc-R antigen expression in pig tissues as well as anti-Gal antibody binding. Other Galα-terminating saccharides (e.g. iGb3 glycolipids and Galα2 determinants) may be present but have not been documented. α3Galactosyl-tranferase gene knock out slightly changes the carbohydrate antigen expression but no "new" antigens recognized by the human immune system have been found. Non-Gal antigens are both of protein and carbohydrate nature but their exact chemical structures are poorly defined. Regarding human non-Gal antibodies our knowledge is as Non-Gal antibodies exist naturally and increase in humans/non-human primate (NHP) receiving WT or GalT-KO pig grafts. Non-Gal antibodies with new antigen epitope recognition can be induced in humans/NHP after challenge by WT or GalT-KO pig grafts. Non-Gal antibodies react with both carbohydrates and proteins. Part of the protein reactivity is directed to glycoprotein carbohydrates chains. Non-Gal antibodies reacting with neuraminic acid terminated saccharides (both N-Acetyl and N-Glycoloyl variants) are present in humans/NHP. Anti-neuraminic acid antibodies are increased, as well as induced, after grafting pig organs into humans/NHP. Non-Gal antibodies does not cause hyperacute xenorejection but can be cytotoxic and cause xenoorgan damage. If humans sensitized to HLA antigens are at a higher risk of rejecting pig xenograft compared with non-sensitized individuals is not fully clarified. Clinical trials are needed to evaluate the relevance of non-Gal antigens/antibodies and for the xenofield to advance.

Prolonged survival of GalT-KO swine skin on baboons

BACKGROUND

Allogeneic skin is currently the best alternative to autologous skin as a temporary treatment for severe burns, but it has several drawbacks. As a potential alternative, we have evaluated GalT-KO swine skin, which lacks expression of the Gal epitope, to investigate the effect of eliminating this epitope on survival of pig-to-baboon skin grafts.

METHODS

Two adult baboons that had fully recovered from previous T cell depletion received simultaneous skin grafts from: (i) GalT-KO swine, (ii) Gal-positive swine, (iii) a third-party baboon, and (iv) self (control skin). Recipients were treated with cyclosporin for 12 days and the survival, gross appearance, and histology of the grafts were compared.

RESULTS

In both baboons, the GalT-KO skin survived longer than either the Gal-positive swine skin or the allogeneic skin. Early rejection of the Gal-positive skin appeared to be mediated by cytotoxic preformed anti-Gal IgM antibodies, while the rejection of GalT-KO skin appeared to result from cellular mechanisms.

CONCLUSIONS

GalT-KO skin may have potential clinical benefits as an alternative to allogeneic skin as a temporary treatment for severe skin injuries.

The anti-nonGal xenoantibody response to alpha1,3-galactosyltransferase gene knockout pig xenografts

PURPOSE OF REVIEW

Anti-nonGal xenoantibodies are a major barrier to the survival of genetically modified porcine xenografts. This review summarizes the contribution of anti-nonGal xenoantibodies to the activation of porcine endothelial cells and graft rejection, and further provides an update on recent advancements in defining the unique features of anti-nonGal xenoantibody structure.

RECENT FINDINGS

Anti-nonGal xenoantibodies pre-exist at low levels in humans and nonhuman primates, and are notably absent in neonates. Exposure of nonhuman primates to alpha1,3-galactosyltransferase gene knockout endothelial cells initiates an induced xenoantibody response that is restricted and encoded by the germline immunoglobulin heavy chain gene IGHV3-21. The target xenoantigen remains undetermined, but several candidate targets have been proposed, including carbohydrate xenoantigens. New advancements in molecular modeling provide insight on the mechanism by which xenoantibodies bind to structurally related carbohydrates.

SUMMARY

Genetic manipulation of porcine donors has significantly prolonged the survival of grafts placed into nonhuman primate recipients, but anti-nonGal xenoantibodies and thrombosis limit the ability of these grafts to function on a long-term basis. Recent developments defining pre-existing anti-nonGal xenoantibody levels, the restriction in the anti-nonGal xenoantibody response and the identification of key sites defining xenoantibody-carbohydrate interactions now provide the information necessary to develop new approaches to preventing xenoantibody-mediated rejection.

Immune responses to alpha1,3 galactosyltransferase knockout pigs

PURPOSE OF REVIEW

To summarize the current knowledge of the immune response generated against xenografts stemming from alpha1,3-galactosyltransferase knockout (GalT-KO) pigs. In particular, we will address the nature of potentially remaining Gal epitopes, the role of non-Gal xenoantigens, and the cellular response directed against GalT-KO tissues.

RECENT FINDINGS

New findings support the view that porcine cells do not express isoglobotrihexosylceramide 3, and GalT-KO pigs, if at all, express negligible levels of Gal. The anti-non-Gal antibody response to GalT-KO cells allowed the identification of several potentially relevant porcine xenoantigens, mainly carbohydrates. Coculture of wildtype pig aortic endothelial cells but not of GalT-KO pig aortic endothelial cells with whole human blood induces the secretion of porcine and human cytokines and the upregulation of E-selectin; in contrast, the transmigration of human leukocytes across porcine endothelium is not regulated by Gal.

SUMMARY

New immunological problems are arising after the elimination of Gal by the generation of GalT-KO pigs; these include non-Gal antibodies and the identification of their elusive antigens, as well as cellular components of the immune system, including neutrophils, macrophages, natural killer cells, and T cells.