Distribution of the alphaGal- and the non-alphaGal T-antigens in the pig kidney: potential targets for rejection in pig-to-man xenotransplantation

Developments in kidney xenotransplantation

The search for kidney xenografts that are appropriate for patients with end-stage renal disease has been ongoing since the beginning of the last century. The major cause of xenograft loss is hyperacute and acute rejection, and this has almost been overcome via scientific progress. The success of two pre-clinical trials of α1,3-galactosyltransferase gene-knockout porcine kidneys in brain-dead patients in 2021 triggered research enthusiasm for kidney xenotransplantation. This minireview summarizes key issues from an immunological perspective: the discovery of key xenoantigens, investigations into key co-stimulatory signal inhibition, gene-editing technology, and immune tolerance induction. Further developments in immunology, particularly immunometabolism, might help promote the long-term outcomes of kidney xenografts.

Proteomic analysis of tissue from α1,3-galactosyltransferase knockout mice reveals that a wide variety of proteins and protein fragments change expression level

A barrier in a pig-to-man xenotransplantation is that the Galα1-3Galβ1-4GlcNAc-R carbohydrate (α-Gal epitope) expressed on pig endothelial cells reacts with naturally occurring antibodies in the recipient’s blood leading to rejection. Deletion of the α1,3-galactosyltransferase gene prevents the synthesis of the α-Gal epitope. Therefore, knockout models of the α1,3-galactosyltransferase gene are widely used to study xenotransplantation. We have performed proteomic studies on liver and pancreas tissues from wild type and α1,3-galactosyltransferase gene knockout mice. The tissues were analyzed by two-dimensional polyacrylamide gel electrophoresis and liquid chromatography - tandem mass spectrometry. The analyses revealed that a wide variety of proteins and protein fragments are differentially expressed suggesting that knockout of the α1,3-galactosyltransferase gene affects the expression of several other genes.

Hormone supplying renal cell sheet in vivo produced by tissue engineering technology

Regenerative medicine is a new medical field and is expected to have a profoundly positive effect in curing difficult-to-treat diseases. Cell sheet fabrication is an important tissue engineering technology used in regenerative medicine. This study investigated the creation of a hormone-releasing tissue using cell sheet technology, which could be utilized in future therapy for chronic renal disease. Renal cell sheets were fabricated on a temperature-responsive cell culture surface with primary renal cells from adult porcine kidney. These sheets contained various kinds of renal cells that showed cyst-like formation. An important renal function is the synthesis of 1,25-dihydroxyvitamin D3, and this was confirmed in the cell sheets in vitro. Erythropoietin (EPO) production is another important renal function. This ability was also observed in the renal cell sheets in vitro, and then again after transplantation in a nude rat. In particular, the relative expression of EPO mRNA increased more under cell sheet culture conditions compared with exponential cell growth conditions. Histological analysis of the implanted renal cell sheets showed them to be Dolichos biflorus agglutinin-positive and to have regenerated renal tubular-like morphology. These results indicated that both functional and morphological regenerative renal tissues were fabricated by cell sheet technology. This study introduces a hormone-supplying treatment for renal dysfunctional diseases using engineered renal tissues. Moreover, since our renal cell sheets developed renal tubular-like structures in vivo, it holds promise for fabricating artificially engineered true renal tissue in the future.

Expression of the carcinoma markers: the sialylated Lewis A and X carbohydrate antigens in normal laryngeal surface epithelium and submucosal glands from old humans

Aberrant surface expression of the carbohydrate ABH and Lewis antigens are often used as markers for the diagnosis of cancer, but while the distribution of these histo-blood group antigens is relatively well-described in tissues and organs from young and middle-aged humans little is known of their expression in old age. The objective for this study was to estimate if the Lewis A and X antigens together with their sialylated modifications, are expressed in sections of normal laryngeal tissue from old humans. Antibodies directed against the tumor markers Sialyl Lewis A and Sialyl Lewis X showed positive reaction in the surface epithelia from normal larynx autopsies obtained from people aged 77-90 years. The sialylated and non-sialylated Lewis A antigens were more frequently expressed in the pseudostratified epithelium than in squamous surface epithelium. Both the sialylated and the non-sialylated carbohydrates were stained in the submucosal glands in all the autopsies. In conclusion, visualization of Lewis tumor markers in the larynx should be interpreted with great care, as they may be present in normal laryngeal epithelial cells from old humans.

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.

Inflammatory responses of tissue-engineered xenografts in a clinical scenario

Acellular tissue-engineered (ATE) xenografts and homografts are used in clinical cardiovascular surgery. The present study examined the specific role of carbohydrate antigen (α-Gal and T-antigen) in immune response after decellularisation in tissue-engineered xenografts (porcine pulmonary artery and bovine jugular vein). An enzyme-linked immunosorbent assay (ELISA) was used to ascertain whether implantation of bioprostheses, ATE xenografts and mechanical valve replacement result in augmentation of anti-α-Gal IgM antibodies within eight days of surgery (each group, n=6). Kinetics of host inflammatory response on surgically explanted ATE xenografts was also studied. Immunostaining for α-Gal and T-antigen detected the presence of them in the native tissue but they were absent in processed ATE xenografts from the same tissue. A significant increase in the concentration of anti-α-Gal IgM antibodies was observed in the serum of bioprosthetic valve recipients as compared to ATE xenograft recipients (P<0.05). Organised collagen, and decreased inflammatory response with increase in endothelisation and vascularisation was evident beyond one year of surgery as compared to early periods in ATE xenografts. This study demonstrates that decellularisation of xenografts and further processing of these tissues enabled reduction of inflammatory stimulus with autologous recellularisation with no calcification.

MUC1 and the simple mucin-type antigens: Tn and Sialyl-Tn are differently expressed in salivary gland acini and ducts from the submandibular gland, the vestibular folds, and the soft palate

OBJECTIVE AND DESIGN

Autopsies of the submandibular gland, the vestibular folds and the soft palate from 65-87 old humans were examined to record the immunohistochemical expression of MUC1 and the simple mucin-type antigens Tn and Sialyl-Tn.

RESULTS

(1) The serous acini in the submucosal glands from the larynx and the soft palate expressed MUC1-associated glycans that were not detectable in the serous acini from the submandibular gland. (2) Virtually all the submucosal acini at oral site of the soft palate are mucous, and in contrast to mucous acini in the vestibular folds and submandibular gland, the palatinal acini in the submucosa underneath the oral mucosa showed a well-defined cytoplasmic reaction with anti-MUC1 antibodies as wells as with anti-Tn. (3) Both the mucous acini and the ducts at the oral site of the soft palate showed reaction for Sialyl-Tn while in the vestibular folds and in the submandibular gland expression for this carbohydrate was observed only in the acini. (4) The staining obtained after incubation with the Tn antibodies showed no cross localization with the staining obtained after incubation with an anti-A blood group antibody. (5) All the autopsies showed reaction in the glands after incubation with the MUC1 antibodies while some autopsies reacted with the anti-Tn antibodies and/or with the anti-Sialyl-Tn antibodies and others did not.

CONCLUSION

The mucin expression in the acini and ducts from the upper human aerodigestive tract strongly depended on the location of the glandular tissue.

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.