The natural anti-Gal antibody

Synthetic Antibody-Rhamnose Cluster Conjugates Show Potent Complement-Dependent Cell Killing by Recruiting Natural Antibodies

Monoclonal antibodies (mAbs) are one of the most rapidly growing drug classes used for the treatment of cancer, infectious and autoimmune diseases. Complement-dependent cytotoxicity (CDC) is one of the effector functions for antibodies to deplete target cells. We report here an efficient chemoenzymatic synthesis of structurally well-defined conjugates of a monoclonal antibody with a rhamnose- and an αGal trisaccharide-cluster to recruit natural anti-rhamnose and anti-αGal antibodies, respectively, to enhance the CDC-dependent targeted cell killing. The synthesis was achieved by using a modular antibody Fc-glycan remodeling method that includes site-specific chemoenzymatic Fc-glycan functionalization and subsequent click conjugation of synthetic rhamnose- and αGal trisaccharide-cluster to provide the respective homogeneous antibody conjugates. Cell-based assays indicated that the antibody-rhamnose cluster conjugates could mediate potent CDC activity for targeted cancer cell killing and showed much more potent efficacy than the antibody-αGal trisaccharide cluster conjugates for CDC effects.

Improved osseointegration using porcine xenograft compared to demineralized bone matrix for the treatment of critical defects in a small animal model

BACKGROUND

Autograft (AG) is the gold standard bone graft due to biocompatibility, osteoconductivity, osteogenicity, and osteoinductivity. Alternatives include allografts and xenografts (XG).

METHODS

We investigated the osseointegration and biocompatibility of a decellularized porcine XG within a critical defect animal model. We hypothesized that the XG will result in superior osseointegration compared to demineralized bone matrix (DBM) and equivalent immune response to AG. Critical defects were created in rat femurs and treated with XG, XG plus bone morphogenetic protein (BMP)-2, DBM, or AG. Interleukin (IL)-2 and IFN-gamma levels (inflammatory markers) were measured from animal blood draws at 1 week and 1 month post-operatively. At 1 month, samples underwent micro-positron-emission tomography (microPET) scans following 18-NaF injection. At 16 weeks, femurs were retrieved and sent for micro-computerized tomography (microCT) scans for blinded grading of osseointegration or were processed for histologic analysis with tartrate resistant acid phosphatase (TRAP) and pentachrome.

RESULTS

Enzyme linked immunosorbent assay testing demonstrated greater IL-2 levels in the XG vs. AG 1 week post-op; which normalized by 28 days post-op. MicroPET scans showed increased uptake within the AG compared to all groups. XG and XG + BMP-2 showed a trend toward increased uptake compared with DBM. MicroCT scans demonstrated increased osseointegration in XG and XG + BMP groups compared to DBM. Pentachrome staining demonstrated angiogenesis and endochondral bone formation. Furthermore, positive TRAP staining in samples from all groups indicated bone remodeling.

CONCLUSIONS

These data suggest that decellularized and oxidized porcine XG is biocompatible and at least equivalent to DBM in the treatment of a critical defect in a rat femur model.

Topical Delivery of Immunosuppression to Prolong Xenogeneic and Allogeneic Split-Thickness Skin Graft Survival

Cadaveric skin allograft is the current standard of treatment for temporary coverage of large burn wounds. Porcine xenografts are viable alternatives but undergo α-1,3-galactose (Gal)-mediated hyperacute rejection and are lost by post-operative day (POD) 3 because of naturally occurring antibodies to Gal in primate recipients. Using baboons, we previously demonstrated that xenografts from GalT-KO swine (lacking Gal) provided wound coverage comparable with allografts with systemic immunosuppression. In this study, we investigate topical immunosuppression as an alternative to prolong xenograft survival. Full-thickness wounds in baboons were created and covered with xenogeneic and allogeneic split-thickness skin grafts (STSGs). Animals were treated with slow-release (TyroSphere-encapsulated) topical formulations (cyclosporine-A [CSA] or Tacrolimus) applied 1) directly to the STSGs only, or 2) additionally to the wound bed before STSG and 1). Topical CSA did not improve either xenograft or allograft survival (median: treated grafts = 12.5 days, control = 14 days; P = 0.27) with similar results when topical Tacrolimus was used. Pretreatment of wound beds resulted in a significant reduction of xenograft survival compared with controls (10 vs 14 days; P = 0.0002), with comparable results observed in allografts. This observation was associated with marked reduction of inflammation on histology with Tacrolimus and not CSA. Prolongation of allograft and xenograft survival after application to full-thickness wound beds was not achieved with the current formulation of topical immunosuppressants. Modulation of inflammation within the wound bed was effective with Tacrolimus pretreatment before STSG application and may serve as a treatment strategy in related fields.

A porcine xenograft-derived bone scaffold is a biocompatible bone graft substitute: An assessment of cytocompatibility and the alpha-Gal epitope

BACKGROUND

Xenografts are an attractive alternative to traditional bone grafts because of the large supply from donors with predictable morphology and biology as well as minimal risk of human disease transmission. Clinical series involving xenograft bone transplantation, most commonly from bovine sources, have reported poor results with frequent graft rejection and failure to integrate with host tissue. Failures have been attributed to residual alpha-Gal epitope in the xenograft which humans produce natural antibody against. To the authors' knowledge, there is currently no xenograft-derived bone graft substitute that has been adopted by orthopedic surgeons for routine clinical use.

METHODS

In the current study, a bone scaffold intended to serve as a bone graft substitute was derived from porcine cancellous bone using a tissue decellularization and chemical oxidation protocol. In vitro cytocompatibility, pathogen clearance, and alpha-Gal quantification tests were used to assess the safety of the bone scaffold intended for human use.

RESULTS

In vitro studies showed the scaffold was free of processing chemicals and biocompatible with mouse and human cell lines. When bacterial and viral pathogens were purposefully added to porcine donor tissue, processing successfully removed these pathogens to comply with sterility assurance levels established by allograft tissue providers. Critically, 98.5% of the alpha-Gal epitope was removed from donor tissue after decellularization as shown by ELISA inhibition assay and immunohistochemical staining.

CONCLUSIONS

The current investigation supports the biologic safety of bone scaffolds derived from porcine donors using a decellularization protocol that meets current sterility assurance standards. The majority of the highly immunogenic xenograft carbohydrate was removed from donor tissue, and these findings support further in vivo investigation of xenograft-derived bone tissue for orthopedic clinical application.

A prophylactic α-Gal-based glycovaccine effectively protects against murine acute Chagas disease

Chagas disease (ChD), caused by the hemoflagellate parasite Trypanosoma cruzi, affects six to seven million people in Latin America. Lately, it has become an emerging public health concern in nonendemic regions such as North America and Europe. There is no prophylactic or therapeutic vaccine as yet, and current chemotherapy is rather toxic and has limited efficacy in the chronic phase of the disease. The parasite surface is heavily coated by glycoproteins such as glycosylphosphatidylinositol (GPI)-anchored mucins (tGPI-mucins), which display highly immunogenic terminal nonreducing α-galactopyranosyl (α-Gal)-containing glycotopes that are entirely absent in humans. The immunodominant tGPI-mucin α-Gal glycotope, the trisaccharide Galα1,3Galβ1,4GlcNAc (Galα3LN), elicits high levels of protective T. cruzi-specific anti-α-Gal antibodies in ChD patients in both the acute and chronic phases. Although glycoconjugates are the major parasite glycocalyx antigens, they remain completely unexplored as potential ChD vaccine candidates. Here we investigate the efficacy of the T. cruzi immunodominant glycotope Galα3LN, covalently linked to a carrier protein (human serum albumin (HSA)), as a prophylactic vaccine candidate in the acute model of ChD, using the α1,3-galactosyltransferase-knockout (α1,3GalT-KO) mouse, which mimics the human immunoresponse to α-Gal glycotopes. Animals vaccinated with Galα3LN-HSA were fully protected against lethal T. cruzi challenge by inducing a strong anti-α-Gal antibody-mediated humoral response. Furthermore, Galα3LN-HSA-vaccinated α1,3GalT-KO mice exhibited significant reduction (91.7–99.9%) in parasite load in all tissues analyzed, cardiac inflammation, myocyte necrosis, and T cell infiltration. This is a proof-of-concept study to demonstrate the efficacy of a prophylactic α-Gal-based glycovaccine for experimental acute Chagas disease.

A vaccine candidate derived from an immunodominant parasitic glycan could offer a much-needed preventive therapy for Chagas disease. The disease, caused by the parasite Trypanosoma cruzi, is endemic to Latin America and an emergent threat to North America and Europe. Current therapies are few, poorly efficacious, and toxic. Igor Almeida, from the University of Texas at El Paso, United States, and his team created a candidate which presents a host with T. cruzi surface-derived α-galactose-containing (α-Gal) glycan covalently linked to a carrier protein. Parasite-derived α-Gal-containing proteins are known to be highly immune-stimulating to humans but were previously unexplored as prophylactics. In a mouse model designed to mimic the human response to Chagas disease, vaccinated animals had a strong antibody response and were fully protected against lethal exposure to T. cruzi. The results offer a promising candidate for future research and validate the method used in this proof-of-concept study.

Synthesis of Galα(1,3)Galβ(1,4)GlcNAcα-, Galβ(1,4)GlcNAcα- and GlcNAc-containing neoglycoproteins and their immunological evaluation in the context of Chagas disease

The protozoan parasite, Trypanosoma cruzi, the etiologic agent of Chagas disease (ChD), has a cell surface covered by immunogenic glycoconjugates. One of the immunodominant glycotopes, the trisaccharide Galα(1,3)Galβ(1,4)GlcNAcα, is expressed on glycosylphosphatidylinositol-anchored mucins of the infective trypomastigote stage of T. cruzi and triggers high levels of protective anti-α-Gal antibodies (Abs) in infected individuals. Here, we have efficiently synthesized the mercaptopropyl glycoside of that glycotope and conjugated it to maleimide-derivatized bovine serum albumin (BSA). Chemiluminescent-enzyme-linked immunosorbent assay revealed that Galα(1,3)Galβ(1,4)GlcNAcα-BSA is recognized by purified anti-α-Gal Abs from chronic ChD patients ∼230-fold more strongly than by anti-α-Gal Abs from sera of healthy individuals (NHS anti-α-Gal). Similarly, the pooled sera of chronic Chagas disease patients (ChHSP) recognized Galα(1,3)Galβ(1,4)GlcNAcα ∼20-fold more strongly than pooled NHS. In contrast, the underlying disaccharide Galβ(1,4)GlcNAcα and the monosaccharide GlcNAcα or GlcNAcβ conjugated to BSA are poorly or not recognized by purified anti-α-Gal Abs or sera from Chagasic patients or healthy individuals. Our results highlight the importance of the terminal Galα moiety for recognition by Ch anti-α-Gal Abs and the lack of Abs against nonself Galβ(1,4)GlcNAcα and GlcNAcα glycotopes. The substantial difference in binding of Ch vs. NHS anti-α-Gal Abs to Galα(1,3)Galβ(1,4)GlcNAcα-BSA suggests that this neoglycoprotein (NGP) might be suitable for experimental vaccination. To this end, the Galα(1,3)Galβ(1,4)GlcNAcα-BSA NGP was then used to immunize α1,3-galactosyltransferase-knockout mice, which produced antibody titers 40-fold higher as compared with pre-immunization titers. Taken together, our results indicate that the synthetic Galα(1,3)Galβ(1,4)GlcNAcα glycotope coupled to a carrier protein could be a potential diagnostic and vaccine candidate for ChD.

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.

“Lost sugars” — reality of their biological and medical applications

The glycan chains attached to cell surfaces or to single proteins are highly dynamic structures with various functions. The glycan chains of mammals and of some microorganisms often terminate in sialic acids or α-1,3-galactose. Although these two sugars are completely distinct, there are several similarities in their biological and medical importance. First, one type of sialic acid, N-glycolylneuraminic acid, and the galactose bound by an α-1,3-linkage to LacNAc, that forms an α-gal epitope, were both eliminated in human evolution, resulting in the production of antibodies to these sugars. Both of these evolutionary events have consequences connected with the consumption of foods of mammalian origin, causing medical complications of varying severity. In terms of ageing, sialic acids prevent the clearance of glycoproteins and circulating blood cells, whereas cryptic α-gal epitopes on senescent red blood cells contribute to their removal from circulation. The efficiency of therapeutic proteins can be increased by sialylation. Another common feature is the connection with microorganisms since sialic acids and α-gal epitopes serve as receptors on host cells and can also be expressed on the surfaces of some microorganisms. Whereas, the sialylation of IgG antibodies may help to treat inflammation, the expression of the α-gal epitope on microbial antigens increases the immunogenicity of the corresponding vaccines. Finally, sialic acids and the α-gal epitope have applications in cancer immunotherapy. N-glycolylneuraminic acid is a powerful target for cancer immunotherapy, and the α-gal epitope increases the efficiency of cancer vaccines. The final section of this article contains a brief overview of the methods for oligosaccharide chain synthesis and the characteristics of sialyltransferases and α-1,3-galactosyltransferase.

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.

In pursuit of xenoreactive antibodies: Where has it gotten us?

Numerous studies have aimed to overcome the barrier to xenotransplantation posed by xenoreactive antibodies and the antigens they recognize. Whether this work will eventually lead to the widespread clinical application of xenotransplantation remains unknown. However, the benefits of this research are already substantial, with research leading to dramatic new developments in fields other than xenotransplantation. Our understanding of natural immunity, particularly the nature and function of natural antibodies, has taken quantum leaps forward, with far-reaching implications. Our improved understanding of the immune response to xenografts has proven invaluable in the characterization of the human immune reaction to commonly used biological therapeutics of xenogeneic origin. Our understanding of cell surface carbohydrates and our ability to modify these carbohydrates in living animals has advanced substantially, with implications for diseases such as cancer and autoimmunity. With this in mind, it is argued that continued work in xenotransplantation is of great value, not only because of the great potential benefits of xenotransplantation, but also because of the more certain benefits that arise from setting our sights on a difficult challenge.

A new kind of carbohydrate array, its use for profiling antiglycan antibodies, and the discovery of a novel human cellulose-binding antibody

In this study, we use a novel glycan array to analyze the glycan-binding antibody repertoire in a pool of affinity-purified IgG collected from a healthy human population. The glycan array used is based on mono- and oligosaccharides covalently linked to the surface via a long linker at their reducing ends. They are thus presented to the medium with a well-defined orientation and are accessible for specific binding by glycan-binding proteins, such as antibodies and lectins. A novel anticellulose antibody was detected that binds specifically to beta4-linked saccharides with a preference for glucopyranose over galactopyranose residues. We also found previously known antiglycan antibodies against mono- and oligosaccharides that are constituents of commonly occurring bacterial polysaccharides. We propose that this array can facilitate high-throughput screening of glycan-binding proteins and the search for biomarkers for personalized medicine.

Human ABO blood group is important in survival and function of porcine working hearts

Pig organs express alphaGal antigen and thus are hyperacutely rejected if perfused by human blood. Human B/A antigens are similar to pig alphaGal antigen, suggesting that the corresponding antibodies may cross-react. Our purpose was to determine if there is a human ABO blood-group difference in porcine-human xenotransplantation. Plasma from six A, five B, seven AB, and six O individuals pooled by blood group were tested in an ex-vivo porcine working heart model. Blood-group A plasma-perfused hearts survived 20 +/- 14 min (n = 5), B 241 +/- 9 min (n = 3), AB 151 +/- 37 min (n = 5), and O 9 +/- 1 min (n = 8). A and O were different (p < 0.001) from B and AB. Function was significantly better in group B. Edema accumulation and creatine kinase change was highest in A and O. All groups had comparable levels of anti-alphaGal antibody, as well as comparable perfusion and operative conditions. Multivariate linear regression analysis showed the anti-B antibody levels to be predictive of survival (p < 0.001). At higher plasma concentrations, hearts perfused with B plasma survived longer (p = 0.01) than AB (218 +/- 45 min, n = 4 vs. 6 +/- 0 min, n = 3). These results suggest a human ABO blood-group difference in porcine-to-human xenotransplantation, which may be mediated by the anti-A and anti-B antibodies.

Human CD59 incorporation into porcine endogenous retrovirus particles: implications for the use of transgenic pigs for xenotransplantation

Transgenic pigs have been engineered to express human CD59 (hCD59) in order to suppress hyperacute rejection of xenotransplants in human recipients. In this study, porcine endogenous retrovirus (PERV) was produced in a porcine cell line expressing hCD59 in order to examine the effect of this complement control protein on PERV neutralization by human sera. hCD59 was found to be incorporated into PERV particles produced from engineered ST-IOWA cells. PERV incorporation of hCD59 resulted in a dramatic inhibition of complement-mediated virolysis by human serum. However, incorporation of hCD59 had no effect on neutralization of PERV by human serum, as measured in infectivity assays. Our results suggest that the use of organs from hCD59 transgenic pigs will inhibit complement-mediated virolysis, but will not compromise the protective effects of human sera on the neutralization of PERV particles.

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.

Exposure to topical bovine thrombin during surgery elicits a response against the xenogeneic carbohydrate galactose alpha1-3galactose

Exposure of humans to topical bovine thrombin has been associated with development of antibodies against bovine and human coagulation factors and blood coagulation abnormalities. However, the nature of this humoral response is unknown. In this study, numerous glycoproteins in the topical bovine thrombin were found to contain the Gal(alpha1)-3Gal epitope, which is known to be highly immunogenic. More importantly, Gal(alpha1)-3Gal is recognized by natural antibodies that are found in all normal individuals and are known to effectively mediate complement activation and subsequent destruction of xenogeneic tissues. Thus, primary exposure of normal individuals to topical bovine thrombin is expected to result in an immediate immune reaction against that reagent. Further, following exposure to topical bovine thrombin, the levels of anti-Gal(alpha1)-3Gal IgG rose to levels tenfold greater than the average level of natural anti-Gal(alpha1)-3Gal IgG in naive individuals. Thus, Gal(alpha1)-3Gal in topical bovine thrombin accounts for, at least in part, the highly immunogenic nature of this reagent.