Decreased agglutinability of methoxy-polyethylene glycol attached red blood cells: significance as a blood substitute

Evaluation of Canine Erythrocyte Surface Antigens and Morphological Alterations Induced by Trypsin Treatment

Dogs have multiple blood type antigens, among which DEA 1, DEA 4, and Dal can induce severe acute hemolytic transfusion reactions. Various antigen modulation techniques have been developed to reduce immunogenicity and transfusion reactions. Recently, trypsin has been suggested as a potential tool for modulating the antigenicity of DEA 1 in veterinary medicine. Following this rationale, this study aims to evaluate the effects of trypsin on the antigenicity of these three antigens. A 50% RBC suspension treated with 1 mg/mL trypsin was incubated at 37 °C for 120 min. The antigenicity of DEA 1, DEA 4, and Dal was assessed using blood typing assays before and after trypsin treatment. As a result, trypsin did not reduce the antigenicity of DEA 1 and DEA 4; instead, trypsin significantly increased their antigenicity (p = 0.008) and promoted agglutination, whereas Dal exhibited a significant reduction in antigenicity (p = 0.008). Quantitative morphological parameters obtained from an automated hematology analyzer revealed no significant differences between trypsin-treated and negative control groups. However, morphological scoring under an optical microscope showed significantly fewer echinocytes in the trypsin-treated group (p = 0.008). Consequently, broad-spectrum proteases like trypsin are unsuitable for universal blood production due to their variable effects on erythrocyte surface antigens.

Enhancement of red blood cell transfusion compatibility using CRISPR-mediated erythroblast gene editing

Regular blood transfusion is the cornerstone of care for patients with red blood cell (RBC) disorders such as thalassaemia or sickle‐cell disease. With repeated transfusion, alloimmunisation often occurs due to incompatibility at the level of minor blood group antigens. We use CRISPR‐mediated genome editing of an immortalised human erythroblast cell line (BEL‐A) to generate multiple enucleation competent cell lines deficient in individual blood groups. Edits are combined to generate a single cell line deficient in multiple antigens responsible for the most common transfusion incompatibilities: ABO (Bombay phenotype), Rh (Rh_null), Kell (K₀), Duffy (Fy_null), GPB (S−s−U−). These cells can be differentiated to generate deformable reticulocytes, illustrating the capacity for coexistence of multiple rare blood group antigen null phenotypes. This study provides the first proof‐of‐principle demonstration of combinatorial CRISPR‐mediated blood group gene editing to generate customisable or multi‐compatible RBCs for diagnostic reagents or recipients with complicated matching requirements.

Red Cell Immunohematology Research Conducted in China

ABO subtypes and RhD variants are the most studied blood groups in China. Some of the polymorphisms in these two blood groups have direct clinical relevance. Molecular diagnosis of blood group polymorphisms is underway in China. In addition, research groups have developed methods such as screening for blood group mimetic peptides using phage display technology. New reagents, akin to antibodies directed against RhD and ABO, are being investigated using aptamer-based techniques. Progress is also being made in the development of synthetic exoglycosidases for conversion of group A and/or B antigens to group O. Development of methoxy-polyethylene-glycol modified red cells has been successful in vitro but has not reached clinical application. In this paper, we summarize red cell immunohematology research that has been conducted in China.

ABO research in the modern era of genomics

Research on ABO has advanced significantly in recent years. A database was established to manage the sequence information of an increasing number of novel alleles. Genome sequencings have identified ABO orthologues and paralogues in various organisms and enhanced the knowledge on the evolution of the ABO and related genes. The most prominent advancements include clarification of the association between ABO and different disease processes. For instance, ABO status affects the infectivity of certain strains of Helicobacter pylori and Noroviruses as well as the sequestration and rosetting of red blood cells infected with Plasmodium falciparum. Genome-wide association studies have conclusively linked the ABO locus to pancreatic cancer, venous thromboembolism, and myocardial infarction in the presence of coronary atherosclerosis. These findings suggest ABO's important role in determining an individual's susceptibility to such diseases. Furthermore, our understanding of the structures of A and B transferases and their enzymology has been dramatically improved. ABO has also become a research subject in neurobiology and the preparation of artificial/universal blood and became a topic in the pseudoscience of "blood type diets." With such new progress, it has become evident that ABO is a critical player in the modern era of genomic medicine. This article provides the most up-to-date information regarding ABO genomics.

Bioengineered skin allografts: a new method to prevent humoral response

We previously reported that a bioengineered interface consisting of a nano-barrier membrane (NB-LVF4) used as an artificial interface between skin allografts and wound surfaces significantly prolonged graft survival without immunosuppression. We now evaluated whether NB-LVF4 could serve as a targeted drug delivery system to further improve outcomes. Fibroblast growth factor-1 (FGF-1) was selected for its known function as a wound hormone. Full-thickness 8-mm skin grafts were cross-transplanted between out-bred mice. Controls were transplanted without treatment. In test groups, the NB-LVF4, with or without FGF-1, was applied to both basolateral skin and wound surfaces with polymerization resulting in a tridimensional transparent membrane. The mice were evaluated for T-cell activation and development of donor-specific antibody. Rejection occurred in controls by 7 days. NB-LVF4 treatment, with or without FGF-1, was found to significantly prolong allograft survival (27 and 28 days, respectively [p < 0.05]). Untreated controls stimulated 10-fold shift in the number of circulating CD4+ cells while test groups exhibited substantially reduced shifts in CD4+ cells. The group treated with FGF-1 did not develop donor-specific antibody. Treatment with the NB-LVF4 membrane delays the onset of allograft rejection in the absence of systemic immunosuppression. FGF-1 appears to prevent the development of a humoral response by preventing B cell activation.

Pretransplant kidney-specific treatment to eliminate the need for systemic immunosuppression

BACKGROUND

Despite significant side effects, chronic systemic immunosuppression remains the backbone of clinical transplantation. We investigated the feasibility of preventing early allorecognition in canine renal allografts using a nonsystemic pretreatment.

METHODS

The renal vasculature was treated with a bioengineered interface consisting of a nano-barrier membrane during 3 hr of ex vivo warm perfusion.

RESULTS

Preliminary feasibility of the immunocloaking technology was established by the following criteria: it is possible to achieve approximately 90% coverage of the vasculature with nano-barrier membrane after 3 hr of ex vivo warm perfusion; covering the luminal surfaces prevents allorecognition as determined by mixed lymphocyte-vascular endothelial reaction; covering the luminal surfaces does not negatively affect renal function as determined by autotransplant outcomes; and graft rejection is significantly postponed in canine kidneys treated with the immunocloaking technology. In the absence of systemic immunosuppression, untreated control dogs experienced a mean onset of rejection on day 6, whereas in the treated dogs with modified renal vascular luminal surfaces, the mean onset of rejection was significantly delayed until day 30.

CONCLUSIONS

The ability to postpone, or eventually eliminate, the allorecognition that occurs immediately on reperfusion could provide a new window of opportunity to introduce adjunct therapies to support tolerance induction. To our knowledge, this is the first time significantly prolonged canine renal allograft survival has been achieved in the absence of systemic immunosuppression or immunologic manipulation of the recipient.

Enhanced cell surface polymer grafting in concentrated and nonreactive aqueous polymer solutions

Macromolecular cell surface modification techniques have shown tremendous utility in various biomedical applications. However, a major drawback concerns inefficient cell surface modification caused by the poor association of hydrophilic macromolecules with cell surfaces. Here, a novel, highly efficient, and universal strategy in which nonreactive "additive" macromolecules are used to modulate the grafting efficiency of cell surface reactive, hydrophilic macromolecules is described. Unprecedented enhanced cell surface modifications by up to 10-fold were observed when various concentrations of a suitable "additive" polymer was present with a constant and low concentration of a "reactive" macromolecule. The importance of this increased efficiency and the possible mechanisms involved are discussed. The cell compatible technique is demonstrated in the case of four different cell types--red blood cells (RBC), leukocytes, platelets, and Jurkat cells. A practical application of grafting macromolecules to cell surfaces in concentrated polymer solutions is demonstrated by the enhanced camouflage of RBC surface antigens for the development of RhD null RBC. In principle, the technique can be adapted to various macromolecular systems and cell types, with significant potential for biomedical applications such as live cell based technologies.

Drug delivery by red blood cells: vascular carriers designed by mother nature

IMPORTANCE OF THE FIELD

Vascular delivery of several classes of therapeutic agents may benefit from carriage by red blood cells (RBC), for example, drugs that require delivery into phagocytic cells and those that must act within the vascular lumen. The fact that several protocols of infusion of RBC-encapsulated drugs are now being explored in patients illustrates a high biomedical importance for the field. AREAS COVERED BY THIS REVIEW: Two strategies for RBC drug delivery are discussed: encapsulation into isolated RBC ex vivo followed by infusion in compatible recipients and coupling therapeutics to the surface of RBC. Studies of pharmacokinetics and effects in animal models and in human studies of diverse therapeutic enzymes, antibiotics and other drugs encapsulated in RBC are described and critically analyzed. Coupling to RBC surface of compounds regulating immune response and complement, affinity ligands, polyethylene glycol alleviating immune response to donor RBC and fibrinolytic plasminogen activators are described. Also described is a new, translation-prone approach for RBC drug delivery by injection of therapeutics conjugated with fragments of antibodies providing safe anchoring of cargoes to circulating RBC, without need for ex vivo modification and infusion of RBC.

WHAT THE READER WILL GAIN

Readers will gain historical perspective, current status, challenges and perspectives of medical applications of RBC for drug delivery.

TAKE HOME MESSAGE

RBC represent naturally designed carriers for intravascular drug delivery, characterized by unique longevity in the bloodstream, biocompatibility and safe physiological mechanisms for metabolism. New approaches for encapsulating drugs into RBC and coupling to RBC surface provide promising avenues for safe and widely useful improvement of drug delivery in the vascular system.

PEGylated Adenoviruses: From Mice to Monkeys

Covalent modification with polyethylene glycol (PEG), a non-toxic polymer used in food, cosmetic and pharmaceutical preparations for over 60 years, can profoundly influence the pharmacokinetic, pharmacologic and toxciologic profile of protein and peptide-based therapeutics. This review summarizes the history of PEGylation and PEG chemistry and highlights the value of this technology in the context of the design and development of recombinant viruses for gene transfer, vaccination and diagnostic purposes. Specific emphasis is placed on the application of this technology to the adenovirus, the most potent viral vector with the most highly characterized toxicity profile to date, in several animal models.

Silencing red blood cell recognition toward Anti-A antibody by means of polyelectrolyte layer-by-layer assembly in a two-dimensional model system

Silencing the antigenic response of red blood cells (RBCs) is a prerequisite toward the development of universal blood transfusion. Using a two-dimensional (2D) model whereby nonfixed RBCs are adsorbed on a human fibronectin (HFN)-coated surface, we demonstrate that the layer-by-layer (LbL) assembly technique of biocompatible polyelectrolytes can be employed to achieve the immunocamouflage of RBCs against the Anti-A antibody while maintaining the integrity and viability of the cells. The multilayered film consisted of a protecting shell (P-shell), containing five bilayers of chitosan-graft-phosphorylcholine (CH-PC) and sodium hyaluronate (HA), covered by a camouflage shell (C-shell) made up of five bilayers of poly-(L-lysine)-graft-poly(ethylene glycol) (PLL-PEG) and alginate (AL). Control experiments in which RBCs were coated by (CH-PC/HA)(10) bilayers indicated that the two polyelectrolytes alone did not prevent immunorecognition. The LbL film formation on RBCs and model substrates was monitored by quartz crystal microbalance with dissipation factor (QCM-D) and analyzed through zeta-potential measurements, atomic force microscopy (AFM), and optical microscopy. Antibody interaction with the coated RBCs was investigated by QCM-D, fluorescence microscopy, and hemolysis assays. Results from these measurements demonstrated that the hybrid LbL system built-up with different sets of polyelectrolytes was able to protect the RBCs from hemolysis and recognition by the Anti-A antibody.

Comparative Analysis of Polymer and Linker Chemistries on the Efficacy of Immunocamouflage of Murine Leukocytes

Membrane grafting of methoxypoly(ethylene glycol) [mPEG] to allogeneic leukocytes attenuates allorecognition and significantly reduces the risk of graft-versus-host disease in mice. To optimize the immunological efficacy of polymer grafting, murine splenocytes were modified using three differing linker chemistries: CmPEG (5 kDa), BTCmPEG (5 and 20 kDa) and TmPEG (5 kDa). In vitro immunocamouflage efficacy was examined by flow cytometic analysis of leukocyte markers and mixed lymphocyte reactions (MLR). In contrast to CmPEG and BTCmPEG, TmPEG exerted significant cellular toxicity. Flow cytometric analysis demonstrated that both CmPEG and BTCmPEG were highly effective at camouflaging cell surface markers while TmPEG was ineffective. Furthermore, CmPEG and BTCmPEG dramatically blocked MLR allorecognition and cellular proliferation. Polymer length was the most critical factor in the immunocamouflage of cells with the BTCmPEG 20 kDa being the most effective. In contrast to other immunomodulatory approaches, immunocamouflage of leukocytes yields a multivalent effect globally interfering with attachment, allorecognition, presentation and costimulation pathways.

Effects of pegylated hamster red blood cells on microcirculation

The objective of this study was to examine the effects of polyethylene glycol (PEG) treated red blood cells (RBCs) on the microcirculation in a hamster back skin window chamber model. Donor hamster RBCs were PEGylated through an incubation with an activated PEG solution, washed, resuspended, and infused through a 10% volume top loading procedure into the carotid artery in an awake Syrian Golden hamster. Eight hamster groups were treated with activated PEG different sizes and concentrations: 0.05 mM-5 kDa PEG, 0.5 mM-5 kDa PEG, 1.1 mM-5 kDa PEG, 2.2 mM-5 kDa PEG, 22 mM-5 kDa PEG, 0.05 mM-20 kDa PEG, 0.5 mM-20 kDa PEG, and 5 mM-20 kDa PEG. Non-treated RBCs were used as control. The microvascular bed under observation was videotaped 30 min before the infusion and followed for 30 min post infusion. The diameter of individual blood vessels and blood flow velocities in selected vessels was measured. Hematocrit and hemoglobin concentration were recorded before infusion and at the end of experiment. Tissue pO(2) was also monitored. Results showed the hamsters tolerated the PEGylated RBCs without apparent ill effects. No significant changes were recorded for the hematocrit, the hemoglobin concentration, the blood vessel diameters, blood flow velocities, and the interstitial partial oxygen pressure (pO(2)) before, during, and after the injections of PEG-RBCs (P > 0.05). Unlike most hemoglobin-based oxygen carrying compounds, which can cause vasoconstriction, the PEGylated RBCs did not produce any measurable vasoactivity. Together with the absence of rouleaux formation and the fact that PEG molecules can mask the surface antigens on RBCs, PEGylation appeared promising as a circulation enhancement treatment.

Modulating the red cell membrane to produce universal/stealth donor red cells suitable for transfusion

Two approaches have been used to produce universal group O donor red blood cells (RBCs) from groups A, B, and AB RBCs. The first involves cleavage of the terminal immunodominant sugars from carbohydrate chains on the RBC membrane, using specific enzymes, to produce so-called enzyme-converted group O (ECO) RBCs. ECO RBCs have been produced from whole units of B RBCs and transfused successfully to humans. Group A RBCs (especially A(1) RBCs) have been more difficult. New sources of enzymes have produced ECO RBCs from A(1) and A(2) RBCs that do not react with powerful monoclonal anti-A. Unfortunately, there are still problems encountered with polyclonal human antibodies (i.e. cross-matching). The second approach interferes with an antibody reaching its specific antigen on the RBC membrane by bonding polyethylene glycol (PEG) to the RBC. PEG will attract water molecules, yielding a combination that may block most RBC antigens, including A and B antigens. Initial excitement generated by preliminary reports of the possibility of producing 'stealth' PEG-RBCs were tempered by the findings of in vitro serological problems and possible reduced in vivo RBC survival. Many of these problems were solved, but recent findings that PEG is immunogenic in animals and humans, and that PEG antibodies can shorten the survival of PEG-RBCs (in rabbits) and pegylated proteins (e.g. PEG-asparaginase) in humans, are disturbing, suggesting that 'stealth' RBCs may never become a reality.

Thiolation mediated pegylation platform to generate functional universal red blood cells

The PEGylation that adds an extension arm on protein amino groups with the conservation of their positive charge masks the A and D antigens of erythrocytes efficiently. In the present study, the efficiency of masking the antigens of RBC by PEGylation protocols that do not conserve the charge with and without adding extension arms is compared. The conjugation of PEG-5000 to RBCs through the addition of extension arms masked the D antigen more efficiently than the other protocol. A combination of PEG-5 K and PEG-20 K is needed to mask the A antigen, irrespective of the PEGylation approach. The oxygen affinity of the PEGylated RBCs increased by the extension arm facilitated PEGylation. The protocol involving the conjugation of PEG-chains without adding extension arm did not alter the oxygen affinity of RBCs. A combination of PEGylation protocols is an alternate strategy to generate universal red blood cells with good levels of oxygen affinity.

Immune complex binding by immunocamouflaged [poly(ethylene glycol)-grafted] erythrocytes

Immune complexes (IC) are constantly formed at low levels in normal individuals. In humans, the red blood cell (RBC) complement receptor 1 (CR1) plays the dominant role in the IC binding and clearance. Over the last several years, we have investigated the potential utility of immunocamouflaged (methoxypoly(ethylene glycol) [mPEG] grafted) RBC to attenuate the risk of alloimmunization. Because the grafted polymer nonspecifically camouflages membrane proteins, its effects on CR1 detection and IC binding were assessed. The dose dependent (0-2.5 mM) effects of activated mPEG (CmPEG, 5 kDa; and BTCmPEG, 5 and 20 kDa) on CR1 detection and the binding of artificially generated IC [C3b coated alkaline phosphatase and antialkaline phosphatase complexes] to control and pegylated RBC was investigated by flow cytometry. In contrast to selected non-ABO blood group antigens, grafted mPEG did not effectively camouflage CR1. Surprisingly, however, even very low grafting concentrations of mPEG (>or=0.3 mM) resulted in a >or=95% loss in IC binding. Further reductions in grafting concentration (0.15 and 0.03 mM mPEG) still yielded decreased IC binding of approximately 60 and 40%, respectively. Importantly, unactivated mPEG had minimal effects on IC binding. These data demonstrate that even small amounts of grafted mPEG interfere with the multivalent CR1-IC interaction necessary for high affinity IC binding, hence large volume transfusions of mPEG-RBC may be contraindicated in patients with pre-existing IC disease. Whether this concern is of clinical significance in healthy humans is less clear due to dilutional effects and the presence of secondary clearance pathways.

Some ethical issues regarding xenotransfusion

BACKGROUND

The use of porcine red blood cells has recently been proposed as a possible solution to the shortage of blood for human transfusion.

OBJECTIVES

The purpose of this paper is to compare some ethical issues regarding xenotransfusion (XTF) with those relating to xenotransplantation (XT) of organs, tissues and cells.

MATERIALS AND METHODS

Various ethical concerns and viewpoints relating to XTF are discussed.

RESULTS

The main ethical obstacles to XT do not apply to XTF. It is much more ethically acceptable to raise pigs for regular blood collection as it doesn't damage the health of the animal. Porcine endogenous retrovirus infection, the major concern associated with XT, does not apply to XTF, since red blood cells have no DNA and have a very short lifespan. Clinical trials will be possible in humans once XTF has been demonstrated to be effective and harmless in non-human primates. Transgenesis is acceptable for pig blood donors because only a limited number of genes are involved, and these animals will never enter into the livestock gene pool or the food chain.

CONCLUSION

Because the need for blood is less pressing than that for organs, tissues or cells, the use of animal blood for human transfusion is not an absolute necessity. However, it represents a real opportunity. The ability to gain access to an unlimited quantity of blood is a reasonable justification for XTF. Because its technical and ethical hurdles are less stringent, XTF could be the first large-scale clinical application of XT.

Xenotransfusions, past and present

The first blood transfusions in humans were xenotransfusions, carried out by Jean-Baptiste Denis beginning in 1667. Richard Lower, Matthäus Purmann and Georges Mercklin also experimented with the use of animal blood for transfusion until this practice was forbidden in 1670, after the death of one of Denis's patients. In the middle of the 19th century, xenotransfusion was rescued from oblivion by the work of Pierre Cyprien Oré. Franz Gesellius and Oscar Hasse fervently defended xenotransfusion, but Emil Ponfick and Leonard Landois stressed the potentially harmful effects of inter-species transfusion from 1874 onward. Xenotransfusion was abandoned completely following the discovery of blood groups by Karl Landsteiner in 1900. From 2000, because of progress in xenotransplantation and the need of blood supply, xenotransfusion is again being considered. Pigs are the best potential donors. The development of alpha-1,3-galactosyltransferase gene-knockout pigs has overcome the first hurdle to xenotransfusion. The main obstacle to porcine red blood cell transfusion is now the cellular response involving macrophages or natural killer cells.

Decreased immunorejection in unmatched blood transfusions by attachment of methoxypolyethylene glycol on human red blood cells and the effect on D antigen

BACKGROUND

Pegylation of red blood cells (RBCs) has been the primary focus of research on the immunocamouflage of cell. The aim of this study was to demonstrate pegylation homogeneity, its shielding effect on D antigens, and its storage stability. In addition, methoxypolyethylene glycol (mPEG)-modified RBCs (mPEG-RBCs) were tested serologically against a panel of serum samples that was difficult to match to find a solution to the difficulty in matching.

STUDY DESIGN AND METHODS

In this study, fluorescein-PEG and a confocal laser scanning microscope were used to monitor PEG attachment on RBC population and observe reaction homogeneity, the stability of mPEG combined with RBCs in vitro was evaluated by the RBC ghost agglutination test, the pegylation sites on membrane were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with two dye methods, and the effect of pegylation on D antigen was detected by immunoblotting techniques. Compatibility tests were carried out between 66 cases of serum with difficulty in blood matching and mPEG-camouflaged RBCs by use of four blood matching methods including direct agglutination, indirect antiglobulin test (IAT), microtyping gel cards (MTS), and the manual polybrene technique (MPT).

RESULTS

The results indicated the homogeneity of pegylation, the absence of RhD protein in mPEG-modified D+ RBCs by Western blotting, and attachment of PEG to RBCs after 30 days of storage, while RBCs still remained antigenically silent. All pegylation RBCs showed a negative reaction with ABO-matched patients' serum samples by direct agglutination, IAT, and MTS, which indicated that pegylation RBCs and patients' serum samples were compatible. MPT was not suitable for detecting blood matching of mPEG-RBCs, because modification changed the RBCs' biophysical properties.

CONCLUSION

In conclusion, mPEG-RBCs have acceptable in vitro properties and provide a useful solution to problems with clinical blood matching, although such masking leaves much to be desired.

Surface decoration of red blood cells with maleimidophenyl-polyethylene glycol facilitated by thiolation with iminothiolane: an approach to mask A, B, and D antigens to generate universal red blood cells

BACKGROUND

The surface decoration of red blood cells (RBCs) by polyethylene glycol (PEG) chains has been an approach developed to camouflage the blood group antigens from their antibodies. A PEGylation protocol, however, that can mask the antigens appropriately to inhibit the agglutination of RBCs with the respective antibodies is not available so far.

STUDY DESIGN AND METHODS

A new approach for PEGylation of RBC membrane proteins has been designed with thiolation-mediated maleimide chemistry. The accessibility of the surface lysine residues of membrane proteins to bulky PEG reagents was increased by linking an extension arm carrying a thiol group.

RESULTS

RBCs have been PEGylated by thiolation-mediated chemistry with maleimidophenyl-PEG (Mal-Phe-PEG) reagents of different chain lengths. Mal-Phe-PEG-5000 chains alone masked the most important antigens of the Rh system (C, c, E, e, and D) from their antibodies. The masking of the A and B antigens needed a combination of Mal-Phe-PEG-5000 and Mal-Phe-PEG-20000 chains to inhibit the agglutination of RBCs completely with anti-A or anti-B.

CONCLUSIONS

Thiolation-mediated PEGylation of RBCs with Mal-Phe-PEG-5000 and Mal-Phe-PEG-20000 converts Group A Rh(D)+ and B Rh(D)+ RBCs into RBCs with serologic behavior comparable to Group O Rh(D)- RBCs that are considered as universal RBCs for transfusion.

Cytoprotection of PEG-modified adult porcine pancreatic islets for improved xenotransplantation

Functional poly(ethylene glycol) (PEG) derivatives, including monosuccinimidyl PEG (MSPEG) with molecular weight (MW) of 2000 (2 kDa) as well as 5 kDa and disuccinimidyl PEG (DSPEG) with MW of 3 and 6 kDa, were synthesized and characterized. They were used to modify the surface of adult porcine islets for cytoprotection. The islets were isolated, purified and modified with functional PEG. Untreated porcine islets were used as control. An in vitro human antibody/complement-mediated cytotoxicity test based on the release of intracellular lactate dehydrogenase was used to evaluate cytotoxicity of human serum to the modified islets. In vitro cell viability was assessed using membrane-integrity straining and islet metabolism in culture. In vitro islet functionality was evaluated by glucose-stimulated insulin release of islets in static incubation with human serum. In vivo islet functionality was evaluated by monitoring non-fasting blood glucose level in streptozotocin-induced diabetic (SCID) immunocompromized mice after intraportal transplantation of porcine islets. Results show that all the PEG derivatives used in the study showed significant in vitro and in vivo cytoprotections against cytotoxic effects elicited by human serum and diabetic SCID mice, respectively, to porcine islets. DSPEG derivatives combined with human albumin exhibited a better cytoprotection, as compared to MSPEG ones, due to the capacity of the succinimidyl groups to selectively react with amino groups of the albumin under physiological conditions. The effects of both MW and concentration of the PEG derivatives on cytoprotection were significant. It appears that this novel biotechnology will be an attractive approach for improved xenotransplantation of islets.

Modification of xenoantigens on porcine erythrocytes for xenotransfusion

BACKGROUND

Problems of supply and disease transmission with blood transfusion may be controlled by the use of an isolated animal donor pool. However, porcine erythrocytes (PRBCs) usually are destroyed rapidly by preformed antibodies in human serum. We examined the impact on PRBC antigenicity by the removal of cell membrane alpha-gal(1-3)beta-galGlcNac epitopes (called alpha-gal) and chemical masking of other xenoantigens.

METHODS

From porcine "low expressors" of alpha-gal, PRBCs were subjected to (1) enzymatic removal of membrane alpha-gal with alpha-galactosidase, (2) covalent attachment of cyanuric acid-linked methoxypolyethylene glycol, or (3) both processes. PRBC integrity was assessed by light microscopy, scanning electron microscopy, osmotic fragility, and determination of oximetric p50. The effects of treatment were measured by hemagglutination, complement fixation, flow cytometric assay of immunoglobulin G/M binding, and clinical cross-match testing to human sera.

RESULTS

Cyanuric acid-linked methoxypolyethylene glycol reduced hemagglutination titers moderately, although alpha-galactosidase treatment reduced hemagglutination titers to levels similar to negative controls. The combination of the treatments was most effective, by the reduction of binding of human immunoglobulin M by 61% compared with controls. RBC morphologic condition, stability, and p50 values were maintained. Clinically used cross-match tests between PRBCs and human sera demonstrated increased compatibility.

CONCLUSIONS

These data suggest that strategies to remove or mask xenoantigens on PRBCs reduce antigenicity sufficiently to allow in vitro cross-match compatibility to human sera.

Beyond the red cell: pegylation of other blood cells and tissues

Immunological recognition of allogeneic tissue is of critical concern in transfusion and transplantation medicine. While the major emphasis of our work on the immunocamouflage of cells has been focused on the erythrocyte, we have extended this research beyond the red blood cell (RBC) to other tissues. Our studies from blood transfusion (i.e., a specialized form of cellular transplantation) suggest that covalent modification of cells and tissues with methoxypoly(ethylene glycol) mPEG can significantly diminish immunologic recognition of other allogeneic tissues and, furthermore, may enhance the induction of tolerance. The mechanisms underlying the mPEG-mediated immunocamouflage of alloantigens is the global camouflaging of antigenic sites, membrane surface charge and the attenuation of receptor-ligand and cell-cell interactions. As a consequence of the immunocamouflage imparted by the grafted mPEG, weak costimulation of alloreactive T cells is observed which subsequently induces apoptosis of these reactive cells. As a result of this clonal deletion, a pro-tolerance state is induced. The potency of immunocamouflage is readily observed in in vivo murine models of transfusion-associated graft versus host disease. Furthermore, initial studies on the in vivo transplantation of pegylated rat and murine pancreatic islets have demonstrated that mPEG-derivatization does not impair the finely tuned signaling necessary for glucose homeostasis. Finally, in contrast to the pharmacological inhibition of the immune response by agents such as cyclosporine, mPEG-mediated immunocamouflage directly attenuates the inherent antigenicity and immunogenicity of the donor tissue itself while leaving the recipient a fully competent immune system.

Polyethylene Glycol Additives Reduce Hemolysis in Red Blood Cell Suspensions Exposed to Mechanical Stress

Mechanical damage to blood cells is of considerable concern in the development and use of circulatory assist devices and other blood contacting systems. Furthermore, hemodilution with saline, dextran, and other plasma expanders applied during extracorporeal circulation and dialysis increases red blood cell (RBC) susceptibility to the high shear stresses associated with these procedures. In this paper, we present polyethylene glycol (PEG) as a potential erythrocyte protective agent against mechanically induced cellular trauma. Bovine RBCs were subjected to mechanical stress induced by rolling stainless steel shots through RBC suspensions for a constant exposure time. The suspensions were prepared at a hematocrit of 30% in various media: PEG (20,000 molecular weight), autologous bovine plasma, Dextran 40 solution, and phosphate buffered saline (PBS). RBC suspensions in Dextran 40 were prepared at a viscosity similar to the PEG suspensions. We found the hemolysis level of RBCs suspended in plasma and in PEG solutions to be several times lower (p < 0.001) than in the Dextran and PBS solutions. No statistically significant difference was found between the hemolysis that occurred in suspensions of RBCs in autologous plasma and in 2.0% PEG solutions. Even PEG concentration as low as 0.1% reduced hemolysis by more than 40% compared with PBS or the same concentration of Dextran in suspension medium. Our data demonstrate the efficacy of PEG molecules in reducing mechanical trauma to erythrocytes and suggest the potential for using PEG in assisted circulation, dialysis, and other procedures where RBCs are subjected to extensive mechanical stress.

Red cell antigens as functional molecules and obstacles to transfusion

Blood group antigens (BGAs) can act as functional molecules but also can evoke autoantibodies and alloantibodies, causing autoimmune hemolytic anemia, hemolytic disease of the newborn and hemolytic transfusion reactions. In Section I, Dr. Marilyn Telen discusses physiologic and pathologic functions of RBC BGA-bearing molecules. She reviews some associations of BGAs with RBC membrane integrity and hemolytic anemia; association of BGAs with enzymatic and transport functions; and adhesion molecules expressed by RBCs, especially with reference to their pathophysiological role in sickle cell disease. In Section II, Dr. Lawrence Petz discusses the problems of providing blood for patients who have RBC autoantibodies. He provides an algorithm for excluding the presence of "hidden" alloantibodies, when all units appear to be incompatible due to the autoantibody. He emphasizes that clinicians should be aware of these approaches and not accept "the least incompatible unit." In Section III, Dr. George Garratty describes two processes, in development, that produce RBCs that result in RBCs that can be described as "universal" donor or "stealth" RBCs. The first process involves changing group A, B, or AB RBCs into group O RBCs by removing the immunospecific sugars responsible for A and B specificity by using specific enzymes. The second process involves covering all BGAs on the RBC surface using polyethylene glycol (PEG). Results of in vitro and in vivo studies on these modified RBCs are discussed.

Transfusion medicine for the pediatrician

In the next decade, many of the methodologies and research reviewed in this article will become clinical practice, making the transfusion of blood products safer and more universally available than they are today. NAT will be standard and will surely be performed on each unit of product, PCR testing for pathogens will evolve, and the pathophysiology and immunology of transfusion-related events such as TRALI and immunomodulation will be elucidated. New methods of preservation and early detection of contamination will extend the life of blood products. Red blood cell antigens may be attenuated, making safe products available to more patients. Clinical vigilance at the bedside and in the blood bank will remain key areas for transfusion safety. As I have told many a resident and patient, blood is not saline; there are and will remain risks inherent in this commonly used medical therapy.

Current and future applications of immunological attenuation via pegylation of cells and tissue

Prevention of immunological rejection of transplanted tissues is of crucial importance in transplantation medicine. Current procedures primarily use pharmacological agents such as cyclosporin, which, while effective, must be typically administered for the life of the individual. Furthermore, the drug-induced global immunosuppression of the patient predisposes the individual to infection and enhances their risk of developing certain forms of cancer. Hence, additional methods are needed to both enhance tissue engraftment and diminish the adverse effects of current immunosuppressive therapy. Studies from blood transfusion (i.e. a specialised form of cellular transplantation) suggest that covalent modification of cells and tissues with methoxypoly(ethylene glycol) [mPEG] can significantly diminish rejection episodes and may further enhance the induction of tolerance to donor tissues. The mechanisms underlying mPEG-mediated immunocamouflage are the loss of antigen recognition, impaired cell-cell interaction, and an inability of endogenous antibodies (e.g. immunoglobulin G) to effectively recognise and bind foreign epitopes. As a consequence of the global camouflage imparted by mPEG, the weak co-stimulation of alloreactive T cells may subsequently induce apoptosis, thus leading to tolerance. Initial studies on the transplantation of pegylated isogeneic rat pancreatic islets demonstrates that mPEG-derivatisation does not impair in vivo cellular signalling and function. Thus, in contrast to the pharmacological inhibition of the recipient's immune response, the mPEG-mediated immunocamouflage directly addresses the inherent antigenicity and immunogenicity of the donor tissue itself while leaving the recipient a fully competent immune system.

Blocking adhesion to cell and tissue surfaces by the chemisorption of a poly-L-lysine-graft-(poly(ethylene glycol); phenylboronic acid) copolymer

A family of graft copolymers that can sterically inhibit interactions between biological surfaces was developed. These copolymers contained phenylboronic acid (PBA) groups as saccharide-binding moieties on a poly-(L-lysine) backbone and poly(ethylene glycol) (PEG) grafted as adhesion-resisting side chains. These copolymers spontaneously chemisorbed to a saccharide-containing resin, and this binding was sterically controlled by the PEG grafting ratio. Copolymers with optimal grafting ratios spontaneously assembled on red blood cell surfaces and sterically prevented their agglutination by lectins and by antibodies to blood groups. The simple conjugation scheme created a PBA moiety with a pKa ca. 6, which can bind cis-diols much more strongly at physiological pH than typical PBA moieties, whose pKas are typically greater than 8. These surfactant copolymers can be employed to PEGylate cell or tissue surfaces by simply incubating the surfaces with an aqueous polymer solution, and have many potential applications such as preventing antibody binding to transplanted cells.

Camouflaged blood cells: low-technology bioengineering for transfusion medicine?

The small number of studies done on the covalent modification of RBC with PEG, or PEG-derivatives, suggests that the immunocamouflage of intact cells significantly reduces the antigenicity and immunogenicity of the foreign cell. Importantly, this protective immunologic effect can be accomplished without adversely affecting the structure, function, or viability of the modified cell (e.g., RBCs and lymphocytes). As a consequence, PEG-RBC may have significant practical value in the treatment of the chronically transfused patient as a prophylactic measure against allosensitization. The PEG-RBC also may be useful in treating the already allosensitized individual. As shown, preexisting antibodies do not effectively recognize nor bind to the modified donor cells. A finding of further interest to transfusion medicine is that pegylation of contaminating lymphocytes within RBC products may prove efficacious in preventing graft-versus-host disease in the immunocompromised patient. However, the main emphasis of our research continues to be the immunocamouflage of RBC for use in chronic transfusion therapy of the SCD and thalassemic patient.

Camouflaging endothelial cells: does it prolong graft survival?

Camouflaging antigens on the surface of cells seems an appealing way to prevent activation of the immune system. We explored the possibility of preventing hyperacute rejection by chemically camouflaging endothelial cells (EC). In vitro as well as in vivo experiments were performed. First, the ability of mPEG coating to prevent antibody-antigen interactions was evaluated. Second, we tested the degree to which mPEG coating prevents activation of EC by stimuli such as TNF-alpha and LPS. Third, in vivo experiments were performed to test the ability of mPEG coating to prolong xenograft survival. We demonstrate that binding of several antibodies to EC or serum proteins can be inhibited by mPEG. Furthermore, binding of TNF-alpha as well as LPS to EC is blocked since mPEG treatment of EC inhibits the subsequent up-regulation of E-selectin by these stimuli. However, in vivo experiments revealed that currently this method alone is not sufficient to prevent hyperacute rejection.

Structural and Functional Consequences of Antigenic Modulation of Red Blood Cells With Methoxypoly(Ethylene Glycol)

We previously showed that the covalent modification of the red blood cell (RBC) surface with methoxypoly(ethylene glycol) [mPEG; MW ∼5 kD] could significantly attenuate the immunologic recognition of surface antigens. However, to make these antigenically silent RBC a clinically viable option, the mPEG-modified RBC must maintain normal cellular structure and functions. To this end, mPEG-derivatization was found to have no significant detrimental effects on RBC structure or function at concentrations that effectively blocked antigenic recognition of a variety of RBC antigens. Importantly, RBC lysis, morphology, and hemoglobin oxidation state were unaffected by mPEG-modification. Furthermore, as shown by functional studies of Band 3, a major site of modification, PEG-binding does not affect protein function, as evidenced by normal SO4− flux. Similarly, Na+ and K+ homeostasis were unaffected. The functional aspects of the mPEG-modified RBC were also maintained, as evidenced by normal oxygen binding and cellular deformability. Perhaps most importantly, mPEG-derivatized mouse RBC showed normal in vivo survival (∼50 days) with no sensitization after repeated transfusions. These data further support the hypothesis that the covalent attachment of nonimmunogenic materials (eg, mPEG) to intact RBC may have significant application in transfusion medicine, especially for the chronically transfused and/or allosensitized patient.