Xenoreactive natural antibodies in the world of natural antibodies: typical or unique?

Anti-Gal IgM, IgA and IgG natural antibodies in childhood

The target for the most abundant xenoreactive natural antibodies in humans is the α-Gal epitope. Anti-Gal could provide natural immune defense against pathogens that express the α-Gal epitope. Anti-Gal natural antibodies are usually studied in adult individuals. Data demonstrating the incidence and concentration of anti-Gal natural antibodies in childhood are in short supply and incomplete. In the present study we prospectively quantified anti-Gal IgM, IgA and IgG levels in different age groups of children from delivery to 24 months of age and compared these levels to the level of these antibodies in their respective mothers. Measurement of anti-Gal antibodies may broaden the spectrum of specific antibodies that are available for determination of specific antibody responses in physiological and pathological conditions in children. Plasma was collected from umbilical cord blood of full term newborn, from blood of infants at age 6, 12 and 24 months and from their respective mothers at time of delivery. Quantitative determination of anti-Gal antibodies IgM, IgA and IgG were made with the enzyme immunoassays Human Anti-Alpha Galactosyl IgM ELISA, IgG ELISA and IgA ELISA. Hemagglutination activity was titrated against rabbit erythrocytes. The kinetic processes for the formation of natural antibodies in the first two years of life, in general, compared with the kinetics for the formation of total immunoglobulins IgM, IgA and IgG. There were no detectable anti-Gal IgM and IgA in the cord blood, whereas anti-Gal IgG were found at similar levels in both neonate cord blood and peripheral blood of their respective mothers. When comparing the percentage of natural antibodies in the plasma of children, the level of natural antibodies in children at the age of two years was approximately 37% for IgM, 25% for IgG and 15% for IgA. The titration of antibodies required for agglutination of rabbit red blood cells over the 24 month period followed the same trend observed for the formation of natural antibodies. Our study demonstrates the kinetics of formation of anti-Gal IgM, IgA and IgG natural antibodies in the first two years of life. The relative lack of these antibodies in this period should be taken into account when assessing for humoral immunodeficiencies, particularly with regards to the potential for children to mount an anti-carbohydrate response.

Boosted rat natural xenoantibodies cross-react with Enterococcus faecalis by targeting melibiose and L-rhamnose

Natural antibodies include a subset described as xenoantibodies considered to be directed at microorganisms and also cross-react with antigens of unrelated species. In this study, we generated T-cell-independent (TI) and T-cell-dependent (TD) xenoantibodies in Lewis rats with hamster and pig blood injections. TI anti-hamster and anti-pig IgM and IgG xenoantibodies cross-reacted with Enterococcus faecalis but not with Escherichia coli isolated from the blood of Lewis rats after cecal ligation and puncture (CLP). TI anti-pig IgM xenoantibodies also showed some reactivity with two human blood isolates of E. faecalis. In contrast, TD xenoantibodies did not show any reactivity with rat or human bacteria. TI and TD anti-hamster and anti-pig IgM and IgG xenoantibodies showed cross-reactivity with lymphocytes and endothelial cells from species distinct to that used for immunization. Glycan array analysis and inhibition assays identified antibodies against melibiose and L-rhamnose as mediators of anti-hamster and anti-porcine xenoantibody cross-reactivity with E. faecalis. A rise in TI anti-hamster and anti-pig xenoantibodies was accompanied by decreased survival of Lewis rats in a low-severity sepsis model of CLP. Therefore, TI xenoantibodies in the rat include anti-carbohydrate antibodies reactive to bacteria of endogenous flora. Enhancement of these antibodies may result in more severe infectious diseases caused by these microorganisms.

Accommodation of grafts: implications for health and disease

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

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.

IgM colocalises with complement and C reactive protein in infarcted human myocardium

AIMS

Reperfusion of ischaemic myocardium after acute myocardial infarction (AMI) can induce ischaemia/reperfusion (I/R) injury, as a result of local activation of the complement system. C reactive protein (CRP) is involved in this activation. This study analysed the potential role of IgM in complement activation in the infarcted human myocardium.

METHODS

Immunochemical analysis was carried out on heart specimens from 59 patients who died from AMI. Serial slides of frozen tissue from the infarction site were stained for IgM, complement factors C3d and C5b-9 (membrane attack complex), and CRP.

RESULTS

IgM deposits were found on the plasma membrane, cross striations, and in the cytoplasm of jeopardised cardiomyocytes in infarcts of one to five days duration. IgM depositions were remarkably similar to those of CRP and both complement factors. The relative staining intensities of IgM and CRP varied greatly among patients.

CONCLUSIONS

Similar to CRP, IgM targets complement locally to jeopardised cardiomyocytes in the human heart after AMI. Localisation patterns and relative staining intensities suggest that IgM and CRP recognise similar epitopes in the ischaemic heart, but that the relative contribution of each protein to complement activation in the ischaemic myocardium differs among patients.

Comparison of the binding properties of the mushroom Marasmius oreades lectin and Griffonia simplicifolia I-B isolectin to alphagalactosyl carbohydrate antigens in the surface phase

The binding of two alpha-galactophilic lectins, Marasmius oreades agglutinin (MOA), and Griffonia simplicifolia I isolectin B(4) (GS I-B(4)) to neoglycoproteins and natural glycoproteins were compared in a surface phase assay. Neoglycoproteins carrying various alpha-galactosylated glycans and laminin from basement membrane of mouse sarcoma that contains the xenogenic Galalpha1-3Gal1-4GlcNAc epitope were immobilized in microtiter plate wells and lectin binding determined with an enzyme-linked assay. After 24 h of incubation, MOA had higher affinity for the xenogenic pentasaccharide (Galalpha1-3Gal1-4GlcNAcbeta1-3Galbeta1-4Glc) than for the Galalpha-monosaccharide. The binding properties of MOA and GS I-B(4) to the xenogenic disaccharide (Galalpha1-3Galbeta1) were comparable while the binding of MOA to the xenogenic pentasaccharide was much stronger than the binding of GS I-B(4) to the same epitope. Non-xenogenic disaccharide-coupled neoglycoproteins having galactose end groups linked alpha1-2 or alpha1-4 to Gal or linked alpha1-3 to GalNAc bound very weakly to MOA, whereas GS I-B(4) recognized all of these disaccharides with similarly high affinity. MOA also showed high affinity for laminin. The results indicate that the Marasmius oreades lectin has nearly the same affinities as does GS I-B(4) for the simple xenogenic carbohydrate antigens, but MOA has greater affinity for the pentasaccharide and is far more specific in its binding preferences than the Griffonia lectin.

The role of antibodies and von Willebrand factor in discordant pulmonary xenotransplantation

Pulmonary xenotransplantation is one potential solution to the paucity of donors but is currently limited by rapid failure of the graft. Unlike cardiac and renal xenotransplants, pulmonary xenografts release large quantities of swine von Willebrand factor (vWF). Swine vWF binds xenoreactive antibodies and is capable of activating primate platelets. The contribution of swine vWF to lung xenograft dysfunction is not entirely clear. To probe the role vWF plays in xenograft dysfunction, we traced the fate of xenoantibodies in vWF+ and von Willebrand factor-deficient (vWFD) swine lungs. These studies showed that the vast majority of xenoantibodies bind the vWF released from the vWF+ swine lung, and thus do not remain bound on lung endothelium. The vWF complexed to xenoantibody remained capable of aggregating primate platelets. With this information, we performed swine-to-baboon lung transplants using vWF+ and vWFD donors. Without vWF present to complex xenoantibodies, a picture of hyperacute rejection more typical of heart and kidney xenografts, with antibody deposition along the graft endothelium, interstitial hemorrhage, and edema occurred. These findings suggest that porcine vWF plays a major role in the pathogenesis of pulmonary xenograft dysfunction, and suggests promising strategies to treat lung xenograft dysfunction.

Quantitation and comparison of anti-Gal-alpha-1,3-Gal antibodies in sera of healthy individuals and patients waiting for kidney transplantation

Highly sensitized patients have poor prospects of being offered an allogeneic donor transplant, therefore, they could become primary candidates for xenogeneic organ transplantation. Hyperacute rejection is initiated by anti-Gal-(alpha)-1,3-Gal antibodies (xenoAb) and this rejection will have to be avoided if xenotransplantation is introduced into clinical practice. Hence, the aim of the study was to determine if sensitized patients against HLA antigens have higher titers of xenoAb than nonsensitized individuals. We determined immunoglobulin (Ig)M and IgG xenoAb in the serum of 100 volunteer blood donors and 98 sensitized patients and compared antibody titers from nonsensitized individuals to those with HLA specific antibodies classified into 5 panel-reactive antibody (PRA) groups: I, 0%; II, >0% and < or =10%; III, >10% and < or =50%; IV, >50% and < or =80%; and V, >80%. Analysis of the isotype specific quantification of the xenoAb revealed great interindividual variation but there were no significant differences in the range of titers of xenoAb. No significant PRA group-related variations were observed for xenoAb. In fact, there was no demonstrable correlation between levels of PRA and levels of xenoAb because the scattergram showed no statistically significant association by nonparametric statistic analysis. The results presented herein revealed that the presence of HLA specific antibody has no demonstrable relationship to the presence of higher titers of xenoAb. Thus, sensitized patients do not present a greater risk of rejection or are more difficult to treat for natural xenoreactivity than nonsensitized patients.

Presence of Gal-alpha1,3Gal epitope on xenogeneic lines: implications for cellular gene therapy based on the encapsulation technology

Exposure to human serum induces the lysis of xenogeneic cells through natural antibodies and complement activation. The carbohydrate Galactose-alpha1,3-Galactose (Gal-alpha1,3-Gal) epitope, has been shown to be the principal antigenic determinant on target cells. This reaction is, therefore, particularly important for xenogeneic cell-based therapy. As a first step toward the evaluation of the impact of this phenomenon for encapsulated xenogeneic cells, we have evaluated the presence of the Gal-alpha1,3Gal epitope on two cell lines currently being used for the systemic delivery of protein in the periphery or the treatment of neurodegenerative diseases. In the second part of the study, we have tested and compared human serum and cerebrospinal fluid (CSF) for the presence of xenoreactive natural antibodies (XNAs) and their potential impact on the survival of xenogeneic cells. Fluorescence-activated cell sorting analysis indicated that baby hamster kidney (BHK) cells expressed low levels of the alpha-Gal epitope, whereas mouse myoblast C2C12 cells were extensively stained with the specific IB4-lectin. There was a direct correlation between serum killing and the level of Gal-alpha1,3-Gal epitope expression on these cells. Importantly, we showed that CSF did not lyse BHK and C2C12 cells as determined by cytotoxic crossmatch assays. The reaction was specific as the addition of soluble Gal-alpha1,3-Gal sugar to human serum effectively reduced cell killing, and the overproduction of alpha-1,3-galactosyltransferase in BHK cells significantly increased inactivation by human serum. To interfere with this antibody-antigen reaction and develop cell lines particularly suitable for cell-based therapy, we either selected C2C12 clones expressing low levels of Gal-alpha1,3-Gal or high levels of alpha-1,2-fucosyltransferase. These cells were found to be resistant to complement-mediated cytolysis. These strategies may, therefore, protect encapsulated xenogeneic cells transplanted in the periphery or the central nervous system even in an unlikely event of a blood-brain barrier breakage and the post-transplantation development of an antibody response.

Lectin interactions with alpha-galactosylated xenoantigens

alpha-Galactosylated xenoantigens (Galalpha1-3Galbeta1-4GlcNAcbeta1 and Galalpha1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc) are often detected with the alpha-Gal specific lectin Griffonia simplicifolia 1 isolectin B4 (GS1 B4). However, this lectin exhibits a broad and variable specificity for carbohydrates terminating in alpha-Gal. Thus, both false positive and false negative results may occur when GS1 B4 is used to determine natural antigens in xeno (pig-to-primate) transplantation research. To refine the tools for detecting alpha-galactosylated antigens we have studied the binding of various alpha-galactophilic lectins to alpha-galactosylated neoglycoproteins. The lectins were: Euonymus europaeus agglutinin (EEA), Griffonia simplicifolia 1 isolectin B4 (GS1 B4), Maclura pomifera agglutinin (MPA) and Pseudomonas aeruginosa agglutinin (PA-IL). Although both GS1 B4 and MPA strongly bound glycoconjugates terminating in Gal there seems to be some differentiation in their sugar binding preferences. MPA was the only lectin that showed high affinity for the pentasaccharide Galalpha1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc and for the Galalpha-glycans on non-primate thyroglobulin. The length of the xenoantigenic carbohydrate chain may influence the nature of the inhibition when a simple sugar is used to inhibit GS1 B4 binding to the xenoantigen. Inhibition studies of MPA GS1 B4 interaction further suggest that both lectins attach to the same site of the carbohydrate antigen and that GS1 B4 in addition binds to at least one other site that has no affinity for MPA. When lectins are used for recognition and investigation of natural Galalpha-antigens, we propose that GS1 B4 and MPA should accompany each other.

Binding of Griffonia simplicifolia 1 isolectin B4 (GS1 B4) to alpha-galactose antigens

Glycoconjugates with terminal Galalpha1-3Galbeta1-4GlcNAc sequences (alpha-galactosyl epitopes, natural xenoreactive antigens) are present on various tissues in pigs and are recognized by human anti-alphagalactosyl (alphaGal) antibodies1. Hence xenotransplantation (pig-to-human) would trigger immune reactions involving complement activation and lead to the hyperactute rejection of the graft. Xenoreactive antigens are often studied by using the lectin Griffonia simplicifolia 1 isolectin B4 (GS1 B4), which shows high affinity to galactose. We here estimate the specificity of GS1 B4 for detecting various galactosyl epitopes by measuring lectin binding to neoglycoproteins, thyroglobulin and pig skeletal muscle. Enzyme linked lectin assays confirmed that GS1 B4 was highly specific to alpha-galactosylated neoglycoproteins while the lectin did not detect a beta-galactosylated ligand. The length of the sugar chains influenced the lectin-carbohydrate interaction. A monosaccharide linked to serum albumin showed higher lectin affinity than did neoglycoproteins with di- and tri-alpha-galactosyl epitopes. When the carbohydrate was extended, as in the xenoreactive pentasaccharide (Galalpha1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc), the carbohydrate- lectin interaction was meagre. Not only the terminal, but also the subterminal sugar affected the lectin binding because the GS1 B4 affinity to Galalpha1-3Gal was much stronger than to Galalpha1-3GalNAc. In bovine and porcine thyroglobulin most alphaGal epitopes appear to be cryptic, but are unmasked by a heat denaturation. In pig skeletal muscle there was lectin reaction not only in the muscle capillaries, but also in the connective tissue and intracellularly in muscle fibres. In Western blots of isolated proteins from pig muscle at least three bands were strongly stained after incubation with lectin.

Removal of bowel aerobic gram-negative bacteria is more effective than immunosuppression with cyclophosphamide and steroids to decrease natural alpha-galactosyl IgG antibodies

Natural alpha-Galactosyl (Gal) antibodies play an important role in the rejection of pig xenografts by humans and Old World monkeys. In this study we investigate the efficacy of two different strategies to reduce the serum level of natural anti-Gal antibodies. On the one hand, removal of aerobic gram-negative bacteria from the intestinal flora, because anti-Gal antibodies appear to be produced as a result of the continuous sensitization by these microorganisms. On the other hand, we studied the effect on these antibodies of an immunosuppressive regimen of cyclophosphamide and steroids. Ten baboons were treated for three months with norfloxacin (Nor Group; n=6) or cyclophosphamide and steroids (CyP Group; n=4). A further four baboons did not receive any treatment (Control Group). Aerobic gram-negative bacteria became negative in stools of the Nor Group after two weeks of treatment, and remained undetectable until week 7. Thereafter, a gradual increase on the fecal concentration of aerobic gram-negative bacteria was observed despite the norfloxacin treatment. The mean anti-Gal IgG in the Nor Group gradually declined from week 4 to 9 to a mean of 62.7 +/- 18% of the baseline level, and during this period were significantly lower than in the CyP (P<0.02) and the Control (P<0.05) groups. No differences were observed between the three groups during the 16 weeks of follow-up in serum levels of anti-Gal IgM, hemolytic anti-pig antibodies, total IgG, IgM and IgA. In conclusion, removal of normal aerobic gram-negative bacteria from the intestinal flora is more effective than immunosuppression with CyP and steroids in reducing the level of natural anti-Gal antibodies, although there is no discernible effect on IgM antibodies.

Human and non-human primate anti-galactosyl response after injection of rat monoclonal antibody bearing galactosyl epitopes

In the case of clinical use of pig-to-human xenografting, any exogenous source of to-galactosyl epitopes will elicit an anti-galactosyl immune response, which could be deleterious for the xenograft. The presence of Galalpha(1-3)Gal residues was thus examined by western blotting on various rat monoclonal antibodies (mAb), which are used in clinical trials. In parallel, the anti-galactosyl humoral response was assessed in the serum of kidney allograft recipients and experimental baboons, which received these mAbs. Galactosyl residues were evidenced on all rat monoclonal antibody tested. The anti-galactosyl response was weak in kidney allograft recipients receiving a basic immunosuppression (Cyclosporine, Azathioprine, Prednisolone) and iterative injections of rat mAbs. In contrast, untreated or immunosuppressed baboons that received rat mAbs developed a major anti-galactosyl humoral response. These results suggest that anti-galactosyl sensitization produced by therapeutic agents will have to be considered in the case of clinical xenotransplantation.

The fourth barrier

At the entrance of a new era, clinical xenotransplantation is a valued and auspicious option in tackling the problem of donor shortage. Because of ethical and anatomical issues, domestic farm animals are considered the most favourable species for organ donation, but transplantation of their organs leads to a complex process of rejection. Mechanistically, three immunological barriers, namely hyperacute rejection, delayed xenograft rejection and a subsequent cellular rejection, are distinguished. A fifth (microbiological) barrier is also being recognised. This review focuses on problems regarding the fourth barrier, i.e. physiology, in possible clinical settings and their corresponding animal models. Besides anatomical differences and posture, biochemical differences may have a severe impact on recipient survival. Differences in blood components and electrolyte and other biochemical concentrations are easily detected throughout the species considered for xenotransplantation. Enzymes and hormones have complex routes of action, activation and inhibition, and their molecular differences can impede function. As infusion or medicine may correct certain imbalances in electrolytes and proteins, problems with complex interactions might be difficult to retrieve and solve. Experimentally, survival of discordant xenografts show promising results, but the first physiological problems have already been detected. So, based upon the few experimental data available and the comparison of veterinary physiology, one might expect differences between the organs grafted, regarding the possible occurrence of physiological problems. Moreover, precautions must be taken to extrapolate long-term survival, because of species specificity.

Immunology of xenotransplantation

The transplantation of tissues and organs between individuals of different species, that is, xenotransplantation, engenders a variety of immune responses. Xenogeneic immune responses mediated by naturally-occurring antibodies and complement lead to hyperacute and acute vascular rejection of vascularized organ grafts and may also cause vascular rejection of cell and tissue grafts. Under some circumstances, however, a vascularized organ graft may evade humoral rejection despite the presence of anti-donor antibodies in the circulation of the recipient; this condition is called accommodation. Xenogeneic immune responses mediated by T lymphocytes and natural killer cells may cause acute cellular rejection. The extent to which cellular rejection of xenografts resembles cellular rejection of allografts remains to be determined. New insights into the molecular mechanisms underlying the immune responses to xenotransplantation has shed light on the pathogenesis of immunological disease and has allowed the development of specific immunomodulatory strategies that may facilitate clinical application of xenotransplantation.

Innate natural antibodies. Primary roles indicated by specific epitopes

Two members of a unique class of natural antibodies have been identified in all of a large cohort of sera from clinically normal humans of broad age distribution. By means of a series of 10-12 mer peptides the epitope for each of those antibodies was characterized with regard to amino acid identity and conformation. Similar epitope specificity was revealed for the IgM isotopes of cord blood and early post natal sera and for IgM and IgG of adult sera, suggesting that the class of natural antibodies represented by the two identified in this study includes those genomically coded for at their effector level of maturation in the B cells of the neonate. Assay of series of specimens from each of four clinically normal adults revealed that those two natural antibodies are present at relatively constant titer, unique to each individual, over four to five and a half year periods. Those observations imply that the primary function of that class of natural antibodies may be related to maintenance of homeostasis and the molecular identity of each of the two epitopes suggests a role, for each, as monitor or control in intracellular traffic. The previous identification of those epitopes in a conserved protein of HIV also provides support for the proposition that a secondary function of natural antibodies, arising from fortuitous coincidence of the identity of the epitopes, may be that of early defense against infectious invaders.

Mapping the active site of CD59

CD59 is a widely distributed membrane-bound inhibitor of the cytolytic membrane attack complex (MAC) of complement. This small (77 amino acid) glycoprotein is a member of the Ly6 superfamily of proteins and is important in protecting host cells from the lytic and proinflammatory activity of the MAC. CD59 functions by binding to C8 and/or C9 in the nascent MAC and interfering with C9 membrane insertion and polymerization. We present data obtained from a combination of molecular modeling and mutagenesis techniques, which together indicate that the active site of CD59 is located in the vicinity of a hydrophobic groove on the face of the molecule opposite to a "hydrophobic strip" suggested earlier. In addition, removal of the single N-linked glycosylation site at Asn18 of CD59 resulted in an enhancement of complement inhibitory activity.

Recent advances in the immunology of xenotransplantation

The transplantation of tissue and organs between individuals of different species, that is xenotransplantation, engenders a variety of severe immune responses. Xenogeneic immune responses mediated by naturally occurring antibodies and complement lead to hyperacute and acute vascular rejection of vascularized organ grafts and may also cause vascular rejection of cell and tissue grafts. Under some circumstances, however, a vascularized organ graft may evade humoral rejection despite the presence of antidonor antibodies in the circulation of the recipient; this condition is called accommodation. Xenogeneic immune responses mediated by T-lymphocytes and natural killer cells may cause acute cellular rejection. The extent to which cellular rejection of xenografts resembles cellular rejection of allografts remains to be determined. New insights into the molecular mechanisms underlying the immune responses to xenotransplantation have shed new light on the pathogenesis of immunological disease and have allowed the development of specific immunomodulatory strategies that may facilitate clinical application of xenotransplantation.