Self tolerance to human A and B histo-blood group antigens exists at the B cell level and cannot be broken by potent polyclonal B cell activation in vitro

Innate and Adaptive Immunity to Transfused Allogeneic RBCs in Mice Requires MyD88

RBC transfusion therapy is essential for the treatment of anemia. A serious complication of transfusion is the development of non-ABO alloantibodies to polymorphic RBC Ags; yet, mechanisms of alloantibody formation remain unclear. Storage of mouse RBCs before transfusion increases RBC immunogenicity through an unknown mechanism. We previously reported that sterile, stored mouse RBCs activate splenic dendritic cells (DCs), which are required for alloimmunization. Here we transfused mice with allogeneic RBCs to test whether stored RBCs activate pattern recognition receptors (PRRs) on recipient DCs to induce adaptive immunity. TLRs are a class of PRRs that regulate DC activation, which signal through two adapter molecules: MyD88 and TRIF. We show that the inflammatory cytokine response, DC activation and migration, and the subsequent alloantibody response to transfused RBCs require MyD88 but not TRIF, suggesting that a restricted set of PRRs are responsible for sensing RBCs and triggering alloimmunization.

The five dimensions of B cell tolerance

B cell tolerance has been generally understood to be an acquired property of the immune system that governs antibody specificity in ways that avoid auto-toxicity. As useful as this understanding has proved, it fails to fully explain the existence of auto-reactive specificities in healthy individuals and contribution these may have to health. Mechanisms underlying B cell tolerance are considered to select a clonal repertoire that generates a collection of antibodies that do not bind self, ie tolerance operates more or less in three dimensions that largely spare autologous cells and antigens. Yet, most B lymphocytes in humans and probably in other vertebrates are auto-reactive and absence of these auto-reactive B cells is associated with disease. We suggest that auto-reactivity can be embodied by extending the concept of tolerance by two further dimensions, one of time and circumstance and one that allows healthy cells to actively resist injury. In this novel concept, macromolecular recognition by the B cell receptor leading to deletion, anergy, receptor editing or B cell activation is extended by taking account of the time of development of normal immune responses (4th dimension) and the accommodation (or tolerance) of normal cells to bound antibody, activation of complement, and interaction with inflammatory cells (fifth dimension). We discuss how these dimensions contribute to understanding B cell biology in health or disease.

Effect of blood type on anti-α-Gal immunity and the incidence of infectious diseases

The identification of factors affecting the susceptibility to infectious diseases is essential toward reducing their burden on the human population. The ABO blood type correlates with susceptibility to malaria and other infectious diseases. Due to the structural similarity between blood antigen B and Galα1-3Galβ1-(3)4GlcNAc-R (α-Gal), we hypothesized that self-tolerance to antigen B affects the immune response to α-Gal, which in turn affects the susceptibility to infectious diseases caused by pathogens carrying α-Gal on their surface. Here we found that the incidence of malaria and tuberculosis, caused by pathogens with α-Gal on their surface, positively correlates with the frequency of blood type B in endemic regions. However, the incidence of dengue fever, caused by a pathogen without α-Gal, was not related to the frequency of blood type B in these populations. Furthermore, the incidence of malaria and tuberculosis was negatively correlated with the anti-α-Gal antibody protective response. These results have implications for disease control and prevention.

Conjugation of A and B Blood Group Structures to Silica Microparticles for the Detection of Antigen-Specific B Cells

Silica microparticles were functionalized with A and B blood group carbohydrate antigens (A type I, A type II, B type I, and B type II) to enable the detection and monitoring of ABO antigen-specific B cells. Microparticles were prepared via the Stöber synthesis, labeled with an Alexafluor fluorescent dye, and characterized via TEM and fluorescence microscopy. The silica microparticles were functionalized with (3-aminopropyl)trimethoxysilane (APTMS), followed by the use of an established fluorenylmethyloxycarbonyl (Fmoc)-protected PEG-based linker. The terminal Fmoc moiety of the PEG-based linker was then deprotected, yielding free amino groups, to which the A and B antigens were coupled. The carbohydrate antigens were synthesized with a p-nitrophenol ester to enable conjugation to the functionalized silica microparticles via an amide bond. The number of free amine groups available for coupling for a given mass of PEG-functionalized silica microparticles was quantified via reaction with Fmoc-glycine. The antigen-functionalized microparticles were then evaluated for their specificity in binding to A and B antigen-reactive B-cells via flow cytometry, and for blocking of naturally occurring antibodies in human serum. Selective binding of the functionalized microparticles to blood group-reactive B cells was observed by flow cytometry and fluorescence microscopy. The modular approach outlined here is applicable to the preparation of silica microparticles containing any carbohydrate antigen and alternative fluorophores or labels. This approach therefore comprises a novel, general platform for screening B cell populations for binding to carbohydrate antigens, including, in this case, the human A and B blood group antigens.

Development of B-cell memory in early childhood and the impact on antigen-specific tolerance after heart transplantation

BACKGROUND

Young children show better survival after heart transplant compared with older individuals and can receive heart transplants safely from ABO-incompatible donors. Children develop immunologic tolerance to donor ABH antigens reflected in persistent absence of specific antibodies. We hypothesized that immature T-independent B-cell response and lack of B-cell memory play a crucial role in tolerance of ABH antigens after ABOi transplants.

METHODS

We determined phenotypes of splenic lymphocytes from adults and children and peripheral blood from ABO-incompatible or ABO-compatible heart transplant recipients and control subjects by flow cytometry. In vitro immune response to T-independent stimulation, erythrocytes, and ABH antigens was assessed using proliferation assays.

RESULTS

A predominant role for CD27(+) B cells in T-independent activation was demonstrated; these cells were significantly less frequent in infants than older subjects. Only IgM(+)CD27(+) B cells proliferated in response to non-self erythrocytes. In blood, IgM(+) and switched IgM(-) memory B cells were rare in infants, increasing to near-adult levels in children 5 years old. IgM(+)CD27(+) B cells were significantly fewer in ABO-incompatible transplant recipients than in ABO-compatible recipients.

CONCLUSIONS

CD27(+) cells play a key role in T-independent B-cell activation. Response to ABH antigens is mediated by IgM(+)CD27(+) B cells, and donor ABO-specific tolerance after ABO-incompatible transplantation in children is facilitated by low prevalence of these cells. The pattern of B-cell memory development is altered after ABO-incompatible transplant. Memory B cells may be quantified to assess eligibility for ABO-incompatible transplant and represent a potential therapeutic target to extend the benefits of the immature immune system to older age groups.

Monitoring B cell subsets and alloreactivity in kidney transplantation

B cells are the precursors of antibody producing plasma cells that can give rise to the formation of donor-specific antibodies. However, recent data suggest that besides their role in antibody production, B cells participate in antibody-independent responses, potentially leading to allograft rejection or allograft tolerance. The presence of CD20(+) B cells in kidney graft biopsies has been shown during severe acute rejection episodes and during chronic rejection. Furthermore, operationally tolerant kidney transplant recipients showed a clear B cell dominated fingerprint of tolerance. Several techniques exist to study B cells on different levels. Numerous classification schemes allow for the distinction of many different B cell subsets using flow cytometry. Regardless, data on B cell subsets during stable graft function, rejection or tolerance remain scarce. To obtain a complete picture of the role of B cells during transplantation, antigen specific B cell assays may be required. Therefore, techniques have now been developed that allow for studying the specificity and frequency of HLA specific B cells. Here, we present an overview of the existent assays, panels and techniques intended to characterize peripheral B cells, and the currently available HLA specific B cell functional assays that may allow for monitoring the humoral alloimmune response in transplant recipients.

Partial tolerance of autoreactive B and T cells to erythrocyte-specific self-antigens in mice

BACKGROUND

Breakdown of humoral tolerance to RBC antigens may lead to autoimmune hemolytic anemia, a severe and sometimes fatal disease. The underlying mechanisms behind the breakdown of humoral tolerance to RBC antigens are poorly understood.

DESIGN AND METHODS

In order to study the pathogenesis of autoimmune hemolytic anemia, we developed a murine model with RBC-specific expression of a model antigen carrying epitopes from hen egg lysozyme and ovalbumin.

RESULTS

Humoral tolerance was observed; this was not broken even by strong immunogenic stimulation (lysozyme or ovalbumin with adjuvant). Autoreactive CD4(+) T cells were detected by tetramer enrichment assays, but failed to activate or expand despite repeat stimulation, indicating a nonresponsive population rather than deletion. Adoptive transfer of autoreactive CD4(+) T cells (OT-II mice) led to autoantibody (anti-lysozyme) production by B cells in multiple anatomic compartments, including the bone marrow.

CONCLUSIONS

These data demonstrate that B cells autoreactive to RBC antigens survive in healthy mice with normal immune systems. Furthermore, autoreactive B cells are not centrally tolerized and are receptive to T-cell help. As the autoreactive T cells are present but non-responsive, these data indicate that factors that reverse T-cell non-responsiveness may be central to the pathogenesis of autoimmune hemolytic anemia.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.

The persistent elimination of B cells responding to blood group A carbohydrates by synthetic group A carbohydrates and B-1 cell differentiation blockade: novel concept in preventing antibody-mediated rejection in ABO-incompatible transplantation

We demonstrated a novel strategy for specific and persistent inhibition of antibody (Ab) production against blood group A or B carbohydrate determinants necessary for successful ABO-incompatible transplantation. Similar to human blood group O or B individuals, mice have naturally occurring Abs against human blood group A carbohydrates in their sera. B cells with receptors for A carbohydrates in mice belonging to the CD5+CD11b+B-1a subset have phenotypic properties similar to those of human B cells. These cells could be temporarily eliminated by injecting synthetic A carbohydrates (GalNAcα1–3, Fucα1–2Gal) conjugated to bovine serum albumin (A-BSA) and anti-BSA Abs. In mice that received the injection of A-BSA/anti-BSA Abs, the serum levels of anti-A IgM were reduced, but immunization with human A erythrocytes resulted in increased serum levels of anti-A Abs. When combined with cyclosporin A (CsA) treatment, which blocks B-1a cell differentiation, and treatment with A-BSA/anti-BSA Abs, the serum levels of anti-A Abs were persistently undetectable in the mice even after the immunization. B cells with receptors for A carbohydrates were markedly reduced in these mice. These results are consistent with the hypotheses that treatment with A-BSA/anti-BSA Abs temporarily depletes B cells responding to A determinants, and CsA treatment prevents the replenishment of these cells.

Synthetic blood group antigens for anti-A removal device and their interaction with monoclonal anti-A IgM

Removal of blood group antibodies against the donor organ prior to ABO-incompatible transplantation can prevent episodes of hyperacute rejection. We are developing a specific antibody filter (SAF) device consisting of immobilized synthetic Atrisaccharide antigens conjugated to polyacrylamide (Atri-PAA) to selectively remove anti-A antibodies directly from whole blood. In this study, we evaluated eight anti-A IgM monoclonal antibodies (mAbs) using Enzyme-Linked Immunosorbent Assay (ELISA) to determine their specificity for binding to Atri-PAA. Five of the eight mAbs met our criteria for specificity by binding to Atri-PAA with at least five times greater affinity compared to the negative controls. These selected mAbs will be studied for their binding characteristics to Atri-PAA which will aid in the development of the SAF. The study of kinetics of antibody removal and quantification of antibody removal will be used in our mathematical model to maximize the antibody removal rate and binding capacity of the SAF.

B-cell Tolerance following ABO-Incompatible Infant Heart Transplantation

Infant heart transplantation is highly successful therapy for severe congenital heart malformations and cardiomyopathies. However the donor organ shortage is particularly crucial for this age group. We have demonstrated that some aspects of human immaturity can be exploited for infant organ transplantation, allowing safe transplantation of organs that would be "incompatible" in mature individuals, and as a window of continued susceptibility to induction of immunologic tolerance.

Histo-blood group antigens as allo- and autoantigens

The science of blood groups has made giant steps forward during the last decade. Blood-group typing of red blood cells (RBCs) is performed on more than 15 million samples per year in Europe, today much less often for forensic reasons than for clinical purposes such as transfusion and organ transplantation. Specific monoclonal antibodies are used with interpretation on the basis of RBC agglutination patterns, and mass genotyping may well be on its way to becoming a routine procedure. The discovery that most blood group systems, whose antigens are by definition found on RBCs, are also expressed in multiple other tissues has sparked the interest of transplantation medicine in immunohematology beyond the HLA system. The one and only "histo-blood group" (HBG) system that is routinely considered in transplantation medicine is ABO, because ABO antigen-incompatible donor/recipient constellations are preferably avoided. However, other HBG systems may also play a role, thus far underestimated. This paper is an up-to-date analysis of the importance of HBG systems in the alloimmunity of transplantation and autoimmune events, such as hemolytic anemia.

NOD/SCID mice engrafted with human peripheral blood lymphocytes can be a model for investigating B cells responding to blood group A carbohydrate determinant

Human antibodies (Abs) against blood group A or B carbohydrate determinant are a major barrier to ABO-incompatible organ transplantation; however, the phenotype and other properties of B cell types responding to A or B carbohydrate epitopes have not been defined. Studies here, which use fluorescein-labeled synthetic A determinant (GalNAcalpha1-3Fucalpha1-2Gal), demonstrate that B cells bearing surface IgM (sIgM) receptors recognizing blood group A carbohydrate determinant are found exclusively in a small B cell subpopulation, i.e. sIgM+ CD11b+ CD5+ B1 cells, in blood group O human peripheral blood mononuclear cells (PBMC). In order to test anti-A Abs producing capacity of the human PBMC, nonobese diabetic (NOD)/severe combined immune-deficient (SCID) mice that have been treated with rabbit anti-asialo GM1 serum to deplete natural killer cells and with 3 Gy of whole body irradiation were engrafted with blood group O or A human PBMC, followed by sensitization of human blood group A red blood cells. Anti-A-specific human Abs were detected in the sera of the mice that received blood group O human PBMC, whereas they were not detected in the sera of the mice that received blood group A human PBMC, indicating profound tolerance of auto-reactive B cells. The human PBMC-NOD/SCID chimera developed by injection of blood group O human PBMC might be a useful in vivo model to test effects of immunosuppressants or other approaches on human B cells that respond to blood group A antigens.

Solid organ transplantation across the ABO histo-blood group barrier: a case report

The ABO blood group system until recently constituted an insuperable barrier for solid organ transplantation, but cases of heart transplantation in infants and kidney transplantation in adults have been reported, wherein ABO-incompatible grafts have been successful. In 1990, the molecular genetic basis of three major alleles at the ABO locus was elucidated; A and B glycosyltransferases are specified by a variety of functional alleles at this locus. The antibody response to ABH antigens, namely, naturally occurring anti-A/B IgM and IgG isotype agglutinins, are controlled preoperatively by recipient conditioning using plasma exchange, immunoadsorption, and immunosuppressive regimens. We report an O-type patient who accidentally received a B-type cardiac allograft in 1997 who survived for 5 years, dying for an unrelated reason. Over a period of 45 months semiquantitatively we monitored the expression of ABO-type antigens in graft heart vessels using monoclonal antibodies on sections of formalin-fixed, paraffin-embedded biopsies. We observed a progressive change in the antigenic profile of graft endothelial cells from B- to O-type, which was first detected at 1 year posttransplant and most prominent 3 years later, the end of the observation period. No temporal relationship was observed between the transition from B to O expression, the anti-B antibody levels or the immunosuppressive regimen.

The origin of anti-GM1 antibodies in neuropathies: the "binding site drift" hypothesis

Elevated titers of serum antibodies against GM1-ganglioside are associated with a variety of autoimmune neuropathies. The origin of these autoantibodies is still unknown, although there is evidence that they are produced by CD5+ B-lymphocytes and that antigen mimicry is involved. Anti-GM, IgM-antibodies in the normal human immunological repertoire are low affinity antibodies that cross-react with other glycoconjugates carrying Gal beta1-3GalNAc and probably do not have GM1-mediated biological activity. Other anti-GM1 IgM-antibodies with higher affinity and/or different fine specificity are present in patients with motor syndromes. Based on our studies of structural requirement for binding, we hypothesize that disease-associated anti-GM1 antibodies originate at random by mutations affecting the binding site of naturally-occurring ones. The hypothesis is conceptually similar to the established phenomenon of "genetic drift" in species evolutionary biology and is therefore termed "binding site drift".

Determination of the frequency of HLA antibody secreting B-lymphocytes in alloantigen sensitized individuals

Sera from prospective transplant patients are usually screened for HLA antibodies prior to transplantation, but presently available tests do not permit quantification of the humoral alloantigen directed response. We adapted a culture system for isolated human B-lymphocytes to assay the secretion of HLA-antibodies on a single cell basis. B-cell supernatants were screened for HLA antibodies by complement dependent cytotoxicity. The assay assigns precursor frequencies for HLA-alloantibody secreting B-lymphocytes (BCPFs), and simultaneously allows for dissection of the humoral alloantigen directed response into its monoclonal components. The lymphocytes of 15 HLA-seropositive multiparous women that were used to validate the assay, were found to contain HLA-BCPFs ranging from 0 to 123 per 10(6) B-lymphocytes (mean: 43 +/- 45 per 10(6) B-lymphocytes). The HLA-specificities of antibodies in the B-cell supernatants were in agreement with serum specificities. Genuine HLA reactivity of B-cell supernatants was confirmed using an ELISA with purified HLA class I antigens. When applied to lymphocytes of patients on transplant waiting lists, the present assay may enable the unraveling of serum specificities in their components, thus supplementing HLA antibody serum screening data.

ABO-incompatible heart transplantation in infants

BACKGROUND

Transplantation of hearts from ABO-incompatible donors is contraindicated because of the risk of hyperacute rejection mediated by preformed antibodies in the recipient to blood-group antigens of the donor. This contraindication may not apply to newborn infants, who do not yet produce antibodies to T-cell-independent antigens, including the major blood-group antigens.

METHODS

We studied 10 infants 4 hours to 14 months old (median, 2 months) who had congenital heart disease or cardiomyopathy and who received heart transplants from donors of incompatible blood type between 1996 and 2000. Serum isohemagglutinin titers were measured before and after transplantation. Plasma exchange was performed during cardiopulmonary bypass; no other procedures for the removal of antibodies were used. Standard immunosuppressive therapy was given, and rejection was monitored by means of endomyocardial biopsy. The results were compared with those in 10 infants who received heart transplants from ABO-compatible donors.

RESULTS

The overall survival rate among the 10 recipients with ABO-incompatible donors was 80 percent, with 2 early deaths due to causes presumed to be unrelated to ABO incompatibility. The duration of follow-up ranged from 11 months to 4.6 years. Two infants had serum antibodies to antigens of the donor's blood group before transplantation. No hyperacute rejection occurred; mild humoral rejection was noted at autopsy in one of the infants with antibodies. No morbidity attributable to ABO incompatibility has been observed. Despite the eventual development of antibodies to antigens of the donor's blood group in two infants, no damage to the graft has occurred. Because of the use of ABO-incompatible donors, the mortality rate among infants on the waiting list declined from 58 percent to 7 percent.

CONCLUSIONS

ABO-incompatible heart transplantation can be performed safely during infancy before the onset of isohemagglutinin production; this technique thus contributes to a marked reduction in mortality among infants on the waiting list.

Normal Human Serum Contains Natural Antibodies Reactive With Autologous ABO Blood Group Antigens

It is widely accepted that the serum of healthy individuals contains natural antibodies only against those blood group A or B antigens that are not expressed on the individual’s red blood cells. The mechanisms involved in tolerance to autologous blood group antigens remain unclear. In the present study, we show that IgM and IgG antibodies reactive with autologous blood group antigens are present in the immunoglobulin fraction of normal human serum. Natural IgG anti-A antibodies purified by affinity chromatography from IgG of individuals of blood group A exhibited an affinity for A trisaccharide antigen in the micromolar range and agglutinated A red cells at sixfold higher concentrations than those required for agglutination with affinity-purified anti-A IgG of individuals of blood group B. Whereas autoantibodies reactive with self A and B antigens are readily detected in purified IgG and IgM fractions, their expression is restricted in whole serum as a result of complementary interactions between variable regions of antibodies. These observations suggest that tolerance to autologous ABO blood group antigens is dependent on peripheral control of antibody autoreactivity.

Normal Human Serum Contains Natural Antibodies Reactive With Autologous ABO Blood Group Antigens

It is widely accepted that the serum of healthy individuals contains natural antibodies only against those blood group A or B antigens that are not expressed on the individual’s red blood cells. The mechanisms involved in tolerance to autologous blood group antigens remain unclear. In the present study, we show that IgM and IgG antibodies reactive with autologous blood group antigens are present in the immunoglobulin fraction of normal human serum. Natural IgG anti-A antibodies purified by affinity chromatography from IgG of individuals of blood group A exhibited an affinity for A trisaccharide antigen in the micromolar range and agglutinated A red cells at sixfold higher concentrations than those required for agglutination with affinity-purified anti-A IgG of individuals of blood group B. Whereas autoantibodies reactive with self A and B antigens are readily detected in purified IgG and IgM fractions, their expression is restricted in whole serum as a result of complementary interactions between variable regions of antibodies. These observations suggest that tolerance to autologous ABO blood group antigens is dependent on peripheral control of antibody autoreactivity.

Type 2 T-cell-independent murine immune response to the human AB0 blood group antigens

The human AB0 blood group determinants are simple carbohydrate structures which are widely distributed in nature. Much work has been done on the structure of the A and B antigens but little is known on the regulation of anti-A and anti-B immune responses. To develop a model system, we have characterized the AB0 immunity of normal Balb/c mice and found a significant level of serum natural anti-A but almost no anti-B. This finding and the known IgM predominance among immune anti-A produced in the Balb/c mouse indicate that the AB0 immunity of this mouse strain is comparable to the one of human blood group B individuals. Following immunization with human red blood cells, similar levels of anti-A were produced in normal and athymic Balb/c mice showing that the anti-A response is T cell independent. Furthermore, no anti-A or anti-B antibodies were produced in CBA/xid mice indicating a type 2 T-cell-independent immune response. These results may contribute to a better understanding of human AB0 responses and establish the mouse as a suitable model to study the immunobiology of AB0 antigens.