Biphasic clearance of incompatible red blood cells through a novel mechanism requiring neither complement nor Fcγ receptors in a murine model

Dynamics of antibody engagement of red blood cells in vivo and in vitro

Exposure to allogenic red blood cells (RBCs), either through pregnancy or transfusion, can result in alloimmunization, which can lead to severe hemolytic transfusion reactions and pregnancy complications. Passively administered antibodies can be used to prevent alloimmunization, where steric hindrance of allogeneic epitopes has been postulated as one mechanism whereby antibody engagement may prevent RBC alloimmunization. However, the dynamics of antibody engagement on the RBC surface has remained difficult to study. To examine this, we leveraged the HOD (HEL, OVA and Duffy) model system and Fcγ receptor knockout recipients to define the dynamics of antibody engagement of the Duffy antigen in the absence of RBC clearance or antigen modulation. Using this approach, the on-rate of antibody engagement of HOD RBCs was very similar in vivo and in vitro, with high levels of antibody binding observed within minutes of HOD RBC exposure. In contrast, the off-rate of HOD RBC bound antibody was relatively slow, with appreciable dissociation not being observed for an hour. However, the dynamics of antibody interactions with HOD changed significantly when antibody decorated HOD RBCs were exposed to free antibody. Despite the presence of prebound antibody, free antibody rapidly associated with HOD RBCs, with the rate of free antibody association observed being faster in vivo than in vitro. Importantly, antibody association and dissociation occurred in the absence of any appreciable changes in RBC clearance, antigen modulation or complement deposition, suggesting that differences in antibody levels observed reflected actual differences in the dynamics of antibody binding. These results suggest that while antibodies appear to be relatively static on the cell surface once bound, antibody engagement can be quite dynamic, especially in the face of free antibody in solution. These results not only have implications in the mechanisms of antibody-mediated immunosuppression, but also the potential use of other antibody-based approaches designed to prevent hemolytic transfusion reactions or target antigens in vivo in general.

The Development and Consequences of Red Blood Cell Alloimmunization

While red blood cell (RBC) transfusion is the most common medical intervention in hospitalized patients, as with any therapeutic, it is not without risk. Allogeneic RBC exposure can result in recipient alloimmunization, which can limit the availability of compatible RBCs for future transfusions and increase the risk of transfusion complications. Despite these challenges and the discovery of RBC alloantigens more than a century ago, relatively little has historically been known regarding the immune factors that regulate RBC alloantibody formation. Through recent epidemiological approaches, in vitro-based translational studies, and newly developed preclinical models, the processes that govern RBC alloimmunization have emerged as more complex and intriguing than previously appreciated. Although common alloimmunization mechanisms exist, distinct immune pathways can be engaged, depending on the target alloantigen involved. Despite this complexity, key themes are beginning to emerge that may provide promising approaches to not only actively prevent but also possibly alleviate the most severe complications of RBC alloimmunization.

ABO blood group antigens and differential glycan expression: Perspective on the evolution of common human enzyme deficiencies

Enzymes catalyze biochemical reactions and play critical roles in human health and disease. Enzyme variants and deficiencies can lead to variable expression of glycans, which can affect physiology, influence predilection for disease, and/or directly contribute to disease pathogenesis. Although certain well-characterized enzyme deficiencies result in overt disease, some of the most common enzyme deficiencies in humans form the basis of blood groups. These carbohydrate blood groups impact fundamental areas of clinical medicine, including the risk of infection and severity of infectious disease, bleeding risk, transfusion medicine, and tissue/organ transplantation. In this review, we examine the enzymes responsible for carbohydrate-based blood group antigen biosynthesis and their expression within the human population. We also consider the evolutionary selective pressures, e.g. malaria, that may account for the variation in carbohydrate structures and the implications of this biology for human disease.

Detection of Phosphatidylserine Exposure on Leukocytes Following Treatment with Human Galectins

Cellular turnover represents a fundamental aspect of immunological homeostasis. While many factors appear to regulate leukocyte removal during inflammatory resolution, recent studies suggest that members of the galectin family play a unique role in orchestrating this process. Unlike cellular removal through apoptotic cell death, several members of the galectin family induce surface expression of phosphatidylserine (PS), a phagocytic marker on cells undergoing apoptosis, in the absence of cell death. However, similar to PS on cells undergoing apoptosis, galectin-induced PS exposure sensitizes cells to phagocytic removal. As galectins appear to prepare cells for phagocytic removal without actually inducing apoptotic cell death, this process has recently been coined preaparesis. Given the unique characteristics of galectin-induced PS exposure in the context of preaparesis, we will examine unique considerations when evaluating the potential impact of different galectin family members on PS exposure and cell viability.

Examination of Whole-Cell Galectin Binding by Solid Phase and Flow Cytometric Analysis

We have utilized simple flow cytometric and fluorescence-based solid phase assays to study the interaction of glycan binding proteins (GBP) to cell surface glycoconjugates. These methods utilize commonly employed flow cytometry techniques and commercially available streptavidin-coated microplates to immobilize various biotinylated ligands, such as glycopeptides, oligosaccharides, and whole cells. Using this approach, fluorescently labeled GBPs, in particular, members of the galectin family, can be interrogated for potential interactions with cell surface carbohydrates, including elucidation of the potential impact of alterations in glycosylation on carbohydrate recognition. Using these approaches, we present examples of flow cytometric and fluorescence-based solid phase assays to study galectin-carbohydrate interactions.

Antigen density dictates RBC clearance, but not antigen modulation, following incompatible RBC transfusion in mice

Incompatible red blood cell (RBC) transfusion can result in life-threatening transfusion complications that can be challenging to manage in patients with transfusion-dependent anemia. However, not all incompatible RBC transfusions result in significant RBC removal. One factor that may regulate the outcome of incompatible RBC transfusion is the density of the incompatible antigen. Despite the potential influence of target antigen levels during incompatible RBC transfusion, a model system capable of defining the role of antigen density in this process has not been developed. In this study, we describe a novel model system of incompatible transfusion using donor mice that express different levels of the KEL antigen and recipients with varying anti-KEL antibody concentrations. Transfusion of KEL+ RBCs that express high or moderate KEL antigen levels results in rapid antibody-mediated RBC clearance. In contrast, relatively little RBC clearance was observed following the transfusion of KEL RBCs that express low KEL antigen levels. Intriguingly, unlike RBC clearance, loss of the KEL antigen from the transfused RBCs occurred at a similar rate regardless of the KEL antigen density following an incompatible transfusion. In addition to antigen density, anti-KEL antibody levels also regulated RBC removal and KEL antigen loss, suggesting that antigen density and antibody levels dictate incompatible RBC transfusion outcomes. These results demonstrate that antibody-induced antigen loss and RBC clearance can occur at distinct antigen density thresholds, providing important insight into factors that may dictate the outcome of an incompatible RBC transfusion.

Red blood cell alloimmunization and sickle cell disease: a narrative review on antibody induction

The high prevalence of red blood cell (RBC) alloantibodies in people with sickle cell disease (SCD) cannot be debated. Why people with SCD are so likely to form RBC alloantibodies, however, remains poorly understood. Over the past decade, a better understanding of non-ABO blood group antigen variants has emerged; RH genetic diversity and the role this diversity plays in RBC alloimmunization is discussed elsewhere. Outside of antigen variants, the immune systems of people with SCD are known to be different than those of people without SCD. Some of these differences are due to effects of free heme, whereas others are impacted by hyposplenism. Descriptive studies of differences in white blood cell (WBC) subsets, platelet counts and function, and complement activation between people with SCD and race-matched controls exist. Studies comparing the immune systems of alloimmunized people with SCD to non-alloimmunized people with SCD to race-matched controls without SCD have uncovered differences in T-cell subsets, monocytes, Fcγ receptor polymorphisms, and responses to free heme. Studies in murine models have documented the role that recipient inflammation plays in RBC alloantibody formation, with human studies reporting a similar association. Murine studies have also reported the importance of type 1 interferon (IFNα/β), known to play a pivotal role in autoimmunity, in RBC alloantibody formation. The goal of this manuscript is to review existing data on factors influencing RBC alloantibody induction in people with SCD with a focus on inflammation and other immune system considerations, from the bench to the bedside.

Examining the Role of Complement in Predicting, Preventing, and Treating Hemolytic Transfusion Reactions

Red blood cell (RBC) transfusion is a critical component of optimal management for a broad range of conditions. Regardless of the indication, pretransfusion testing is required to appropriately match RBC donors and recipients to provide immunologically compatible blood. Although this approach is effective in the vast majority of situations, occasionally, patients will inadvertently receive an incompatible RBC transfusion, which can result in a hemolytic transfusion reaction (HTR). In addition, patients with life-threatening anemia and a complex alloantibody profile, which precludes rapid procurement of compatible RBCs, may also receive incompatible RBCs, placing them at risk for an HTR. Despite the rarity of these clinical situations, when incompatible blood transfusion results in an HTR, the consequences can be devastating. In this review, we will explore the challenges associated with actively preventing and treating acute HTRs following incompatible RBC transfusion. In doing so, we will focus primarily on the role of complement, not only as a key player in HTRs, but also as a potential target for the prevention and treatment of HTRs.

Challenges in preventing and treating hemolytic complications associated with red blood cell transfusion

Red blood cell (RBC) transfusion support represents a critical component of sickle cell disease (SCD) management. However, as with any therapeutic intervention, RBC transfusion is not without risk. Repeat exposure to allogeneic RBCs can result in the development of RBC alloantibodies that can make it difficult to find compatible RBCs for future transfusions and can directly increase the risk of developing acute or delayed hemolytic transfusion reactions, which can be further complicated by hyperhemolysis. Several prophylactic and treatment strategies have been employed in an effort to reduce or prevent hemolytic transfusion reactions. However, conflicting data exist regarding the efficacy of many of these approaches. We will explore the challenges associated with predicting, preventing and treating different types of hemolytic transfusion reactions in patients with SCD in addition to describing future strategies that may aid in the management of the complex transfusion requirements of SCD patients.

Transfusion-related red blood cell alloantibodies: induction and consequences

Blood transfusion is the most common procedure completed during a given hospitalization in the United States. Although often life-saving, transfusions are not risk-free. One sequela that occurs in a subset of red blood cell (RBC) transfusion recipients is the development of alloantibodies. It is estimated that only 30% of induced RBC alloantibodies are detected, given alloantibody induction and evanescence patterns, missed opportunities for alloantibody detection, and record fragmentation. Alloantibodies may be clinically significant in future transfusion scenarios, potentially resulting in acute or delayed hemolytic transfusion reactions or in difficulty locating compatible RBC units for future transfusion. Alloantibodies can also be clinically significant in future pregnancies, potentially resulting in hemolytic disease of the fetus and newborn. A better understanding of factors that impact RBC alloantibody formation may allow general or targeted preventative strategies to be developed. Animal and human studies suggest that blood donor, blood product, and transfusion recipient variables potentially influence which transfusion recipients will become alloimmunized, with genetic as well as innate/adaptive immune factors also playing a role. At present, judicious transfusion of RBCs is the primary strategy invoked in alloimmunization prevention. Other mitigation strategies include matching RBC antigens of blood donors to those of transfusion recipients or providing immunomodulatory therapies prior to blood product exposure in select recipients with a history of life-threatening alloimmunization. Multidisciplinary collaborations between providers with expertise in transfusion medicine, hematology, oncology, transplantation, obstetrics, and immunology, among other areas, are needed to better understand RBC alloimmunization and refine preventative strategies.

Challenges in the treatment and prevention of delayed hemolytic transfusion reactions with hyperhemolysis in sickle cell disease patients

BACKGROUND

Delayed hemolytic transfusion reactions (DHTRs) are serious complications of RBC transfusion that can occur in previously alloimmunized patients. Patients who require episodic transfusions during heightened inflammatory states, such as patients with sickle cell disease (SCD), are particularly prone to alloimmunization and developing DHTRs with hyperhemolysis. While efforts to mitigate these hemolytic episodes via immunosuppressive drugs can be employed, the relative efficacy of various treatment options remains incompletely understood.

CASE REPORTS

In this study, we explored five patients with SCD and multiple RBC alloantibodies who received various forms of immunosuppressive therapy in an attempt to prevent or treat severe DHTRs.

RESULTS

The clinical course for these five patients provides insight into the difficulty of effectively treating and preventing DHTRs in patients with SCD with currently available immunosuppressive therapies.

CONCLUSION

Based on our experience, and the current literature, it is difficult to predict the potential impact of various immunosuppressive therapies when seeking to prevent or treat DHTRs. Future mechanistic studies are needed to identify the optimal treatment options for DHTRs in the presence or absence of distinct alloantibodies in patients with SCD.

Consequences of hemolysis: Pro-inflammatory cytokine response to erythrophagocytosis

Red blood cells (RBCs) can be cleared from the circulation either intravascularly or extravascularly. In the setting of an IgG-mediated delayed hemolytic transfusion reaction (HTR), most RBC clearance is typically extravascular, presumably by phagocytosis by liver and splenic macrophages. Animal models of HTRs suggest that this RBC clearance is associated with a pro-inflammatory cytokine response. Although IgG-mediated HTRs are typically benign, they can lead to vaso-occlusive crises and further complications, such as hyperhemolysis, in patients with sickle cell disease (SCD). Why the manifestations of HTRs are more severe in the setting of SCD has yet to be determined; however, in this symposium paper, we provide a review of the evidence that robust RBC phagocytosis results in a pro-inflammatory cytokine response, which may induce vaso-occlusive crises and further complications in the setting of SCD.

Recipient priming to one RBC alloantigen directly enhances subsequent alloimmunization in mice

Individuals that become immunized to red blood cell (RBC) alloantigens can experience an increased rate of antibody formation to additional RBC alloantigens following subsequent transfusion. Despite this, how an immune response to one RBC immunogen may impact subsequent alloimmunization to a completely different RBC alloantigen remains unknown. Our studies demonstrate that Kell blood group antigen (KEL) RBC transfusion in the presence of inflammation induced by poly (I:C) (PIC) not only enhances anti-KEL antibody production through a CD4⁺ T-cell-dependent process but also directly facilitates anti-HOD antibody formation following subsequent exposure to the disparate HOD (hen egg lysozyme, ovalbumin, fused to human blood group antigen Duffy b) antigen. PIC/KEL priming of the anti-HOD antibody response required that RBCs express both the KEL and HOD antigens (HOD × KEL RBCs), as transfusion of HOD RBCs plus KEL RBCs or HOD RBCs alone failed to impact anti-HOD antibody formation in recipients previously primed with PIC/KEL. Transfer of CD4⁺ T cells from PIC/KEL-primed recipients directly facilitated anti-HOD antibody formation following (HOD × KEL) RBC transfusion. RBC alloantigen priming was not limited to PIC/KEL enhancement of anti-HOD alloantibody formation, as HOD-reactive CD4⁺ T cells enhanced anti-glycophorin A (anti-GPA) antibody formation in the absence of inflammation following transfusion of RBCs coexpressing GPA and HOD. These results demonstrate that immune priming to one RBC alloantigen can directly enhance a humoral response to a completely different RBC alloantigen, providing a potential explanation for why alloantibody responders may exhibit increased immune responsiveness to additional RBC alloantigens following subsequent transfusion.

Antibody-mediated immunosuppression can result from RBC antigen loss independent of Fcγ receptors in mice

BACKGROUND

Anti-RhD administration can prevent de novo anti-RhD formation following RhD+ red blood cell (RBC) exposure, termed antibody-mediated immunosuppression (AMIS). Recent studies suggest that AMIS may occur through target antigen alterations, known as antigen modulation. However, studies suggest that AMIS may occur independent of antigen modulation. In particular, AMIS to RBCs that transgenically express the fusion hen egg lysozyme-ovalbumin-Duffy (HOD) antigen have been shown to occur independent of activating Fcγ receptors (FcγRs) thought to be required for antigen modulation. Therefore, we sought to determine the mechanism behind AMIS following HOD RBC exposure.

STUDY DESIGN AND METHODS

Following transfer of HOD RBCs into wild-type or FcγR-chain knockout recipients in the presence or absence of monoclonal anti-hen egg lysozyme (HEL) antibody, individually or in combination, HOD antigen levels and anti-HOD antibody formation were examined.

RESULTS

Our results demonstrate that anti-HEL antibodies individually or in combination suppressed anti-HOD IgM, which correlated with the rate of detectable decrease in HEL on HOD RBCs. Furthermore, exposure to anti-HEL antibodies alone or in combination equally suppressed anti-HOD IgG formation. Unexpectedly, combination or individual anti-HEL antibodies induced AMIS and antigen modulation in an FcγR-independent manner. Pre-exposure of HOD RBCs to anti-HEL antibodies reduced antigen levels and suppressed anti-HOD antibody formation following HOD RBC exposure.

CONCLUSION

These results suggest that antibody-mediated antigen modulation may reflect a mechanism of AMIS that can occur independent of activating FcγRs and may provide a surrogate to identify antibodies capable of inducing AMIS against different RBC antigens.

Marginal Zone B Cells Induce Alloantibody Formation Following RBC Transfusion

Red blood cell (RBC) alloimmunization represents a significant immunological challenge for some patients. While a variety of immune constituents likely contribute to the initiation and orchestration of alloantibodies to RBC antigens, identification of key immune factors that initiate alloantibody formation may aid in the development of a therapeutic modality to minimize or prevent this process. To define the immune factors that may be important in driving alloimmunization to an RBC antigen, we determined the specific immune compartment and distinct cells that may initially engage transfused RBCs and facilitate subsequent alloimmunization. Our findings demonstrate that the splenic compartment is essential for formation of anti-KEL antibodies following KEL RBC transfusion. Within the spleen, transfused KEL RBCs are found within the marginal sinus, where they appear to specifically co-localize with marginal zone (MZ) B cells. Consistent with this, removal of MZ B cells completely prevented alloantibody formation following KEL RBC transfusion. While MZ B cells can mediate a variety of key downstream immune pathways, depletion of follicular B cells or CD4 T cells failed to similarly impact the anti-KEL antibody response, suggesting that MZ B cells may play a key role in the development of anti-KEL IgM and IgG following KEL RBC transfusion. These findings highlight a key contributor to KEL RBC-induced antibody formation, wherein MZ B cells facilitate antibody formation following RBC transfusion.

Antibody-mediated immune suppression by antigen modulation is antigen-specific

Alloantibodies developing after exposure to red blood cell (RBC) alloantigens can complicate pregnancy and transfusion therapy. The only method currently available to actively inhibit RBC alloantibody formation is administration of antigen-specific antibodies, a phenomenon termed antibody-mediated immune suppression (AMIS). A well-known example of AMIS is RhD immune globulin prophylaxis to prevent anti-D formation in RhD- individuals. However, whether AMIS is specific or impacts alloimmunization to other antigens on the same RBC remains unclear. To evaluate the specificity of AMIS, we passively immunized antigen-negative recipients with anti-KEL or anti-hen egg lysozyme (HEL) antibodies, followed by transfusion of murine RBC expressing both the HEL-ovalbumin-Duffy (HOD) and human KEL antigens (HOD × KEL RBC). Significant immunoglobulin G deposition on transfused HOD × KEL RBC occurred in all passively immunized recipients. Complement deposition and antigen modulation of the KEL antigen occurred on transfused RBC only in anti-KEL-treated recipients, whereas HEL antigen levels decreased only in the presence of anti-HEL antibodies. Western blot analysis confirmed the specificity of antigen loss, which was not attributable to RBC endocytosis and appears distinct for the 2 antigens. Specifically, removal of KEL was attenuated by clodronate treatment, whereas loss of HEL was unaffected by clodronate in vivo but sensitive to protease treatment in vitro. Antigen-specific modulation correlated with antigen-specific AMIS, with anti-KEL treated recipients forming antibodies to the HOD antigen and anti-HEL-treated recipients developing antibodies to the KEL antigen. Together, these results demonstrate that passively administered antibodies can selectively inhibit the immune response to a specific antigen.

Hemoglobin A clearance in children with sickle cell anemia on chronic transfusion therapy

BACKGROUND

Chronic transfusion therapy for sickle cell anemia reduces disease complications by diluting sickle-erythrocytes with hemoglobin A (HbA)-containing erythrocytes and suppressing erythropoiesis. Minor antigen mismatches may result in alloimmunization, but it is unknown if antigen mismatches or recipient characteristics influence HbA clearance posttransfusion.

STUDY DESIGN AND METHODS

Children with sickle cell anemia on chronic transfusion therapy were followed prospectively for 12 months. All patients received units serologically matched for C/c, E/e, and K; patients with prior red blood cell (RBC) antibodies had additional matching for Fy^a , Jk^b , and any previous alloantibodies. Patients' RBC antigen genotypes, determined by multiplexed molecular assays (PreciseType Human Erythrocyte Antigen, and RHCE and RHD BeadChip, Immucor) were compared to genotypes of transfused RBC units to assess for antigen mismatches. Decline in hbA (ΔHbA) from posttransfusion to the next transfusion was calculated for each transfusion episode.

RESULTS

Sixty patients received 789 transfusions, 740 with ΔHbA estimations, and 630 with donor Human Erythrocyte Antigen genotyping. In univariate mixed-model analysis, ΔHbA was higher in patients with past RBC antibodies or splenomegaly and lower in patients with splenectomy. RBC antigen mismatches were not associated with ΔHbA. In multivariate linear mixed-effects modeling, ΔHbA was associated with RBC antibodies (2.70 vs. 2.45 g/dL/28 d, p = 0.0028), splenomegaly (2.87 vs. 2.28 g/dL/28 d, p = 0.019), and negatively associated with splenectomy (2.46 vs. 2.70 g/dL/28 d, p = 0.011).

CONCLUSIONS

HbA decline was increased among patients with sickle cell anemia with prior immunologic response to RBC antigens and decreased among those with prior splenectomy, demonstrating that recipient immunologic characteristics influenced the clearance of transfused RBCs.

CD4 Depletion or CD40L Blockade Results in Antigen-Specific Tolerance in a Red Blood Cell Alloimmunization Model

Approximately 3-10% of human red blood cell (RBC) transfusion recipients form alloantibodies to non-self, non-ABO blood group antigens expressed on donor RBCs, with these alloantibodies having the potential to be clinically significant in transfusion and pregnancy settings. However, the majority of transfused individuals never form detectable alloantibodies. Expanding upon observations that children initially transfused with RBCs at a young age are less likely to form alloantibodies throughout their lives, we hypothesized that "non-responders" may not only be ignorant of antigens on RBCs but instead tolerized. We investigated this question in a reductionist murine model, in which transgenic donors express the human glycophorin A (hGPA) antigen in an RBC-specific manner. Although wild-type mice treated with poly IC and transfused with hGPA RBCs generated robust anti-hGPA IgG alloantibodies that led to rapid clearance of incompatible RBCs, those transfused in the absence of an adjuvant failed to become alloimmunized. Animals depleted of CD4+ cells or treated with CD40L blockade prior to initial hGPA RBC exposure, in the presence of poly IC, failed to generate detectable anti-hGPA IgG alloantibodies. These non-responders to a primary transfusion remained unable to generate anti-hGPA IgG alloantibodies upon secondary hGPA exposure and did not prematurely clear transfused hGPA RBCs even after their CD4 cells had returned or their CD40L blockade had resolved. This observed tolerance was antigen (hGPA) specific, as robust IgG responses to transfused RBCs expressing a third-party antigen occurred in all studied groups. Experiments completed in an RBC alloimmunization model that allowed evaluation of antigen-specific CD4+ T-cells (HOD (hen egg lysozyme, ovalbumin, and human duffyb)) demonstrated that CD40L blockade prevented the expansion of ovalbumin 323-339 specific T-cells after HOD RBC transfusion and also prevented germinal center formation. Taken together, our data suggest that recipients may indeed become tolerized to antigens expressed on RBCs, with the recipient's immune status upon initial RBC exposure dictating future responses. Although questions surrounding mechanism(s) and sustainability of tolerance remain, these data lay the groundwork for future work investigating RBC immunity versus tolerance in reductionist models and in humans.

Clinically significant anti-KEL RBC alloantibodies are transferred by breast milk in a murine model

BACKGROUND AND OBJECTIVES

Fetuses affected by maternal RBC alloantibodies may have prolonged anaemia after birth, leading one to question whether maternal alloantibody transfer may occur outside the placenta. In response to a recent publication describing breast milk transfer of clinically significant amounts of maternal antiplatelet IgA antibodies from mother to nursing infant, we hypothesized that maternal RBC alloantibodies may also be capable of being transferred in breast milk.

MATERIALS AND METHODS

The presence and clinical significance of breast milk alloantibody transfer were tested through a series of pregnancy, fostering and transfusion experiments, using a murine model in which transgenic RBCs express the human KEL glycoprotein.

RESULTS

Maternal anti-KEL immunoglobulins, induced through transfusion or pregnancy, were detected in the aqueous phase of breast milk. Further, efficient transfer of maternal anti-KEL IgG and IgA to nursing pups was observed in fostering experiments. The breast milk-acquired alloantibodies were clinically significant in wild-type pups in a transfusion setting, binding to 'incompatible' KEL RBCs and leading to premature clearance from the circulation. Although breast milk-acquired alloantibodies also bound to the RBCs of transgenic KEL-positive fostered pups, no anaemia resulted.

CONCLUSIONS

Taking these murine data in combination with recently published human data of maternal antiplatelet IgA antibodies in breast milk leading to sequelae in some infants, it is theoretically possible that maternal anti-RBC IgA alloantibodies may also be transferred in human breast milk and may lead to sequelae in some infants under some circumstances.

The role of inflammation in intravenous immune globulin-mediated hemolysis

Intravenous immune globulin (IVIG) therapy has shown great success in a number of autoimmune and inflammatory conditions and its use continues to increase worldwide. There is growing awareness of significant side effects of high-dose IVIG: however, particularly severe hemolysis in patients that are non-group O. It has been proposed that IVIG-associated hemolysis may be heralded by an existing inflammatory condition. In the work presented herein, we have provided a review of the pathophysiology of inflammation, particularly as it applies in immune-mediated red blood cell hemolysis, and a summary of previous publications that suggest an association between IVIG-mediated hemolysis and a state of existing inflammation. In addition, preliminary results from a prospective study to address the mechanism of IVIG-associated hemolysis are provided. These preliminary data support the idea of an existing inflammatory condition preceding overt hemolysis after high-dose IVIG therapy that: 1) is restricted to non-group O patients, 2) is seen when using IVIG doses of more than 2 g/kg, 3) involves an activated mononuclear phagocyte system, 4) may be presaged by a significant increase in the anti-inflammatory cytokine interleukin-1 receptor agonist, and 5) is independent of secretor status.

Detection of phosphatidylserine exposure on leukocytes following treatment with human galectins

Cellular turnover represents a fundamental aspect of immunological homeostasis. While many factors appear to regulate leukocyte removal during inflammatory resolution, recent studies suggest that members of the galectin family play a unique role in orchestrating this process. Unlike cellular removal through apoptotic cell death, several members of the galectin family induce surface expression of phosphatidylserine (PS), a phagocytic marker on cells undergoing apoptosis, in the absence of cell death. However, similar to PS on cells undergoing apoptosis, galectin-induced PS exposure sensitizes cells to phagocytic removal. As galectins appear to prepare cells for phagocytic removal without actually inducing apoptotic cell death, this process has recently been coined preaparesis. Given the unique characteristics of galectin-induced PS exposure in the context of preaparesis, we will examine important considerations when evaluating the potential impact of different galectin family members on PS exposure and cell viability.

Factors Influencing RBC Alloimmunization: Lessons Learned from Murine Models

Red blood cell (RBC) alloimmunization may occur following transfusion or pregnancy/delivery. Although observational human studies have described the immunogenicity of RBC antigens and the clinical significance of RBC alloantibodies, studies of factors influencing RBC alloimmunization in humans are inherently limited by the large number of independent variables involved. This manuscript reviews data generated in murine models that utilize transgenic donor mice, which express RBC-specific model or authentic human blood group antigens. Transfusion of RBCs from such donors into nontransgenic but otherwise genetically identical recipient mice allows for the investigation of individual donor or recipient-specific variables that may impact RBC alloimmunization. Potential donor-related variables include methods of blood product collection, processing and storage, donor-specific characteristics, RBC antigen-specific factors, and others. Potential recipient-related variables include genetic factors (MHC/HLA type and polymorphisms of immunoregulatory genes), immune activation status, phenotype of regulatory immune cell subsets, immune cell functional characteristics, prior antigen exposures, and others. Although murine models are not perfect surrogates for human biology, these models generate phenomenological and mechanistic hypotheses of RBC alloimmunization and lay the groundwork for follow-up human studies. Long-term goals include improving transfusion safety and minimizing the morbidity/mortality associated with RBC alloimmunization.

Examination of whole cell galectin binding by solid phase and flow cytometric analysis

We have utilized simple flow cytometric and fluorescence-based solid phase assays to study the interaction of glycan-binding proteins (GBP) to cell surface glycoconjugates. These methods utilize commonly employed flow cytometry techniques and commercially available streptavidin-coated microplates to immobilize various biotinylated ligands, such as glycopeptides, oligosaccharides, and whole cells. Using this approach, fluorescently labeled GBPs, in particular, members of the galectin family, can be interrogated for potential interactions with cell surface carbohydrates, including elucidation of the potential impact of alterations in glycosylation on carbohydrate recognition. Using these approaches, we present examples of flow cytometric and fluorescence-based solid phase assays to study galectin-carbohydrate interactions.

Antigen modulation confers protection to red blood cells from antibody through Fcγ receptor ligation

Autoantibodies and alloantibodies can damage self-tissue or transplanted tissues through either fixation of complement or ligation of FcγRs. Several pathways have been described that imbue self-tissues with resistance to damage from complement fixation, as a protective measure against damage from these Abs. However, it has been unclear whether parallel pathways exist to provide protection from FcγR ligation by bound Abs. In this article, we describe a novel pathway by which cell surface Ag is specifically decreased as a result of Ab binding (Ag modulation) to the extent of conferring protection to recognized cells from Fcγ-dependent clearance. Moreover, the Ag modulation in this system requires FcγR ligation. Together, these findings provide unique evidence of self-protective pathways for FcγR-mediated Ab damage.

A novel role for C3 in antibody-induced red blood cell clearance and antigen modulation

Hemolytic transfusion reactions (HTRs) due to incompatible red blood cell (RBC) transfusions are a leading cause of transfusion associated death. Although many transfused incompatible RBCs are cleared, some remain in circulation despite the presence of RBC-specific antibodies, potentially due to "antigen modulation." With a goal of better understanding incompatible RBC clearance, we generated a murine model with RBC-specific expression of a clinically significant human antigen (KEL2) known to be involved in antigen modulation as well as in HTRs. Wild-type (WT) recipients transfused with transgenic KEL2 RBCs generated anti-KEL glycoprotein alloantibodies, which fixed complement, led to intravascular hemolysis, and resulted in decreased levels of KEL2 antigen detectable on cells remaining in circulation. Antigen modulation did not appear to solely reflect removal of RBCs with higher antigen expression, because cells continued to display antigen modulation in the absence of significant clearance. Recipients genetically lacking complement exhibited lesser degrees of incompatible RBC clearance and antigen modulation in comparison with WT or FcγR knock-out (KO) animals, suggesting a role for complement in RBC clearance. In summary, this HTR model may serve as a platform to test strategies to downmodulate antigen and inhibit incompatible RBC clearance, thus potentially mitigating transfusion dangers.

Alloantibodies to a paternally derived RBC KEL antigen lead to hemolytic disease of the fetus/newborn in a murine model

Exposure to nonself red blood cell (RBC) antigens, either from transfusion or pregnancy, may result in alloimmunization and incompatible RBC clearance. First described as a pregnancy complication 80 years ago, hemolytic disease of the fetus and newborn (HDFN) is caused by alloimmunization to paternally derived RBC antigens. Despite the morbidity/mortality of HDFN, women at risk for RBC alloimmunization have few therapeutic options. Given that alloantibodies to antigens in the KEL family are among the most clinically significant, we developed a murine model with RBC-specific expression of the human KEL antigen to evaluate the impact of maternal/fetal KEL incompatibility. After exposure to fetal KEL RBCs during successive pregnancies with KEL-positive males, 21 of 21 wild-type female mice developed anti-KEL alloantibodies; intrauterine fetal anemia and/or demise occurred in a subset of KEL-positive pups born to wild type, but not agammaglobulinemic mothers. Similar to previous observations in humans, pregnancy-associated alloantibodies were detrimental in a transfusion setting, and transfusion-associated alloantibodies were detrimental in a pregnancy setting. This is the first pregnancy-associated HDFN model described to date, which will serve as a platform to develop targeted therapies to prevent and/or mitigate the dangers of RBC alloantibodies to fetuses and newborns.

Antibody specific for the glycophorin A complex mediates intravenous immune globulin-resistant anemia in a murine model

BACKGROUND

Therapy for patients with autoimmune hemolytic anemia (AHA) remains a major challenge. Patients with glycophorin A (GPA)-specific immunoglobulin G antibodies can have severe hemolysis, which may occur by mechanisms independent from traditional macrophage-dependent Fcγ receptor (FcγR)-mediated extravascular hemolysis. As intravenous immune globulin (IVIG) is known to display its beneficial effects in FcγR-mediated cytopenias, and IVIG responses in AHA are inconsistent at best, we sought to gain insight into the mechanism of anemia by a GPA complex-specific monoclonal antibody (TER119) in a mouse model of immune hemolytic anemia and evaluate the therapeutic effect of IVIG.

STUDY DESIGN AND METHODS

The anemic effect of the TER119 antibody was studied in vitro by incubation of mouse RBC with the antibody and in vivo by infusing the antibody into normal mice versus mice genetically deficient for the Fc receptor γ chain (Fcγ), complement C3, mice naturally deficient in complement C5, and splenectomized mice. IVIG efficacy in anemia was determined by treating mice with an intensive IVIG dosing regimen.

RESULTS

The TER119-mediated anemia was independent of classical FcγR-, C3-, and C5-dependent mechanisms, but occurred by a mechanism consistent with RBC agglutination. In accordance with agglutination, the presence of the spleen accelerated the anemia observed but anemia could still occur in splenectomized mice. IVIG did not significantly affect the induction of anemia by TER119.

CONCLUSION

The mechanism of anemia induced by AHA-causing antibodies may be an important factor to consider in the response to therapy with IVIG.

Transfusion of murine red blood cells expressing the human KEL glycoprotein induces clinically significant alloantibodies

BACKGROUND

Red blood cell (RBC) alloantibodies to nonself antigens may develop after transfusion or pregnancy, leading to morbidity and mortality in the form of hemolytic transfusion reactions or hemolytic disease of the newborn. A better understanding of the mechanisms of RBC alloantibody induction, or strategies to mitigate the consequences of such antibodies, may ultimately improve transfusion safety. However, such studies are inherently difficult in humans.

STUDY DESIGN AND METHODS

We recently generated transgenic mice with RBC-specific expression of the human KEL glycoprotein, specifically the KEL2 or KEL1 antigens. Herein, we investigate recipient alloimmune responses to transfused RBCs in this system.

RESULTS

Transfusion of RBCs from KEL2 donors into wild-type recipients (lacking the human KEL protein but expressing the murine KEL ortholog) resulted in dose-dependent anti-KEL glycoprotein immunoglobulin (Ig)M and IgG antibody responses, enhanced by recipient inflammation with poly(I:C). Boostable responses were evident upon repeat transfusion, with morbid-appearing alloimmunized recipients experiencing rapid clearance of transfused KEL2 but not control RBCs. Although KEL1 RBCs were also immunogenic after transfusion into wild-type recipients, transfusion of KEL1 RBCs into KEL2 recipients or vice versa failed to lead to detectable anti-KEL1 or anti-KEL2 responses.

CONCLUSIONS

This murine model, with reproducible and clinically significant KEL glycoprotein alloantibody responses, provides a platform for future mechanistic studies of RBC alloantibody induction and consequences. Long-term translational goals of these studies include improving transfusion safety for at-risk patients.

Initiation and regulation of complement during hemolytic transfusion reactions

Hemolytic transfusion reactions represent one of the most common causes of transfusion-related mortality. Although many factors influence hemolytic transfusion reactions, complement activation represents one of the most common features associated with fatality. In this paper we will focus on the role of complement in initiating and regulating hemolytic transfusion reactions and will discuss potential strategies aimed at mitigating or favorably modulating complement during incompatible red blood cell transfusions.

Resistance of a subset of red blood cells to clearance by antibodies in a mouse model of incompatible transfusion

BACKGROUND

Alloimmunization to antigens on transfused red blood cells (RBCs) represents a major barrier to chronic transfusion. In extreme cases of multiple alloimmunization, clinicians may be faced with the decision of transfusing incompatible RBCs or risking death from lack of transfusion. The disastrous results of hemolytic transfusion reactions are well understood, and major pathways of clearance have been described. However, well described but poorly understood is the survival of a subset of incompatible donor RBCs during hemolysis, despite antibody binding.

STUDY DESIGN AND METHODS

We utilize a tractable murine model of incompatible transfusion in which RBCs from transgenic donor mice expressing human glycophorin A (hGPA) are transfused into recipients passively immunized with anti-hGPA.

RESULTS

As in humans, the majority of RBCs are cleared but a subset of incompatible donor RBCs persist in circulation, despite being bound by antibodies. Data contained herein reject the hypothesis that lack of clearance is due to insufficient antibody or overwhelming of phagocytic machinery; rather, we establish that surviving RBCs represent a distinct population resistant to clearance.

CONCLUSIONS

These studies demonstrate that surviving RBCs during incompatible transfusion can represent a population that is resistant to clearance.