Red blood cell alloimmunization: new findings at the bench and new recommendations for the bedside

Polyinosinic: polycytidylic acid induced inflammation enhances while lipopolysaccharide diminishes alloimmunity to platelet transfusion in mice

Introduction

Alloimmune responses against platelet antigens, which dominantly target the major histocompatibility complex (MHC), can cause adverse reactions to subsequent platelet transfusions, platelet refractoriness, or rejection of future transplants. Platelet transfusion recipients include individuals experiencing severe bacterial or viral infections, and how their underlying health modulates platelet alloimmunity is not well understood.

Methods

This study investigated the effect of underlying inflammation on platelet alloimmunization by modelling viral-like inflammation with polyinosinic-polycytidylic acid (poly(I:C)) or gram-negative bacterial infection with lipopolysaccharide (LPS), hypothesizing that underlying inflammation enhances alloimmunization. Mice were pretreated with poly(I:C), LPS, or nothing, then transfused with non-leukoreduced or leukoreduced platelets. Alloantibodies and allogeneic MHC-specific B cell (allo-B cell) responses were evaluated two weeks later. Rare populations of allo-B cells were identified using MHC tetramers.

Results

Relative to platelet transfusion alone, prior exposure to poly(I:C) increased the alloantibody response to allogeneic platelet transfusion whereas prior exposure to LPS diminished responses. Prior exposure to poly(I:C) had equivalent, if not moderately diminished, allo-B cell responses relative to platelet transfusion alone and exhibited more robust allo-B cell memory development. Conversely, prior exposure to LPS resulted in diminished allo-B cell frequency, activation, antigen experience, and germinal center formation and altered memory B cell responses.

Discussion

In conclusion, not all inflammatory environments enhance bystander responses and prior inflammation mediated by LPS on gram-negative bacteria may in fact curtail platelet alloimmunization.

Increased red blood cell alloimmunization rates in transfused aplastic anemia and myelofibrosis patients

BACKGROUND

Red blood cell (RBC) alloimmunization (AI) is a well-known complication of RBC transfusions, which results in the formation of alloantibodies to non-self antigens on donor RBCs, putting patients at risk of transfusion-related complications. The rate of AI with RBC transfusions in the general hospitalized population is estimated to be 2%-3%. However, some patients who are deemed "transfusion-dependent" require regular transfusions of blood products due to persistently low cell counts, putting them at even greater risk of RBC AI and increased morbidity. However, few studies currently exist investigating RBC AI in some transfusion-dependent patient populations, e.g., aplastic anemia (AA) and myelofibrosis (MF).

STUDY DESIGN AND METHODS

We conducted a 5-year retrospective review to investigate the prevalence of RBC AI, alloantibody incidence, and the number of RBC transfusions in AA and MF patients, who received RBC transfusions within our hospital system.

RESULTS

During the study period, 64 AA and 93 MF patients received 1301 and 2766 RBC transfusions, respectively. Compared to the RBC AI rate in the generalized hospitalized patient population (1%-2%), patients with AA and MF had an increased rate of RBC AI incidence rate at 14.1% and 12.9%, respectively. Furthermore, patients with primary MF demonstrated an isolated increased RBC AI incidence rate of 13.3%. The most common alloantibodies produced were anti-E and anti-K.

DISCUSSION

Within our institution, patients with AA and MF had increased incidence rates of RBC AI compared to the general hospitalized patient population and may benefit from an antigen-matched protocol to minimize AI-related complications.

Antigen-specific IgG subclass composition in recipient mice can indicate the degree of red blood cell alloimmunization as well as discern between primary and secondary immunization

BACKGROUND

Despite the vast antigen disparity between donor and recipient red blood cells (RBCs), only 2%-6% of transfusion patients mount an alloantibody response. Recently, RBC antigen density has been proposed as one of the factors that can influence alloimmunization, however, there has been no characterization of the role of antigen density along with RBC dose in primary and secondary immunization.

STUDY DESIGN AND METHODS

To generate RBCs that express distinct antigen copy numbers, different quantities of hen egg lysozyme (HEL) were coupled to murine RBCs. The HEL-RBCs were subsequently transfused into recipient mice at different RBC doses and their HEL-specific IgM, IgG, and IgG subclass response was evaluated.

RESULTS

Productive immune responses could be generated through a high copy number antigen transfused at low RBC doses or a low copy number transfused at high RBC doses. Further, primary but submaximal humoral immunization predominantly induced the IgG2b and IgG3 subclasses. In contrast, a maximal primary immunization or a secondary immunization induced all four IgG subclasses.

DISCUSSION

Our results confirm the existence of an antigen threshold for productive immune responses but indicate that a high antigen copy number alone might not be enough to induce a response, but rather a combination of both antigen copy number and cell dosage may determine the outcome of immunization. Further, this study provides a proof of concept that the IgG subclass composition can be an indicator of the level of RBC alloimmunization as well as discern between primary and secondary immunization at least in this murine model.

The role of RBC antigen transgene integration sites on RBC biology in mice

BACKGROUND

Transgenic mice expressing RBC specific antigens are widely used in mechanistic studies of RBC alloimmunization. Existing RBC donor strains have random transgene integration, potentially disrupting host elements that can confound biological interpretation.

STUDY DESIGN AND METHODS

Integration site and genomic alterations were characterized by both targeted locus amplification and congenic backcrossing in the five most commonly used RBC alloantigen donor strains (KEL-K2_hi , KEL-K2_med , and KEL-K2_lo , and KEL-K1). A targeted transgenic approach was developed to allow RBC specific transgene expression from a safe harbor locus (ROSA26). Alloimmune responses were assessed by transfusing alloantigen expressing RBCs into wild-type recipients and measuring alloantibodies by flow cytometry.

RESULTS/FINDINGS

Four of the five analyzed strains had at least one gene disrupted by the transgene integration but none of the disrupted genes are known to be involved in RBC biology. The integration of KEL-K2_med potentially altered the immunological properties of RBCs, although the biological significance of the observed changes is unclear. The ROSA26 targeted approach resulted in a single copy of the transgene that maintains RBC specific expression without random disruption of genomic elements.

CONCLUSION

These findings provide a detailed characterization of genomic disruption by transgene integration found in commonly used RBC donor strains that is relevant to numerous previous publications as well as future studies. With the possible exception of KEL-K2_med , transgene integration is not predicted to affect RBC biology in existing models, and new models can avoid this concern using the described targeted transgenic approach.

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.

Structure and function of the immune system in the spleen

The spleen is the largest secondary lymphoid organ in the body and, as such, hosts a wide range of immunologic functions alongside its roles in hematopoiesis and red blood cell clearance. The physical organization of the spleen allows it to filter blood of pathogens and abnormal cells and facilitate low-probability interactions between antigen-presenting cells (APCs) and cognate lymphocytes. APCs specific to the spleen regulate the T and B cell response to these antigenic targets in the blood. This review will focus on cell types, cell organization, and immunologic functions specific to the spleen and how these affect initiation of adaptive immunity to systemic blood-borne antigens. Potential differences in structure and function between mouse and human spleen will also be discussed.

American Society of Hematology 2020 guidelines for sickle cell disease: transfusion support

BACKGROUND

Red cell transfusions remain a mainstay of therapy for patients with sickle cell disease (SCD), but pose significant clinical challenges. Guidance for specific indications and administration of transfusion, as well as screening, prevention, and management of alloimmunization, delayed hemolytic transfusion reactions (DHTRs), and iron overload may improve outcomes.

OBJECTIVE

Our objective was to develop evidence-based guidelines to support patients, clinicians, and other healthcare professionals in their decisions about transfusion support for SCD and the management of transfusion-related complications.

METHODS

The American Society of Hematology formed a multidisciplinary panel that was balanced to minimize bias from conflicts of interest and that included a patient representative. The panel prioritized clinical questions and outcomes. The Mayo Clinic Evidence-Based Practice Research Program supported the guideline development process. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach was used to form recommendations, which were subject to public comment.

RESULTS

The panel developed 10 recommendations focused on red cell antigen typing and matching, indications, and mode of administration (simple vs red cell exchange), as well as screening, prevention, and management of alloimmunization, DHTRs, and iron overload.

CONCLUSIONS

The majority of panel recommendations were conditional due to the paucity of direct, high-certainty evidence for outcomes of interest. Research priorities were identified, including prospective studies to understand the role of serologic vs genotypic red cell matching, the mechanism of HTRs resulting from specific alloantigens to inform therapy, the role and timing of regular transfusions during pregnancy for women, and the optimal treatment of transfusional iron overload in SCD.

The role of genomics in transfusion medicine

PURPOSE OF REVIEW

To summarize recent advances in red blood cell (RBC) blood group genotyping, with an emphasis on advances in the use of NGS next generation sequencing (NGS) to detect clinically relevant blood group gene variation.

RECENT FINDINGS

Genetic information is useful in predicting RBC blood group antigen expression in several clinical contexts, particularly, for patients at high-risk for allosensitization, such as multiple transfused patients. Blood group antigen expression is directed by DNA variants affecting multiply genes. With over 300 known antigens, NGS offers the attractive prospect of comprehensive blood group genotyping. Recent studies from several groups show that NGS reliably detects blood group gene single nucleotide variants (SNVs) with good correlation with other genetic methods and serology. Additionally, new custom NGS methods accurately detect complex DNA variants, including hybrid RH alleles. Thus, recent work shows that NGS detects known and novel blood group gene variants in patients, solves challenging clinical cases, and detects relevant blood group variation in donors.

SUMMARY

New work shows that NGS is particularly robust in identifying SNVs in blood group genes, whereas custom genomic tools can be used to identify known and novel complex structural variants, including in the RH system.

Delayed haemolytic and serologic transfusion reactions: pathophysiology, treatment and prevention

PURPOSE OF REVIEW

The aim of this study was to summarize the basic epidemiology, pathophysiology and management of delayed serologic and delayed haemolytic transfusion reactions (DHTRs), as well as recent developments in our understanding of these adverse events.

RECENT FINDINGS

Several studies have identified risk factors for DHTRs, including high alloantibody evanescence rates among both general patient groups and those with sickle cell disease (SCD). Antibody detection is also hampered by the phenomenon of transfusion record fragmentation. There have also been enhancements in understanding of what may contribute to the more severe, hyperhaemolytic nature of DHTRs in SCD, including data regarding 'suicidal red blood cell death' and immune dysregulation amongst transfusion recipients with SCD. With growing recognition and study of hyperhaemolytic DHTRs, there have been improvements in management strategies for this entity, including a multitude of reports on using novel immunosuppressive agents for preventing or treating such reactions.

SUMMARY

Delayed serologic and haemolytic reactions remain important and highly relevant transfusion-associated adverse events. Future directions include further unravelling the basic mechanisms, which underlie DHTRs and developing evidence-based approaches for treating these reactions. Implementing practical preventive strategies is also a priority.

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.

Red blood cell alloimmunization and delayed hemolytic transfusion reactions in patients with sickle cell disease

Red blood cell (RBC) alloimmunization is more common in patients with sickle cell disease (SCD) than in any other studied patient population. The high prevalence of RBC alloimmunization is multi-factorial, likely involving the chronic hemolysis and inflammatory status of SCD itself, the transfusion burden of patients, and the RH genetic diversity of patients and blood donors, among other reasons. Antibody evanescence, or the decrease of RBC alloantibodies below levels detectable by blood bank testing, occurs frequently with fewer than 30% of alloantibodies estimated to be detected by current screening practices. Evanescence increases the likelihood that a patient with SCD will have a delayed hemolytic transfusion reaction upon future RBC exposure, with previously undetected alloantibodies coming roaring back in an anamnestic manner after exposure to the cognate RBC antigen. A subset of patients having delayed hemolytic transfusion reactions go on to experience hyperhemolysis; some but not all cases of hyperhemolysis are associated with previously evanescent RBC alloantibodies. There is an increasing appreciation of the association between RBC alloantibodies and RBC autoantibodies, as well as involvement of the alternative complement pathway in some instances of hyperhemolysis. A case report in this manuscript describes a highly alloimmunized patient with SCD who experiences a delayed hemolytic transfusion reaction with bystander hemolysis due to a previously evanescent, complement binding anti-M RBC alloantibody. Additional studies, including those involving multiple centers and countries, are needed to further understand RBC alloimmunization in patients with SCD and to develop strategies to prevent or mitigate potentially life-threatening hemolytic transfusion reactions.

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.

Platelet Transfusion Induces Alloimmunization to D and Non-D Rhesus Antigens

Background

Platelet concentrates (PC) contain residual contaminating red blood cells (RBC), being higher in pooled buffy coat PC (BC-PC) than in apheresis units (AP-PC). Data about PC-induced alloimmunization against non-D Rhesus (Rh) antigens are limited.

Methods

For all newly detected RhD and non-D alloantibodies between August 2015 and September /2017 we prospectively evaluated if they were triggered through PC by analyzing for incompatible RBC and/or PC transfusions.

Results

We found 5,799 positive results in 89,190 antibody screening tests. We identified 13 newly detectable Rh antibodies through incompatible PCs in 11 patients: 6× anti-D, 4× anti-E, 2× anti-c, 1× anti-f. They received a total of 156 PC (83 BC-PC; 73 AP-PC): 5 patients received incompatible BC-PC only, 1 patient received incompatible AP-PC only, 5 patients received incompatible BC-PC and AP-PC. Quality control showed a mean (range) of 0.304 (0.152-1.662) and 0.014 (0.003-0.080) × 10⁹ RBC/l for BC-PC and AP-PC, respectively. Ten of the 11 patients received RBC transfusions, all of them being antigen-negative for the alloantibodies identified.

Conclusions

PC transfusions may not only induce RhD alloimmunization, but also immunization against further Rh antigens such as c, E, and f. The risk seems higher for BC-PC than for AP-PC. The results may have impact on future recommendations of PC transfusion with respect to Rh compatibility and upper limits of RBC contamination.

Blood and Blood Components: From Similarities to Differences

Blood transfusion is made possible because, in most countries and organizations, altruistic individuals voluntarily, anonymously, and generously donate (without compensation) either whole blood or separated components that are then processed and distributed by professionals, prior to being allocated to recipients in need. Being part of modern medicine, blood transfusion uses so-called standard blood components when relative to cellular fractions and fresh plasma. However, as will be discussed in this paper, strictly speaking, such so-called labile blood components are not completely standard. Furthermore, the prevalent system based on voluntary, non-remunerated blood donation is not yet universal and, despite claims by the World Health Organization that 100% of blood collection will be derived from altruistic donations by 2020 (postponed to 2025), many obstacles may hinder this ambition, especially when relative to the collection of the enormous amount of plasma destined for fractionation into plasma derivative or drugs. Finally, country organizations also vary due to the economy, sociology, politics, and epidemiology. This paper then, discusses the particulars (of which ethical considerations) of blood transfusion diversity and the consequences for donors, patients, and society.

Murine Red Blood Cells Lack Ligands for B Cell Siglecs, Allowing Strong Activation by Erythrocyte Surface Antigens

CD22 and sialic acid-binding Ig-like lectin (Siglec)-G are members of the Siglec family of inhibitory coreceptors expressed on B cells that participate in enforcement of peripheral B cell tolerance. We have shown previously that when a BCR engages its cognate Ag on a cell surface that also expresses Siglec ligands, B cell Siglecs are recruited to the immunological synapse, resulting in suppression of BCR signaling and B cell apoptosis. Because all cells display sialic acids, and CD22 and Siglec-G have distinct, yet overlapping, specificities for sialic acid-containing glycan ligands, any cell could, in principle, invoke this tolerogenic mechanism for cell surface Ags. However, we show in this article that C57BL/6J mouse RBCs are essentially devoid of CD22 and Siglec-G ligands. As a consequence, RBCs that display a cell surface Ag, membrane-bound hen egg lysozyme, strongly activate Ag-specific B cells. We reasoned that de novo introduction of CD22 ligands in RBCs should abolish B cell activation toward its cognate Ag on the surface of RBCs. Accordingly, we used a glyco-engineering approach wherein synthetic CD22 ligands linked to lipids are inserted into the membrane of RBCs. Indeed, insertion of CD22 ligands into the RBC cell surface strongly inhibited B cell activation, cytokine secretion, and proliferation. These results demonstrate that the lack of Siglec ligands on the surface of murine RBCs permits B cell responses to erythrocyte Ags and show that Siglec-mediated B cell tolerance is restricted to cell types that express glycan ligands for the B cell Siglecs.