The spleen plays a central role in primary humoral alloimmunization to transfused mHEL red blood cells

Complement C3 and marginal zone B cells promote IgG-mediated enhancement of RBC alloimmunization in mice

Administration of anti-RhD immunoglobulin (Ig) to decrease maternal alloimmunization (antibody-mediated immune suppression [AMIS]) was a landmark clinical development. However, IgG has potent immune-stimulatory effects in other settings (antibody-mediated immune enhancement [AMIE]). The dominant thinking has been that IgG causes AMIS for antigens on RBCs but AMIE for soluble antigens. However, we have recently reported that IgG against RBC antigens can cause either AMIS or AMIE as a function of an IgG subclass. Recent advances in mechanistic understanding have demonstrated that RBC alloimmunization requires the IFN-α/-β receptor (IFNAR) and is inhibited by the complement C3 protein. Here, we demonstrate the opposite for AMIE of an RBC alloantigen (IFNAR is not required and C3 enhances). RBC clearance, C3 deposition, and antigen modulation all preceded AMIE, and both CD4+ T cells and marginal zone B cells were required. We detected no significant increase in antigen-specific germinal center B cells, consistent with other studies of RBC alloimmunization that show extrafollicular-like responses. To the best of our knowledge, these findings provide the first evidence of an RBC alloimmunization pathway which is IFNAR independent and C3 dependent, thus further advancing our understanding of RBCs as an immunogen and AMIE as a phenomenon.

Distinct RBC alloantibody responses in type 1 interferon-dependent and -independent lupus mouse models

During transfusion of red blood cells (RBCs), recipients are exposed to both ABO and non-ABO 'minor' antigens. RBC donor units and recipient RBCs are not routinely matched for non-ABO antigens. Thus, recipients are exposed to many RBC alloantigens that can lead to RBC alloantibody production and subsequent clinically significant hemolysis. RBC alloantibodies also significantly limit the provision of compatible RBC units for recipients. Prior studies indicate that the frequency of RBC alloimmunization is increased during inflammatory responses and in patients with autoimmune diseases. Still, mechanisms contributing to alloimmune responses in patients with autoimmunity are not well understood. More than half of adult patients with systemic lupus erythematosus (SLE) produce type 1 interferons (IFNα/β) and express IFNα/β stimulated genes (ISGs). Previously, we reported that IFNα/β promote RBC alloimmune responses in the pristane mouse model, which develops a lupus-like phenotype that is dependent on IFNα/β signaling. However, it is unclear whether IFNα/β or the lupus-like phenotype induces alloimmunization in lupus models. Therefore, we tested the hypothesis that IFNα/β promotes RBC alloimmune responses in lupus by examining alloimmune responses in IFNα/β-independent (MRL-lpr) and IFNα/β-dependent (pristane) lupus models. Whereas pristane treatment significantly induced interferon-stimulated genes (ISGs), MRL-lpr mice produced significantly lower levels that were comparable to levels in untreated WT mice. Transfusion of murine RBCs that express the KEL antigen led to anti-KEL IgG production by pristane-treated WT mice. However, MRL-lpr mice produced minimal levels of anti-KEL IgG. Treatment of MRL-lpr mice with recombinant IFNα significantly enhanced alloimmunization. Collectively, results indicate that a lupus-like phenotype in pre-clinical models is not sufficient to induce RBC alloantibody production, and IFNα/β gene signatures may be responsible for RBC alloimmune responses in lupus mouse models. If these findings are extended to alternate pre-clinical models and clinical studies, patients with SLE who express an IFNα/β gene signature may have an increased risk of developing RBC alloantibodies and may benefit from more personalized transfusion protocols.

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.

The ongoing challenge of RBC alloimmunization in the management of patients with sickle cell disease

RBC transfusion remains a cornerstone in the treatment of sickle cell disease (SCD). However, as with many interventions, transfusion of RBCs is not without risk. Allogeneic RBC exposure can result in the development of alloantibodies, which can make it difficult to find compatible RBCs for future transfusion and increases the likelihood of life-threatening complications. The development of RBC alloantibodies occurs when a patient's immune system produces alloantibodies against foreign alloantigens present on RBCs. Despite its longstanding recognition, RBC alloimmunization has increasingly become a challenge when caring for patients with SCD. The growing prominence of alloimmunization can be attributed to several factors, including expanded indications for transfusions, increased lifespan of patients with SCD, and inadequate approaches to prevent alloimmunization. Recognizing these challenges, recent observational studies and preclinical models have begun to elucidate the immune pathways that underpin RBC alloimmunization. These emerging data hold promise in paving the way for innovative prevention strategies, with the goal of increasing the safety and efficacy of RBC transfusion in patients with SCD who are most vulnerable to alloimmunization.

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.

Marginal zone B cells mediate a CD4 T-cell-dependent extrafollicular antibody response following RBC transfusion in mice

Red blood cell (RBC) transfusions can result in alloimmunization toward RBC alloantigens that can increase the probability of complications following subsequent transfusion. An improved understanding of the immune mechanisms that underlie RBC alloimmunization is critical if future strategies capable of preventing or even reducing this process are to be realized. Using the HOD (hen egg lysozyme [HEL] and ovalbumin [OVA] fused with the human RBC antigen Duffy) model system, we aimed to identify initiating immune factors that may govern early anti-HOD alloantibody formation. Our findings demonstrate that HOD RBCs continuously localize to the marginal sinus following transfusion, where they colocalize with marginal zone (MZ) B cells. Depletion of MZ B cells inhibited immunoglobulin M (IgM) and IgG anti-HOD antibody formation, whereas CD4 T-cell depletion only prevented IgG anti-HOD antibody development. HOD-specific CD4 T cells displayed similar proliferation and activation following transfusion of HOD RBCs into wild-type or MZ B-cell-deficient recipients, suggesting that IgG formation is not dependent on MZ B-cell-mediated CD4 T-cell activation. Moreover, depletion of follicular B cells failed to substantially impact the anti-HOD antibody response, and no increase in antigen-specific germinal center B cells was detected following HOD RBC transfusion, suggesting that antibody formation is not dependent on the splenic follicle. Despite this, anti-HOD antibodies persisted for several months following HOD RBC transfusion. Overall, these data suggest that MZ B cells can initiate and then contribute to RBC alloantibody formation, highlighting a unique immune pathway that can be engaged following RBC transfusion.

Whole-blood phenotyping to assess alloimmunization status in transfused sickle cell disease patients

It is essential to limit hemolytic transfusion reactions in polytransfused individuals, and the prevention of alloimmunization is a key solution. CD4+ T lymphocyte (TL) markers, particularly follicular T helper (Tfh) cells, may differentiate between responder and nonresponder alloimmunization statuses. We tested this hypothesis by studying the phenotype of CXCR5+PD1+ TLs in whole blood. Our results suggest that high levels of CXCR5+PD1+CD4+ TLs in whole blood may be a characteristic of nonalloimmunized patients. However, these cells did not display the phenotypic characteristics of active Tfh cells. Instead, a decrease in blood quiescent Tfh-cell levels was observed in nonalloimmunized polytransfused patients. High levels of CXCR5+PD1+CD4+ TLs may be associated with inhibitory signaling functions of T cells, as reflected by the low levels of PD1+ICOS+ cells in the nonalloimmunized polytransfused group. The description of these particular phenotypes, and their comparison among groups of patients, responders, and nonresponders, suggests that new immunological components should be considered when trying to understand posttransfusion alloimmunization.

Autoantigen presentation by splenic dendritic cells is required for RBC-specific autoimmunity

BACKGROUND

Failure of humoral tolerance to red blood cell (RBC) antigens may lead to autoimmune hemolytic anemia (AIHA), a severe and sometimes fatal disease. Previous studies have shown that although tolerance is robust in HOD mice, autoantibodies are generated upon adoptive transfer of OTII CD4+ T cells, which are specific for an epitope contained within the HOD antigen. These data imply that antigen-presenting cells (APCs) are presenting RBC-derived autoantigen(s) and are capable of driving T-cell activation. Given that multiple APCs participate in erythrophagocytosis, we used a transgenic approach to determine which cellular subsets were required for autoantigen presentation and subsequent autoreactive T-cell activation.

STUDY DESIGN AND METHODS

HOD mice, which express an RBC-specific antigen consisting of hen egg lysozyme, ovalbumin, and human blood group molecule Duffy, were bred with IAbfl/fl and Cre-expressing transgenic animals to generate mice that lack I-Ab expression on particular cell subsets. OTII CD4+ T cell proliferation was assessed in vivo in HOD+ I-Abfl/fl xCre+ mice and in vitro upon coculture with sorted APCs.

RESULTS

Analysis of HOD+ I-Abfl/fl xCre+ mice demonstrated that splenic conventional dendritic cells (DCs), but not macrophages or monocytes, were required for autoantigen presentation to OTII CD4+ T cells. Subsequent in vitro coculture experiments revealed that both CD8+ and CD8- DC subsets participate in erythrophagocytosis, present RBC-derived autoantigen and stimulate autoreactive T-cell proliferation.

CONCLUSION

These data suggest that if erythrocyte T-cell tolerance fails, DCs are capable of initiating autoimmune responses. As such, targeting DCs may be a fruitful strategy for AIHA therapies.

Blood microparticles are a component of immune modulation in red blood cell transfusion

Patients may display alloimmunization following transfusion. Microparticles (MPs) released into the blood are present in transfusion products. We show that MPs can modulate the immune system, CD4⁺ T-cell, and humoral responses, through their concentration, cellular origin and phenotype, and should therefore be considered to reduce the immune impact of transfusion.

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.

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.

A locus on chromosome 5 shows African ancestry-limited association with alloimmunization in sickle cell disease

Red blood cell (RBC) transfusion remains a critical therapeutic intervention in sickle cell disease (SCD); however, the apparent propensity of some patients to regularly develop RBC alloantibodies after transfusion presents a significant challenge to finding compatible blood for so-called alloimmunization responders. Predisposing genetic loci have long been thought to contribute to the responder phenomenon, but to date, no definitive loci have been identified. We undertook a genome-wide association study of alloimmunization responder status in 267 SCD multiple transfusion recipients, using genetic estimates of ancestral admixture to bolster our findings. Analyses revealed single nucleotide polymorphisms (SNPs) on chromosomes 2 and 5 approaching genome-wide significance (minimum P = 2.0 × 10-8 and 8.4 × 10-8, respectively), with local ancestry analysis demonstrating similar levels of admixture in responders and nonresponders at implicated loci. Association at chromosome 5 was nominally replicated in an independent cohort of 130 SCD transfusion recipients, with meta-analysis surpassing genome-wide significance (rs75853687, P meta = 6.6 × 10-9), and this extended to individuals forming multiple (>3) alloantibodies (P meta = 9.4 × 10-5). The associated variant is rare outside of African populations, and orthogonal genome-wide haplotype analyses, contingent on local ancestry, revealed genome-wide significant sharing of a ∼60-kb haplotype of African ancestry at the chromosome 5 locus (Bayes Factor = 4.95). This locus overlaps a putative cis-acting enhancer predicted to regulate transcription of ADRA1B and the lncRNA LINC01847, both members of larger ontologies associated with immune regulation. Our findings provide potential insights to the pathophysiology underlying the development of alloantibodies and implicate non-RBC ancestry-limited loci in the susceptibility to alloimmunization.

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.

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.

Cellular immune responses in red blood cell alloimmunization

In excess of 340 blood group antigens have now been described that vary between individuals. Thus, any unit of blood that is nonautologous represents a significant dose of alloantigen. Most blood group antigens are proteins, which differ by a single amino acid between donors and recipients. Approximately 1 out of every 70 individuals are transfused each year (in the United States alone), which leads to antibody responses to red blood cell (RBC) alloantigens in some transfusion recipients. When alloantibodies are formed, in many cases, RBCs expressing the antigen in question can no longer be safely transfused. However, despite chronic transfusion, only 3% to 10% of recipients (in general) mount an alloantibody response. In some disease states, rates of alloimmunization are much higher (eg, sickle cell disease). For patients who become alloimmunized to multiple antigens, ongoing transfusion therapy becomes increasingly difficult or, in some cases, impossible. While alloantibodies are the ultimate immune effector of humoral alloimmunization, the cellular underpinnings of the immune system that lead to ultimate alloantibody production are complex, including antigen consumption, antigen processing, antigen presentation, T-cell biology, and B-cell biology. Moreover, these cellular processes differ to some extent with regard to transfused RBCs as compared with other better-studied immune barriers (eg, infectious disease, vaccines, and solid organ transplantation). The current work focuses on illustrating the current paradigm of humoral immunity, with a specific focus on particulars of RBC alloimmunization and recent advances in the understanding thereof.

Alteration of lymphocyte phenotype and function in sickle cell anemia: Implications for vaccine responses

Individuals with sickle cell anemia (SCA) have increased susceptibility to infections, secondary to impairment of immune function. Besides the described dysfunction in innate immunity, including impaired opsonization and phagocytosis of bacteria, evidence of dysfunction of T and B lymphocytes in SCA has also been reported. This includes reduction in the proportion of circulating CD4+ and CD8+ T cells, reduction of CD4+ helper: CD8+ suppressor T cell ratio, aberrant activation and dysfunction of regulatory T cells (Treg ), skewing of CD4+ T cells towards Th2 response and loss of IgM-secreting CD27 + IgM(high) IgD(low) memory B cells. These changes occur on the background of immune activation characterized by predominance of memory CD4+ T cell phenotypes, increased Th17 signaling and elevated levels of C-reactive protein and pro-inflammatory cytokines IL-6 and TNF-α, which may affect the immunogenicity and protective efficacy of vaccines available to prevent infections in SCA. Thus, in order to optimize the use of vaccines in SCA, a thorough understanding of T and B lymphocyte functions and vaccine reactivity among individuals with SCA is needed. Studies should be encouraged of different SCA populations, including sub-Saharan Africa where the burden of SCA is highest. This article summarizes our current understanding of lymphocyte biology in SCA, and highlights areas that warrant future research. Am. J. Hematol. 91:938-946, 2016. © 2016 Wiley Periodicals, Inc.

Bridging channel dendritic cells induce immunity to transfused red blood cells

Calabro et al. show that 33D1⁺ dendritic cells present in the bridging channel of the spleen are essential for alloantibody response to transfused red blood cells.

Red blood cell (RBC) transfusion is a life-saving therapeutic tool. However, a major complication in transfusion recipients is the generation of antibodies against non-ABO alloantigens on donor RBCs, potentially resulting in hemolysis and renal failure. Long-lived antibody responses typically require CD4⁺ T cell help and, in murine transfusion models, alloimmunization requires a spleen. Yet, it is not known how RBC-derived antigens are presented to naive T cells in the spleen. We sought to answer whether splenic dendritic cells (DCs) were essential for T cell priming to RBC alloantigens. Transient deletion of conventional DCs at the time of transfusion or splenic DC preactivation before RBC transfusion abrogated T and B cell responses to allogeneic RBCs, even though transfused RBCs persisted in the circulation for weeks. Although all splenic DCs phagocytosed RBCs and activated RBC-specific CD4⁺ T cells in vitro, only bridging channel 33D1⁺ DCs were required for alloimmunization in vivo. In contrast, deletion of XCR1⁺CD8⁺ DCs did not alter the immune response to RBCs. Our work suggests that blocking the function of one DC subset during a narrow window of time during RBC transfusion could potentially prevent the detrimental immune response that occurs in patients who require lifelong RBC transfusion support.

Red blood cell alloimmunization is influenced by the delay between Toll-like receptor agonist injection and transfusion

Murine models of red blood cell transfusion show that inflammation associated with viruses or methylated DNA promotes red blood cell alloimmunization. In vaccination studies, the intensity of antigen-specific responses depends on the delay between antigen and adjuvant administration, with a short delay limiting immune responses. In mouse models of alloimmunization, the delay between the injection of Toll-like receptor agonists and transfusion is usually short. In this study, we hypothesized that the timing of Toll-like receptor 3 agonist administration affects red blood cell alloimmunization. Poly(I:C), a Toll-like receptor 3 agonist, was administered to B10BR mice at various time points before the transfusion of HEL-expressing red blood cells. For each time point, we measured the activation of splenic HEL-presenting dendritic cells, HEL-specific CD4(+) T cells and anti-HEL antibodies in serum. The phenotype of activated immune cells depended on the delay between transfusion and Toll-like receptor-dependent inflammation. The production of anti-HEL antibodies was highest when transfusion occurred 7 days after agonist injection. The proportion of HEL-presenting CD8α(+) dendritic cells producing interleukin-12 was highest in mice injected with poly(I:C) 3 days before transfusion. Although the number of early-induced HEL-specific CD4(+) T cells was similar between groups, a high proportion of these cells expressed CD134, CD40 and CD44 in mice injected with poly(I:C) 7 days before transfusion. This study clearly shows that the delay between transfusion and Toll-like receptor-induced inflammation influences the immune response to transfused red blood cells.

Immunophenotypic parameters and RBC alloimmunization in children with sickle cell disease on chronic transfusion

Alloimmunization against red blood cell (RBC) antigens is a cause of morbidity and mortality in transfused patients with sickle cell disease (SCD). To investigate distinguishing characteristics of patients who develop RBC alloantibodies after transfusion (responders) versus those who do not (non-responders), a cross-sectional study of 90 children with SCD on chronic RBC transfusion therapy at a single institution was conducted in which 18 immune parameters (including T and B cell subsets) were tested via flow cytometry, and medical records were reviewed. RBC alloimmunization was present in 26/90 (29%) patients, with anti-E, K, and C among the most commonly detected alloantibodies despite prophylactic matching for these antigens at the study institution. In addition, RBC autoantibodies had been detected in 18/26 (69%) of alloimmunized versus 7/64 (11%) of non-alloimmunized patients (P < 0.0001). Alloimmunized patients were significantly older (median 13.0 years vs. 10.7 years, P = 0.010) and had more RBC unit exposures (median 148 U vs. 82 U, P = 0.020) than non-alloimmunized patients. Sex, age at initiation of chronic transfusion, splenectomy, stroke, and transfusion outside of the study institution were not significantly associated with RBC alloimmunization. Alloimmunized patients had a significantly increased percentage of CD4+ T memory cells compared to non-alloimmunized patients (57% vs. 49%, P = 0.0047), with no other significant differences in immune cell subsets or laboratory values detected between these groups. Additional research of RBC alloimmunization is needed to optimize transfusion therapy and to develop strategies to prevent alloimmunization.

Phenotypic differences of CD4(+) T cells in response to red blood cell immunization in transfused sickle cell disease patients

Alloimmunization against red blood cells (RBCs) is the main immunological risk associated with transfusion in patients with sickle cell disease (SCD). However, about 50-70% of SCD patients never get immunized despite frequent transfusion. In murine models, CD4(+) T cells play a key role in RBC alloimmunization. We therefore explored and compared the CD4(+) T-cell phenotypes and functions between a group of SCD patients (n = 11) who never became immunized despite a high transfusion regimen and a group of SCD patients (n = 10) who had become immunized (at least against Kidd antigen b) after a low transfusion regimen. We studied markers of CD4(+) T-cell function, including TLR, that directly control lymphocyte function, and their spontaneous cytokine production. We also tested responders for the cytokine profile in response to Kidd antigen b peptides. Low TLR2/TLR3 expression and, unexpectedly, strong expression of CD40 on CD4(+) T cells were associated with the nonresponder status, whereas spontaneous expression of IL-10 by CD4(+) T cells and weak Tbet expression were associated with the responder status. A Th17 profile was predominant in responders when stimulated by Jb(k) . These findings implicate CD4(+) T cells in alloimmunization in humans and suggest that they may be exploited to differentiate responders from nonresponders.

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.

Immunologic characterization suggests reduced alloimmunization in a murine model of thalassemia intermedia

BACKGROUND

Transfusion therapy remains a mainstay of treatment for patients with thalassemia major and to a lesser extent for the less anemic patients with thalassemia intermedia. We have previously reported a role for regulatory T cells (Tregs) in the control of antibody responses in wild-type C57BL/6 (WT) mice exposed to allogeneic red blood cell transfusions. As an initial step to study and characterize immune regulation in thalassemias, we performed an immunologic cell-type characterization of C57BL/6 Hbb(th-1)/Hbb(th-1) mouse model of thalassemia intermedia (Thal) in steady state as well as after transfusions with allogeneic blood.

STUDY DESIGN AND METHODS

The myeloid and lymphocyte compartments including Tregs and T helper (Th) responses were analyzed in transfusion naive Thal and WT mouse spleens. The effect of allogeneic transfusions on Treg and global T helper responses was also measured.

RESULTS

We found elevated levels and activity of splenic Tregs in Thal mice with lower Th type 1/Th type 2 ratios before as well as after transfusion. Furthermore, pretransfused Thal mice had altered ratios of the splenic myeloid compartment with increased proportion of macrophages but lower frequency of conventional dendritic cells. Surprisingly, transfusions resulted in lower alloimmunization levels in Thal compared to WT mice.

CONCLUSION

These data suggest that this experimental model of thalassemia intermedia has an intrinsic alteration in splenic immunoregulation with an increased resistance to alloimmunization, raising the possibility that studying this animal model may help to identify potential immunoregulatory networks to inhibit alloimmunization.

Red blood cell alloimmunization mitigation strategies

Hemolytic transfusion reactions due to red blood cell (RBC) alloantibodies are a leading cause of transfusion-associated death. In addition to reported deaths, RBC alloantibodies also cause significant morbidity in the form of delayed hemolytic transfusion reactions. These alloantibodies may also cause morbidity in the form of anemia, with compatible RBC units at times being unable to be located for highly alloimmunized patients, or in the form of hemolytic disease of the newborn. Thus, preventing RBC alloantibodies from developing in the first place, or mitigating the dangers of existing RBC alloantibodies, would decrease transfusion-associated morbidity and mortality. A number of human studies have evaluated the impact on RBC alloimmunization rates of providing partially phenotypically or genotypically matched RBCs for transfusion, and a number of animal studies have evaluated the impact of single variables on RBC alloimmunization. The goal of this review is to take a comprehensive look at existing human and animal data on RBC alloimmunization, focusing on strategies that may mitigate this serious hazard of transfusion. Potential factors that impact initial RBC alloimmunization, on both the donor and recipient sides, will be discussed. These factors include, but are not limited to, exposure to the antigen and an ability of the recipient's immune system to present that antigen. Beyond these basic factors, coexisting "danger signals," which may come from the donor unit itself or which may be present in the recipient, also likely play a role in determining which transfusion recipients may become alloimmunized after RBC antigen exposure. In addition, to better understanding factors that influence the development of RBC alloantibodies, this review will also briefly discuss strategies to decrease the dangers of existing RBC alloantibodies.

Recovery and Biodistribution of Ex Vivo Expanded Human Erythroblasts Injected into NOD/SCID/IL2Rγ mice

Ex vivo expanded erythroblasts (EBs) may serve as advanced transfusion products provided that lodgment occurs in the macrophage-niche of the marrow permitting maturation. EBs expanded from adult and cord blood expressed the receptors (CXCR4, VLA-4, and P-selectin ligand 1) necessary for interaction with macrophages. However, 4-days following transfusion to intact NOD/SCID/IL2Rγ(null) mice, CD235a(pos) EBs were observed inside CD235a(neg) splenic cells suggesting that they underwent phagocytosis. When splenectomized and intact NOD/SCID/IL2Rγ(null) mice were transfused using retrovirally labeled human EBs, human cells were visualized by bioluminescence imaging only in splenectomized animals. Four days after injection, human CD235a(pos) cells were detected in marrow and liver of splenectomized mice but only in spleen of controls. Human CD235a(pos) erythrocytes in blood remained low in all cases. These studies establish splenectomized NOD/SCID/IL2Rγ(null) mice as a suitable model for tracking and quantification of human EBs in vivo.

[Murine models in blood transfusion: allo-immunization, hemolysis]

Mice represent an animal model that can be easily manipulated. Mice have been used to model many human diseases. This review addresses murine models of immunity directed against red blood cell antigens as well as models of antibody and non-antibody mediated hemolysis. These models allow for a better understanding of the side effects of transfusion, such as red blood cell allo-immunization and post-transfusional hemolytic reactions. They also help explore strategies to treat and prevent these side effects in ways that would not be available using clinical research alone.

HLA alloimmunization is associated with RBC antibodies in multiply transfused patients with sickle cell disease

BACKGROUND

Alloimmunization to minor red blood cell (RBC) antigens occurs commonly in sickle cell disease (SCD). Patients with alloimmunization demonstrate increased risk for new alloantibody formation with subsequent transfusion. Alloimmunization to human leukocyte antigens (HLA) can occur with RBC transfusion and may result in graft rejection during stem cell or organ transplantation. The prevalence and risk factors for HLA alloimmunization in multiply transfused pediatric SCD patients are unknown.

PROCEDURE

A cross-sectional study of HLA alloimmunization in SCD patients aged 3-21 years with a history of >or=3 RBC transfusions was performed to test the hypothesis that HLA alloimmunization is associated with RBC alloimmunization. Antibodies to class I and class II HLA were measured by Flow Panel Reactive Antibody (FlowPRA).

RESULTS

Seventy-three SCD patients (30 with RBC antibodies) were tested. HLA antibodies were detected in 25/73 (34%) patients; class I HLA antibodies occurred in 24/73 (33%) and class II HLA antibodies occurred in 3 (4%). Among patients with RBC antibodies, 16/30 (53%) had HLA antibodies, while 9/43 (21%) patients without RBC antibodies had HLA antibodies (OR 4.32 [1.6-12.1]). In a multivariate analysis, antibodies to RBC antigens were an independent predictor of HLA alloimmunization (P = 0.041). The association of RBC and HLA immunization was strongest among patients with no history of chronic transfusion therapy.

CONCLUSIONS

This analysis is the first description of HLA alloimmunization in pediatric SCD patients who receive primarily leukoreduced RBC transfusions and demonstrates that HLA alloimmunization tendency is associated with antibodies to RBC antigens.

Use of mouse models to study the mechanisms and consequences of RBC clearance

Mice provide tractable animal models for studying the pathophysiology of various human disorders. This review discusses the use of mouse models for understanding red-blood-cell (RBC) clearance. These models provide important insights into the pathophysiology of various clinically relevant entities, such as autoimmune haemolytic anaemia, haemolytic transfusion reactions, other complications of RBC transfusions and immunomodulation by Rh immune globulin therapy. Mouse models of both antibody- and non-antibody-mediated RBC clearance are reviewed. Approaches for exploring unanswered questions in transfusion medicine using these models are also discussed.