The Development and Consequences of Red Blood Cell Alloimmunization

Abstract

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.

Keywords: alloimmunization; hemolysis; red blood cell; sickle cell disease; transfusion medicine.

Figure 1

Red blood cell (RBC) alloimmunization. (a) More than 300 different alloantigens on RBCs have been described. In contrast to the spontaneous formation of anti-ABO(H) alloantibodies, individuals can develop additional alloantibodies as a result of direct exposure to RBC alloantigens (due to the lack of immunological tolerance toward the alloantigens not expressed by the transfusion recipient). RBC alloantigens can differ in density, chemical composition, and overall function. Some alloantigens reflect carbohydrate modifications [ABO(H)], while others represent the presence or absence of an entire protein (RhD). Most are due to a single amino acid substitution as shown for S/s, RhC/c, RhE/e, KEL (Js^b/Js^a, Kp^a/Kp^b, k/K), Duffy (Fy^a/Fy^b), and Kidd (Jk^a/Jk^b); some reflect changes in several amino acids as illustrated by M/N (M = Ser, Ser, Thr, Thr, Gly; N = Leu, Ser, Thr, Thr, Glu). (b) The general concept of the RBC alloimmune response suggests that RBC alloimmunization results from the uptake of transfused RBCs by antigen-presenting cells, such as dendritic cells (DCs), allowing the processing and presentation of alloantigen peptides in the context of major histocompatibility antigens to CD4 T cells. B cells also engage allogeneic RBCs and, with the help of CD4 T cells, can be driven to become antibody secreting cells (plasma cells) that produce IgG anti-RBC alloantibodies. However, despite many transfusion exposures, some patients, referred to as nonresponders, never generate detectable alloantibodies even though they are negative for a variety of RBC alloantigens to which they could, in theory, generate alloantibodies. In contrast, patients who have made alloantibodies following transfusion are called responders. The underlying basis for why responders generate alloantibodies and nonresponders do not has been the subject of many studies, where a variety of possibilities have emerged. It is certainly possible that nonresponders could generate alloantibodies under certain conditions, and therefore a hard line between responders and nonresponders may not exist. Instead, responder status may more likely reflect a continuum that is influenced by recipient genetics such as human leukocyte antigens (HLAs), donor unit variability, and the status of the immune system at the time of transfusion; ultimately, these factors may all dictate the likelihood that alloantibodies form following allogeneic RBC exposure.

Figure 2

Hemin can regulate immune cell function in a responder- or nonresponder-specific manner. The apparent proclivity of some individuals to respond to allogeneic red blood cell (RBC) transfusion by generating alloantibodies, while others do not, could in part be influenced by common functional differences in immune cell behavior between responders and nonresponders. Hemin not only appears to impair the ability of monocytes and monocyte-derived dendritic cells (DCs) to activate CD4 T cells but also enhances the development of regulatory T cells (Tregs), which can further inhibit immune function. Hemin also decreases the ability of CD4 T cells to simulate B cells to become antibody-secreting cells. Elevated levels of hemin, especially during disease exacerbations in patients with sickle cell disease, may therefore suppress the ability of many immune cells involved in alloantibody production to facilitate RBC alloimmunization. In contrast, several studies have demonstrated that alloantibody responders are not as sensitive to hemin-induced inhibition of immune function, possibly leading to an increased likelihood that alloantibodies form following RBC transfusion in this setting.

Figure 3

Cellular players and innate signaling pathways proposed to regulate red blood cell (RBC) alloimmunization. Allogeneic RBCs can be engaged by a variety of effector systems to induce alloantibodies. RBC removal by 33D1⁺ bridging channel dendritic cells (DCs) can result in the trafficking of antigens to the T cell zone in the white pulp, where direct activation of CD4 T cells, such as T follicular helper (Tfh) cells, can occur. RBCs can also engage marginal zone (MZ) B cells, which can result in the formation of immunoglobulin M (IgM), which can variably fix complement depending on the target antigen involved. Complement component 3 (C3)-coated RBCs may then engage distinct complement receptors to further promote alloimmunization. MZ B cells can generate IgG antibodies in the absence of CD4 T cells through a C3 and type 1 interferon receptor (IFNR)-dependent process. In contrast, CD4 T cell–dependent alloimmunization can likewise occur through a MZ B cell–dependent pathway, where the interleukin-6 receptor (IL-6R) on CD4 T cells and Toll-like receptors (TLRs) are involved. The distinct immune pathways engaged may have consequences on the persistence of alloantibodies following initial alloimmunization and the likelihood of an anamnestic alloimmune response should alloantigen reexposure occur.

Figure 4

Alloantibody engagement of red blood cells (RBCs) following an incompatible transfusion can result in varied outcomes. Alloantibodies can induce complement activation, Fc gamma receptor (FcγR) ligation, or both, which can lead to extravascular and/or intravascular hemolysis. The antibody effector system involved in RBC removal also appears to facilitate antibody-induced loss of the target antigen. Loss of the target antigen lowers antibody binding, reducing the ability of antibody effector systems to facilitate extravascular and/or intravascular hemolysis and allowing transfused cells that have experienced antigen loss to persist in circulation despite the presence of the offending alloantibody. Complement split products generated following complement activation can further activate systemic immune responses, while heme released following a hemolytic transfusion reaction (HTR) can induce additional complement and immune cell activation through activation of innate immune receptors, including complement receptors and Toll-like receptor 4 (TLR4). Recipient variation in phagocytic function and/or complement levels may also influence each of these pathways. The consequences of incompatible RBC transfusion can therefore be quite variable depending on the alloantibodies and alloantigens involved, which can impact the extent of RBC clearance versus antigen loss, the activation of complement, and the release of free heme, all of which can influence the clinical sequelae of an incompatible transfusion. As a result of the varied consequences of antibody binding to RBCs, it can be difficult to predict the outcome of a given incompatible RBC transfusion a priori or devise common approaches to prevent or treat all HTRs should only incompatible RBCs be available.