Unmasking delayed hemolytic transfusion reactions in patients with sickle-cell disease: Challenges and opportunities for improvement

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.

Harnessing the potential of red blood cells in immunotherapy

Red blood cell (RBC) transfusion represents one of the earliest and most widespread forms of cellular therapy. While the primary purpose of RBC transfusions is to enhance the oxygen-carrying capacity of the recipient, RBCs also possess unique properties that make them attractive vehicles for inducing antigen-specific immune tolerance. Preclinical studies have demonstrated that RBC transfusion alone, in the absence of inflammatory stimuli, often fails to elicit detectable alloantibody formation against model RBC antigens. Several studies also suggest that RBC transfusion without inflammation may not only fail to generate a detectable alloantibody response but can also induce a state of antigen-specific non-responsiveness, a phenomenon potentially influenced by the density of the corresponding RBC alloantigen. The unique properties of RBCs, including their inability to divide and their stable surface antigen expression, make them attractive platforms for displaying exogenous antigens with the goal of leveraging their ability to induce antigen-specific non-responsiveness. This could facilitate antigen presentation to the host's immune system without triggering innate immune activation, potentially enabling the induction of antigen-specific tolerance for therapeutic applications in autoimmune disorders, preventing immune responses against protein therapeutics, or reducing alloreactivity in the setting of transfusion and transplantation.

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.

Delayed haemolytic transfusion reaction in paediatric patients with sickle cell disease: A retrospective study in a French national reference centre

Delayed haemolytic transfusion reaction (DHTR) is a life-threatening haemolytic anaemia following red blood cell transfusion in patients with sickle cell disease, with only scarce data in children. We retrospectively analysed 41 cases of DHTR in children treated between 2006 and 2020 in a French university hospital. DHTR manifested at a median age of 10.5 years, symptoms occurred a median of 8 days after transfusion performed for an acute event (63%), before surgery (20%) or in a chronic transfusion programme (17%). In all, 93% of patients had painful crisis. Profound anaemia (median 49 g/L), low reticulocyte count (median 140 ×10⁹ /L) and increased lactate dehydrogenase (median 2239 IU/L) were observed. Antibody screening was positive in 51% of patients, and more frequent when there was a history of alloimmunisation. Although no deaths were reported, significant complications occurred in 51% of patients: acute chest syndrome (12 patients), cholestasis (five patients), stroke (two patients) and kidney failure (two patients). A further transfusion was required in 23 patients and corticosteroids were used in 21 to reduce the risk of additional haemolysis. In all, 13 patients subsequently received further transfusions with recurrence of DHTR in only two. The study affords a better overview of DHTR and highlights the need to establish guidelines for its management in children.