Pre- and Post-transfusion Complement Activation in Transfusion-dependent β-thalassaemia

What is the role of complement in bystander hemolysis? Old concept, new insights

INTRODUCTION

Bystander hemolysis occurs when antigen-negative red blood cells (RBCs) are lysed by the complement system. Many clinical entities including passenger lymphocyte syndrome, hyperhemolysis following blood transfusion, and paroxysmal nocturnal hemoglobinuria are complicated by bystander hemolysis.

AREAS COVERED

The review provides data about the role of the complement system in the pathogenesis of bystander hemolysis. Moreover, future perspectives on the understanding and management of this syndrome are described.

EXPERT OPINION

Complement system can be activated via classical, alternative, and lectin pathways. Classical pathway activation is mediated by antigen-antibody (autoantibodies and alloantibodies against autologous RBCs, infectious agents) complexes. Alternative pathway initiation is triggered by heme, RBC microvesicles, and endothelial injury that is a result of intravascular hemolysis. Thus, C5b is formed, binds with C6-C9 compomers, and MAC (C5b-9) is formulated in bystander RBCs membranes, leading to cell lysis. Intravascular hemolysis, results in activation of the alternative pathway, establishing a vicious cycle between complement activation and bystander hemolysis. C5 inhibitors have been used effectively in patients with hyperhemolysis syndrome and other entities characterized by bystander hemolysis.

Complement in Sickle Cell Disease: Are We Ready for Prime Time?

Sickle cell disease (SCD) is a widely spread inherited hemoglobinopathy that includes a group of congenital hemolytic anemias, all characterized by the predominance of sickle hemoglobin (HbS). Its features are anemia, predisposal to bacterial infections and complications such as vaso-occlusive crisis (VOC) or delayed hemolytic transfusion reaction (DHTR), which lead to increased rate of morbidity and mortality even in the era of hydroxyurea. The interaction between sickle cells, neutrophils, platelets or endothelial cells in small vessels results in hemolysis and has been considered the disease’s main pathophysiological mechanism. Complement activation has been reported in small cohorts of SCD patients, but the governing mechanism has not been fully elucidated. This will be important to predict the patient group that would benefit from complement inhibition. Until now, eculizumab-mediated complement inhibition has shown beneficial effects in DHTR, with limited reports in patients with VOC. In the meantime, several innovative agents are under clinical development Our state-of-the-art review summarizes current data on 1) complement activation in SCD both in steady state and crisis, 2) underlying mechanisms of complement over-activation for the clinician in the context of SCD, 3) actions of hydroxyurea and new therapeutic approaches including indirect involvement in complement activation, and 4) novel paradigms in complement inhibition.

Complement in sickle cell disease and targeted therapy: I know one thing, that I know nothing

Sickle cell disease (SCD) is a common inherited clinical syndrome, characterized by the presence of hemoglobin S. Anemia, susceptibility to infections and episodes of vaso-occlusive crisis (VOC) are among its features. Since SCD complications (VOC or delayed hemolytic transfusion reaction/DHTR) lead to significant morbidity and mortality, a number of studies have addressed their pathophysiology Although SCD pathophysiology has been mainly attributed to the interaction between sickle cells and neutrophils, platelets or endothelial cells in small vessels leading to hemolysis, the role of complement activation has been increasingly investigated. Importantly, complement inhibition with eculizumab has shown beneficial effects in DHTR. Given the unmet clinical need of novel therapeutics in SCD, our review summarizes current understanding of (a) complement system for the clinician, (b) complement activation in SCD both in asymptomatic state and severe clinical manifestations, (c) probable underlying mechanisms of complement activation in SCD, and (d) new therapeutic perspective of complement inhibition.

Successful Outcome of Hyperhemolysis in Sickle Cell Disease following Multiple Lines of Treatment: The Role of Complement Inhibition

Delayed hemolytic transfusion reaction (DHTR) is a life-threatening complication in patients with sickle cell disease, characterized by difficulties in diagnosis and management. Certain reports have suggested successful salvage treatment with the terminal complement inhibitor, eculizumab. We here report evidence of complement activation and successful complement inhibition with one dose of eculizumab in an adult sickle cell disease patient presenting DHTR with hyperhemolysis. A 21-year old female sickle cell disease patient [Hb S (HBB: c.20A>T)/β-thalassemia (β-thal)] presented at our Adult Thalassaemia Unit, Hippokration General Hospital of Thessaloniki, Thessaloniki, Greece, with headaches, perioral numbness and pain in both antibrachia. The patient was admitted with the diagnosis of vaso-occlussive crisis (VOC) and treated symptomatically. On her third day of admission, due to hemoglobin (Hb) value of 6.9 g/dL with increase in reticulocytes, the patient was transfused with one unit of compatible leukodepleted red blood cells (RBC). The following day, hemolytic parameters increased, although Coombs, panel antibodies and screening tests were negative. Five days later, she again received a unit of RBCs, resulting in another increase of hemolytic parameters. During the following 2 days, there was a dramatic decrease of Hb levels to 5.4 g/dL with reticulocytes at 6.0%, negative Coombs testing and negative alloantibodies. Based on these findings, we recognized the syndrome of DHTR with hyperhemolysis. Given the lack of immediate access to other treatment, we administered intravenous rituximab, immunoglobulins and corticosteroids. As there was no response, one dose of eculizumab (900 mg) was then administered with ciprofloxacin as prophylaxis. The patient remains well 6 months post treatment.