Complement Activation and Thrombotic Microangiopathies

Hyperhemolytic Transfusion Reaction in Non-Hemoglobinopathy Patients and Terminal Complement Pathway Activation: Case Series and Review of the Literature

Hyperhemolytic transfusion reaction (HHTR) is a severe, life-threatening hemolytic transfusion reaction where hemoglobin value after red blood cell (RBC) transfusion is lower than the pre-transfusion value. When HHTR occurs, mainly in patients with hemoglobinopathy, complement activation up to membrane attack complex (MAC) is strongly suspected. However, our knowledge of HHTR in patients without hemoglobinopathy is limited. In the present study, we retrospectively reviewed patients with the diagnosis of HHTR who were attended at our hospital between 2013 and 2016. We also performed a literature search to identify other reported cases of HHTR. Finally, the role of terminal complement pathway activation in the pathogenesis of HHTR was assessed by exposing endothelial cells in vitro to activated-patient plasma to analyze C5b-9 deposits by immunofluorescence. HHTR was diagnosed in 3 patients according to current criteria. Patients were treated with intravenous immunoglobulins (alone or in conjunction with rituximab and plasma exchange), and all of them recovered successfully. We retrieved from literature search 10 patients without hemoglobinopathy who developed HHTR. A marked increase of C5b-9 (MAC) deposition on endothelial cells (almost 2.5-fold increase versus control, P < .05) was observed with the plasma sample obtained from one of our patients. In conclusion, HHTR was a rare transfusion reaction that occurred in patients without hemoglobinopathy. We add more evidence that complement cascade activation up to MAC might play a role in the pathogenesis of HHTR.

Complement as the enabler of carfilzomib-induced thrombotic microangiopathy

Carfilzomib has been associated with the development of thrombotic microangiopathy (TMA) in relapsed/refractory multiple myeloma patients, a severe disease with no currently available aetiological treatment. We evaluated the potential role of terminal complement pathway in four patients with carfilzomib-induced TMA. Membrane attack complex (C5b-9) deposition on endothelial cells in culture exposed to plasma from patients during the acute phase of the disease suggests complement overactivation as a mechanism of potential endothelial damage in three out of four patients. If confirmed in larger cohorts, C5b-9 evaluation will allow early identification of patients who could benefit from complement blockade and treatment monitoring.

[The complement system-a "hot topic" not only for kidney diseases]

Increasing interest in the role of the complement system in systemic and renal disease is based on new pathophysiological and therapeutic insights of the recent past and particularly in genetic analyses in children with atypical hemolytic uremic syndrome (aHUS). aHUS is the prototypical systemic disease associated with excessive activation of the alternative complement pathway and manifests in the kidney, but also in other organs as thrombotic microangiopathy (TMA). Pathomechanisms discovered to induce the overactivation of the alternative complement pathway in aHUS led to the first successful therapeutic application of a C5b9 inhibitor. This suppression of the terminal complement cascade succeeded in inhibiting local tissue damage. Thereafter, thanks to advanced modern technologies, further systemic and renal diseases associated with mutations or auto-antibodies targeting the complement pathway were identified. Hereby, disease onset is frequently associated with an additional trigger, e.g. infection or hormonal alterations/imbalances, against the background of a pre-existing predisposition of the patient.Due to the growing understanding of the regulation, and thus the possibility of therapeutic modulation of the different complement pathways, and due to the increasing availability of a variety of drugs inhibiting the complement system, interest in complement-mediated systemic and renal disease has been steadily increasing, making it a "hot-topic" in medicine in recent years.

Complement activation in sickle cell disease: Dependence on cell density, hemolysis and modulation by hydroxyurea therapy

The complement system is an innate immune defense cascade that can cause tissue damage when inappropriately activated. Evidence for complement over activation has been reported in small cohorts of patients with sickle cell disease (SCD). However, the mechanism governing complement activation in SCD has not been elucidated. Here, we observe that the plasma concentration of sC5b-9, a reliable marker for terminal complement activation, is increased at steady state in 61% of untreated SCD patients. We show that greater complement activation in vitro is promoted by SCD erythrocytes compared to normal ones, although no significant differences were observed in the regulatory proteins CD35, CD55, and CD59 in whole blood. Complement activation is positively correlated with the percentage of dense sickle cells (DRBCs). The expression levels of CD35, CD55, and CD59 are reduced in DRBCs, suggesting inefficient regulation when cell density increases. Moreover, the surface expression of the complement regulator CD46 on granulocytes was inversely correlated with the plasma sC5b-9. We also show increased complement deposition in cultured human endothelial cells incubated with SCD serum, which is diminished by the addition of the heme scavenger hemopexin. Treatment of SCD patients with hydroxyurea produces substantial reductions in complement activation, measured by sC5b-9 concentration and upregulation of CD46, as well as decreased complement activation on RBCs in vitro. In conclusion, complement over activation is a common pathogenic event in SCD that is associated with formation of DRBCs and hemolysis. And, it affects red cells, leukocytes and endothelial cells. This complement over activation is partly alleviated by hydroxyurea therapy.