Expression pattern of CD55 and CD59 on red blood cells in sickle cell disease

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.

Therapeutic Lessons to be Learned From the Role of Complement Regulators as Double-Edged Sword in Health and Disease

The complement system is an important part of the innate immune system, providing a strong defense against pathogens and removing apoptotic cells and immune complexes. Due to its strength, it is important that healthy human cells are protected against damage induced by the complement system. To be protected from complement, each cell type relies on a specific combination of both soluble and membrane-bound regulators. Their importance is indicated by the amount of pathologies associated with abnormalities in these complement regulators. Here, we will discuss the current knowledge on complement regulatory protein polymorphisms and expression levels together with their link to disease. These diseases often result in red blood cell destruction or occur in the eye, kidney or brain, which are tissues known for aberrant complement activity or regulation. In addition, complement regulators have also been associated with different types of cancer, although their mechanisms here have not been elucidated yet. In most of these pathologies, treatments are limited and do not prevent the complement system from attacking host cells, but rather fight the consequences of the complement-mediated damage, using for example blood transfusions in anemic patients. Currently only few drugs targeting the complement system are used in the clinic. With further demand for therapeutics rising linked to the wide range of complement-mediated disease we should broaden our horizon towards treatments that can actually protect the host cells against complement. Here, we will discuss the latest insights on how complement regulators can benefit therapeutics. Such therapeutics are currently being developed extensively, and can be categorized into full-length complement regulators, engineered complement system regulators and antibodies targeting complement regulators. In conclusion, this review provides an overview of the complement regulatory proteins and their links to disease, together with their potential in the development of novel therapeutics.

A case-control analysis of hyperhemolysis syndrome in adults and laboratory correlates of complement involvement

BACKGROUND

Hyperhemolysis syndrome (HS) is a poorly understood, severe hemolytic anemia provoked by transfusion. Both host and donor RBCs are destroyed in HS; thus, transfusion paradoxically worsens anemia. Risk factors and mechanism of HS are unknown.

STUDY DESIGN AND METHODS

A retrospective case-control analysis was performed on adults with HS. Patients with HS were matched 1:1 with matched, transfused controls, and HS risk factors were analyzed with multivariable logistic regression. HS samples were analyzed for complement deposition by flow cytometry, and an in vitro model of bystander hemolysis was developed.

RESULTS

Forty-one patients with 54 episodes of HS were identified in a 26-year period from 1992 to 2018. Of the HS episodes, only 18.5% were associated with a new alloantibody, and such patients were more tolerant of additional transfusion in the acute episode (p = 0.005). Thirteen percent of episodes were fatal, and HS recurred in 52.6%. Alloimmunization (odds ratio [OR], 17.3), non-B blood type (OR, 9.8), D antigen (OR, 9.1), and infection (OR, 5.5) were associated with HS on multivariable analysis. Hyperbilirubinemia was predictive of fatal HS (OR, 33.6). Increased complement was observed on RBCs during HS episodes, and the in vitro model of bystander hemolysis recapitulated complement decoration of sickled RBCs.

CONCLUSIONS

HS is associated with significant morbidity, mortality, and recurrence. Risk factors such as known alloimmunization, blood group, and infection predispose to HS. Bystander complement activation may drive HS. These factors may help physicians refine risk-benefit assessments for transfusion and guide further therapeutic development.

Critical role of C5a in sickle cell disease

Innate immune complement activation may contribute to sickle cell disease (SCD) pathogenesis. Ischemia-reperfusion physiology is a key component of the inflammatory and vaso-occlusive milieu in SCD and is associated with complement activation. C5a is an anaphylatoxin, a potent pro-inflammatory mediator that can activate leukocytes, platelets, and endothelial cells, all of which play a role in vaso-occlusion. We hypothesize that hypoxia-reoxygenation (H/R) in SCD mice activates complement, promoting inflammation and vaso-occlusion. At baseline and after H/R, sickle Townes-SS mice had increased C3 activation fragments and C5b-9 deposition in kidneys, livers and lungs and alternative pathway Bb fragments in plasma compared to control AA-mice. Activated complement promoted vaso-occlusion (microvascular stasis) in SS-mice; infusion of zymosan-activated, but not heat-inactivated serum, induced substantial vaso-occlusion in the skin venules of SS-mice. Infusion of recombinant C5a induced stasis in SS, but not AA-mice that was blocked by anti-C5a receptor (C5aR) IgG. C5a-mediated stasis was accompanied by inflammatory responses in SS-mice including NF-κB activation and increased expression of TLR4 and adhesion molecules VCAM-1, ICAM-1, and E-selectin in the liver. Anti-C5aR IgG blocked these inflammatory responses. Also, C5a rapidly up-regulated Weibel-Palade body P-selectin and von Willebrand factor on the surface of human umbilical vein endothelial cells in vitro and on vascular endothelium in vivo. In SS-mice, a blocking antibody to P-selectin inhibited C5a-induced stasis. Similarly, an antibody to C5 that blocks murine C5 cleavage or an antibody that blocks C5aR inhibited H/R-induced stasis in SS-mice. These results suggest that inhibition of C5a may be beneficial in SCD.

Altered Expression Levels of CD59, but Not CD55, on Red Blood Cells in Stored Blood

OBJECTIVE

Red blood cells (RBCs) in storage undergo structural and biochemical changes that may cause functional effects. Studies exploring structural changes affecting the expression levels of CD55 and CD59 on RBCs are limited. The aim of this study was to investigate the pattern of CD55 and CD59 expression on RBCs in stored blood from Arab donors.

MATERIALS AND METHODS

Flow-cytometric analysis was performed on RBCs from 92 packed RBC (PRBC) units, stored for varying times, and from 56 nonstored RBC from healthy controls using the commercial REDQUANT kit.

RESULTS

The proportions of CD55- and CD59-deficient RBCs from stored PRBC units did not significantly differ when compared with those from healthy controls; however, the mean fluorescent intensity (MFI) of CD59 expression, but not MFI of CD55 expression, on RBCs from stored PRBC units was significantly reduced when compared to the expression of RBCs from healthy controls (p = 0.02). MFI of CD55 expression on RBCs from PRBC units did not significantly differ among the 3 groups of stored RBC; however, there was a statistically significant time-dependent preferential decline in MFI of CD59 expression on RBCs from stored PRBC units (p < 0.01).

CONCLUSION

There is a preferential time-dependent decline in the expression of CD59, but not of CD55, on stored RBCs, the in vivo significance of which in relation to the response to PRBC transfusion needs further investigation.

Consequences of dysregulated complement regulators on red blood cells

The complement system represents the first line of defense that is involved in the clearance of pathogens, dying cells and immune complexes via opsonization, induction of an inflammatory response and the formation of a lytic pore. Red blood cells (RBCs) are very important for the delivery of oxygen to tissues and are continuously in contact with complement proteins in the blood plasma. To prevent complement activation on RBCs, various complement regulatory proteins can be found in plasma and on the cell membrane. RBCs are special cells without a nucleus and having a slightly different make-up of complement regulators than nucleated cells, as membrane cofactor protein (MCP) is not expressed and complement receptor 1 (CR1) is highly expressed. Decreased expression and/or function of complement regulatory proteins may result in unwanted complement activation and accelerated removal of RBCs. This review describes complement regulation on RBCs and the consequences when this regulation is out of balance.

Altered Expression Pattern of CD55 and CD59 on Red Blood Cells in Anemia of Chronic Kidney Disease

OBJECTIVE

The aim of this study was to investigate the expression pattern of CD55 and CD59 on red blood cells (RBCs) in anemic chronic kidney disease (CKD) patients, and factors that might influence their expression.

SUBJECTS AND METHODS

Nighty-one adult anemic CKD patients and 80 healthy controls (HCs) were enrolled. Anemic CKD patients were divided into 3 subgroups based on receiving erythropoietin and renal replacement therapies. Flow cytometric analysis of CD55 and CD59 expression was performed on RBCs from blood samples obtained from CKD patients and HCs.

RESULTS

CD59 deficiency was significantly higher among CKD patients than HCs (n = 68, 74.7%, vs. n = 13, 16.3%, respectively; p < 0.001). The median proportions of CD55- and CD59-deficient RBCs in CKD patients were significantly higher compared to HCs (0.34 vs. 0.15, and 4.3 vs. 2.0, p < 0.001 and p < 0.001, respectively). The mean fluorescence intensity (MFI) of CD55 and CD59 expression was significantly lower in CKD patients compared to HCs (1.2 vs. 2.8, and 17.0 vs. 20.3, p < 0.04 and p < 0. 001, respectively). The hemoglobin level was inversely correlated with the proportions of CD55- and CD59-deficient RBCs (r = -0.37, p < 0.001, and r = -0.22, p < 0.02, respectively). The number of CD59-deficient patients was significantly different between the 3 subgroups of CKD patients (p = 0.001), and a significant difference was present in the MFI of CD55 and CD59 expression among the 3 subgroups (p = 0.04 and p = 0.03, respectively).

CONCLUSION

The expression pattern of CD55 and CD59 on RBCs is altered in anemic CKD patients, which could play a role in the pathogenesis of anemia in CKD.