Critical role of C5a in sickle cell disease

Abstract

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.

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

“…Hemolysis‐derived products or anti‐endothelial antibodies present in the serum could explain this activation. This complement over activation is concordant with the complement‐mediated cytotoxicity from SCD patients' sera (modified Ham's test), the enhanced C3 deposits on ECs exposed to SCD RBC microvesicles, and the endothelial C3 deposits in tissues of SCD mice and biopsies of patients . This over activation is indeed at least in part heme‐dependent since it was inhibited by the heme scavenger hemopexin.…”

“…Taken together, recent mouse models and data from patients suggest that complement plays a key role in the SCD disease process and is a potential therapeutic target . The HU treatment resulted in prevention of complement activation on blood cells.…”

“…Clinical observations have shown markers of complement activation in the circulation of SCD patients and increased levels of surface‐bound C3 fragments on RBCs . Mouse models have demonstrated that complement plays a key role in vaso‐occlusion and tissue injury, since complement blockade at the level of C5 or C5aR1 had a protective effect . The mechanism of this complement activation is not yet fully elucidated.…”

“…The mechanism of this complement activation is not yet fully elucidated. in vitro studies and mouse models have provided evidence that the activation of the cascade in SCD could proceed by different mechanisms: on ischemic tissue, via cell‐free heme/heme‐carrying RBC microvesicles, or directly on the surface of the SCD RBCs . The clinical relevance of the progression of the complement cascade to its terminal steps has not been investigated in detail in a large cohort of SCD patients.…”

The Red Blood Cell—Inflammation Vicious Circle in Sickle Cell Disease

“…Vercellotti et al (55) demonstrated increased C3 activation fragments and C5b-9 in the kidneys, lungs and liver of sickle cell mice, compared to control mice, and Lombardi et al (56) found increased microvascular deposition of C5b-9 on skin biopsies in SCD patients. Increased alternative pathway Bb fragments have also been reported in the plasma of both sickle cell mice and sickle cell patients (55,57). The infusion of recombinant C5a has been shown to cause blood stasis and inflammation in the liver of sickle cell mice (through NF-κB activation and increased expression of TLR4 and several adhesion molecules), but this response was reversed by an anti-C5a receptor IgG (55).…”