https://orcid.org/0000-0003-4808-5587
Key Points
Complement is activated in SCD and contributes to disease pathogenesis.
Hydroxyurea therapy may attenuate complement activation.
Sickle cell disease (SCD) is an understudied, life-threatening genetic disorder affecting around 300,000 infants yearly with limited treatment options. The complement system, a critical part of innate immunity, has emerged as a contributor to SCD pathophysiology, thus presenting a potential new treatment target. Our aim was to assess complement activity in children with SCD receiving hydroxyurea (HU) therapy during vaso-occlusive crisis (VOC) and steady state. Blood samples were collected from 46 paediatric SCD patients during VOC (early and late) and steady state, with control samples from healthy volunteers. Clinical data were obtained from patient records, and patient heme levels were measured using colorimetric assay. Complement deposition on endothelial cells (ECs) was quantified using high throughput automated immunofluorescence imaging. Complement protein concentration was measured using ELISA and multiplex assays. We found that, in vitro, heme drove C3b and C5b-9 deposition on ECs. Patients had increased heme levels during both VOC and steady state compared to healthy controls. However, complement activation correlated with total hemoglobin concentration in steady state patients, but not heme levels. C5b-9 deposition on ECs was significantly higher in early crisis compared to late crisis, suggesting heightened complement activity early in VOC, with C5b-9 deposition also strongly correlating with circulating sC5b-9 levels. A significant increase in the C3b/C3 ratio further indicated early complement activation during VOC. In conclusion, complement activity is likely highest in early VOC in SCD patients and presents a critical potential treatment target, but is overall attenuated by HU therapy despite elevated heme or hemoglobin levels.
