Complement Activation during Vaso-Occlusive Pain Crisis in Pediatric Sickle Cell Disease

Background: Sickle cell disease (SCD) affects millions of individuals worldwide with substantial morbidity and mortality. The sickle hemoglobin (HbS) polymerizes upon deoxygenation, causing rigid and adhesive red blood cells (RBCs), triggering vascular occlusion, greatly shortened RBC lifespan, and chronic hemolysis. Amongst acute complications in SCD, vaso-occlusive pain crisis (VOC) is the leading cause of hospitalization, with supportive care being the primary approach to management. We and others have recently demonstrated important contributions of complement to the pathophysiology of SCD. When the complement pathway (CP) is activated during SCD crises, inhibition at C5 using eculizumab, has been successful in treating various acute complications in SCD (Chonat et al, Haematologica). In this study, we prospectively analyzed the extent of CP activation among children with SCD presenting with VOC.

Methods: Patients aged 0-21 years old managed at Children's Healthcare of Atlanta with homozygous sickle cell (SS) or S beta zero thalassemia genotypes were enrolled in an IRB-approved research study. Inclusion criteria included those requiring intravenous opioids for VOC, and excluded those with chronic pain, >6 VOC admission in the previous 12 months or on chronic transfusions. Blood samples were collected within 48 hours of VOC presentation, and steady-state levels were obtained at a 4-week clinic follow-up. Data was analyzed using a paired t-test, and receiver operator characteristic (ROC) curves were generated comparing intra-person complement levels during acute VOC versus respective steady-state levels.

Results: Sixty-four patients have been enrolled thus far, of which 43 (67%) had steady-state samples collected. The majority of patients (90.5%) have SS genotype with a mean (SD) patient age of 14.15 (4.68) years. Fifty-three (84.1%) patients reported taking hydroxyurea (HU). Fifty-nine (93.7%) patients had at least one VOC admission in the past 12 months, with an average of 2.98 (1.67) VOC admissions. Pain Score reported on 55 patients averaged 4.93 (4.78) on a pain scale of 0 to 10. Mean values during VOC and steady-state of hemoglobin (Hb) were 8.12 and 9.01 g/dL, platelet count 431 and 511, and lactate dehydrogenase (LDH) 549 and 483 U/L, respectively. Seventeen patients had complement work-up performed during acute and steady-state, and 4 of them had additional samples collected during subsequent VOC. Complement protein levels C3, C4, C5, properdin, factor B, and complement regulatory proteins factor H and I were unremarkable during VOC and steady-state. However, complement activation markers, specifically anaphylatoxins C3a, C5a and Bb were significantly elevated during VOC compared to steady-state (see Table 1) suggesting activation of alternative CP during VOC (see Table 1 and Figure 1A-C). Terminal complement complex (C5b9) was not statistically different between VOC and steady-state (Figure 1D, red dotted lines signify normal ranges). Remarkably, patients who re-presented with acute VOC exhibited similar increases in their C3a/C5a (Figure 1E-F), substantiating the increases related to their VOC. Hemoglobin and LDH (Figure 1G-H) were similarly significant, suggestive of intravascular hemolysis. Three (7.1%) patients developed acute chest syndrome, two of whom experienced respiratory failure requiring intensive care management, and all exhibited significant CP activation. The area under the curve (AUC) of the ROC curve was analyzed to determine the ability of complement biomarkers to differentiate VOC from steady-state. Based on the AUC of these biomarkers, complement anaphylatoxins C3a and C5a exhibited the highest AUC of 0.76 and 0.87, respectively.

Discussion: To our knowledge, this is the first prospective and comprehensive evaluation of CP in patients with SCD during VOC and steady-states. These preliminary findings suggest CP activation is present in a large proportion of patients during VOC, with increased activation of alternative and common CP, associated with intravascular hemolysis. Minimal increase in C5b9 could be explained by a significant proportion (> 80%) of our patients being on HU therapy, similar to prior data (Roumenina et al, AJH). Specifically, C3a/C5a, along with other biomarkers, could not only predict disease activity in patients during VOC, but provide pharmacological targets in VOC, which need further validation.

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