Serum Haptoglobin and Hemopexin Levels in Pediatric SS and SC Disease Patients: Biomarker of Hemolysis and Inflammation

INTRODUCTION: Hemolysis is a hallmark of sickle cell disease (SCD) pathogenesis, with the release of hemoglobin (Hb) and heme into the plasma that can promote vasculopathy, inflammation, thrombosis, and renal injury. Plasma haptoglobin (Hp) and hemopexin (Hpx) tightly bind free Hb and heme, respectively, thwarting these clinical sequelae. Hp-Hb and Hpx-heme complexes bind to CD163 and CD91 receptors found primarily on macrophages and hepatocytes, respectively, and are taken up by receptor-mediated endocytosis. In SCD patients it is generally believed that chronic hemolysis depletes plasma haptoglobin and hemopexin levels, however we found only one paper in the literature that has reported plasma Hp and Hpx levels in SCD patients, albeit in a limited number of SS (n=5) and SC (n=3) patients (Muller-Eberhard U et al. Blood 1968). Here we report serum Hp and Hpx levels in 272 Brazilian SS, SC, and AA children along with systemic biomarkers of hemolysis, iron metabolism, inflammation leukocyte and platelet counts, and lipid metabolism. We hypothesize that serum Hp and Hpx levels will be significantly lower and biomarkers of hemolysis, iron metabolism, and inflammation will be significantly higher in SS and SC children compared to AA children. METHODS: The present study used blood samples collected prospectively from 272 unrelated steady-state SCD children with SS (n=179) and SC (n=93) disease. Patients were followed at Bahia Pediatrics Centers, during January 2010 to November 2015. The study included 28 healthy AA volunteers randomly selected from the Clinical Laboratory of the Faculdade de Farmácia da Universidade Federal da Bahia. The study was approved by the Fundação de Pesquisa Oswaldo Cruz research board, and all procedures were in accordance with the Declaration of Helsinki of 1975, as revised in 2000. Biochemical and inflammatory analyses were performed by immunochemistry assay and immunoassay. Hematological analyses were carried out using an electronic cell counter and the Hb profile was investigated by high performance liquid chromatography. Serum Hp and Hpx levels were measured by enzyme-linked immunoassay following the manufacturer's instructions. RESULTS: Evaluating the Hp and Hpx levels, we found that SS and SC patients have lower Hp and Hpx levels (SS<SC<AA) than AA controls (p<.001, Table 1). Similarly, SS and SC patients have lower hemoglobin and hematocrits (SS<SC<AA) and increased reticulocyte counts and serum LDH (SS>SC>AA) than AA controls (p<.001, Table 1). Total, direct, and indirect bilirubin, markers of heme degradation, followed a similar pattern (SS>SC>AA, p<.001, Table 1). These data may reflect more intravascular hemolysis in the following pattern: SS>SC>AA. Likewise, serum ferritin levels, a marker of body iron stores, were higher in SS and SC patients than AA controls (SS>SC>AA, p<.001, Table 1). CRP levels and white blood cells count, markers of inflammation, also reflected the same pattern (SS>SC>AA, p<.001, Table 1). In a different pattern, platelet counts were highest in SS patients and lowest in SC patients (SS>AA>SC). Finally, Total, HDL, and LDL cholesterol levels were lowest in patients with more hemolysis (SS<SC<AA, p<.001, Table 1). CONCLUSIONS: We report lower levels of Hp and Hpx in children with SCD (SS<SC<AA). These data support Muller-Eberhard's 1968 brief report on Hp and Hpx levels in hemolytic conditions and demonstrate significant correlations of serum Hp and Hpx with biomarkers of hemolysis, inflammation, and cholesterol. We speculate that serum Hp and Hpx levels in SCD may serve as clinical indicators of hemolysis, inflammation, and disease severity.

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