Plasma ‘Free’ HB Is Related to Red Cell Derived Vesicle Numbers in Sickle Cell Anemia and Thalassemia Intermedia: Implications for Nitric Oxide (NO) Scavenging and Pulmonary Hypertension.

Abstract Plasma hemoglobin (Hb) is a scavenger of nitric oxide (NO), which is a likely contributor to the pathogenesis and treatment of clinical abnormalities such as pulmonary hypertension and vasocclusive episodes in sickle cell anemia (SCA) and possibly in thalassemia syndromes, where pulmonary hypertension has been described most frequently in splenectomised patients with thalassemia intermedia (TI). Since plasma Hb consists of both free Hb and RBC-derived microvesicle Hb (Greenwalt. Vox Sang1991;61:14), the relationship between plasma Hb and circulating vesicles may be significant. We have measured plasma Hb and plasma vesicle levels in adults with SCA and thalassemia intermedia (TI). Patients with SCA were all untransfused and TI patients had no transfusions during the previous 3 months. Plasma Hb values in healthy adult controls (1.77±0.2, n=7) were significantly lower than in SCA (11.21±2.08mg/dl, n=15, p=0.003) or in splenectomised TI patients (48.46 ±3.66mg/dl, n=5, p=0.0038). Plasma Hb levels were significantly greater in TI as compared to levels in SCA (p=0.001). Vesicle numbers in SCA were 12.59±3.65 x103/ul (n=21, p< 0.001, SCA v control) and in TI were 24.19 ±12.23 (n=7, p< 0.001, TI v control). Thus both plasma Hb and circulating vesicle levels in SCA and TI were significantly greater than healthy controls. Plasma Hb was significantly greater in TI than in SCA and circulating vesicles markedly greater in TI than in SCA. Furthermore there was a significant correlation between plasma Hb and vesicle numbers in SCA, TI and normal controls (n=26, R2=0.59, p=0.0015). An analysis of splenectomised versus non-splenectomised TI patients revealed a further trend; both plasma Hb and vesicle numbers were significantly higher in splenectomised (n=5, n=7 respectively) than in non-splenectomised patients (n=4, n=4 respectively). Plasma Hb and vesicle numbers were respectively 48.5±3.6 and 24.19±12.2 in splenectomised patients compared to 17.18±5.58 (p=0.014) and 4.36±0.81 (p=0.008) in non-splenectomized TI patients. These findings show that raised plasma Hb levels are related to increments in vesicle numbers in TI and SCA. Splenectomy in TI, which is associated with increased risk of pulmonary hypertension and thrombosis, is associated with increased vesicle numbers and plasma Hb. We suggest that the scavenger characteristics of Hb containing vesicles for NO may differ from the scavenger characteristics of free Hb and thus require detailed study..