β-Thalassemia is associated with several abnormalities of the innate immune system. Neutrophils in particular are defective, predisposing patients to life-threatening bacterial infections. The molecular and cellular mechanisms involved in impaired neutrophil function remain incompletely defined. We used the Hbb β-thalassemia mouse and hemoglobin E (HbE)/β-thalassemia patients to investigate dysregulated neutrophil activity. Mature neutrophils from Hbb mice displayed a significant reduction in chemotaxis, opsonophagocytosis, and production of reactive oxygen species, closely mimicking the defective immune functions observed in β-thalassemia patients. In Hbb mice, the expression of neutrophil CXCR2, CD11b, and reduced NAD phosphate oxidase components (p22phox, p67phox, and gp91phox) were significantly reduced. Morphological analysis of Hbb neutrophils showed that a large percentage of mature phenotype neutrophils (Ly6GLy6C) appeared as band form cells, and a striking expansion of immature (Ly6GLy6C) hyposegmented neutrophils, consisting mainly of myelocytes and metamyelocytes, was noted. Intriguingly, expression of an essential mediator of neutrophil terminal differentiation, the ets transcription factor PU.1, was significantly decreased in Hbb neutrophils. In addition, in vivo infection with failed to induce PU.1 expression or upregulate neutrophil effector functions in Hbb mice. Similar changes to neutrophil morphology and PU.1 expression were observed in splenectomized and nonsplenectomized HbE/β-thalassemia patients. This study provides a mechanistic insight into defective neutrophil maturation in β-thalassemia patients, which contributes to deficiencies in neutrophil effector functions.
Summary
Severe bacterial infection is a major complication causing morbidity and mortality in β‐thalassaemia/HbE patients. Innate immunity constitutes the first line of defence against bacterial infection. This study aimed to comprehensively investigate the innate immune phenotype and function related to factors predisposing to infection in non‐transfusion‐dependent (NTD) β°‐thalassaemia/HbE patients. Twenty‐six patients and 17 healthy subjects were recruited to determine complement activity (C3, C4, mannose‐binding lectin and CH50) and surface receptor expression including markers of phagocytosis (CD11b, CD16 and C3bR), inflammation (C5aR) and migration (CD11b, CXCR1 and CXCR2) on neutrophils and monocytes. In addition, phagocytosis and oxidative burst activity of neutrophils and monocytes against Escherichia coli and neutrophil migration were examined. Decreased C3 and surface expression of CD11b and C3bR on neutrophils were found in patients. However, phagocytosis of neutrophils in patients was still in the normal range. Interestingly, patients displayed a significant reduction of surface expression of CXCR2 [1705 ± 217 mean fluorescent intensity (MFI)] on neutrophils, leading to impaired neutrophil migration (9·2 ± 7·7%) when compared to neutrophils from healthy subjects (2261 ± 627 MFI and 27·8 ± 9% respectively). Moreover, surface expression of CXCR2 on neutrophils was associated with splenectomy status, serum ferritin and haemoglobin levels. Therefore, impaired neutrophil migration could contribute to the increased susceptibility to infection seen in NTD β°‐thalassaemia/HbE patients.
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