Increased GM-CSF Levels in Sickle Cell Disease Are Associated with Increased Leukocyte Counts and Are Reversed by Hydroxyurea.

The hematopoietic cytokine, GM-CSF, stimulates the proliferation and differentiation of progenitor cells to macrophages, eosinophils and megakaryocytes. Increased levels of GM-CSF have previously been reported in sickle cell disease (SCD) patients with low fetal hemoglobin (HbF) levels (Croizat, Br J Haematol. 87:592; 1994); however, a clearcut association between GM-CSF and HbF levels in SCD has yet to be established, since there is some evidence that levels may depend on other factors (Haider et al., Acta Haematol. 102:140; 1999). To further investigate the possible role of GM-CSF in SCD, plasma samples were collected from normal individuals (AA), steady-state sickle cell anemia patients (SS) and sickle cell patients on hydroxyurea therapy (SSHU; 20-30 mg/kg/day). Plasma GM-CSF levels were measured using a specific immunoenzymatic assay. HbF levels were quantified by ion-exchange HPLC and hematological measurements obtained using an Advia Hematology System. Plasma GM-CSF levels seem to be higher in SS patients than in normal controls (0.828 ± 0.215 pg/ml, n=39; 0.230 ± 0.081 pg/ml, n=9; respectively). In contrast, GM-CSF was significantly lower in SSHU patients than in SS without HU (0.288 ± 0.104 pg/ml; n=14; P = 0.045 compared to SS). A negative correlation between plasma GM-CSF and HbF levels was seen in patients (r = −0.332; P = 0.026; n=45; SS and SSHU groups) and, notably, a positive correlation between GM-CSF and white blood cell counts was observed (r = 0.302; P = 0.037; n= 48; SS and SSHU groups). Furthermore, GM-CSF (2.5 ng/ml) abolished the production of γ globin during erythropoietin-stimulated differentiation of TF-1 hematopoietic cells. These results provide further support to the hypothesis that plasma GM-CSF correlates negatively with HbF in sickle cell disease. Interestingly, in vitro data suggest that increased GM-CSF may, in fact, diminish HbF levels rather than reflect increased haematopoietic stress as a consequence of low HbF. Importantly, GM-CSF appears to correlate with leukocyte numbers in SCD, and levels were significantly decreased in patients taking HU, indicating that this cytokine may contribute to the leukocytosis seen in some SCD patients and may play a role in leukocyte count reduction by HU.