Hydroxyurea-Induced Changes of Components Involved In the Modulation of Adenosine Levels, In Blood Cells From Sickle Cell Disease Patients

Recent studies have demonstrated the role of high adenosine levels in priapism episodes in a mouse model of sickle cell disease (SCD). Interestingly, adenosine signaling is related to several physiopathological processes that may relate to clinical features observed in patients with SCD. Adenosine (ADO) is a purine nucleoside that plays diverse roles in distinct physiological contexts. Extracellular ADO production occurs sequentially by the ectonucleotidases CD39 (which converts ATP and ADP to 5′-AMP) and CD73 (which convert 5′-AMP to ADO). Moreover, ADO levels are controlled by its conversion to inosine by the enzyme Adenosine Deaminase (ADA). ADA can be anchored in the cell membrane by CD26, leading to an increased localized action and consequently, to reduced local concentrations of adenosine. Hydroxyurea (HU) is the only drug approved by FDA to reduce vaso-occlusive episodes in patients with SCD, partly by the induction of fetal hemoglobin (HbF) and reduction of polymerization of HbS. However, the clinical improvement of patients is not always associated with increased HbS levels, indicating the potential effect of HU on other processes. Given the known (or proposed) contribution of distinct blood cell types in the physiopathology of SCD, in this study, we aimed to evaluate the possible modulation in the expression of CD39, CD73 and CD26 on lymphocytes and monocytes from SCD patients, in HU treated patients.

The expression of CD39, CD73 and CD26 was evaluated by flow cytometry on total lymphocytes (CD3+) and monocytes (CD14+) in the peripheral blood (PB) of 12 patients treated with HU, 21 untreated and seven control healthy individuals.

On average, while less than 0.3% and 1.7% of monocytes of controls and untreated patients express CD26, respectively; in patients treated with HU, more than 10% of the monocytes express CD26 (p=0.0171, unpaired T-test). Additionally, in treated patients, a significantly lower percentage of lymphocytes express CD39, as compared to untreated (p=0.0431, unpaired T-test). The CD73 protein was not expressed by monocytes, and there was no modulation of its levels in lymphocytes.

During inflammation (a processes associated with the physiopathology of SCD), the extracellular concentration of adenosine is increased and distinct blood cell types localize to the affected tissue. The results indicate a potential mechanism of action of HU in SCD patients, mediated by the increased expression of CD26 on monocytes (with subsequent co-localization of the enzyme ADA) and by the decreased expression of CD39 on lymphocytes. As a result of the observed changes, a decrease in the local synthesis of adenosine, associated with its increased conversion to inosine, would be expected. Thus, HU may drive the reduction of adenosine levels, thereby reducing the aggravating effects of this molecule in different physiopathological processes affected in patients with SCD. Supported by: FAPESP, CNPQ, FINEP and INSERM.

No relevant conflicts of interest to declare.