Recovery of Thymic Function after Allogeneic Hematopoietic Stem Cell Transplantation in with Sickle Cell Anemia Patients

Introduction: Sickle cell anemia (SCA) is a hereditary chronic disorder of hemoglobin (Hb) caused by a single mutation in the human β-globin gene leading to an amino acid substitution (Glu → Val) in the sixth position of the β-globin chain. The polymerization of the resulting abnormal hemoglobin (HbS) under hypoxic conditions is a key event in the complex pathophysiology of SCA (Hebbel, 2011). Vaso-occlusive crisis (VOC) is the main clinical manifestation and caused by a mechanical obstruction of small blood vessels by rigidly distorted red blood cells (RBCs) and local inflammation (Raphael; Vichinsky, 2005). SCA represents a serious public health problem in many countries. Currently, allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only curative treatment for SCA. However, allo-HSCT can have severe drawbacks, such as the development of graft-versus-host disease (GvHD). Another main concern is the profound and long-lasting immunodeficiency consecutive to transplantation procedure. A potent immune reconstitution (IR) after HSCT is essential to limit the infection risk and disease relapse (Toubert et al., 2012). The reconstitution of a diverse peripheral T cell repertoire is a long and continuous process that depends on a functional thymus to produce naïve T cells. Therefore, thymus plays a crucial role in the quality of IR and clinical outcome after allo-HSCT. The aim of this study was to evaluate the thymic function in SCA patients treated with allo-HSCT and analyze the impact of acute GvHD on patient's thymic function.

Methods and Patients: Twenty-nine patients SCA patients were treated with allo-HSCT and evaluated in this work. Peripheral blood mononuclear cells were collected before transplantation (day 0) and at 1, 3, 6, 12, 24 and >24 months post-transplantation periods. Fifteen healthy individuals (afrodescendants) matched by age/gender were enrolled as controls. Thymic function was evaluated by quantification of signal joint (sj)-T-cell receptor excision circles (sjTREC) and βTREC in the genomic DNA by real-time quantitative PCR. Intra-thymic T-cell division (n) was calculated by the formula: n= LOG(sjTREC/βTREC)/LOG2. Nonparametric Mann-Whitney test was used and the results were considered statistically significant when p < 0.05.

Results: sjTREC and β-TREC levels were significantly reduced one month after transplantation. However, at 6 months post-transplantation their levels increased substantially. At 12 months post-HSCT, significantly higher sjTREC and β-TREC levels were found, compared to the first periods after transplantation. The intra-thymic cell division was significantly reduced at 1 month after transplantation, compared to the control group and pre-transplantation period. Patients who developed acute GvHD showed decreased sjTREC and β-TREC levels at 6 months post-transplantation, compared to control group and to patients without GvHD.

Conclusions: Our findings demonstrated that SCA patients have, despite of the inflammatory environment, a preserved thymus and present rebound of its important function after undergoing allo-HSCT. In addition, we observed that patients undergoing acute GvHD have a decreased thymic function, compromising their immune status during early immune reconstitution phase.