Split chimerism between nucleated and red blood cells after bone marrow transplantation for haemoglobinopathies

Previous studies have shown that a stable presence of both donor and recipient haematopoietic derived cells after allogeneic haematopoietic stem cell transplantation (HSCT) occurs in approximately ten percent of the patients affected by β-Thalassemia. Once achieved this condition, defined as persistent mixed chimerism (PMC), the patients do not require additional red blood cells (RBCs) support and, regardless of the presence in some cases of an extremely low percentage of donor-derived nucleated cells, they are clinically cured by an incomplete, but functional graft. Most of the published papers have, however, investigated the impact of donor engraftment in the nucleated cells rather than in the mature erythrocytes. We have recently published a paper showing that in four long-term transplanted patients affected by hemoglobinopathies, characterized by the presence of few donor engrafted nucleated cells-both in the peripheral blood and in the bone marrow-the majority of the erythrocytes were of donor origin. Moreover we showed that the proportion of donor-derived erythroid precursors, determined by analyzing singularly picked-up burst-forming unit erythroid colonies, was equivalent to that observed in the mature nucleated cells rather than in the red blood cells. These results suggest that in patients characterized by the presence of PMC after HSCT a selective advantage of the donor erythroid precursors maturation might successfully contrast the problems bound to the recipient ineffective erythropoiesis. When genetically modified HSCT will be a possible option for treating Thalassemia Major, the co-existence of the repaired cells with those still expressing the genetic defect will be an expected scenario, not in an allogeneic, but in an autologous environment.