Quantitatively different red cell/nucleated cell chimerism in patients with long-term, persistent hematopoietic mixed chimerism after bone marrow transplantation for thalassemia major or sickle cell disease

Background. Persistent mixed chimerism represents a state wherein recipient and donor cells stably co-exist after haematopoietic stem cell transplantation. However, since in mostly of the studies reported in literature the engraftment state was observed in the nucleated cells, in this paper we determined the donor origin in the mature erythrocytes of patients with persistent mixed chimerism after transplantation for haemoglobinopathies. Results were compared with the engraftment state observed in singularly picked-up burst-forming unit-erythroid colonies and in the nucleated cells collected from the peripheral blood and from the marrow. Design and Methods. The donor origin of the erythrocytes was determined analyzing differences on the surface antigens of the erythrocytes suspension after incubation with anti-ABO and/or anti-C, -c, -D, -E and -e monoclonal antibodies by a flow cytometer. Short tandem repeats analysis was used to determine the donor origin of nucleated cells and burst-forming unit-erythroid colonies singularly picked up after 14 days incubation. Results. A proportion of donor-derived nucleated cells of 71%, 46%, 15% and 25% was observed at day 1364, 1385, 1314 and 932 respectively, in four transplanted patients affected by haemoglobinopathies. Similar results were also obtained in the erythroid precursors, analyzing the donor/recipient origin of the burst-forming unit-erythroid colonies, while on the contrary, at the same days of observation, a proportion of 100%, 100%, 73% and 90% donor-derived erythrocytes was observed in the four patients with persistent mixed chimerism. Conclusions. Our results showed that mostly of the erythrocytes present in four long-term transplanted patients affected by haemoglobinopathies and characterized by the presence of few donor engrafted nucleated cells were of donor origin. The indication that small proportions of donor engrafted cells might be sufficient to clinical control the disease in patients affected by haemoglobinopathies is relevant, although the biological mechanisms underlying these observations need to be further investigated.