Engraftment of early erythroid progenitors is not delayed after non-myeloablative major ABO-incompatible haematopoietic stem cell transplantation.

We hypothesized that patients undergoing major ABO-incompatible non-myeloablative haematopoietic stem cell transplantation (nm-HSCT) might experience prolonged haemolysis after transplant due to the delayed disappearance of host plasma cells producing anti-donor isohaemagglutinins (HAs). To address this question, we analysed data from 107 consecutive patients transplanted with allogeneic peripheral blood stem cells from human leucocyte antigen-matched (related, n = 84; unrelated, n = 23) donors after non-myeloablative conditioning (200 cGy total body irradiation +/- fludarabine). In total, 23 out of the 107 patients received major or major/minor ABO-incompatible transplants. Red blood cell (RBC) transfusion requirements during the first 120 d post transplant were higher in major ABO-mismatched than in ABO-matched recipients (0.12 vs 0.03 median units RBC concentrate/d, P = 0.04). Two patients developed transient pure red cell aplasia, which had resolved spontaneously by 9 months after transplant. Major ABO incompatibility did not influence rates of engraftment. Patients with sustained engraftment experienced gradual declines of anti-donor HAs, and the estimated median time to reaching IgM and IgG titres of < 1:1 was at least 133 d in evaluable patients, approximately twice longer than reported after myeloablative conditioning. There was a strong correlation between degrees of donor chimaerism in erythroid burst-forming units, granulocyte macrophage colony-forming units and granulocytes, indicating that donor erythroid engraftment, defined by early erythroid progenitors, was as prompt as myeloid engraftment. In conclusion, our data suggest that major ABO-incompatibility is not a barrier to successful non-myeloablative HSCT.