Flow cytometric monitoring of red blood cell chimerism after bone marrow transplantation

Chimerism after bone marrow transplantation (BMT) was investigated by flow cytometry analysis of red blood cells (RBCs) and of reticulocytes using a series of selected monoclonal or polyclonal antibodies directed against ABO, Rhesus, Kell, Duffy or MNSs antigens. The method allows the routine detection of less than 0.1% of positive cells in artificial mixed field populations. Blood samples from 135 patients undergoing BMT were investigated around days 15, 30, 45, 60, 90, 180, and then every 6 months after transplantation. Characteristic patterns showing expression of donor red blood cell antigens (expansion markers) and concomitant decrease of recipient specific antigens (depletion markers) within days 16-20 were observed for 125 successfully engrafted patients. Distinct patterns were obtained in 10 patients. A delay in engraftment was evidenced in four patients by the absence of chimerism during the first 6 months without any sign of relapse. Re-appearance of recipient RBCs and reticulocytes was observed in five patients; it was consistent with relapse that was later confirmed by clinical, haematological and cytogenetic studies. Finally, a stable and partial chimerism with 20% of RBCs expressing a marker from the recipient was observed in one patient without any sign of relapse. The reported investigation demonstrated that flow cytometry of RBCs and reticulocytes represents a powerful method to efficiently monitor bone marrow transplanted patients on a long-term basis.