The assessment of the hematopoietic reservoir after immunosuppressive therapy or bone marrow transplantation in severe aplastic anemia.

We investigated the hematopoietic reservoir in 43 severe aplastic anemia (SAA) patients following immunosuppression (IS) (n = 15) or bone marrow transplantation (BMT) (n = 28), at a median interval of 5 years (range, 2-20) from treatment. All patients had normal blood counts, good marrow cellularity, and normal numbers of colony forming unit-granulocyte macrophages (CFU-GM). Burst forming unit-erythroid (BFU-E) and colony forming unit-granulocyte erythroid megakaryocyte macrophages (CFU-GEMM) numbers were reduced when compared with normal controls. However, the most pronounced defect was observed at the level of long-term culture-initiating cells (LTC-IC), which significantly differed from controls (P < .00001) both for IS and BMT patients. Their number did not improve with time and was not affected by transplant or treatment-related variables. When IS patients were compared with BMT we found comparable numbers of CFU-GEMM (P = .8) and LTC-IC (P = .9), but lower numbers of BFU-E and CFU-GM (P = .05 and P = .004, respectively), suggestive of a persistent suppressive mechanism. These data indicate that LTC-IC numbers are severely reduced in BMT and IS patients, contradicting the common belief that the former are fully reconstituted as compared with the latter. In addition, the number of mature cells and committed progenitors does not seem to reflect the real size of the hematopoietic reservoir and few stem cells may be sufficient to guarantee normal hematopoiesis long term.