RIC regimens followed by allogeneic hematopoietic stem cell transplantation (HSCT) were evaluated in patients with hematologic malignancies who were not candidates for conventional transplantation because of age or medical co-morbidities. In this kind of transplantation, the analysis of chimerism kinetics remains fundamental. The main aims of this study were to evaluate and compare the impact of pre- and post-transplant variables and chimerism on transplantation outcome. Chimerism status was evaluated in total blood (TB) and in purified CD3+ cells using quantitative PCR (STR/SNP) for all. We analysed donor type and kinetics chimerism of 52 patients (32 M - 20 F) who had undergone allogeneic HSCT after RIC for hematological malignancies. Median age was 40 years (range 25–67 years). Diagnosis were multiple myeloma: 16, acute myeloid leukemia: 13, myelodysplasia: 6, chronic lymphocytic leukemia: 5, Non Hodgkin’s lymphoma: 5, acute lymphoblastic leukemia: 1, chronic myeloid leukemia: 2 and primitive myelofibrosis: 4. Conditioning regimens were Fludarabine + Busulfan in 33 patients, Fludarabine + 2 Gys total body irradiation 15, Cyclophasphamide 3 (2 alone/one with Busulfan) and one other chemotherapy. Stem cell source was PB in all except one who received cord blood. Fourty-eight were identical sibling and 4 unrelated donor transplantations. At transplant, 17 patients were in CR, 21 PR and 14 in evolutive diseases. All patients except one engrafted. Twenty-five developed aGVHD ≥ Grade II (16 Grade III-IV). At the last follow-up 22 patients died (9 disease progression and 13 of transplant related mortality, 30 were alive (18 developed chronic graft vs Host disease (cGVHD) 12 extensive/6 limited cGHVD). Among 52 patients, only 49 had a long-term chimerism documentation of CD3 subpopulation (43 TB and CD3+ cells): 15 (14 TB) were full donor chimerism (FDC) throughout the follow-up, and 34 remained mixed chimerism (MC). Among these 34 (29 TB) MC patients at the latest follow-up, 7 (5 TB) remained in MC, 2 (5 TB) converted to recipient profile and 25 (19 TB) converted to donor profile. Univariate analysis demonstrated the close correlation between chimerism status evaluated on PB CD3+ cells only at any time post-transplant and the onset of aGVHD (p = 0.0391) but not cGVHD. Multivariate analysis according to linear regression method did not find any impact of the following variables on chimerism kinetics after RIC transplant: disease status before transplant, age, sex, type of RIC regimen, number of days of ATG, aGVHD (p ≥ 0.11). In conclusion, this study underlines the tight correlation that exists between chimerism status and kinetics on CD3+ PB subpopulations after RIC transplant and acute GVHD development that impacts on transplant outcome.

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