The Different Dynamics of Chimerism Evolution Determines the Success of Myeloablative, Reduced Intensity Conditioning and T-Cell Depleted Allogeneic Stem Cell Transplantation.

Background: The dynamics of chimerism evolution determines the success of allogeneic stem cell transplantation (SCT). Several factors such as the intensity of the conditioning regimen, the T-cell content of the graft or the GVHD prophylaxis, influence the degree of chimerism after SCT. Objective: To evaluate the dynamics of chimerism after different SCT settings (ablative, reduced intensity conditioning -RIC- and T-cell depleted -TCD-) and its influence in the success of the procedure.

Patients and Methods: The study includes 68 SCT: 32 ablative (including 7 from HLA matched unrelated donors -UD-), 19 RIC and 17 TCD (including 8 from haploidentical donors and 2 from UD with 1 HLA disparity). Chimerism analysis was performed by FISH for the sex chromosomes or STR-PCR (sensitivity 1 Samples obtained on days +30, +100, +180, +365 and once a year thereafter, included bone marrow (BM) and peripheral blood (PB). Moreover, chimerism was analyzed in PB and leukocyte lineages (T lymphocytes CD3+, B lymphocytes CD19+ and myeloid cells CD15+ isolated (purity >95%) by immunomagnetic means, AutoMACS, Miltenyi Biotec), every 2 weeks, starting on day +15 (except in ablative), and until complete chimerism (CC) was achieved. Results of chimerism follow-up were censored once the diagnosis of relapse or rejection was established.

Results: The incidence (% patients) of mixed chimerism (MC) on day +30 (ablative: BM 27%, PB 15%; RIC: BM 40%, PB 41%, CD3 40%; TCD: BM 31%, PB 33%, CD3 54%), as well as its dynamics (MC on day +100: ablative BM 8%, PB 4%; RIC BM 8%, PB 15%, CD3 22%; TCD BM 20%, PB 40%, CD3 37%) were different in the three SCT settings. All except 2 CD15 samples analyzed showed CC. Moreover, the percentage of recipient cells (%R) was significantly higher after RIC and TCD than after ablative SCT, as well as in T lymphocytes than in BM or PB (7/8 cases with simultaneous studies showed MC in T lymphocytes and CC in PB). All RIC SCT evolved to CC by day +180 while TCD SCT showed persistent MC (2 patients with stable MC after one year). The incidence of rejection was greater after RIC (2/19) and TCD (4/17) than after ablative SCT (2/32). All these patients showed MC, mainly in T lymphocytes, which allowed early diagnosis and successful treatment with immunosuppression withdrawal and donor leukocyte infusion. Patients with CC in PB/T lymphocytes on day +30 had a higher incidence of GVHD>I than those with MC. In the present series, however, a relationship between chimerism and relapse, disease free survival or overall survival, was not observed.

Conclusions: SCT with greater incidence of MC (RIC and TCD) favor immune tolerance between donor and recipient which reduces the risk/severity of GVHD at the expense of a higher incidence of graft rejection.