Engraftment of Donor Cells after Allogeneic Stem Cell Transplantation: Comparison and Impact of Chimerism in Whole Blood and Peripheral CD3+ T-Cells

Introduction: After allogeneic stem cell transplantation (allo-SCT), engraftment can be assessed by quantitative polymerase chain reaction (qPCR) using differing donor/recipient markers (Alizadeh et al. Blood, 2002), identified in peripheral blood DNA cells before transplant. We report here on the concomitant examination of the proportions of donor and recipient DNA in peripheral whole blood (WB) and sorted CD3⁺ T-cells on days +60 and +90, looking at their impact on survival.

Patients, material and methods: This monocentric study evaluated the impact on outcomes of early WB and sorted CD3+ T cells chimerism independently and of the four possible combinations of chimerism between WB and sorted CD3+ T-cells. All follow-up chimerism samples from allo-SCT patients performed in adults at Nantes University Hospital between October 2009 and October 2016 were reviewed, focusing on those where both PB and/or CD3⁺ T-cells were evaluated on days +60 (45-75) and/or +90 (75-120) after allo-SCT. A global cohort of 229 patients (239 grafts) was retrieved, which includes 52 patients evaluable on day +60 only, 67 evaluable on day +90 only and 120 evaluable on both days +60 and +90. A threshold of >95% donor DNA was considered for complete chimerism. Disease free survival (DFS) was calculated from the date of graft until relapse, death or last follow-up. Overall survival (OS) was calculated from the date of graft until death or last follow-up. Chi square tests were used to compare incidences. Log rang test and Kaplan Meier were used to evaluate DFS and OS.

Results: The whole cohort comprised 62% males and had a median age of 58 years old (20-74) at the time of allo-SCT. Patients were treated for myeloid-lineage disease in 59% of the cases. Reduced-intensity conditioning was used in 89% (n=212), donors were familial in 45% (n=107), registry in 48% (n=114). Unrelated cord blood units were used in 8% of the cases (n=18). Post-transplant cyclophosphamide (PTCY) was performed in 48 procedures including 33 and 15 with haplo (HG) and matched donors respectively. Considering the 239 allograft procedures, the median follow-up was 5.8 years (95% CI: 3.1-5.8), the rate of relapse 27% and the rate of death 31%.

Complete WB chimerism was observed for 80% and 71% of the cases on day +60 and day +90 respectively. Complete CD3+ chimerism was present for 53% and 51% of the grafts at days +30 and +90 respectively. Thus, cases displaying both complete WB and CD3+ chimerism on days +60 and +90 were 53% and 51% respectively, while 27% and 20% were documented with full WB and mixed CD3⁺ chimerism on days +60 and +90. Mixed chimerism was observed in both WB and CD3⁺ cells in 14% of the cases on day +60 and 22% on day +90. Finally, a small proportion of patients (6% and 7% at days +60 and +90) displayed an intriguing complete chimerism in CD3⁺ cells yet mixed WB chimerism. None of these features appeared associated to disease lineage (lymphoid or myeloid) nor cord blood allo-SCT. Interestingly, of the 27 grafts with myeloablative conditioning, only 14 had full WB/CD3⁺ engraftment on day +60 or +90, and thus all 27 were retained for the study.

None of the four WB/CD3 chimerism combinations at the two times considered had an impact on DFS in this cohort. Surprisingly, although full or mixed WB chimerism had no impact on DFS and OS at days +60 and +90, the presence of a mixed CD3+ chimerism (vs full) at day+90 was associated with a significantly better OS (median: 5.8 months years [95%CI: -not reached] versus 3.1 years [95%CI: 2.2- 3.1]; p=0.025). CD3+ chimerism at day+60 has no impact on OS.

All HG resulted in full CD3+ chimerism at both time points compared to non HG (100% vs 52%, p<0.0001). The same was almost true when considering PTCY procedures: 90% at day+60 and 92% at day +90. Of note, there was no influence on DFS nor OS of WB or CD3+ chimerism status when considering only HG or PTCY grafts vs others in this series.

Discussion: In this large series, early WB chimerism status did not predict outcome. Surprisingly, mixed CD3+ chimerism at day+90 appears to be significantly associated with a longer OS, suggesting that remaining recipient memory lymphocytes could be beneficial. This result has to be confirmed prospectively. It remains also to define the place of donor lymphocyte infusions (DLI) to prevent relapse in patients with full or mixed CD3+ chimerism post-transplant (analyses of DLI received in our patients are on-going).