Long-Term Persistence of Mixed Full Donor Chimerism after Double Cord Blood Transplantation. Example of Tolerance in Allogeneic Hematopoietic Cell Transplantation Setting

Background

Double umbilical cord blood transplantation (UCBT) is recognized as an alternative source of hematopoietic donor cells for allogeneic haematopoietic cell transplantation (HCT), particularly in adult setting. Because of important infectious complications, UCBT are rarely utilized and more and more replaced by haplo-identical cell transplantation for adult patients. Nevertheless, some very good results were observed with a long-term follow-up. In most cases, after several weeks, one unit turns immuno-dominant and thus, the second one is no longer detected.

Patients and Methods

We reported three cases of long-term mixed full donor chimerism associated to documented complete remission of haematological malignancies after double UCBT. The indications of UCBT were acute myeloid leukaemia for 2 patients [1 AML MRC (female of 65 years) and 1 transformation of Fanconi disease (female of 20 years)] and acute lymphoid leukaemia for 1 patient (female of 22 years). AML received a reduced intensity and ALL a standard conditioning. The general characteristics of recipients and UCB are presented in Table 1. The chimerism follow-up were performed by STR and qPCR techniques with a sensitivity of 5 and 0.2% respectively. Early after transplantation, patient 1 developed a grade I graft-versus-host disease (GVHD), patient 3 a grade II GVHD treated by extracorporeal photopheresis, and patient 2 did not develop any GVHD. The follow-up of these patients were 14, 13 and 8 years respectively after UCBT without any episodes of relapse and at the last follow-up still a total donor mixed chimerism.

Results

Results of STR and qPCR after transplantation early revealed the presence of the two transplant units with a total donor chimerism (Table 1).

Patient 1 presented a perfectly balanced chimerism with 54 % of the majority unit. The other two patients showed an imbalance in the proportions of their two UCB with 83% and 72% of their majority units respectively but a long-term persistence of 17% and 28% of their second UCB.

The study of total blood chimerism during their follow-up revealed a stabilization of the proportions of the two units during the first year (Figure 1).

A more in-depth study of circulating cell subsets (CD3+, CD15+ and CD19+) chimerism of patient 1 revealed a very strong initial imbalance of the two units, which ended up being balanced at two years after transplantation. In the other two patients, the respective study of CD3+ and CD33+ cells showed a stabilization of the two unit percentage around six months after transplantation.

At five years after transplantation, patient 1 had an in-depth study of circulating cells sub-populations (CD4+ T lymphocytes, CD8+ T lymphocytes, B-lymphocytes, natural killer cells, regulatory T cells, monocytes, naive CD4+ and CD8+ T lymphocytes) by flow cytometry, after separation of the two units cells by specific monoclonal antibody directed against HLA mismatching antigen (HLA-A2). This analysis revealed an imbalance in the percentage of NK and monocytes. The immune-dominant unit had three times more NK cells than the other one and this imbalance ratio increased by twenty-two for monocytes. A co-culture study of the CD4+ lymphocytes demonstrated a lack of reactivity of both unit against the other one but a normal ability to react against an allogeneic third-part cell.

Conclusions

This expected tolerance, highlighted in co-culture experience, showed rare cases of naturally induced graft tolerance. A study of tolerance markers and underlying mechanisms in these special cases of persistent mixed full donor chimerism is necessary to acquire better knowledge and to define more investigations ways for the induction of tolerance in patients receiving hematopoietic cell transplantation or organ transplantations.

No relevant conflicts of interest to declare.