Increased Incidence of Mixed Chimerism with the Use of Fludarabine – Treosulfan Based Conditioning in Patients Undergoing Allogeneic Stem Cell Transplantation for Thalassaemia Major

Patients and Methods: This is a retrospective analysis of patients with thalassaemia major who underwent allogeneic stem cell transplantation at CMC Hospital, Vellore India between November 2009 and December 2013 using a uniform Fludarabine/Treosulfan/Thiotepa based conditioning regimen. Graft source was either unmanipulated bone marrow or G-CSF primed peripheral blood stem cells (PBSC). Donors were either HLA identical sibling/family or unrelated donors. Whole blood chimerism was performed using VNTRs on Day 30, 60-90, and subsequently based upon physician decision. >95% donor cells was considered as complete chimerism. If there was a drop in chimerism, reduction of immunosuppression and donor lymphocyte infusion (DLI) was considered.

Results: 132 patients underwent HSCT for thalassemia major during this period of which 120 who survived more than 4 weeks post HSCT were evaluable for chimerism. There were 70 males and 50 females with a median age was 9.3 years (range: 3 – 21). Majority had Lucarelli Class III thalassaemia. 113 (94.1%) had matched sibling or family donors while 7 had unrelated donors. Graft source was predominantly PBSC in 97 (80.8%). At day 28 post HSCT, 97 (80.8%) had complete chimerism (CC) while graft rejection occurred in 3 and 20 (16.6%) had mixed chimerism (MC) (Level 1 in 12, level 2 and level 3 had 4 each). Of the 97 patients with CC, 23 (23.7%) developed mixed chimerism between 2-4 months after HSCT. This included level 1 MC in 14, level 2 MC in 7 and level 3 MC in 2. The overall incidence of MC occurring at any time post HSCT was 35.8%. Tapering and cessation of cyclosporine was done in all patients and 5 patients had 1-3 courses of DLI. On follow up of the 43 patients with MC, 24 achieved complete chimerism, 10 had stable mixed chimerism and 9 had rejection (mostly level 3 MC).  Two patients developed mild graft versus host disease after DLI.

Conclusion: Occurrence of mixed chimerism is common after allogeneic HSCT for thalassaemia after Fludarabine/Treosulfan/Thiotepa conditioning. Rapid tapering of immunosuppression and judicious use of DLI helps in reducing the risk of secondary graft rejection. Closer monitoring of chimerism after HSCT needs to be done when such conditioning regimens are used.