Impact of CD 34 Cell Dose and Conditioning Regimens On Donor T-Cell and Myeloid Chimerism After an Alemtuzumab-Based in-Vivo T-Cell Depleted Allogeneic Stem Cell Transplant (SCT)

Background: Phase II studies suggested that pts who received an in-vivo T-cell depleted (TCD) alloSCT with Alemtuzumab (AL) tended to have less aGVHD.However, the utility of this approach is limited by increased incidence of mixed chimerism which is associated with increased disease recurrence and often require DLI.

Hypothesis: We hypothesize that sub-optimal CD 34 +ve cell dose contribute to mixed chimerism after an in-vivo TCD alloSCT using AL. By identifing an optimal CD 34 +ve cell dose; an in-vivo TCD protocol could maintain the benefit of lowering the incidences of GVHD and minimize mixed chimerism. We also evaluate other patient & treatment characteristics that may contribute to mixed chimerism.

Method: We conducted a retrospective study to evaluate the impact of CD 34 +ve cell dose and patient and treatment characteristics on T-cell and Myeloid chimerism after an in-vivo TCD alloSCT. Between 5/11 & 5/12, 32 consecutive pts underwent an in-vivo TCD alloSCT. F/M: 18/14. The diagnosis included 21 AML/MDS, 4 NHL, 3 MDS & 7 others. Conditioning regimens consisted of Fludarabine/Melphalan:22, Fludarabine/Busulfan: 7 and others in 3. Upon achievement of engraftment, we monitor engraftment by Short tandem repeat monthly with T-cell and Myeloid subsets analysis. We utilize of AL for in-vivo TCD (20 MRD: 30 mg on Day -1; 12 MUD: 30 mg daily on Day -2 & Day -1). All patients also received cyclosporine.

Results: Pts received a median of 4.96 x 10e6 per kg CD 34 positive cells (range 2.45 – 27.22). 30/32 pts achieved neutrophils engraftment except 1 died shortly after SCT from CHF & 1 experienced primary graft failure with day 30 STR showing only 3% donor T-cell chimerism. On Day 30 post-SCT, all evaluable pts had predominately donor T-cell (median 99%; 22/31 (71%) ≥ 99%; range 82 to > 99%) and Myeloid Chimerism (28/31 (90%) ≥ 99%. 25 pts survived beyond Day 90 and have engraftment data available for analysis . Between Days 90 & 120 analysis, most pts maintain predominant donor myeloid chimerism (> 95%; median 99%); only 11/25 (44%) had maintained ≥ 91% donor T-cell chimerism. 7/25 (28%) and 7/25 (28%) had 71-90 % and ≤ 70% donor T-cell chimerism respectively. CD 34 +ve Cell dose, disease type, disease status, prior treatment, age, sources of stem cells (MRD vs MUD ) and conditioning regimen did not predict for mixed chimerism. While 43% of the patients received Melphalan/Fludarabine; 60% of the patients received a busulfan/fludarbine had donor T-cell chimerism dropped below 80% between day 90-120. Five pts (17%) developed acute GVHD with only 1 (4%) Grade IV aGVHD.  

Conclusion: 1. Mixed chimerism is common after in-vivo TCD alloSCT; 2. CD 34 Cell dose, disease type, prior treatment, age, sources of stem cells and conditioning regimen did not predict for mixed chimerism & 3.The high incidences of significant mixed chimerism (< 80%) on day 90 in pts received Bu/Flu is of concerns that warrant close monitoring in future studies.