Correlation between MRD and Chimerism Kinetics in CLL Patients after Myeloablative and Non-Myeloablative Allogeneic Hematopoïetic Stem Cell Transplantation.
Authors: Plesa Adriana , Le Quoc-Hung , Revesz Daniela , Thomas Xavier , Ducastelle Sophie , Nicolini Franck E. , Michallet Anne-Sophie , Mollet Isabelle , Dubois Valerie , Clark Melisa , Dumontet Charles , Raus Nicole , Michallet Mauricette 
Published: October 2019

Abstract
This study concerns the correlation between chimerism and minimal residual disease (MRD) documentation of 12 patients (9 males and 3 females − 51 years) who underwent allogeneic hematopoietic stem cell transplantation (HSCT) for chronic lymphocytic leukemia (CLL). Chimerism was assessed from total peripheral blood (PB), bone marrow (BM) and CD3+ cells by STR-PCR (sensitivity [S]:5%) followed by SNP-PCR (S:0.2%) and MRD from PB and BM by an international standardized multicolor flow cytometric approach (S:0.01%). At diagnosis, there were 4 Binet stage A, 7 stage B and 1 stage C. Four patients were in CR, 7 in PR and 1 in progressive disease pre-transplant. Eleven patients underwent 1 HSCT and one patient 2 HSCT from HLA identical sibling donors (10 PBSC and 3 BM); 2 patients received a myeloablative regimen and 10 reduced intensity conditioning. After transplant, 6 patients developed an acute GVHD≥ grade 2 and 8 a chronic GVHD. The chimerism kinetics were followed in all patients and showed a conversion from mixed chimerism (MC) into full donor chimerism (FDC) at day 30 post transplant in 6 patients, between day 30 and day 120 in 4 patients, 2 patients never converted to FDC and remained either in stable MC or progressed to recipient profile. MRD documentation was introduced more recently and we have MRD kinetics only for 5 patients showing a slower MRD decrease when compared to chimerism kinetics. The median follow-up was 39 months and at the last follow-up, among the 11 patients who received 1 HSCT, 2 patients died from infection and they showed FDC and MRD negativity, 9 patients are alive (7 in CR according to NCI criteria and 2 were too early). Among the 7 CR patients, 6 were in FDC and MRD negative and 1 in MC and MRD negative. The patient who received 2 allogeneic transplants had in fact relapsed 24 months after the first allogeneic transplant and the second transplant allowed a CR associated to a MRD negative status and FDC, followed by a recent increase of MRD level in spite of FDC maintenance at last follow-up. We studied 246 samples for chimerism and 88 for MRD. To correlate quantitative MRD levels with chimerism data only 56 samples were comparable. Considering kinetics, we observed a concordance in 50/56 (89%) with 16/50 (32%) both positive early after transplant and 34/50 (68%) both negative later. Among the 6 non concordant samples which belonged to 2 patients, 5/6 (83%) were MC and MRD negative explained by the mixed chimerism profile of CD3+ cells and 1/6 (relapsed patient) was MRD positive and FDC explained by the better sensitivity of MRD. Relation between MRD and chimerism for PB and BM was tested using a mixed effect linear regression to account for repeated measures for each patient. This analysis showed a significant correlation between PB MRD and chimerism (p<0.001) and between BM MRD and chimerism (p<0.001). In conclusion, because of the better sensitivity, specificity and advantageous cost of MRD flow we could recommend in the future to combine chimerism and MRD documentation during the first 90 days and to limit to the flow cytometry MRD approach past the 90 days follow-up after allogeneic HSCT for CLL patients.