Donor-Host Lineage-Specific Chimerism Monitoring and Analysis in Pediatric Patients Following Allogeneic Stem Cell Transplantation: Influence of Pretransplantation Variables and Correlation with Post-Transplantation Outcomes.

The impact of donor-host chimerism in post-hematopoietic stem cell transplantation (HSCT) outcomes is poorly understood. We were interested in studying whether pre-HSCT variables influenced lineage-specific donor-host chimerism and how lineage-specific chimerism impacts post-HSCT outcomes. Our main objective was to study pre-HSCT variables as predictors of lineage-specific donor-host chimerism patterns and to better characterize the relationship between post-HSCT lineage-specific chimerism and adverse outcomes, including graft failure and disease relapse. We conducted a retrospective data analysis of all patients who underwent allogeneic HSCT at the Pediatric Transplantation and Cellular Therapy service at Memorial Sloan Kettering Cancer Center between January 2010 and June 2015 and had at least 2 measurements of split-lineage chimerism. The trend of lineage-specific donor-host chimerism post-HSCT and the impact of age, disease, graft type, and pretransplantation conditioning regimen on chimerism at 3 months and 12 months post-HSCT were studied. The Wilcoxon signed-rank test, Mann-Whitney-Wilcoxon test, and Cox proportional hazard models were used for statistical analyses. A total of 137 patients were included (median age, 11.3 years). Most patients had a hematologic malignancy (n = 95), and fewer had a nonmalignant disorder (n = 27) or primary immune deficiency (n = 15). Myeloablative conditioning regimens (n = 126) followed by T cell-depleted (TCD) peripheral blood stem cell or bone marrow grafts (n = 101) were most commonly used. Mixed chimerism (MC) of total peripheral blood leukocytes (PBLs) did not predict loss of donor chimerism in all lineages and when stable was not associated with graft failure or rejection in this analyses. Split chimerism with complete donor chimerism (CC) of myeloid, B, and natural killer cells, but not T cells, occurred early post-HSCT, but full donor T cell chimerism was achieved at 12 months post-HSCT by most patients. MC within the T cell lineage was the major contributor to PBL MC, with lower median donor T cell chimerism at 3 months than at 12 months (91%) post-HSCT (51% versus 91%; P < .0001). Predictors of MC at 3 and 12 months were (1) age <3 years (P = .01 for PBLs and P = .003 for myeloid lineage); (2) nonmalignant disorder (P = .007 for PBLs); and (3) the use of reduced-intensity conditioning regimens. TCD grafts produced lower donor T cell chimerism at 3 months post-HSCT compared with unmodified grafts (P < .0001), where T cell lineage CC was achieved early post-HSCT. The donor T cell chimerism was similar at 12 months in the 2 types of grafts. Umbilical cord blood grafts had CC in all lineages at all time points post-HSCT. Loss of donor B cell chimerism was associated with increased risk of relapse in hematologic malignancies (hazard ratio, 1.33; P = .05). Age, underlying disease, conditioning regimen, and graft manipulation can impact post-HSCT donor-host chimerism and be predictors for early MC. MC in total PBLs and T cells was not related to graft failure or disease relapse. Whole-blood PBL chimerism analysis is not sufficient to assess the significance of post-HSCT donor-host status; rather, lineage-specific chimerism, particularly for myeloid, T, and B cells, should be analyzed to guide interventions and inform outcomes.