Lineage-Specific Chimerism Analysis After Allogeneic Hematopoietic Cell Transplantation in Patients with Myeloid Neoplasms: Current Evidence and Considerations in the Post-Transplant Cyclophosphamide Setting

Background: Chimerism analysis is a key tool for monitoring donor-cell engraftment and the risk of relapse and graft-versus-host disease (GVHD) following allogeneic hematopoietic cell transplantation (allo-HCT). The advantage of lineage-specific chimerism assessment, and its dynamics in patients receiving post-transplant cyclophosphamide (PTCy)-based GVHD prophylaxis, remains unclear. Objective: This review summarizes the current state of the art on chimerism analysis in patients with myeloid neoplasms undergoing allo-HCT with PTCy, with emphasis on lineage-specific testing and modern methodologies. Methods: A structured literature review was conducted to assess chimerism dynamics in whole blood (WB), bone marrow, and peripheral blood (PB) subpopulations, including T-cells, CD34+, myeloid, B, and NK (natural killer) cells, and their association with clinical outcomes following PTCy. Results: Lineage-specific PB chimerism, particularly in T-cells, myeloid lineage and CD34+ cells, is more sensitive than WB chimerism for predicting relapse. Declining donor myeloid chimerism or persistent myeloid mixed donor chimerism (MDC) may precede hematologic relapse and provide an early signal of graft instability or ineffective graft-versus-leukemia activity. T-cell MDC has been associated with an increased risk of relapse and a lower risk of GVHD, although persistent T-cell MDC in some patients may instead indicate immune tolerance. Declining CD34+ donor chimerism correlates with a higher risk of relapse and inferior survival outcomes and may therefore complement measurable residual disease testing. Data regarding B-cell and NK-cell chimerism remain inconsistent, likely influenced by delayed immune reconstitution. Compared to anti-thymocyte globulin, PTCy may promote higher donor T-cell chimerism, though findings across studies are variable. Next-generation sequencing (NGS) enables more sensitive detection of microchimerism and relapse prediction. Conclusions: Chimerism analysis, particularly when lineage-specific and NGS-based, offers valuable prognostic insight in allo-HCT with PTCy. Further prospective studies are needed to standardize testing and guide personalized post-HCT strategies.