title: Double umbilical cord blood transplantation: relevance of persistent mixed-unit chimerism.
abstract: Double umbilical cord blood transplantation (UCBT) was developed as a strategy to circumvent the cell dose limitation of single UCBT with a concomitant potential benefit of lowering the rate of leukemia relapse. Sustained hematopoiesis after double UCBT usually is derived from a single donor unit, as only a few patients have been reported to display stable mixed-unit chimerism for varying periods of time. Explanations for the 1 unit dominance, predictors for identifying unit superiority, and persistence of long-term mixed-unit chimerism remain elusive. Review of published literature revealed only 11 of 280 patients (4%) with mixed-unit chimerism for at least 1 year after transplantation, with 3 patients receiving reduced-intensity conditioning regimens. Mixed-unit chimerism was more likely if both units were closely HLA matched to each other. Outcome data for patients with stable mixed-unit chimerism, for the most part, were scarcely reported. Analysis of the small sample size revealed a potential advantage of stable mixed-unit chimerism on enhancing the graft-versus-leukemia effect; however, definitive conclusions cannot be made on the effect of mixed-unit chimerism on the rates of graft-versus-host disease. Therefore, gathering outcome data prospectively in larger clinical series will help answer the question of whether stable mixed-unit chimerism is either beneficial and, therefore, should be strived for, detrimental and, thus, needs to be eliminated, or if it is of no clinical consequence.
authors.affiliation: Department of Pediatrics, Case Western Reserve University, Cleveland, Ohio.
authors.forename: Hasan
authors.initials: H
authors.lastname: Hashem
authors.affiliation: Department of Medicine, Case Western Reserve University, Cleveland, Ohio. Electronic address: hillard.lazarus@case.edu.
authors.forename: Hillard M
authors.initials: HM
authors.lastname: Lazarus
mesh_terms: D064591:Allografts; D036101:Cord Blood Stem Cell Transplantation; D006086:Graft vs Host Disease; D020544:Graft vs Leukemia Effect; D006680:HLA Antigens; D006801:Humans; D018183:Transplantation Chimera
chemical_list: D006680:HLA Antigens
keywords: Dominance; Double; Mixed chimerism; Transplantation; Umbilical cord blood