Analysis of Chimerism Induction Following Allogeneic Hematopoietic Stem Cell Transplantation (HSCT) with a Reduced-Intensity Regimen.

<Background> Although the analysis of chimerism induction has become an important diagnostic tool for providing better clinical management of patients undergoing allogeneic HSCT, the techniques have not been fully standardized. Moreover, it is currently unknown whether the onset of graft-versus-host disease (GVHD) is related to the status of mixed chimerism (MC) or complete donor-type chimerism (CDC).

<Methods> First, to validate our chimerism analysis system, we performed experiments with artificially mixed cell samples from healthy volunteers to examine the reliability of short tandem repeat (STR) determination by quantitative polymerase chain reaction (PCR). We confirmed a linear correlation between the proportion of mixed cells and the calculated ratio, with a correlation coefficient of 0.99, which enables the detection of target cells at 3% (median standard deviation, 1.6%). Next, using this validated system, we prospectively evaluated the kinetics of chimerism in CD3+, CD19+, and peripheral blood mononuclear cells (PBMC), for correlation with the occurrence of GVHD in 19 patients with various hematological diseases (median age, 53y) who had received allogeneic HSCT from an HLA-identical sibling donor between July 2003 and February 2004. The preparative regimen was fludarabine/busulfan (BU) (n=12) or cladribine/BU (n=7). GVHD prophylaxis consisted of cyclosporin alone (n=8), cyclosporin plus short-term methotrexate (n=7), or tacrolimus (n=4). Chimerism analysis was repeated weekly after transplantation.

<Results> We evaluated 405 consecutive blood samples from these 19 recipients, 12 of whom developed acute GVHD. Six of these 12 patients showed MC, i.e. 91% (83–94%; MC) donor cells in the CD3+ fraction at the onset of GVHD, but all except one subsequently achieved CDC within a median of 15 (7–33) days without additional DLI. The remaining patient showed persistent MC and relapsed 158 days after transplantation.

<Conclusion> We found that the presence of MC in the CD3+ fraction is rather common at the onset of acute GVHD, but GVHD subsequently eradicates residual host hematopoietic cells. Alternatively, GVHD is part of a clinical manifestation of an immune reaction that is related to the induction of CDC.