Clinical relevance of serial quantitative analysis of hematopoietic chimerism after allogeneic stem cell transplantation in children for severe aplastic anemia W Hoelle 1 ,JFBeck 2 ,GDueckers 1 , H Kreyenberg 1 ,PLang 1 ,BGruhn 4 ,MFu¨hrer 5 , D Niethammer 1 , T Klingebiel 3 and P Bader 1 1 University Children’s Hospital, Tu¨bingen, Germany; 2 University Children’s Hospital, Greifswald, Germany; 3 University Children’s Hospital, Frankfurt, Germany; 4 University Children’s Hospital, Jena, Germany; and 5 University Children’s Hospital, Munich, Germany Summary: Allogeneic stem cell transplantation (allo-SCT) is a well- established treatment modality for children with severe aplastic anemia (SAA). Treatment failures are rare and mostly caused by graft rejection. Increasing mixed chimerism represents a stage at the very beginning of graft rejection, where immunological intervention might be an effective prophylactic approach. To substantiate this, we: (1) monitored peripheral blood cells from children with SAA after allo-SCT and performed pre-emptive immunotherapy in patients with increasing MC. In all, 23/ 34 courses of 32 children with SAA after allo-SCT showed a complete chimerism (CC) throughout and 10/34 developed different types of mixed chimerism (MC). Altogether, 4/10 with MC spontaneously developed decreasing MC, 2/10 courses persisted with low propor- tions of autologous cells below 30% (stable-MC), 4/10 developed increasing MC and one patient showed an autologous recovery. All patients with CC, decreasing MC or stable MC remained in continuous complete remission (CCR). In all, 2/4 patients with increasing MC developed graft rejection. Based on these observations, 2/4 new patients with increasing MC received low-dose DLIs prophylactically, and remained in CCR without any GVHD. These results substantiate that low-dose DLI in children with SAA and increasing MC can prevent graft rejection with a calculable risk to induce severe GVHD. Bone Marrow Transplantation (2004) 33, 219–223. doi:10.1038/sj.bmt.1704337 Published online 1 December 2003 Keywords: increasingmixedchimerism;immunotherapy; graft rejection; SAA; children Allogeneic stem cell transplantation (allo-SCT) is a convincing treatment modality in children and young adults with severe aplastic anemia (SAA). 1 Best results can be achieved with stem cells from a matched family donor (MFD), but in the last few years other allo-SCT procedures using stem cells from matched unrelated, mismatched unrelated or haploidentical donors (MUD, MMUD or haplo) were applied with good success, as well. 2–5 Treatment failures in SAA are rare and are often caused by graft rejection. 5,6 Divers studies focused on individual chimerism development showed that the mixed chimerism (MC) phenotype is linked to graft rejection. 7–10 In addition, we recently demonstrated that patients with different types of acute leukemias and increasing MC after allo-SCT are at the highest risk to relapse, and that early immunotherapeutical intervention in this stage significantly improvestheoutcome. 11–13 Inthisstudy,weaskedwhether the phenotype of increasing MC might also be a reliable indicator for threatening graft rejection in SAA patients afterallo-SCT,andwhethergraftrejectioncanbeprevented by additional immunotherapy, as well. Patients, materials and methods Patients In total, peripheral blood samples from 32 children, who received allo-SCT for SAA, were referred from nine pediatrictransplantcentersinGermanytothestudycenter inTu¨bingen.ThestudyperiodlastedfromJanuary1993to June2002.ThestudyprotocolwasapprovedbytheClinical Ethics Committee of the University of Tu¨bingen, and conductedaccordingtotheprinciplesoftheDeclarationof Helsinki. Informed assent and consent were obtained from the patients and parents, respectively, according to institu- tional guidelines. Data were obtained for analysis until December 2002. The median follow-up was 42 months (2.5–108months).Patientcharacteristics,typesofstemcell donors,sourcesofstemcellsandconditioningregimensare summarized in Table 1. All patients were diagnosed and treated according to the guidelines of the actual SAA protocol of the Received 08 May 2003; accepted 20 August 2003; published online 1 December 2003 Correspondence:DrPBader,DepartmentofPediatricHematologyand Oncology, University Children’s Hospital, Hoppe-Seyler-Strae 1, Tu¨bingen D-72070, Germany; E-mail: peter.bader@med.uni-tuebingen.de Bone Marrow Transplantation (2004) 33, 219–223 & 2004 Nature Publishing Group All rights reserved 0268-3369/04 $25.00 www.nature.com/bmt