A systematic approach to molecular quantitative determination of mixed chimaerism following allogeneic bone marrow transplantation: an analysis of its applicability in a group of patients with severe aplastic anaemia

Mixed chimaerism (MC) following allogeneic bone marrow transplantation (allo-BMT) is defined as the persistent cohabitation of haematopoietic cells from recipients and donors. Its kinetics, clinical implications and more efficient laboratory approaches for MC detection are the object of ongoing research in view of the possibility of developing useful markers. Here we describe a sequential analysis of chimaerism using variable number of tandem repeat (VNTR) polymerase chain reaction (PCR) followed by quantitative, fluorescent labelled, short tandem repeat (STR) PCR. A set of four, highly discriminative VNTR and four STR markers was used to assess chimaerism. Sensitivity and regression analysis indicated that this approach was reliable for routine application in a single BMT centre. We studied 12 patients with severe aplastic anaemia (SAA) who had received allo-BMT, and had been conditioned with cyclosphosphamide (Cy) with or without anti-thymocyte globulin (ATG). We found a 50% prevalence of MC in the whole group, with levels between 4% and 37% of recipient cells. A sustained stable MC pattern after BMT was characteristic of the Cy-only conditioned patients but was also recorded in one patient treated with the Cy + ATG regime who showed a sustained MC pattern over a period of 24 months post-BMT. In none of our patients, MC was associated with an increased risk of graft rejection in a median follow-up of 39.5 months.