The availability of several methods to unambiguously mark individual cells has strongly fostered the understanding of clonal developments in hematopoiesis and other stem cell driven regenerative tissues. While cellular barcoding is the method of choice for experimental studies, patients that underwent gene therapy carry a unique insertional mark within the transplanted cells originating from the integration of the retroviral vector. Close monitoring of such patients allows accessing their clonal dynamics, however, the early detection of events that predict monoclonal conversion and potentially the onset of leukemia are beneficial for treatment. We developed a simple mathematical model of a self-stabilizing hematopoietic stem cell population to generate a wide range of possible clonal developments, reproducing typical, experimentally and clinically observed scenarios. We use the resulting model scenarios to suggest and test a set of statistical measures that should allow for an interpretation and classification of relevant clonal dynamics. Apart from the assessment of several established diversity indices we suggest a measure that quantifies the extension to which the increase in the size of one clone is attributed to the total loss in the size of all other clones. By evaluating the change in relative clone sizes between consecutive measurements, the suggested measure, referred to as maximum relative clonal expansion (mRCE), proves to be highly sensitive in the detection of rapidly expanding cell clones prior to their dominant manifestation. This predictive potential places the mRCE as a suitable means for the early recognition of leukemogenesis especially in gene therapy patients that are closely monitored. Our model based approach illustrates how simulation studies can actively support the design and evaluation of preclinical strategies for the analysis and risk evaluation of clonal developments.

Unsere Untersuchungen belegen, dass nichtmyeloablative Stammzelltransplantationen (NST) bei Patienten mit Hochrisiko-ALL oder -AML eine neue therapeutische Option darstellen. Die NST ermöglichte eine allogene Stammzelltransplantation bei Transplantationskandidaten mit Kontraindikationen gegen eine hochdosierte Strahlen- und Chemotherapie (Standardtransplantation). Nach NST kam es häufig zur Entwicklung von Infektionen und einer spät auftretenden akuten Transplantat-gegen-Wirt Reaktion (GvHD), diese waren aber mit einer geringeren Mortalität verbunden. Das erkrankungsfreie Überleben und das Gesamt-Überleben nach NST und nach Standardtransplantation waren fast gleich. Der höheren Rezidivrate nach NST stand eine höhere transplantationsassoziierte Mortalität nach Standardtransplantation gegenüber. Das Konditionierungsregime selber war ohne Einfluss auf das Überleben der Patienten. Sequenzielle Chimärismusuntersuchungen von Leukozytensubpopulationen erlaubten eine frühe Diagnose eines gemischten Chimärismus, der einen prädiktiven Wert für das Auftreten eines Rezidivs hatte. Ein stabiler gemischter Chimärismus wurde nicht beobachtet. Durch eine adoptive Immuntherapie mit Spenderlymphozyten (DLI) konnte bei der Mehrzahl der transplantierten Patienten mit gemischtem Chimärismus eine Chimärismuskonversion und eine lang anhaltende komplette Remission induziert werden, ein wichtiger Hinweis auf einen Transplantat-gegen-Leukämie Effekt (GvL) der Spenderzellen. Durch die NST rückt der immunologische Effekt der Transplantation gegenüber der Zytoreduktion bei der Standardtransplantation stärker in den Vordergrund. Unsere Ergebnisse sprechen für die Wirksamkeit eines GvL-Effektes bei akuten Leukämien auch nach NST. Die Erfahrungen mit der NST bei akuten Leukämien haben zu einer Anwendung bei refraktären Nierenzellkarzinomen geführt. Eine Tumorregression wurde erst nach Chimärismuskonversion und/oder Entwicklung einer GvHD beobachtet; diese Befunde sind vereinbar mit einem Transplantat-gegen-Tumor Effekt (GvT). Die transplantationsassoziierte Morbidität war allerdings bei diesen meist älteren Patienten erheblich. Die Therapie sollte ausschließlich im Rahmen klinischer Studien erfolgen, da es sich nach wie vor um ein experimentelles Therapieverfahren handelt. Die Analyse minimal residueller Erkrankung und der Einsatz hochauflösender Chimärismusuntersuchungen von Leukozytensubpopulationen sollte die Bedeutung des gemischten Chimärismus weiter klären und zu einem gezielteren Einsatz von DLI führen. Prospektive vergleichende Studien müssen in naher Zukunft den Stellenwert der NST untersuchen. / Our clinical investigations demonstrate, that nonmyeloablative stem cell transplantation (NST) is a novel therapeutic option in patients with high-risk ALL or AML. NST can be administered to patients eligible for allogeneic stem cell transplantation with contraindications against high-dose radio- and chemotherapy (standard SCT). After NST, infections and late onset acute graft-versus-host disease (GvHD) frequently occurred, but transplant-related mortality was low in contrast to standard SCT. Leukemia-free survival and overall survival were similar after NST and standard SCT. A higher relapse rate after NST was balanced by a higher transplant-related mortality after standard SCT. The conditioning regimen itself had no relevant impact on survival. Sequential chimerism analyses of leukocyte subpopulations resulted in early diagnosis of mixed chimerism, which proved to be predictive for later relapses. Stable mixed chimerism was not established in these patients. Adoptive immunotherapy in patients, who were in hematologic remission but had mixed chimerism after transplantation, induced conversion to complete donor chimerism and long-lasting complete remissions in the majority of patients, a strong hint to a graft-versus-leukemia-effect (GvL) of donor T-lymphocyte infusions. A change in the character of stem cell transplantation was achieved by NST with a shift from the predominantly cytoreductive effect of standard SCT towards an emphasis on the immunologic GvL-effect. Our results demonstrated the efficacy of a GvL-effect in acute leukemias after NST. The experiences with NST in acute leukemias have prompted studies in patients with refractory advanced renal cell cancer. Tumor regressions were observed only after chimerism conversion and / or development of GvHD, these results being compatible with a graft-versus-tumor effect (GvT). However, transplant-related morbidity was substantial in these mostly elderly patients. Therapy within clinical studies is mandatory, because allogeneic stem cell transplantation still has to be regarded an experimental procedure in these patients. The analysis of minimal residual disease and application of high-resolution chimerism analysis of leukocyte subpopulations by microarrays could lead to a more profound understanding of mixed chimerism and to a more rational use of DLI in the near future. Prospective randomized trials should be conducted to evaluate the role of NST.

The availability of several methods to unambiguously mark individual cells has strongly fostered the understanding of clonal developments in hematopoiesis and other stem cell driven regenerative tissues. While cellular barcoding is the method of choice for experimental studies, patients that underwent gene therapy carry a unique insertional mark within the transplanted cells originating from the integration of the retroviral vector. Close monitoring of such patients allows accessing their clonal dynamics, however, the early detection of events that predict monoclonal conversion and potentially the onset of leukemia are beneficial for treatment. We developed a simple mathematical model of a self-stabilizing hematopoietic stem cell population to generate a wide range of possible clonal developments, reproducing typical, experimentally and clinically observed scenarios. We use the resulting model scenarios to suggest and test a set of statistical measures that should allow for an interpretation and classification of relevant clonal dynamics. Apart from the assessment of several established diversity indices we suggest a measure that quantifies the extension to which the increase in the size of one clone is attributed to the total loss in the size of all other clones. By evaluating the change in relative clone sizes between consecutive measurements, the suggested measure, referred to as maximum relative clonal expansion (mRCE), proves to be highly sensitive in the detection of rapidly expanding cell clones prior to their dominant manifestation. This predictive potential places the mRCE as a suitable means for the early recognition of leukemogenesis especially in gene therapy patients that are closely monitored. Our model based approach illustrates how simulation studies can actively support the design and evaluation of preclinical strategies for the analysis and risk evaluation of clonal developments.