4.1. Rejection
The rejection of the graft is associated with the presence of immunocompetent host cells surviving conditioning and capable of mounting the immune response directed against the allogeneic graft [15]. We hypothesize that such cells should be numerous enough to be detected by FISH analysis. However, others suggested that graft could be rejected even with no changes in the number of host cells detectable [5]. In our cohort of patients, rejection was always preceded by an increase in the number or reappearance of host cells detectable by FISH. In two patients, the process was acute and irreversible despite countermeasures applied. But in four other patients we were able to reverse the trend and most probably prevent rejection. None of the patients with complete or decreasing chimerism rejected the graft. We conclude, certainly taking into account the limited number of recruited patients, that FISH technique was sensitive enough to detect the imminent rejection and to mount counteraction in selected patients. We argue that when poor graft function is diagnosed with no host cells detected, causes other than rejection should be considered.
4.2. Relapse
Despite marked improvement in transplant procedures, relapse remains the major cause of treatment failure after alloSCT in malignancies. Established relapse after this treatment modality is rarely curable regardless of the therapeutic approach [16]. Immunotherapy offers a promise of cure when applied before overt hematological recurrence [17]. Due to the association with a considerably high risk of complications including GvHD, immunotherapy should be applied only if the probability of relapse can be clearly established. When a molecular marker of the disease is lacking, chimerism monitoring allows to identify patients at risk. However, the major limitation of chimerism in MRD detection is its low sensitivity. Nevertheless, the sensitivity of the assay can be improved when chimerism is monitored in population of cells selected in accordance with the immunophenotype of leukemia at diagnosis [18]. Among our patients only one child experienced an extramedullary relapse while being MC(–) both in PB and BM. This might indicate, that chimerism is not capable of detecting malignant cells in CNS sanctuary.
Acknowledgements
Contributions. D. Turkiewicz provided concept and design, provided analysis and interpretation of the data. E. Gorczynska obtained funding and provided analysis and interpretation of the data. J. Toporski and K. Kalwak provided provision of study materials/patients. B. Rybka provided collection and assembly of data. D. Noworolska provided technical support. J. Boguslawska-Jaworska provided critical revision of the article. A. Chybicka provided final approval of the article. This study was supported in part by grant AM 686 from Wroclaw University of Medicine. The authors would like to express their gratitude to Dr. Rupert Handgretinger for reviewing the manuscript.