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Abstract 1306 Introduction Acute lymphoblastic leukemia (ALL) can rarely be manifested before its diagnosis as pancytopenia and bone-marrow aplasia followed by spontaneous transient hematological remission. A leukemic clone can be detected during this particular period, making it possible to follow its dynamics in size, as well as independent changes leading to the overt disease. Patients and methods We initially selected 5 cases of ALL, in which the pre-diagnostic period of anemia had occurred. In all pre-diagnostic samples, the immunoglobulin (Ig) and/or T-cell receptor (TCR) gene rearrangements were investigated using quantitative real-time PCR. In addition, SNP array and FISH were performed in some cases in order to reveal other aberrations specific to leukemia. To backtrack the preleukemic cells three different approaches were employed - qPCR, FISH and next generation sequencing (NGS). Results Immunoreceptor gene rearrangements were detected in all 5 patients in the diagnostic sample. At least one of the diagnostic rearrangements was found in all preleukemic bone-marrow samples. The levels of positivity differed from 10−1 to 10−6. In 3 patients more than one pre-diagnostic sample was made available for our examination. A surprising dynamics in the size of the leukemic clone was revealed by the investigation. Although a steady increase of blasts was expected, particularly high levels occurred in the first available preleukemic samples (10% and more clonal Ig/TCR positive cells compared to the diagnosis, in 4/5 of patients), followed by remarkable decrease 1 to 3 months before the diagnosis in 3 patients with more than one sample available. This dynamics points to possible immune system interventions during preleukemic phase. In 2 patients NGS (targeted to Ig/TCR gene rearrangements, Adaptive Biotechnologies, Seattle) was applied. The acquired data correlated with the results of qPCR backtracking. In addition, more detailed information was gained about gene rearrangement spectra as well as about possible immunological background of the leukemia progression. In both these patients, the examination revealed a significant fraction of non-malignant γΔ T cell-clone carrying an identical Vg9-Jg1.2 gene rearrangement, which has been reported previously to play an important role in anti-tumour and anti-microbe immune responses. Interestingly, the dynamics of this particular clone seemed to correlate negatively with the leukemic cell levels, pointing out its putative role in tumour surveillance. Consequently, SNP array was performed to define the aberrations acquired, followed by FISH investigation to confirm the above conclusions. While some aberrations were confirmed already in the pre-diagnostic samples (namely ETV6 deletion in the patient with ETV6/RUNX1 positive ALL), others occurred in the diagnostic sample, yet not in the preleukemic cells several months before the diagnosis. Conclusion Preleukemic cells present during the aplastic phase preceding leukemia onset are not just a gradually growing clone. Their specific dynamics has been shown, which supports our previous data acquired during the investigation of secondary leukemias. Furthermore, at least in some cases, this clone indeed should be considered ‘preleukemic’ as it is to undergo at least one more hit or subclonal selection to progress into an active disease. Disclosures: No relevant conflicts of interest to declare.