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Background CXCR4 (CD184) is a receptor specific to the Stromal Derived Factor 1 (SDF-1), a ligand also known as CXCL12. The ligand-receptor interaction has a pleiotropic effect on hematopoietic cell proliferation, migration and activation through several signaling pathways. CXCR4 expression on neoplastic cells might be responsible for their dissemination to particular organs with cells expressing CXCL12 (e.g. lymph nodes, bones, and within bone marrow). In B-cell precursor acute lymphoblastic leukemia (BCP-ALL), expression of CXCR4 was associated with higher capacity of leukemic blasts to seed into bone marrow niches. Aim of the study The study aimed at thorough analysis of CXCR4 expression on BCP-ALL blasts and correlation of CXCR4 expression with the expression of other antigens such as CD66c, CD34, CD10, CD38, CD20 and CD45 as well as with the levels of minimal residual disease on day 15. Patients and Methods The study group consisted of 198 consecutive children aged 0-18 years (median 4.4 years) treated for BCP-ALL in the centers of the Polish Pediatric Leukemia/Lymphoma Study Group. Bone marrow samples obtained at initial diagnosis were stained with monoclonal antibodies (CD58, CD66c, CD34, CD19, CD10, CD38, CD20, CD45, CXCR4) in two 8-color tubes and analyzed with multiparameter flow cytometry (BD FACSCanto II, Becton Dickinson, San Jose, CA, USA) according to the EuroFlow standard protocols. The expression of particular antigens on BCP-ALL blasts was defined by median fluorescence. In 177 patients the samples from day 15 were available and analyzed for the presence of minimal residual disease (MRD) with multicolor flow cytometry. Infinicyt software (Cytognos, Salamanca, Spain) was used for more detailed analyses of the flow cytometric data. Results The expression of CXCR4 in BCP-ALL was highly variable with median fluorescence ranging from 252 to 24 388 (median 4011). There was no obvious correlation of CXCR4 expression with immunophenotype and with the expression of other analyzed markers (CD66, CD34, CD10, CD38 i CD45). The only borderline significant correlation found was between CXCR4 and CD20 expression. On day 15, 70 children (39%) demonstrated MRD levels below 0.1%, which is consistent with MRD-based low-risk group. Among these patients, 41 children had undetectable MRD already at this time point. In contrary, MRD levels > 10% were recorded in 21 patients (12%), who were stratified to high-risk group, accordingly. Maximal MRD levels recorded at day 15 were 85.6%. In remaining 86 children (49%), MRD levels at day 15 were in-between 0.1% and 10%, which reflects intermediate response to the treatment. There was no correlation between CXCR4 expression and MRD levels at day 15. Conclusion CXCR4 expression on BCP-ALL blasts is highly heterogeneous and is not associated with particular leukemia immunophenotype. Further analyses should characterize clinical features of leukemia and treatment response with regard to CXCR4 expression. The study was supported by Polish National Center of Science grant N N407 687040. Disclosures: No relevant conflicts of interest to declare.