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Abstract 1627 Poster Board I-653 Background Leukemic blasts from B-cell acute lymphoblastic leukemia (B-ALL) and T-ALL circulate through the blood stream and may infiltrate different organs. Extramedullary organs may act as sanctuaries for lymphoblasts, preventing the exposure to adequate levels of chemotherapeutic drugs. Typical extramedullary relapses are seen in testes and in central nervous system (CNS). We aimed at determining whether chemokines may play a role in the infiltration of the CNS by leukemic blasts in childhood ALL. We studied the expression of chemokine receptors in ALL blasts in marrow, as well as the levels of chemokine ligands in the cerebrospinal fluid (CSF) of children with B- or T-ALL. If chemokines had a role in CNS leukemic infiltration, the following should be confirmed: leukemic blasts should express high levels of the chemokine receptor/s for which high levels of its corresponding ligand/s were detected at the CNS. Methods This prospective study was approved by the local ethical committees for clinical research. Samples from 80 children in 10 Spanish pediatric oncology units were obtained. We detected the presence of leukemic blasts in CFS by flow cytometry. We defined leukemic infiltration of CFS samples as the presence of cells with the same immunophenotype as the leukemia in the marrow aspirates. We detected the expression levels of 9 CCR and 6 CXCR molecules in ALL blasts by flow cytometry in marrow aspirates. Levels of chemokine ligands were quantitated by Cytometric Bead Array or by commercial ELISA kits in CSF samples. Results We found that chemokine receptors expression and levels of chemokine ligands varied depending upon the lineage (T versus B), the maturation state (pre-T versus T; pro-B versus pre-B versus common-B) and the risk-status (high versus non-high) of the leukemia. T-ALL patients with high levels of CNS leukemic infiltration expressed significantly higher levels of CXCL10 compared to the same parameters of T-ALL patients with low/absent levels of CNS disease (p=0,049). Common B-ALL patients with high levels of CNS leukemic infiltration expressed higher levels of CCL22 compared to that of common B-ALL patients with low/absent levels of CNS disease (p=0,059). Among the 4 patients with a CNS relapse, we detected higher levels of CXCR3 (p=0,0038) and of its ligand, CXCL10 (p=0,0169), compared to patients who did not relapse. Conclusions Our study suggests that the CXCR3/CXCL10 axis may be involved in the CNS relapse of high-risk ALL in children: high expression of CXCR3 on marrow blasts plus high levels of CXCL10 in the CNS was associated with leukemic CNS relapse. Disclosures No relevant conflicts of interest to declare.