PS914 CONDITIONAL MEF2C EXPRESSION IN THE MOUSE THYMUS DRIVES DEVELOPMENT OF T/B-BIPHENOTYPIC LYMPHOID LEUKEMIA/LYMPHOMA

Abstract

Background: Central Nervous System (CNS) relapses are still one of the main causes of morbimortality in childhood acute lymphoblastic leukaemia (ALL) patients despite the reduction achieved by CNS-directed prophylactic chemotherapy. Although CNS colonization by ALL cells has been already associated to both increased survival and enhanced chemoresistance of blasts to intrathecal therapy, the mechanisms of protection and tumour cell maintenance in this compartment, as well as the interactions established between ALL and CNS cells, still need to be better understood. Aims: We propose in vivo and in vitro studies aimed at determining the location of leukaemia cells in the choroid plexus (CP) and the relevance of their interaction with stromal fibroblastic cells to create a tumour supportive microenvironment. Also, we explore the contribution of stromal cells to the ALL chemoresistance. Methods: Brain histopathology evaluation of immunodeficient NSG mice transplanted with human primary ALL cells or the Nalm-6 cell line was performed by using electron and fluorescence microscopy. Leukemic co-cultures with human CP cells in the presence or absence of different concentrations of chemotherapeutic drugs were used for viability assays. Changes in CP microenvironment were further tested by RT-qPCR, flow cytometry, ELISA and CBA assays. Results: The histological analysis of a xenograft model of ALL revealed the presence of leukaemic cells not only in the cerebral ventricles floating in the cerebrospinal fluid, but also interacting with the CP epithelium and within the CP stroma of those animals with CNS infiltration. The proximity of ALL cells to CP capillaries suggests they may easily enter through this fenestrated endothelium. Evaluation of interactions between leukaemic cells and stromal cells showed an upregulated expression of very late antigen-4 (VLA-4) in ALL cells and vascular cell adhesion molecule (VCAM1) in CP fibroblasts. Furthermore, in these co-cultures, ALL cells exhibited a reduced cell proliferation and an enhanced cell viability following treatment with chemotherapeutic agents. However, the blockade of VCAM1-VLA-4 interaction was found to impair those effects suggesting that VCAM1-VLA-4 interactions are, at least in part, responsible of protecting leukemic cells from cell death. Summary/Conclusion: Choroid plexus is proposed as a possible sanctuary for leukemic cells. There, ALL cells may remain attached to fibroblasts in a dormant state and protected from intrathecal chemotherapy treatment, thus favouring later relapses.