The results are expressed relative to the value of untreated sample, assigned to an arbitrary value 1. apoptosis in leukaemic, but not in normal B cells. Cytotoxicity of SEL24\B489 was similar in wild\type cells. Finally, inhibition of PIM kinases decreased CXCR4\mediated cell chemotaxis in two related mechanisms\by decreasing CXCR4 phosphorylation and surface expression, and by limiting CXCR4\triggered mTOR pathway activity. Importantly, PIM and mTOR inhibitors similarly impaired migration, indicating that CXCL12\triggered mTOR is required for CLL cell chemotaxis. Given the microenvironment\modulated PIM expression, their pro\survival function and a role of PIMs in CXCR4\induced migration, inhibition of these kinases might override microenvironmental protection and be an attractive therapeutic strategy in this disease. and other cytogenetic abnormalities, ZAP70 expression and immunoglobulin heavy variable (mutations distinguish two main biologically distinct subtypes of the disease, with different underlying genetic lesions, degree of clonal evolution, epigenetic changes and activated signalling pathways. The mutated subtype is associated with a good prognosis and the unmutated subtype with a poor prognosis.1 While majority of circulating CLL cells are arrested in the G0 phase of the cell cycle, replenishment of the leukaemic population is dependent on a proliferating fraction in the bone marrow and lymphoid tissues.3 In these compartments CLL cells interact with multiple bystander cell types, including bone marrow stromal cells (BMSCs), nurse\like cells (NLCs), follicular dendritic cells (FDCs), endothelial cells and T cells.4 These microenvironment components create niches that communicate with CLL cells direct contact and paracrine signals, protecting them from spontaneous and drug\induced apoptosis, and fostering proliferation. Consistent with this, primary CLL cells isolated from lymph nodes exhibit gene expression signatures characterized by activation of the B\cell receptor (BCR) pathway, NFB pathway and increased expression of E2F target genes.5 Trafficking of neoplastic B cells to these proliferation\conducive compartments is controlled by chemokines.6, 7 One of the key chemokines involved in CLL cells homing DIAPH2 is CXCL12 (formerly stromal\cell derived factor 1, SDF1). Activation of CXCR4 induces CLL cells chemotaxis, transendothelial migration and exhibits direct anti\apoptotic effects.8, 9, 10, 11 Given the role of CXCR4 in CLL cell motility and viability, mechanisms regulating CXCR4 activity and CXCR4\triggered signal transduction are particularly interesting as potential therapeutic targets. Accordingly, highly active B\cell receptor signalling inhibitors, such as ibrutinib, lead to egress BYK 204165 of CLL cells from the BYK 204165 lymphoid compartments to a periphery in a mechanism that involves decrease of surface CXCR4 expression.8 CXCR4 surface expression and recycling are regulated by PIM (provirus integration site for Moloney murine leukaemia virus) kinases, which phosphorylate CXCR4 on serine 339.9 PIMs have been postulated as a key mechanism downstream of BCR, responsible for modulation of CXCR4 in CLL.8, 10 The family of PIM proteins involves three conserved oncogenic serine/threonine kinases, PIM1, PIM2 and PIM3. PIMs phosphorylate a broad range of substrates, which are engaged in cell growth, metabolism, proliferation, migration and drug resistance.12, 13, 14 Increased activity of PIM kinases consolidates multiple oncogenic pathways by phosphorylation and inactivation of Forkhead box O (FOXO) family tumour suppressors, inactivation of proapoptotic Bcl\2\associated death promoter (BAD) and MYC stabilization.15 Moreover, PIM kinases phosphorylate 4E\binding protein 1 (4EBP1) and thus promote protein translation and tumour growth.16, 17, 18 Given these pleiotropic effects, inhibition of PIM kinases appeared a highly promising BYK 204165 therapeutic strategy in multiple human malignancies, including lymphoma. In this study, we investigated the expression of PIM kinases in CLL patients and further characterized the consequences of their inhibition. We demonstrate that PIMs expression is induced by the microenvironment\derived signals. Blocking PIMs activity with a newly developed small molecule inhibitor SEL24\B489 overrides protective microenvironment signals and induces CLL cell death. PIM inhibition blocks CLL cells migration in the CXCL12 chemokine gradient by affecting CXCR4 surface expression and CXCR4\dependent mTOR activation. Consistent with these pathogenetic findings, we demonstrate that expression of individual PIM isoforms is higher in patients with more aggressive and advanced disease. Thus, PIM kinases directly support CLL cell survival and participate in the cross\talk between leukaemic cells and their microenvironment. 2.?METHODS 2.1. CLL patient samples and cell culture The study enrolled 141 newly diagnosed and 9 relapsed CLL patients, and was conducted after local bioethics committee approval and according to Declaration of Helsinki. Patient baseline characteristics are given in Table?1. Peripheral blood mononuclear cells were separated by Ficoll gradient centrifugation. B cells were isolated with the B cell isolation kit II (Miltenyi Biotec). After isolation CLL cells were maintained in RPMI\1640 medium supplemented with 10% autologous plasma, 10% FBS, 1% penicillin\streptomycin and 25?mmol/L HEPES buffer, at a density of 1 1??107?cells/mL. For co\culture experiments CLL cells were layered over the 30%\confluent HS5 stromal cells and treated as indicated. After 48?hours CLL cells were harvested by gentle BYK 204165 agitation and further processed as described. Table 1 Baseline characteristics of CLL patients included in the study loci showed significantly higher PIM1 transcript level than patients with mutated genes (Figure?1C). In contrast,.