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Metabolic reprogramming is a key hallmark of cancer, but less is known about metabolic plasticity of the same tumor at different sites. Here, we investigated the metabolic adaptation of leukemia in two different microenvironments, the bone marrow and the central nervous system (CNS). We identified a metabolic signature of fatty-acid synthesis in CNS leukemia, highlighting Stearoyl-CoA desaturase (SCD1) as a key player. In vivo SCD1 overexpression increases CNS disease, whilst genetic or pharmacological inhibition of SCD1 decreases CNS load. Overall, we demonstrated that leukemic cells dynamically rewire metabolic pathways to suit local conditions and that targeting these adaptations can be exploited therapeutically.

Prevention of central nervous system (CNS) relapse is critical for cure of childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Despite this, mechanisms of CNS infiltration are poorly understood, and the timing, frequency, and properties of BCP-ALL blasts entering the CNS compartment are unknown. We investigated the CNS-engrafting potential of BCP-ALL cells xenotransplanted into immunodeficient NOD.Cg- ITALIC! Prkdc (ITALIC! scid) ITALIC! Il2rg (ITALIC! tm1Wjl)/SzJ mice. CNS engraftment was seen in 23 of 29 diagnostic samples (79%): 2 of 2 from patients with overt CNS disease and 21 of 27 from patients thought to be CNS negative by diagnostic lumbar puncture. Histologic findings mimic human pathology and demonstrate that leukemic cells transit the blood-cerebrospinal fluid barrier situated close to the dural sinuses, the site of recently discovered CNS lymphatics. Retrieval of blasts from the CNS showed no evidence for chemokine receptor-mediated selective trafficking. The high frequency of infiltration and lack of selective trafficking led us to postulate that CNS tropism is a generic property of leukemic cells. To test this, we performed serial dilution experiments which showed CNS engraftment in 5 of 6 mice after transplant of as few as 10 leukemic cells. Clonal tracking techniques confirmed the polyclonal nature of CNS-infiltrating cells, with multiple clones engrafting in both the CNS and periphery. Overall, these findings suggest that subclinical seeding of the CNS is likely to be present in most BCP-ALL patients at original diagnosis, and efforts to prevent CNS relapse should concentrate on effective eradication of disease from this site rather than targeting entry mechanisms.

Central nervous system acute lymphoblastic leukemia (CNS-ALL) is a major clinical problem. Prophylactic therapy is neurotoxic, and a third of the relapses involve the CNS. Increased expression of interleukin 15 (IL-15) in leukemic blasts is associated with increased risk for CNS-ALL. Using in vivo models for CNS leukemia caused by mouse T-ALL and human xenografts of ALL cells, we demonstrate that expression of IL-15 in leukemic cells is associated with the activation of natural killer (NK) cells. This activation limits the outgrowth of leukemic cells in the periphery, but less in the CNS because NK cells are excluded from the CNS. Depletion of NK cells in NOD/SCID mice enabled combined systemic and CNS leukemia of human pre-B-ALL. The killing of human leukemia lymphoblasts by NK cells depended on the expression of the NKG2D receptor. Analysis of bone marrow (BM) diagnostic samples derived from children with subsequent CNS-ALL revealed a significantly high expression of the NKG2D and NKp44 receptors. We suggest that the CNS may be an immunologic sanctuary protected from NK-cell activity. CNS prophylactic therapy may thus be needed with emerging NK cell-based therapies against hematopoietic malignancies.

Central nervous system acute lymphoblastic leukaemia (CNS-ALL) is a major clinical problem. CNS-directed 'prophylactic' chemo- or radio - therapy is associated with significant early and long-term toxicity. Moreover, greater than a third of the relapses occur in the CNS. To design specific, more effective and less toxic therapy and for personalized precise adjustment of prophylactic therapy there is a need for better understanding of the biology of this disease. Specifically, the precise neurotropic mechanisms of ALL are currently unclear, as is the pathogenesis of CNS relapse. Here we review and contrast the recent findings with earlier studies of pathogenesis of CNS leukaemia. We also describe the challenges in research of this devastating complication of ALL.

Endothelin-1 (ET-1) receptor antagonists increase plasma immunoreactive ET-1 levels. However, their effect on preproendothelin-1 (PPE-1) mRNA levels is still controversial. Few studies have found a decrease in PPE-1 mRNA levels in endothelial cells treated with the nonselective ETA/B receptor antagonist, and others demonstrated that an ETB blockade by the selective antagonist BQ788 increases PPE-1 mRNA levels. We studied the effect of ETA and ETB selective receptor antagonists on PPE-1 transcription, both in vitro and in vivo. Endothelial cells, transiently transfected with PPE-1 luciferase plasmid, were treated with ET-1 receptor antagonists. Bosentan, a dual ETA/B receptor antagonist, and BQ788 (ETB receptor antagonist) treatment resulted in a 1.6-fold and 1.3-fold increase, respectively in luciferase activity as compared with the untreated control. In contrast, the ETA receptor antagonist, BQ123, had no effect on luciferase activity. Transgenic mice that express the luciferase gene under the control of PPE-1 promoter were treated with Bosentan. Luciferase activity, PPE-1 mRNA levels, and plasma immunoreactive ET-1 levels were increased by 1.6-fold to 2.0-fold in the Bosentan-treated group compared with the untreated, control group. ET-1 receptor blockade increased PPE-1 transcription both in vitro and in vivo. The increased transcription can be attributed to ETB receptor blockade, because BQ-788, but not BQ-123, increased PPE-1 transcription.

Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy accounting for 80% of leukemias. The involvement of the central nervous system (CNS) by ALL is a major clinical problem and occurs in about 50% of the children without adequate treatment. The introduction of CNS-directed therapy consisting of intrathecal and high dose systemic chemotherapy and, occasionally, cranial irradiation, reduced relapse rate to less than 5% and has become a prerequisite for treating children with ALL. However, substantial neurotoxicity associated with this therapy is a major concern. Moreover the CNS is involved in up to a third from all relapses. To date very little is known about the pathogenesis of CNS leukemia. Our research was promoted by the previous observation that high mRNA expression of interleukin 15 (IL15) in leukemic blasts is associated with increased risk for CNS involvement (Cario et al JCO 2007;25:4813-20). As IL15 is a strong stimulant of Natural Killer (NK) cells, we hypothesized that the increased expression of IL15 may activate NK cells which, in turn, will control residual ALL cells in the peripheral blood but not in the relatively protected central nervous system. To investigate this hypothesis, we utilized two mouse models, a S49-derived T lymphoblastic leukemia syngeneic model and a novel human xenograft ALL model in immune-deficient mice. We found that constitutive expression of IL15 in mouse T lymphoblastic leukemia cells transplanted in neonatal Balb/c mice markedly slowed the development of systemic disease and caused CNS leukemia characterized by pronounced clinical CNS symptoms and subarachnoid infiltration of leukemia cells. This phenotype was accompanied by increase in activated natural killer (NK) cells (from 0.16% to 18.6% P Taken together we show here, for the first time, a crucial role for NK cells in the control of CNS leukemia. We suggest that the association between IL15 expression in ALL blasts and isolated CNS relapse might be explained by activation of NK cells leading to increased surveillance of residual leukemia in the bone marrow but not in the CNS which serve as a sanctuary site for tumor growth. Disclosures: No relevant conflicts of interest to declare.