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<0.01). Similarly, we demonstrated that transplantation of the human B-cell precursor ALL line 018Z cause isolated CNS leukemia in NOD/SCID mice but induced aggressive systemic and CNS leukemia in NK-deficient NOD scid Il2rg null (NSG) mice. We further found that 018Z cells express and secrete endogenous IL15 and activated NK cells in-vivo in NOD/SCID mice. Remarkably, transplantation of 018Z cells in NOD/SCID mice after antibody mediated depletion of NK cells, resulted in a combined peripheral and CNS leukemia as observed in NSG mice. Mechanistic studies showed that 018Z leukemic blasts are efficiently killed by primary human NK cells and highly express ligands for the NKG2D and the NKp44 activating receptors. Blocking of the NKG2D receptor significantly reduced killing by NK cells.
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.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.