MMP9 and central nervous system leukemia: In fact, in an elegant study using primary human brain microvascular endothelial cells as an in vitro model of blood brain barrier (BBB) and a mouse model of CNS leukemia, Feng and co-workers64 demonstrated the ability of matrix metalloproteinases MMP-2 and MMP-9, secreted by leukemic cells, to induce a breakdown of the BBB by down-regulating TJs proteins and altering their architecture.