Circulating Histone Is Elevated in Acute Leukemia and Protects Leukemic Cell Death through Enhancement of Leukemic Cell Adhesion to Endothelium

Background: Leukocytes release nuclear contents into extracellular milieu called by extracellular traps (ET) when they are stimulated by reactive oxygen species (ROS). The nuclear contents mainly compose of histone-DNA complex and neutrophil elastase. This study investigated whether the leukemic cells could release ET and the released histone could induce endothelial activation, finally resulting in leukemic progression.

Methods: The circulating ET biomarkers (histone-DNA complex, cell free DNA, neutrophil elastase) were measured in 80 patients with hematologic diseases and 40 healthy controls by ELISA. ET formation and ROS level were investigated during leukemic cell proliferation in vitro. Histone-induced endothelial adhesion molecule expression and survival were measured by flow cytometry.

Results: Acute leukemia showed high ET biomarkers, which correlated with peripheral blast count. Leukemic cells produced high ROS and release extracellular histone, which was significantly blocked by antioxidants. Histone significantly induced 3 endothelial adhesion molecules expression, promoting leukemic cell adhesion to endothelial cells, which was inhibited by histone inhibitors (heparin, polysialic acid, activated protein C), 3 neutralizing antibodies against adhesion molecules, and Toll like receptor(TLR)9 antagonist. When the leukemic cell co-cultured with endothelial cells, the adherent leukemic cells showed better survival than the non-adherent, demonstrating that histone-treated endothelial cells protected leukemic cells from both spontaneous and chemotherapy-induced death.

Conclusion: Our data for the first time supports that extracellular histone could be released from leukemic cells through ROS dependent mechanism. The released histone could promote leukemic cell adhesion through endothelial adhesion molecule induction and finally protect from leukemic cell death.