Physical contact with endothelial cells through β1- and β2- integrins rescues chronic lymphocytic leukemia from spontaneous and drug-induced apoptosis and induces a peculiar gene expression profile on leukemic cells.

Background: Chronic lymphocytic leukemia B-cells display prolonged survival in vivo, but when cultured in vitro rapidly undergo spontaneous apoptosis. We hypothesize that interaction with endothelial cells in infiltrated tissues and during recirculation may have a pathogenetic role in chronic lymphocytic leukemia.Design and Methods: We evaluated apoptosis of leukemic cells after co-culture on HUVEC monolayer with addition of Fludarabine and blocking adhesion antibodies. Then, we compared microarray-based expression profiles between leukemic cells at baseline and after co-culture.ùResults: We found that endothelial layer protected leukemic cells from apoptosis inducing a 2-fold mean decrement in apoptotic cells after 2 days co-culture. Moreover, endothelial layer decreased sensitivity of chronic lymphocytic leukemia B-cells to Fludarabine-induced apoptosis. Physical contact with endothelium mediated by both β1- and β2- integrins is essential for survival advantage. In particular, blocking CD106 on endothelial cells or CD18 on leukemic B-cells determined the almost complete abrogation of survival advantage (>70% inhibition of viability). Conversely, a reduction of apoptosis was also measured in leukemic cells cultured in conditioned medium collected after 2 days of co-culture, implying that survival is partially mediated by soluble factors. Overall, the contact with endothelial cells modulated 1,944 genes on chronic lymphocytic leukemia B-cells, establishing a peculiar gene expression profile: up-regulation of angiogenesis-related genes, increase of genes involved in TGFβ and Wnt signalling pathways, secretion of cytokines recruiting stromal cells and macrophages and increase in anti-apoptotic molecules such as Bcl2 and Survivin. Conclusion: Our study supports the notion that endothelial cells are major players in chronic lymphocytic leukemia microenvironment. Adhesion to endothelium strongly sustains survival, protects from drug-induced apoptosis and widely modifies gene expression profile of leukemic cells.