FAS and acute lymphoblastic leukemia: In search of the physiological outcome of the CD154/α5β1 interaction in the function of malignant T cells, and based on earlier observations showing that binding of collagen, an extracellular matrix protein, to some members of the β1 integrin inhibits Fas-induced cell death [27], we evaluated the impact of sCD154 on Fas-induced apoptosis in T-ALL cell lines.