Nalm-6 GFP secrete OPN and express functional OPN receptors. (A) Flow cytometry analysis of Nalm-6 GFP reveals cell surface expression of the OPN receptors β₁, α₄, α₅, and α₉β_1, but no expression of the β₇, α_V or CD44 receptors. (B) Nalm-6 GFP adhere strongly to thrombin-cleaved OPN (tcOPN) in vitro, but weakly to full-length hOPN, *P = .015, **P = .035. (C) Nalm-6 GFP adhesion to the α₄β₁ ligand fibronectin in vitro is unaffected by OPN neutralization, whereas Nalm-6 GFP adhesion to tcOPN is specifically inhibited by OPN-neutralizing antibodies, *P < .001. (D) Anti-VLA-4 receptor antibodies inhibit almost all Nalm-6 GFP adhesion to tcOPN in vitro, demonstrating that VLA-4 is the major integrin receptor responsible for Nalm-6 tcOPN adhesion, *P < .0001. (E) RT-PCR analysis shows human OPN (hOPN) mRNA transcripts in Nalm-6 GFP. K562, an AML cell line, was used as a negative control. (F) hOPN ELISA analysis of conditioned media from Nalm-6 GFP demonstrates their secretion of OPN, *P < .001.