Activation of the β₂ integrin Mac-1 (CD11b/CD18) by an endogenous lipid mediator of human neutrophils and HL-60 cells

β₂ integrins (CD11/CD18) play a key role in the adhesion, activation, migration and phagocytosis of human neutrophils. In order to exert their functions, β₂ integrins require activation, which results in an enhancement of ligand affinity. This functional up-regulation is probably due to a conformational change of the β₂ integrins, but the mechanisms of inside-out signaling that trigger this activation are still under investigation. In the present study, the effect of cellular lipids on the affinity state of β₂ integrins was investigated. Lipids were extracted from human neutrophils and HL-60 cells after stimulation with IL-8 or phorbol ester, respectively. The extracts were purified by anion exchange chromatography and/or HPLC fractionation. The lipid extracts induced the adhesion of neutrophils to fibrinogen and, in a cell-free assay system, the binding of C3bi-coated zymosan-particles by purified β₂ integrin Mac-1 (CD11b/CD18). The integrin up-regulating activity was resistant to ester hydrolysis, eluted as one particular HPLC-fraction, and showed an absorption maximum at 194 ± 2 nm. Taken together, these data support the concept that activated neutrophils and HL-60 cells can generate an endogenous lipid mediator, which up-regulates ligand binding activity of β₂ integrins.