Surfactant modulates intracellular signaling of the adhesion receptor L-selectin

Intraalveolar leukocyte accumulation is one of the hallmarks during respira tory distress. In the intraalveolar space, leukocyte activation is mediated by pathogens, cytokines, and different ligands binding to adhesion molecul es. Leukocyte stimulation via the adhesion molecule L-selectin is specifica lly induced by ligands expressed on leukocytes, platelets, endothelial cell s, or lipopolysaccharide. Recently, we have demonstrated that leukocyte act ivation by L-selectin transmits several intracellular signaling cascades re sulting in capping and cytoskeletal changes, the activation of kinases and neutral sphingomyelinase, the recruitment of adaptor proteins to the cell m embrane, the activation of the small G-proteins Ras and Rac, and the releas e of oxygen. In the present study, we examined the effects of surfactant on L-selectin-induced signal transduction in leukocytes. Using fluorescence m icroscopy, we provide evidence that preincubation of leukocytes with surfac tant significantly inhibits receptor capping; 28 +/-7% of cells show cappin g after L-selectin stimulation versus 8+/-5% and 3+/-1% of cells after prei ncubation with Exosurf and Curosurf, respectively (p < 0.05). The activity of the neutral sphingomyelinase in cell lysates is also modulated by surfac tant. In addition, we show that the activation of the tyrosine kinase p561c k is diminished by approximately 50% after surfactant treatment. This resul ts in inhibition in tyrosine phosphorylation of certain intracellular prote ins. The interaction of the L-selectin molecule with its antibody was not i nfluenced by surfactant as shown by how cytometry. Surfactant inhibits intr acellular signaling events of the L-selectin receptor in leukocytes and mig ht therefore contribute to the modulatory effects of surfactant on immune f unction.