L-selectin mediates downregulation of neutrophil TNF receptors

Tumor necrosis factor (TNF) is a potent activator of neutrophil granulocytes, which acts via two cell-surface receptors: the p55-TNF receptor (TNF-R55) and the p75-TNF receptor (TNF-R75). Proteolytic cleavage of the extracellular region of the receptors results in formation of soluble TNF-binding proteins, TNF-R55-BP and TNF-R75-BP. We recently reported that adherence alone, without any further stimuli, causes release of both TNF-R55-BP and TNF-R75-BP and that both leukocyte-integrin-dependent and non-integrin-dependent adherence mechanisms can modulate TNF receptor expression. In the present work we show that crosslinking of a mAb to the adhesion protein L-selectin (TQ1) on the surface of neutrophils results in downregulation of TNF-receptor binding capacity. Furthermore, when the fluctuations of cytosolic free calcium found in adherent neutrophils were blocked with the cell-permeable calcium chelator BAPTA, adherence-induced release of TNF-R55-BP was inhibited. We have shown that adherence, via mechanisms involving two adhesion proteins, L-selectin and the CD11/CD18 leukocyte integrins, and fluctuations of cytosolic free calcium, can result in downregulation of neutrophil TNF-receptors.