Shedding of tumor necrosis factor receptors by activated human neutrophils

Abstract

The capacity of human neutrophils (PMN) to bind tumor necrosis factor (TNF) was rapidly lost when the cells were incubated in suspension with agents that can stimulate their migratory and secretory responses. Both physiological (poly)peptides (FMLP, C5a, CSF-GM) and pharmacologic agonists (PMN, calcium ionophore A23187) induced the loss of TNF receptors (TNF-R) from the cell surface. Half-maximal loss in TNF-R ensued after only ∼2 min with 10-7 M FMLP at 37°C, and required only 10-9 M FMLP during a 30-min exposure. However, there were no such changes even with prolonged exposure of PMN to FMLP at 4° or 16°C. Scatchard analysis revealed loss of TNF-binding sites without change in their affinity (Kd ∼0.4 nM) as measured at incompletely modulating concentrations of FMLP, C5a, PMA, or A23187. The binding of anti-TNF-R mAbs to PMN decreased in parallel, providing independent evidence for the loss of TNF-R from the cell surface. At the same time, soluble TNF-R appeared in the medium of stimulated PMN. This inference was based on the PMN- and FMLP-dependent generation of a nonsedimentable activity that could inhibit the binding of TNF to fresh human PMN or to mouse macrophages, and the ability of mAbs specific for human TNF-R to abolish inhibition by PMN-conditioned medium of binding of TNF to mouse macrophages. Soluble TNF-R activity was associated with a protein of Mr ∼28,000 by ligand blot analysis of cell-free supernatants of FMLP-treated PMN. Thus, some portion of the FMLP-induced loss of TNF-R from human PMN is due to shedding of TNF-R. Shedding was unaffected by inhibitors of serine and thiol proteases and could not be induced with phosphatidylinositol-specific phospholipase C. Loss of TNF-R from PMN first stimulated by other agents may decrease their responsiveness to TNF. TNF-R shed by PMN may be one source of the TNF-binding proteins found in body fluids, and may blunt the actions of the cytokine on other cells.