A Chondroitin Sulfate Proteoglycan on Human Neutrophils Specifically Binds Platelet Factor 4 and Is Involved in Cell Activation

Abstract

Platelet factor 4 (PF-4), a member of the α-chemokine subfamily of cytokines, activates human neutrophils independently of intracellular free calcium mobilization or binding to IL-8R. In the present study, we have identified and partially characterized a receptor for PF-4 on human neutrophils, which displays weak cross-reactivity with the IFN-γ-inducible protein 10, but not with other α-chemokines such as IL-8, neutrophil-activating peptide 2, or melanoma growth-stimulatory activity (GROα). Binding studies revealed that human neutrophils express a high number of receptors (Bmax ∼ 7.6 × 106 sites/cell) of moderate affinity (Kd ≈ 650 nM). The kinetics of PF-4-binding correlates with the proportion of PF-4 tetramers in solution and with the activation of neutrophils for exocytosis. Reduction of PF-4 binding and PF-4-induced exocytosis in the presence of various glycosaminoglycans or following treatment of cells with chondroitinase ABC (but not other glycosaminoglycan-degrading enzymes) altogether demonstrates that the PF-4 receptor is a proteoglycan of the chondroitin sulfate class. Cross-linking experiments with radiolabeled PF-4 revealed a receptor-ligand complex of ∼250 kDa. Taken together, our data show that a distinct chondroitin sulfate proteoglycan represents specific receptors for tetrameric PF-4 on human neutrophils.