Senescent human neutrophil binding to thrombospondin (TSP): evidence for a TSP-independent pathway of phagocytosis by macrophages


Dr Joseph F.Murphy Department of Clinical Pharmacology, Royal College of Surgeons in Ireland, 123 St Stephen's Green, Dublin 2, Ireland.


This study investigated the interaction of apoptotic polymorphonuclear neutrophils (PMN) with thrombospondin (TSP), an important event mediating the clearance of apoptotic neutrophils by macrophages. We developed an in vitro assay to examine this interaction. Based on this assay, we found that apoptotic but not fresh PMN bound specifically to surface-immobilized TSP (33 ± 0.03 × 103 cells/well) compared to fibrinogen, fibronectin or laminin (8.0 ± 0.3 × 103 cells/well). Moreover, the binding was specific for surface bound but not soluble TSP and appeared to be divalent cation dependent, was not significantly inhibited by heparin and was sensitive to cycloheximide (CHX) treatment of senescent PMN (>90% inhibition at 10 μM CHX). In contrast to the binding studies, phagocytosis of senescent PMN by macrophages was not affected by EDTA or cycloheximide. Phosphatidyl- L-serine liposomes, phospho- L-serine, glucosamine, galactosamine, and the acetylated sugars had no effect on phagocytosis. We conclude that: (i) there was specific binding of senescent human PMN to immobilized TSP, which is divalent cation dependent and requires new protein synthesis in the PMN during senescence; (ii) in addition to the recently defined TSP-dependent pathway, there is a TSP-independent pathway mediating phagocytosis of senescent PMN by macrophages. The identity of this pathway remains to be defined.