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

Abstract

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 x 10(3) cells/well) compared to fibrinogen, fibronectin or laminin (8.0 +/- 0.3 x 10(3) 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 microM 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.