SENESCENT HUMAN NEUTROPHIL BINDING TO THROMBOSPONDIN (TSP) - EVIDENCEFOR A TSP-INDEPENDENT PATHWAY OF PHAGOCYTOSIS BY MACROPHAGES

This study investigated the interaction of apoptotic polymorphonuclear neutrophils (PMN) with thrombospondin (TSP), an important event media ting the clearance of apoptotic neutrophils by macrophages. Mie develo ped an in vitro assay to examine this interaction. Based on this assay , we found that apoptotic but not fresh PMN bound specifically to surf ace-immobilized TSP (33 +/- 0.03 x 10(3) cells/well) compared to fibri nogen, fibronectin or laminin (8.0 +/- 0.3 x 10(3) cells/well). Moreov er, the binding was specific for surface bound but not soluble TSP and appeared to be divalent cation dependent, was not significantly inhib ited by heparin and was sensitive to cycloheximide (CHX) treatment of senescent PMN (>90% inhibition at 10 mu M CHX). In contrast to the bin ding studies, phagocytosis of senescent PMN by macrophages was not aff ected by EDTA or cycloheximide. Phosphatidyl-L-serine liposomes, phosp ho-L-serine, glucosamine, galactosamine, and the acetylated sugars had no effect on phagocytosis. We conclude that: (i) there was specific b inding of senescent human PMN to immobilized TSP, which is divalent ca tion dependent and requires new protein synthesis in the PMN during se nescence; (ii) in addition to the recently defined TSP-dependent pathw ay, there is a TSP-independent pathway mediating phagocytosis of senes cent PMN by macrophages. The identity of this pathway remains to be de fined.