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We have investigated the cell surface recognition mechanisms used by human monocyte-derived macrophages (MO ) in phago- cytosis of intact aging human neutrophils (PMNs) undergoing apoptosis. This study shows that the adhesive protein thrombo- spondin (TSP) was present in the interaction, both associated with the MO surface and in solution at a mean concentration of 0.59 ,g/ml. The interaction was inhibited by treatment of MO (but not aged PMN) with cycloheximide, but could be "res- cued" by replenishment with exogenous TSP. Under control conditions, MO recognition of aged PMNs was specifically po- tentiated by purified platelet TSP at 5 ,g/ml, present either in the interaction or if preincubated with either cell type, suggest- ing that TSP might act as a "molecular bridge" between the two cell types. In support, both aged PMN and MO were found to adhere to TSP, and phagocytosis of aged PMN was specifically inhibited by (a) excess soluble TSP; (b) antibodies to TSP that also inhibit TSP-mediated adhesion to aged PMN; and (c) down-regulation of M4 receptors for TSP by plating MO on TSP-coated surfaces. Furthermore, inhibition with mAbs / Arg-Gly-Asp-Ser peptide of the candidate MO receptors for TSP, CD36, and a,83 exerted synergistic effects on both MO recognition of aged PMN and MO adhesion to TSP, indicating that "two point" adhesion of TSP to these MO structures is involved in phagocytosis of aged PMN. Our findings indicate newly defined roles for TSP and CD36 in phagocytic clearance of senescent neutrophils, which may limit inflammatory tissue injury and promote resolution. ( J. Clin. Invest.
Savill et al. (Thu,) studied this question.