Platelet-activating factor primes endotoxin-stimulated macrophage procoagulant activity

Abstract

Macrophage procoagulant activity (PCA) at the site of inflammation may be induced by several stimuli including bacteria and endotoxin (LPS). The local factors controlling PCA induction are poorly defined. The lipid mediator platelet-activating factor (PAF) is ubiquitous to inflammatory sites. To determine the effect of PAF on LPS-induced PCA, thioglycolate-elicited murine peritoneal macrophages were exposed to PAF (10 −7 M) or control medium for 30 min and then stimulated with LPS (10 μg/ml) for 2, 4, or 6 hr. The ability of macrophages to shorten the clotting time of plasma (ie., PCA) was then measured and clotting times were converted to PCA units using a thromboplastin standard. Cytosolic calcium ([Ca 2+] i) measurements were made using the calcium-sensitive fluorescent dye indo-1. PAF alone did not induce a rise in PCA expression (medium alone, 47 ± 11 mU/10 6 cells; PAF alone, 49 ± 12 mU/10 6 cells at t = 4 hr), but PAF treatment prior to LPS exposure resulted in a significant increase in the LPS-stimulated expression of PCA (LPS alone, 190 ± 29 mU/10 6 cells; PAF/LPS, 329 ± 57 mU/10 6 cells at t = 4 hr, P < 0.05). This priming effect was reversed by the PAF antagonist WEB 2086 (WEB/PAF/LPS, 196 ± 31 mU/2 × 10 6 cells). Stimulation of cells with PAF alone resulted in a rapid rise in [Ca 2+] i (resting, 213 ± 19 nmole; peak, 577 ± 35 nmole). This effect was also inhibited by WEB 2086. These data suggest that PAF plays an important role in the modulation of PCA production by macrophages. The rise in [Ca 2+] i stimulated by PAF may represent a cellular activation mechanism for PCA expression by peritoneal macrophages and hence may be important in fibrin deposition at inflammatory sites.