Fibrinolytic activity of normal human blood monocytes

Abstract

Neutrophils (PMN) are important in the cellular phase of blood fibrino lytic activity (FA). The contribution of monocytes (MC), which have FA, is unclear. To determine the relative roles of these cells to activity in normal blood, we examined, by solid phase radiofibrin assay, FA of normal blood and plasma, and of purified PMN and MC, with and without plasminogen (PLG), mini-plasminogen (mPLG), the other major elastase fragment of PLG, or autologous plasma. PMN alone (0.5 × 10 6/ml) had striking activity (292 ± 25 SEM ng fibrin lysed/h; n = 10 normal subjects) while MC alone (0.5 × 10 6/ml) had mean FA of 32 ± 4 ng/h, which could be accounted for by contaminating PMN (36 ± 8 ng/h). Thus, in a 1 h assay (when cellular FA accounts for 70–80% of FA in whole blood), normal numbers of MC (0.5 × 10 6/ml) had no detectable FA when assayed with PLG or normal plasma. With longer assay times (2–6 h), PLG-dependent (plasminogen activator, PA) activity was demonstrated with mixtures of MC and PLG or plasma. This PA activity was released into the medium and required prior contact of MC and an intact, soluble PLG molecule for PA activity to be detected in medium (suggesting a PLG-MC triggering mechanism), since activity was reduced or absent when MC were exposed to mPLG, the other major elastase fragment of PLG, or solid phase PLG. Exposure of MC to solid phase fibrin did not result in PA release. MC PA activity was little affected by cycloheximide pretreatment, indicating preformed rather than newly synthesized PA. By SDS-PAGE and fibrin zymography, MC extracts revealed a single PA band with features of pro-urokinase (single chain urinary-type PA): M r 55,000, inhibition by antiurokinase antibody (but not by anti-tPA), and resistance to inhibition by DFP. By ELISA assay, approximate normal monocyte content of this PA (as M r 55,000 urokinase) was 0.03 fg (3.3 × 10 8 molecules) per cell.