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Fibrin deposition is characteristic of inflammatory diseases. The monocytes is central to the inflammatory response and can affect fibrinolysis by expression of urokinase (u-PA) and plasminogen activator inhibitor types 1 and 2 (PAI-1 and PAI-2, respectively). This study examines whether thrombin, which promotes fibrin deposition, can contribute to fibrin persistence by modulating expression of proteins of the fibrinolytic system. Monocytes were isolated from human peripheral blood and analyzed for PAI-2, PAI-1, and u-PA antigens by enzyme-linked immunosorbent assay (ELISA). Monocytes responded to thrombin by increased expression of PAI-2 in a dose- and time-dependent manner, with maximal synthesis at a concentration of 1 U/mL to 10 U/mL. This trend was also evident for PAI-1, which was present at much lower levels. Thrombin and lipopolysaccharide (LPS) stimulated comparable levels of PAI-2, studied at the antigen and mRNA level. The dose effet of LPS on PAI-2 and PAI-1 was found to differ from that of thrombin. The level of u-PA was undetectable by ELISA and zymography in all samples. Thrombin stimulates PAI-2 synthesis by human monocytes, therefore creating an imbalance in the fibrinolytic system. This may contribute to persistence of fibrin, deposited during inflammation.

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Small, N‐ to C‐terminal cyclized peptides containing the leucyl‐aspartyl‐valine (LDV) motif from fibronectin connecting segment‐1 (CS‐1) have been investigated for their effects on the adhesion of human T‐lymphoblastic leukaemia cells (MOLT‐4) to human plasma fibronectin in vitro mediated by the integrin Very Late Antigen (VLA)‐4 (α4β1, CD49d/CD29). Cyclo(‐isoleucyl‐leucyl‐aspartyl‐valyl‐aminohexanoyl‐) (c(ILDV‐NH(CH2)5CO)) was approximately 5 fold more potent (IC50 3.6±0.44 μM) than the 25‐amino acid linear CS‐1 peptide. Cyclic peptides containing two more or one less methylene groups had similar potency to c(ILDV‐NH(CH2)5CO) while a compound containing three less methylene groups, c(ILDV‐NH(CH2)2CO), was inactive at 100 μM. c(ILDV‐NH(CH2)5CO) had little effect on cell adhesion mediated by two other integrins, VLA‐5 (α5,β1, CD49e/CD29) (K562 cell adhesion to fibronectin) or Leukocyte Function Associated molecule‐1 (LFA‐1, αLβ2, CD11a/CD18) (U937 cell adhesion to Chinese hamster ovary cells transfected with intercellular adhesion molecule‐1) at concentrations up to 300 μM. c(ILDV‐NH(CH2)5CO) inhibited ovalbumin delayed‐type hypersensitivity or oxazolone contact hypersensitivity in Balb/c mice when dosed continuously from subcutaneous osmotic mini‐pumps (0.1–10 mg kg−1 day−1). Maximum inhibition (approximately 40%) was similar to that caused by the monoclonal antibody PS/2 (7.5 mg kg−1 i.v.) directed against the α4 integrin subunit. c(ILDV‐NH(CH2)5CO) also inhibited oxazolone contact hypersensitivity when dosed intravenously 20 h after oxazolone challenge (1–10 mg kg−1). Ear swelling was reduced at 3 h and 4 h but not at 1 h and 2 h post‐dose (10 mg kg−1). Small molecule VLA‐4 inhibitors derived from c(ILDV‐NH(CH2)5CO) may be useful as anti‐inflammatory agents. British Journal of Pharmacology (1999) 126, 1751–1760; doi: