Role of blood coagulation in acute inflammation

Plasminogen: an enigmatic zymogen

Plasminogen is an abundant plasma protein that exists in various zymogenic forms. Plasmin, the proteolytically active form of plasminogen, is known for its essential role in fibrinolysis. To date, therapeutic targeting of the fibrinolytic system has been for 2 purposes: to promote plasmin generation for thromboembolic conditions or to stop plasmin to reduce bleeding. However, plasmin and plasminogen serve other important functions, some of which are unrelated to fibrin removal. Indeed, for >40 years, the antifibrinolytic agent tranexamic acid has been administered for its serendipitously discovered skin-whitening properties. Plasmin also plays an important role in the removal of misfolded/aggregated proteins and can trigger other enzymatic cascades, including complement. In addition, plasminogen, via binding to one of its dozen cell surface receptors, can modulate cell behavior and further influence immune and inflammatory processes. Plasminogen administration itself has been reported to improve thrombolysis and to accelerate wound repair. Although many of these more recent findings have been derived from in vitro or animal studies, the use of antifibrinolytic agents to reduce bleeding in humans has revealed additional clinically relevant consequences, particularly in relation to reducing infection risk that is independent of its hemostatic effects. The finding that many viruses harness the host plasminogen to aid infectivity has suggested that antifibrinolytic agents may have antiviral benefits. Here, we review the broadening role of the plasminogen-activating system in physiology and pathophysiology and how manipulation of this system may be harnessed for benefits unrelated to its conventional application in thrombosis and hemostasis.

A high-affinity receptor for urokinase plasminogen activator on human keratinocytes: characterization and potential modulation during migration

Low passage cultures of normal human keratinocytes produce several components of the plasminogen activator/plasmin proteolytic cascade, including urokinase plasminogen activator (uPA), tissue plasminogen activator (tPA), and two specific inhibitors. Studies here presented demonstrate that these cells also contain a high-affinity (Kd = 3 x 10(-10) M) plasma membrane-binding site for uPA. High molecular weight uPA, either as the single-chain precursor or two-chain activated form, bound to the receptor; however, low molecular weight (33 kD) uPA, tPA, or epidermal growth factor did not compete for binding, demonstrating specificity. Acid treatment, which removed endogenous uPA from the receptor, was required to detect maximal binding (45,000 sites per cell). To investigate the possibility that the uPA receptor on keratinocytes may be involved in epithelial migration during wound repair, cultures were wounded and allowed to migrate into the wounded site. Binding sites for uPA were localized by autoradiographic analysis of 125I-uPA binding as well as by immunocytochemical studies using anti-uPA IgG. With both techniques uPA binding sites were detected selectively on the plasma membrane of cells at the leading edge of the migrating epithelial sheet. This localization pattern suggests that uPA receptor expression on keratinocytes may be coupled to cell migration during cutaneous wounding.

Pulmonary fibrin deposition and increased microvascular permeability to protein following fibrin microembolism in dogs: a structure-function relationship

The effects of fibrin microembolism were examined using an infusion of a prothrombin activator (Echis carinatus venom, ECV; 30 min, 0.5 NIH thrombin equivalent units/kg) in acute mongrel dogs prepared with a pulmonary lymph cannula (n = 6, 12.3-21.5 kg). Lymph flow increased approximately 2.5-fold after 1-1.5 hr of elevated left atrial pressure (Pla = 20 cm H2O; 26 +/- 7 to 63 +/- 16 microliter/min, P less than 0.01) and the plasma to lymph protein concentration ratio (CP/CL) declined from 0.66 +/- .04 to 0.54 +/- .16 (P less than 0.01, x +/- SE). After Pla was reduced to control levels, the initiation of fibrin microembolism was associated with an approximate 2.7-fold elevation of lymph flow (62 +/- 8 microliters/min, P less than 0.01) and the CP/CL was not changed (0.56 +/- 0.04, P = ns). When Pla was increased following microembolism, lymph flow more than doubled to 117 +/- 24 microliter/min (P less than 0.01) and the CP/CL remained unaltered (0.56 +/- 0.03, P = ns). These changes were associated with afibrinogenemia and the appearance of fibrin degradation products (FDP) in plasma (150 +/- 50 micrograms/ml) and lymph (80 micrograms/ml) in three of the animals tested. No consistent pattern was seen in the CL/CP of separate endogenous plasma proteins after each intervention. These data support the view that pulmonary fibrin microembolism without inhibition of the fibrinolytic system was associated with an early increased pulmonary microvascular permeability to protein. In a separate group of similarly prepared animals (n = 8, 13-21.5 kg) without a lymph catheter, scanning electron microscopic observations showed branching fibrin microemboli to partially occlude some pulmonary arterioles. Mixed thrombus formations in larger precapillary blood vessels were also seen. Ultrastructural observations revealed the deposition of fibrin strands (periodicity = 220-230 A) within the pulmonary capillaries. Some of these deposits were overlaid by lamellar pseudopodia from endothelial cells and the fibrin appeared to be within these cells. Although plasmalemmal vesicles seemed to be more numerous in the endothelial cells with adjacent fibrin deposits, no gaps or breaks were seen in the densely stained interendothelial cell junctions and/or the endothelial cell membrane of the affected lung capillaries. Activated neutrophils and platelets were more numerous in the pulmonary capillaries following EVC. These data suggest that the presence of FDP and/or fibrin deposits within the pulmonary microvasculature may influence the early functional integrity of pulmonary endothelial cells at sites of fibrin accumulation.

Dermal and epidermal involvement in the evolution of acute eruptive guttate psoriasis vulgaris

A light and electron microscopic study of the evolution of acute eruptive guttate psoriasis vulgaris (AEGP) following penicillin-treated streptococcal throat infection is presented. The earliest recognizable changes, distinguished in clinically normal psoriatic skin (CNPS) from patients with psoriasis of 2 days' duration, comprised mast cell degranulation (Type I MCD), a vascular pattern showing endothelial cell gaps in postcapillary venules and postcapillary venules with endothelial cell hypertrophy and compressed lumen as well as epidermal involvement with punctiform spongiotic areas (PSAs). These early dermal and epidermal changes suggest that Type I MCD represents a primary morphologic event. Inflammatory infiltrate of mononuclear cells and exocytosis of mononuclear cells into the PSAs appeared when the concomitant overt psoriasis was 5-21 days old, and these changes were persistent in psoriatic lesions (PLs) of 2 days' duration. They are suggested to be precursors of overt psoriasis. In 2-day-old PLs, MCD (Types I and II) was a prominent feature. It was associated with (1) more extensive vascular changes, (2) inflammatory infiltrate of mononuclear cells and scanty polymorphonuclear leukocytes, (3) epidermal hyperplasia, and (4) migration of a few polymorphonuclear leukocytes through the epidermis with formation of Munro microabscesses in parakeratotic areas of stratum corneum. From the morphologic viewpoint, the progression from 2-day-old to fully evolved PLs seemed basically to be quantitative. The demonstration of MCD as a salient feature in the evolution of AEGP may have future therapeutic and preventive implications for psoriasis.

Platelet-tissue interaction: role of platelet-activating factor (PAF-acether)

The platelet-activating factor, or PAF-acether, is a phospholipid derivative that is a potent aggregating agent. It is formed by platelets themselves and also by basophils, neutrophils, monocytes and macrophages including alveolar macrophages. A role for PAF-acether during inflammation is envisaged since its intravenous administration induces hypotension, thrombocytopenia and bronchoconstriction in anaesthetized guinea-pigs. It is hypothesized that platelets, which contain and synthetize many pro-inflammatory substances, play a primary role in some pathological states.

The leucocoyte disappearance reaction in non-immune acute inflammation

Injection of a variety of irritants (saline, ovalbumin, compound 48/80 and powdered glass) into the rat pleural cavity induced the disappearance of pleural leucocytes during the first two hours of the reaction. This phenomenon, termed the leucocyte disappearance reaction (LDR), was suppressed by treatment with the anticoagulants heparin and warfarin. The in-vitro incubation of normal, or inflammatory pleural leucocytes resulted in the deposition of dense interconnecting meshwork of fibrin only upon addition of fibrinogen to the culture medium. It is suggested from these results that the LDR is related to the clotting system, involving leucocyte-derived enzyme(s) analogous to those of the clotting system (e.g., tissue thromboplastin), which convert fibrogen to fibrin, resulting in cell-trapping and subsequent "disappearance" of pleural leuococytes. Similarities were observed betweeen the LDR in non-immune inflammation and the macrophage disappearance reaction of cell-mediated immunity. The significance of these phenomena in the inflammatory process, both immune and non-immune, is discussed.

Phagocytosis: a review

Primitive unicellular organisms depend greatly on internalization of particulate matter for nourishment. In metazoa, this process is further developed to play a major role in mechanisms of defense. This review analyzes, mainly in mammalian systems, the various phenomena surrounding the phagocytic act. Much of the emphasis is placed on experimental work which has recently elucidated some of its features. Both the structural and functional aspects of phagocytosis are considered throughout the review, which is subdivided into an examination of chemotaxis and the various agents inducing it, the mode of recognition of particles to be phagocytized, and the mechanisms of ingestion. The last includes a discussion of the possible means whereby recognition is translated into ingestion, the modes of adhesion of particles onto the surface of phagocytes, the formation and fusion of pseudopodia during engulfment and ingestion, and process and significance of degranulation. In addition, the metabolic changes in phagocytes during the processes of chemotaxis, ingestion, and digestion are described. A discussion of the various ways phagocytes may destroy microorganisms incorporates an appreciation of the importance of the microbicidal action of the acidic environment of the phagosome, the various lysosomal contents, hydrogen peroxide, superoxide, singlet oxygen, and chemiluminescence. The interdependence and interrelationship of the induction and cooperation of these mechanisms are examined.

Secretion of plasminogen activator by rheumatoid and nonrheumatoid synovial cells in culture

The secretion of plasminogen activator by human synovial cultures established from explants was studied. Thirteen cultures established from patients with rheumatoid disease were hyposecretors of plasminogen activator. Seven cultures derived from patients with nonrheumatoid joint disorders all secreted high levels of this enzyme. Rheumatoid and nonrheumatoid cells contained comparable levels of intracellular activator. It is suggested that the secretion of plasminogen activator is a function of normal synovium and that a defect in this function in the rheumatoid synovium perpetuates the inflammatory process.

Incorporation of radiolabelled fibrinogen into venous thrombi induced in dogs

The time course and locus of radiolabelled fibrinogen (RLF) incorporation into venous thrombi were studied in dogs. Incorporation of radiolabelled fibrinogen (RLF) into fresh venous thrombi induced in vivo was studied by serial autoradiography, well-counting and surface counting. Autoradiographs disclosed diffuse incorporation of RLF into thrombi formed before and after RLF injection. Well-counting data also reflected thrombus RLF accumulation in both instances, although this accumulation was discernible earlier in thrombi induced in the presence of circulating RLF. Thrombi induced prior to RLF injection were detectable by surface counting after six hours. These studies support the potential value of RLF in detecting human venous thrombosis but also point out the limitations of the technique as well as questions which require clarification.