Tissue factor pathway inhibitor and bacterial infection

Community acquired pneumonia (CAP) is the eighth leading cause of death in the United States (2004 statistics), but the sixth leading cause of death in those over age 65, and the number one cause of death from infectious diseases. There are 1.3 million hospitalizations for pneumonia in the United States (2005 statistics) at an estimated annual cost of over $40 billion [1]. The mortality rate for community acquired pneumonia varies depending on the severity of the disease ranging from 5% in those treated as outpatients to12% in hospitalized patients and up to 30% in patients admitted to intensive care units [2]. Streptococcus pneumoniae is the most common identifiable cause of CAP followed by Haemophilus influenzae and Moraxella catarrhalis.

Tissue Factor Pathway Inhibitor (TFPI) is an anticoagulant protein that acts as a direct inhibitor of factor Xa (fXa) and a fXa dependent inhibitor of factor VIIa/tissue factor (fVIIa/TF). In addition to its anti-coagulant properties, TFPI also is thought to have anti-inflammatory properties most likely through its ability to limit thrombin generation and, thereby, the proinflammatory intracellular signaling mediated through activation of protease activated receptors (PAR). Because of its anticoagulant and anti-inflammatory activity, TFPI has been used in clinical trials as a treatment for severe sepsis. Historically, the potential therapeutic effects of TFPI were first identified by Sandset and colleagues who demonstrated that immunodepletion of TFPI sensitized rabbits to TF induced disseminated intravascular coagulopathy [3]. Later studies would reveal that endotoxin and inflammatory cytokines could induce intravascular TF expression which led to the demonstration of a direct relationship between TF, endotoxin, bacteria, and DIC in rabbits and baboons [4,5]. TFPI was then demonstrated to rescue baboons from septic shock following intravenous infusion of E.coli and characterized by reduced fibrinogen consumption, attenuated the increase in IL-6 levels, and diminished markers of cell injury [6].

In 2001, a Phase II study was performed to assess the safety and therapeutic effect of recombinant TFPI (rTFPI) in patients with severe sepsis [7]. The outcome of the Phase II trial was promising and prompted the Phase 3 OPTIMIST trial [8]. However, in the OPTIMIST trial there were no differences in the overall 28 day mortality rate in the rTFPI treatment and placebo groups. Subsequent subgroup analysis of study results revealed that in patients with severe CAP there was a trend toward improved survival in the rTFPI treatment group over those patients in the placebo group. This survival trend became statistically significant when subgroup analysis was limited to patients not treated with heparin in which an infectious organism was identified [9]. The results of this subgroup analysis have compelled further investigations of how TFPI impacts the biological relationship between coagulation, inflammation and the therapeutic outcome of the patient.

In the article “Recombinant Human Tissue Factor Pathway Inhibitor exerts anticoagulant, anti-inflammatory and antimicrobial effects in murine pneumococcal pneumonia”, van den Boogaard and colleagues discuss the biological and therapeutic effects of tissue factor pathway inhibitor (TFPI) in a mouse model of Streptococcus pneumoniae pneumonia [10]. The innate immune system is the first line of defense for CAP caused by Streptococcus pneumoniae and the chronologies of pneumococcus pneumonia in this mouse model are well defined [11]. This group has previously performed studies using rodent models of pneumonia demonstrating the importance of the fVIIa/TF pathway and the pathogenesis of Streptococcal pneumonia [12]. In this present study, the authors evaluated the therapeutic benefit of rTFPI with or without concurrent ceftriaxone initiated either eight or 24 hours following intranasal inoculation of mice with 2 × 10⁵ CFU of Streptococcus pneumoniae. Therapeutic effects of rTFPI were evaluated measuring bacterial load, plasma thrombin-antithrombin complexes, lung inflammation and inflammatory cytokine responses. The authors found that rTFPI reduced the pneumonia related coagulopathy, including significant further attenuation of the coagulopathy over that of ceftriaxone alone. The anti-inflammatory, bacterial load effects, cytokine effects, and lung neutrophil counts were most dramatic with ceftriaxone and only modest additional effects seen with the addition of rTFPI. Additional studies will need to be performed to determine whether the anticoagulant effects of TFPI enhance survival over that of ceftriaxone alone. As observed in clinical cases of CAP, the initial bacterial load in this mouse model has a significant effect on disease outcome. By varying the bacterial load given to the mice, investigators will be able to assess animals that recover from the disease and those which show a progressive disease course thus providing optimal therapeutic approaches for specific patient populations in rTFPI combination therapy.

The exact physiological mechanisms responsible for the decreased TFPI activity observed during bacterial infections are unclear [13]. Since TFPI is very sensitive to proteolytic inactivation, one common hypothesis is that TFPI is cleaved by either endogenous of pathogen produced proteases. Tang and colleagues examined TFPI activity in a baboon model and found there was a severe decrease in TFPI activity in the lungs of baboons with E.coli sepsis and subsequent tissue factor up regulation that resulted in increased tissue-bound active thrombin, platelet activation and fibrin deposition in the lungs. They further demonstrated that the decrease in TFPI coincides with the release of tissue plasminogen activator and the peak of plasmin generation suggesting that plasmin is the physiological protease that cleaves TFPI in this model [14]. Similarly, Yun and colleagues have described how omptins, outer membrane aspartyl proteases from gram negative bacteria, proteolytically inactivate TFPI and suggested this activity may increase the virulence of the pathogen [15]. The cleavage of TFPI by leukocyte elastase was first described by Higuchi and colleagues [16]. In contrast to the potential detrimental consequences to the host that may result from proteolytic inactivation of TFPI hypothesized by Tang and Yun, Massberg and colleagues reported a host antimicrobial defense mechanism in which TFPI is proteolytically cleaved by two major neutrophil serine proteases, elastase and cathepsin G promotes coagulation that results in compartmentalization of bacteria within blood vessels with an associated decreased tissue invasion [17].

Regardless of whether there is an increased or decreased TFPI activity to act in favor of the host or the pathogen, there is increasing evidence of extensive cross-talk between the coagulation and inflammatory systems involving tissue factor, thrombin, and their inhibitors. Whether the decrease in TFPI activity is caused directly by the invading bacteria, by coagulation proteins induced by the bacteria, or by the endogenous inflammatory cells that are up-regulated secondary to bacterial infection, TFPI plays a central role in the regulation of tissue factor and there by its downstream production of thrombin. Studies such as the report by van den Boogaard presented here using the CAP mouse model to investigate how recombinant TFPI therapy alters the course of bacterial infections will lead to new understanding of the physiological mechanisms that underlie host-pathogen interactions.