Monocytes augment bacterial species- and strain-dependent induction of tissue factor activity in bacterium-infected human vascular endothelial cells

In bacterial endocarditis (BE), intravascular infection with Staphylococcus aureus, Streptococcus sanguis, or Staphylococcus epidermidis can lead to formation of a fibrin clot on the inner surface of the heart and cause heart dysfunction. The events that start the coagulation in the early stage of the disease are largely unknown. We have recently shown that human endothelial cells (EC) upon binding and internalization of S. aureus, but not S. sanguis or S. epidermidis, express tissue factor (TF)-dependent procoagulant activity (TFA). The present study shows that infection of EC with these three pathogens induces surface expression of intracellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) and monocyte adhesion. Subsequent coculture of these cells synergistically enhanced TFA, which was exclusively dependent on TF molecules that were expressed on EC during coculture. TFA induction required direct contact between monocytes and bacterium-infected EC, but the signals for this response were not generated by the binding of monocytes through their beta(2)- or alpha(4)-integrins to ICAM-1 or VCAM-1, respectively, on infected EC. The mechanism by which monocytes induce TFA in bacterium-infected EC was partly mediated by the proinflammatory cytokine interleukin-1 produced by the cells during coculture. Endogenous tumor necrosis factor alpha was not involved. This modulating effect of monocytes on species- and strain-dependent TFA of bacterium-infected EC supports our hypothesis that in an early stage in the pathogenesis of BE, as well as other intravascular infections that lead to detrimental fibrin formation, the coagulation cascade can be activated on the surfaces of EC as a consequence of specific interactions between pathogenic bacteria, EC, and monocytes.

Interleukin-10 regulates the tissue factor activity of monocytes in an in vitro model of bacterial endocarditis

Monocytes are important effector cells in the pathogenesis of bacterial endocarditis since they provide the tissue factor that activates the coagulation system and maintains established vegetations. Monocytes secrete cytokines that can modulate monocyte tissue factor activity (TFA), thereby affecting the formation and maintenance of vegetations. In this study, we show that monocytes cultured for 4 h on a Streptococcus sanguis-infected fibrin matrix mimicking the in vivo vegetational surface express high levels of TFA. This was accompanied by secretion of the proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha), interleukin-1 alpha (IL-1 alpha), and IL-1 beta. After a 24-h incubation period the anti-inflammatory cytokine IL-10 could also be detected. Our data show that, whereas TNF-alpha and IL-1 have a minor role in the induction of TFA by monocytes cultured on a fibrin matrix, TNF-alpha but not IL-1 plays an important role in the induction of IL-10 by these cells. In turn, our data show that IL-10 is an important factor in the downregulation of monocyte TFA. In summary, we conclude that IL-10 is an important factor in the control of monocyte TFA in endocardial vegetations.

Role of monocytes in experimental Staphylococcus aureus endocarditis

In the pathogenesis of bacterial endocarditis (BE), the clotting system plays a cardinal role in the formation and maintenance of the endocardial vegetations. The extrinsic pathway is involved in the activation of the coagulation pathway with tissue factor (TF) as the key protein. Staphylococcus aureus is a frequently isolated bacterium from patients with BE. We therefore investigated whether S. aureus can induce TF activity (TFA) on fibrin-adherent monocytes, used as an in vitro model of BE. We also assessed in vivo in rabbits with catheter induced vegetations, the effect of S. aureus infection on vegetational TFA. In vitro experiments showed that adherent S. aureus induced TFA on fibrin-adherent monocytes which was optimal at a bacterium/monocyte ratio of 1 to 1. Monocyte damage occurred when this ratio exceeded 4 to 1 (visually) or 6 to 1 (propidium iodide influx) Consequently, TFA decreased. In vivo S. aureus led to very high bacterial numbers in the vegetations and a significant increase of their weight. However, TFA of infected vegetations was the same as of sterile ones. This may be due to the high bacteria to monocyte ratio as well as bacterium-induced monocyte damage. Teicoplanin treatment of infected rabbits reduced bacterial numbers in the blood and in the vegetations. Two-day treatment resulted in an increase of vegetational TFA, but after four-day treatment vegetational TFA dropped, most probably due to a suboptimal bacterium/monocyte ratio. S. aureus endocarditis in etoposide (Vepesid)-treated rabbits, leading to a selective monocytopenia, caused a rapid death of the animals. In these rabbits no vegetations were found at all. We conclude that, like Streptococcus sanguis and Staphylococcus epidermidis, S. aureus is able to induce TFA in fibrin-adherent blood monocytes. In addition, monocytes have a protective effect during the course of S. aureus endocarditis.

Altered gene expression in Staphylococcus aureus upon interaction with human endothelial cells

Staphylococcus aureus is isolated from a substantial number of patients with infective endocarditis who are not known to have predisposing heart abnormalities. It has been suggested that the infection is initiated by the direct binding of S. aureus to human vascular endothelium. To determine the mutual response of the endothelial cells and the bacteria, we studied the interaction between S. aureus and human vascular endothelium. Scanning electron microscopic analyses showed that binding of S. aureus to human umbilical vein endothelial cells (HUVEC) mainly occurred via thread-like protrusions extending from the cell surface. Bound bacteria appeared to be internalized via retraction of the protrusions into newly formed invaginations of the endothelial cell surface. The growth phase of S. aureus had a major impact on the interaction with HUVEC. Logarithmically growing bacteria showed increased binding to, and were more readily internalized by, HUVEC compared to stationary-phase bacteria. To assess the bacterial response to the cellular environment, an expression library of S. aureus was used to identify genes whose expression was induced after 4 h of exposure to HUVEC. The identified genes could be divided into different categories based on the functions of the encoded proteins (transport, catabolism, biosynthesis, and DNA repair). Further analyses of five of the S. aureus transposon clones showed that HUVEC as well as human serum are stimuli for triggering gene expression in S. aureus.

Bacterial species- and strain-dependent induction of tissue factor in human vascular endothelial cells

A cardinal process in bacterial endocarditis (BE) is the activation of the clotting system and the formation of a fibrin clot on the inner surface of the heart, the so-called endocardial vegetation. The processes that lead to the activation of the clotting system on endothelial surfaces upon exposure to bacteria are largely unknown. In the present study, we investigated in an in vitro model whether infection of human endothelial cells (EC) with bacteria that are relevant to BE, such as Staphylococcus aureus, Streptococcus sanguis, and Staphylococcus epidermidis, leads to induction of tissue factor (TF)-dependent procoagulant activity (TFA) and whether this process is influenced by host factors, such as interleukin-1 (IL-1), that are produced in response to the bacteremia in vivo. The results show that S. aureus binds to and is internalized by EC, resulting in expression of TF mRNA and TF surface protein as well as generation of TFA within 4 to 8 h after infection. No TFA was found when EC were exposed to UV-irradiated S. aureus or bacterial cell wall fragments. S. sanguis and S. epidermidis, although also binding to EC, did not induce endothelial TFA. This indicates a species and strain dependency. EC also expressed TFA after exposure to IL-1. The enhanced TFA of EC after exposure to S. aureus was not prevented by IL-1 receptor antagonist, arguing against an auto- or paracrine contribution of endogenous IL-1. When IL-1 was applied together with bacteria, this had a synergistic effect on the induction of EC TFA. This was found in particular with S. aureus but also, although to a lesser degree, with S. sanguis and S. epidermidis. This influence of IL-1 on the species- and strain-dependent induction of EC TFA suggests that bacterial factors as well as host factors orchestrate the induction of coagulation in an early stage in the pathogenesis of endovascular disease, such as BE.

Role of monocytes and bacteria in Staphylococcus epidermidis endocarditis

The endocardial vegetation which is formed in the course of bacterial endocarditis (BE) contains tissue factor (TF)-dependent procoagulant activity. Earlier studies showed that monocytes are the main source of TF in the vegetations. The TF activity (TFA) of vegetations isolated from Streptococcus sanguis-infected rabbits depended on the numbers of bacteria as well as monocytes in the vegetation. In this study, we investigated whether for Staphylococcus epidermidis, a frequent pathogen in BE, an effect similar to that found for S. sanguis could be shown. In vitro, S. epidermidis was found to stimulate TFA of fibrin adherent monocytes significantly. This stimulation was maximal at a bacterium-to-monocyte ratio of 7. In vivo, TFA was found to be significantly higher in S. epidermidis-infected than in sterile catheter-induced vegetations. Reduction of vegetational bacterial numbers by teicoplanin treatment lead to a small but significant decrease of TFA. Reduction of monocyte numbers by etoposide did not affect vegetational TFA. Comparison of data for S. epidermidis and S. sanguis revealed that at equivalent bacterial numbers, vegetational TFAs were approximately the same for both microorganisms. Combining the results of the present study with those of a previous study using S. sanguis, we conclude that the main factor determining monocyte-dependent vegetational TFA is the number of vegetation-associated bacteria. The lower TFA found for S. epidermidis-infected than for S. sanguis-infected vegetations can be explained by the significantly lower bacterial numbers in the infected vegetations and consequently a lower stimulation of vegetation-associated monocytes.

Role of phagocytosis in activation of the coagulation system in Streptococcus sanguis endocarditis

The formation of vegetations consisting of fibrin, cellular elements, humoral factors, and bacteria is the central event in the pathogenesis of bacterial endocarditis. Fibrin formation occurs on the vegetation, the coagulation system being activated locally via the expression of tissue factor (TF) on fibrin-adherent monocytes. This study was performed to assess the importance of phagocytosis of fibrin-adherent Streptococcus sanguis in the stimulation of TF expression on fibrin-adherent monocytes, as well as a role for "frustrated" phagocytosis. With the latter process, these cells are unable to remove bacteria from the fibrin surface but nonetheless might be activated to generate TF. We found that serum was not required for the stimulation of TF expression by fibrin-adherent monocytes in the presence of S. sanguis in an in vitro model for bacterial endocarditis. The bacterial adhesin dextran did not influence the TF activity (TFA) of fibrin-adherent monocytes: TFA was the same after stimulation with a dextran-positive streptococcus as with its dextran-negative mutant. Furthermore, dextran did not influence the TFA of endocardial vegetations, which was the same for vegetations isolated from rabbits infected either with dextran-positive S. sanguis or its dextran-negative mutant. These results do not support the hypothesis that in bacterial endocarditis (frustrated) phagocytosis significantly contributes to TF expression on vegetation-adherent monocytes. Fibronectin, however, although not influencing the fibrin binding of the streptococci, did enhance the TFA of monocytes in a concentration-dependent manner. We conclude that although streptococci do enhance expression of TFA on monocytes, phagocytosis and bacterial adhesins do not play a major role in this process. Stimulation of monocyte TFA may be more dependent on interactions between monocytes and the vegetational surface via fibronectin receptors, such as VLA 4 and VLA 5 (very late antigens 4 and 5).

Influence of monocytes and antibiotic treatment on tissue factor activity of endocardial vegetations in rabbits infected with Streptococcus sanguis

A main feature in the pathogenesis of bacterial endocarditis is the activation of the coagulation system via the extrinsic pathway, resulting in the formation of infected endocardial vegetations. Earlier studies gave indirect evidence that monocytes play an important role in the procoagulant response during the course of the disease. In this study, we assessed the role of monocytes more directly. We compared weights and tissue factor activities (TFA) of endocardial vegetations of normal rabbits infected with Streptococcus sanguis with those of rabbits which were treated with the cytostatic drug etoposide (Vepesid; Bristol-Myers Squibb B.V.) to induce a selective monocytopenia. Furthermore, the importance of the presence of bacteria was determined through the influence of antibiotic treatment on TFA, vegetational weight, and infection of the vegetations. The TFA of the vegetations was measured chromogenically by monitoring the factor VII-dependent activation of factor X with an amidolytic assay for factor Xa. We found that the degree of infection and the weight of vegetations of rabbits treated with the cytostatic drug etoposide did not differ from that of untreated rabbits. Their TFA, however, was significantly lower than the TFA of vegetations of rabbits not treated with etoposide. We also found that, as with the monocytopenic rabbits, the weight of the vegetations was not reduced in penicillin G-treated rabbits. The degree of infection and TFA, however, were significantly lower. We conclude that monocytes indeed are involved in the activation of the coagulation system during the course of bacterial endocarditis and that the degree of infection is positively correlated to the TFA of the vegetations.