Effect of pneumococci on blood clotting, platelets, and polymorphonuclear leukocytes

Platelets, Bacterial Adhesins and the Pneumococcus

Systemic infections with pathogenic or facultative pathogenic bacteria are associated with activation and aggregation of platelets leading to thrombocytopenia and activation of the clotting system. Bacterial proteins leading to platelet activation and aggregation have been identified, and while platelet receptors are recognized, induced signal transduction cascades are still often unknown. In addition to proteinaceous adhesins, pathogenic bacteria such as Staphylococcus aureus and Streptococcus pneumoniae also produce toxins such as pneumolysin and alpha-hemolysin. They bind to cellular receptors or form pores, which can result in disturbance of physiological functions of platelets. Here, we discuss the bacteria-platelet interplay in the context of adhesin–receptor interactions and platelet-activating bacterial proteins, with a main emphasis on S. aureus and S. pneumoniae. More importantly, we summarize recent findings of how S. aureus toxins and the pore-forming toxin pneumolysin of S. pneumoniae interfere with platelet function. Finally, the relevance of platelet dysfunction due to killing by toxins and potential treatment interventions protecting platelets against cell death are summarized.

Evaluation of Immature Platelet Fraction in Lower Respiratory Tract Infections: A Retrospective Study

Introduction

Immature platelet fraction (IPF) is a parameter of an automated hematologic analyzer and is related to platelet size and cytoplasmic RNA content. It reflects thrombopoiesis and is often used as the marker of platelet activity. IPF has been evaluated mostly in hematologic disorders and has also been evaluated in patients with gestational hypertension, sepsis, autoimmune diseases and in hospitalised patients with neutrophilia. Platelets, asides from the maintenance of hemostasis, release inflammatory mediators that can modify leukocyte and endothelial responses to various inflammatory stimuli. Lower respiratory tract infections are the leading cause of death from infections worldwide. The role of platelets in lower respiratory tract infections has been reported in many studies. IPF, which is related to platelet activation, has not been evaluated in patients with lower respiratory tract infections.

Methods

The study involved patients who fulfilled the criteria of community-acquired pneumonia (CAP) and aspiration pneumonia (AP). In addition, age and sex-matched healthy controls were involved. Whole blood samples were collected from healthy controls and from the patients on admission. The mean IPF% and C-reactive protein (CRP) levels were measured in patients with CAP, in patients with AP and in healthy controls. The mean IPF% values in patients with infection were compared to mean IPF% values in healthy controls. The mean IPF% values were compared to mean CRP levels in patients with infection. Additionally, the mean IPF% values in patients that died in the first 14 days were compared to the mean IPF% values in patients that were alive. The statistical analysis of data was performed with the Statistical Package for the Social Sciences (SPSS) for Windows, Version 13.0 (SPSS Inc, Chicago, IL). 

Results

The study population consisted of 45 patients (27 patients with CAP and 18 patients with AP), 27 males and 18 females, with a mean age of 72.11 ± 16.4 years and 39 healthy controls, 22 males and 17 females with a mean age of 64.2 ± 14.8 years. The mean CRP levels in patients with infection were 155.2±119.1 mg/dl. The mean IPF% value of patients with infection was 2.76 ± 2.27 and the mean IPF% value of controls was 1.72 ± 0.77 (p < 0.006). The IPF% value in patients with CAP was 2.55 ± 2.02 and in patients with AP 3.07 ± 2.64 (p = 0.595). The mean IPF% value in patients with infection had no linear correlation with CRP value in these patients (r = 0.076, p = 0.62). The mean IPF% value in all patients that died in the first 14 days was 3.75 ± 2.44 and the mean IPF% value in all patients alive was 2.35 ± 2.11 (p = 0.06). The mean IPF% value in patients with CAP who died in the first 14 days of hospitalisation was 5.54 ± 3.17 and in patients with CAP who were alive was 1.87 ± 0.72 (p = 0.06). The mean IPF% value in patients with AP who died was 2.63 ± 0.85 and in patients with AP who were alive was 3.41 ± 3.51 (p = 0.554).

Conclusions

Mean IPF% value is greater in patients with lower respiratory tract infections, including CAP and AP, compared to healthy controls. There is no linear correlation between IPF values and CRP values in patients with lower respiratory tract infections. In addition, there is a difference in mean IPF% value between patients who died in the first 14 days of hospitalisation compared to those who were alive, but not statistically significant.

Understanding Platelets in Infectious and Allergic Lung Diseases

Emerging evidence suggests that platelets, cytoplasmic fragments derived from megakaryocytes, can no longer be considered just as mediators in hemostasis and coagulation processes, but as key modulators of immunity. Platelets have received increasing attention as the emergence of new methodologies has allowed the characterization of their components and functions in the immune continuum. Platelet activation in infectious and allergic lung diseases has been well documented and associated with bacterial infections reproduced in several animal models of pulmonary bacterial infections. Direct interactions between platelets and bacteria have been associated with increased pulmonary platelet accumulation, whereas bacterial-derived toxins have also been reported to modulate platelet function. Recently, platelets have been found extravascular in the lungs of patients with asthma, and in animal models of allergic lung inflammation. Their ability to interact with immune and endothelial cells and secrete immune mediators makes them one attractive target for biomarker identification that will help characterize their contribution to lung diseases. Here, we present an original review of the last advances in the platelet field with a focus on the contribution of platelets to respiratory infections and allergic-mediated diseases.

Review manuscript: Mechanisms of platelet activation by the pneumococcus and the role of platelets in community-acquired pneumonia

There is increasing recognition of the involvement of platelets in orchestrating inflammatory responses, driving the activation of neutrophils, monocytes and vascular endothelium, which, if poorly controlled, may lead to microvascular dysfunction. Importantly, hyperreactivity of platelets has been implicated in the pathogenesis of myocardial injury and the associated particularly high prevalence of acute cardiovascular events in patients with severe community-acquired pneumonia (CAP), of which Streptococcus pneumoniae (pneumococcus) is the most commonly encountered aetiologic agent. In this context, it is noteworthy that a number of studies have documented various mechanisms by which the pneumococcus may directly promote platelet aggregation and activation. The major contributors to platelet activation include several different types of pneumococcal adhesin, the pore-forming toxin, pneumolysin, and possibly pathogen-derived hydrogen peroxide, which collectively represent a major focus of the current review. This is followed by an overview of the limited experimental studies together with a larger series of clinical studies mainly focused on all-cause CAP, which have provided evidence in support of associations between alterations in circulating platelet counts, most commonly thrombocytopenia, and a poor clinical outcome. The final section of the review covers, albeit briefly, systemic biomarkers of platelet activation which may have prognostic potential.

The interaction of bacterial pathogens with platelets

In recent years, the frequency of serious cardiovascular infections such as endocarditis has increased, particularly in association with nosocomially acquired antibiotic-resistant pathogens. Growing evidence suggests a crucial role for the interaction of bacteria with human platelets in the pathogenesis of cardiovascular infections. Here, we review the nature of the interactions between platelets and bacteria, and the role of these interactions in the pathogenesis of endocarditis and other cardiovascular diseases.

Degradation of connective tissue proteins by serine proteases from Streptococcus pneumoniae

The proteolytic activity of pneumococcal culture supernatants was investigated. Phenylmethylsulfonyl fluoride and diisopropylfluorophosphate inhibited the proteolytic activity by 94% indicating that the enzymes are serine proteases. Zymogram analysis with inhibitors utilizing a non-denaturing gelatin substrate gel revealed two classes of serine proteases; one sensitive to calcium chelators and one resistant. Enzymes from the culture supernatant cleaved fibronectin, fibrinogen, elastin, and laminin; whereas bovine albumin, and the human immunoglobulins, IgG, IgM, and IgA, were not cleaved. These results indicate that pneumococci produce previously unrecognized serine proteases that degrade several tissue and blood proteins.

Splenic function: physiology and splenic hypofunction

A wide variety of disorders can result in diminished splenic function. The pathophysiology appears to be clearly defined in some instances, such as congenital asplenia and disorders of splenic vascular obstruction or congestion. In others, such as the autoimmune and GI disorders, the mechanism remains poorly defined. Further research is needed. The hyposplenia which occurs in many of these disorders has been associated with an increased risk of life-threatening, overwhelming bacterial sepsis. In other instances, this complication has not been reported. This certainly should not be interpreted to mean that it cannot occur. The risk of septicemia in hyposplenic disorders is rarely above 10 to 15%. In disorders with minimal inhibition of splenic function, the incidence of sepsis would presumably be less than the 1.5% incidence following surgical splenectomy for trauma. Considering these data, a very large number of patients would have to become asplenic before it would be likely that one would develop sepsis. Furthermore, the lack of awareness of the possibility of hyposplenia-related sepsis in many of these disorders may cause such occurrences to go unrecognized. Finally, since the risk of sepsis is probably less in hyposplenic adults as compared to children, studies on adults may underestimate the incidence of this complication in children. Many of the disorders reported to cause hyposplenia in adults have not been noted to do so in children. In instances such as celiac disease, it may take many years for the complication to manifest so that it would be unlikely for a child to manifest hyposplenia during childhood. However, in other instances, not enough children have been studied to be confident that the hyposplenia and its associated risk of sepsis are not complications that occur in children. Hyposplenia-related bacterial septicemia is a catastrophic complication. If a patient develops a disorder that is potentially associated with hyposplenia, the patient should be observed for signs of asplenia in the peripheral blood. If the technique is available, quantitation of red cell pits should be performed. If not, other studies of splenic function such as radionuclide scans should be considered, depending on the incidence of hyposplenia in that particular disorder. If evidence of asplenia develops, pneumococcal vaccine should be administered, penicillin prophylaxis should be considered, significant febrile episodes should be managed aggressively, and probably most importantly, the patient and family should be carefully educated about this complication. Most deaths from hyposplenia-related septicemia are preventable.

Mechanism for nonspecific immunity of Listeria monocytogenes in rats mediated by platelets and the clotting system

A proposed mechanism for nonspecific immunity to Listeria monocytogenes in rats based on the existence of an activatable lysin is described. Using a deoxyribonucleic acid release assay, we found lysin activity in serum made from whole blood but not in serum made from platelet-free plasma. Washed platelets and platelet lysates exhibited only partial activity as compared with that in serum. This activity was amplified by the addition of platelet-free plasma serum. The activity of the lysin was unaffected by heparin, dialysis, a serine esterase inhibitor, or heating to 56 degrees C for 30 min. Effective inhibitors were ethylenediaminetetraacetic acid and stronger heating (to 65 degrees C). Listeria organisms were found to reduce the recalcified clotting time to platelet-rich plasma in a dose-dependent fashion, indicating that the organisms can exhibit procoagulant activity. The susceptibility of rats to Listeria infection was enhanced by anticoagulant treatment. Rats were infected with Listeria organisms with and without administration of heparin. Heparin-treated rats developed bacteremia, and some died. None of the control rats developed bacteremia or died. These results suggest that natural immunity to Listeria infection is partly due to a platelet-dependent lysin which is activated during clotting and is, in turn, promoted by the Listeria organisms themselves.

Effect of warfarin on the induction and course of experimental Staphylococcus epidermidis endocarditis

The effect of warfarin treatment on an experimental Staphylococcus epidermidis endocarditis was studied. Warfarin was found to affect both the induction and course of the infection of catheter-induced endocardial vegetations. In warfarin-treated rabbits, larger bacterial inocula were needed to induce an infection, and the degree of infection of the vegetations was also significantly lower, eventually resulting in the total elimination of the bacteria from the vegetations. Thus, warfarin treatment seems to have an inhibitory effect on the induction and development of an S. epidermidis infection of the endocardium. The results differ from previous findings in studies done with Streptococcus anguis, where warfarin was found to have no effect on the induction or course of the infection of endocardial vegetations, which suggests that different mechanisms are involved in the pathogenesis of endocarditis caused by these two species of bacteria.