The thrombotic potential of oral pathogens

Thrombus Aspirates From Patients With Acute Ischemic Stroke Are Infiltrated by Viridans Streptococci

BACKGROUND

Acute ischemic stroke may be due to embolism from ruptured atherosclerotic carotid arteries. DNA of oral bacteria, mainly the viridans streptococci group, has been detected in thrombus aspirates of patients with ischemic stroke as well as in carotid endarterectomy samples. Because viridans streptococci are known to possess thrombogenic properties, we studied whether their presence in thrombus aspirates and in carotid artery specimens can be confirmed using bacterial immunohistochemistry.

METHODS AND RESULTS

Thrombus aspirates from 61 patients with ischemic stroke (70.5% men; mean age, 66.8 years) treated with mechanical thrombectomy, as well as carotid endarterectomy samples from 20 symptomatic patients (65.0% men; mean age, 66.2 years) and 48 carotid artery samples from nonstroke autopsy cases (62.5% men; mean age, 66.4 years), were immunostained with an antibody cocktail against 3 species (Streptococcus sanguinis, Streptococcus mitis, and Streptococcus gordonii) of viridans streptococci. Of the thrombus aspirates, 84.8% were immunopositive for viridans streptococci group bacteria, as were 80.0% of the carotid endarterectomy samples, whereas immunopositivity was observed in 31.3% of the carotid artery samples from nonstroke autopsies. Most streptococci were detected inside neutrophil granulocytes, but there were also remnants of bacterial biofilm as well as free bacterial infiltrates in some samples.

CONCLUSIONS

Oral streptococci were found in aspirated thrombi of patients with acute ischemic stroke as well as in carotid artery samples. Our results suggest that viridans streptococci group bacteria may play a role in the pathophysiology of ischemic stroke.

Association between Oral Pathology, Carotid Stenosis, and Oral Bacterial DNA in Cerebral Thrombi of Patients with Stroke

Methods

Thrombus aspirates and control arterial blood were taken from 71 patients (70.4% male; mean age, 67.4 years) with acute ischemic stroke. Tooth pathology was registered using CT scans. Carotid stenosis was estimated with CTA and ultrasonography. The presence of bacterial DNA from aspirated thrombi was determined using quantitative PCR. We also analyzed the presence of these bacterial DNAs in carotid endarterectomies from patients with peripheral arterial disease.

Results

Bacterial DNA was found in 59 (83.1%) of the thrombus aspirates (median, 8.6-fold). Oral streptococcal DNA was found in 56 (78.9%) of the thrombus aspirates (median, 5.1-fold). DNA from A. actinomycetemcomitans and P. gingivalis was not found. Most patients suffered from poor oral health and had in median 19.0 teeth left. Paradoxically, patients with better oral health had more oral streptococcal DNA in their thrombus than the group with the worst pathology (p = 0.028). There was a trend (OR 7.122; p = 0.083) in the association of ≥50% carotid artery stenosis with more severe dental pathology. Oral streptococcal DNA was detected in 2/6 of carotid endarterectomies.

Conclusions

Stroke patients had poor oral health which tended to associate with their carotid artery stenosis. Although oral streptococcal DNA was found in thrombus aspirates and carotid endarterectomy samples, the amount of oral streptococcal DNA in thrombus aspirates was the lowest among those with the most severe oral pathology. These results suggest that the association between poor oral health and acute ischemic stroke is linked to carotid artery atherosclerosis.

Oral Bacterial Signatures in Cerebral Thrombi of Patients With Acute Ischemic Stroke Treated With Thrombectomy

Background

Chronic infections have been reported to be risk factors for both coronary heart disease and ischemic stroke. DNA of oral bacteria, mainly from the viridans streptococci group, has been detected in coronary thrombus aspirates of myocardial infarction and cerebral aneurysms. Viridans streptococci are known to cause infective endocarditis and possess thrombogenic properties. We studied the presence of oral bacterial DNA in thrombus aspirates of patients with acute ischemic stroke treated with mechanical thrombectomy.

Methods and Results

Thrombus aspirates and arterial blood were taken from 75 patients (69% men; mean age, 67 years) with acute ischemic stroke. The presence of Streptococcus species, mainly the Streptococcus mitis group, belonging to viridans streptococci as well as Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans in samples were determined using a quantitative polymerase chain reaction with specific primers and probes. The relative amount of bacterial DNA in a sample was determined with the comparative threshold cycle method. Bacterial DNA was detected in 84% (n=63) of aspired thrombi, and 16% (n=12) of samples were considered bacterial DNA negative. DNA of Streptococcus species, mainly the S mitis group, was found in 79% (n=59) of samples. The median relative amount of Streptococcus species DNA was 5.10‐fold higher compared with the control blood samples from the same patients. All thrombi were negative for both P gingivalis and A actinomycetemcomitans.

Conclusions

This is the first study showing the common presence of bacterial DNA from viridans streptococci in aspired thrombi of patients with acute ischemic stroke. Streptococcal bacteria, mostly of oral origin, may contribute to the progression and thrombotic events of cerebrovascular diseases.

Porphyromonas gingivalis lipopolysaccharide activates platelet Cdc42 and promotes platelet spreading and thrombosis

BACKGROUND

Periodontitis confers an increased risk for cardiovascular diseases, including thrombosis. However, the molecular mechanisms that potentially link periodontitis with thrombosis are undefined. Here we test the hypothesis that Gram-negative periodontal infection promotes pathological platelet activation and amplifies shape change. We focus specifically on lipopolysaccharide (LPS) signaling to platelets.

METHODS

Platelets were isolated from blood samples and allowed to spread on coverslips in the presence or absence of LPS purified from the periodontal pathogen Porphyromonas gingivalis. Platelets were fixed and stained with Alexa-488-phalloidin to label the actin cytoskeleton. The degree of platelet spreading and shape change was quantified by confocal microscopy. In a translational pilot study, blood samples were obtained from human subjects exhibiting generalized severe periodontitis (SP) or healthy periodontium (HP). Rotational thromboelastometry was used to quantify the rate of clot formation via the intrinsic coagulation pathway.

RESULTS

LPS-treated platelets exhibited significantly (P < 0.05) greater spreading and higher numbers of actin-rich filopodia (cell extensions) than controls. We also found that LPS stimulation of platelets promoted the activation of Cdc42, the small GTPase responsible for filopodia formation. Exposure of whole blood samples to LPS significantly (P < 0.05) reduced clotting times. Blood from SP patients clotted significantly (P < 0.05) more rapidly and exhibited shorter partial thromboplastin times compared with HP controls.

CONCLUSIONS

This is the first study to suggest a mechanism by which LPS stimulation drives Cdc42 activation and platelet spreading. These data are consistent with the notion that periodontitis promotes accelerated clot formation and an increased risk of thrombosis.

Bacteria-induced intracellular signalling in platelets

Multiple studies have now shown that various species of bacteria can stimulate platelets; many in a strain and donor-dependent manner. The signalling pathways underlying this platelet activation has been the subject of scrutiny for the last decade. The best-delineated pathway is that in response to Streptococcal species, such as Streptococcus sanguinis (S. sanguinis), Streptococcus gordonii (S. gordonii) and Streptococcus oralis (S. oralis), where a pathway is initiated by the engagement of the low affinity IgG receptor, FcγRIIA. This leads to and involves the tyrosine kinase Syk, the adaptor protein Linker of Activated T Cells (LAT) and subsequently both phospholipase Cγ2 (PLCγ2) and phosphatidylinositol-3-kinase (PI-3-K). Finally, this leads to the expression of the αIIbβ3 integrin, the synthesis and release of thromboxane A2 (T × A2) and the exocytosis of PF4, each of which plays a crucial role in secondary signalling and full platelet activation. Roles for other signalling pathways in Streptococcal-induced platelet activation are less clear, although an ADP-mediated inhibition of adenylyl cyclase, a glycoprotein Ib/IX/V-mediated pathway and perhaps a complement-induced pathway have each been proposed. Platelet activation by Porphyromonas gingivalis (P. gingivalis) at least partially shares the FcγRIIA/Syk/PLCγ2/PI-3-K mechanism utilised by Streptococcal species. However, it has also been suggested that P. gingivalis activates platelets by two additional methods; stimulation of the protease-activated receptors leading to activation of phospholipase Cβ (PLCβ), and the engagement of Toll-like receptors 2 and 4 by released lipopolysaccharide leading to an ill-defined pathway which may involve PI-3-K. Consequently, it appears that bacteria can stimulate platelets by eliciting multiple signalling pathways some of which are common, and some unique, to individual species.

Platelet activation by Streptococcus sanguinis is accompanied by MAP kinase phosphorylation

There is increasing interest in the role of infections in atherothrombotic conditions. In particular, bacteria, notably those of oral origin, have been shown to activate platelets using a variety of mechanisms. Previous studies have shown that S. sanguinis strain 2017-78 induces platelet aggregation which requires the presence of both vWF and IgG. This aggregation is accompanied by the consecutive phosphorylation/desphosphorylation/rephosphorylation of several signalling proteins. The first two phases are thromboxane-dependent whereas the rephosphorylation phase is mediated by engagement of the αIIbβ3 integrin. Here signalling events, specifically the potential role of MAP kinases, associated with S. sanguinis strain 2017-78-induced platelet activation have been further examined using an immunoblotting approach. The addition of S. sanguinis strain 2017-78 caused a similar triphasic phosphorylation profile of the platelet MAP kinase Erk2 to that seen with other phosphoproteins. Pretreatment with aspirin or RGDS did not affect 2017-78-induced Erk2 phosphorylation or desphosphorylation but both inhibited the rephosphorylation phase. In contrast the level of 2017-78-induced platelet MAP kinase p38 phosphorylation remained at an elevated level, and this was unaffected by aspirin. Similarly, 2017-78-induced cPLA(2) phosphorylation remained above basal levels during the aggregation process. The p38 inhibitor SB203580 inhibited S. sanguinis-induced aggregation with no effect on the phosphorylation of either p38 or cPLA(2). Thus the current study demonstrates the activation of both the Erk2 and p38 forms of MAP kinases, and of cPLA(2), in platelets stimulated with S. sanguinis strain 2017-78, and is consistent with a role for Erk2, but not for p38, in the cPLA(2) phosphorylation in response to S. sanguinis.