Inhibitory effects of lipoteichoic acid from Staphylococcus aureus on platelet function and platelet-monocyte aggregation

In Vitro Antimicrobial Properties and Their Mechanisms in Relation to Reactive Oxygen Species of Canine Platelet-Rich Fibrin

Platelet-rich fibrin (PRF), which has been shown to promote wound and bone regeneration, has demonstrated antimicrobial properties against periodontal pathogens. However, in veterinary medicine, no study has determined the antimicrobial effects of canine platelet-rich fibrin (cPRF). Therefore, this study aimed to determine the antimicrobial effect of cPRF against E. coli and S. pseudintermedius found in dogs' wounds and against the standard strain S. aureus. Additionally, the mechanism of the existing antibacterial activity of cPRF, which involves the formation of reactive oxygen species (ROS), was tested. Blood samples from six dogs were processed for cPRF. The antimicrobial properties of three groups (growth control, cPRF, and drug control) were evaluated at 0.5, 4, 8, and 24 h using a time-kill assay. The killing mechanisms involving ROS were evaluated using horseradish peroxidase (HRP) to suppress ROS production in PRF (PRF-SR). Subsequently, tests for antimicrobial properties and ROS generation were compared to those of the growth control and cPRF groups. The results showed that cPRF had significant antimicrobial properties against E. coli but no antimicrobial properties against S. pseudintermedius. After the ROS suppression, PRF-SR did not show an antimicrobial property against E. coli. Moreover, cPRF-treated bacteria exhibited significantly greater intracellular ROS than PRF-SR. In conclusion, canine PRF showed an antimicrobial effect against E. coli, and its antibacterial mechanism was related to releasing ROS.

A Champion of Host Defense: A Generic Large-Scale Cause for Platelet Dysfunction and Depletion in Infection

Thrombocytopenia is commonly associated with sepsis and infections, which in turn are characterized by a profound immune reaction to the invading pathogen. Platelets are one of the cellular entities that exert considerable immune, antibacterial, and antiviral actions, and are therefore active participants in the host response. Platelets are sensitive to surrounding inflammatory stimuli and contribute to the immune response by multiple mechanisms, including endowing the endothelium with a proinflammatory phenotype, enhancing and amplifying leukocyte recruitment and inflammation, promoting the effector functions of immune cells, and ensuring an optimal adaptive immune response. During infection, pathogens and their products influence the platelet response and can even be toxic. However, platelets are able to sense and engage bacteria and viruses to assist in their removal and destruction. Platelets greatly contribute to host defense by multiple mechanisms, including forming immune complexes and aggregates, shedding their granular content, and internalizing pathogens and subsequently being marked for removal. These processes, and the nature of platelet function in general, cause the platelet to be irreversibly consumed in the execution of its duty. An exaggerated systemic inflammatory response to infection can drive platelet dysfunction, where platelets are inappropriately activated and face immunological destruction. While thrombocytopenia may arise by condition-specific mechanisms that cause an imbalance between platelet production and removal, this review evaluates a generic large-scale mechanism for platelet depletion as a repercussion of its involvement at the nexus of responses to infection.

Staphylococcus epidermidis lipoteichoic acid: exocellular release and ltaS gene expression in clinical and commensal isolates

PURPOSE

Staphylococcus epidermidis ATCC12228 lipoteichoic acid (LTA) inhibits TNFα production from keratinocytes that are activated with poly I:C. However, this effect has not been proven in clinical or commensal isolates.

METHODOLOGY

The <10 kDa fractions of S. epidermidis isolates from ocular infections (n=56), healthy skin (n=35) and healthy conjunctiva (n=32) were obtained. TNFα production was determined by elisa in HaCaT keratinocytes stimulated with poly I:C and with the <10 kDa fractions. LTA in the cytoplasmic membrane and in the <10 kDa fractions of the isolates was determined during bacterial growth by flow cytometry, Western blot and electrospray ionization mass spectrometry. The expression levels of ugtP, ltaA and ltaS were evaluated.

RESULTS

Two populations of isolates were found: a population that inhibited TNFα production (TNFα-inhibitor isolates) and a population that did not inhibit it (TNFα non-inhibitor isolates). The cells from the TNFα-inhibitor isolates had less LTA in the cytoplasmic membrane compared to the cells from the TNFα non-inhibitor isolates (P<0.05). Similarly, LTA was detected in the supernatants of TNFα-inhibitor isolates, and it was absent in TNFα non-inhibitor isolates. High expression levels of the ugtP and ltaA genes in the 1850I (TNFα-inhibitor isolate) and 37HS (TNFα non-inhibitor isolate) isolates were found during bacterial growth. However, the ltaS gene had a low expression level (P<0.05) in the 37HS isolate.

CONCLUSION

The TNFα-inhibitor isolates release LTA due to high expression of the LTA synthesis genes. By contrast, TNFα non-inhibitor isolates do not release LTA due to low expression level of the ltaS gene.

Platelet Interaction with Innate Immune Cells

Beyond their traditional role in haemostasis and thrombosis, platelets are increasingly recognised as immune modulatory cells. Activated platelets and platelet-derived microparticles can bind to leukocytes, which stimulates mutual activation and results in rapid, local release of platelet-derived cytokines. Thereby platelets modulate leukocyte effector functions and contribute to inflammatory and immune responses to injury or infection. Platelets enhance leukocyte extravasation, differentiation and cytokine release. Platelet-neutrophil interactions boost oxidative burst, neutrophil extracellular trap formation and phagocytosis and play an important role in host defence. Platelet interactions with monocytes propagate their differentiation into macrophages, modulate cytokine release and attenuate macrophage functions. Depending on the underlying pathology, platelets can enhance or diminish leukocyte cytokine production, indicating that platelet-leukocyte interactions represent a fine balanced system to restrict excessive inflammation during infection. In atherosclerosis, platelet interaction with neutrophils, monocytes and dendritic cells accelerates key steps of atherogenesis by promoting leukocyte extravasation and foam cell formation. Platelet-leukocyte interactions at sites of atherosclerotic lesions destabilise atherosclerotic plaques and promote plaque rupture. Leukocytes in turn also modulate platelet function and production, which either results in enhanced platelet destruction or increased platelet production. This review aims to summarise the key effects of platelet-leukocyte interactions in inflammation, infection and atherosclerosis.

Platelet interaction with bacterial toxins and secreted products

Bacteria that enter the bloodstream will encounter components of the cellular and soluble immune response. Platelets contribute to this response and have emerged as an important target for bacterial pathogens. Bacteria produce diverse extracellular proteins and toxins that have been reported to modulate platelet function. These interactions can result in complete or incomplete platelet activation or inhibition of platelet activation, depending on the bacteria and bacterial product. The nature of the platelet response may be highly relevant to disease pathogenesis.