Platelets, Bacterial Adhesins and the Pneumococcus

Abstract

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.

Keywords: MSCRAMMs; Staphylococcus aureus; Streptococcus pneumoniae; platelet activation; platelet killing; pneumolysin; pore formation; surface proteins; toxin.

Figure 1

Scheme illustrating different platelet functions in the immune response. Platelets sense and bind invading bacteria and injured endothelium, resulting in platelet activation. Upon activation, platelets release chemokines and cytokines such as RANTES, triggering leukocyte recruitment and PMVs acting on gene expression of monocytes and monocyte (MC)-derived cells such as dendritic cells (DC). In addition, neutrophils are attracted, and NET formation occurs at the site of infection via platelet-dependent mechanisms. Created with BioRender.com (accessed on 21 March 2022).

Figure 2

Binding of S. pneumoniae (blue) to platelets (red). Scanning electron microscopy of single platelets (upper left), single pneumococci (bottom left), and platelets incubated with the pneumococcal TIGR4 strain for 1 h (right). The right image shows binding of pneumococci to platelets. In addition, pneumolysin pores are formed in platelet membranes (arrows), and released granule content is visible (circle).

Figure 3

Binding of bacteria to platelets occurs either directly or indirectly. Bacterial adhesins with specific repeating units can utilize ECM proteins such as fibronectin (Fn), fibrinogen (Fg), TSP-1, or vWF as molecular bridges to bind to, e.g., integrin αIIβ3 or other complexes of glycoproteins. Furthermore, some bacterial factors can directly bind to integrins, TLRs, or other platelet surface proteins. Bacteria already covered by IgGs are recognized by FcγRIIa. Created with BioRender.com (accessed on 21 March 2022).

Figure 4

Scheme of different interactions between platelets and pneumococci. Individual pneumococcal surface proteins can induce direct activation of platelets. On the other hand, the intracellular pneumolysin (Ply) kills platelets by extensive pore formation in platelet membranes, as shown by the SEM image of a Ply-treated platelet on the right side. Pneumolysin is released in the circulation upon autolysis of pneumococci, and its action on platelets can be neutralized by the addition of pharmaceutical IgG preparations. Created with BioRender.com (accessed on 21 March 2022).

Figure 5

Individual pneumococcal proteins and pneumococcal lysates directly activate human platelets. Washed platelets of a defined set of donors were incubated with different concentrations of pneumococcal proteins (A) (Table 1) for 30 min or pneumococcal lysates (B) with the indicated genetic backgrounds for 60 min at 37 °C. CD62P was used as an activation marker and was detected by flow cytometry, using a PE-Cy5-labelled P-selectin antibody. PBS was used as negative control, and 20 µM TRAP-6 was used as a positive control. The data are presented as geometric mean of fluorescence intensity (GMFI) of positive gated events multiplied with the percentage of positive gated events in the dot plots.