Infective Endocarditis is a microbial infection characterised by the presence of septic vegetations on the surface of the endocardium (Moreillon and Que, 2004). Infection most commonly occurs on the heart valves that has been damaged by congenital defects such as previous disease or trauma (Durack, 1995). As a result these sites have the ability to generate turbulent blood flow which in turn can cause damage to inner most lining of the blood vessels, the endothelium, which causes surface damage leading to exposure of underlying matrix protein (Ruggeri, 2009). Once exposed this highly thrombogenic surface leads to rapid platelet deposition and the formation of a fibrin network. Circulating bacteria from a transient bacteremia in turn binds to this sterile platelet fibrin nidus which allows a secondary accumulation of platelets that encase the bacteria leading to stable thrombus formation (Moreillon and Que, 2004).
Despite improvements in medical and surgical therapy, invasive staphylococcal disease causing infective endocarditis is still associated with a severe prognosis and remains a significant therapeutic challenge. Once a disease primarily affecting younger patients presenting with rheumatic heart disease, modern times see a significant increase in newer ‘at risk’ categories including patients with long term indwelling central venous catheters, patients undergoing haemodialysis and invasive intravascular procedures such as arthroplasty, immunocompromized patients and intraveneous drug abusers (Thuny et al., 2012). Treatment of infective endocarditis usually requires a multidisciplinary approach involving specialists in infectious disease, cardiologists and cardiac surgeons. Current treatment regimes consist of aggressive prolonged antibiotic therapy, frequently combined with surgery (Prendergast and Tornos, 2010, Wilson et al., 2007). Prolonged antibiotic use is often less than successful as 40% of patients relapse within 2 months of finishing clinically effective therapy. Furthermore, prolonged exposure to antibiotics leads to a greater risk of adding to the global problem of multiple antibiotic resistant strains of bacteria. Surgery is a costly and risky alternative, however necessary in up to 47% of patients (Castillo et al., 2000, Murdoch et al., 2009). In many cases surgery is not preferable due to risks associated with cardiac failure, further spread of infection leading to persistent sepsis due to surgical removal of an infected thrombus and/or life threatening embolisation (Jault et al., 1997, Heiro et al., 2000, Thuny et al., 2012, Remadi et al., 2007).
2. The Staphylococcus
3. Platelets play a critical role in thrombosis and haemostasis
Platelets are small anucleate cell fragments of the larger haematopoietic precursor cell, the megakaryocyte (Thon and Italiano, 2010) and are crucial mediators of haemostasis. Platelets have no control over gene expression as they do not possess a nucleus however they have got limited capabilities in translational protein synthesis (Lindemann et al., 2001b). The primary role of platelets in haemostasis is to police the integrity of the endothelium to prevent blood loss (Nieswandt et al., 2009). Platelets circulate close to the endothelial cell surface at high shear as individual entities that ordinarily do not interact with any other cell types. A transition from this resting state to an activated state can be rapidly initiated if platelets are exposed to an appropriate stimulus. Disruption of the endothelial cell lining due to trauma or injury to the vascular endothelium platelets rapidly accumulate at the site of injury (Gawaz et al., 2005). Recruitment is a highly controlled event that is initiated by the adhesive interaction between the exposed extracellular matrix proteins in damaged endothelium and specific membrane receptors on the platelet (Tabuchi and Kuebler, 2008). Collagen (Santoro and Zutter, 1995), vonWillebrand factor (vWf) (Ruggeri, 1999), fibronectin (Savage et al., 1998, Kasirer-Friede et al., 2007) and thrombospondin (Jurk et al., 2003) constitute the exposed matrix proteins at the site of injury. Athough plasma proteins such as fibrinogen/fibrin and vitronectin are not synthesized by endothelial cells they can bind to exposed matrix proteins and increase adhesiveness at the damaged site (Ruggeri et al., 2006, Ruggeri and Mendolicchio, 2007).
Platelets express a vast array of membrane receptors that play a critical role in recognition of matrix proteins. The initial interaction of platelets with the injured vessel wall occurs between GPIbα and immobilised vonWillebrand factor (Chesterman and Berndt, 1986). This interaction initiates the tethering of circulating platelets to the vessel wall. Platelets typically ‘roll’ over the vWf in the direction of flow driven by shear forces experienced by the vasculature (Ruggeri, 2009). A loss of interaction between GPIb and vWf on one side of the platelet leads to the formation of another GPIb-vWf interaction on the other side of the platelet which gives rise to a rolling phenomenon. This rolling mechanism is critical to slowing down the platelet long enough for a second interaction that anchors the platelet to the damaged site. This firm adhesion can be mediated by several membrane receptors, some of which will have become activated as a result of platelet rolling and others who are expressed on the platelet surface as a result platelet activation (Jackson et al., 2009). Once firmly adhered, the platelets rearrange cytoskeletal components which results in filopodia and llamelipodia extension leading to flattening or spreading of the platelet. Platelet spreading is critical following firm adhesion as it firstly allows the platelet withstand the shear forces experienced in the vasculature and secondly it increases the platelet surface area thus covering more of the damaged site.
Following attachment, platelets undergo a series of highly controlled intracellular signalling events that lead to the release reaction where platelets release the contents of its stored intracellular granules. Alpha granules contain proteins such as P-selectin which mediates adhesion of platelets to monocytes, neutrophils and lymphocytes, resulting in the formation of platelet leukocyte complexes (Diacovo et al., 1996a, Diacovo et al., 1996b, Larsen et al., 1989). These granules also contain many chemotactic agents which lead to the recruitment of various inflammatory cells; platelet derived growth factor (PDGF) and 12-hydroxyeicosatetraenoic acid (12-HETE) which recruit neutrophils (Herd and Page, 1994, Mannaioni et al., 1997); platelet factor 4 and platelet derived histamine releasing factor (PDHRF) which recruit eosinophils in airway disease (Brindley et al., 1983, Frigas and Gleich, 1986); PDGF and transforming growth factor β (TGF-β) which recruit monocytes and macrophages and TGF-β which recruits fibroblasts (Deuel et al., 1982, Tzeng et al., 1985, Wahl et al., 1987). Platelet granules also contain several mediators of tissue damage such as oxygen free radicals and hydrolytic enzymes. Dense granules release cationic proteins that initiate vascular permeability and mediators that enhance aggregate formation such as adenosine diphosphate (ADP) and serotonin (5-HT) (Rendu and Brohard-Bohn, 2001). Bioactive amines are also secreted from platelets following activation including Thromboxane A2 (TxA2) and platelet activating factor (PAF) (McIntyre et al., 2003, Patrono et al., 2001).More recently it has been shown that platelet granules contain many antimicrobial peptides such as beta-lysin, platelet microbial protein (PMP), neutrophil activating peptide (NAP-2), released upon activation normal T-cell expressed and secreted (RANTES) and fibrinopeptides A and B (Johnson and Donaldson, 1968, Donaldson and Tew, 1977, Kameyoshi et al., 1992, Yeaman et al., 1997, Krijgsveld et al., 2000).
Once activation is complete the platelet forms a new surface for additional platelets to adhere, predominantly through GPIIb/IIIa crosslinking adjacent platelets through a fibrinogen bridge, resulting in aggregate formation. The final step sees and effective plug at the site of injury that is reinforced by the conversion of fibrinogen to fibrin through the coagulation cascade (Ruggeri et al., 2006).
4. The growing role of platelets in infection and immunity
Platelets are poorly recognised for their role in infection and immunity even though just like professional phagocytes (neutrophils, macrophages and dendritic cells) platelets are derived from the same haematopoietic stem cell, undergo chemotaxis (Clemetson et al., 2000), phagocytose foreign particles (Youssefian et al., 2002), and secrete a multitude of products including inflammatory mediators (Kameyoshi et al., 1992), cytokines (Lindemann et al., 2001a, Antczak et al., 2010) and antimicrobial peptides (Tang et al., 2002, Mercier et al., 2004), all while directing and recruiting several members of the innate immune system to the infected area (Cox et al., 2011, Semple and Freedman, 2010). In addition, toll like receptors (TLR) which are a family of pattern recognition receptors expressed by several professional phagocytes recognise conserved molecular motifs expressed on different classes of infectious agent (Janeway and Medzhitov, 2002, Armant and Fenton, 2002). To date at least 13 TLRs have been described in various immune and nonimmune cells in both human and mice. Recently human platelet have been shown to express TLR1,2,4,6,8 and 9, reinforcing their role as primitive immune cells in host defence (Cognasse et al., 2005, Shiraki et al., 2004, Aslam et al., 2006, Zhang et al., 2009, Garraud and Cognasse, 2010, Andonegui et al., 2005, Keane et al., 2010). More recent studies have also demonstrated that TLRs are also responsible for lipopolysaccharide (LPS)-induced thrombocytopenia (Andonegui et al., 2005, Aslam et al., 2006).
5. Mechanisms of interaction
Bacteria can interact with platelet in two ways, they can either support platelet adhesion or they can induce platelet aggregation. Platelet adhesion to immobilised bacteria is a measure of the strength of the interaction, whereas platelet aggregation induced by bacteria is a measure of the quality of the interaction. In contrast to typical platelet aggregation induced by physiological agonists such as adenosine diphosphate (ADP), collagen or thrombin, bacteria induce an all or nothing response. This means that the bacteria either induce a maximal aggregation or they don’t induce platelet aggregation at all, there is no intermediate response. Another unique feature of bacteria induced platelet aggregation is a distinct pause in time before aggregation takes place. This is typically called the lag time. Increasing the concentration of bacteria shortens the lag time but never eliminates it. The average lag time to platelet aggregation following addition of Staphylococci is between 5-12 minutes. This is in contrast to the lag time observed upon the addition of typical platelet agonists ADP or thrombin which have a lag time less than 10 seconds.
There are 3 main interactions between bacteria and platelets. In the first interaction bacteria express proteins that can directly interact with a surface receptor on the platelet. In this case the bacterial protein express ligand mimetic domains that act as agonists on the platelet receptor thus triggering an intracellular signal that culminates in platelet activation. In the second interaction bacterial proteins bind a plasma protein that is a natural ligand for a platelet receptor. For example, bacteria can bind antibody which in turn bridges the bacteria to the antibody receptor (FcγRIIa) expressed on the platelet. Once engaged the receptor results in the generation of an intracellular signal leading to platelet activation. Finally bacteria may have the ability to secrete products or toxins that in turn activate platelets. Engagement of the product or toxin with a platelet receptor results in activation. These different mechanisms of interaction may help explain the lag time to platelet aggregation. For example, the lag time could be representative of the time taken to trigger a response or bind a plasma protein. A major challenge in studying platelet bacterial interactions is that most bacteria can interact with platelets using multiple mechanisms. This makes it incredibly difficult to identify either the platelet receptors or the bacterial proteins involved in triggering thrombus formation. Moreover not only are the interactions species specific but strain specific as well.
6. Staphylococci interactions with platelets
6.1. Indirect interaction (Released products)
Lipoteichoic acid (LTA) is an essential component of the cell wall of
6.2. Indirect interaction (Cell wall proteins)
There are numerous cell wall proteins expressed on the surface of
More recent studies demonstrated that multiple cell wall proteins expressed on
A critical part of
As discussed in chapter 2 serine rich proteins expressed by viridans streptococci play a critical role in inducing platelet aggregation.
While all of these studies are critical to our understanding of the molecular mechanisms involved in aggregate formation, one must be aware of the relevance of these findings to physiological conditions experienced in the vasculature. For example, almost all of the studies carried out to date have been carried out under non-physiological stirring or using static adhesion assays, neither of which are representative of the conditions experienced in the vasculature. Many reports in the literature in recent times have clearly demonstrated that the local fluid environment in the circulation critically affects the molecular pathways of cell-cell interactions (Varki, 1994). To address this several attempts have been made to create an environment more representative of conditions experienced in the circulation. Rheology is a useful technique that can be employed to shear cells at physiological rates. Using a cone and plate viscometer, Pawar et al. demonstrated that when
6.3. Direct interaction (Cell wall proteins)
A growing concern about studies to date is the apparent lack of contrast with conditions experienced physiologically.
7. Final thoughts and future directions
Infective endocarditis is notoriously difficult to treat as antibiotics are incapable of penetrating the growing thrombus to reach the encased microorganisms. As a result of this the in-hospital mortality rate can be as high as 36% (Botelho-Nevers et al., 2009). Even with treatment, 40% of patients with infective endocarditis relapse within 2 months of finishing clinically effective therapy (Netzer et al., 2002). Furthermore, approximately 25% of patients with infective endocarditis eventually require surgery, usually within 2 years after completing therapy (Olaison and Pettersson, 2003). These statistics reflect the poor delivery and penetration of antibiotic into the growing thrombus. The costs associated with hospitalization (of which the average stay in hospital is 30 days), surgery and prolonged antibiotic treatment is extremely high placing a severe burden on already over-stretched healthcare systems though out the world. The danger of
Potential drug targets identified from studies over the years suggest that blocking the interaction between IgG and platelet FcγRIIa may indeed prevent platelet receptor clustering and thus inhibit thrombus formation. Blockade of the platelet FcγRIIa receptor has distinct advantages over other anti-platelet agents as inhibitors of FcγRIIa do not affect the platelet response to other agonist and therefore does not compromise essential platelet functions.