Platelet interactions with viruses and parasites

Abstract While the interactions between Gram-positive bacteria and platelets have been well characterized, there is a paucity of data on the interaction between other pathogens and platelets. However, thrombocytopenia is a common feature with many infections especially viral hemorrhagic fever. The little available data on these interactions indicate a similarity with bacteria-platelet interactions with receptors such as FcγRIIa and Toll-Like Receptors (TLR) playing key roles with many pathogens. This review summarizes the known interactions between platelets and pathogens such as viruses, fungi and parasites.


Introduction
The role of platelets as part of the innate immune system has become well established, due to their role in responding to infection [1,2]. Central to this is the ability of platelets to bind to pathogens and either kill them or clear them from the circulation [3]. This is not a novel concept and even in the early 1960's there were reports of bacteria toxins activating platelets. Probably the earliest report of a pathogen activating platelets was by  where they showed platelet aggregation and secretion in response to zymosan (yeast-derived surface glucan) [4]. This was then expanded to include bacteria, especially the oral bacterial Streptococcus sanguinis by a series of studies from the Herzberg group [5] and the Douglas group [6]. Then, in a series of papers from our group, we dissected the mechanism of platelet aggregation induced by Staphylococcus aureus [7]. Subsequently, there has been a rapid increase in papers on bacteria-platelet interactions. Most of the research have focused on the interaction between platelets and Gram-positive bacteria although there have been a number of studies on the Gramnegative Helicobacter pylori [8]. Despite the earliest study reporting on yeast-induced platelet aggregation there is a paucity of studies on the interaction of pathogens other than bacteria with platelets. In this review we will focus on what is known about the interaction of platelets with non-bacterial pathogens.
The mechanisms that facilitate platelet-bacteria interactions can generally apply to other pathogens ( Figure 1 and Table 1). Thus, platelets become activated either by an interaction with the pathogen or by a product secreted by the pathogen. Pathogens can either directly bind to platelets or the binding can be facilitated by the pathogen binding a platelet-binding plasma protein [1,9,10].
Pathogen-platelet interactions are facilitated by receptors on the platelet surface. As part of the innate immune system, platelets express many different receptors that are involved in immune function rather than thrombosis. Thus, platelets express pathogen-recognition molecules such as Toll-Like Receptors (TLRs), especially TLR2 [11], TLR4 [12] and TLR7 [13] as well as DC-SIGN [14]. They also express FcgRIIa, a receptor for the Fc-portion of IgG, which is typically expressed on phagocytic cells [15] (there is evidence of limited phagocytic ability for platelets [16][17][18][19]) and that has been identified on platelets and found to be functional [20]. While both immune-mediated and hemostasis-mediated platelet activation results in activated platelets, the platelet response is quite different in both cases [21]. For the majority of bacteria that activate platelets, binding of IgG is required. This binds to FcgRIIa on the platelet surface triggering platelet activation [22].

Platelets in viral infection
Thrombocytopenia and in severe cases disseminated intravascular coagulation (DIC) are not uncommon in viral infection. While viral hemorrhagic fever typically comes to mind in the context of virus-associated coagulopathy, other virus can have effects on platelet function.
Dengue hemorrhagic fever is unusual as it typically occurs in response to a secondary infection with the primary infection producing relatively minor flu-like symptoms. In fact, there are four serotypes of DENV and it is infection with a second serotype that leads to dengue hemorrhagic fever. This suggests that the presence of anti-DENV antibodies is necessary for DHF to occur and this process is known as antibody-dependent enhancement (ADE) [33,34]. These antibodies have been shown to enhance virus uptake and replication through an interaction with Fc receptors [34][35][36][37]. However, just as antibody binding to bacteria can trigger platelet activation, it is likely that antibody binding to DENV will also activate platelets in an FcgRIIa-dependent manner. This platelet activation has been shown to lead to enhanced endothelial permeability [38]. Bone marrow infection occurs in animal models of Dengue [39] which could lead to thrombocytopenia due to impaired platelet production.

Filoviruses
These are primarily represented by Ebola and Marburg viruses both of which cause very severe VHF and are transmitted by fruit bats. Very little is known about the pathogenesis of filovirus-induced VHF although not surprisingly there is evidence of platelet activation [40,41]. Sudan virus (SUDV) has been shown to be associated with an increase in von Willebrand factor (vWf) levels which is associated with poor outcome as well as hemorrhagic presentation [42].

Bunyaviruses
This family includes the Phleboviruses (Rift Valley fever virus and severe fever with thrombocytopenia syndrome virus), Nairoviruses (Crimean-Congo Hemorrhagic fever, CCHF) and Hantaviruses (Hantaan virus). Thrombocytopenia and an increase in mean platelet volume are associated with some bunyavirus infections and are predictors of poor outcome [43][44][45]. Direct binding to platelets was demonstrated [46]. The use of steroids and IVIg was found to be successful in the treatment of CCHF in twelve patients [47] and as IVIg has been shown to act as an inhibitor of FcgRIIa [48] it suggests a potential role for FcgRIIa in CCHF.

Arenaviruses
These include the Old World Lassa fever virus and lymphocytic choriomeningitis virus (LCMV) and the New World Junin, Guanarito, Machupo and Sabia viruses [49]. Although there is evidence that partial platelet depletion increases disease severity in LCMV infection, little is known about the pathogenesis of Arenavirus VHF [50][51][52].

Other viral infections
Thrombocytopenia is a feature of infection with non-VHF viruses where the extent of the coagulopathy is not so severe. Coxsackieviruses B virus has been shown to directly infect platelets which appears to be protective as thrombocytopenic mice have a higher mortality rate [53]. Encephalomyocarditis virus activates platelets via TLR-7 [13] and cytomegalovirus binds to platelet TLR-2 [54]. Both influenza A H 1 N 1 [55,56] and Dengue virus [20] form immune complexes that can trigger platelet activation in a FcgRIIa-dependent manner. Adenovirus Type 3 enhanced ADP-induced platelet activation [57] as does  DC-SIGN Dengue [20] HIV 1 [62] Toll-Like Receptors (TLR) Encephalomyocarditis virus [13] Cytomegalovirus [54] Fungi [123] CLEC-2 HIV 1 [63] GPIV Plasmodium falciparum [89][90] Hepatitis B infection [58]. Co-infection of influenza H1N1 and S. aureus greatly increased the chances of developing DIC [59]. Platelets interact with neutrophil extracellular traps (NETs) and facilitate their ability to neutralize poxvirus [60]. In hepatitis there is evidence that platelet are being sequestered to the liver which may play a significant role in hepatitis-associated thrombocytopenia [61].

HIV
Platelet DC-SIGN is implicated in the binding of HIV-1 [62] in conjunction with CLEC-2 [63]. This leads to secretion of CXCL4, which prevents HIV-1 infection of neighboring T-cells [64] as well as secretion of other pro-inflammatory factors such as CD40L [65]. HIV-1 infection increases platelet-monocyte interactions which is associated with neuro-inflammation [66]. This results in monocyte activation and enhanced levels of extravasation especially in the brain microvasculature. It is proposed that this may play a role in the cognitive decline seen in AIDS [66][67][68]. The enhanced platelet activity seen in HIV patients can be reduced by treatment with aspirin [69] or with anti-retroviral therapy [70]. The enhanced inflammation and endothelial cell activation seen in HIV [71] has been shown to persist even being present 12-years after anti-retroviral therapy [72]. HIV-1-derived Tat has been shown to directly bind to platelets and activate them in a process dependent on CCR3 and b 3 -integrins [73].

Conclusion
The platelet response to viral infection has many of the similarities of the response to bacterial infection. The primary purpose of this is host-defense and in this context there is evidence that platelets act as sponges to absorb the viruses, which are then cleared from the circulation with the activated platelets. However, thrombocytopenia can also arise from infection of the endothelium, which binds platelets and removes them from the circulation. There is evidence of multiple effects on platelets with each virus as is the case with bacterial interactions with platelets [1,10,74].

Parasites and fungi
Thrombocytopenia can also occur with both parasite and fungal infections. Malaria has attracted most of the attention in this area [75].

Malaria
Malaria is a mosquito-borne parasite infection (Plasmodium), which is transmitted to humans through the Anopheles mosquito. Malaria is a major cause of morbidity and mortality in the developing world with 207 million cases of malaria in 2012 and an estimated 627 000 deaths, mostly children under five and pregnant women who live in Sub-Saharan Africa (WHO 2013). Malaria is caused by infection with Plasmodium falciparum, P. vivax, P. ovale, P. malariae or P. knowlesi. Most of the deaths are due to infection by P. falciparum as it is the most severe infection as well as the dominant infection. Plasmodium bergei and P. chabaudi infect mice and are frequently used for animal studies. Most cases of malaria present as uncomplicated malaria with characteristic symptoms of fever, nausea and aches, however, some can present with severe malaria that involves impaired function of various organs. The most serious form of severe malaria is cerebral malaria, which is estimated to occur in 10% of hospitalized cases and is associated with 80% of deaths. Cerebral malaria occurs when infected red blood cells (RBCs) occlude cerebral blood vessels [76].

Thrombocytopenia in malaria
Thrombocytopenia is a common feature in malaria [77,78] and is a predictor of outcome [79][80][81]. In fact, it is considered to be diagnostic in suspect febrile patients [82,83]. There is evidence in patients with malaria that thrombocytopenia is due to platelet activation and could also be related to direct interaction with the parasites [84,85]. Platelets have been shown to be involved in clumping of parasitized red cells [86] and they have been found to accumulate in the brains of patients with cerebral malaria [86,87].

Plasmodium and platelets interaction
GPIV (CD36) is a glycosylated protein [88] present in platelets and other cells such as macrophages, dendritic cells, adipocytes, muscle and some types of endothelial cells, and is a cell receptor for P. falciparum-infected erythrocytes [89,90] although other receptors may be involved [80,91]. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is the protein that binds to CD36 [92]. While PfEMP1 can directly bind to CD36 and a number of CD36 ligands can induce platelet aggregation, there is no evidence that PfEMP1 triggers platelet activation. However, P. falciparum does trigger clumping of infected erythrocytes that is mediated by platelets in a CD36-dependent manner [80], and thus it is possible that this may be due to PfEMP1-CD36 mediated platelet activation.

Role of the endothelium
The endothelium plays an important role in the pathogenesis of malaria. The clumped RBC's bind to the endothelium and can ultimately occlude smaller blood vessels, especially in the brain. Activated endothelium is a key component of cerebral malaria and has been shown to occur in children [93]. Overproduction of cytokines plays a major role in the activation of the endothelium [94,95]. One of the key cytokines involved is TNF which is produced by macrophages in response to malaria antigens [96], possibly acting on TNFR2 [97]. Platelets play a significant role in the destruction of TNF-activated endothelial cells [98][99][100] while TGFb 1 released from activated platelets can kill TNF-activated endothelial cells [101]. A recent model has been proposed that draws together many of these observations in malaria. Activated endothelial cells secrete high molecular weight vWf, which form strings under high shear. Platelets bind to these strings, which also bind to the activated endothelial cells. Infected RBCs can then in turn bind to the immobilized platelets ultimately occluding the blood vessel which if in the cerebral micro-circulation leads to cerebral malaria [102].

Platelets as a double-edged sword
The role of platelets in malaria is complex. On one hand, platelet activation is critical in mediating the binding of infected RBCs to the endothelium and subsequent aggregate formation that is the cause of cerebral malaria and animal studies suggest that blocking platelet function in mice is beneficial [95,103,104]. On the other hand, platelets as part of the innate immune system play an important role in mediating the initial immune response to infection. Studies suggest that platelets play a beneficial role in malaria [105,106] and mice rendered thrombocytopenic prior to infection had a much higher mortality rate compared with normal mice and thrombocytopenia only occurred in normal rats and not in splenectomized although mortality was much higher in splenectomized rats [105,107]. Platelets also have been shown to be cytotoxic to plasmodium [108] and that platelet factor 4 (CXCL4) plays a key role in mediating this cytotoxicity [109,110].

Other parasites
Schistosomes are trematodes and a major pathogen that causes over 200 million cases of schistosomiasis per year. Thrombocytopenia is a common symptom of infection with Schistosoma mansoni primarily due to platelets gathering in the spleen [111]. Induction of thrombocytopenia prior to infection has been shown to significantly increase S. mansoni growth and platelets have been shown to bind to and kill the schistosomes [112,113].
Trypanosoma cruzi infection causes Chagas disease (a cardiac disease) [114] and is associated with thrombocytopenia [115], thromboembolism leading to stroke [114] and increased levels of platelet-leucocyte complexes associated with cerebral microvasculopathy [116]. Interestingly pentamidine which is often used to treat Chagas disease is also an anti-platelet agent (GPIIb/IIIa antagonist) [117] and T. cruzi binds fibronectin which could facilitate an interaction with platelets [118]. Trans-sialidase secreted from the trypanosome has been implicated in Chagasassociated thrombocytopenia as it cleaves sialic acid residues from platelets, which are then cleared from the circulation by Kupffer cells [119].

Fungi
Platelets are early responders to fungal infection as with any other pathogen and their activation is designed to inhibit fungal growth [120]. Thrombocytopenia is known to occur in response to fungal infections [121] such as with Candida albicans [122]. Platelets can bind directly to fungi and become activated or the fungi can secrete platelet-activating factors and the formation of plateletmonocyte complexes in a TLR-dependent manner [123]. Aspergillus fumigatus secretes a serine protease and gliotoxin both of which can activate platelets [124] although gliotoxin from C. albicans was found to inhibit platelet aggregation [125]. Aspergillus fumigatus also directly activates platelets [126]. Zymaosan induces platelet aggregation and secretion [4] in a complement [127] and antibody/FcgRIIa-dependent manner [128] similar to that seen with bacteria [6,129,130].

Conclusion
Platelets are an integral part of the immune system and are often the first responders to any invading pathogen be it bacteria, virus or parasite. Like many immune cells platelets can bind to pathogens through a direct interaction with pathogen-recognition molecules such as TLRs or DC-SIGN. Alternatively plasma proteins can also bind to the pathogens facilitating platelet binding via receptors such as GPIIb/IIIa and GPIb. Platelet binding to pathogens serves two functions. Firstly, it removes the pathogen from the circulation and reduces the pathogen burden. Pathogen binding to platelets usually leads to platelet activation, which is typically an FcgRIIa-dependent process. Platelet activation leads to the secretion of anti-microbial peptides as well as facilitating clearance from the circulation. Thus, for many pathogens thrombocytopenic animals are more susceptible to infection and have a worse outcome than normal animals.
Some pathogens can overcome the platelet response to infection. Typically these organisms are resistant to the microbicidal effects of platelets and they can replicate faster than the platelets can clear them. In this case two problems arise. The pathogen continues to grow and secondly platelet activation to the pathogen also continues. This excessive platelet activation can lead to disseminated intravascular coagulation such as occurs in an extreme form in VHF or septicemia. At this stage the platelets have become part of the problem and correcting this DIC is critical for survival of the patient. In the case of a chronic infection such as HIV the low-level sustained platelet activation by the virus is a risk factor for cardiovascular disease in these patients.
There is some preliminary evidence that inhibiting this platelet activation with anti-platelet agents may be beneficial in VHF, sepsis and even HIV, however, as they also inhibit platelet activity they may be counter-productive if DIC has become established. A better strategy is to identify the mechanisms involved in the thrombocytopenia and to develop an inhibitor of the pathogenplatelet interaction without compromising platelet function. FcgRIIa is a potential drug target here as it is important in bacteria-platelet interactions and there is evidence that it may play a role in some viral infections. Platelets are a double-edged sword and their important role in the innate immune system suggests that drugs that target the platelet-pathogen interaction should be restricted to patients with evidence of disease progression to ensure that their ability to control the early stages of the disease is not compromised.
While most of the research on platelet-pathogen interactions has focused on Gram-positive bacteria the current Ebola epidemic highlights the importance in understanding the role of platelets in responding to viruses as well. Understanding these interactions may produce new drugs that will help to deal with emerging pathogens such as Ebola, Dengue and other VHFs.