Activated Platelets Mediate Monocyte Killing of Klebsiella pneumoniae.

Platelets are known for essential activities in hemostasis and for their important contribution to protection against infectious pathogens. Klebsiella pneumoniae is an opportunistic pathogen widely known to cause nosocomial infections. Recently, hypervirulent strains of K. pneumoniae have been emerging, which can cause severe infections in immunocompetent individuals. Combined with the increase in antibiotic resistance, it is important to understand how K. pneumoniae affects components of the immune system. We studied the interactions of human platelets with several K. pneumoniae strains (the wild type encapsulated strain, and a nonencapsulated mutant). Thrombin-stimulated whole human and mouse blood significantly inhibited bacterial growth compared to unstimulated whole blood. Furthermore, we investigated the effect of K. pneumoniae on platelet activation. Both strains induced significant increase in activation of both unstimulated and thrombin-stimulated human platelets. Additionally, only the nonencapsulated mutant increased aggregation of platelets in response to ADP. K. pneumoniae killing assays were then performed with washed platelets in the presence or absence of thrombin. Surprisingly, washed platelets failed to exhibit any effects on the growth of K. pneumoniae. We further explored the impact of platelets on monocyte-mediated killing of K. pneumoniae. Importantly, we found that activated platelets significantly enhanced monocyte-mediated killing of K. pneumoniae. This effect was likely due to the formation of platelet-monocyte aggregates in blood upon thrombin stimulation. Overall, this study highlights the role of platelets in mediating a protective response against K. pneumoniae and reinforces the importance of platelets in modulating leukocyte behavior.

Myogenic Cell Expression of Intercellular Adhesion Molecule-1 Contributes to Muscle Regeneration after Injury.

This study investigated intercellular adhesion molecule-1 (ICAM-1), a membrane protein that mediates cell-to-cell adhesion and communication, as a mechanism through which the inflammatory response facilitates muscle regeneration after injury. Toxin-induced muscle injury to tibialis anterior muscles of wild-type mice caused ICAM-1 to be expressed by a population of satellite cells/myoblasts and myofibers. Myogenic cell expression of ICAM-1 contributed to the restoration of muscle structure after injury, as regenerating myofibers were more abundant and myofiber size was larger for wild-type compared with Icam1-/- mice during 28 days of recovery. Contrastingly, restoration of muscle function after injury was similar between the genotypes. ICAM-1 facilitated the restoration of muscle structure after injury through mechanisms involving the regulation of myofiber branching, protein synthesis, and the organization of nuclei within myofibers after myogenic cell fusion. These findings provide support for a paradigm in which ICAM-1 expressed by myogenic cells after muscle injury augments their adhesive and fusogenic properties, which, in turn, facilitates regenerative and hypertrophic processes that restore structure to injured muscle.

Impact of Digestive Inflammatory Environment and Genipin Crosslinking on Immunomodulatory Capacity of Injectable Musculoskeletal Tissue Scaffold.

The paracrine and autocrine processes of the host response play an integral role in the success of scaffold-based tissue regeneration. Recently, the immunomodulatory scaffolds have received huge attention for modulating inflammation around the host tissue through releasing anti-inflammatory cytokine. However, controlling the inflammation and providing a sustained release of anti-inflammatory cytokine from the scaffold in the digestive inflammatory environment are predicated upon a comprehensive understanding of three fundamental questions. (1) How does the release rate of cytokine from the scaffold change in the digestive inflammatory environment? (2) Can we prevent the premature scaffold degradation and burst release of the loaded cytokine in the digestive inflammatory environment? (3) How does the scaffold degradation prevention technique affect the immunomodulatory capacity of the scaffold? This study investigated the impacts of the digestive inflammatory environment on scaffold degradation and how pre-mature degradation can be prevented using genipin crosslinking and how genipin crosslinking affects the interleukin-4 (IL-4) release from the scaffold and differentiation of naïve macrophages (M0). Our results demonstrated that the digestive inflammatory environment (DIE) attenuates protein retention within the scaffold. Over 14 days, the encapsulated protein released 46% more in DIE than in phosphate buffer saline (PBS), which was improved through genipin crosslinking. We have identified the 0.5 (w/v) genipin concentration as an optimal concentration for improved IL-4 released from the scaffold, cell viability, mechanical strength, and scaffold porosity, and immunomodulation studies. The IL-4 released from the injectable scaffold could differentiate naïve macrophages to an anti-inflammatory (M2) lineage; however, upon genipin crosslinking, the immunomodulatory capacity of the scaffold diminished significantly, and pro-inflammatory markers were expressed dominantly.

Stepping Up to the Plate(let) against Candida albicans.

Candida albicans is a pervasive commensal fungus that is the most common pathogen responsible for invasive fungal infection (IFI). With incidence of IFI on the rise due to increasing susceptible populations, it is imperative that we investigate how Candida albicans interacts with blood components. When stimulating either human or mouse whole blood with thrombin, we saw a significant decrease in C. albicans survival. We then repeated Candida killing assays with thrombin-stimulated or unstimulated washed platelets and saw a similar decrease in CFU. To investigate whether killing was mediated through surface components or releasable products, platelets were pretreated with an inhibitor of actin polymerization (cytochalasin D [CytoD]). CytoD was able to abrogate C. albicans killing. Moreover, dilution of releasates from thrombin-stimulated platelets showed that the toxicity of the releasates on C. albicans is concentration dependent. We then investigated C. albicans actions on platelet activation, granule release, and aggregation. While C. albicans does not appear to affect alpha or dense granule release, C. albicans exerts a significant attenuation of platelet aggregation to multiple agonists. These results illustrate for the first time that platelets can directly kill C. albicans through release of their granular contents. Additionally, C. albicans can also exert inhibitory effects on platelet aggregation.

Platelets Mediate Host Defense against Staphylococcus aureus through Direct Bactericidal Activity and by Enhancing Macrophage Activities.

Platelets are the chief effector cells in hemostasis. However, recent evidence suggests they have multiple roles in host defense against infection. Reports by us and others showed that platelets functionally contribute to protection against Staphylococcus aureus infection. In the current study, the capacity of mouse platelets to participate in host defense against S. aureus infection was determined by assessing two possibilities. First, we determined the ability of platelets to kill S. aureus directly; and, second, we tested the possibility that platelets enhance macrophage phagocytosis and intracellular killing of S. aureus In this study we report evidence in support of both mechanisms. Platelets effectively killed two different strains of S. aureus. A clinical isolate of methicillin-resistant S. aureus was killed by platelets (>40% killing in 2 h) in a thrombin-dependent manner whereas a methicillin-sensitive strain was killed to equal extent but did not require thrombin. Interestingly, thrombin-stimulated platelets also significantly enhanced peritoneal macrophage phagocytosis of both methicillin-resistant S. aureus and methicillin-sensitive S. aureus by >70%, and restricted intracellular growth by >40%. Enhancement of macrophage anti-S. aureus activities is independent of contact with platelets but is mediated through releasable products, namely IL-1ß. These data confirm our hypothesis that platelets participate in host defense against S. aureus both through direct killing of S. aureus and enhancing the antimicrobial function of macrophages in protection against S. aureus infection.

Platelets Enhance Dendritic Cell Responses against Staphylococcus aureus through CD40-CD40L.

Staphylococcus aureus is a major human pathogen that can cause mild to severe life-threatening infections in many tissues and organs. Platelets are known to participate in protection against S. aureus by direct killing and by enhancing the activities of neutrophils and macrophages in clearing S. aureus infection. Platelets have also been shown to induce monocyte differentiation into dendritic cells and to enhance activation of dendritic cells. Therefore, in the present study, we explored the role of platelets in enhancing bone marrow-derived dendritic cell (BMDC) function against S. aureus We observed a significant increase in dendritic cell phagocytosis and intracellular killing of a methicillin-resistant Staphylococcus aureus (MRSA) strain (USA300) by thrombin-activated platelets or their releasates. Enhancement of bacterial uptake and killing by DCs is mediated by platelet-derived CD40L. Coculture of USA300 and BMDCs in the presence of thrombin-activated platelet releasates invokes upregulation of the maturation marker CD80 on DCs and enhanced production of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α), interleukin 12 (IL-12), and IL-6. Overall, these observations support our hypothesis that platelets play a critical role in the host defense against S. aureus infection. Platelets stimulate DCs, leading to direct killing of S. aureus and enhanced DC maturation, potentially leading to adaptive immune responses against S. aureus.

Identification of a novel mode of complement activation on stimulated platelets mediated by properdin and C3(H2O).

Elevated numbers of activated platelets circulate in patients with chronic inflammatory diseases, including atherosclerosis and coronary disease. Activated platelets can activate the complement system. Although complement activation is essential for immune responses and removal of spent cells from circulation, it also contributes to inflammation and thrombosis, especially in patients with defective complement regulation. Proinflammatory activated leukocytes, which interact directly with platelets in response to vascular injury, are among the main sources of properdin, a positive regulator of the alternative pathway. The role of properdin in complement activation on stimulated platelets is unknown. Our data show that physiological forms of human properdin bind directly to human platelets after activation by strong agonists in the absence of C3, and bind nonproportionally to surface CD62P expression. Activation of the alternative pathway on activated platelets occurs when properdin is on the surface and recruits C3b or C3(H2O) to form C3b,Bb or a novel cell-bound C3 convertase [C3(H2O),Bb], which normally is present only in the fluid phase. Alternatively, properdin can be recruited by C3(H2O) on the platelet surface, promoting complement activation. Inhibition of factor H-mediated cell surface complement regulation significantly increases complement deposition on activated platelets with surface properdin. Finally, properdin released by activated neutrophils binds to activated platelets. Altogether, these data suggest novel molecular mechanisms for alternative pathway activation on stimulated platelets that may contribute to localization of inflammation at sites of vascular injury and thrombosis.

FcγRIIa ligation induces platelet hypersensitivity to thrombotic stimuli.

Platelets are known for their important role in hemostasis, however their significance in other functions, including inflammation and infection, are becoming more apparent. Patients with systemic lupus erythematosus (SLE) are known to have circulating IgG complexes in their blood and are highly susceptible to thrombotic events. Because platelets express a single receptor for IgG, we tested the hypothesis that ligation of this receptor (FcγRIIa) induces platelet hypersensitivity to thrombotic stimuli. Platelets from SLE patients were considerably more sensitive to thrombin compared to healthy volunteers, and this correlated with elevated levels of surface IgG on SLE platelets. To test whether FcγRIIa ligation stimulated thrombin hypersensitivity, platelets from healthy volunteers were incubated with buffer or heat-aggregated IgG, then stimulated with increasing concentrations of thrombin. Interestingly, heat-aggregated IgG-stimulated platelets, but not buffer-treated platelets, were hypersensitive to thrombin, and hypersensitivity was blocked by an anti-FcγRIIa monoclonal antibody (mAb). Thrombin hypersensitivity was not due to changes in thrombin receptor expression (GPIbα or PAR1) but is dependent on activation of shared signaling molecules. These observations suggest that ligation of platelet FcγRIIa by IgG complexes induces a hypersensitive state whereby small changes in thrombotic stimuli may result in platelet activation and subsequent vascular complications such as transient ischemic attacks or stroke.

Immunity to pathogenic mucosal C. albicans infections mediated by oral megakaryocytes activated by IL-17 and candidalysin.

The fungus Candida albicans can cause mucosal infections including oropharyngeal candidiasis (OPC) in immunocompromised patients. In humans, an increased risk of fungal infections correlates with thrombocytopenia. However, our understanding of platelets and megakaryocytes (Mks) in mucosal fungal infections is almost entirely unknown. When megakaryocyte- and platelet-depleted mice were infected with OPC, the tongue showed higher fungal burden, due to decreased neutrophil accumulation. Protection depended on a distinct population of oral-resident Mks. Interleukin-17, important in antifungal immunity, was required since mice lacking the IL-17 receptor had decreased circulating platelets and their oral Mks did not expand during OPC. The secretion of the peptide toxin candidalysin activated human Mks to release platelets with antifungal capacity. Infection with a candidalysin-deficient strain resulted in decreased expansion of tongue Mks during OPC. This is the first time that a distinct megakaryocyte population was identified in the oral mucosa which is critical for immunity against fungal infection.

FcγRIIa requires lipid rafts, but not co-localization into rafts, for effector function.

OBJECTIVE: To determine if receptor localization into lipid rafts, or the lipid rafts themselves, are important for FcγRIIa effector functions. MATERIAL: Wild-type FcγRIIa or mutant FcγRIIa(C208A) that does not translocate to lipid rafts were transfected into Chinese hamster ovary (CHO) cells which have been shown to be reliable cells for studying FcγR function. TREATMENT: Cells were treated with buffer or methyl-ß-cyclodextrin (MßCD) to deplete cholesterol and dissolve the structure of lipid rafts. METHODS: To evaluate lipid raft association, transfected CHO cells were lysed and centrifuged over a sucrose gradient. Fractions were run on SDS-PAGE and blotted for FcγRIIa or sphingolipid GM1 to illustrate the lipid raft fractions. Lateral mobility of GFP-tagged wild-type or mutant FcγRIIa was assessed using fluorescence recovery after photobleaching (FRAP) microscopy. Internalization of IgG-opsonized erythrocytes was assessed by fluorescence microscopy and uptake of heat-aggregated IgG (haIgG) was measured using flow cytometry. RESULTS: We observed that FcγRIIa(C208A) did not localize into lipid rafts. However, the mutant FcγRIIa retained lateral mobility and effector function similar to wild-type FcγRIIa. However, mutant FcγRIIa function was abolished upon treatment with MßCD. CONCLUSIONS: Lipid rafts provide an essential component required for effector activities independent of receptor localization.

SSRIs: Applications in inflammatory lung disease and implications for COVID-19.

Selective serotonin reuptake inhibitors (SSRIs) have anti-inflammatory properties that may have clinical utility in treating severe pulmonary manifestations of COVID-19. SSRIs exert anti-inflammatory effects at three mechanistic levels: (a) inhibition of proinflammatory transcription factor activity, including NF-κB and STAT3; (b) downregulation of lung tissue damage and proinflammatory cell recruitment via inhibition of cytokines, including IL-6, IL-8, TNF-α, and IL-1ß; and (c) direct suppression inflammatory cells, including T cells, macrophages, and platelets. These pathways are implicated in the pathogenesis of COVID-19. In this review, we will compare the pathogenesis of lung inflammation in pulmonary diseases including COVID-19, ARDS, and chronic obstructive pulmonary disease (COPD), describe the anti-inflammatory properties of SSRIs, and discuss the applications of SSRIS in treating COVID-19-associated inflammatory lung disease.

IgG-complex stimulated platelets: a source of sCD40L and RANTES in initiation of inflammatory cascade.

Platelets are a crucial element in maintenance of hemostasis. Other functions attributable to platelets are now being appreciated such as their role in inflammatory reactions and vascular remodeling. Platelets have been reported to bind immunological stimuli like IgG-complexes and the understanding that platelets may participate in immunological reactions has been speculated for nearly 50years. In previous observations, we demonstrated that platelets could bind and internalize aggregated IgG-complexes without inducing platelet aggregation or granule release. To characterize this observation further, we tested the hypothesis that aggregated IgG-complexes do not activate platelets. To this end, platelets were stimulated with IgG-complexes or thrombin as a positive control and evaluated for activation by aggregation, expression of surface markers and production of cytokines. Activation with thrombin resulted in aggregation, expression of high levels of CD62P (P-selectin) expression and activation of the fibrinogen receptor, alpha(IIb)beta(3). Furthermore, stimulation with thrombin resulted in significant amounts of sCD40L (CD154) and RANTES (CCL5). However, platelets stimulated with IgG-complexes resulted in no aggregation and low levels of CD62P expression. Surprisingly, platelets stimulated with aggregated IgG-complexes released similar amounts of sCD40L and RANTES as platelets activated by thrombin. These data suggest that platelets are capable of secreting inflammatory molecules in response to IgG-complexes.

Differential requirement of lipid rafts for FcγRIIA mediated effector activities.

Immunoglobulin G (IgG) dependent activities are important in host defense and autoimmune diseases. Various cell types including macrophages and neutrophils contribute to pathogen destruction and tissue damage through binding of IgG to Fcγ receptors (FcγR). One member of this family, FcγRIIA, is a transmembrane glycoprotein known to mediate binding and internalization of IgG-containing targets. FcγRIIA has been observed to translocate into lipids rafts upon binding IgG-containing targets. We hypothesize that lipid rafts participate to different extents in binding and internalizing targets of different sizes. We demonstrate that disruption of lipid rafts with 8mM methyl-ß-cyclodextrin (MßCD) nearly abolishes binding (91% reduction) and phagocytosis (60% reduction) of large IgG-coated targets. Conversely, binding and internalization of small IgG-complexes is less dependent on lipid rafts (49% and 17% inhibition at 8mM MßCD, respectively). These observations suggest that differences between phagocytosis and endocytosis may arise as early as the initial stages of ligand recognition.

From Classical to Unconventional: The Immune Receptors Facilitating Platelet Responses to Infection and Inflammation.

Platelets have long been recognized for their role in maintaining the balance between hemostasis and thrombosis. While their contributions to blood clotting have been well established, it has been increasingly evident that their roles extend to both innate and adaptive immune functions during infection and inflammation. In this comprehensive review, we describe the various ways in which platelets interact with different microbes and elicit immune responses either directly, or through modulation of leukocyte behaviors.

Variation of stress levels, burnout, and resilience throughout the academic year in first-year medical students.

Medical student wellness is of great concern in the health care field. A growing number of studies point to increases in suicide, depression, anxiety, mood disorders, and burnout related to physician lifestyles. Mental health issues commencing in medical school have been suggested to have a significant impact on future physician lifestyle and burnout. Tracking the mental health of medical students at the University of Toledo College of Medicine and Life Sciences (UTCOMLS) with standardized indices will help elucidate triggers of poor mental health. Anonymous surveys were developed and distributed to preclinical medical students at five strategic time points throughout the 2018 2019 academic year. Surveys collected basic demographic information as well as inventories measuring perceived stress, burnout, resilience, and mindfulness. 172 M1s (83 males and 89 females) were included in the study and average response rate for the first 4 (out of 5) surveys averaged 74.8%. M1 males and females had on average increased personal burnout over time with females consistently scoring higher. Both males and females had an increase in stress from August to each subsequent month (p<0.05). Females reported a higher level of perceived stress than males in the beginning and middle of the academic year (p<0.05). Both males and females report a gradual decrease in resiliency throughout the academic year. These surveys demonstrated over half of males and females in medical school reported higher perceived stress scores than their gender-matched peers in the general United States population. Our study strengthens documented trends in resiliency, perceived stress, and burnout amongst medical students. More study in designing targeted approaches to ameliorate these findings in the medical student population is warranted.

Platelet-T cell aggregates in lung cancer patients: Implications for thrombosis.

Platelet-leukocyte aggregates (PLAs) are associated with increased thrombosis risk. The influence of PLA formation is especially important for cancer patients, since thrombosis accounts for approximately 10% of cancer-associated deaths. Our objective was to characterize and quantify PLAs in whole blood samples from lung cancer patients compared to healthy volunteers with the intent to analyze PLA formation in the context of lung cancer-associated thrombosis. Consenting lung cancer patients (57) and healthy volunteers (56) were enrolled at the Dana Cancer Center at the University of Toledo Health Science Campus. Peripheral blood samples were analyzed by flow cytometry. Patient medical history was reviewed through electronic medical records. Most importantly, we found lung cancer patients to have higher percentages of platelet-T cell aggregates (PTCAs) than healthy volunteers among both CD4+ T lymphocyte and CD8+ T lymphocyte populations. Our findings demonstrate that characterization of PTCAs may have clinical utility in differentiating lung cancer patients from healthy volunteers and stratifying lung cancer patients by history of thrombosis.

Characterization of a transgenic mouse model of chronic conditional platelet depletion.

BACKGROUND: Platelets are widely recognized for their role in maintaining hemostasis. Recently, platelets have become appreciated for their varying roles in immunity, neuroprotection, and other physiological processes. While there are currently excellent methods to transiently deplete platelets and models of thrombocytopenia, studying the roles of platelets in chronic processes can be challenging. OBJECTIVE: Phenotypic characterization of the PF4-DTR mouse model of conditional platelet depletion compared to antibody depletion. METHODS: We describe the ability of the PF4-DTR mouse to maintain chronic platelet depletion, along with examining the bleeding phenotype compared to antibody-mediated platelet depletion. RESULTS: Systemic administration of diphtheria toxin resulted in >99% platelet depletion that can be maintained for >2 weeks. When compared to an antibody depletion model, PF4-DTR mice showed similar phenotypes when challenged with tail bleed and saphenous vein measurements of hemostasis. Mice depleted with diphtheria toxin were also able to undergo adoptive transfer of platelets. If the frequency and amount of diphtheria toxin is reduced, mice can be maintained at >40% depletion for >28 days, showing that this model is tunable. CONCLUSIONS: When compared to the gold standard of antibody-mediated depletion, PF4-DTR mice showed similar phenotypes and should be considered an important tool for examining the impact of thrombocytopenia over longer periods of time.

Differential kinase requirements in human and mouse Fc-gamma receptor phagocytosis and endocytosis.

Fc gamma receptors (FcgammaRs) contribute to the internalization of large and small immune complexes through phagocytosis and endocytosis, respectively. The molecular processes underlying these internalization mechanisms differ dramatically and have distinct outcomes in immune clearance and modulation of cell function. However, it is unclear how the same receptors (FcgammaR) binding to identical ligands (IgG) can elicit such distinct responses. We and others have shown that Syk kinase, Src-related tyrosine kinases (SRTKs) and phosphatidyl inositol 3-kinases (PI3K) play important roles in FcgammaR phagocytosis. Herein, we demonstrate that these kinases are not required for FcgammaR endocytosis. Endocytosis of heat-aggregated IgG (HA-IgG) by COS-1 cells stably transfected with FcgammaRIIA or chimeric FcgammaRI-gamma-gamma (EC-TM-CYT) was not significantly altered by PP2, piceatannol, or wortmannin. In contrast, phagocytosis of large opsonized particles (IgG-sensitized sheep erythrocytes, EA) was markedly reduced by these inhibitors. These results were confirmed in primary mouse bone marrow-derived macrophages and freshly isolated human monocytes. Levels of receptor phosphorylation were similar when FcgammaRIIA was cross-linked using HA-IgG or EA. However, inhibition of FcgammaR phosphorylation prevented only FcgammaR phagocytosis. Finally, biochemical analyses of PI3K(p85)-Syk binding indicated that direct interactions between native Syk and PI3K proteins are differentially regulated during FcgammaR phagocytosis and endocytosis. Overall, our results indicate that FcgammaR endocytosis and phagocytosis differ dramatically in their requirement for Syk, SRTKs, and PI3K, pointing to striking differences in their signal transduction mechanisms. We propose a competitive inhibition-based model in which PI3K and c-Cbl play contrasting roles in the induction of phagocytosis or endocytosis signaling cascades.

Platelet FcgammaRIIA binds and internalizes IgG-containing complexes.

OBJECTIVE: The physiologic role of platelet FcgammaRIIA, the only Fc receptor for IgG on human platelets, is largely unknown. FcgammaRIIA is also expressed on phagocytes such as monocytes and neutrophils, where it mediates the binding and internalization of both soluble IgG-containing complexes and IgG-coated cells. We previously reported the creation and characterization of a transgenic mouse that expresses human FcgammaRIIA on platelets and macrophages at levels comparable to that seen in humans. Using the transgenic mouse model, we observed that FcgammaRIIA mediates the clearance of IgG-coated cells. With the hypothesis that FcgammaRIIA on platelets may serve to remove IgG complexes from the circulation, we studied the capacity of human platelet FcgammaRIIA to bind and internalize such complexes. METHODS: We demonstrated by flow cytometry and electron microscopy that human platelets at 37 degrees C can bind and endocytose IgG complexes. We also utilized platelets from FcgammaRIIA transgenic mice to study endocytosis of IgG complexes by platelet FcgammaRIIA. RESULTS: Wild-type mouse platelets do not express Fcgamma receptors. While platelets from wild-type mice did not bind or endocytose IgG complexes, the presence of transgenic FcgammaRIIA on mouse platelets allowed the platelets to bind and endocytose IgG complexes. CONCLUSION: Our data indicate that platelet FcgammaRIIA binds and internalizes IgG complexes and suggest that human platelets may function to clear soluble IgG complexes from the circulation.

Cancer and Thrombosis: The Platelet Perspective.

Platelets are critical to hemostatic and immunological function, and are key players in cancer progression, metastasis, and cancer-related thrombosis. Platelets interact with immune cells to stimulate anti-tumor responses and can be activated by immune cells and tumor cells. Platelet activation can lead to complex interactions between platelets and tumor cells. Platelets facilitate cancer progression and metastasis by: (1) forming aggregates with tumor cells; (2) inducing tumor growth, epithelial-mesenchymal transition, and invasion; (3) shielding circulating tumor cells from immune surveillance and killing; (4) facilitating tethering and arrest of circulating tumor cells; and (5) promoting angiogenesis and tumor cell establishment at distant sites. Tumor cell-activated platelets also predispose cancer patients to thrombotic events. Tumor cells and tumor-derived microparticles lead to thrombosis by secreting procoagulant factors, resulting in platelet activation and clotting. Platelets play a critical role in cancer progression and thrombosis, and markers of platelet-tumor cell interaction are candidates as biomarkers for cancer progression and thrombosis risk.