Small but mighty

2.1 Platelet production, maturation and structure

Platelets are the smallest cells (2‐4 µm) in the blood stream, which lack a nucleus but are rich in granules and organelles such as mitochondria and endosomal,11 falling off from mature megakaryocytes (MKs) and entering the circulation as pro‐platelets.31 The normal platelet counts in human blood ranges from 150,000 to 450,000 per microliter, and the lifespan of platelets is only 7–10 days. Megakaryocytes are considered as multifunctional hematopoietic cells that not only produce a large number of platelets but also exist in different subtypes, capable of regulating the body's immune functions.32 MKs are the largest cells in the bone marrow (50–100 µm). To assemble and release platelets, MKs undergo endomitosis, where the number of chromosomes increases, and the volume of the cytoplasm enlarges without cell division, resulting in polyploidy.33 During this process, most of the cytoplasm of MKs is remodeled and packaged into units called pro‐platelets, and then undergo fission to produce platelets34 (Figure 1, left side). Numerous pieces of evidence suggest that cytoskeletal reorganization provides the driving force for platelet extension,47 and cytoplasmic dynein is utilized to drive the extension of pro‐platelets.48 Although platelets are anucleate cell fragments, resting platelets contain a variety of organelles and granules, including mitochondria, lysosomes, α‐granules, and dense granules.49 Resting platelets typically contain about 5‐8 mitochondria, and the number of α‐granules is linearly correlated with platelet size.50 Platelets generate energy through glycolysis and mitochondrial respiration. Interestingly, recent studies have shown that platelet mitochondria undergo fission events during activation, and this fission is dependent on dynamin‐related protein 1 (Drp1).51 Additionally, MutT Homolog 1 (MTH1) has been recently identified as providing protection against mitochondrial oxidative stress, and platelets lacking MTH1 exhibit reduced aggregation and thrombin‐induced calcium mobilization.52

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2.2 Platelet activation and adhesion

Platelets activation is amplified in different cancers and inflammatory response which describes the processes that platelets change from a smooth and nonviscous state to a viscous, irregular shape.53 Although there are many types of platelet receptors, including integrins, leucine‐rich repeats receptors, selectins, tyrosine kinase receptors, transmembrane receptors, prostaglandin receptors, lipid receptors, immunoglobulin superfamily receptors, tetraspanins and other platelet receptors54 (Figure 1). The most important glycoproteins are considered to be GPIb‐IX‐V complex, integrin αIIbβ3 and GPVI, which play important roles in platelet activation, adhesion and aggregation (Figure 1). Early platelet adhesion is coordinated and mediated by the binding of GPIbα to von Willebrand factor (VWF) and integrins to collagen. These glycoprotein receptors bind thrombin, and the signals are transmitted through PARs, a specific family of G protein‐coupled receptors (GPCRs), to activate platelets41^, 42^, 43^, 44^, 45^, 46 (Figure 1). The extracellular matrix protein collagen which acts as another strong platelet aggregation inducer exposed by injured vessel endothelium is able to interact with the collagen receptor on the platelet surface. Additionally, GPVI plays a major role in platelet‐collagen adhesion while also activating different adhesion‐related receptors, including integrin α2β1, which further enhances this adhesion process.37^, 38^, 39 It has been proved that galectin‐9 (Gal‐9) stimulates aggregation in human and mouse platelets in a dose‐dependent way, which is mediated by GPVI and CLEC‐2.55 GPVI acts as the primary receptor for collagen,56 and upon binding to collagen, it triggers downstream signaling pathways, such as the phospholipase Cγ2 (PLCγ2) pathway. This leads to the release of calcium ions and the activation of protein kinase C (PKC), resulting in platelet shape change and the secretion of pro‐coagulant substances, including ADP and TXA₂. Subsequently, TXA₂ binds to TXA₂ receptors, while ADP interacts with P2Y1 and P2Y12 receptors on the platelet surface, thereby further enhancing platelet activation40 (Figure 1). However, high concentrations of collagen can induce weak platelet aggregation without the participation of TXA₂ and ADP.57 ADP, one of the components in the dense granules released from activated platelets,58^, 59 acts as a weak agonist connecting to GPCR‐P2Y₁ and P2Y₁₂, and there is an intricate signal interaction between P2Y₁ and P2Y₁₂ to maintain the delicate balance between the activation and inhibition of platelets, and the adequate platelet aggregation cannot be achieved without their co‐activation60^, 61^, 62^, 63 (Figure 1). High shear stress in the blood vessels could also lead to the activation of platelets.64 vWF is a kind of glycoprotein present in platelet α‐granules and subendothelial connective tissue, which plays an essential role in hemostasis.65^, 66 When vascular injury occurs, vWF is exposed and binds to the exposed collagen. This binding provides an anchoring point for platelets, and the bridging function of vWF and its interaction with the GPIb‐IX‐V complex ensure that platelets can effectively adhere and aggregate, particularly under high shear stress conditions, thereby forming a stable thrombus.35^, 36 Binding of fibrinogen to GPIIb/IIIa on the surface of activated platelets enables platelets adhesion to endothelial cells, resulting in the interaction between and intercellular adhesion molecule (ICAM)‐1 receptors of activated endothelial cells.67

3.1 Platelet in innate immune response

Platelets play an indispensable role in the innate immune system, serving as signaling messengers for leukocytes and amplifiers of inflammation (Figure 2). Previous studies have reported that platelet activation promotes various inflammatory conditions, including sepsis, peritonitis, and pancreatitis.73^, 74^, 75

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Platelets can act as cellular scavengers by using adhesion receptors on their surface to scan for pathogens on the vascular surface, enhancing the activity of phagocytic cells, and exacerbating tissue damage due to inflammation.76 Additionally, platelets can form transient interactions through their surface GPIb with vWF expressed by Kupffer cells (KCs). This interaction allows KCs to rapidly capture bacteria adhered to platelets and activate the platelets, enabling GPIIb to adhere to KCs, thereby encapsulating the bacteria70 (Figure 2).

Under certain inflammatory stimuli, platelets can directly interact with leukocytes through the surface expression of P‐selectin or CD40, binding to PSGL‐1 and CD40L on the leukocyte surface, forming platelet‐leukocyte aggregates. Platelets primarily form these aggregates with circulating monocytes or neutrophils, promoting the innate immune response71 (Figure 2, left side). In humans, neutrophils are the most abundant leukocyte in blood, which take an important part in the innate immune system. And the interaction between platelets and neutrophils are crucial for inflammation and immunity.77 The interaction between platelets and neutrophils happens through P‐selectin and PSGL‐1. Platelets in the bloodstream can be activated via TLR4, TLR2, and TLR1,68^, 69 which can induce the formation of neutrophil‐platelet aggregates (NPA), leading to the robust activation of neutrophils and the formation of Neutrophil extracellular traps (NETs) (Figure 2, right side).

Moreover, the stimulation of TLR2 can increase the upregulation of P‐selectin expression thereby further promoting inflammation and the innate immune response.69 High‐mobility group box 1 (HMGB1), released by activated platelets, enhances inflammation by interacting with TLR on immune cells, contributing to reactive oxygen species (ROS) burst and inflammation.73 In local inflammatory responses, platelets activate neutrophils by promoting adhesion and secretion of IL‐8, leading to neutrophil recruitment to infection sites and promoting the formation of NETs,78 aiding in pathogen capture and neutralization.

NETs are web‐like structures released by activated neutrophils consist of DNA fibers, histones and various antimicrobial proteins, playing a crucial role in immobilizing and killing microbes and facilitating their destruction by neutrophils.79 Platelet adhesion, activation, and aggregation within NETs promote thrombus formation,80^, 81 which further elucidates the close association between inflammation and thrombosis.

Monocytes, serving as a bridge between innate and adaptive immunity, are demonstrated to form platelet‐monocyte aggregates and regulated by platelets through vari influencing inflammation and pathogen clearance.6^, 82^, 83^, 84 Upon platelet activation, the formation of platelet‐monocyte aggregates is initiated by the interaction between P‐selectin and PSGL‐1.85 Additionally, GPIbα, αIIbβ3, ICAM‐2, and JAM‐3 on the platelet surface can further bridge with Mac‐1 on monocytes, thereby facilitating the formation of platelet‐leukocyte aggregates86^, 87^, 88^, 89^, 90 (Figure 2, left side). Furthermore, activated platelets locally release transforming growth factor‐β (TGF‐β), inducing CD16 upregulation in monocytes, promoting their differentiation into M2‐like macrophages and participating in antibody‐dependent cellular phagocytosis (ADCP).91^, 92

Macrophages, derived from monocytes and mainly distributed in tissues, play a crucial role in phagocytosis and digestion of cellular debris and pathogens. Platelet factor 4 (PF4) is an abundant chemokine released from activated platelet α‐granules, demonstrated to have numerous effects on myeloid leukocytes, proved to be a ligand for the integrin Mac‐1,93 can turn monocytes to macrophages with high capacity for unspecific phagocytosis, and mediate oxidative burst.94 Chatterjee et al. found enhanced aggregation of platelets and platelet‐macrophage aggregates in peritoneal fluid after inducing peritonitis in mice, with platelet‐derived CXCL12 modulating monocyte‐macrophage function through interactions with PF4 and CXCR7.95

Additionally, PMPs are the most abundant microparticles in blood, which contain multiple kinds of bioactive molecules such as cytokines, lipids and RNAs, induce transcriptional changes in macrophages, reprogramming their functions towards a phagocytic phenotype96 (Figure 2, right side).

Atherosclerosis, representing the interaction between arterial lipid deposition and unresolved inflammation, is a chronic vascular inflammatory disease, which is closely linked to the formation of monocyte‐platelet aggregates (MPA). Barrett et al. found that platelets induce monocyte migration and recruitment to atherosclerotic plaques, leading to the formation of MPA within the plaque, promoting SOCS3 protein expression in macrophages, enhancing inflammatory cytokine production, and impairing macrophage function, contributing to plaque growth.97 Moreover, PF4 plays an important role in accelerating atherosclerosis by recruiting monocytes on endothelial cells in conjunction with CCL5,98 suppressing CD163 expression in macrophages and upregulating endothelial E‐selectin expression, further promoting atherosclerosis.72

Platelets also induce monocyte differentiation into SMCs/smooth muscle‐like cells, further promoting plaque stability.99 During arterial thrombus formation, eosinophils rapidly recruit and interact with platelets, forming extracellular eosinophil traps (EETs), which further activate platelets, promoting atherosclerotic plaque formation and thrombus generation.100

3.2 Platelets in adaptive immune response

Our current understanding of platelet interactions with immune cells has primarily focused on the connection between platelets and innate immune cells. However, platelets also play diverse direct or indirect roles in adaptive immunity. A key aspect of adaptive immunity is the timely presentation of antigens and the rapid recruitment of lymphocytes to sites of infection or tissue injury. In a murine model of allergic inflammation, the interaction between P‐selectin on the platelet surface and PSGL‐1 promotes the formation of platelet‐leukocyte aggregates, which, along with eosinophils, participate in the recruitment of lymphocytes.101

Circulating lymphocytes can enter lymph nodes and be rapidly transported to pathogen‐exposed sites due to their ability to roll along High Endothelial Venules (HEVs). Activated platelets can bind to circulating lymphocytes via P‐selectin on their surface, mediating their rolling within HEVs.102

T‐cells are broadly classified into CD8⁺ cytotoxic T cells and CD4⁺ T helper cells. CD8⁺ cytotoxic T cells produce effector cytokines and release cytotoxic granules upon antigen stimulation, directly killing target cells.103 CD4⁺ T helper cells are further classified based on their secreted cytokines into Th1, Th2, Th17, and T regulatory (Treg) cells.104 T helper (Th) cells are the primary effector cells in transplant rejection, with the chemokine PF4 capable of limiting Th17 differentiation, thereby reducing the immune response to transplantation.105 PF4‐deficient and platelet‐deficient mice demonstrate a heightened immune response to cardiac transplantation, characterized by an increased number of Th17 cells and elevated plasma IL‐17 levels. However, when human CD4⁺ T cells were cocultured with autologous platelets, platelets enhanced the secretion of IL‐10 and other characteristic cytokines by Th1 (secreting IFN‐γ) and Th17 (IL‐17) cells, as well as the differentiation of Th1, Th17, and Treg cells106 (Figure 3i).

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These experimental findings suggest that platelets and PF4 play crucial roles in the development and differentiation of CD4⁺ T cells. CD4⁺ Tregs can suppress inflammation following severe trauma, and the disruption of PAR4 on platelets led to increased activation of CD4⁺ Tregs post‐trauma in a mouse model with third‐degree burn injury of 25% of the total body area.111 In a mouse pneumonia model, platelet‐Treg aggregates were found to be recruited to the lungs during the resolution of inflammation, with the formation of these aggregates mediated by sCD40L and PSGL‐1 expressed on Tregs. Platelets induced Tregs to produce IL‐10 and TGF‐β, further polarizing alveolar macrophages towards a pro‐resolving phenotype, thus contributing to the termination of inflammation107 (Figure 3ii). During sepsis, both human and mouse platelets, as well as platelets generated in vitro from human megakaryocytes stimulated with LPS and IFN‐γ, showed significantly increased MHC‐I expression, which enhanced antigen cross‐presentation and binding to antigen‐specific CD8⁺ T cells, regulating CD8⁺ T cell function and numbers112 (Figure 3iii).

Moreover, platelets are essential for immune cell development, as demonstrated by severe proliferation defects in lymphatic endothelial cells of CLEC‐2‐deficient mice, resulting in the absence of lymph nodes at birth.113 In systemic lupus erythematosus (SLE) patients, activated platelets contribute to autoimmunity by interacting with plasmacytoid dendritic cells (pDCs) via the CD40‐CD40L pathway, forming immune aggregates and enhancing IFN‐α secretion.109 Platelets also activate the cross‐presentation program in peripheral blood monocytes through P‐selectin and PSGL‐1, rapidly driving monocytes to differentiate into phDCs, indicating that platelets can mediate cytokine‐independent DC maturation110 (Figure 3v).

In vitro coculture experiments show that platelets and B lymphocytes engage in mutual activation via CD40‐CD40L, with increased expression of CD62p on platelets and CD86 on B cells, alongside elevated secretion of IgG1, IgG2, and IgG3 by B cells108 (Figure 3iv), suggesting that platelets may have additional, yet undiscovered, roles in adaptive immunity.

Moreover, recent findings have shown that in patients with metastatic colorectal cancer, Erbin expression is significantly elevated in platelets. Knockdown of Erbin in platelets or megakaryocytes can enhance B cell‐mediated antitumor immune responses via the Erbin–mitochondria axis, thereby inhibiting pulmonary metastasis in mice.114 This suggests that there may be additional, yet undiscovered roles for platelets in adaptive immunity.

4.1 Tumor‐induced platelet activation

Tumor cells can secrete soluble mediators such as ADP and TXA₂ to promote tumor cell‐induced platelet aggregation (TCIPA).115 TXA₂ is generally considered as a potent agonist for platelet activation and a regulator of platelet aggregation, exerting its effects by binding to the thromboxane prostanoid receptor (TP).116 ADP from ‐granules released by activated platelets can further stabilize platelet aggregation, while ADP produced by tumor cells can also promote platelets activation and the formation of TCIPA.117 Haemmerle et al. demonstrated that the hypoxia‐induced ADP production and platelets infiltration of tumor would promote tumor growth after discontinuation of antiangiogenic drugs in a mouse model of ovarian cancer, while focal adhesion kinase (FAK) in platelets could promote the migration of platelets into the tumor microenvironment (TME).118 More studies also demonstrated that the manner in which platelet activation triggered by colon cancer cells depends on tissue factor (TF) expression, thrombin generation, activation of the protease‐activated receptor 4 (PAR4) on platelets and consequent release of ADP and TXA₂.119 The expressions of TF, thrombin and PAR1 is upregulated in breast fibroblasts before the infiltration of breast cancer cells120 and thrombin accelerated tumor progression by enhancing the adhesion of tumor cells and increasing their metastatic potential.121 As a crucial promoter of coagulation response, TF is commonly overexpressed in tumor cells. TF, considered as a transmembrane protein, which is primarily expressed in subendothelial cells, triggers the coagulation cascade by forming a complex with its endogenous ligand coagulation factor VII (FVII), mediating the conversion from prothrombin to thrombin.122 Multiple tumor cells express TF and spontaneously releasing TF⁺ microparticles (MPs) into bloodstream, which may explain the increased probability of VTE in patients with cancer.123 There are other mechanisms by which tumor cells mediate platelet activation and aggregation. For instance, P‐selectin known to promote TCIPA,124 which sends signals through its receptor, P‐selectin glycoprotein ligand 1 (PSGL‐1), on leukocytes, inducing the production of high procoagulant TF⁺MPs, and it is also associated with fibrin formation.125^, 126 Soluble P‐selectin has been clinically proven to be strongly associated with VTE and can serve as an evaluation factor for cancer‐associated thrombosis127 (Figure 4). In vivo studies have found that P‐selectin can promote platelet deposition within solid tumors and enhance tumor growth by activating platelet integrins.134

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C‐type lectin‐like receptor 2 (CLEC‐2) plays a significant role in platelet activation and can be activated by the transmembrane protein podoplanin (PDPN) on the surface of tumor cells, which is upregulated in various tumor types135^, 136 (Figure 4). This interaction signals through ITAM can activate downstream signaling molecules protein kinase C and integrin‐αIIbβ 3 as well as the subsequent granule release.137 In vivo experiments in mice have shown that inhibiting this interaction can effectively suppress platelet aggregation and tumor metastasis.138^, 139^, 140^, 141

Furthermore, matrix metalloproteinases (MMPs) on tumor cells are proved to be involved in the formation of TCIPA,142 and tumor cell ‐released HMGB1 was found to mediate platelet‐tumor interaction by interacting with TLR4 on platelets, promoting tumor progression and metastasis143 (Figure 4).

Platelets are capable of efficiently absorbing tumor‐derived extracellular vesicles (Figure 4). In vitro studies have demonstrated that, following the uptake of tumor cell‐derived exosomes, platelets undergo robust activation via a CD63‐RPTPα‐dependent pathway.144 Moreover, through this mechanism, platelets can acquire tumor‐associated RNA,145 functional proteins,146 and even experience alterations in their transcriptional profile.147

4.2 Platelet‐tumor cell adhesion

For patients with malignancy, cancer‐associated thrombosis (CAT) is the leading cause of death besides cancer progression itself. The connection between cancer and thrombosis was proposed more than 100 years ago by Dr. Armand Trousseau,7 who highlighted the link between cancer and the formation of platelet‐rich microthrombi in the vascular system, hence the disease is also called Trousseau syndrome. Since then, a lot of studies have confirmed the increased probability of VTE in patients with cancer, and it is considered that the development of CAT is associated with bad prognosis.148 According to current studies, it was demonstrated that the amounts of leukocytes, platelets, and TF and microvesicles (MVs) are factors which increase CAT in patients with cancer, and soluble P‐selectin can also be recognized as a biomarker of CAT.149^, 150 Moreover, platelets activated by surgical stress may enhance the formation of platelet‐tumor cell aggregates.151 The primary reason for the elevated risk of CAT is that tumor cells can activate platelets through various mechanisms,152 forming TCIPA, a phenomenon observed in multiple cancers, including lung, colorectal, breast, and pancreatic cancers.21^, 153^, 154^, 155 Platelets protect CTCs by forming clots, which create a physical barrier that shields CTCs from the high shear stress of blood flow and immune cell attack, thereby promoting tumor metastasis to some extent152 (Figure 2). Numerous studies have demonstrated that some substances released by tumor cells can activate platelets and thus trigger the formation of TCIPA.

Platelets adhere to tumor cells through multiple pathways, a process mediated by several platelet activation‐related receptors, including GPVI, integrins, CLEC‐2, and P‐selectin. GPVI is a platelet‐specific receptor that interacts with collagen and fibrin, triggering platelet activation via the ITAM signaling pathway, which promotes platelet adhesion and aggregation. In vivo studies have demonstrated that platelet GPVI interacts with Galectin‐3 expressed on tumor cells, facilitating the extravasation of colorectal and breast cancer cells.128 Knockout of GPVI expression significantly reduces platelet accumulation within metastatic tumors.156 Additionally, CLEC‐2 on the platelet surface can bind to transmembrane protein PDPN expressed on various tumor cells, promoting tumor metastasis.157 Furthermore, in vivo research has shown that integrin α6β1 significantly enhances lung metastasis by binding to A disintegrin and A metalloprotease (ADAM) 9 on tumor cells, while targeting α6β1 effectively inhibits tumor metastasis,129 and ADAM9 has been found in various solid tumors158 (Figure 4). P‐selectin on the platelet surface plays a critical role in platelet‐tumor adhesion and aggregation. Platelet P‐selectin can interact with tumor cells in a manner independent of PSGL‐1 and CD24, and integrin α_IIbβ₃ on the platelet surface can stabilize this interaction into a firm adhesion, thereby promoting the formation of TCIPA159 (Figure 4). Moreover, integrin α_Vβ₃ on tumor cells cooperates with platelets promoting tumor cells metastasis and extravasation,159 which can be mediated by GPIIb/IIIa on platelets160 (Figure 4). Mucins, which are heavily glycosylated proteins, are associated with poor prognosis in tumor cells,161^, 162^, 163 and their binding to platelet P‐selectin may mediate the production of TCIPA (Figure 4). Injection of cancer‐associated mucins into P‐selectin‐deficient mice results in a significant reduction in microthrombus formation.

4.3 Platelet‐mediated CTC immune evasion

Distal metastasis first requires tumor cells to shed from primary tumor site and enter the vascular system as CTCs, where the tumor cells can arrest on the vessel wall and then invade into the distant tissues, thus CTCs are also described as the seeds of tumor metastasis.164 Only a very small number of CTCs could survive under the high shear forces of blood stream and the killing of immune system,165 indicating that the interplay between platelets and lymphocytes can significantly influence tumor progression. Platelet‐derived selectins facilitate interactions between tumor cells and platelets/leukocytes, activating endothelial cells and inducing the production of chemokine CCL5, contributing to the formation of a pro‐metastatic microenvironment.166 Furthermore, platelet‐induced formation of TEPs promotes M2 polarization of tumor‐associated macrophages (TAMs), reshaping the tumor microenvironment and facilitating pulmonary metastasis.167

Upon entering the bloodstream and turning into CTCs, tumor cells, now stripped of the protective TME, are exposed to the immune system as well as the threat of high shear forces in the blood stream, making them susceptible to cytolytic lymphocytes.168 However, it was found in vivo that the coculture of platelets with lymphocytes decreased the production of IFN‐γ, TNF‐α, T lymphocytes proliferation and the expression of CD25, PD‐L1, which reduced inflammatory responses.169

NK cells are vital cytotoxic immune cells of the innate immune system, play a crucial role in eliminating tumor cells and mitigating tumor metastasis. Nieswandt et al. first demonstrated that platelets protect CTCs from NK cell lysis in mice.15 Initially, it was posited that platelets provide physical protection to CTCs from NK cells by forming a dense coat around the tumor cells. However, more mechanisms of platelets have been found to immediate the immune evasion of tumor cells. Placke et al. showed that platelets coating on the surface of tumor cells at the ultrastructural level may cause transfer of platelet‐derived MHC‐I onto the tumor cells surface, thereby disrupting NK cell recognition of tumors and impeding IFN‐γ production16 (Figure 5). The interaction between G proteins (Gi and G13) on the surface of platelets and GPCRs on tumor cells can promote tumor growth and metastasis.175 In a mouse lung metastasis model, it was observed that mice deficient in fibrin(ogen) and Gq exhibited significantly reduced lung metastasis. This reduction, however, could be reversed by the depletion of NK cells, which demonstrated that the platelet‐fibrin (ogen) axis enhances metastatic potential by impeding the elimination of tumor cells by NK cells.176 Additionally, they also demonstrated that TF‐mediated metastasis might be facilitated through fibrinogen and platelet‐dependent restriction of micrometastasis clearance by NK cells.177

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Besides, Platelets upregulate RANKL expression upon contact with tumor cells, and platelet‐derived RANKL has been found to inhibit the tumor‐killing activity of normal NK cells178 (Figure 5). Similarly, ligands of immune checkpoint GITR, GITRL has been demonstrated to be upregulated on platelet surface and the formation of tumor cell‐platelet coating confers tumor cells GITRL pseudo‐expression, which inhibits the toxicity of NK cells and IFN‐ secretion, and upregulation of platelet GITRL expression in patients with breast cancer is found to be specifically regulated, having connection with tumor progression179^, 180 (Figure 5). Interestingly, recent studies also demonstrated that platelet coating will reduce the expression of NKG2D ligands on the surface of tumor cells, resulting in impaired recognition of NK cells and, thus, reduced lysis of tumor cells, which could suggest that platelet‐mediated shedding of NKG2DL may be involved in the immune escape of metastatic tumor cells from NK cells, and, further studies found that shed NKG2D ligands are induced to be secreted into the TME and promote immune evasion.181^, 182 However, our recent research found that platelet‐derived RGS18 could drive upregulation of HLA‐E expression on CTCs, which acquire RGS18 by endocytosing platelets, evading immune surveillance from NK cells through a special immune checkpoint molecule pair, HLA‐E:CD94‐NKG2A, which overturns the previous perception and suggests a novel mechanism by which platelets promote tumor metastasis173 (Figure 5).

It was suggested that there might be a connection between tumor PD‐L1 expression and platelet count, which help regulate tumor progression,183 and that PD‐L1 is expressed on platelets of both patients with cancer and healthy individuals.184 Platelet‐derived PD‐L1 can protect PD‐L1‐negative solid tumors from cytotoxicity of T lymphocytes, thereby modulating the immune system to support tumor growth.185 Furthermore, when tumor cells interact with platelets, PD‐L1 is transferred from tumor cells to platelets via fibronectin 1, integrin α_Vβ₁ and GPIbα‐dependent ways, which resulting in high PD‐L1 expression in the platelets of non‐small cell lung cancer (NSCLC) patients, which subsequently suppresses the activity of CD4⁺ and CD8⁺ T lymphocytes174 (Figure 5). In addition, platelets can also inactivate antitumor immune responses by upregulated PD‐L1 expression on ovarian cancer cells through contact‐dependent NF‐κB signaling pathway and the contact‐independent TFG‐βR1/Smad signaling pathway.186 Additionally, platelet‐derived factors such as PF4 can influence T cell differentiation, thereby modulating immune responses.187^, 188^, 189

Platelet is a major source of TGF‐β and activation of TGF‐β1 occurs when subjected to agitation or shear stress.190 The platelet‐specific deletion of Lrrc32, the gene coding glycoprotein A repetitions predominant (GARP), reduces TGF‐β activity at tumor sites, enhancing protective immunity against melanoma and colorectal cancer, revealing that platelets can suppress the immunity of T lymphocytes and promote tumor growth via the GARP–TGF‐β axis.191 Moreover, it has been demonstrated that thrombin cleaves GARP, resulting in liberation of active TGF‐β1 from the GARP–LTGF‐β1 complex.192 TGF‐β1 has also been shown to downregulate the surface expression of NKp30, significantly inhibiting NK cell‐mediated killing of DCs, and reduce NK cell cytotoxicity by downregulating the activation receptor NKG2D,171^, 172 leading to diminished cytotoxicity and IFN‐ production.170 Moreover, the expression of CD226 and CD96 on NK cells and their associated ligands on tumor cells can also be inhibited by platelet‐derived TGF‐β, thereby affecting the immune function of NK cells.181

Platelets can also influence immune responses by affecting the efficiency of antigen presentation, thereby further promoting tumor metastasis. The endoplasmic reticulum‐resident heat shock protein Gp96 mediates the process of antigen presentation and pro‐inflammatory cytokine secretion by DCs. However, when platelets specifically bind to Gp96, they reduce the secretion of pro‐inflammatory cytokines and DC activation, impairing antigen presentation and subsequent T lymphocyte activation.193

4.4 Platelet‐mediated tumor cell invasion and transendothelial migration

Platelets facilitate tumor cell migration to nearby vascular regions and promote tumor metastasis by enhancing EMT.194 In vitro experiments have shown that platelet treatment enhances the adhesion of colorectal adenocarcinoma cells to human umbilical vein endothelial cells (HUVECs).12 The interaction between platelets and the endothelium, particularly the P‐selectin‐mediated interaction, is considered a key factor in the adhesion of platelet‐tumor cell aggregates to the vascular wall.193 Additionally, ADAM15 expressed on vascular endothelial cells can bind to GPIIb‐IIIa on the platelet surface, further promoting platelet activation, secretion, and adhesion.195 Platelet‐derived TGF and direct platelet‐tumor cell connection can activate the TGF‐Smad and NF‐ B pathways in cancer cells, resulting in promoting the progression of EMT.17 Platelets can infiltrate from blood vessels to adjacent tumor tissues via FAK,196 and their expression of integrin α₆β₁ interacts with ADAM9 expressed by tumor cells to significantly promote tumor metastasis.197 In the colorectal cancer tumor microenvironment, platelets bind to TAMs through P‐selectin‐PSGL‐1, activating the JNK/STAT1 pathway, and facilitating CRC growth and metastasis.198

The final step in hematogenous metastasis is distal colonization, in which platelets play an important role. Platelets directly contact circulating tumor cells in the vasculature and form an envelope on the surface of tumor cells, and the activated platelet receptor integrin αIIbβ3 mediates the adhesion of tumor cells to the vasculature.199 It has been shown that direct interaction between platelet‐derived lysophosphatidic acid (LPA) and the tumor antigen CD97 stimulates platelet activation and leads to granule secretion, ATP release, induces changes in vascular permeability and promotes transendothelial migration.200 Schumacher et al. found that activation of platelets by tumor cells causes ATP release, which activates endothelial P2Y2 receptors, which induces the opening of the endothelial barrier and disrupts endothelial cell junctions, facilitate tumor cells migration across the endothelium and promoting cancer cells extravasation.201

In the local vasculature, platelets aid in recruiting immune cells that contribute to the formation of a pre‐metastatic microenvironment conducive to CTCs metastasis and promote tumor cell extravasation. Platelet‐derived cytokines may recruit monocytes and TAMs, further enhancing endothelial activation and indirectly promoting tumor extravasation.202 The direct interaction between circulating tumor cells, platelets and leukocytes activates endothelial cells, inducing the production of C‐C chemokine ligand 5 (CCL5), promoting extracellular matrix remodeling and tumor migration,203 with P‐selectin playing a crucial role in this process. In addition, Labelle et al. found that platelets assist CTCs in rapidly recruiting granulocytes to form an early metastatic microenvironment, and granulocytes secrete CXCL5 and CXCL7 upon platelet‐dependent contact with tumor cells to promote metastatic foci.204 Monocytes and macrophages further secrete vascular endothelial growth factor (VEGF) and cathepsins that can further help circulating tumor cells to cross the endothelial barrier and form early metastatic foci.

4.5 Platelets and tumor angiogenesis

The ‐granules released upon platelet activation contain high levels of proangiogenic factors such as VEGF, angiopoietin‐1 and anti‐vascular growth factors like endostatin.205 When platelets are activated, a large number of angiogenesis‐related factors are released into the tumor microenvironment including VEGF, platelet‐derived growth factor (PDGF), fibroblast growth factor (FGF), MMPs, and others206 (Figure 6). Platelets, as the major physiologic transporters of the most important proangiogenic factor VEGF, release VEGF upon activation209 and at sites of vascular injury to promote angiogenesis.210 Platelets are activated in malignant tumors,211 with ADP‐activated platelets observed to promote angiogenesis in HUVECs in vitro, and breast cancer cells could stimulate platelet VEGF secretion.212

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Interestingly, it has been found that platelets in patients with malignant tumors are able to selectively take up and sequester certain angiogenic regulators.213 Joseph et al. found that proangiogenic and antiangiogenic proteins are segregated in different subpopulations of ‐granules secreted by platelets and macrophages. These different subpopulations of ‐granules may exert proangiogenic and antiangiogenic roles at different sites to exert proangiogenic and inhibitory angiogenic effects at different sites,214 which may be due to differential packaging of ‐granules.215 Additionally, Kuznetsov et al. found that certain luminal breast cancer (LBC) cells are able to recruit platelets to take up the cytokines they release and transport them to painless metastatic tumor foci, stimulating angiogenesis and promoting tumor growth.216

Moreover, the presence of platelets not only stimulates angiogenesis but also plays a crucial role in preventing hemorrhage within newly formed blood vessels and maintaining vascular integrity207^, 208(Figure 6). Platelets help stabilize tumor vasculature. In mouse models of lung cancer and melanoma, inducing severe acute thrombocytopenia led to rapid destabilization of tumor blood vessels and intratumoral hemorrhage within 30 min, indicating that platelets help maintain tumor homeostasis.217 Inhibition of the GPIV function on the platelet surface further induced rapid intratumoral bleeding and increased the accumulation of chemotherapeutic drugs within the tumor, resulting in significant antitumor effects.218

In summary, inhibiting angiogenesis within tumors and destabilizing tumor vasculature represent effective antitumor therapeutic strategies, with platelets serving as the most critical target.

4.6 Platelet‐mediated tumor cell metabolic reprogramming

Multiple studies have now shown that platelets can influence tumor cells by transferring lipids, proteins, and RNA,219 altering the expression profiles of tumor cells, and even leading to metabolic reprogramming. Platelets are rich in mitochondria, and recent studies have discovered that osteosarcoma cells can acquire platelet mitochondria through the PINK/Parkin‐Mfn2 pathway, reprogramming the cells into a metastatic state. Platelet mitochondria can also regulate the GSH/GSSG balance and ROS levels within tumor cells, thereby promoting tumor metastasis.220 Thrombin‐mediated hydrolysis is an important determinant of metastatic potential and tumor growth in colorectal cancer and found that PAR1 and fibrinogen or fibrin could promote tumor growth.221 A recent study found that medium‐sized extracellular vesicles (mEVs) released from thrombin‐activated platelets in patients with colorectal cancer promote prometastatic and prothrombotic phenotypes of tumor cells.222 Platelets shed extracellular vesicles known as PMPs into the extracellular matrix, which contain a wealth of bioactive substances, including RNA, transcription factors, and proteins.130^, 131^, 132^, 133 When chronic lymphocytic leukemia (CLL) cells are cocultured with PMPs in vitro, it induces metabolic reprogramming in the CLL cells, leading to increased oxygen consumption rate (OCR), ATP levels, and ROS production.223 Additionally, a newly discovered subpopulation of PMPs, termed mitoMPs, contains functional mitochondria. Tumor cells cocultured with mitoMPs acquire these exogenous mitochondria, exhibiting enhanced migratory and invasive characteristics.224

ACKNOWLEDGMENTS

Graphical abstract and Figures 1, 2, 3, 4, 5 and 6 in this review were created using BioRender.com (https://biorender.com). The study received no funding.