Complement and platelets: Mutual interference in the immune network

Platelets and diseases: signal transduction and advances in targeted therapy

Platelets are essential anucleate blood cells that play pivotal roles in hemostasis, tissue repair, and immune modulation. Originating from megakaryocytes in the bone marrow, platelets are small in size but possess a highly specialized structure that enables them to execute a wide range of physiological functions. The platelet cytoplasm is enriched with functional proteins, organelles, and granules that facilitate their activation and participation in tissue repair processes. Platelet membranes are densely populated with a variety of receptors, which, upon activation, initiate complex intracellular signaling cascades. These signaling pathways govern platelet activation, aggregation, and the release of bioactive molecules, including growth factors, cytokines, and chemokines. Through these mechanisms, platelets are integral to critical physiological processes such as thrombosis, wound healing, and immune surveillance. However, dysregulated platelet function can contribute to pathological conditions, including cancer metastasis, atherosclerosis, and chronic inflammation. Due to their central involvement in both normal physiology and disease, platelets have become prominent targets for therapeutic intervention. Current treatments primarily aim to modulate platelet signaling to prevent thrombosis in cardiovascular diseases or to reduce excessive platelet aggregation in other pathological conditions. Antiplatelet therapies are widely employed in clinical practice to mitigate clot formation in high-risk patients. As platelet biology continues to evolve, emerging therapeutic strategies focus on refining platelet modulation to enhance clinical outcomes and prevent complications associated with platelet dysfunction. This review explores the structure, signaling pathways, biological functions, and therapeutic potential of platelets, highlighting their roles in both physiological and pathological contexts.

Checkpoint Inhibitors, CAR T Cells, and the Hemostatic System: What Do We Know So Far?

Immune checkpoint inhibitors (ICIs) and chimeric antigen receptor (CAR) T cells are novel therapeutic strategies that enhance anticancer immunity by activating or engineering cancer-targeting T cells. The resulting hyperinflammation carries several side effects, ranging from autoimmune-like symptoms to cytokine release syndrome (CRS), with potentially severe consequences. Recent findings indicate that ICIs increase the risk of venous and arterial thromboembolic adverse events. Patients with prior VTE might be at higher risk of developing new events under ICI while other risk factors vary across studies. So far, data on CAR T-linked coagulopathies are limited. Hypofibrinogenemia in the presence of CRS is the most commonly observed dysregulation of hemostatic parameters. A rare but particularly severe adverse event is the development of disseminated intravascular coagulation activation, which can occur in the setting of CRS and may be linked to immune effector cell-associated hemophagocytic lymphohistiocytosis. While the increasing number of studies on thromboembolic complications and coagulation alterations under ICIs and CAR T therapies are concerning, these results might be influenced by the retrospective study design and the heterogeneous patient populations. Importantly, numerous promising new T cell-based immunotherapies are currently under investigation for various cancers and are expected to become very prominent therapy options in the near future. Therefore, coagulopathies and thrombosis under T cell-directed immuno- and anti-cancer therapies is important. Our review provides an overview of the current understanding of ICI- and CAR T-associated thromboembolism. We discuss pathogenic mechanisms of inflammation-associated coagulation activation and explore potential biomarkers for VTE.

The concept of immunothrombosis in pancreas transplantation

Early failure of a pancreatic allograft due to complete thrombosis has an incidence of approximately 10% and is the main cause of comorbidity in pancreas transplantation. Although several risk factors have been identified, the exact mechanisms leading to this serious complication are still unclear. In this review, we define the roles of the individual components involved during sterile immunothrombosis-namely endothelial cells, platelets, and innate immune cells. Further, we review the published evidence linking the main risk factors for pancreatic thrombosis to cellular activation and vascular modifications. We also explore the unique features of the pancreas itself: the vessel endothelium, specific vascularization, and relationship to other organs-notably the spleen and adipose tissue. Finally, we summarize the therapeutic possibilities for the prevention of pancreatic thrombosis depending on the different mechanisms such as anticoagulation, anti-inflammatory molecules, endothelium protectors, antagonism of damage-associated molecular patterns, and use of machine perfusion.

Immunity and Coagulation in COVID-19

Discovered in late 2019, the SARS-CoV-2 coronavirus has caused the largest pandemic of the 21st century, claiming more than seven million lives. In most cases, the COVID-19 disease caused by the SARS-CoV-2 virus is relatively mild and affects only the upper respiratory tract; it most often manifests itself with fever, chills, cough, and sore throat, but also has less-common mild symptoms. In most cases, patients do not require hospitalization, and fully recover. However, in some cases, infection with the SARS-CoV-2 virus leads to the development of a severe form of COVID-19, which is characterized by the development of life-threatening complications affecting not only the lungs, but also other organs and systems. In particular, various forms of thrombotic complications are common among patients with a severe form of COVID-19. The mechanisms for the development of thrombotic complications in COVID-19 remain unclear. Accumulated data indicate that the pathogenesis of severe COVID-19 is based on disruptions in the functioning of various innate immune systems. The key role in the primary response to a viral infection is assigned to two systems. These are the pattern recognition receptors, primarily members of the toll-like receptor (TLR) family, and the complement system. Both systems are the first to engage in the fight against the virus and launch a whole range of mechanisms aimed at its rapid elimination. Normally, their joint activity leads to the destruction of the pathogen and recovery. However, disruptions in the functioning of these innate immune systems in COVID-19 can cause the development of an excessive inflammatory response that is dangerous for the body. In turn, excessive inflammation entails activation of and damage to the vascular endothelium, as well as the development of the hypercoagulable state observed in patients seriously ill with COVID-19. Activation of the endothelium and hypercoagulation lead to the development of thrombosis and, as a result, damage to organs and tissues. Immune-mediated thrombotic complications are termed "immunothrombosis". In this review, we discuss in detail the features of immunothrombosis associated with SARS-CoV-2 infection and its potential underlying mechanisms.

A Perspective on How Fibrinaloid Microclots and Platelet Pathology May be Applied in Clinical Investigations

Microscopy imaging has enabled us to establish the presence of fibrin(ogen) amyloid (fibrinaloid) microclots in a range of chronic, inflammatory diseases. Microclots may also be induced by a variety of purified substances, often at very low concentrations. These molecules include bacterial inflammagens, serum amyloid A, and the S1 spike protein of severe acute respiratory syndrome coronavirus 2. Here, we explore which of the properties of these microclots might be used to contribute to differential clinical diagnoses and prognoses of the various diseases with which they may be associated. Such properties include distributions in their size and number before and after the addition of exogenous thrombin, their spectral properties, the diameter of the fibers of which they are made, their resistance to proteolysis by various proteases, their cross-seeding ability, and the concentration dependence of their ability to bind small molecules including fluorogenic amyloid stains. Measuring these microclot parameters, together with microscopy imaging itself, along with methodologies like proteomics and imaging flow cytometry, as well as more conventional assays such as those for cytokines, might open up the possibility of a much finer use of these microclot properties in generative methods for a future where personalized medicine will be standard procedures in all clotting pathology disease diagnoses.

Platelets and inter-cellular communication in immune responses: Dialogue with both professional and non-professional immune cells

Platelets, derived from bone marrow megakaryocytes, are essential for vascular integrity and play multifaceted roles in both physiological and pathological processes within the vasculature. Despite their small size and absence of a nucleus, platelets are increasingly recognized for their diverse immune functions. Recent research highlights their pivotal role in interactions with various immune cells, including professional cells like macrophages, dendritic cells, natural killer cells, T cells, and B cells, influencing host immune responses. Platelets also engage with non-professional immune cells, contributing to immune responses and structural maintenance, particularly in conditions like inflammation and atherosclerosis. This review underscores the emerging significance of platelets as potent immune cells, elucidating their interactions with the immune system. We explore the mechanisms of platelet activation, leading to diverse functions, such as aggregation, immunity, activation of other immune cells, and pathogen clearance. Platelets have become the predominant immune cells in circulation, involved in chronic inflammation, responses to infections, and autoimmune disorders. Their immunological attributes, including bioactive granule molecules and immune receptors, contribute to their role in immune responses. Unlike professional antigen-presenting cells, platelets process and present antigens through an MHC-I-dependent pathway, initiating T-cell immune responses. This review illuminates the unique features of platelets and their central role in modulating host immune responses in health and disease.

Platelet-rich Plasma as Effectiveness in the Treatment of Osteoarthrosis of the Knee in a Tertiary Care Center

Introduction

Osteoarthrosis (OA) is a condition that develops slowly but eventually causes considerable morbidity. Many medical specialties have employed platelet-rich plasma (PRP) since it is the best autologous biological blood-derived product, can be exogenously supplied to tissues, and releases high concentrations of platelet-derived growth factors to promote tendon, bone, and wound healing. This prospective research aims to clinically assess the effectiveness of PRP injection therapy for treating knee osteoarthrosis.

Materials and Methods

A clinical prospective study was conducted at a tertiary care center for a period of a year. Subjects older than 50 years were selected for both genders who had a symptomatic primary knee OA. PRP is administered to and evaluated in 80 patients with knee osteoarthrosis. Using the VAS and WOMAC questionnaire tools, primary data was collected. The observations were compared using the appropriate statistical tools while considering significance at 95%.

Results

Eighty-four subjects participated in the study till the end of the study period of 3 months. All study participants filled out questionnaires before injection, eight weeks after injection, and 12 weeks after. The obtained scores for the WOMAC and the VAS were compared, and it was observed that after 12 weeks, the WOMAC scores lowered from 77 to 33 (P < 0.001) and the VAS scores lowered to 6 to 1.5 (P < 0.001) and these alterations were significant.

Conclusion

According to the most recent research, PRP therapy is a less expensive and more straightforward therapeutic option for the management of knee pain in patients who have OA, and it has also improved patients' ability to participate in everyday activities.

Leishmania major-derived lipophosphoglycan influences the host's early immune response by inducing platelet activation and DKK1 production via TLR1/2

Background

Platelets are rapidly deployed to infection sites and respond to pathogenic molecules via pattern recognition receptors (TLR, NLRP). Dickkopf1 (DKK1) is a quintessential Wnt antagonist produced by a variety of cell types including platelets, endothelial cells, and is known to modulate pro-inflammatory responses in infectious diseases and cancer. Moreover, DKK1 is critical for forming leukocyte-platelet aggregates and induction of type 2 cell-mediated immune responses. Our previous publication showed activated platelets release DKK1 following Leishmania major recognition.

Results

Here we probed the role of the key surface virulence glycoconjugate lipophosphoglycan (LPG), on DKK1 production using null mutants deficient in LPG synthesis (Δlpg1- and Δlpg2-). Leishmania-induced DKK1 production was reduced to control levels in the absence of LPG in both mutants and was restored upon re-expression of the cognate LPG1 or LPG2 genes. Furthermore, the formation of leukocyte-platelet aggregates was dependent on LPG. LPG mediated platelet activation and DKK1 production occurs through TLR1/2.

Conclusion

Thus, LPG is a key virulence factor that induces DKK1 production from activated platelets, and the circulating DKK1 promotes Th2 cell polarization. This suggests that LPG-activated platelets can drive innate and adaptive immune responses to Leishmania infection.

Properdin inhibition ameliorates hepatic ischemia/reperfusion injury without interfering with liver regeneration in mice

Hepatic ischemia/reperfusion injury (IRI) often causes serious complications in liver surgeries, including transplantation. Complement activation seems to be involved in hepatic IRI; however, no complement-targeted intervention has been clinically applied. We investigated the therapeutic potential of Properdin-targeted complement regulation in hepatic IRI. Male wild-type mice (B10D2/nSn) were exposed to 90-minute partial hepatic IRI to the left and median lobes with either monoclonal anti-Properdin-antibody (Ab) or control-immunoglobulin (IgG) administration. Since the complement system is closely involved in liver regeneration, the influence of anti-Properdin-Ab on liver regeneration was also evaluated in a mouse model of 70% partial hepatectomy. Anti-Properdin-Ab significantly reduced serum transaminases and histopathological damages at 2 and 6 hours after reperfusion (P <0.001, respectively). These improvements at 2 hours was accompanied by significant reductions in CD41+ platelet aggregation (P =0.010) and ssDNA+ cells (P <0.001), indicating significant amelioration in hepatic microcirculation and apoptosis, respectively. Characteristically, F4/80+ cells representing macrophages, mainly Kupffer cells, were maintained by anti-Properdin-Ab (P <0.001). Western blot showed decreased phosphorylation of only Erk1/2 among MAPKs (P =0.004). After 6 hours of reperfusion, anti-Properdin-Ab significantly attenuated the release of HMGB-1, which provokes the release of proinflammatory cytokines/chemokines (P =0.002). Infiltration of CD11b+ and Ly6-G+ cells, representing infiltrating macrophages and neutrophils, respectively, were significantly alleviated by anti-Properdin-Ab (both P <0.001). Notably, anti-Properdin-Ab did not affect remnant liver weight and BrdU+ cells at 48 hours after 70% partial hepatectomy (P =0.13 and 0.31, respectively). In conclusion, Properdin inhibition significantly ameliorates hepatic IRI without interfering with liver regeneration.

The role of platelets in immune-mediated inflammatory diseases

Immune-mediated inflammatory diseases (IMIDs) are characterized by excessive and uncontrolled inflammation and thrombosis, both of which are responsible for organ damage, morbidity and death. Platelets have long been known for their role in primary haemostasis, but they are now also considered to be components of the immune system and to have a central role in the pathogenesis of IMIDs. In patients with IMIDs, platelets are activated by disease-specific factors, and their activation often reflects disease activity. Here we summarize the evidence showing that activated platelets have an active role in the pathogenesis and the progression of IMIDs. Activated platelets produce soluble factors and directly interact with immune cells, thereby promoting an inflammatory phenotype. Furthermore, platelets participate in tissue injury and promote abnormal tissue healing, leading to fibrosis. Targeting platelet activation and targeting the interaction of platelets with the immune system are novel and promising therapeutic strategies in IMIDs.

Pediatric pulmonary valve replacements: Clinical challenges and emerging technologies

Congenital heart diseases (CHDs) frequently impact the right ventricular outflow tract, resulting in a significant incidence of pulmonary valve replacement in the pediatric population. While contemporary pediatric pulmonary valve replacements (PPVRs) allow satisfactory patient survival, their biocompatibility and durability remain suboptimal and repeat operations are commonplace, especially for very young patients. This places enormous physical, financial, and psychological burdens on patients and their parents, highlighting an urgent clinical need for better PPVRs. An important reason for the clinical failure of PPVRs is biofouling, which instigates various adverse biological responses such as thrombosis and infection, promoting research into various antifouling chemistries that may find utility in PPVR materials. Another significant contributor is the inevitability of somatic growth in pediatric patients, causing structural discrepancies between the patient and PPVR, stimulating the development of various growth-accommodating heart valve prototypes. This review offers an interdisciplinary perspective on these challenges by exploring clinical experiences, physiological understandings, and bioengineering technologies that may contribute to device development. It thus aims to provide an insight into the design requirements of next-generation PPVRs to advance clinical outcomes and promote patient quality of life.

Platelet extracellular vesicles: Darkness and light of autoimmune diseases

Autoimmune diseases are characterized by a breakdown of immune tolerance, leading to inflammation and irreversible end-organ tissue damage. Platelet extracellular vesicles are cellular elements that are important in blood circulation and actively participate in inflammatory and immune responses through intercellular communication and interactions between inflammatory cells, immune cells, and their secreted factors. Therefore, platelet extracellular vesicles are the "accelerator" in the pathological process of autoimmune diseases; however, this robust set of functions of platelet extracellular vesicles has also prompted new advances in therapeutic strategies for autoimmune diseases. In this review, we update fundamental mechanisms based on platelet extracellular vesicles communication function in autoimmune diseases. We also focus on the potential role of platelet extracellular vesicles for the treatment of autoimmune diseases. Some recent studies have found that antiplatelet aggregation drugs, specific biological agents can reduce the release of platelet extracellular vesicles. Platelet extracellular vesicles can also serve as vehicles to deliver drugs to targeted cells. It seems that we can try to silence or inhibit microRNA carried by platelet extracellular vesicles transcription and regulate the target cells to treat autoimmune diseases as platelet extracellular vesicles can transfer microRNA to other cells to regulate immune-inflammatory responses. Hopefully, the information presented here will provide hope for patients with autoimmune diseases.

Platelet-monocyte aggregates: molecular mediators of thromboinflammation

Platelets, key facilitators of primary hemostasis and thrombosis, have emerged as crucial cellular mediators of innate immunity and inflammation. Exemplified by their ability to alter the phenotype and function of monocytes, activated platelets bind to circulating monocytes to form monocyte-platelet aggregates (MPA). The platelet-monocyte axis has emerged as a key mechanism connecting thrombosis and inflammation. MPA are elevated across the spectrum of inflammatory and autoimmune disorders, including cardiovascular disease, systemic lupus erythematosus (SLE), and COVID-19, and are positively associated with disease severity. These clinical disorders are all characterized by an increased risk of thromboembolic complications. Intriguingly, monocytes in contact with platelets become proinflammatory and procoagulant, highlighting that this interaction is a central element of thromboinflammation.

Complement as a vital nexus of the pathobiological connectome for acute respiratory distress syndrome: An emerging therapeutic target

The hallmark of acute respiratory distress syndrome (ARDS) pathobiology is unchecked inflammation-driven diffuse alveolar damage and alveolar-capillary barrier dysfunction. Currently, therapeutic interventions for ARDS remain largely limited to pulmonary-supportive strategies, and there is an unmet demand for pharmacologic therapies targeting the underlying pathology of ARDS in patients suffering from the illness. The complement cascade (ComC) plays an integral role in the regulation of both innate and adaptive immune responses. ComC activation can prime an overzealous cytokine storm and tissue/organ damage. The ARDS and acute lung injury (ALI) have an established relationship with early maladaptive ComC activation. In this review, we have collected evidence from the current studies linking ALI/ARDS with ComC dysregulation, focusing on elucidating the new emerging roles of the extracellular (canonical) and intracellular (non-canonical or complosome), ComC (complementome) in ALI/ARDS pathobiology, and highlighting complementome as a vital nexus of the pathobiological connectome for ALI/ARDS via its crosstalking with other systems of the immunome, DAMPome, PAMPome, coagulome, metabolome, and microbiome. We have also discussed the diagnostic/therapeutic potential and future direction of ALI/ARDS care with the ultimate goal of better defining mechanistic subtypes (endotypes and theratypes) through new methodologies in order to facilitate a more precise and effective complement-targeted therapy for treating these comorbidities. This information leads to support for a therapeutic anti-inflammatory strategy by targeting the ComC, where the arsenal of clinical-stage complement-specific drugs is available, especially for patients with ALI/ARDS due to COVID-19.

Study of associations of blood proteins with development of unstable atherosclerotic plaques in coronary arteries by quantitative proteomics

Aim. To study the associations of blood proteins with the presence of unstable atherosclerotic plaques in the arteries in patients with coronary artery disease using the quantitative proteomic analysis.

Materials and methods. The study included patients with coronary artery disease (n = 40); the average age of patients was 58 ± 7 years. Material for the study was blood serum. Protein concentrations in serum samples were determined using the PeptiQuant Plus Proteomics Kit (Cambridge Isotope Laboratories, USA). Protein fractions were identified using the liquid chromatograph and tandem mass spectrometer Q-TRAP 6500.

Results. Mass spectrometry revealed an increased concentration of proteins, such as fibrinogen, fibulin-1, and complement factor H, in the serum samples of patients with unstable atherosclerotic plaques. It took place with a simultaneous decrease in the levels of α 2-antiplasmin, heparin cofactor II, coagulation factor XII, plasminogen, prothrombin, vitronectin, complement proteins (C1, C3, C7, C9), and complement factor B. The differences were considered significant at p < 0.05. It was revealed that the presence of unstable atherosclerotic plaques was associated with the level of fibulin-1 (Exp(B) = 1.008; р = 0.05), plasminogen (Exp(В) = 0.995; р = 0.027), and coagulation factor X (Exp(В) = 0.973; р = 0.037).

Conclusion. An increased concentration of fibulin-1 can be considered as a potential biomarker of unstable atherosclerotic plaque development in coronary artery disease. The possibility of using the studied proteins as biomarkers of unstable atherosclerotic plaques requires further studies on their potential role in the development of this disease.

The complement alternative pathway and hemostasis

The complement and hemostatic systems are complex systems, and both involve enzymatic cascades, regulators, and cell components-platelets, endothelial cells, and immune cells. The two systems are ancestrally related and are defense mechanisms that limit infection by pathogens and halt bleeding at the site of vascular injury. Recent research has uncovered multiple functional interactions between complement and hemostasis. On one side, there are proteins considered as complement factors that activate hemostasis, and on the other side, there are coagulation proteins that modulate complement. In addition, complement and coagulation and their regulatory proteins strongly interact each other to modulate endothelial, platelet and leukocyte function and phenotype, creating a potentially devastating amplifying system that must be closely regulated to avoid unwanted damage and\or disseminated thrombosis. In view of its ability to amplify all complement activity through the C3b-dependent amplification loop, the alternative pathway of complement may play a crucial role in this context. In this review, we will focus on available and emerging evidence on the role of the alternative pathway of complement in regulating hemostasis and vice-versa, and on how dysregulation of either system can lead to severe thromboinflammatory events.

Indications that the Antimycotic Drug Amphotericin B Enhances the Impact of Platelets on Aspergillus

Platelets are currently thought to harbor antimicrobial functions and might therefore play a crucial role in infections, e.g., those caused by Aspergillus or mucormycetes. The incidence of invasive fungal infections is increasing, particularly during the coronavirus disease 2019 (COVID-19) pandemic, and such infections continue to be life-threatening in immunocompromised patients. For this reason, the interaction of antimycotics with platelets is a key issue to evaluate modern therapeutic regimens. Amphotericin B (AmB) is widely used for the therapy of invasive fungal infections either as deoxycholate (AmB-D) or as a liposomal formulation (L-AmB). We showed that AmB strongly activates platelets within a few minutes. AmB concentrations commonly measured in the blood of patients were sufficient to stimulate platelets, indicating that this effect is highly relevant in vivo. The stimulating effect was corroborated by a broad spectrum of platelet activation parameters, including degranulation, aggregation, budding of microparticles, morphological changes, and enhanced adherence to fungal hyphae. Comparison between the deoxycholate and the liposomal formulation excluded the possibility that the liposomal part of L-Amb is responsible for these effects, as no difference was visible. The induction of platelet activation and alteration by L-AmB resulted in the activation of other parts of innate immunity, such as stimulation of the complement cascade and interaction with granulocytes. These mechanisms might substantially fuel the antifungal immune reaction in invasive mycoses. On the other hand, thrombosis and excessive inflammatory processes might occur via these mechanisms. Furthermore, the viability of L-AmB-activated platelets was consequently decreased, a process that might contribute to thrombocytopenia in patients.

Coagulation and complement: Key innate defense participants in a seamless web

In 1969, Dr. Oscar Ratnoff, a pioneer in delineating the mechanisms by which coagulation is activated and complement is regulated, wrote, “In the study of biological processes, the accumulation of information is often accelerated by a narrow point of view. The fastest way to investigate the body’s defenses against injury is to look individually at such isolated questions as how the blood clots or how complement works. We must constantly remind ourselves that such distinctions are man-made. In life, as in the legal cliché, the devices through which the body protects itself form a seamless web, unwrinkled by our artificialities.” Our aim in this review, is to highlight the critical molecular and cellular interactions between coagulation and complement, and how these two major component proteolytic pathways contribute to the seamless web of innate mechanisms that the body uses to protect itself from injury, invading pathogens and foreign surfaces.

Copy number variation analysis using next-generation sequencing identifies the CFHR3/CFHR1 deletion in atypical hemolytic uremic syndrome: a case report

OBJECTIVES

Atypical hemolytic uremic syndrome (aHUS) is characterized by a triad of thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure resulting from platelet thrombi in the microcirculation of the kidney and other organs, in the absence of a preceding diarrheal illness. This report describes a case in which copy number variation (CNV) analysis using next-generation sequencing (NGS) identified the CFHR3/CFHR1 deletion in a patient with aHUS.

METHODS

A 49-year-old Korean female was diagnosed with aHUS based on clinical findings, including schistocytes in peripheral blood and marked thrombocytopenia, suggesting the presence of thrombotic microangiopathy, elevated serum lactate dehydrogenase, and acute kidney injury. Sequence variants and CNV generated from NGS data were estimated to determine if there was a potential genetic cause. Multiplex ligation-dependent probe amplification (MLPA) was conducted to confirm the CFHR3/CFHR1 deletion identified by NGS with CNV analysis.

RESULTS

No known or novel pathogenic single nucleotide variant or small insertion/deletion that would be predicted to have damaging effects that could lead to aHUS were identified. However, CNV analysis of NGS data identified the heterozygous CFHR3/CFHR1 deletion. MLPA confirmed this loss of one copy number between the CFHR3 and the CFHR1 genes on chromosome 1q31.3.

CONCLUSION

We genetically diagnosed a Korean woman harboring a heterozygous CFHR3/CFHR1 deletion of a known causative gene for aHUS. Our report emphasizes the need for CNV analysis of NGS data and gene dosage assays, such as MLPA, to evaluate large-scale deletions or duplications and generate hybrid CFH genes in patients with suspected aHUS.

Current Status of and Global Trends in Platelet Transfusion Refractoriness From 2004 to 2021: A Bibliometric Analysis

Platelet transfusion refractoriness (PTR) is common in patients with hematology and oncology and is becoming an important barrier in the treatment of thrombocytopenia and hemorrhage. Bibliometrics is an effective method for identifying existing research achievements, important breakthroughs, current research hotspots, and future development trends in any given field. In recent years, research on PTR has received increasing attention, but a bibliometric analysis of this field has not yet been reported. In this study, we applied bibliometrics to analyze the existing literature on PTR research over the past 17 years. On November 1, 2021, we began a publications analysis of PTR research using the Science Citation Index Expanded of the Web of Science Core Collection with collection dates from 2004 to 2021. This research aimed to summarize the state of PTR research using Bibliometrix to identify connections between different elements (i.e., authors, institutions, countries, journals, references, and keywords) using VOS viewer analyses to visualize key topics and trends in PTR research using Cite Space and gCLUTO. The results of all 310 studies showed that the annual number of publications focused on PTR is steadily increasing, with the United States of America and Japan making significant contributions. We noted that the research group led by Dr. Sherrill J. Slichter was prominent in this field, while Estcourt Lise may become the most influential newcomer. Transfusion was the most popular journal, and Blood was the most cited journal. Using various analyses, including co-cited analysis, historiography analysis, citation burst analysis, and factorial analysis, we pointed out and discussed contributing publications. According to occurrence analysis, co-word biclustering analysis, landform map, thematic evolution, and thematic map, we believe that “activation,” “p-selection,” “CD36 deficiency,” “gene-frequencies,” “CD109,” “HPA-1,” and “beta (3) integrin” may become new trends in PTR research. The outcome of our bibliometric analyses has, for the first time, revealed profound insights into the current state and trends in PTR research. The systematic analysis provided by our study clearly demonstrates the field's significant advancements to all researchers who are interested in a quick and comprehensive introduction to the field.

Enrichment of complement, immunoglobulins and autoantibody targets in the proteome of platelets from patients with Systemic Lupus Erythematosus (SLE)

Background  Systemic lupus erythematosus (SLE) is a complex disease characterized by autoimmunity toward apoptotic cells, excessive amounts of circulating immune complexes, and complement activation. A decreased platelet size has been observed in SLE and their nonhemostatic functions may play an active role in the disease. The main objective of this study was to find clues that could explain their decreased size and functional role, analyzing the entire platelet proteome.

Methods  Platelets were isolated from 23 patients with SLE. The five individuals with the highest and lowest average platelet forward scatter were selected for further analysis. Platelet protein content was analyzed using liquid chromatography with tandem mass spectrometry (LC-MS/MS) and compared with platelets from five healthy controls. Data are available via ProteomeXchange with identifier PXD031202.

Results  Out of 2,572 proteins identified, 396 had significantly different levels (ANOVA q -value ≤ 0.01). Forty proteins, including immunoglobulin-, complement- and phosphatidylserine-binding proteins had higher abundance in platelets from SLE patients, largely independent of size (fold difference of ≥1.5 and a t -test p -value of ≤0.05 as cut-off). Functional characterization revealed increased degranulation and skewed hemostatic balance in platelets from SLE patients. In the SLE proteome, immunoglobulin proteins were negatively correlated to serum complement C3 and C4 and the highest relative levels were detected in platelets of normal size.

Conclusion  Platelets from SLE patients shared a specific protein profile, including immunoglobulins, complement proteins, and autoantigens, largely independent of the platelet size and in agreement with an integrated role for platelets in SLE.

Disseminated intravascular coagulation and its immune mechanisms

Disseminated intravascular coagulation (DIC) is a syndrome triggered by infectious and noninfectious pathologies characterized by excessive generation of thrombin within the vasculature and widespread proteolytic conversion of fibrinogen. Despite diverse clinical manifestations ranging from thrombo-occlusive damage to bleeding diathesis, DIC etiology commonly involves excessive activation of blood coagulation and overlapping dysregulation of anticoagulants and fibrinolysis. Initiation of blood coagulation follows intravascular expression of tissue factor or activation of the contact pathway in response to pathogen-associated or host-derived, damage-associated molecular patterns. The process is further amplified through inflammatory and immunothrombotic mechanisms. Consumption of anticoagulants and disruption of endothelial homeostasis lower the regulatory control and disseminate microvascular thrombosis. Clinical DIC development in patients is associated with worsening morbidities and increased mortality, regardless of the underlying pathology; therefore, timely recognition of DIC is critical for reducing the pathologic burden. Due to the diversity of triggers and pathogenic mechanisms leading to DIC, diagnosis is based on algorithms that quantify hemostatic imbalance, thrombocytopenia, and fibrinogen conversion. Because current diagnosis primarily assesses overt consumptive coagulopathies, there is a critical need for better recognition of nonovert DIC and/or pre-DIC states. Therapeutic strategies for patients with DIC involve resolution of the eliciting triggers and supportive care for the hemostatic imbalance. Despite medical care, mortality in patients with DIC remains high, and new strategies, tailored to the underlying pathologic mechanisms, are needed.

Biomaterial and cellular implants: foreign surfaces where immunity and coagulation meet

Exposure of blood to a foreign surface in the form of a diagnostic or therapeutic biomaterial device or implanted cells or tissue elicits an immediate, evolutionarily conserved thromboinflammatory response from the host. Primarily designed to protect against invading organisms after an injury, this innate response features instantaneous activation of several blood-borne, highly interactive, well-orchestrated cascades and cellular events that limit bleeding, destroy and eliminate the foreign substance or cells, and promote healing and a return to homeostasis via delicately balanced regenerative processes. In the setting of blood-contacting synthetic or natural biomaterials and implantation of foreign cells or tissues, innate responses are robust, albeit highly context specific. Unfortunately, they tend to be less than adequately regulated by the host's natural anticoagulant or anti-inflammatory pathways, thereby jeopardizing the functional integrity of the device, as well as the health of the host. Strategies to achieve biocompatibility with a sustained return to homeostasis, particularly while the device remains in situ and functional, continue to elude scientists and clinicians. In this review, some of the complex mechanisms by which biomaterials and cellular transplants provide a "hub" for activation and amplification of coagulation and immunity, thromboinflammation, are discussed, with a view toward the development of innovative means of overcoming the innate challenges.

TSST-1 protein exerts indirect effect on platelet activation and apoptosis

Thrombocytopenia or platelet dysfunction is a risk factor for severe infection. Staphylococcus aureus (S. aureus) releases a variety of virulence factors especially toxic shock syndrome toxin 1 (TSST-1), which may cause toxic shock syndrome. S. aureus, when carrying the tst gene, is more prone to cause toxic shock syndrome and is responsible for an especially high rate of mortality. However, the effect of TSST-1 protein on platelets is unknown. Patients with the tst gene positive S. aureus bacteremia showed more serious infection, higher mortality and lower platelet count. The tst gene positive S. aureus strains induce more platelet apoptosis and activation and corresponding up-regulation of Bak and down-regulation of Bcl-XL in addition to the activation of Caspase-3. C57BL/6 mice infected with the tst gene positive strains resulted in both a decrease in platelet count and an increase in platelet apoptosis and/or activation events and mortality. Moreover, TSST-1 protein, encoded by tst gene, caused the decrease of platelet count, the increase of platelet apoptosis and activation events and the level of inflammatory cytokines in vivo. However, TSST-1 protein was unable to induce traditional activation and apoptosis on human platelets in vitro. These results suggested that TSST-1 protein may exert indirect effects on platelet activation and apoptosis in vivo.

Vascular Dementia and Crosstalk Between the Complement and Coagulation Systems

Vascular Dementia (VaD) is a neurocognitive disorder caused by reduced blood flow to the brain tissue, resulting in infarction, and is the second most common type of dementia. The complement and coagulation systems are evolutionary host defence mechanisms activated by acute tissue injury to induce inflammation, clot formation and lysis; recent studies have revealed that these systems are closely interlinked. Overactivation of these systems has been recognised to play a key role in the pathogenesis of neurological disorders such as Alzheimer's disease and multiple sclerosis, however their role in VaD has not yet been extensively reviewed. This review aims to bridge the gap in knowledge by collating current understanding of VaD to enable identification of complement and coagulation components involved in the pathogenesis of this disorder that may have their effects amplified or supressed by crosstalk. Exploration of these mechanisms may unveil novel therapeutic targets or biomarkers that would improve current treatment strategies for VaD.

COVID-19, Pre-Eclampsia, and Complement System

COVID-19 is characterized by virus-induced injury leading to multi-organ failure, together with inflammatory reaction, endothelial cell (EC) injury, and prothrombotic coagulopathy with thrombotic events. Complement system (C) via its cross-talk with the contact and coagulation systems contributes significantly to the severity and pathological consequences due to SARS-CoV-2 infection. These immunopathological mechanisms overlap in COVID-19 and pre-eclampsia (PE). Thus, mothers contracting SARS-CoV-2 infection during pregnancy are more vulnerable to developing PE. SARS-CoV-2 infection of ECs, via its receptor ACE2 and co-receptor TMPRSS2, can provoke endothelial dysfunction and disruption of vascular integrity, causing hyperinflammation and hypercoagulability. This is aggravated by bradykinin increase due to inhibition of ACE2 activity by the virus. C is important for the progression of normal pregnancy, and its dysregulation can impact in the form of PE-like syndrome as a consequence of SARS-CoV-2 infection. Thus, there is also an overlap between treatment regimens of COVID-19 and PE. C inhibitors, especially those targeting C3 or MASP-2, are exciting options for treating COVID-19 and consequent PE. In this review, we examine the role of C, contact and coagulation systems as well as endothelial hyperactivation with respect to SARS-CoV-2 infection during pregnancy and likely development of PE.

The strategy of modulation blood responses by surface modification with different functional groups on polyester film

A main problem in the design of blood-contacting biomaterials has been the deficiency of a systematic understanding of blood-biomaterial interactions and the strategy to modulate blood responses. In this work, different functional groups including carboxyl (COOH), hydroxyl (OH) and zwitterionic sulfobetaine group (⊕N((CH₃ )₂ )(CH₂ )₃ SO_3-○- , SMDB) were grafted on the poly (butylene terephthalate) (PBT) film to study how the functional groups modulate blood responses and in terms of interaction with the coagulation system, the complement system, and platelets. The results showed protein absorption and platelet adhesion was stronger on the PBT bearing COOH group than PBT films bearing OH and zwitterionic sulfobetaine groups (total protein (μg/cm² ): 32.92 ± 5.89 vs. 22.02 ± 1.44 vs. 19.09 ± 1.59; platelet adhesion (/mm² ): 1,626.7 ± 120.1 vs. 1,395.6 ± 363.3 vs. 1,102.2 ± 373.7), which had a rougher and negatively charged surface, and the coagulation system was inhibited by binding fibrinogen (Fg) and coagulation factors. Meanwhile, PBT-PSMDB showed anticoagulant property and induced platelet activation. As a result, complement formation on these two films were less than PBT bearing OH groups by inhibiting the coagulation system (C3a (ng/ml): 3,745.4 ± 143.9 vs. 3,290.9 ± 249.7 vs. 4,887.9 ± 88.9; C5a (ng/ml): 22.1 ± 2.6 vs. 22.3 ± 1.8 vs. 27.9 ± 2.0). On the other hand, PBT bearing OH groups did not facilitate remarkable platelet adhesion and activation, and had no influence on platelet aggregation, hypotonic shock response, and coagulation system. The above results showed that the blood responses were highly interlinked, and could be modulated by grafting with different functional groups on the biomaterial surfaces. These findings may help identify a strategy to design materials with better hemocompatibility for blood contact, filtration, and purification applications.

Complement, inflammation and thrombosis

A mutual relationship exists between immune activation and mechanisms of thrombus formation. In particular, elements of the innate immune response such as the complement system can modulate platelet activation and subsequently thrombus formation. Several components of the complement system including C3 or the membrane attack complex have been reported to be associated with platelets and become functionally active in the micromilieu of platelet activation. The exact mechanisms how this interplay is regulated and its consequences for tissue inflammation, damage or recovery remain to be defined. This review addresses the current state of knowledge on this topic and puts it into context with diseases featuring both thrombosis and complement activation. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Complement 5 Inhibition Ameliorates Hepatic Ischemia/reperfusion Injury in Mice, Dominantly via the C5a-mediated Cascade

BACKGROUND

Hepatic ischemia/reperfusion injury (IRI) is a serious complication in liver surgeries, including transplantation. Complement activation seems to be closely involved in hepatic IRI; however, no complement-targeted intervention has been clinically applied. We investigated the therapeutic potential of Complement 5 (C5)-targeted regulation in hepatic IRI.

METHODS

C5-knockout (B10D2/oSn) and their corresponding wild-type mice (WT, B10D2/nSn) were exposed to 90-minute partial (70%) hepatic ischemia/reperfusion with either anti-mouse-C5 monoclonal antibody (BB5.1) or corresponding control immunoglobulin administration 30 minutes before ischemia. C5a receptor 1 antagonist was also given to WT to identify which cascade, C5a or C5b-9, is dominant.

RESULTS

C5-knockout and anti-C5-Ab administration to WT both significantly reduced serum transaminase release and histopathological damages from 2 hours after reperfusion. This improvement was characterized by significantly reduced CD41+ platelet aggregation, maintained F4/80+ cells, and decreased high-mobility group box 1 release. After 6 hours of reperfusion, the infiltration of CD11+ and Ly6-G+ cells, cytokine/chemokine expression, single-stranded DNA+ cells, and cleaved caspase-3 expression were all significantly alleviated by anti-C5-Ab. C5a receptor 1 antagonist was as effective as anti-C5-Ab for reducing transaminases.

CONCLUSIONS

Anti-C5 antibody significantly ameliorated hepatic IRI, predominantly via the C5a-mediated cascade, not only by inhibiting platelet aggregation during the early phase but also by attenuating the activation of infiltrating macrophages/neutrophils and hepatocyte apoptosis in the late phase of reperfusion. Given its efficacy, clinical availability, and controllability, C5-targeted intervention may provide a novel therapeutic strategy against hepatic IRI.

MASP2 levels are elevated in thrombotic microangiopathies: association with microvascular endothelial cell injury and suppression by anti-MASP2 antibody narsoplimab

Involvement of the alternative complement pathway (AP) in microvascular endothelial cell (MVEC) injury characteristic of a thrombotic microangiopathy (TMA) is well documented. However, the role of the lectin pathway (LP) of complement has not been explored. We examined mannose-binding lectin associated serine protease (MASP2), the effector enzyme of the LP, in thrombotic thrombocytopenic purpura, atypical hemolytic uremic syndrome and post-allogeneic hematopoietic stem cell transplantation (alloHSCT) TMAs. Plasma MASP2 and terminal complement component sC5b-9 levels were assessed by enzyme-linked immunosorbent assay (ELISA). Human MVEC were exposed to patient plasmas, and the effect of the anti-MASP2 human monoclonal antibody narsoplimab on plasma-induced MVEC activation was assessed by caspase 8 activity. MASP2 levels were highly elevated in all TMA patients versus controls. The relatively lower MASP2 levels in alloHSCT patients with TMAs compared to levels in alloHSCT patients who did not develop a TMA, and a significant decrease in variance of MASP2 levels in the former, may reflect MASP2 consumption at sites of disease activity. Plasmas from 14 of the 22 TMA patients tested (64%) induced significant MVEC caspase 8 activation. This was suppressed by clinically relevant levels of narsoplimab (1·2 μg/ml) for all 14 patients, with a mean 65·7% inhibition (36.8-99.4%; P < 0·0001). In conclusion, the LP of complement is activated in TMAs of diverse etiology. Inhibition of MASP2 reduces TMA plasma-mediated MVEC injury in vitro. LP inhibition therefore may be of therapeutic benefit in these disorders.

Docked severe acute respiratory syndrome coronavirus 2 proteins within the cutaneous and subcutaneous microvasculature and their role in the pathogenesis of severe coronavirus disease 2019

The purpose of this study was to examine the deltoid skin biopsy in twenty-three patients with coronavirus disease 2019 (COVID-19), most severely ill, for vascular complement deposition and correlate this with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral RNA and protein localization and ACE2 expression. Deltoid skin microvascular complement screening has been applied to patients with various systemic complement-mediated microvascular syndromes, best exemplified by atypical hemolytic uremic syndrome. In 21 of 23 cases, substantial microvascular deposition of complement components was identified. The two patients without significant complement deposition included one patient with moderate disease and a severely ill patient who although on a ventilator for a day was discharged after 3 days. The dominant microvascular complement immunoreactant identified was the terminal membranolytic attack complex C5b-9. Microvascular complement deposition strongly colocalized in situ with the SARS-CoV-2 viral proteins including spike glycoproteins in the endothelial cells as well as the viral receptor ACE2 in lesional and nonlesional skin; viral RNA was not evident. Microvascular SARS-CoV-2 viral protein, complement, and ACE2 expression was most conspicuous in the subcutaneous fat. Although the samples from severely ill patients with COVID-19 were from grossly normal skin, light microscopically focal microvascular abnormalities were evident that included endothelial cell denudement, basement membrane zone reduplication, and small thrombi. It is concluded that complement activation is common in grossly normal skin, especially in the subcutaneous fat which may provide a link between severe disease and obesity, in people with severe COVID-19, and the strong colocalization with the ACE2 receptor and viral capsid proteins without viral RNA suggests that circulating viral proteins (ie, pseudovirions) may dock onto the endothelial of these microvessels and induce complement activation.

Aspergillus-Derived Galactosaminogalactan Triggers Complement Activation on Human Platelets

Invasive fungal infections caused by Aspergillus (A.) and Mucorales species still represent life-threatening diseases in immunocompromised individuals, and deeper knowledge about fungal interactions with elements of innate immunity, such as complement and platelets, appears essential for optimized therapy. Previous studies showed that galactosaminogalactan secreted by A. fumigatus and A. flavus is deposited on platelets, thereby inducing their activation. Since the altered platelet surface is a putative trigger for complement activation, we aimed to study the interplay of platelets with complement in the presence of fungal GAG. Culture supernatants (SN) of A. fumigatus and A. flavus both induced not only GAG deposition but also subsequent deposition of complement C3 fragments on the platelet surface. The SN of a Δuge3 mutant of A. fumigatus, which is unable to synthesize GAG, did not induce complement deposition on platelets, nor did the SN of other Aspergillus species and all tested Mucorales. Detailed analysis revealed that GAG deposition itself triggered the complement cascade rather than the GAG-induced phosphatidylserine exposure. The lectin pathway of complement could be shown to be crucially involved in this process. GAG-induced complement activation on the platelet surface was revealed to trigger processes that might contribute to the pathogenesis of invasive aspergillosis by A. fumigatus or A. flavus. Both pro-inflammatory anaphylatoxins C3a and C5a arose when platelets were incubated with SN of these fungal species; these processes might favor excessive inflammation after fungal infection. Furthermore, platelets were stimulated to shed microparticles, which are also known to harbor pro-inflammatory and pro-coagulant properties. Not only did early processes of the complement cascade proceed on platelets, but also the formation of the terminal complement C5b-9 complex was detected on platelets after incubation with fungal SN. Subsequently, reduced viability of the platelets could be shown, which might contribute to the lowered platelet numbers found in infected patients. In summary, fungal GAG initiates an interplay between complement and platelets that can be supposed to contribute to excessive inflammation, thrombocytopenia, and thrombosis, which are important hallmarks of fatal invasive mycoses.

Complementopathies and precision medicine

The renaissance of complement diagnostics and therapeutics has introduced precision medicine into a widened field of complement-mediated diseases. In particular, complement-mediated diseases (or complementopathies) with ongoing or published clinical trials of complement inhibitors include paroxysmal nocturnal hemoglobinuria, cold agglutinin disease, hemolytic uremic syndrome, nephropathies, HELLP syndrome, transplant-associated thrombotic microangiopathy, antiphospholipid antibody syndrome, myasthenia gravis, and neuromyelitis optica. Recognizing that this field is rapidly expanding, we aim to provide a state-of-the-art review of (a) current understanding of complement biology for the clinician, (b) novel insights into complement with potential applicability to clinical practice, (c) complement in disease across various disciplines (hematology, nephrology, obstetrics, transplantation, rheumatology, and neurology), and (d) the potential future of precision medicine. Better understanding of complement diagnostics and therapeutics will not only facilitate physicians treating patients in clinical practice but also provide the basis for future research toward precision medicine in this field.

Platelets: emerging facilitators of cellular crosstalk in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease in which a variety of circulating pro-inflammatory cells and dysregulated molecules are involved in disease aetiology and progression. Platelets are an important cellular element in the circulation that can bind several dysregulated molecules (such as collagen, thrombin and fibrinogen) that are present both in the synovium and the circulation of patients with RA. Platelets not only respond to dysregulated molecules in their environment but also transport and express their own inflammatory mediators, and serve as regulators at the boundary between haemostasis and immunity. Activated platelets also produce microparticles, which further convey signalling molecules and receptors to the synovium and circulation, thereby positioning these platelet-derived particles as strategic regulators of inflammation. These diverse functions come together to make platelets facilitators of cellular crosstalk in RA. Thus, the receptor functions, ligand binding potential and dysregulated signalling pathways in platelets are becoming increasingly important for treatment in RA. This Review aims to highlight the role of platelets in RA and the need to closely examine platelets as health indicators when designing effective pharmaceutical targets in this disease.

Interactions of Alginate-Deferoxamine Conjugates With Blood Components and Their Antioxidation in the Hemoglobin Oxidation Model

While deferoxamine (DFO) has long been used as an FDA-approved iron chelator, its proangiogenesis ability attracts increasing number of research interests. To address its drawbacks such as short plasma half-life and toxicity, polymeric conjugated strategy has been proposed and shown superiority. Owing to intravenous injection and application in blood-related conditions, however, the blood interactions and antioxidation of the DFO-conjugates and the mechanisms underlying these outcomes remain to be elucidated. In this regard, incubating with three different molecular-weight (MW) alginate-DFO conjugates (ADs) red blood cells (RBCs), coagulation system, complement and platelet were investigated. To prove the antioxidant activity of ADs, we used hemoglobin oxidation model in vitro. ADs did not cause RBCs hemolysis while reversible aggregation and normal deformability ability were observed. However, the coagulation time, particularly APTT and TT, were significantly prolonged in a dose-dependent manner, and fibrinogen was dramatically decreased, suggesting ADs could dominantly inhibit the intrinsic pathways in the process of coagulation. The dose-dependent anticoagulation might be related with the functional groups along the alginate chains. The complements, C3a and C5a, were activated by ADs in a dose-dependent manner through alternative pathway. For platelet, ADs slightly suppressed the activation and aggregation at low concentration. Based on above results, the cross-talking among coagulation, complement and platelet induced by ADs was proposed. The antioxidation of ADs through iron chelation was proved and the antioxidant activity was shown in a MW-dependent manner.

The role of anti-complement factor H antibodies in the development of atypical haemolytic uremic syndrome: a possible contribution to abnormality of platelet function

Atypical haemolytic uremic syndrome (aHUS) is associated with complement system abnormality, such as production of complement factor H (CFH) autoantibodies. The growing evidence indicates complement overactivation on platelets is intimately involved in aHUS pathogenesis, besides endothelial injury. We here showed plasma from patients with anti-CFH antibodies induced aggregation of washed platelets, while purified anti-CFH antibodies suppressed aggregation. This suggested anti-CFH antibody itself suppressed thrombosis, while other plasma factor including complement factors could overactivate the platelets, leading to aggregation, which augmented the notion the state of complement activation influenced by anti-CFH antibodies is important in the aggregation of platelets in aHUS.

Platelets and platelet extracellular vesicles in hemostasis and sepsis

Platelets, cell fragments traditionally thought of as important only for hemostasis, substantially and dynamically contribute to the immune system's response to infection. In addition, there is increasing evidence that externally active platelet entities, including platelet granules and platelet extracellular vesicles (PEVs), play a role not only in hemostasis, but also in inflammatory actions previously ascribed to platelets themselves. Given the functions of platelets and PEVs during inflammation and infection, their role in sepsis is being investigated. Sepsis is a condition marked by the dysregulation of the body's normal activation of the immune system in response to a pathogen. The mechanisms for controlling infection locally become detrimental to the host if they are applied systemically. Similar to cells traditionally ascribed to the immune system, including neutrophils, lymphocytes, and macrophages, platelets are instrumental in helping a host clear an infection, but are also implicated in the uncontrolled amplification of the immune response that leads to sepsis. Clearly, the function of platelets is more complicated than its simple structure and primary role in hemostasis initially suggest. This review provides an overview of platelet and platelet extracellular vesicle structure and function, highlighting the complex role platelets and PEVs play in the body in the context of infection and sepsis.

Complement as an Immune Barrier in Platelet Transfusion Refractoriness

Patients with hematological cancers often have low platelet counts because of progressing bone marrow failure or cytostatic therapy. A large fraction of those patients need platelet transfusions, which can be life-saving if bleedings occur and also allow diagnostic and therapeutic interventions. The outcomes of platelet transfusions are not always easy to predict in terms of bleeding control or increase in platelet count. Reasons could be disease-specific factors, fever, or infections leading to platelet consumption, but the immune system may also be involved, in particular, in patients previously immunized against foreign human leukocyte antigens (HLA). Mechanisms underlying immune-mediated platelet destruction in the presence of antibodies again HLA are not well understood in clinical situations. This review discusses the role of complement in platelet refractoriness, with a focus on HLA antibody-mediated platelet refractoriness. We summarize recent work in this area, discuss complement-platelet interactions in general terms, and a suggest a possible role of complement in platelet transfusion in general.

Platelets and Complement Cross-Talk in Early Atherogenesis

Atherosclerosis remains a ubiquitous and serious threat to human health. The initial formation of the atherosclerotic lesion (atheroma) is driven by pro-inflammatory signaling involving monocytes and vascular endothelial cells; later stages of the disease involve rupture of well-established atherosclerotic plaques, thrombosis, and blood vessel occlusion. While the central role of platelets in thrombosis is undisputed, platelets exhibit pro-inflammatory activities, and contribute to early-stage atheroma formation. Platelets also engage components of the complement system, an essential element of innate immunity that contributes to vascular inflammation. Here we provide an overview of the complex interplay between platelets and the complement system, with a focus on how the crosstalk between them may impact on the initiation of atheroma formation.

The non-haemostatic role of platelets in systemic lupus erythematosus

Dysregulation of lymphocyte function, accumulation of autoantibodies and defective clearance of circulating immune complexes and apoptotic cells are hallmarks of systemic lupus erythematosus (SLE). Moreover, it is now evident that an intricate interplay between the adaptive and innate immune systems contributes to the pathogenesis of SLE, ultimately resulting in chronic inflammation and organ damage. Platelets circulate in the blood and are chiefly recognized for their role in the prevention of bleeding and promotion of haemostasis; however, accumulating evidence points to a role for platelets in both adaptive and innate immunity. Through a broad repertoire of receptors, platelets respond promptly to immune complexes, complement and damage-associated molecular patterns, and represent a major reservoir of immunomodulatory molecules in the circulation. Furthermore, evidence suggests that platelets are activated in patients with SLE, and that they could contribute to the circulatory autoantigenic load through the release of microparticles and mitochondrial antigens. Herein, we highlight how platelets contribute to the immune response and review evidence implicating platelets in the pathogenesis of SLE.

Mannose-Binding Lectin Drives Platelet Inflammatory Phenotype and Vascular Damage After Cerebral Ischemia in Mice via IL (Interleukin)-1α

Supplemental Digital Content is available in the text.

Objective—

Circulating complement factors are activated by tissue damage and contribute to acute brain injury. The deposition of MBL (mannose-binding lectin), one of the initiators of the lectin complement pathway, on the cerebral endothelium activated by ischemia is a major pathogenic event leading to brain injury. The molecular mechanisms through which MBL influences outcome after ischemia are not understood yet.

Approach and Results—

Here we show that MBL-deficient (MBL^−/−) mice subjected to cerebral ischemia display better flow recovery and less plasma extravasation in the brain than wild-type mice, as assessed by in vivo 2-photon microscopy. This results in reduced vascular dysfunction as shown by the shift from a pro- to an anti-inflammatory vascular phenotype associated with MBL deficiency. We also show that platelets directly bind MBL and that platelets from MBL^−/− mice have reduced inflammatory phenotype as indicated by reduced IL-1α (interleukin-1α) content, as early as 6 hours after ischemia. Cultured human brain endothelial cells subjected to oxygen-glucose deprivation and exposed to platelets from MBL^−/− mice present less cell death and lower CXCL1 (chemokine [C-X-C motif] ligand 1) release (downstream to IL-1α) than those exposed to wild-type platelets. In turn, MBL deposition on ischemic vessels significantly decreases after ischemia in mice treated with IL-1 receptor antagonist compared with controls, indicating a reciprocal interplay between MBL and IL-1α facilitating endothelial damage.

Conclusions—

We propose MBL as a hub of pathogenic vascular events. It acts as an early trigger of platelet IL-1α release, which in turn favors MBL deposition on ischemic vessels promoting an endothelial pro-inflammatory phenotype.

Role of complement system in pathological remodeling of the vascular wall

Cardiovascular diseases (CVD) remain the major cause of morbidity and mortality in Europe. The clinical complications associated to arterial wall rupture involve intimal cap rupture in complicated atherosclerotic plaques and medial rupture in abdominal aortic aneurysm (AAA). The mechanisms underlying pathological vascular remodeling include lipid accumulation, cell proliferation, redox imbalance, proteolysis, leukocyte infiltration, cell death, and eventually, thrombosis. The complement system could participate in vascular remodeling by several mechanisms, from an initial protective response that aims in the clearing of cell debris to a potential deleterious role participating in leukocyte chemotaxis and cell activation and bridging innate and adaptive immunity. We have reviewed the presence and distribution of complement components, as well as the triggers of complement activation in atherosclerotic plaques and AAA, to later assess the functional consequences of complement modulation in experimental models of pathological vascular remodeling and the potential role of complement components as potential circulating biomarkers of CVD. On the whole, complement system is a key mechanism involved in vascular remodelling, which could be useful in the diagnostic/prognostic setting, as well as a potential therapeutic target, of CVD.

The Human Platelet as an Innate Immune Cell: Interactions Between Activated Platelets and the Complement System

Platelets play an essential role in maintaining homeostasis in the circulatory system after an injury by forming a platelet thrombus, but they also occupy a central node in the intravascular innate immune system. This concept is supported by their extensive interactions with immune cells and the cascade systems of the blood. In this review we discuss the close relationship between platelets and the complement system and the role of these interactions during thromboinflammation. Platelets are protected from complement-mediated damage by soluble and membrane-expressed complement regulators, but they bind several complement components on their surfaces and trigger complement activation in the fluid phase. Furthermore, localized complement activation may enhance the procoagulant responses of platelets through the generation of procoagulant microparticles by insertion of sublytic amounts of C5b9 into the platelet membrane. We also highlight the role of post-translational protein modifications in regulating the complement system and the critical role of platelets in driving these reactions. In particular, modification of disulfide bonds by thiol isomerases and protein phosphorylation by extracellular kinases have emerged as important mechanisms to fine-tune complement activity in the platelet microenvironment. Lastly, we describe disorders with perturbed complement activation where part of the clinical presentation includes uncontrolled platelet activation that results in thrombocytopenia, and illustrate how complement-targeting drugs are alleviating the prothrombotic phenotype in these patients. Based on these clinical observations, we discuss the role of limited complement activation in enhancing platelet activation and consider how these drugs may provide opportunities for further dissecting the complex interactions between complement and platelets.

Platelets in Skin Autoimmune Diseases

Systemic lupus erythematosus (SLE), systemic sclerosis (SSc), and small vessel vasculitis are three autoimmune diseases frequently manifested in the skin. They share common pathogenic features, including production of autoantibodies, loss of tolerance to self-antigens, tissue necrosis and fibrosis, vasculopathy and activation of the coagulation system. Platelets occupy a central part within the coagulation cascade and are well-recognized for their hemostatic role. However, recent cumulative evidence implicates their additional and multifaceted immunoregulatory functions. Platelets express immune receptors and they store growth factors, cytokines, and chemokines in their granules enabling a significant contribution to inflammation. A plethora of activating triggers such as damage associated molecular patterns (DAMPs) released from damaged endothelial cells, immune complexes, or complement effector molecules can mediate platelet activation. Activated platelets further foster an inflammatory environment and the crosstalk with the endothelium and leukocytes by the release of immunoactive molecules and microparticles. Further insight into the pathogenic implications of platelet activation will pave the way for new therapeutic strategies targeting autoimmune diseases. In this review, we discuss the inflammatory functions of platelets and their mechanistic contribution to the pathophysiology of SSc, ANCA associated small vessel vasculitis and other autoimmune diseases affecting the skin.

Platelet-rich plasma in osteoarthritis treatment: review of current evidence

Platelet-rich plasma (PRP) is defined as a volume of plasma with a platelet concentration higher than the average in peripheral blood. Many basic, preclinical and even clinical case studies and trials report PRP’s ability to improve musculoskeletal conditions including osteoarthritis, but paradoxically, just as many conclude it has no effect. The purpose of this narrative review is to discuss the available relevant evidence that supports the clinical use of PRP in osteoarthritis, highlighting those variables we perceive as critical. Here, recent systematic reviews and meta-analyses were used to identify the latest randomized controlled trials (RCTs) testing a PRP product as an intra-articular treatment for knee osteoarthritis, compared with an intra-articular control (mostly hyaluronic acid). Conclusions in the identified RCTs are examined and compared. In total, five recent meta-analyses and systematic reviews were found meeting the above criteria. A total of 19 individual trials were identified in the five reviews but only 9 were level of evidence I RCTs, and many had moderate or high risks of bias. At present, results from these RCTs seem to favor PRP use over other intra-articular treatments to improve pain scales in the short and medium term (6–12 months), but the overall level of evidence is low. As a result, clinical effectiveness of PRP for knee osteoarthritis treatment is still under debate. This is, prominently, the result of a lack of standardization of PRP products, scarceness of high quality RCTs not showing high risks of bias, and poor patient stratification for inclusion in the RCTs.

Anti-HLA antibodies with complementary and synergistic interaction geometries promote classical complement activation on platelets

High titers of HLA antibodies are associated with platelet refractoriness, causing poor platelet increments after transfusions in a subset of patients with HLA antibodies. Currently, we do not know the biological mechanisms that explain the variability in clinical responses in HLA alloimmunized patients receiving platelet transfusions. Previously we showed that a subset of anti-HLA IgG-antibodies induces FcγRIIa-dependent platelet activation and enhanced phagocytosis. Here, we investigated whether anti-HLA IgG can induce complement activation on platelets. We found that a subset of anti-HLA IgG induced complement activation via the classical pathway, causing C4b and C3b deposition and formation of the membrane-attack complex. This resulted in permeabilization of platelet membranes and increased calcium influx. Complement activation also caused enhanced α-granule release, as measured by CD62P surface exposure. Blocking studies revealed that platelet activation was caused by FcγRIIa-dependent signaling as well as HLA antibody induced complement activation. Synergistic complement activation employing combinations of monoclonal IgGs suggested that assembly of oligomeric IgG complexes strongly promoted complement activation through binding of IgGs to different antigenic determinants on HLA. In agreement with this, we observed that preventing anti-HLA-IgG hexamer formation using an IgG-Fc:Fc blocking peptide, completely inhibited C3b and C4b deposition. Our results show that HLA antibodies can induce complement activation on platelets including membrane attack complex formation, pore formation and calcium influx. We propose that these events can contribute to fast platelet clearance in vivo in patients refractory to platelet transfusions with HLA alloantibodies, who may benefit from functional-platelet matching and treatment with complement inhibitors.

Candida: Platelet Interaction and Platelet Activity in vitro

Over the last 2 decades, platelets have been recognized as versatile players of innate immunity. The interaction of platelets with fungal pathogens and subsequent processes may critically influence the clinical outcome of invasive mycoses. Since the role of platelets in Candida infections is poorly characterized and controversially discussed, we studied interactions of human platelets with yeast cells, (pseudo-)hyphae, biofilms and secretory products of human pathogenic Candida species applying platelet rich plasma and a whole blood model. Incubation of Candida with platelets resulted in moderate mutual interaction with some variation between different species. The rate of platelets binding to -Candida (pseudo-) hyphae and candidal biofilm was comparably low as that to the yeast form. Candida-derived secretory products did not affect platelet activity - neither stimulatory nor inhibitory. The small subset of platelets that bound to Candida morphotypes was consequently activated. However, this did not result in reduced growth or viability of the different Candida species. A whole blood model simulating in vivo conditions confirmed platelet activation in the subpopulation of Candida-bound platelets. Thus, the inability of platelets to efficiently react on Candida presence might favor fungal survival in the blood and contribute to high morbidity of Candida sepsis.

Properdin: a tightly regulated critical inflammatory modulator

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non‐physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin‐centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long‐term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.