Platelet, a key regulator of innate and adaptive immunity

How I treat patients who are refractory to platelet transfusions

ABSTRACT

Patients with thrombocytopenia requiring ongoing platelet transfusion support may develop inadequate platelet count increments, referred to as platelet refractoriness (PR), which further complicates their care. The underlying etiologies of PR can be broadly divided into immune and nonimmune causes. A high index of suspicion is required to initiate testing for alloimmunization, and the leading culprit in immune PR is the development of class I HLA antibodies. The approach to diagnosis of immune PR has changed over recent years with new technologies, but questions regarding the clinical significance and interpretation of these methods have not been conclusively answered. The provision of HLA-matched platelets requires close and timely coordination between transfusion services and clinical teams; however, the true impact of their provision on clinical outcomes is not clear. This paper reviews diagnostic and management challenges, appraises the existing data available to support treatment options, and identifies research gaps.

Vaccination and Platelet Biology: Unraveling the Immuno-Hemostatic Interplay

Platelets, which have been traditionally associated with hemostasis and thrombosis functions, now receive attention for their role in immune responses that may affect vaccine development and effectiveness. Through their interactions with immune cells and modulation of inflammation alongside their role in antigen presentation, platelets become integral components of both innate and adaptive immune systems. New research shows platelets can improve vaccine effectiveness while reducing adverse side effects. During vaccine administration, platelets release cytokines and chemokines, which attract and stimulate immune cells to the injection site. Platelets work together with dendritic cells and T cells to support antigen processing and presentation, which leads to strong immune activation. Platelets' pro-inflammatory mediators strengthen local immune responses to boost protective immunity generation. Significant attention has been given to platelet involvement in vaccine-related thrombotic events, including vaccine-induced immune thrombotic thrombocytopenia (VITT). The rarity and severity of these events demonstrate the need to investigate the complex interplay between vaccine mechanisms and platelet activation. Exploration of the platelet-immune axis can lead to new methods for improving both the effectiveness and safety of vaccines. Researchers are working on creating innovative approaches for treatments that target platelet receptors and thrombosis pathways without interfering with the regular hemostatic functions of platelets. New vaccine development methods and personalized immunization strategies can emerge from targeting platelets with adjuvants and immune modulators.

Complex gene-dependent and-independent mechanisms control daily rhythms of hematopoietic cells

The abundance and behaviour of all hematopoietic components display daily oscillations, supporting the involvement of circadian clock mechanisms. The daily variations of immune cell functions, such as trafficking between blood and tissues, differentiation, proliferation, and effector capabilities are regulated by complex intrinsic (cell-based) and extrinsic (neuro-hormonal, organism-based) mechanisms. While the role of the transcriptional/translational molecular machinery, driven by a set of well-conserved genes (Clock genes), in nucleated immune cells is increasingly recognized and understood, the presence of non-transcriptional mechanisms remains almost entirely unexplored. Studies on anucleate hematopoietic components, such as red blood cells and platelets, have shown that auto-sustained redox reaction cycles persist and operate in mammals. This opens to the possibility that transcriptional and non-transcriptional circadian mechanisms might coexist in nucleated immune cell populations, potentially complementing each other. It is becoming increasingly clear that disruption of the circadian rhythm at the central level (core clock) is strongly implicated in a plethora of diseases that are associated with maladaptive immune responses. On the other hand, several evidence imply that dysregulated immune activity (e.g. excessive inflammation) may alter/disrupt the proper functioning of peripheral clocks. This knowledge paves the way to the exploitation of chronobiological concepts in clinical practice. A better comprehension of various transcriptional/translational and biochemical mechanisms that maintain rhythmicity in immune system activities, as well as the many factors (host-derived, microbiota-derived, environment) that can alter or disrupt these processes, will facilitate the development of novel chrono-immunotherapeutic approaches.

Human B-cells can form Hetero-aggregates with Blood Platelets: A Novel Insight into Adaptive Immunity Regulation in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic autoimmune disease characterized by inflammation and neurodegeneration. Our original study analyzes the interactions between blood platelets and leukocytes in MS, focused on their potential role in modulating immune responses. We demonstrated, for the first time, a significant increase in leukocyte migration towards platelets, indicating their higher chemotactic capabilities in MS. This novel finding is supported by microscopic imaging of platelet-leukocyte hetero-aggregates (PLAs). Our study included platelet activation status and platelet-lymphocyte cross-talk analysis distinguishing lymphocytic subpopulation in patients with relapsing-remitting (RRMS) and secondary progressive MS (SPMS) compared to healthy controls (HC). Flow cytometry method revealed an elevated expression of platelet activation typical markers i.e. PAC-1 and CD62P in both phenotypes of MS, especially in RRMS, and higher GPVI level in SPMS. Detailed immunophenotyping and confocal imaging showed an increased pool of platelet-lymphocyte aggregates (PLAs-Ly), particularly involving B-cells over T-cells across both MS phenotypes. The study also explored the involvement of the CD40-CD40L pathway, discovering significant correlations between platelet CD40L expression and lymphocytic antigen CD40, especially on B-cells in SPMS. This novel finding may indicate the special significance of platelet-B-cell cross-talk in progressive disease phenotype. Our research identified potential platelet-leukocyte interaction pathways that may influence the lymphocyte-mediated immune response in MS, highlighting the unexplored formation of platelet-B cell hetero-aggregates (PLAs-LyB).

Platelet-Specific Deletion of TGF-β1 Impairs Septic Thrombosis in Mice-Brief Report

BACKGROUND

Sepsis is featured as a systemic inflammation and thrombosis induced by infection. TGF-β (transforming growth factor-β) 1 is mainly secreted from platelets and plays a role in immune response and inflammation. Whether platelet-derived TGF-β1 participates in sepsis remains unclear. This study intends to investigate its role in sepsis in mice.

METHODS

Platelet-specific TGF-β1 knockout mice received cecal ligation and puncture surgery to induce sepsis followed by the analysis of survival time, platelets number, pathology changes of lung and liver, liver function, the recruitment of platelets, neutrophils and monocytes, and neutrophil extracellular traps' formation. In addition, adoptive transfer of wild-type platelets into platelet-specific TGF-β1 knockout mice was performed to further evaluate the role of TGF-β1 in the pathogenesis of sepsis.

RESULTS

TGF-β1 level was gradually increased in the lung during the progress of sepsis, and platelets are the major source of the elevated TGF-β1 level in the lung after sepsis. Deficiency of platelet-derived TGF-β1 prolonged the survival of sepsis mice, inhibited the drop of platelet number and bacterial growth, impaired the thrombus formation in the lung and liver, and improved liver function. In addition, platelet TGF-β1 deficiency also decreased the recruitment of neutrophils and monocytes to the lung and impaired neutrophil extracellular trap formation. However, the adoptive transfer of normal platelets to platelet-specific TGF-β1 knockout mice significantly reduced the number of circulating platelets, increased thrombosis in the lung and liver, and promoted the neutrophil extracellular trap formation.

CONCLUSIONS

Deficiency of platelet-derived TGF-β1 inhibits septic thrombosis and prolongs survival time, indicating that it might be a novel therapeutic target for the treatment of sepsis.

Platelet signaling in immune landscape: comprehensive mechanism and clinical therapy

Platelets are essential for blood clotting and maintaining normal hemostasis. In pathological conditions, platelets are increasingly recognized as crucial regulatory factors in various immune-mediated inflammatory diseases. Resting platelets are induced by various factors such as immune complexes through Fc receptors, platelet-targeting autoantibodies and other platelet-activating stimuli. Platelet activation in immunological processes involves the release of immune activation stimuli, antigen presentation and interaction with immune cells. Platelets participate in both the innate immune system (neutrophils, monocytes/macrophages, dendritic cells (DCs) and Natural Killer (NK) cells and the adaptive immune system (T and B cells). Clinical therapeutic strategies include targeting platelet activation, platelet-immune cell interaction and platelet-endothelial cell interaction, which display positive development prospects. Understanding the mechanisms of platelets in immunity is important, and developing targeted modulations of these mechanisms will pave the way for promising therapeutic strategies.

Mechanisms of HIV-immunologic non-responses and research trends based on gut microbiota

With the increasing number of people with HIV (PWH) and the use of antiretroviral treatment (ART) for PWH, HIV has gradually become a chronic infectious disease. However, some infected individuals develop issues with immunologic non-responses (INRs) after receiving ART, which can lead to secondary infections and seriously affect the life expectancy and quality of life of PWH. Disruption of the gut microbiota is an important factor in immune activation and inflammation in HIV/AIDS, thus stabilizing the gut microbiota to reduce immune activation and inflammation and promoting immune reconstitution may become a direction for the treatment of HIV/AIDS. This paper, based on extensive literature review, summarizes the definition, mechanisms, and solutions for INRs, starting from the perspective of gut microbiota.

Measurement of platelet thrombus formation in patients following severe thermal injury

Severe thermal injury significantly impacts upon hemostasis and is associated with classical changes to the circulating platelet count with a nadir followed by a rebound thrombocytosis at days ~3 and ~15 post-injury, respectively. To date, few studies have assessed platelet function following thermal injury as platelet tests often require large quantities of blood, are not representative of normal platelet pathophysiology, and are usually dependent on a normal platelet count. The purpose of this study was to measure platelet thrombus formation in vitro using a whole blood flow chip-based system following thermal injury and to study how platelet counts may impact upon the measurement. Adult (≥16 years) patients (N = 10) with ≥ 20% total burn surface area (TBSA) burn were recruited within 24 h of injury. Healthy controls (N = 25) were also recruited. Whole blood counts were measured using a hematology analyzer (Sysmex XN-1000). Platelet function was measured using the Total Thrombus-formation Analyzer System (T-TAS) within chips coated with tissue factor and collagen at shear rates of either 600 sec-1 (AR chips) or 1200 sec-1 (HD chips), the latter test being independent of platelet count. We confirmed the classical nadir in platelet counts following severe thermal injury at days 2, 3, 4 (p < 0.0001) and day 5 (p < 0.01) post-injury compared to healthy controls. Physiological platelet thrombus formation was significantly (p < 0.01) abnormal at day 3 post-injury using the AR chips but was related to the platelet count. However, although platelet dysfunction was not significant using HD chips, some of the results were independent of platelet count. A small number of samples, however, still gave abnormal results suggesting that there can be an underlying acquired platelet functional abnormality. Furthermore, the AR chip Area Under the Curve (AUC) was significantly lower on day 1 post-injury and negatively associated with severity of injury (TBSA, p < 0.05) and higher platelet function (AUC) positively associated with survival (p < 0.05). This study suggests that measuring platelet dysfunction within a more physiological in vitro test may have potential clinical utility. Larger studies are required to fully understand the impact of platelet dysfunction following severe thermal injury.

Prognostic gene landscapes and therapeutic insights in sepsis-induced coagulopathy

BACKGROUND

Sepsis is a common and critical condition encountered in clinical practice that can lead to multi-organ dysfunction. Sepsis-induced coagulopathy (SIC) significantly affects patient outcomes. However, the precise mechanisms remain unclear, making the identification of effective prognostic and therapeutic targets imperative.

METHODS

The analysis of transcriptome data from the whole blood of sepsis patients, facilitated the identification of key genes implicated in coagulation. Then we developed a prognostic model and a nomogram to predict patient survival. Consensus clustering classified sepsis patients into three subgroups for comparative analysis of immune function and immune cell infiltration. Single-cell sequencing elucidated alterations in intercellular communication between platelets and immune cells in sepsis, as well as the role of the coagulation-related gene FYN. Real-time quantitative PCR determined the mRNA levels of critical coagulation genes in septic rats' blood. Finally, administration of a FYN agonist to septic rats was observed for its effects on coagulation functions and survival.

RESULTS

This study identified four pivotal genes-CFD, FYN, ITGAM, and VSIG4-as significant predictors of survival in patients with sepsis. Among them, CFD, FYN, and ITGAM were underexpressed, while VSIG4 was upregulated in patients with sepsis. Moreover, a nomogram that incorporates the coagulation-related genes (CoRGs) risk score with clinical features of patients accurately predicted survival probabilities. Subgroup analysis of CoRGs expression delineated three molecular sepsis subtypes, each with distinct prognoses and immune profiles. Single-cell sequencing shed light on heightened communication between platelets and monocytes, T cells, and plasmacytoid dendritic cells, alongside reduced interactions with neutrophils in sepsis. The collagen signaling pathway was found to be essential in this dynamic. FYN may affect platelet function by modulating factors such as ELF1, PTCRA, and RASGRP2. The administration of the FYN agonist can effectively improve coagulation dysfunction and survival in septic rats.

CONCLUSIONS

The research identifies CoRGs as crucial prognostic markers for sepsis, highlighting the FYN gene's central role in coagulation disorders associated with the condition and suggesting novel therapeutic intervention strategies.

Distinct targeting and uptake of platelet and red blood cell-derived extracellular vesicles into immune cells

Blood-derived extracellular vesicles (EVs) hold great therapeutic potential. As blood contains mixed EV populations, it is challenging to study EVs originating from different cells separately. Blood cell concentrates manufactured in blood banks offer an excellent non-invasive source of blood cell-specific EV populations. To study blood cell-specific EVs, we isolated EVs from platelet (TREVs) and red blood cell (EryEVs) concentrates and characterized them using nanoparticle tracking analysis, imaging flow cytometry, electron microscopy and western blot analysis and co-cultured them with peripheral blood mononuclear cells (PBMCs). Our aim was to use imaging flow cytometry to investigate EV interaction with PBMCs as well as study their effects on T-lymphocyte populations to better understand their possible biological functions. As a conclusion, TREVs interacted with PBMCs more than EryEVs. Distinctively, TREVs were uptaken into CD11c+ monocytes rapidly and into CD19+ B-lymphocytes in 24 h. EryEVs were not uptaken into CD11c+ monocytes before the 24-h time point, and they were only seen on the surface of lymphocytes. Neither TREVs nor EryEV were uptaken into CD3+ T-lymphocytes and no effect on T-cell populations was detected. We have previously seen similar differences in targeting PC-3 cancer cells. Further studies are needed to address the functional properties of blood cell concentrate-derived EVs. This study demonstrates that imaging flow cytometry can be used to study the distinctive differences in the interaction and uptake of EVs. Considering our current and previous results, EVs present a new valuable component for the future development of blood-derived therapeutics.

Platelets in Renal Disease

Kidney disease is a major global health concern, affecting millions of people. Nephrologists have shown interest in platelets because of coagulation disorders caused by renal diseases. With a better understanding of platelets, it has been found that these anucleate and abundant blood cells not only play a role in hemostasis, but also have important functions in inflammation and immunity. Platelets are not only affected by kidney disease, but may also contribute to kidney disease progression by mediating inflammation and immune effects. This review summarizes the current evidence regarding platelet abnormalities in renal disease, and the multiple effects of platelets on kidney disease progression. The relationship between platelets and kidney disease is still being explored, and further research can provide mechanistic insights into the relationship between thrombosis, bleeding, and inflammation related to kidney disease, and elucidate targeted therapies for patients with kidney disease.