Interactions Between Platelets, Leukocytes, and the Endothelium

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).

Regulatory Roles of SWI/SNF Chromatin Remodeling Complexes in Immune Response and Inflammatory Diseases

The switch/sucrose non-fermentable (SWI/SNF) chromatin remodeling complexes (also referred to as BAF complexes) are composed of multiple subunits, which regulate the nucleosome translocation and chromatin accessibility. In recent years, significant advancements have been made in understanding mutated genes encoding subunits of the SWI/SNF complexes in cancer biology. Nevertheless, the role of SWI/SNF complexes in immune response and inflammatory diseases continues to attract significant attention. This review presents a summary of the significant functions of SWI/SNF complexes during the overall process from the development to the activation of innate and adaptive immune cells. In addition, the correlation between various SWI/SNF subunits and diverse inflammatory diseases is explored. Further investigations are warranted in terms of the mechanism of SWI/SNF complexes' preference for binding sites and opposite pro-/anti-inflammatory effects. In conclusion, further efforts are needed to evaluate the druggability of targeting SWI/SNF complexes in inflammatory diseases, and we hope this review will inspire the development of novel immune modulators in clinical practice.

Immunothrombosis and its underlying biological mechanisms

The evolutionary conserved link between coagulation and innate immunity is a biological process characterized by the thrombosis formation stimulus of immune cells and specific thrombosis-related molecules. In physiological settings, the relationship between the immune system and thrombosis facilitates the recognition of pathogens and damaged cells and inhibits pathogen proliferation. However, when deregulated, the interplay between hemostasis and innate immunity becomes a pathological process named immunothrombosis, which is at the basis of several infectious and inflammation-related thrombotic disorders, including coronavirus disease 2019 (COVID-19). In advanced stages, alterations in both coagulation and immune cell function due to extreme inflammation lead to an increase in blood coagulability, with high rates of thrombosis and mortality. Therefore, understanding underlying mechanisms in immunothrombosis has become decisive for the development of more efficient therapies to treat and prevent thrombosis in COVID-19 and in other thrombotic disorders. In this review, we outline the existing knowledge on the molecular and cellular processes involved in immunothrombosis, focusing on the role of neutrophil extracellular traps (NETs), platelets and the coagulation pathway. We also describe how the deregulation of hemostasis is associated with pathological conditions and can significantly aggravate a patient's condition, using COVID-19 as a clinical model.

Platelet, a key regulator of innate and adaptive immunity

Platelets, anucleate blood components, represent the major cell type involved in the regulation of hemostasis and thrombosis. In addition to performing haemostatic roles, platelets can influence both innate and adaptive immune responses. In this review, we summarize the development of platelets and their functions in hemostasis. We also discuss the interactions between platelet products and innate or adaptive immune cells, including neutrophils, monocytes, macrophages, T cells, B cells and dendritic cells. Activated platelets and released molecules regulate the differentiation and function of these cells via platelet-derived receptors or secreting molecules. Platelets have dual effects on nearly all immune cells. Understanding the exact mechanisms underlying these effects will enable further application of platelet transfusion.

A microfluidic device for assessment of E-selectin-mediated neutrophil recruitment to inflamed endothelium and prediction of therapeutic response in sickle cell disease

Neutrophil recruitment to the inflamed endothelium is a multistep process and is of utmost importance in the development of the hallmark vaso-occlusive crisis in sickle cell disease (SCD). However, there lacks a standardized, clinically feasible approach for assessing neutrophil recruitment to the inflamed endothelium for individualized risk stratification and therapeutic response prediction in SCD. Here, we describe a microfluidic device functionalized with E-selectin, a critical endothelial receptor for the neutrophil recruitment process, as a strategy to assess neutrophil binding under physiologic flow in normoxia and clinically relevant hypoxia in SCD. We show that hypoxia significantly enhances neutrophil binding to E-selectin and promotes the formation of neutrophil-platelet aggregates. Moreover, we identified two distinct patient populations: a more severe clinical phenotype with elevated lactate dehydrogenase levels and absolute reticulocyte counts but lowered fetal hemoglobin levels associated with constitutively less neutrophil binding to E-selectin. Mechanistically, we demonstrate that the extent of neutrophil activation correlates with membrane L-selectin shedding, resulting in the loss of ligand interaction sites with E-selectin. We also show that inhibition of E-selectin significantly reduces leukocyte recruitment to activated endothelial cells. Our findings add mechanistic insight into neutrophil-endothelial interactions under hypoxia and provide a clinically feasible means for assessing neutrophil binding to E-selectin using clinical whole blood samples, which can help guide therapeutic decisions for SCD patients.

Comprehensive phenotyping of human platelets by single-cell cytometry

Platelets are small anucleate blood cells that contribute to hemostasis, immunity, and inflammation. Circulating platelets are heterogeneous in size, age, receptor expression, and reactivity. They inherit many features from megakaryocytes and are further modified on exposure to bioactive substances in the bloodstream. Among these substances, prothrombotic agonists, vasodilators, and bloodborne pathogens modulate platelet phenotypes via distinct signaling cascades. The ability of platelets to respond to (patho)physiologic signals is incompletely understood but likely depends on their repertoire of surface receptors, which may partition them into discrete subsets with specialized functions and divergent abilities. The single-cell resolution of flow and mass cytometry is ideal for immunophenotyping and allows the identification of platelet subsets in remarkable detail. In this report, we describe the surface markers and gating strategies needed for the comprehensive characterization of platelets.

Platelet Immunophenotyping by High-Dimensional Mass Cytometry

Platelets are small blood cells that contribute to hemostasis, immunity, and inflammation. Characterization of platelet surface markers allows for differentiation of activated platelets from resting platelets, diagnosis of platelet disorders, and investigation of platelet biology and pathology. In this article, we describe the use of mass cytometry or "CyTOF" (mass spectroscopy detection of metal-tagged antibodies on individual cells) to measure a large number of markers on each platelet and to identify platelet subsets based on the shared expression of multiple markers. This powerful new approach provides a vastly more detailed picture of platelet immunophenotypes than conventional flow cytometry and enables investigation of the roles of platelet subsets in health and disease. © 2021 Wiley Periodicals LLC. Basic Protocol: Platelet immunophenotyping by high-dimensional mass cytometry Support Protocol: Data preprocessing.