https://onlinelibrary.wiley.com/doi/pdf/10.1111/jth.15534

Platelets contribute to disease severity in COVID-19.

Single-cell analysis of ploidy and the transcriptome reveals functional and spatial divergency in murine megakaryopoiesis

/pdf/platelet-activation-by-sars-cov-2-implicates-the-release-of-2eko6fk6.pdf

Platelet activation by SARS-CoV-2 implicates the release of active tissue factor by infected cells

Platelet MHC Class I Mediates CD8+ T Cell Suppression During Sepsis.

Systemic inflammation has been suggested to have a pivotal role in atherothrombosis, but the factors that trigger systemic inflammation have not been fully elucidated. Lipopolysaccharide (LPS) is a component of the membrane of Gram-negative bacteria present in the gut that can translocate into the systemic circulation, causing non-septic, low-grade endotoxaemia. Gut dysbiosis is a major determinant of low-grade endotoxaemia via dysfunction of the intestinal barrier scaffold, which is a prerequisite for LPS translocation into the systemic circulation. Experimental studies have demonstrated that LPS is present in atherosclerotic arteries but not in normal arteries. In atherosclerotic plaques, LPS promotes a pro-inflammatory status that can lead to plaque instability and thrombus formation. Low-grade endotoxaemia affects several cell types, including leukocytes, platelets and endothelial cells, leading to inflammation and clot formation. Low-grade endotoxaemia has been described in patients at risk of or with overt cardiovascular disease, in whom low-grade endotoxaemia was associated with atherosclerotic burden and its clinical sequelae. In this Review, we describe the mechanisms favouring the development of low-grade endotoxaemia, focusing on gut dysbiosis and changes in gut permeability; the plausible biological mechanisms linking low-grade endotoxaemia and atherothrombosis; the clinical studies suggesting that low-grade endotoxaemia is a risk factor for cardiovascular events; and the potential therapeutic tools to improve gut permeability and eventually eliminate low-grade endotoxaemia. In this Review, the authors describe the mechanisms underlying lipopolysaccharide translocation from the gut to the systemic circulation and the ensuing low-grade endotoxaemia, discuss studies suggesting a role for low-grade endotoxaemia in atherothrombosis and cardiovascular complications, and highlight potential therapeutic strategies to improve gut permeability and eliminate low-grade endotoxaemia. 

/pdf/gut-derived-low-grade-endotoxaemia-atherothrombosis-and-2lim9l7b.pdf

Gut-derived low-grade endotoxaemia, atherothrombosis and cardiovascular disease

Although platelets are the cellular mediators of thrombosis, they are also immune cells. Platelets interact both directly and indirectly with immune cells, impacting their activation and differentiation, as well as all phases of the immune response. Megakaryocytes (Mks) are the cell source of circulating platelets, and until recently Mks were typically only considered bone marrow-resident (BM-resident) cells. However, platelet-producing Mks also reside in the lung, and lung Mks express greater levels of immune molecules compared with BM Mks. We therefore sought to define the immune functions of lung Mks. Using single-cell RNA sequencing of BM and lung myeloid-enriched cells, we found that lung Mks, which we term MkL, had gene expression patterns that are similar to antigen-presenting cells. This was confirmed using imaging and conventional flow cytometry. The immune phenotype of Mks was plastic and driven by the tissue immune environment, as evidenced by BM Mks having an MkL-like phenotype under the influence of pathogen receptor challenge and lung-associated immune molecules, such as IL-33. Our in vitro and in vivo assays demonstrated that MkL internalized and processed both antigenic proteins and bacterial pathogens. Furthermore, MkL induced CD4+ T cell activation in an MHC II-dependent manner both in vitro and in vivo. These data indicated that MkL had key immune regulatory roles dictated in part by the tissue environment.

/pdf/lung-megakaryocytes-are-immune-modulatory-cells-44sr914x6y.pdf

Lung megakaryocytes are immune modulatory cells.

Classically, platelets have been described as the cellular blood component that mediates hemostasis and thrombosis. This important platelet function has received significant research attention for >150 years. The immune cell functions of platelets are much less appreciated. Platelets interact with and activate cells of all branches of immunity in response to pathogen exposures and infection, as well as in response to sterile tissue injury. In this review, we focus on innate immune mechanisms of platelet activation, platelet interactions with innate immune cells, as well as the intersection of platelets and adaptive immunity. The immune potential of platelets is dependent in part on their megakaryocyte precursor providing them with the molecular composition to be first responders and immune sentinels in initiating and orchestrating coordinated pathogen immune responses. There is emerging evidence that extramedullary megakaryocytes may be immune differentiated compared with bone marrow megakaryocytes, but the physiological relevance of immunophenotypic differences are just beginning to be explored. These concepts are also discussed in this review. The immune functions of the megakaryocyte/platelet lineage have likely evolved to coordinate the need to repair a vascular breach with the simultaneous need to induce an immune response that may limit pathogen invasion once the blood is exposed to an external environment.

Platelet and Megakaryocyte Roles in Innate and Adaptive Immunity

A state of the art lecture titled "Megakaryocytes in the Lung" was presented at the London International Society on Thrombosis and Haemostasis congress in 2022. This lecture highlighted that although most medical teaching presents platelets as bone marrow megakaryocyte-derived cellular mediators of thrombosis, platelets are also a critical part of the immune system with direct roles in responses to sterile tissue injury and pathogens. Bone marrow megakaryocytes differentiate from hematopoietic stem cells and package platelets with immune molecules. Activated platelets, therefore, initiate or accelerate the progression of vascular inflammatory pathologies, as well as being regulators of immune responses to infectious agents. Platelets are now known to have mechanistic roles in immune responses to disease processes, such as heart transplant rejection, myocardial infarction, aortic aneurysm, peripheral vascular disease, and infections. From these studies comes the concept that megakaryocytes are immune cell progenitors and recent emerging information highlights that megakaryocytes may themselves be immune cells. Despite megakaryocytes being described in the lung for >100 years, lung megakaryocytes have only recently been shown to be platelet producing and lung megakaryocytes are immune-differentiated in both phenotype and function. What is still not known is the origin of lung megakaryocytes and roles of lung megakaryocytes in health and disease. This review will discuss the long history of lung megakaryocytes in the literature and potential models for megakaryocyte origins and immune functions. Finally, we summarize relevant new data related to this topic that was presented during the 2022 International Society on Thrombosis and Haemostasis Congress. 

Megakaryocytes in the lung: History and future perspectives.

In addition to their well-studied hemostatic functions, platelets are immune cells. Platelets circulate at the interface between the vascular wall and leukocytes, and transient platelet-leukocyte complexes are found in both healthy and disease states, positioning platelets to provide physiologic cues of vascular health and injury. Roles for activated platelets in inducing and amplifying immune responses have received an increasing amount of research attention, but our past studies also showed that normal platelet counts are needed in healthy conditions to maintain immune homeostasis. We have now found that thrombocytopenia (a low platelet count) leads to monocyte dysfunction, independent of the cause of thrombocytopenia, in a manner that is dependent on direct platelet-monocyte CD47 interactions that regulate monocyte immunometabolism and gene expression. Compared to monocytes from mice with normal platelet counts, monocytes from thrombocytopenic mice had increased toll-like receptor (TLR) responses, including increased IL-6 production. Furthermore, ex vivo co-incubation of resting platelets with platelet naïve bone marrow monocytes, induced monocyte metabolic programming and durable changes in TLR agonist responses. Assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-Seq) on monocytes from thrombocytopenic mice showed persistently open chromatin at LPS response genes and resting platelet interactions with monocytes induced histone methylation in a CD47 dependent manner. Using mouse models of thrombocytopenia and sepsis, normal platelet numbers were needed to limit monocyte immune dysregulation and IL6 expression in monocytes from human patients with sepsis also inversely correlated with patient platelet counts. Our studies demonstrate that in healthy conditions, resting platelets maintain monocyte immune tolerance by regulating monocyte immunometabolic processes that lead to epigenetic changes in TLR-related genes. This is also the first demonstration of sterile cell interactions that regulate of innate immune-metabolism and monocyte pathogen responses.

Alison C. Livada

Papers

Lung megakaryocytes are immune modulatory cells.

Platelet and Megakaryocyte Roles in Innate and Adaptive Immunity

Megakaryocytes in the lung: History and future perspectives.

Thrombocytopenia Independently Leads to Monocyte Immune Dysfunction