Megakaryocytes in pulmonary diseases

Immune dysregulation in COVID-19 induced ARDS in kidney transplant recipients revealed by single-cell RNA sequencing

BACKGROUND

Since the emergence of COVID-19 at the end of 2019, the disease has led to widespread acute respiratory distress syndrome (ARDS), particularly among kidney transplant recipients (KTRs), who are at increased risk due to long-term immunosuppressive therapy. This study aims to explore the differences in immune responses between kidney transplant recipients and non-kidney transplant recipients in COVID-19-induced ARDS to identify potential therapeutic targets for improving outcomes. Single-cell RNA sequencing was performed on 108,320 cells derived from peripheral blood samples to construct a global single-cell map of COVID-19 induced ARDS in kidney transplant recipients(ARDSKT), COVID-19 induced ARDS in non transplant recipients(ARDSNKT), and healthy controls. Subsequently, using cellular clustering analysis, we obtained single-cell maps of different cell types. We employed enrichment analysis to determine the pathways involved in different subpopulations and focused on the role of key immune cells such as monocytes, megakaryocytes, B cells, and CD8+ T cells in the pathogenesis of ARDS. Significant immune differences were observed between ARDSKT and ARDSNKT. In ARDSKT, the S100A9+ MK subpopulation, which activates the NF-κB signaling pathway, was elevated, promoting inflammation. In contrast, the S100A12+ monocyte subpopulation that activates the chemokine signaling pathway was more abundant in ARDSNKT, reflecting a stronger inflammatory response, while its abundance was reduced in ARDSKT due to immunosuppression. The CXCR4+ B subpopulation, crucial for adaptive immunity, was significantly reduced in ARDSKT. Additionally, the XAF1+ Teff subpopulation, associated with apoptosis, was more abundant in ARDSKT, potentially impairing immune recovery. This study highlights the immune differences between ARDSKT and ARDSNKT, revealing the impact of immunosuppression on immune dysregulation. These findings suggest that targeting specific immune pathways can improve therapeutic strategies for ARDSKT.

Megakaryocyte in sepsis: the trinity of coagulation, inflammation and immunity

BACKGROUND

Megakaryocytes are traditionally recognized as cells responsible for platelet production. However, beyond their role in thrombopoiesis, megakaryocytes also participate in inflammatory responses and regulate immune system functions. Sepsis, characterized by life-threatening organ dysfunction due to a dysregulated response to infection, prominently features coagulopathy, severe inflammation, and immune dysfunction as key pathophysiological aspects.

AIM OF REVIEW

Given the diverse functions of megakaryocytes, we explore their roles in coagulation in the context of sepsis, and also in inflammatory and immune regulation. We try to infer future research directions and potential strategies for sepsis prevention and treatment based on the properties of megakaryocytes.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The purpose of this review is to both highlight and provide an update on the functions of megakaryocytes and pathophysiological changes in sepsis. Specific emphasis is given to the role of megakaryocytes in sepsis, which suggests value of future research and clinical application.

Immunological face of megakaryocytes

Megakaryocytes (MKs), which are traditionally known for their role in platelet production, are now emerging as unique immune cells with diverse capabilities. They express immune receptors, participate in pathogen recognition and response, phagocytose pathogens, contribute to antigen presentation, and interact with various immune cell types. When encountering inflammatory challenges, MKs exhibit intricate immune functions that can either promote or inhibit inflammation. These responses are mediated through mechanisms, such as the secretion of either anti-inflammatory or pro-inflammatory cytokines and release of immunomodulatory platelets according to specific conditions. This intricate array of responses necessitates a detailed exploration to determine whether the immune functions of MKs are carried out by the entire MK population or by a specific subpopulation. Breakthroughs in single-cell RNA sequencing have uncovered a unique "immune MK" subpopulation, revealing its distinct characteristics and immunoregulatory functions. This review provides latest insights into MKs' immune attributes and their roles in physiological and pathological contexts and emphasizes the discovery and functions of "immune MKs".

Research progress of megakaryocytes and platelets in lung injury

The lung is an important site of extramedullary platelet formation, and megakaryocytes in the lung participate in immune responses in addition to platelet production. In acute lung injury and chronic lung injury, megakaryocytes and platelets play a promoting or protective role through different mechanisms. The authors reviewed the role of megakaryocytes and platelets in common clinical lung injuries with different course of disease and different pathogenic factors in order to provide new thinking for the diagnosis and treatment of lung injuries.

Clinical, laboratory and immunohistochemical characterization of in situ pulmonary arterial thrombosis in fatal COVID-19

Background

COVID-19 patients carry an increased rate of thrombosis. It is controversial to which extent thrombi in the pulmonary arterial tree really contribute to disease severity with hypoxemia secondary to microvascular/lung parenchymal damage with viral alveolitis considered to play the main role in critical disease.

Objectives

The primary objective was to compare post-mortem lung disease from fatal COVID-19 pneumonia in patients with macroscopically evident pulmonary arterial tree thrombosis and patients without, by characterizing the immunohistochemical nature of thrombi, and by comparing clinical and laboratory features of these patients with other COVID-19 patients who died but without evidence of pulmonary arterial thrombosis (controls).

Patients and methods

13 COVID-19 pneumonia cases (mean age ± standard deviation: 74 ± 6.5 years) with macroscopically visible pulmonary arterial thrombosis were compared to 14 controls. Hematoxylin and Eosin stained slides were reviewed choosing those with visible pulmonary thrombi which were further characterized by immunohistochemistry, in particular for the inflammatory infiltrates. Ante mortem serum markers relevant to pulmonary embolism were evaluated in both groups.

Results

Twenty arterial thrombi (5 cases with multiple thrombi) were selected for study and were composed by white blood cells (WBC) [median, IQR range: 10 % (5–12.25)], mainly neutrophils [58 % (35.2–64.5)]. Cases with thrombosis showed significantly higher levels of platelet count [median, IQR range: 195000/mmc (157750–274,500) vs 143,500 (113000–175,250), p = 0.011], LDH [854 U/L (731–1315) vs 539 (391.5–660), p = 0.003] at admission, and D-dimer at ICU transfer [25,072 FEU (6951–50,531) vs 1024 (620–5501), p = 0.003].

Conclusions

Immunothrombotically driven arterial thrombi in COVID-19 patients are associated with D-Dimer and LDH elevations, thus linking inflammation, coagulopathy and organ damage in fatal COVID-19.