Responses of human mast cells and epithelial cells following exposure to influenza A virus

IL-33 Induces an Antiviral Signature in Mast Cells but Enhances Their Permissiveness for Human Rhinovirus Infection

Mast cells (MCs) are classically associated with allergic asthma but their role in antiviral immunity is unclear. Human rhinoviruses (HRVs) are a major cause of asthma exacerbations and can infect and replicate within MCs. The primary site of HRV infection is the airway epithelium and MCs localise to this site with increasing asthma severity. The asthma susceptibility gene, IL-33, encodes an epithelial-derived cytokine released following HRV infection but its impact on MC antiviral responses has yet to be determined. In this study we investigated the global response of LAD2 MCs to IL-33 stimulation using RNA sequencing and identified genes involved in antiviral immunity. In spite of this, IL-33 treatment increased permissiveness of MCs to HRV16 infection which, from the RNA-Seq data, we attributed to upregulation of ICAM1. Flow cytometric analysis confirmed an IL-33-dependent increase in ICAM1 surface expression as well as LDLR, the receptors used by major and minor group HRVs for cellular entry. Neutralisation of ICAM1 reduced the IL-33-dependent enhancement in HRV16 replication and release in both LAD2 MCs and cord blood derived MCs. These findings demonstrate that although IL-33 induces an antiviral signature in MCs, it also upregulates the receptors for HRV entry to enhance infection. This highlights the potential for a gene-environment interaction involving IL33 and HRV in MCs to contribute to virus-induced asthma exacerbations.

Mast Cell-Derived Proteases Induce Endothelial Permeability and Vascular Damage in Severe Fever with Thrombocytopenia Syndrome

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging hemorrhagic fever acquired by tick bites. Whether mast cells (MCs), the body's first line of defense against pathogens, might influence immunity or pathogenesis during SFTS virus (SFTSV) infection remained unknown. Here, we found that SFTSV can cause MC infection and degranulation, resulting in the release of the vasoactive mediators, chymase, and tryptase, which can directly act on endothelial cells, break the tight junctions of endothelial cells and threaten the integrity of the microvascular barrier, leading to microvascular hyperpermeability in human microvascular endothelial cells. Local activation of MCs (degranulation) and MC-specific proteases-facilitated endothelial damage were observed in mouse models. When MC-specific proteases were injected subcutaneously into the back skin of mice, signs of capillary leakage were observed in a dose-dependent manner. MC-specific proteases, chymase, and tryptase were tested in the serum collected at the acute phase of SFTS patients, with the higher level significantly correlated with fatal outcomes. By performing receiver operator characteristic curve (ROC) analysis, chymase was determined as a biomarker with the area under the curve value of 0.830 (95% CI = 0.745 to 0.915) for predicting fatal outcomes in SFTS. Our findings highlight the importance of MCs in SFTSV-induced disease progression and outcome. An emerging role for MCs in the clinical prognosis and blocking MC activation as a potential drug target during SFTSV infection was proposed. IMPORTANCE We revealed a pathogenic role for MCs in response to SFTSV infection. The study also identifies potential biomarkers that could differentiate patients at risk of a fatal outcome for SFTS, as well as novel therapeutic targets for the clinical management of SFTS. These findings might shed light on an emerging role for MCs as a potential drug target during infection of other viral hemorrhagic fever diseases with similar host pathology as SFTS.

Divergent Mast Cell Responses Modulate Antiviral Immunity During Influenza Virus Infection

Influenza A virus (IAV) is a respiratory pathogen that infects millions of people each year. Both seasonal and pandemic strains of IAV are capable of causing severe respiratory disease with a high risk of respiratory failure and opportunistic secondary infection. A strong inflammatory cytokine response is a hallmark of severe IAV infection. The widespread tissue damage and edema in the lung during severe influenza is largely attributed to an overexuberant production of inflammatory cytokines and cell killing by resident and infiltrating leukocytes. Mast cells (MCs) are a sentinel hematopoietic cell type situated at mucosal sites, including the lung. Poised to react immediately upon detecting infection, MCs produce a vast array of immune modulating molecules, including inflammatory cytokines, chemokines, and proteases. As such, MCs have been implicated as a source of the immunopathology observed in severe influenza. However, a growing body of evidence indicates that MCs play an essential role not only in inducing an inflammatory response but in suppressing inflammation as well. MC-derived immune suppressive cytokines are essential to the resolution of a number of viral infections and other immune insults. Absence of MCs prolongs infection, exacerbates tissue damage, and contributes to dissemination of the pathogen to other tissues. Production of cytokines such as IL-10 and IL-6 by MCs is essential for mitigating the inflammation and tissue damage caused by innate and adaptive immune cells alike. The two opposing functions of MCs—one pro-inflammatory and one anti-inflammatory—distinguish MCs as master regulators of immunity at the site of infection. Amongst the first cells to respond to infection or injury, MCs persist for the duration of the infection, modulating the recruitment, activation, and eventual suppression of other immune cells. In this review, we will discuss the immune modulatory roles of MCs over the course of viral infection and propose that the immune suppressive mediators produced by MCs are vital to minimizing immunopathology during influenza infection.

Histamine receptors and COVID-19

OBJECTIVE

Reports that the over-the-counter histamine H2 receptor antagonist famotidine could help treat the novel coronavirus disease (COVID-19) appeared from April 2020. We, therefore, examined reports on interactions between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and histamine receptor antagonists.

METHODS

A systematic literature search was performed by 19 September 2020, and updated on 28 October 2020, in PubMed, Scopus, Cochrane Library and Google Scholar using (COVID-19 OR coronavirus OR SARS-CoV-2) AND (histamine antagonist OR famotidine OR cimetidine). ClinicalTrials.gov was searched for COVID-19 and (famotidine or histamine).

RESULTS

Famotidine may be a useful addition in COVID-19 treatment, but the results from prospective randomized trials are as yet awaited. Bioinformatics/drug repurposing studies indicated that, among several medicines, H1 and H2 receptor antagonists may interact with key viral enzymes. However, in vitro studies have to date failed to show a direct inhibition of famotidine on SARS-CoV-2 replication.

CONCLUSIONS

Clinical research into the potential benefits of H2 receptor antagonists in managing COVID-19 inflammation began from a simple observation and now is being tested in multi-centre clinical trials. The positive effects of famotidine may be due to H2 receptor-mediated immunomodulatory actions on mast cell histamine-cytokine cross-talk, rather than a direct action on SARS-CoV-2.