Mast Cell Tryptase and Implications for SARS-CoV-2 Pathogenesis

Mast cells (MCs) are heterogenous innate leukocytes producing many inflammatory mediators during viral infections that can be protective or damaging to the host, as is seen in the infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the pathogen responsible for the coronavirus disease that was first identified in 2019 (COVID-19). MCs can sense viruses by diverse mechanisms. They express angiotensin-converting enzyme 2 (ACE2), known as the principal entry receptor for SARS-CoV-2, and tryptase that can promote SARS-CoV-2 infection. Tryptase is one of the most abundant serine proteases released by MCs during degranulation and is reported to have both beneficial and detrimental roles in respiratory diseases. Reviewed here are the potential roles of MC-derived tryptase during COVID-19, the implications it has in the pathogenesis of SARS-CoV-2, and the possibility of treating COVID-19 by targeting tryptase.

The Roles of Neutrophils in Cytokine Storms.

A cytokine storm is an abnormal discharge of soluble mediators following an inappropriate inflammatory response that leads to immunopathological events. Cytokine storms can occur after severe infections as well as in non-infectious situations where inflammatory cytokine responses are initiated, then exaggerated, but fail to return to homeostasis. Neutrophils, macrophages, mast cells, and natural killer cells are among the innate leukocytes that contribute to the pathogenesis of cytokine storms. Neutrophils participate as mediators of inflammation and have roles in promoting homeostatic conditions following pathological inflammation. This review highlights the advances in understanding the mechanisms governing neutrophilic inflammation against viral and bacterial pathogens, in cancers, and in autoimmune diseases, and how neutrophils could influence the development of cytokine storm syndromes. Evidence for the destructive potential of neutrophils in their capacity to contribute to the onset of cytokine storm syndromes is presented across a multitude of clinical scenarios. Further, a variety of potential therapeutic strategies that target neutrophils are discussed in the context of suppressing multiple inflammatory conditions.

Cytokine Storm Syndrome in SARS-CoV-2 Infections: A Functional Role of Mast Cells.

Cytokine storm syndrome is a cascade of escalated immune responses disposing the immune system to exhaustion, which might ultimately result in organ failure and fatal respiratory distress. Infection with severe acute respiratory syndrome-coronavirus-2 can result in uncontrolled production of cytokines and eventually the development of cytokine storm syndrome. Mast cells may react to viruses in collaboration with other cells and lung autopsy findings from patients that died from the coronavirus disease that emerged in 2019 (COVID-19) showed accumulation of mast cells in the lungs that was thought to be the cause of pulmonary edema, inflammation, and thrombosis. In this review, we present evidence that a cytokine response by mast cells may initiate inappropriate antiviral immune responses and cause the development of cytokine storm syndrome. We also explore the potential of mast cell activators as adjuvants for COVID-19 vaccines and discuss the medications that target the functions of mast cells and could be of value in the treatment of COVID-19. Recognition of the cytokine storm is crucial for proper treatment of patients and preventing the release of mast cell mediators, as impeding the impacts imposed by these mediators could reduce the severity of COVID-19.