Mast cells release DNA Extracellular Traps (DETs) in response to SARS-CoV-2 and its spike protein

Infection by SARS-CoV-2 is associated with an uncontrolled and damaging inflammatory response during severe COVID-19 disease, during which immune cells, such as neutrophils, monocytes, and macrophages, release pro-inflammatory mediators leading to the development of acute respiratory distress syndrome. Mast cells may also contribute to the pathogenesis of COVID-19, as increased serum levels of their proteases are associated with the severity of the disease. Mast cells are strategically located in tissues that interface with the external environment, such as the skin, respiratory tract, and gastrointestinal mucosa, exhibiting microbicidal activities, including phagocytosis and the release of DNA embedded with granular proteins, known as DNA extracellular traps (DETs).

We show that the inactivated SARS-CoV-2 virions and the SARS-CoV-2 recombinant Spike protein induce DET formation in the human mast cell line HMC-1, with participation of TLR2 and TLR4 recognition. Using pharmacological inhibitors, we demonstrate the involvement of reactive oxygen species (ROS), NF-кB, peptidyl arginine deiminase (PAD), calcium, and serine proteases in the formation of DETs by either stimulus. DETs were toxic to pulmonary epithelial and endothelial cells, with tryptase, H3 citrullinated histone, and DNA scaffold contributing to cytotoxicity by apoptosis. Exposure of infectious virions to DETs reduced SARS-CoV-2 infection in Calu-3 cells, suggesting that the viral particles were trapped and killed by DETs. Furthermore, we also detected DET structures, characterized by colocalization of tryptase, citrullinated histone H3, and DNA, in the lung biopsies of COVID-19 patients.

Taken together, our results suggest a dual role for mast cell DETs during SARS-CoV-2 infection, as they damage both pulmonary epithelial and endothelial cells while capturing SARS-CoV-2 virions and inhibiting viral infection. Our findings are consistent with the assumption that mast cell DETs control the viral load by reducing SARS-CoV-2 infectivity.