Serine proteases neutrophil elastase (NE), protease 3 (PR3), cathepsin G (CatG), and neutrophil serine protease 4 (NSP4) are released by activated neutrophils swarming around the place of pathogen invasion to provoke an immune response. However, uncontrolled proteolytic activity of proteases results in various human diseases, including cardiovascular diseases, thrombosis, and autoimmunity. In addition, proteases can be hijacked by several viruses to prime virus-derived surface proteins and evade immune detection by entering into the host cell. Indeed, porcine elastase increases the suitability of host cells to be infected by SARS-CoV-1. We compared the cleavage sites of human NE, PR3, and CatG as well as porcine-derived trypsin within the amino acid sequence of the proteolytic sensitive activation loop at the interface of S1/S2 of the spike protein (S protein) of SARS-CoV-1 as well as SARS-CoV-2. As a result, NE and PR3, but not CatG, hydrolyze the scissile peptide bond adjacent to the polybasic amino acid sequence of the S1/S2 interface of SARS-CoV-2, which is distinctive from SARS-CoV-1. These findings suggest that neutrophil-derived NE and PR3 participate in priming of the S1/S2 interface during an immune response.

The serine proteases neutrophil elastase (NE), proteinase 3 (PR3), cathepsin G (CatG), and neutrophil serine protease 4 (NSP4) are secreted by activated neutrophils as a part of the innate immune response against invading pathogens. However, these serine proteases might be adopted by viruses to mediate viral surface protein priming resulting in host cell entrance and productive infection. Indeed, NE and PR3 hydrolyze the scissile peptide bond within the proteolytically sensitive polybasic sequence of the activation loop of SARS-CoV-2 located at the S1/S2 interface of the Spike (S) protein; an amino acid motif which differs from SARS-CoV-1. The occurrence of novel SARS-CoV-2 variants and substitution of distinct amino acids at the polybasic sequence prompts serious concerns regarding increased transmissibility. We propose that a novel cleavage site by CatG of the Omicron variant and the increased substrate turnover of the Delta variant by furin within the polybasic sequence should be considered for increased transmission of SARS-CoV-2 variants.

During inflammation neutrophils become activated and segregate neutrophil serine proteases (NSPs) to the surrounding environment in order to support a natural immune defense. However, an excess of proteolytic activity of NSPs can cause many complications, such as cardiovascular diseases and chronic inflammatory disorders, which will be elucidated on a biochemical and immunological level. The application of selective serine protease inhibitors is the logical consequence in the management of the indicated comorbidities and will be summarized in this briefing.

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