SARS-CoV-2 can lead to serious respiratory illness and death, but its ability to spread asymptomatically sets it apart from SARS-CoV-1 and MERS-CoV. In the latter, when these viruses enter the target cell, the spike protein interacts with the ACE2 cellular receptor. But the SARS-CoV-2 spike is more refined in its approach to ACE2 than that of SARS-CoV-1. In SARS-CoV-2 and MERS-CoV, the spike undergoes cleavage (maturation stopped) by furine (an enzyme important in the process of maturation of albumin or insulin), in order to optimise binding. This specificity has probably contributed to the emergence of this phenomenom in humans. The mechanism is also exploited by other viruses such as flu or HIV.
To increase understanding of this enzyme’s role in the COVID-19 pandemic, American researchers at Galveston, University of Texas recreated an entire SARS-CoV-2 virus from its sequence (inverse genetics). They removed the spike’s capacity for furine cleavage by introducing mutations. The viral replication of this mutant was 25 times greater than that of the naturally occurring, non-mutated virus in Vero E6 cells from monkey kidneys, a model often used in the study of this virus. In these cells, cleavage in the mutant was greatly lessened, though not completely eliminated, indicating that enzymes other than furine are able to cleave the spike.
However, mutant virus replication is reduced 10-fold in human respiratory cells. The researchers showed that this alternative cleavage is carried out by TMPRSS2, an enzyme expressed in the cellular membrane whose functions are little understood. Human respiratory cells express this enzyme more than the Vero E6 cells, and when this enzyme is artificially over-abundant in cells, the mutant replicates like the naturally occurring virus. TMPRSS2 therefore has an effect on viral entry.
In vivo, the mutant virus appears less pathogenic than the natural virus in hamsters (unchanged behaviour, and no weight loss). The mutant self-replicates efficiently in these animals, but is eliminated more quickly and symptoms are less serious. When the hamsters were re-infected 28 days later with the natural virus, all the animals, whether infected initially by the mutant or by the natural virus, were protected. This indicates that the mutant virus stimulates the production of neutralizing antibodies just like the natural virus.