Generation of a human ACE2-expressing pig as a model of Covid-19

ACE2 is the main receptor which interacts with SARS-CoV-2 in humans. A pig which expressed human ACE2 (hACE2) may therefore be susceptible to COVID-19, and could be used as a model to answer a variety of questions such as how the disease occurs and why different people get different disease severity. The model could also be used during the development of a vaccine or testing of antiviral therapies.

Other animal models for SARS have been used over the last 16 years. Young lab mice are not naturally susceptible, and although older mice are slightly susceptible, no severe disease or mortality has been seen. Mice engineered to carry hACE2 receptors are susceptible to more severe disease and death due to brain infection. Non-human primates have also been used, but also show no severe disease or mortality.

The same animal models for SARS-CoV-2 have not all yet been trialled, but we know that young lab mice and mice carrying hACE2 do not suffer from severe disease. However, rhesus monkeys are susceptible to mild infections, and have been shown to gain immunity from one of the vaccines being trialled.

There are various advantages and disadvantages of using pigs as a disease model. The advantages include them having similar gross anatomy as humans, including the lungs. Pigs are also good intubation models for respiratory studies, and are less expensive to work with than non-human primates. The main disadvantage of pigs is that they are not susceptible to SARS-CoV-2, so will need to be engineered to carry hACE2, which will be costly and time consuming. Additionally, even with hACE2, pigs still might not be susceptible to the virus, or might not show the same symptoms. However, a number of previous studies suggest that the viral receptor is the main factor that determines species-specificity, supporting this approach.

There are three main objectives for this project:

1) Establish whether pig cells are susceptible to SARS-CoV-2 when they overexpress human ACE2, and are challenged by high levels of the virus. If so, this is good evidence that pigs with hACE2 will also be susceptible.

2) Produce chimeric ACE2 constructs, including different mixes of human and pig. This may help to show which bits of hACE2 are important for SARS-CoV-2 susceptibility.

3) CRISPR editing of ACE2, replacing pieces of pig ACE2 with human ACE2 to generate pigs cells that are infectible with SARS-CoV-2.