Human ACE2 Models for COVID-19 Preclinical Research

Learn how the
hACE2 Mouse Supports SARS-CoV-2 Research

black mouse being held
Although COVID-19 vaccines are finally rolling out in many parts of the world, scientists continue research to better understand this disease. The reality is that COVID-19 still poses a major health threat during and perhaps even after the vaccine roll out, and additional targeted therapies will be needed to treat those who fall ill or who are not able to mount a sufficient immune response after vaccination. Interest is also rising in studies on the sequelae of SARS-CoV-2 infection, including so-called "long COVID" in which people experience persistent, long-term health problems after COVID-19 infection, often following very mild cases.

Animal models of SARS-CoV-2 infection

The World Health Organization (WHO) assembled a global panel of experts in early 2020 to share knowledge about appropriate animal models for the study of COVID-19. One outcome of that panel was a September 2020 publication in Nature1 describing the then-current state of COVID-19 animal research. The panel reviewed in detail mouse models, Syrian hamsters, ferrets and non-human primates, along with some information on less-commonly used models such as mink and poultry.

Ferrets and hamsters have been long used to study respiratory infectious diseases and are useful in SARS-CoV-2 research. From a logistical standpoint, these are models which are already familiar to scientists with established respiratory disease programs, but for researchers pivoting to COVID-19 research, there may be challenges in housing and working with these species as many institutions have limited or no housing for these species, particularly at higher biosafety levels . And while these models have been extraordinarily helpful in the development of the currently-available vaccines and therapeutics, they may not be as useful for studies directed at understanding the impact of co-morbidities and genetic risk factors on disease progression and outcomes. For that type of work, the mouse has unparalleled value as a model organism.

The power of the mouse as a model organism

“Researchers may face challenges in housing and working with ferrets and hamsters as many institutions have limited or no housing for these species, particularly at higher biosafety levels.”
The laboratory mouse, Mus musculus, has been leveraged in biomedical research for well over 100 years. That century of scientific exploration has led to the development of more than 100,000 strains, including many with genetic mutations designed to reveal the role of one or more genes in human disease. This vast resource of strains and knowledge simply makes the mouse the most powerful model organism for the dissection of normal and pathological biological states.

Beyond a vaccine — how hACE2 mice can uncover the mysteries of COVID-19

Age and certain co-morbidities have emerged as risk factors for poor outcomes in COVID-19, but the mechanisms involved are not yet clear. A mechanistic understanding of disease is often the required first step for development of a new therapeutic, as it allows identification of potential drug targets. It is this type of research where mouse models shine. At a basic level, mutant models can elucidate the role of cell surface receptors on viral entry and how particular immune cells react in either helpful or adverse ways. Perhaps more interestingly, mice can clarify how genetic and other risk factors are involved in COVID-19 pathogenesis. How does disease differ in an obese mouse versus a lean mouse? How do various human APOE isoforms contribute to disease outcomes? These are all questions which genetically engineered mice will help answer in the coming years.

hACE2 mice as SARS-CoV-2 infection models

A particular subset of genetically engineered mice has already been widely used in SARS-CoV-2 infection studies: mice expressing human ACE2. ACE2 is the cell surface receptor involved in SARS-CoV-2 viral entry, and the binding affinity of SARS-CoV-2 spike protein to human ACE2 is much higher than to mouse ACE2. In general, normal laboratory mice are not susceptible to infection by SARS-CoV-2 whereas mice expressing human ACE2 are.

In the human population, COVID-19 disease in response to SARS-CoV-2 infection can range from asymptomatic, to relatively mild, to severe/lethal. Mouse models which can replicate this range of disease response are needed. Several unique human ACE2 transgenic mouse strains have been generated and are now being used in COVID-19 research to better understand mechanisms of infection and to develop therapeutics. Taconic Biosciences distributes two strains, hACE2 AC70 and hACE2 AC22, which represent independent founder lines carrying the same ubiquitously expressed transgene encoding human ACE2 cDNA . While AC70 is an incredibly sensitive lethal model of SARS-CoV-2 infection, with a lethal dose of >10 virions, AC22 is a lethality resistant model in which the majority of mice develop clinical illness after intranasal infection with 105 TCID50 of SARS-CoV-2 (US_WA-1/2020) and then recover, with some proportion of mice exhibiting lethal infection. Additional research is underway to further explore the viral dose response of AC22.

coronavirus toolkitExplore more about Taconic's:

Susceptibility to SARS-CoV-2
hACE2 StrainTaconic model #NomenclatureTransgene LocationTransgene Copy NumberSARS-CoV-2 Dose (US_WA-1/2020)MortalitySurvival (days post-infection)Clinical SignsSite of Viral ReplicationSex DifferencesOther Information
AC7018222B6;C3-Tg(CAG-ACE2)70CtktX chromosome1103-106 TCID50100%4-5Severe weight loss, lethal infectionPrimarily lung and brainMinimal sex differences observed 
101 TCID50100%6-10Moderate-severe weight loss, lethal infectionPrimarily lung and brain LD50: 3 TCID50
ID50: 0.5 TCID50
AC2218225B6;C3(C)-Tg(CAG-ACE2)22Ctktan unplaced scaffold region, presumably on chromosome 10~30-40105 TCID5030-40%7Moderate weight loss, lethal infection in some micePrimarily lung and brain ID50: 101.5 TCID50
(~30 TCID50)

These two models are complementary and can support a range of COVID-19 studies. Lethal infection models such as AC70 or K18-hACE2 mice (infected above a threshold dose) are useful for clear readouts on challenge studies to evaluate prophylactic vaccines. AC70 mice may also be useful for assessment of therapeutic efficacy, though the rapid onset of morbidity and mortality (~5-7 days after infection) may in some cases not provide sufficient time for a therapy to impact the course of disease or even to assess target engagement. Mortality in these mice may result from infection and inflammation in the brain3 rather than respiratory illness, the latter of which is the most common direct cause of death in COVID-19 patients4,5.

Mice which exhibit sublethal infection may be more appropriate for certain types of studies. Of note, the majority of people infected with COVID-19 do not die, but rather recover from infection, sometimes with long-term sequelae. Mice with sublethal infection may be useful to investigate the recovery process in detail, tissue by tissue. Public health systems now face what could be an epidemic of patients suffering from long-term post-COVID-19 symptoms6, and research is needed to understand this syndrome and treat it effectively. This may be one emerging application for mouse models of sublethal SARS-CoV-2 infection.

Future directions of COVID-19 mouse modeling

Several approaches are available to study genetic factors and co-morbidities. While some manipulations can easily be applied to existing hACE2 mice, such as inducing obesity through high fat diet conditioning, study of genetic factors may require generation of strains with multiple mutant alleles through crossbreeding or other methods. Alternatively, relevant mutant lines which lack human ACE2 can be used with mouse-adapted SARS-CoV-2 virus.

1. Muñoz-Fontela C, Dowling WE, Funnell SGP, Gsell PS, Riveros-Balta AX, Albrecht RA, Andersen H, Baric RS, Carroll MW, Cavaleri M, Qin C, Crozier I, Dallmeier K, de Waal L, de Wit E, Delang L, Dohm E, Duprex WP, Falzarano D, Finch CL, Frieman MB, Graham BS, Gralinski LE, Guilfoyle K, Haagmans BL, Hamilton GA, Hartman AL, Herfst S, Kaptein SJF, Klimstra WB, Knezevic I, Krause PR, Kuhn JH, Le Grand R, Lewis MG, Liu WC, Maisonnasse P, McElroy AK, Munster V, Oreshkova N, Rasmussen AL, Rocha-Pereira J, Rockx B, Rodríguez E, Rogers TF, Salguero FJ, Schotsaert M, Stittelaar KJ, Thibaut HJ, Tseng CT, Vergara-Alert J, Beer M, Brasel T, Chan JFW, García-Sastre A, Neyts J, Perlman S, Reed DS, Richt JA, Roy CJ, Segalés J, Vasan SS, Henao-Restrepo AM, Barouch DH. Animal models for COVID-19. Nature. 2020 Oct;586(7830):509-515. doi: 10.1038/s41586-020-2787-6.
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3. Kumari P, Rothan HA, Natekar JP, Stone S, Pathak H, Strate PG, Arora K, Brinton MA, Kumar M. Neuroinvasion and Encephalitis Following Intranasal Inoculation of SARS-CoV-2 in K18-hACE2 Mice. Viruses. 2021 Jan 19;13(1):132. doi: 10.3390/v13010132.
4. Grippo F, Navarra S, Orsi C, Manno V, Grande E, Crialesi R, Frova L, Marchetti S, Pappagallo M, Simeoni S, Di Pasquale L, Carinci A, Donfrancesco C, Lo Noce C, Palmieri L, Onder G, Minelli G; Italian National Institute of Health Covid-Mortality Group. The Role of COVID-19 in the Death of SARS-CoV-2-Positive Patients: A Study Based on Death Certificates. J Clin Med. 2020 Oct 27;9(11):3459. doi: 10.3390/jcm9113459.
5. Contou D, Cally R, Sarfati F, Desaint P, Fraissé M, Plantefève G. Causes and timing of death in critically ill COVID-19 patients. Crit Care. 2021 Feb 23;25(1):79. doi: 10.1186/s13054-021-03492-x.
6. Mandal S, Barnett J, Brill SE, Brown JS, Denneny EK, Hare SS, Heightman M, Hillman TE, Jacob J, Jarvis HC, Lipman MCI, Naidu SB, Nair A, Porter JC, Tomlinson GS, Hurst JR; ARC Study Group. 'Long-COVID': a cross-sectional study of persisting symptoms, biomarker and imaging abnormalities following hospitalisation for COVID-19. Thorax. 2020 Nov 10:thoraxjnl-2020-215818. doi: 10.1136/thoraxjnl-2020-215818.

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