The COVID-19 pandemic continues to threaten the world with second waves of infection, even as several spots which seemed to have brought it under control experience resurgences of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). An effective vaccine or antiviral is desperately needed to control the spread of the virus.
A new study published on the preprint server bioRxiv* in August 2020 presents a cynomolgus macaque model that develops symptomatic SARS-CoV-2 infection in a mild form, with low or no antibody responses, making it suitable to study human responses to vaccines and drugs targeting the virus.
This scanning electron microscope image shows SARS-CoV-2 (round gold objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2, also known as 2019-nCoV, is the virus that causes COVID-19. The virus shown was isolated from a patient in the U.S. Image captured and colorized at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Montana. Credit: NIAID
The Role of Animal Models
In vitro studies are invaluable in assessing preventive and therapeutic drug candidates against SARS-CoV-2, but in vivo efficacy can be estimated only with live animal studies. The need is, therefore, for animal models that respond to the virus by developing disease symptoms and signs in a similar fashion and mechanism to humans.
Several studies have reported transgenic mouse models that express human angiotensin-converting enzyme (ACE) 2, the viral receptor, and thus develop an experimental infection with the virus, where wildtype mice do not. The former showed the features of interstitial pneumonia. However, the researchers noted that if both human and mouse ACE2 are expressed together, COVID-19 features may appear instead.
Another animal model is the Syrian hamster, which is easily infected by the virus to develop viral pneumonia. The issue is the lack of antibodies, and the rapid resolution of the illness in two weeks, which limits the ability to understand how severe COVID-19 develops. Rhesus and cynomolgus macaques also develop lung inflammation progressing to pneumonia when exposed to the virus. Studies have followed up such infections for up to 21 days.
Experimental Infection in Cynomolgus Macaques
The current study aims to observe infected cynomolgus macaques (CMs) for 28 weeks, regarding the T cell immune response directed against viral antigens. Since their immune system and metabolism is similar to that of humans, this could be a useful model for COVID-19 drugs and vaccines.
Prior research by the same team has successfully recorded experimental flu infection in these animals, including the potential emergence of drug-resistant variants. This helped to evaluate flu vaccines and drugs.
Based on this, the researchers introduced SARS-CoV-2 into CMs as a preclinical animal model of infection. They inoculated the virus into the conjunctiva, nasal cavity, oral cavity, and trachea. Swabs from the nose and mouth were consistently positive up to day 7 but not after day 10, up to 28 days when the animals were euthanized and autopsied. Thus, the virus replicates mainly in the nose and mouth of CMs.
They found that it spreads in the lungs, resulting in raised temperatures in all the animals. A third of the animals showed viral pneumonia on chest X-rays, as well as neutralizing antibodies (NAbs) against the virus. One animal showed the presence of a clot in the lung. All these findings have been reported in human patients as well.
There was no significant alteration in biochemical values following infection. There were changes in the white cell count and composition in peripheral blood.
Low Symptomatic Infection and NAb Development
The incidence of NAb detection in CMs was low with SARS-CoV-2 infection compared to influenza. Also, it seems that the virus was cleared without the development of NAbs, since interferon (IFN)-γ responses are observed, but IL-2 levels are slow to rise. An Immunoglobulin G (IgG) response was detected in two CMs, but not the third, which also had a smaller bronchus-associated lymphoid tissue (BALT) mass.
The low rate of symptomatic infection in these animals correlates with the 80% asymptomatic infection rate in humans. In two symptomatic animals, NAbs were not detected even at day 28 from infection, but specific antibodies were found on day 14 in the majority of the CMs. This recapitulates the observed delay in IgG and IgM responses in human patients who develop COVID-19.
Implications and Future Directions
The researchers suggest that this means that patients without severe or complicated disease will be slow to develop antibodies, and also that the failure to seroconvert does not mean the animal or human was uninfected. A rise in IFN-γ with low IL-2 responses does not result in neutralizing antibody response but could induce a Th1-type reaction that eliminates the virus in the absence of a Nab. Also, the researchers suggest, the early IFN-γ production could have been the result of cross-reactive memory Th1 cells due to prior exposure to other coronaviruses. This is an area for future study, as coronaviruses that cause the common cold might be responsible for this immunity.
The researchers conclude that the experimental induction of clinical disease, along with the delayed and low antibody responses, indicate that “cynomolgus macaques are an appropriate animal model of SARS-CoV-2 infection for the development of vaccines and antiviral drugs.” Additionally, the study indicates the need for highly immunogenic vaccines and multi-dose regimens to achieve successful immunization of humans against this virus.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.