The novel human coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a pandemic. Critical to the rapid evaluation of vaccines and antivirals against SARS-CoV-2 is the development of tractable animal models to understand the adaptive immune response to the virus. To this end, the use of common laboratory strains of mice is hindered by significant divergence of the angiotensin-converting enzyme 2 (ACE2), which is the receptor required for entry of SARS-CoV-2. In the current study, we designed and utilized an mRNA-based transfection system to induce expression of the hACE2 receptor in order to confer entry of SARS-CoV-2 in otherwise non-permissive cells. By employing this expression system in an in vivo setting, we were able to interrogate the adaptive immune response to SARS-CoV-2 in type 1 interferon receptor deficient mice. In doing so, we showed that the T cell response to SARS-CoV-2 is enhanced when hACE2 is expressed during infection. Moreover, we demonstrated that these responses are preserved in memory and are boosted upon secondary infection. Importantly, using this system, we functionally identified the CD4+ and CD8+ structural peptide epitopes targeted during SARS-CoV-2 infection in H2 b restricted mice and confirmed their existence in an established model of SARS-CoV-2 pathogenesis. We demonstrated that, identical to what has been seen in humans, the antigen-specific CD8+ T cells in mice primarily target peptides of the spike and membrane proteins, while the antigen-specific CD4+ T cells target peptides of the nucleocapsid, membrane, and spike proteins. As the focus of the immune response in mice is highly similar to that of the humans, the identification of functional murine SARS-CoV-2-specific T cell epitopes provided in this study will be critical for evaluation of vaccine efficacy in murine models of SARS-CoV-2 infection. Author summary The development of tractable small animal models is critical to gain an understanding of the immune response to the novel human coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and for the evaluation of vaccines against the virus. However, the development of murine models of infection has been hindered due to the lack of expression of the human angiotensin-converting enzyme 2 (hACE2), which is the receptor required for entry of SARS-CoV-2. In this study, we cloned the hACE2 gene into an mRNA expression vector and demonstrated that transfection with this mRNA allowed for SARS-CoV-2 entry and replication. We utilized this novel method of hACE2 expression in mice by in vivo mRNA transfection to characterize the adaptive immune response to SARS-CoV-2. This unique and tractable model allowed for the first ever characterization of the murine SARS-CoV-2 specific T cell response. This information will be critical to determining the correlates of protection against the virus and for the evaluation of vaccines.
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