Dear Editor, Since the outbreak of a novel coronavirus disease (COVID-19) in late 2019, it has spread rapidly and developed into a global pandemic. As of August 12, 2020, more than 215 countries and territories around the world have reported more than 20.5 million confirmed COVID-19 cases with over 745,693 deaths (https://www. worldometers. info/coronavirus/# countries). Such harsh conditions urged scientists across the world to gear up to develop vaccines and antiviral drugs against COVID-19, which also lead to massive requirement for experimental animals. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative pathogen of COVID-19. It has been demonstrated that SARS-CoV-2 uses angiotensin converting enzyme 2 (ACE2) as cellular receptor for entry into target cells. Mouse model is the most commonly used animal model for studying human diseases. However, SARS-CoV-2 fails to invade and replicate in this traditional animal model due to the structural differences in mouse ACE2 (mACE2) compared with human ACE2 (hACE2), 1 which has become the major hurdle for COVID-19 study. Currently, several strategies have been developed to overcome this receptor incompatibility by:(i) generating transgenic mice bearing hACE2 receptor, 2-4 (ii) establishing adenovirus hACE2 mouse model with recombinant adenovirus expressing hACE2, 5 and (iii) adapting the SARS-CoV-2 by serial passages in the respiratory tract of mice. 6-8 In this study, we used an alternative strategy to generate a SARS-CoV-2-sensitive mouse model by exogenous delivery of hACE2 with Venezuelan equine encephalitis replicon particles (VEEV-VRP-hACE2)(Supplementary information, Fig. S1a). VEEV is a positive sense, single-stranded RNA virus which belongs to the genus Alphavirus, family Togaviridae. Alphavirus replicon particles (VRPs), including VEEV-VRPs, represent efficient vectors for gene delivery and have been applied to studies of vaccine development, gene therapy and cell transduction. They contain self-replicating RNA-encoding viral replicase proteins (nsP1-nsP4) and express the gene of interest in place of viral structural protein genes. 9 By providing viral structural proteins in trans, the replicon RNA is packaged into VEEV-VRPs for in vitro and in vivo gene delivery. 10 Due to their intrinsic biological properties, VEEV-VRPs offer several advantages with a broad range of susceptible host cells, high expression level of cytoplasmic proteins and easy manipulation of recombinant RNA molecules using cDNA clones. 10, 11 Here, Venezuelan equine encephalitis virus (VEEV) replicon expressing hACE2 with a C-terminal S-tag was packaged into VRPs using the helper RNAs encoding VEEV capsid and envelope proteins to produce VEEV-VRP-hACE2 (Supplementary information, Fig. S1). MLE-12 cells (mouse lung type II epithelial cell line) were used to evaluate the availability of VEEV-VRP-hACE2 for SARS-CoV-2-sensitive cells establishment. After confirming hACE2 expression in MLE-12 cells transduced with VEEV-VRP-hACE2 (VRP-hACE2) through indirect immunofluorescence assay (IFA)(Fig. 1 a) and western blotting (Fig. 1 b), MLE-12 cells were administered with VEEV-VRP-hACE2 at 12h prior to SARS-CoV-2 (WIV04) 1 infection. The SARS-CoV-2 NP-specific IFA-positive cells were only observed in VEEV-VRP-hACE2-transduced MLE-12 cells but not in the DMEM-treated cells (Fig. 1 c). Similar results were obtained using BHK-21 cells (Fig. 1 c). These results demonstrated that VEEVVRP-hACE2 could efficiently deliver hACE2 in vitro and convert nonpermissive cells into SARS-CoV-2-permissive cells. We then …