RESUMO
SARS-CoV-2 nucleocapsid protein (N) induces strong antibody and T cell responses. Although considered to be localized in the cytosol, we readily detect N on the surface of live cells. N released by SARS-CoV-2 infected cells or N-expressing transfected cells binds to neighboring cells by electrostatic high-affinity binding to heparan sulfate and heparin, but not other sulfated glycosaminoglycans. N binds with high affinity to 11 human chemokines, including CXCL12{beta}, whose chemotaxis of leukocytes is inhibited by N from SARS-CoV-2, SARS-CoV-1, and MERS CoV. Anti-N Abs bound to the surface of N expressing cells activate Fc receptor-expressing cells. Our findings indicate that cell surface N manipulates innate immunity by sequestering chemokines and can be targeted by Fc expressing innate immune cells. This, in combination with its conserved antigenicity among human CoVs, advances its candidacy for vaccines that induce cross-reactive B and T cell immunity to SARS-CoV-2 variants and other human CoVs, including novel zoonotic strains.
RESUMO
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, the most severe pandemic in a century. The virus gains access to host cells when the viral Spike protein (S-protein) binds to the host cell-surface receptor angiotensin-converting enzyme 2 (ACE2). Studies have attempted to understand SARS-CoV-2 S-protein interaction with vertebrate orthologs of ACE2 by expressing ACE2 orthologs in mammalian cells and measuring viral infection or S-protein binding. Often these cells only transiently express ACE2 proteins and levels of ACE2 at the cell surface are not quantified. Here, we describe a cell-based assay that uses stably transfected cells expressing ACE2 proteins in a bi-cistronic vector with an easy to quantify reporter protein to normalize ACE2 expression. We found that both binding of the S-protein receptor-binding domain (RBD) and infection with a SARS-CoV-2 pseudovirus is proportional to the amount of human ACE2 expressed at the cell surface, which can be inferred by quantifying the level of reporter protein, Thy1.1. We also compared different ACE2 orthologs which were expressed in stably transfected cells expressing equivalent levels of Thy1.1. When ranked for either viral infectivity or RBD binding, mouse ACE2 had a weak to undetectable affinity for S-protein while human ACE2 was the highest level detected and feline ACE2 had an intermediate phenotype. The generation of stably transfected cells whose ACE2 level can be normalized for cross-ortholog comparisons allows us to create a reusable cellular library useful for measuring emerging SARS-CoV-2 variants ability to potentially infect different animals. ImportanceSARS-CoV-2 is a zoonotic virus responsible for the worst global pandemic in a century. An understanding of how the virus can infect other vertebrate species is important for controlling viral spread and understanding the natural history of the virus. Here we describe a method to generate cells stably expressing equivalent levels of different ACE2 orthologs, the receptor for SARS-CoV-2, on the surface of a human cell line. We find that both binding of the viral Spike protein receptor binding domain (RBD) and infection of cells with a SARS-CoV-2 pseudovirus are proportional to ACE2 levels at the cell surface. Adaptation of this method will allow for the creation of a library of stable transfected cells expressing equivalent levels of different vertebrate ACE2 orthologs which can be repeatedly used for identifying vertebrate species which may be susceptible to infection with SARS-CoV-2 and its many variants.
RESUMO
Serological tests are an indispensable tool to understand the epidemiology of the SARS-CoV-2 pandemic, particularly in areas where molecular diagnostics are limited. Poor assay performance may hinder the utility of these tests, including high rates of false-positivity previously reported in sub-Saharan Africa. From 312 Malian samples collected prior to 2020, we measured antibodies to the commonly tested SARS-CoV-2 antigens and four other betacoronaviruses by ELISA, and assessed functional cross-reactivity in a subset by SARS-CoV-2 pseudovirus neutralization assay. We then evaluated the performance of an ELISA developed in the US, using two-antigen SARS-CoV-2 spike protein and receptor-binding domain. To optimize test performance, we compared single and two-antigen approaches using existing assay cutoffs and population-specific cutoffs for Malian control samples (positive and negative). Background reactivity to SARS-CoV-2 antigens was common in pre-pandemic samples compared to US controls (43.4% (135/311) for spike protein, 22.8% (71/312) for RBD, and 33.9% (79/233) for nucleocapsid protein). SARS-CoV-2 reactivity correlated weakly with other betacoronavirus reactivity, varied between Malian communities, and increased with age. No pre-pandemic samples demonstrated functional activity. Regardless of the cutoffs applied, specificity improved using a two-antigen approach. Test performance was optimal using a two-antigen assay with population-specific cutoffs derived from ROC curve analysis [Sensitivity: 73.9% (51.6-89.8), Specificity: 99.4% (97.7-99.9)]. In the setting of high background reactivity, such as sub-Saharan Africa, SARS-CoV-2 serological assays need careful qualification is to characterize the epidemiology of disease, prevent unnecessary harm, and allocate resources for targeted control measures.
