The causative virus of COVID-19, SARS-CoV-2, was identified as a new member of the Coronaviruses subfamily (CoVs; corona – viridae) in the family Coronaviridae. This family belongs to the superfamily of Nidovirus (Nidovirales). SARS-CoV-2 is the seventh known pathogenic coronavirus, out of which three have caused severe epidemics, namely SARS, MERS, and now Covid-19. Common symptoms of COVID-19 include fever, cough, muscle aches, expectoration, headache, hemoptysis, and diarrhea. SARS-CoV-2 shares 79% genome sequence identity with SARS-CoV and 50% genome sequence identity with MERS-CoV. Its genome structure is shared with other beta coronaviruses. Six functional open reading frames (ORFs) are arranged in order from 5′ to 3′: replicase (ORF1a/ORF1b), spike (S), envelope (E), membrane (M) and nucleocapsid (N). In addition, there are 7 putative ORFs (encoding accessory proteins) interspersed between the structural genes.
For SARS-CoV-2, it can only be performed in a Biosafety Level 3 (BSL3) laboratory. The clear advantage of pseudotyped viruses is safety, as these studies can be performed in standard BSL2 laboratories. Several serological diagnostic techniques to assess specific immunity to pathogens have been developed for SARS-CoV-2, including virus neutralization test (VNT), pseudotyped virus neutralization test (pVNT), and enzyme-linked immunosorbent assay (ELISA). VNT is generally considered to be the 'gold standard' for serological testing because the results show that the infectious virus has been inactivated, so VNT is a strong correlative indicator of disease prevention. The level of neutralization may depend on a variety of factors, including the cell type used in the assay, the species from which the blood sample derives, and the virus isolate used in the assay. ELISA provides another platform to detect antibodies in serum or plasma samples to determine the presence of specific antibodies following infection with SARS-CoV-2. Existing SARS-CoV-2 ELISA platforms generally utilize SARS-CoV-2 spikes or nucleocapsid recombinant antigens to identify the presence of binding antibodies. Sequential detection of IgM followed by IgG antibody over time has been described in human patients, with early IgM declining over time to leave a longer-lasting IgG response.
Figure 1. Entry and replication of SARS-CoV-2 inside the host cells.
SARS-CoV-2 is a single-stranded, positive-sense, non-segmented, enveloped RNA virus with a size of approximately 29.9 kB and a diameter of 50-200 nm. Structurally, it has a double-layered lipid envelope, including spike glycoprotein, envelope protein, membrane glycoprotein, and nucleocapsid protein. viral spike glycoproteins have receptor-binding domains (RBDs) that interact with host cell receptors. Membrane glycoproteins are responsible for the assembly of viral particles. The virus spike glycoprotein binds to the cellular receptor ACE2. This binding results in a conformational change at the RBD that enables the coreceptor to bind to the transmembrane protease serine-2 (TMPRSS2), which, with the help of cellular endosomes, promotes viral internalization through endocytosis. Internalization results in the uncoating of viral genomic RNA into the cytoplasm. Genomic RNA binds to host ribosomes, resulting in the formation of polyproteins containing transcriptional complexes, including viral RNA-dependent RNA polymerase (RdRP). RdRP generates viral genomic RNA and several subgenomic mRNAs, which are translated into relevant viral proteins. Translated proteins translocate to the endoplasmic reticulum membrane and transit through the ER-to-Golgi intermediate compartment, where they interact with the encapsulated newly generated genomic RNA, resulting in budding into the lumen of the secretory vesicular compartments. The newly formed viral particles (virions) are then released from the cell by exocytosis.
The COVID-19 pandemic continues to create a massive global health crisis, killing more than 3.5 million people. The overall case fatality rate of COVID-19 is about 1%, and about 3-20% of COVID-19 patients require hospitalization, and a significant portion (about 10-30%) of them require intensive care, putting enormous pressure on the health system. Currently, there is no specific treatment for COVID-19, which highlights our limited understanding of the pathogenesis of COVID-19 and compels us to conduct in-depth antiviral research. Creative Diagnostics supports clients in SARS-CoV-2 antiviral research and development with extensive knowledge, including the pathogenesis of SARS-CoV-2, interactions with host cells, and the role of the immune system in the development of severe disease.
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