Commercial immunoassays for detection of anti-SARS-CoV-2 spike and RBD antibodies: urgent call for validation against new and highly mutated variants

Measuring the level of protection conferred by anti-SARS-CoV-2 (trimeric) spike or RBD (receptor binding domain) antibodies (especially total and IgG) is a suitable and reliable approach for predicting biological protection against the risk of infection and severe coronavirus disease 2019 (COVID-19) illness. Nonetheless, SARS-CoV-2 has undergone a broad process of recombination since the identification of the prototype lineage in 2019, introducing a huge number of mutations in its genome and generating a vast array of variants of interest (VoI) and concern (VoC). Many of such variants developed several mutations in spike protein and RBD, with the new Omicron (B.1.1.529) clade displaying over 30 changes, 15 of which concentrated in the RBD. Besides their impact on virus biology, as well as on the risk of detection failure with some molecular techniques (i.e., S gene dropout), recent evidence suggests that these mutations may also jeopardize the reliability of currently available commercial immunoassays for detecting anti-SARS-CoV-2 antibodies. The antigen (either spike or RBD) and epitopes of the prototype SARS-CoV-2 coated in some immunoassays may no longer reflect the sequence of circulating variants. On the other hand, anti-SARS-CoV-2 antibodies elicited by highly mutated SARS-CoV-2 variants may no longer be efficiently recognized by the currently available commercial immunoassays. Therefore, beside the compelling need to regularly re-evaluate and revalidate all commercially available immunoassays against live virus neutralization assays based on emerging VoCs or VoIs, diagnostic companies may also consider to redevelop their methods, replacing former SARS-CoV-2 antigens and epitopes with those of the new variants.

S glycoprotein diversity of the Omicron variant

On the backdrop of ongoing Delta variant infection and vaccine-induced immunity, the emergence of the new Variant of Concern, the Omicron, has again fuelled the fears of COVID-19 around the world. Currently, very little information is available about the S glycoprotein mutations, transmissibility, severity, and immune evasion behaviour of the Omicron variant. In the present study, we have performed a comprehensive analysis of the S glycoprotein mutations of 309 strains of the Omicron variant and also discussed the probable effects of observed mutations on several aspects of virus biology based on known available knowledge of mutational effects on S glycoprotein structure, function, and immune evasion characteristics.

Secondary structure of subgenomic RNA M of SARS-CoV-2

SARS-CoV-2 belongs the Coronavirinae family. As other coronaviruses, SARS-CoV-2 is enveloped and possesses positive-sense, single-stranded RNA genome of ~30 kb. Genome RNA is used as the template for replication and transcription. During these processes, positive-sense genomic RNA (gRNA) and subgenomic RNAs (sgRNAs) are created. Several studies showed importance of genomic RNA secondary structure in SARS-CoV-2 replication. However, the structure of sgRNAs have remained largely unsolved so far. In this study, we performed probing of sgRNA M of SARS-CoV-2 in vitro. This is the first experimentally informed secondary structure model of sgRNA M, which presents features likely to be important in sgRNA M function. The knowledge about sgRNA M provides insights to better understand virus biology and could be used for designing new therapeutics.

Infectivity of three Mayaro Virus (Genus Alphavirus, Family Togaviridae) geographic isolates in human cell lines

Mayaro virus (MAYV) is an emergent arthropod-borne virus that causes an acute febrile illness accompanied by arthralgia, similar to chikungunya virus. Increasing urbanization of MAYV outbreaks in the Americas has led to concerns that this virus could further expand its geographic range. Given the potential importance of this pathogen, we sought to fill some critical gaps in knowledge regarding MAYV infectivity and geographic variation. This study describes the cytopathogenicity of MAYV in human dermal fibroblasts, human skeletal muscle satellite cells, human embryonic kidney cells (HEK), peripherally derived human macrophages, and Vero cells. MAYV strain isolated from Bolivia (MAYV-U) infected cells more rapidly compared to MAYV strains isolated in Peru and Brazil (MAYV-P; MAYV-B), with high titers (1×108 pfu/ml) peaking at 37 hours post infection. MAYV-U also caused the most cytopathic effect in a time dependent manner. Furthermore, differently from the other two prototypic strains, MAYV-U harbors unique mutations in the E2 protein, D60G and S205F, likely to interact with the host cell receptor, which may explain the observed differences in infectivity. We further demonstrate that pre-treatment of cells with interferon-β inhibited viral replication in a dose-dependent manner. Together, these findings advance our understanding of MAYV infection of human target cells and provide initial data regarding MAYV phenotypic variation according to geography.Author SummaryArthropod-borne viruses are of great public health concern, causing epidemics worldwide due to climate change, changes in land use, rapid urbanization, and the expanding geographic ranges of suitable vectors. Among these viruses, Mayaro is an emerging virus for which little is currently known. This study aims to answer fundamental questions of Mayaro virus biology using three geographically distinct viral strains to examine variability in infection kinetics and infectivity in susceptible cell types. We found one geographic isolate to have accelerated infection kinetics and increased cell damage because of infection. To better understand what was unique about this isolate, we compared their envelope protein, which is critical for entry into a cell. We found that the isolate with increased replication kinetics possessed mutations at sites that may promote viral entry, which could explain these findings. Together, these findings further our understanding of Mayaro virus biology and provide insight into factors that contribute to Mayaro transmission and infectivity.

Top-Down and Bottom-Up Proteomics Methods to Study RNA Virus Biology

RNA viruses cause a wide range of human diseases that are associated with high mortality and morbidity. In the past decades, the rise of genetic-based screening methods and high-throughput sequencing approaches allowed the uncovering of unique and elusive aspects of RNA virus replication and pathogenesis at an unprecedented scale. However, viruses often hijack critical host functions or trigger pathological dysfunctions, perturbing cellular proteostasis, macromolecular complex organization or stoichiometry, and post-translational modifications. Such effects require the monitoring of proteins and proteoforms both on a global scale and at the structural level. Mass spectrometry (MS) has recently emerged as an important component of the RNA virus biology toolbox, with its potential to shed light on critical aspects of virus–host perturbations and streamline the identification of antiviral targets. Moreover, multiple novel MS tools are available to study the structure of large protein complexes, providing detailed information on the exact stoichiometry of cellular and viral protein complexes and critical mechanistic insights into their functions. Here, we review top-down and bottom-up mass spectrometry-based approaches in RNA virus biology with a special focus on the most recent developments in characterizing host responses, and their translational implications to identify novel tractable antiviral targets.

SARS-CoV-2 viral RNA is not viral load

Several recent studies used results of SARS-CoV-2 RT-qPCR, Ct (threshold cycle), as proxies of viral load1. Unfortunately, an important aspect of this virus’ biology is neglected: Coronaviruses being (+)ssRNA viruses the form of RNA they use for replication is identical to the form used for transcription. It is therefore not obvious which process, replication or transcription, is quantified by RT-qPCR. To make matters more complicated, Coronaviruses produce several kinds of mRNA, genomic (= full size) and subgenomic (carrying only some genes), hence modulating their gene expression1. As shown by Finkel et al. the different mRNAs of SARS-CoV-2 occur at different densities in cell cultures. Because of their location on the viral genome the different targets of RT-qPCR are differentially affected, some being carried by more types of mRNA than others. Further, gene expression being affected by environmental and genetic factors, the quantity of RNA revealed by RT-qPCR may consequently vary due to differences in replication rates, in expression rates, or in both. It is thus unclear how good a proxy of viral load Ct values are, or what differences in Ct values may reflect.Even though the process underlying them is poorly characterized, and despite additional known biases in sample quality and RT-PCR protocols, quantitative analyses of Ct may nevertheless be highly informative e.g. in allowing to detect patterns in ‘levels of RNA’ in patients with different properties (gender, age, severe vs. mild disease, stage of infection) or in allowing to relate these patterns to epidemic properties in populations. For example, given that a priori replication levels should be the same for all genes of these monopartite viruses, differences in Ct among markers lying in different viral genes for should reflect different expression profiles. Such observations could thus help reveal interesting, and potentially epidemiologically significant, variations e.g. among SARS-CoV-2 variants.