Studies using human and animal-derived cell lines suggest a human origin for SARS-CoV-2 Omicron variants

In a recent study posted on Bio Rxiv*Sar, researchers examined the replication capacity of seven severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) isolates in 17 cell lines to understand phenotypic variation and host range. rice field. Variants monitored in this study included the ancestral SARS-CoV-2 strain B.1, variants of concern (VOCs) alpha, beta, gamma, delta, omicron (BA.1), and previous interest One variant (VOI) Zeta was included.

Background

SARS-CoV-2, the etiologic agent of coronavirus disease 2019 (COVID-19), has shown high genetic plasticity since initiating the first pandemic wave in late 2019. By 2020, SARS-CoV-2 will generate multiple VOCs, VOIs with distinct genetic, clinical, and epidemiological features.

Interestingly, most SARS-CoV-2 variants evolved independently from their ancestral strains, rather than each other in a definite pattern. For example, the Omicron VOC, which emerged in 2022, was genetically most similar to the SARS-CoV-2 strain prevalent in mid-2020. Another example is VOI zeta, which occurred briefly in South America with gamma VOCs.

About research

In this study, researchers used differentiated three-dimensional (3D) tissues of human airways and organoids to in vivo situation. In addition, they used a series of immortalized cell lines of domestic and wildlife species, especially of European origin.

These cell lines included primary human airway epithelial cells (HAE) derived from nasal epithelial and lung-derived cell lines. They used animal cell lines derived from bat, rodent, insectivore, and carnivore species. In addition, the researchers obtained all SARS-CoV-2 isolates after one passage of his Vero-E6 cells. Vero-E6 is less tolerant to beta VOCs, so he isolated in the A549-ACE-2 cell line after his second passage in mixed Vero-E6:A549-ACE-2 (1:1) cells. Did. These isolate infected cell cultures at a multiplicity of infection (MOI) of 0.1 at 37 °C and 33 °C. The team performed all cell cultures at 37°C and 5% CO2.₂Under similar conditions, they performed all SARS-CoV-2 infection assays.

The team used real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) to quantify the SARS-CoV-2 ribonucleic acid (RNA) load in the samples and presented the results as mean logarithms._Ten RNA copy number/mL (RNAc/mL).

Survey results

The authors observed a distinct Omicron BA.1 phenotype with shorter but faster and more efficient replication capacity and infectivity. viral shedding Nasal HAE cell model. All of these properties, in addition to immune evasion properties, may contribute to Omicron’s infectivity and high rate of secondary attacks in the real world.

Although less efficient replication in the lower respiratory tract explains the reduced clinical severity of Omicron BA.1, early efficient replication contributes to its efficient community spread. There is a possibility that Also, improved angiotensin-converting enzyme 2 (ACE-2) binding and more efficient endocytosis contribute to efficient cell entry and replication of Omicron. The HAE model has allowed us to study early stages of SARS-CoV-2 replication that remain undetected by diagnostic tests using clinical samples collected after the onset of symptoms. Furthermore, adaptive immunity to mitigate viral replication was lacking in the HAE model.

Notably, Omicron RNA levels and plaque-forming unit (PFU) titers decreased rapidly in HAE cells compared to Delta at 96 hours.in humans ex vivo Bronchial, Omicron BA.1 again showed early and rapid replication, but not in the lung parenchyma. The overall replication efficiency of SARS-CoV-2 was lower in lung-derived cells than in HAE cultures. An ancestral SARS-CoV-2 strain with the D614G S mutation showed more efficient replication in nasal and lung in vitro models. Alpha and beta VOCs and zeta VOIs replicate and expel infectious virions better at lower temperatures than delta and omicron.

Conservation of the host receptor ACE-2 across mammalian species facilitates interspecies transmission of SARS-CoV-2. Perhaps this is why we continue to establish more and more new animal reservoirs. In this context, the researchers found that both Delta and Omicron BA.1 did not show increased host range in animal cell lines derived from multiple European small mammals (e.g. bats of the bat family). I noticed that rhino family). Moreover, no signs of replication were seen in cells from the more ubiquitous non-rhinoceros bats. P. pipistrellus.

Rabbit kidney-derived cell lines alone were sensitive to replication of Omicron, B.1, and Delta. Surprisingly, although SARS-CoV-2 infects both species, other rabbit cell lines or cells derived from mink and ferret showed no susceptibility to SARS-CoV-2.There may be several explanations for the discrepancies observed in vitro Findings and natural infection and animal studies. For example, cell culture models do not accurately reflect cell locations. in vivo Duplicate and have decreased receptor expression.

SARS-CoV-2 also uses different receptors in some animal species. Although not fully reflected, in vivo Susceptibility, phenotypic assessment in cell lines may complement bioinformatic studies seeking to identify susceptible animal species by comparing ACE-2 sequences.

Conclusion

Research data showed that Omicron has the most striking phenotypic differences compared to all other SARS-CoV-2 variants. Moreover, SARS-CoV-2 is of human origin as it does not readily replicate in bat cells, which like RaTG13 has a diverse receptor-binding domain sequence.

Furthermore, this study highlighted how cell culture models can help us better understand the phenotypic differences and infectivity of SARS-CoV-2 variants in humans. despite the fact that in vivo These models are suitable for assessing the risk of zoonotic spillback of SARS-CoV-2.

*Important Notices

bioRxiv publishes non-peer-reviewed, preliminary scientific reports and should not be considered conclusive, to guide clinical practice/health-related actions, or to be treated as established information .