Once a virus infects a cell, neighboring cells are likely to become infected by interactions that are dependent on the cell and the virus in question. For example, cell-to-cell spread is facilitated in HIV by viral synapses, which also allow for escape from neutralizing antibodies. Low-level drug inhibitors are able to suppress infection if applied before the first cell is infected, but not after, demonstrating the lower sensitivity towards antiviral therapy-induced by this cell-to-cell transfer method.
Concerning severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the causative pathogen of coronavirus disease 2019 (COVID-19) – cell-to-cell spread has been observed by a process of cell fusion, involving major changes in cytoskeleton regulation and expression of the spike protein on the cell surface, allowing interaction with neighbors. Syncytia, single cells containing several nuclei formed by cell fusion, are frequently seen in the lungs of COVID-19 patients, indicating this method of cell-to-cell spread.
In a new study by researchers in the UK, Germany and South Africa, this process is visualized using time-lapse microscopy of a human cell lung line, demonstrating that SARS-CoV-2 can induce cell-to-cell fusion within 6 hours post-infection. Further, the application of neutralizing monoclonal antibodies or convalescent plasma was unable to prevent the spread of the virus by this mechanism, suggesting that removing SARS-CoV-2 from cells permissive of cell membrane fusion could be more difficult than other types of cell.
A preprint version of the study is available on the bioRxiv* server, while the article undergoes peer review.
How was the study performed?
The group began by engineering a human lung cell line capable of infection with SARS-CoV-2, with fluorescent nuclei that can be easily visualized by microscopy. In particular, an angiotensin-converting enzyme 2 (ACE2) receptor expressing H1299 non-small cell carcinoma line bearing yellow protein labeled histone tags was utilized, providing sufficient but non-overexpressed fluorescent tagging of the nucleus.
Upon introducing one such cell infected with SARS-CoV-2 to a culture of uninfected cells, the group observed syncytia formation, which did not occur in the absence of infection. Interestingly, the nuclei were seen to cluster into organized ring structures, and by 36 hours post-infection, around 20% of the nuclei had been drawn into such structures. Cells that had become infected underwent little cell division, evidenced by the minimally rising then falling number of nuclei in the culture induced by initial division of neighboring cells before the infection spreads.
To examine whether the ability of neutralizing antibodies to suppress the virus was dependent on the method of cell-to-cell spread, the authors utilized SARS-CoV-2 bearing the D614G mutation seen in the earliest variants of concern, such as the UK B.1.1.7 variant, which was compared with later lineages such as B.1.351 that have additional mutations to the spike protein: L18F, K417N, E484K, and N501Y. When applying convalescent plasma sourced from individuals that have recovered from the former, the latter frequently exhibits improved escape from neutralizing antibodies. This was confirmed by the group by in vitro infection studies, with the B.1.351 variant better avoiding capture by antibodies generated against B.1.1.7. However, the reverse was also true, with antibodies generated against B.1.351 being equally effective towards the earlier strain also. After concluding that these antibodies, both sourced from convalescent individuals or as monoclonal antibodies, could neutralize the cell-free virus, they were applied to infected H1299 cells as above. It was seen that they had little to no effect in suppressing cell-to-cell transmission.
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