The cytopathic effect (CPE) triggered by viral infections in host cells can play a key role in the biology of viruses. The most commonly described CPEs are rounding of the host cell, fusion among cells to form a syncytium, inclusion bodies and cell lysis (1). In many cases, these modifications of cell physiology and morphology are directly related to manipulation of the host cell by viral factors, which favours viral spread to new hosts. 

Since 2003, the infection of amoebae by giant viruses has been increasingly described, revealing unimaginable structural and genomic complexity (2-4). The main cytopathic effect observed in amoebae infected with giant viruses is rounding followed by cell lysis. However, we recently described a giant virus able to trigger an atypical CPE, by forming giant bunches of host cells. This observation inspired the investigation of this giant virus beginning in the soda lakes of the Pantanal (wetlands) in Brazil and in the depths of the Atlantic Ocean, from where we discovered the tupanviruses (5). Electron microscopy of tupanviruses revealed structural characteristics never seen before among amoebae giant viruses, including a long tail attached to the capsid, which can reach lengths of up to 2.3 µm. 

Tupanvirus particles visualized by scanning electron microscopy

Tupanvirus genomic characterization revealed the most complete set of translation-related genes of the known virosphere, including 20 aminoacyl-tRNA synthetases, up to 70 tRNAs and several other genes related to peptide synthesis (5). Even with such significant features observed in tupanviruses, the basis of this paper began with a modest optical microscopic investigation of the cytopathic effects associated with tupanvirus infection. It was observed that tupanvirus triggers a distinct cytopathic effect in Acanthamoeba castellanii by forming giant bunches of host cells. The mechanism by which tupanviruses induce this atypical effect was unknown. 

Tupanvirus-induced bunches, Acanthamoeba castellanii, red-highlighted.

Previous studies demonstrated that a mannose-binding protein (MBP) in amoebae is essential for promoting an amoeba’s adhesion to surfaces and to other cells (6-9).  A subsequent extensive bioinformatics search revealed that tupanviruses also present an MBP gene. We showed that bunches formation correlated with altered viral and amoebal MBP gene expression. Furthermore, the presence of free mannose in the medium negatively affected infection-induced MBP gene expression, which consequently inhibited bunches formation in a dose-dependent way. Considering that the interaction between amoebas may occur through interactions between their MBP receptors, we hypothesized that tupanvirus promotes the formation of large bunches by inducing the expression of MBP genes, and the inhibition of mannose receptor expression leads to a lower potential for interaction among amoebae cells. 

In nature, one of the main challenges for viral particles is to find permissive hosts. Interestingly, we observed that tupanvirus-infected amoebas interacted with uninfected amoebas, forming large agglomerations of cells (bunches). In this way, we suggested that infected cells act like “zombies” searching and attaching to uninfected cells. It is possible that bunches formation induced by tupanvirus infection may be important to the recruitment of cells, improving the chances of viral progeny to find a new host cell that may be scarce in the environment. Bunch formation might reduce the dilution effect in aquatic environments, thereby allowing the release of more particles near the host, which could also facilitate encounters between viral particles and their hosts. This unprecedented mechanism illustrates that a plethora of viral dissemination strategies remains to be discovered in nature.

Our paper can be found at https://www.nature.com/articles/s41598-018-36552-4.

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