Shape-Shifting Virus Poses Vaccine Difficulty

Molecular basis of dengue virus serotype 2 morphological switch from 29°C to 37°C

Dengue virus is a plus-strand RNA virus that infects approximately 400 million people annually, especially in the tropics of South and Southeast Asia.¹ As a plus strand RNA virus, the genome that enters the infected host cell can act directly to start translating proteins, making the replication and proliferation of more viral particles rapid as compared with many other types of viruses. Dengue poses a serious health threat to a large proportion of the world as it can cause hemorrhagic fever and shock. The human immune system attempts to eradicate the virus, however, it faces multiple complications as the virus is constantly evolving to escape the immune system.

It is extremely difficult to create a vaccine for dengue virus for three reasons. Primarily, there are 4 circulating serotypes in the human population. What this means is that not every type of dengue virus is exactly the same and, therefore, the body’s response to each serotype is slightly different, making the production of a successful vaccine difficult. Another reason why it is difficult to create a vaccine is because there is evidence that dengue can rely on antibody-dependent enhancement (ADE). ADE occurs when a slightly different version of the virus becomes more competent at infecting hosts because similar antibodies are able to bind to the virus, but not able to neutralize it, facilitating viral entry into cells.² What’s an antibody, you ask. Antibodies are one way the human immune system recognizes specific infections based on structure and responds to block the virus from entering cells. Finally, there is more recent evidence that certain serotypes of the virus can infect individuals with 2 distinct shapes that are temperature dependent. The outer protein, or envelope protein, is what the antibodies recognize and bind to in a highly specific manner.³ When the shape of the envelope protein changes, the antibodies can no longer bind as tightly. This paper examines the reasons why there are multiple different structures for the dengue virus.

The virion, or shell of the virus, is constructed from 180 copies of the E protein encoded by the viral genome. In the “smooth” virion, the E proteins form dimers and “rafts” whereas in the “bumpy” virion, the inter-dimer and raft interactions, and some intra-dimer interactions are broken. It was found that DENV2 (dengue) will initiate the switch to the bumpy virion at either 37°C or 40°C, dependent on the strain. It was found that the strain New Guinea C-2 (NGC-2) switched their virions to become bumpy at 37°C while NGC-1 remained smooth at the same temperature. The E protein between these two strains was found to be different at 5 amino acids, or protein building blocks. The researchers created mutations at these identified amino acids to discover which one was most important at initiating the virion change from smooth to bumpy. It was found that a mutant with a single, fairly minor change, at the sixth amino acid was sufficient to confer the change from smooth to bumpy at 37°C in the NGC-1 strain without changing any other proteins. The 6th amino acid is present at the interface between dimers, suggesting the NGC-2 bumpy strain is less stable than than NGC-1.

Figure 1: NGC-1 dengue virus remains "smooth" at both 29°C and 37°C while NGC-2 changes from smooth at 29°C to "bumpy" at 37°C.  

Furthermore, the authors observed whether there were differences in the replication cycles of the different strains. They found that after 24 hours post infection, the bumpy virus had higher growth rates than the smooth virus. At 40°C, all of the virions, no matter the amino acid sequence would switch to the bumpy phenotype. This is significant to the replication cycle because dengue causes fever and as the body temperature rises, the virus is actually becoming more effective as it changes to the bumpy phenotype. In additional amino acid change was found to be important in clinical strains of the virus at position 262. This amino acid is present at the intra-dimer interface of the E protein. This demonstrates that dimerization of the E protein is necessary for the smooth conformation at 37°C. Similar to the replication rate results, bumpy virions were also better at attaching and entering into human cells.

Figure 2: Mutants 4 and 1, containing the amino acid change at position 6 and a bumpy conformation at 37°C, have the highest replication rates. Mutants that remain smooth at 37°C have lower replication rates. 

            Interestingly, previous studies have found that DENV infectivity decreases with increased temperature, contrary to the evidence that bumpy virions have greater attachment and entry and faster replication. The authors suggest that this decreased infectivity may be due to the decreased stability at increased temperatures. Another potential reason for decreased infectivity at high temperatures may be due to the greater structure of the bumpiness that antibodies would be able to bind to. An in vivo study would better be able to demonstrate how the bumpy and smooth phenotypes interact with the immune system at these increased temperatures. Future research should investigate the mutant virions in vivo to confirm whether the bumpy or smooth virus would be more successful at high, feverish temperatures. 

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Lim, X., Shan, C., Marzinek, J. K., Dong, H., Ng, T. S., Ooi, J. S. G., … Lok, S. (2019). Molecular basis of dengue virus serotype 2 morphological switch from 29°C to 37°C. PLoS Pathogens, 15(9), 1-25. 

Halstead, S. B. & O’Rourke, E. J. (1977). Dengue viruses and mononuclear phagocytes: infection enhancement by non-neutralizing antibody. Journal of Experimental Medicine, 146, 201-217.

Bhatt, S., Gething, P. W., Brady, O. J., Messina, J. P., Farlow, A. W., … Hay, S. I. (2013). The global distribution and burden of dengue. Nature Letter, 496, 504-507. 

Fibriansah G, Tan J.L., Smith S.A., de Alwis A.R., Ng T.S., Kostyuchenko V.A., et al. A potent anti-dengue human antibody preferentially recognizes the conformation of E protein monomers assembled on the virus surface. EMBO Mol Med. 2014; 6(3):358–71.