New vaccine candidate against Dengue virus- MTase mutants

 Greetings

     Almost an year ago, I had posted on Dengue vaccine (Link). I had put up some of the known facts at that time on effectiveness of CYD TDV. Now a paper published looks at an alternative approach for dengue vaccine, (experiment in its primary stage). That would be the topic of today's post

     Dengue Virus (DENV) is a Flavivirus member, transmitted by mosquito (Aedes species) bite. There are 4 serotypes named from DENV 1- 4. Many variants exist within serotypes owing to high mutation rates of virus. All of them are associated with clinically significant infections, with very little cross protection. As with other infectious diseases, vaccine hunt is on for dengue (Visit Dengue vaccine initiative for details; Link). More recent studies have been indicative of growing incidence of infection, and thus has become a international vaccine priority. Multiple different companies and scientists have vested their interests in developing vaccine of which many have entered clinical trials. A list of successful candidate vaccines are aggregated in Table 1.

Table 1: Various dengue vaccines under investigation and clinical trials.

      Of the seven Non structural proteins that the gene code, NS5 is a multifunctional protein, with the RNA-dependent RNA polymerase and methyltransferase (MTase) activities, responsible for RNA cap formation. Capping of RNA is an important process, (as the readers may know) is a hall mark of eukaryotic RNA. A lack of RNA cap is usually identified by the cellular immune process. That sums up to say that  MTase (also known as 2'-O-methyltransferase) is responsible for escaping cellular immune detection. Moreover, it has been shown earlier that NS5 can stimulate the production of IL-8. IL-8, is a proinflammatory CXC chemokine, which induces a respiratory burst, and can counteract the antiviral effects of IFN-α and thus enhance viral replication of many viruses.

Fig 1: Dengue- 2 NS5 protein highlighting
the methyl-transferase region. Source

    The DENV-2 NS5 methyltransferase has an N-terminal subdomain, a core subdomain, a C-terminal subdomain, a K-D-K-E motif, and a "S-adenosyl methionine-dependent methyltransferase fold" structure which is essentially a "sandwich" of αβα sheets in the N-terminal domain. The N-terminal domain has the capacity to bind GTP, hold the guanosine of the viral cap structure, and synthesize two different methylation reactions that are required for the formation of the RNA cap. A GTP-binding site in the N-terminal domain is suggested to be a cap-binding site for the Dengue-2-methyltransferase. The C-terminal subdomain is an RNA-dependent-RNA polymerase (RdRp) domain. The core subunit is responsible for Ado-Met binding and catalytic activity due to the GTP-binding pocket. Taken from Massana etal.

      The above detail forms the basis of the new vaccine design approach by Zust etal. The researchers created MTase mutants containing Ala-substitutions at the K-D-K-E tetrad motif (2 types, E217A and K61A+E217A). They then grew the virus in BHK21 cell lines. The mutants still retained polymerase activity and hence could reproduce. This provided a stable MTase mutant virus. The same mutation was introduced in all the 4 serotypes. These new virus were able to infect the animal models actively but were very weak, since they lacked the support of MTase. Moreover, lack of MTase allowed immune detection, and hence develop immunity. So when the animal was challenged with wild virus, it conferred a good protection. An added advantage of this study was to show that these mutants were incompetent in infecting the mosquito.

       This study is a proof of concept experiment that the MTase mutant DENV can be used safely and provide a good immune protection. A tetravalent formulation with same attenuating mutation in all four serotype recombinant vaccine strains, presents with difficult conditions of reversion via recombination. The authors also report that they couldn't see the problem of ADE (Antibody-dependent enhancement of infection), which is otherwise a problem attributed to failure of many earlier vaccine studies.

       Dr Katja Fink commented, "There is still no clinically approved vaccine or specific treatment available for dengue, so we are very encouraged by the positive results with this novel vaccine strategy. Our next step will be to work on a vaccine formulation that will confer full protection from all four serotypes with a single injection. If this proves to be safe in humans, it can be a major breakthrough for the dengue vaccine field." Source

       The study has got us new insights and hopefully will be entering to clinical trials soon.

Guy B, Barrere B, Malinowski C, Saville M, Teyssou R, & Lang J (2011). From research to phase III: preclinical, industrial and clinical development of the Sanofi Pasteur tetravalent dengue vaccine. Vaccine, 29 (42), 7229-41 PMID: 21745521

Herrero etal (2013). Dengue virus therapeutic intervention strategies based on viral, vector and host factors involved in disease pathogenesis. Vaccine. 2011 Sep 23; 29 (42): 7229-41. PMID: 21745521

Coller BA, & Clements DE (2011). Dengue vaccines: progress and challenges. Current opinion in immunology, 23 (3), 391-8 PMID: 21514129

Medin CL, Fitzgerald KA, & Rothman AL (2005). Dengue virus nonstructural protein NS5 induces interleukin-8 transcription and secretion. Journal of virology, 79 (17), 11053-61 PMID: 16103156

Züst R etal. Rational design of a live attenuated dengue vaccine: 2'-o-methyltransferase mutants are highly attenuated and immunogenic in mice and macaques. PLoS Pathogens. 2013 Aug;9(8):e1003521. PMID: 23935499