This is the first of two papers on mRNA vaccines for Zika by this group. Click here to read the second. 

Summary:

In the recent years, Zika virus (ZIKV) infection has become a global health threat. This infection has been known to cause multiple symptoms in adults including myalgia and conjunctivitis. More recent studies have also linked ZIKV with congenital malformations and miscarriage when pregnant mothers get infected and transmit the infection to the fetus. There is currently no approved treatment or prevention means for ZIKV and with the virus’ epidemiologic profile rapidly growing the need for an effective vaccine is greatly felt. ZIKV is an enveloped positive sense RNA virus encoding structural proteins of capsid/ membrane/ envelope and a few non-structural proteins. Developing a vaccine against ZIKV is especially challenging because of its sequence similarity with Dengue virus (DENV) and the possibility of antibody cross-reactivity in the case of DENV exposure. This can lead to antibody-dependent enhancement (ADE) of infection and cause severe conditions such as lethal shock syndrome. Using the NanoAssemblr^TM Benchtop, the Diamond lab at Washington University in St. Louis, in conjuction with  Valera LLC, a Moderna Venture, have developed a versatile Lipid Nanoparticle (LNP) platform with the capacity of immunization against different serotypes of ZIKV while avoiding the potential DENV antibody cross reactivity. LNPs have been previously thoroughly characterized and established for the delivery of siRNA.  In this article, the Diamond lab has applied a modified version of the siRNA-LNP formulation to encapsulate mRNA which led to ZIKV immunization following intramuscular injection in mice. Unlike inactivated-virus vaccines, mRNA vaccines can be developed relatively quickly, and can be engineered to enhance immunogenicity or reduce negative reactions such as antibody cross-reactivity and thus ADE, making mRNA-LNP vaccines desirable agents for immunization against pathogens.

The Diamond lab encapsulated a modified mRNA encoding a signal sequence from human IgE, the pre-membrane and envelope proteins (IgE_sig-prM-E). The vaccine efficacy was first assessed in immunocompromised mice and the results indicated that the mice receiving the IgE_sig-prM-E LNP with a booster dose had the strongest serum neutralizing response against ZIKV. When challenged with ZIKV at 42 days after vaccination, these mice all survived infection. Vaccine efficacy was then tested in wildtype mice which resulted in the same outcome. The authors then aimed to vary the mRNA sequence to minimize antibody cross-reactivity with DENV. For this purpose, they engineered 4 mutations in a conserved epitope of the envelope protein which are known to eliminate the mentioned cross-reactivity. Loss of cross-reactivity of the mutant mRNA -LNP was then confirmed in vitro and in vivo and the vaccine was further tested for efficacy in vivo. While the mutant mRNA-LNP minimized ADE, the authors found that FL mutations induced lower neutralizing responses compared to the WT vaccine.

Overall, the Diamond lab successfully built an mRNA vaccine platform for ZIKV. Their wild type mRNA-LNP formulation is the first ZIKV vaccine that confers sterilizing immunity (combined absence of virus in target tissues and lack of anamnestic humoral response). The mutant mRNA-LNP vaccine they generated abolished ADE, but at the cost of reduced likelihood of conferring sterilizing immunity. The authors suggest that the lower immunogenicity of the mutant mRNA-LNP vaccine can be improved with a different array of mutations in the said conserved region. Nevertheless, the ZIKV mRNA-LNP vaccine is a highly potent platform that can be adjusted rapidly based on the epidemiology profiles. This study describes the first non-replicating mRNA-LNP vaccine and opens doors for development of future vaccines where such an approach can improve the safety and efficacy profiles.

Abstract:

The emergence of ZIKV infection has prompted a global effort to develop safe and effective vaccines. We engineered a lipid nanoparticle (LNP) encapsulated modified mRNA vaccine encoding wild-type or variant ZIKV structural genes and tested immunogenicity and protection in mice. Two doses of modified mRNA LNPs encoding prM-E genes that produced virus-like particles resulted in high neutralizing antibody titers (∼1/100,000) that protected against ZIKV infection and conferred sterilizing immunity. To offset a theoretical concern of ZIKV vaccines inducing antibodies that cross-react with the related dengue virus(DENV), we designed modified prM-E RNA encoding mutations destroying the conserved fusion-loop epitope in the E protein. This variant protected against ZIKV and diminished production of antibodies enhancing DENV infection in cells or mice. A modified mRNA vaccine can prevent ZIKV disease and be adapted to reduce the risk of sensitizing individuals to subsequent exposure to DENV, should this become a clinically relevant concern.