Genital herpes, caused by the herpes simplex virus 2 (HSV-2) is a very common sexually transmitted infection (STI). It affects 14% of Americans aged 15-49 years. There is no herpes vaccine at present. Many failed attempts at vaccine production have centered around HSV proteins.
Now, a new vaccine precursor, reported in the journal Science Immunology on September 20, 2019, has reported complete protection (called “sterilizing immunity”) of immunized animals against a high dose of the virus. This is the highest possible level of protection and ensures that the virus cannot replicate within the immunized host any longer, preventing disease and transmission simultaneously.
The new formulation is based on a modification of the viral RNA which is encapsulated within fat nanoparticles. This mRNA encodes three essential HSV-2 proteins that have glucose molecules attached (glycoproteins). In guinea pig studies, the vaccine was found to prevent the development of genital herpes and reduce the virus load in the lesions. The animals had high levels of protective antibodies and immune cells. This could be the precursor to more advanced preclinical trials.
Digital illustration of Herpes virus Image Credit: Kateryna Kon / Shutterstock
What is HSV?
HSV causes painful blisters to form along the course of a single nerve in the genital region, and is easily transmitted even when the original infection is asymptomatic (or subclinical), both between sexual partners and from a mother to her baby. Once infected, the virus persists for life, and can cause recurrences, either painful or subclinical, with the same risk of transmission to others. In babies, it causes neonatal herpes, affecting about 14,000 babies worldwide. This can result in neonatal encephalitis, a devastating brain inflammation in newborns, or in liver or lung inflammation. The consequences may be lifelong and crippling. The presence of herpetic genital lesions also increases the risk of HIV transmission to and from the patient by three to four times.
Many researchers have tried to use the protein components of the vaccine to create a vaccine that prevents virus entry, but though these helped to delay the initial presentation of the disease, they did not protect against infection.
Why this approach?
In the current study researchers from the Perelman School of Medicine at the University of Pennsylvania used three virus glycoproteins, namely C, D and E, to stimulate antibody production. Of these, C and E are molecules that cheat the host immune system. D is required for the entry of the virus into the cell. Thus a single vaccine can block all three functions, making the vaccine more powerful.
They combined the three proteins into two formulations for testing: either the proteins themselves, along with CpG and alum as adjuvants, to make them more strongly immunogenic; and nucleoside-modified mRNA within fat nanoparticles, that is, the genetic tools used by the host cells to make the identical proteins within the body.
Previous work has shown that protein vaccines containing these three glycoproteins protected almost 100% of the animals against the lesions but only 47% against vaginal shedding. However, mRNA is more effective than the foreign proteins at inducing immunity. This motivated them to use the modified mRNA to encode the same proteins in order to see if this strategy would make the vaccine more effective against preventing viral replication itself.
The use of mRNA with modified nucleosides is a promising branch of vaccine research, because it allows more effective protein production and reduce the incidence of inflammation. This is highly desirable as the presence of inflammation stops translation, or protein production, in response to immune-mediated gene inhibition. The use of lipid nanoparticles (LNPs) is important in helping the modified mRNA to enter the cells and protecting it against the ribonuclease enzymes that break it down.
How was the study performed?
In the first experiment, 64 mice were first immunized with the modified mRNA via intradermal injections in two doses at a gap of 28 days. One group received only the mRNA encoding the D glycoprotein. Another received the trivalent mRNA. A third received only nonspecific modified mRNA. All three were LNP preparations. A comparison group were infected with the virus and then immunized by intramuscular injections of the protein vaccine, three doses being given at intervals of two weeks.
28 days after the last immunization, both groups were exposed to standard infecting doses of HSV-2 in the vagina in two titers, one tenfold higher than the other.
The experiment was repeated in 10 guinea pigs which offer a stricter test of vaccine efficacy, and also allow evaluation of latent HSV-2 infection because of the occurrence of recurrent lesions and vaginal shedding, which is not seen in mice.
The study outcome
With both types of vaccine, the modified mRNA was found to produce the modified glycoprotein after cell entry. In both mice and guinea pigs, it completely prevented the emergence of herpes lesions in the animals tested, but the picture was different when it came to preventing subclinical infection. In mice, the protein vaccine and the mRNA vaccine had a success rate of 77% and 98% in preventing subclinical infection, respectively. Vaginal culture was negative in all 39 animals which received trivalent mRNA. When it came to finding virus particles in the shed vaginal cells, 1/19 animals and 5/10 animals in the trivalent mRNA and protein antigen had dormant infection. However, both groups failed to yield virus capable of replication, and the vaginal fluid was not infectious when inoculated into naïve mice.
With guinea pigs, no animal in the trivalent mRNA or trivalent protein antigen groups developed herpes lesions. Vaginal cultures became negative for the virus after day 2 in all animals. When virus shedding was evaluated, 50% of animals were positive on 9% of observation days after receiving the protein vaccine, in contrast to 10% of animals on 2% of observation days with the mRNA vaccine.
The trivalent mRNA vaccine prevented infection equally well even when the infecting HSV-2 dose was increased tenfold. No animal had clinical or subclinical infection, no virus on vaginal culture, and no viral shedding. Repeating the experiment with intramuscular injection in both types of vaccine yielded the same results. Using the single D-encoding mRNA was significantly less effective.
Overall, only 1/64 mice and 2/10 guinea pigs had subclinical infection following trivalent mRNA immunization. Even then, the virus recovered from the vaginal secretions in all cases was not capable of replication, indicating a negligible rate of transmission.
An immunology screen showed a much greater antibody and immune cell response with the mRNA vaccine. The cellular immune responses were enhanced in three areas: the CD4+ T cells which are directly involved in T cell attack on virally infected cells, the T follicular helper cells that push the antibody response, and the B cell responses in the germinal center that produce the antibody proper.
It seems likely that the superior efficacy of the mRNA vaccine is because it enhances multiple pathways of immune response, as well as allowing a longer exposure to the trivalent antigen. The 100% prevention of lesions, and 80% reduction in viral shedding, using the trivalent mRNA vaccine in guinea pigs, is far better than that using the D protein vaccine that has already been tested in human trials.
Researcher Harvey Friedman says, “We're extremely encouraged. Based on these results, it is our hope that this vaccine could be translated into human studies to test both the safety and efficacy of our approach.”
Nucleoside-modified mRNA encoding HSV-2 glycoproteins C, D, and E prevents clinical and subclinical genital herpes. Sita Awasthi1, Lauren M. Hook1, Norbert Pardi1, Fushan Wang1, Arpita Myles2, Michael P. Cancro2, Gary H. Cohen3, Drew Weissman1 and Harvey M. Friedman. Science Immunology. 20 Sep 2019: Vol. 4, Issue 39, eaaw7083. DOI: 10.1126/sciimmunol.aaw7083. https://immunology.sciencemag.org/content/4/39/eaaw7083