Lauren M. Hook

Our vaccine candidate for genital herpes includes three immunogens involved in virus entry and immune evasion. HSV-2 glycoprotein C (gC2) is one of the immune evasion molecules that inhibits complement activation by binding C3b, and is the focus of this manuscript. Mice were immunized with 0.25, 0.5, 1, 10, or 30 µg of gC2 lipid nano particle (LNP)-encapsulated nucleoside-modified mRNA and challenged intravaginally with HSV-2. The gC2 mRNA-LNP, even at the lowest dose, was highly protective as a single immunogen. We measured antibody responses to six gC2 epitopes. Both neutralizing and C3b binding epitopes on gC2 were targeted. We passively immunized mice with monoclonal antibodies (mAbs) to gC2 and determined that the mAbs that enhance complement activation by blocking C3b binding were protective, while the mAb that neutralizes the virus, but does not block C3b binding, failed to protect. These results highlight the importance of a vaccine immunogen that induces antibodies that block the ability of gC to inhibit complement activation.

: The complement cascade is part of the first line of defence against viral infections, and many viruses have evolved to block it. For example, glycoprotein C (gC) from Herpes Simplex Virus 1 and 2 (gC1 and gC2) facilitates infection by modulating the complement cascade through an interaction with C3b. gC is also involved in attachment and other viral processes. However, our understanding of the molecular mechanisms of gC have been limited due to the absence of a structure. AlphaFold predicts that gC contains a disordered N-terminus and three immunoglobulin-like domains. Here, we generated various gC2 constructs and demonstrated that gC2 domains 1 and 2 are necessary and sufficient to interact with C3b and block the alternative pathway. A gC2 construct lacking the N-terminus in complex with C3b was characterised by cryo-EM at 3.6 Å, providing the first structure for gC2, and revealing that the interaction is predominantly driven by gC2 domain 2 and the MG8 domain of C3b. This structure was confirmed by cross-linking mass spectrometry and by using C3b-blocking antibodies that recognised gC2 linear epitopes at the interface with C3b. Overall, the gC-C3b interaction is different from other C3b-interacting partners, providing a novel mechanism to regulate the complement cascade.

Seroconversion is defined as a four-fold or greater rise in antibody titers. This assay is used in human prophylactic vaccine trials to confirm HSV as the cause of genital lesions and detect subclinical latent infection. We evaluated the accuracy of seroconversion in detecting infection using a guinea pig model of genital infection. Not all animals intravaginally inoculated with HSV-2 become infected, particularly if vaccinated; therefore, we need to establish criteria to determine whether an animal is infected. Our primary analysis involved considering animals to be infected if they had any of the following: (a) genital lesions; (b) HSV-2 DNA in vaginal secretions four or more weeks after HSV-2 inoculation as a marker of reactivation from latency; or (c) HSV-2 DNA in dorsal root ganglia, the site of latency. In the second analysis, we considered animals to be infected if they had positive virus cultures from vaginal swabs obtained on day two or four post HSV-2 inoculation. In the third analysis, we considered animals to be infected if they had any condition included in the first two analyses. We collected sera prior to HSV-2 inoculation and two months later and tested the first 57 animals for seroconversion using Western blotting and gG2 IgG ELISA. The results were concordant in 54 of 57 animals (95%), and when discordant, the gG2 ELISA matched infection results as defined by the primary analysis. The remaining animals were evaluated by gG2 IgG ELISA only. A total of 43 animals were inoculated with HSV-2 but not vaccinated (No vaccine group), and 224 were vaccinated with glycoprotein or mRNA vaccines prior to HSV-2 inoculation (Vaccine group). In the No vaccine group, we detected no false positives (seroconversion without infection) but 24% to 29% false negatives (no seroconversion despite infection) depending on the criteria used to define infection. In the Vaccine group, we detected 8% to 22% false positives and 31% to 37% false negatives. The accuracy of seroconversion was 74% to 79% in the No vaccine group and 71% to 76% in the Vaccine group. These results raise concerns about using seroconversion as a diagnostic test in human vaccine trials. Alternate approaches, such as subject home swabbing for HSV DNA, should be considered as a possible replacement.

HSV-1 disease is a significant public health burden causing orofacial, genital, cornea, and brain infection. We previously reported that a trivalent HSV-2 gC2, gD2, gE2 nucleoside-modified mRNA-lipid nanoparticle (LNP) vaccine provides excellent protection against vaginal HSV-1 infection in mice. Here, we evaluated whether this HSV-2 gC2, gD2, gE2 vaccine is as effective as a similar HSV-1 mRNA LNP vaccine containing gC1, gD1, and gE1 in the murine lip and genital infection models. Mice were immunized twice with a total mRNA dose of 1 or 10 µg. The two vaccines produced comparable HSV-1 neutralizing antibody titers, and surprisingly, the HSV-2 vaccine stimulated more potent CD8⁺ T-cell responses to gE1 peptides than the HSV-1 vaccine. Both vaccines provided complete protection from clinical disease in the lip model, while in the genital model, both vaccines prevented death and genital disease, but the HSV-1 vaccine reduced day two vaginal titers slightly better at the 1 µg dose. Both vaccines prevented HSV-1 DNA from reaching the trigeminal or dorsal root ganglia to a similar extent. We conclude that the trivalent HSV-2 mRNA vaccine provides outstanding protection against HSV-1 challenge at two sites and may serve as a universal prophylactic vaccine for HSV-1 and HSV-2.

Herpes simplex virus type 2 (HSV-2) is a leading cause of genital ulcer disease and a major risk factor for acquisition and transmission of HIV. Frequent recurrent genital lesions and concerns about transmitting infection to intimate partners affect the quality of life of infected individuals. Therapeutic vaccines are urgently needed to reduce the frequency of genital lesions and transmission. S-540956 is a novel vaccine adjuvant that contains CpG oligonucleotide ODN2006 annealed to its complementary sequence and conjugated to a lipid that targets the adjuvant to lymph nodes. Our primary goal was to compare S-540956 administered with HSV-2 glycoprotein D (gD2) with no treatment in a guinea pig model of recurrent genital herpes (studies 1 and 2). Our secondary goals were to compare S-540956 with oligonucleotide ODN2006 (study1) or glucopyranosyl lipid A in a stable oil-in-water nano-emulsion (GLA-SE) (study 2). gD2/S-540956 reduced the number of days with recurrent genital lesions by 56%, vaginal shedding of HSV-2 DNA by 49%, and both combined by 54% compared to PBS, and was more efficacious than the two other adjuvants. Our results indicate that S-540956 has great potential as an adjuvant for a therapeutic vaccine for genital herpes, and merits further evaluation with the addition of potent T cell immunogens.

The toxicity of mRNA-lipid nanoparticle (LNP) vaccines depends on the total mRNA-LNP dose. We established that the maximum tolerated dose of our trivalent mRNA-LNP genital herpes vaccine was 10 μg/immunization in mice. We then evaluated one of the mRNAs, gD2 mRNA-LNP, to determine how much of the 10 μg total dose to assign to this immunogen. We immunized mice with 0.3, 1.0, 3.0, or 10 μg of gD2 mRNA-LNP and measured serum IgG ELISA, neutralizing antibodies, and antibodies to six crucial gD2 epitopes involved in virus entry and spread. Antibodies to crucial gD2 epitopes peaked at 1 μg, while ELISA and neutralizing titers continued to increase at higher doses. The epitope results suggested no immunologic benefit above 1 μg of gD2 mRNA-LNP, while ELISA and neutralizing titers indicated higher doses may be useful. We challenged the gD2 mRNA-immunized mice intravaginally with HSV-2. The 1-μg dose provided total protection, confirming the epitope studies, and supported assigning less than one-third of the trivalent vaccine maximum dose of 10 μg to gD2 mRNA-LNP. Epitope mapping as performed in mice can also be accomplished in phase 1 human trials to help select the optimum dose of each immunogen in a multivalent vaccine.

Nucleoside-modified mRNA vaccines have gained global attention because of COVID-19. We evaluated a similar vaccine approach for preventing a chronic, latent genital infection rather than an acute respiratory infection. We used animal models to compare an HSV-2 trivalent nucleoside-modified mRNA vaccine with the same antigens prepared as proteins, with an emphasis on antigen-specific memory B cell responses and immune correlates of protection. In guinea pigs, serum neutralizing-antibody titers were higher at 1 month and declined far less by 8 months in mRNA- compared with protein-immunized animals. Both vaccines protected against death and genital lesions when infected 1 month after immunization; however, protection was more durable in the mRNA group compared with the protein group when infected after 8 months, an interval representing greater than 15% of the animal’s lifespan. Serum and vaginal neutralizing-antibody titers correlated with protection against infection, as measured by genital lesions and vaginal virus titers 2 days after infection. In mice, the mRNA vaccine generated more antigen-specific memory B cells than the protein vaccine at early times after immunization that persisted for up to 1 year. High neutralizing titers and robust B cell immune memory likely explain the more durable protection by the HSV-2 mRNA vaccine.

HSV-1 causes 50% of first-time genital herpes infections in resource-rich countries and affects 190 million people worldwide. A prophylactic herpes vaccine is needed to protect against genital infections by both HSV-1 and HSV-2. Previously our laboratory developed a trivalent vaccine that targets glycoproteins C, D, and E present on the HSV-2 virion. We reported that this vaccine protects animals from genital disease and recurrent virus shedding following lethal HSV-2 challenge. Importantly the vaccine also generates cross-reactive antibodies that neutralize HSV-1, suggesting it may provide protection against HSV-1 infection. Here we compared the efficacy of this vaccine delivered as protein or nucleoside-modified mRNA immunogens against vaginal HSV-1 infection in mice. Both the protein and mRNA vaccines protected mice from HSV-1 disease; however, the mRNA vaccine provided better protection as measured by lower vaginal virus titers post-infection. In a second experiment, we compared protection provided by the mRNA vaccine against intravaginal challenge with HSV-1 or HSV-2. Vaccinated mice were totally protected against death, genital disease and infection of dorsal root ganglia caused by both viruses, but somewhat better protected against vaginal titers after HSV-2 infection. Overall, in the two experiments, the mRNA vaccine prevented death and genital disease in 54/54 (100%) mice infected with HSV-1 and 20/20 (100%) with HSV-2, and prevented HSV DNA from reaching the dorsal root ganglia, the site of virus latency, in 29/30 (97%) mice infected with HSV-1 and 10/10 (100%) with HSV-2. We consider the HSV-2 trivalent mRNA vaccine to be a promising candidate for clinical trials for prevention of both HSV-1 and HSV-2 genital herpes. Herpes simplex virus type 1 (HSV-1) is an important cause of genital herpes infection, although worldwide herpes simplex virus type 2 (HSV-2) is the most common cause. Herpes infections persist for life and there is no cure. A preventative vaccine is the best approach to reduce new genital herpes infections. An optimal vaccine should protect against both HSV-1 and HSV-2 infection. Our vaccine targets HSV-2 glycoproteins C, D, and E administered either as proteins or mRNA encapsulated in lipid nanoparticles. We compared the vaccine delivered as mRNA or proteins for prevention of HSV-1 genital infection in mice. Both vaccines prevented genital disease but the mRNA vaccine was better at limiting virus replication in the genital tract. We then compared protection by the HSV-2 mRNA vaccine against genital HSV-1 and HSV-2 infection. Mice infected with either virus were totally protected from genital disease. Importantly, in two experiments, the mRNA vaccine prevented HSV invasion of the dorsal root ganglia, the site of virus latency, in 39/40 (97.5%) mice infected with either HSV-1 or HSV-2. We conclude that the HSV-2 trivalent mRNA vaccine provides potent protection against both HSV-1 and HSV-2 genital infection and is a promising vaccine candidate for human trials.

The goals of a genital herpes vaccine are to prevent painful genital lesions and reduce or eliminate subclinical infection that risks transmission to partners and newborns. We evaluated a trivalent glycoprotein vaccine containing herpes simplex virus type 2 (HSV-2) entry molecule glycoprotein D (gD2) and two immune evasion molecules: glycoprotein C (gC2), which binds complement C3b, and glycoprotein E (gE2), which blocks immunoglobulin G (IgG) Fc activities. The trivalent vaccine was administered as baculovirus proteins with CpG and alum, or the identical amino acids were expressed using nucleoside-modified mRNA in lipid nanoparticles (LNPs). Both formulations completely prevented genital lesions in mice and guinea pigs. Differences emerged when evaluating subclinical infection. The trivalent protein vaccine prevented dorsal root ganglia infection, and day 2 and 4 vaginal cultures were negative in 23 of 30 (73%) mice compared with 63 of 64 (98%) in the mRNA group (P = 0.0012). In guinea pigs, 5 of 10 (50%) animals in the trivalent subunit protein group had vaginal shedding of HSV-2 DNA on 19 of 210 (9%) days compared with 2 of 10 (20%) animals in the mRNA group that shed HSV-2 DNA on 5 of 210 (2%) days (P = 0.0052). The trivalent mRNA vaccine was superior to trivalent proteins in stimulating ELISA IgG antibodies, neutralizing antibodies, antibodies that bind to crucial gD2 epitopes involved in entry and cell-to-cell spread, CD4⁺ T cell responses, and T follicular helper and germinal center B cell responses. The trivalent nucleoside-modified mRNA-LNP vaccine is a promising candidate for human trials.