Global sensing of the antigenic structure of herpes simplex virus gD using high-throughput array-based SPR imaging

Selection, activity, and nuclease stabilization of cross-neutralizing DNA aptamers targeting HSV-1 and HSV-2

Cross-neutralizing aptamers targeting both HSV-1 and HSV-2 were developed by selecting against the ectodomains of glycoprotein D (gD) from both viruses in parallel as well as sequentially using the SELEX method. Since gD facilitates viral invasion, sterically blocking the host-receptor interaction prevents infection. Candidate aptamers were screened, and lead aptamers were identified that exhibited exceptional neutralizing activity against both viruses in vitro. The specificity of the aptamers was confirmed by comparing their activity to scrambled versions of themselves. Modifications of the lead compounds were tested to define critical motifs to guide development. Stability of the aptamers was increased using phosphorothioate backbone linkages, and 2' methoxy substitutions of terminal and key internal bases. Aptamers were applied in a guinea pig vaginal HSV-2 infection model and found to reduce both the viral load of infected animals and the severity of the resulting disease. These results suggest that cross-neutralizing aptamers can be developed into on-demand antiviral interventions effective against both HSV-1 and HSV-2.

Receptor Binding-Induced Conformational Changes in Herpes Simplex Virus Glycoprotein D Permit Interaction with the gH/gL Complex to Activate Fusion

Herpes simplex virus (HSV) requires four essential virion glycoproteins-gD, gH, gL, and gB-for virus entry and cell fusion. To initiate fusion, the receptor binding protein gD interacts with one of two major cell receptors, HVEM or nectin-1. Once gD binds to a receptor, fusion is carried out by the gH/gL heterodimer and gB. A comparison of free and receptor-bound gD crystal structures revealed that receptor binding domains are located within residues in the N-terminus and core of gD. Problematically, the C-terminus lies across and occludes these binding sites. Consequentially, the C-terminus must relocate to allow for both receptor binding and the subsequent gD interaction with the regulatory complex gH/gL. We previously constructed a disulfide bonded (K190C/A277C) protein that locked the C-terminus to the gD core. Importantly, this mutant protein bound receptor but failed to trigger fusion, effectively separating receptor binding and gH/gL interaction. Here, we show that "unlocking" gD by reducing the disulfide bond restored not only gH/gL interaction but fusion activity as well, confirming the importance of C-terminal movement in triggering the fusion cascade. We characterize these changes, showing that the C-terminus region exposed by unlocking is: (1) a gH/gL binding site; (2) contains epitopes for a group (competition community) of monoclonal antibodies (Mabs) that block gH/gL binding to gD and cell-cell fusion. Here, we generated 14 mutations within the gD C-terminus to identify residues important for the interaction with gH/gL and the key conformational changes involved in fusion. As one example, we found that gD L268N was antigenically correct in that it bound most Mabs but was impaired in fusion, exhibited compromised binding of MC14 (a Mab that blocks both gD-gH/gL interaction and fusion), and failed to bind truncated gH/gL, all events that are associated with the inhibition of C-terminus movement. We conclude that, within the C-terminus, residue 268 is essential for gH/gL binding and induction of conformational changes and serves as a flexible inflection point in the critical movement of the gD C-terminus.

Moving beyond Titers

Vaccination to prevent and even eliminate disease is amongst the greatest achievements of modern medicine. Opportunities remain in vaccine development to improve protection across the whole population. A next step in vaccine development is the detailed molecular characterization of individual humoral immune responses against a pathogen, especially the rapidly evolving pathogens. New technologies such as sequencing the immune repertoire in response to disease, immunogenomics/vaccinomics, particularly the individual HLA variants, and high-throughput epitope characterization offer new insights into disease protection. Here, we highlight the emerging technologies that could be used to identify variation within the human population, facilitate vaccine discovery, improve vaccine safety and efficacy, and identify mechanisms of generating immunological memory. In today’s vaccine-hesitant climate, these techniques used individually or especially together have the potential to improve vaccine effectiveness and safety and thus vaccine uptake rates. We highlight the importance of using these techniques in combination to understand the humoral immune response as a whole after vaccination to move beyond neutralizing titers as the standard for immunogenicity and vaccine efficacy, especially in clinical trials.

A potent neutralizing and protective antibody against a conserved continuous epitope on HSV glycoprotein D

Infections caused by herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) remain a serious global health issue, and the medical countermeasures available thus far are limited. Virus-neutralizing monoclonal antibodies (NAbs) are crucial tools for studying host-virus interactions and designing effective vaccines, and the discovery and development of these NAbs could be one approach to treat or prevent HSV infection. Here, we report the isolation of five HSV NAbs from mice immunized with both HSV-1 and HSV-2. Among these were two antibodies that potently cross-neutralized both HSV-1 and HSV-2 with the 50% virus-inhibitory concentrations (IC₅₀) below 200 ng/ml, one of which (4A3) exhibited high potency against HSV-2, with an IC₅₀ of 59.88 ng/ml. 4A3 neutralized HSV at the prebinding stage and prevented HSV infection and cell-to-cell spread. Significantly, administration of 4A3 completely prevented weight loss and improved survival of mice challenged with a lethal dose of HSV-2. Using structure-guided molecular modeling combined with alanine-scanning mutagenesis, we observed that 4A3 bound to a highly conserved continuous epitope (residues 216 to 220) within the receptor-binding domain of glycoprotein D (gD) that is essential for viral infection and the triggering of membrane fusion. Our results provide guidance for developing NAb drugs and vaccines against HSV.

Antibodies to Crucial Epitopes on HSV-2 Glycoprotein D as a Guide to Dosing an mRNA Genital Herpes Vaccine

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.

Is the ZIKV Congenital Syndrome and Microcephaly Due to Syndemism with Latent Virus Coinfection?

The emergence of the Zika virus (ZIKV) mirrors its evolutionary nature and, thus, its ability to grow in diversity or complexity (i.e., related to genome, host response, environment changes, tropism, and pathogenicity), leading to it recently joining the circle of closed congenital pathogens. The causal relation of ZIKV to microcephaly is still a much-debated issue. The identification of outbreak foci being in certain endemic urban areas characterized by a high-density population emphasizes that mixed infections might spearhead the recent appearance of a wide range of diseases that were initially attributed to ZIKV. Globally, such coinfections may have both positive and negative effects on viral replication, tropism, host response, and the viral genome. In other words, the possibility of coinfection may necessitate revisiting what is considered to be known regarding the pathogenesis and epidemiology of ZIKV diseases. ZIKV viral coinfections are already being reported with other arboviruses (e.g., chikungunya virus (CHIKV) and dengue virus (DENV)) as well as congenital pathogens (e.g., human immunodeficiency virus (HIV) and cytomegalovirus (HCMV)). However, descriptions of human latent viruses and their impacts on ZIKV disease outcomes in hosts are currently lacking. This review proposes to select some interesting human latent viruses (i.e., herpes simplex virus 2 (HSV-2), Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), human parvovirus B19 (B19V), and human papillomavirus (HPV)), whose virological features and co-exposition with ZIKV may provide evidence of the syndemism process, shedding some light on the emergence of the ZIKV-induced global congenital syndrome in South America.

Using Split Luciferase Assay and anti-HSV Glycoprotein Monoclonal Antibodies to Predict a Functional Binding Site Between gD and gH/gL

Herpes simplex virus (HSV) entry and cell-cell fusion require glycoproteins gD, gH/gL, and gB. HSV entry begins with gD binding its receptor (nectin-1), which then activates gH/gL to enable the conversion of pre-fusion gB to its active form to promote membrane fusion. Virus-neutralizing monoclonal antibodies (Mabs) interfere with one or more of these steps and localization of their epitopes identifies functional sites on each protein. Utilizing this approach, we have identified the gH/gL binding face on gD and the corresponding gD binding site on gH/gL. Here, we used combinations of these Mabs to define the orientation of gD and gH/gL relative to each other. We reasoned that if two Mabs, one directed at gD and the other at gH/gL, block fusion more effectively than when either were used alone (additive), then their epitopes would be spatially distanced and binding of one would not directly interfere with binding of the other during fusion. However, if the two Mabs blocked fusion with equal or lesser efficacy that when either were used alone (indifferent), we propose that their epitopes would be in close proximity in the complex. Using a live cell fusion assay, we found that some Mab pairings blocked the fusion with different mechanisms while other had a similar mechanisms of action. Grouping the different combinations of antibodies into indifferent and additive groups, we present a model for the orientation of gD vis-à-vis gH/gL in the complex.Importance: Virus entry and cell-cell fusion mediated by HSV require four essential glycoproteins, gD, gH/gL, gB and a gD receptor. Virus-neutralizing antibodies directed against any of these proteins bind to residues within key functional sites and interfere with essential steps in the fusion pathway. Thus, the epitopes of these Mabs overlap and point to critical, functional sites on their target proteins. Here, we combined gD and gH/gL antibodies to determine whether they work in an additive or non-additive (indifferent) fashion to block specific events in glycoprotein-driven cell-cell fusion. Identifying combinations of antibodies that have additive effects will help in the rational design of an effective therapeutic "polyclonal antibody" to treat HSV disease. In addition, identification of the exact contact regions between gD and gH/gL can inform the design of small molecules that would interfere with the gD-gH/gL complex formation, thus preventing the virus from entering the host cell.

Serum and Cervicovaginal Fluid Antibody Profiling in Herpes Simplex Virus (HSV) Seronegative Recipients of the HSV529 Vaccine

Previous HSV2 vaccines have not prevented genital herpes. Concerns have been raised about the choice of antigen, the type of antibody induced by the vaccine, and whether antibody is present in the genital tract where infection occurs. We reported results of a trial of an HSV2 replication-defective vaccine, HSV529, that induced serum neutralizing antibody responses in 78% of HSV1 -/HSV2 - vaccine recipients. Here we show that HSV1 -/HSV2 - vaccine recipients developed antibodies to epitopes of several viral proteins; however, fewer antibody epitopes were detected in vaccine recipients compared with naturally infected persons. HSV529 induced antibodies that mediated HSV2-specific NK cell activation. Depletion of gD-binding antibody from sera reduced neutralizing titers by 62% and NK cell activation by 81%. HSV2 gD antibody was detected in cervicovaginal fluid at about one-third the level of that in serum. A vaccine that induces potent serum antibodies transported to the genital tract might reduce HSV genital infection.

Localization of the Interaction Site of Herpes Simplex Virus Glycoprotein D (gD) on the Membrane Fusion Regulator, gH/gL

A cascade of protein-protein interactions between four herpes simplex virus (HSV) glycoproteins (gD, gH/gL, and gB) drive fusion between the HSV envelope and host membrane, thereby allowing for virus entry and infection. Specifically, binding of gD to one of its receptors induces a conformational change that allows gD to bind to the regulatory complex gH/gL, which then activates the fusogen gB, resulting in membrane fusion. Using surface plasmon resonance and a panel of anti-gD monoclonal antibodies (MAbs) that sterically blocked the interaction, we previously showed that gH/gL binds directly to gD at sites distinct from the gD receptor binding site. Here, using an analogous strategy, we first evaluated the ability of a panel of uncharacterized anti-gH/gL MAbs to block binding to gD and/or inhibit fusion. We found that the epitopes of four gD-gH/gL-blocking MAbs were located within flexible regions of the gH N terminus and the gL C terminus, while the fifth was placed around gL residue 77. Taken together, our data localized the gD binding region on gH/gL to a group of gH and gL residues at the membrane distal region of the heterodimer. Surprisingly, a second set of MAbs did not block gD-gH/gL binding but instead stabilized the complex by altering the kinetic binding. However, despite this prolonged gD-gH/gL interaction, "stabilizing" MAbs also inhibited cell-cell fusion, suggesting a unique mechanism by which the fusion process is halted. Our findings support targeting the gD-gH/gL interaction to prevent fusion in both therapeutic and vaccine strategies against HSV.IMPORTANCE Key to developing a human HSV vaccine is an understanding of the virion glycoproteins involved in entry. HSV employs multiple glycoproteins for attachment, receptor interaction, and membrane fusion. Determining how these proteins function was resolved, in part, by structural biology coupled with immunological and biologic evidence. After binding, virion gD interacts with a receptor to activate the regulator gH/gL complex, triggering gB to drive fusion. Multiple questions remain, one being the physical location of each glycoprotein interaction site. Using protective antibodies with known epitopes, we documented the long-sought interaction between gD and gH/gL, detailing the region on gD important to create the gD-gH/gL triplex. Now, we have identified the corresponding gD contact sites on gH/gL. Concurrently we discovered a novel mechanism whereby gH/gL antibodies stabilize the complex and inhibit fusion progression. Our model for the gD-gH/gL triplex provides a new framework for studying fusion, which identifies targets for vaccine development.

Characterizing Epitope Binding Regions of Entire Antibody Panels by Combining Experimental and Computational Analysis of Antibody: Antigen Binding Competition

Vaccines and immunotherapies depend on the ability of antibodies to sensitively and specifically recognize particular antigens and specific epitopes on those antigens. As such, detailed characterization of antibody-antigen binding provides important information to guide development. Due to the time and expense required, high-resolution structural characterization techniques are typically used sparingly and late in a development process. Here, we show that antibody-antigen binding can be characterized early in a process for whole panels of antibodies by combining experimental and computational analyses of competition between monoclonal antibodies for binding to an antigen. Experimental "epitope binning" of monoclonal antibodies uses high-throughput surface plasmon resonance to reveal which antibodies compete, while a new complementary computational analysis that we call "dock binning" evaluates antibody-antigen docking models to identify why and where they might compete, in terms of possible binding sites on the antigen. Experimental and computational characterization of the identified antigenic hotspots then enables the refinement of the competitors and their associated epitope binding regions on the antigen. While not performed at atomic resolution, this approach allows for the group-level identification of functionally related monoclonal antibodies (i.e., communities) and identification of their general binding regions on the antigen. By leveraging extensive epitope characterization data that can be readily generated both experimentally and computationally, researchers can gain broad insights into the basis for antibody-antigen recognition in wide-ranging vaccine and immunotherapy discovery and development programs.

Nucleoside-modified mRNA encoding HSV-2 glycoproteins C, D, and E prevents clinical and subclinical genital herpes

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.

Vaccines to prevent genital herpes

Genital herpes increases the risk of acquiring and transmitting Human Immunodeficiency Virus (HIV), is a source of anxiety for many about transmitting infection to intimate partners, and is life-threatening to newborns. A vaccine that prevents genital herpes infection is a high public health priority. An ideal vaccine will prevent both genital lesions and asymptomatic subclinical infection to reduce the risk of inadvertent transmission to partners, will be effective against genital herpes caused by herpes simplex virus types 1 and 2 (HSV-1, HSV-2), and will protect against neonatal herpes. Three phase 3 human trials were performed over the past 20 years that used HSV-2 glycoproteins essential for virus entry as immunogens. None achieved its primary endpoint, although each was partially successful in either delaying onset of infection or protecting a subset of female subjects that were HSV-1 and HSV-2 uninfected against HSV-1 genital infection. The success of future vaccine candidates may depend on improving the predictive value of animal models by requiring vaccines to achieve near-perfect protection in these models and by using the models to better define immune correlates of protection. Many vaccine candidates are under development, including DNA, modified mRNA, protein subunit, killed virus, and attenuated live virus vaccines. Lessons learned from prior vaccine studies and select candidate vaccines are discussed, including a trivalent nucleoside-modified mRNA vaccine that our laboratory is pursuing. We are optimistic that an effective vaccine for prevention of genital herpes will emerge in this decade.

Point Mutations in Retargeted gD Eliminate the Sensitivity of EGFR/EGFRvIII-Targeted HSV to Key Neutralizing Antibodies

Effective oncolytic virotherapy may require systemic delivery, tumor targeting, and resistance to virus-neutralizing (VN) antibodies. Since herpes simplex virus (HSV) glycoprotein D (gD) is the viral attachment/entry protein and predominant VN target, we examined the impact of gD retargeting alone and in combination with alterations in dominant VN epitopes on virus susceptibility to VN antibodies. We compared the binding of a panel of anti-gD monoclonal antibodies (mAbs) that mimic antibody specificities in human HSV-immune sera to the purified ectodomains of wild-type and retargeted gD, revealing the retention of two prominent epitopes. Substitution of a key residue in each epitope, separately and together, revealed that both substitutions (1) blocked retargeted gD recognition by mAbs to the respective epitopes, and, in combination, caused a global reduction in mAb binding; (2) protected against fusion inhibition by VN mAbs reactive with each epitope in virus-free cell-cell fusion assays; and (3) increased the resistance of retargeted HSV-1 to these VN mAbs. Although the combined modifications of retargeted gD allowed bona fide retargeting, incorporation into virions was partially compromised. Our results indicate that stacking of epitope mutations can additively block retargeted gD recognition by VN antibodies but also that improvements in gD incorporation into virus particles may be required.

Dynamic organization of Herpesvirus glycoproteins on the viral envelope revealed by super-resolution microscopy

The processes of cell attachment and membrane fusion of Herpes Simplex Virus 1 involve many different envelope glycoproteins. Viral proteins gC and gD bind to cellular receptors. Upon binding, gD activates the gH/gL complex which in turn activates gB to trigger membrane fusion. Thus, these proteins must be located at the point of contact between cellular and viral envelopes to interact and allow fusion. Using super-resolution microscopy, we show that gB, gH/gL and most of gC are distributed evenly round purified virions. In contrast, gD localizes essentially as clusters which are distinct from gB and gH/gL. Upon cell binding, we observe that all glycoproteins, including gD, have a similar ring-like pattern, but the diameter of these rings was significantly smaller than those observed on cell-free viruses. We also observe that contrary to cell-free particles, gD mostly colocalizes with other glycoproteins on cell-bound particles. The differing patterns of localization of gD between cell-free and cell-bound viruses indicates that gD can be reorganized on the viral envelope following either a possible maturation of the viral particle or its adsorption to the cell. This redistribution of glycoproteins upon cell attachment could contribute to initiate the cascade of activations leading to membrane fusion.

The envelopes of Herpesvirus particles contain a variety of different proteins that allow them to infect specific cell types. An essential core set of these proteins is designed to allow viral entry into the cell after adsorption by binding to specific receptors and ultimately inducing fusion between the viral and cellular membranes in a regulated way through a succession of interactions between receptor-binding and fusion-triggering viral proteins. We have identified here for the first time the localization patterns of these essential proteins at the surface of purified virions and we describe how their localization changes after cell attachment. These results illustrate how the dynamics of viral proteins at the surface of the viral particle could participate in optimizing the all-important process of cell binding and membrane fusion.

Surface Plasmon Resonance Reveals Direct Binding of Herpes Simplex Virus Glycoproteins gH/gL to gD and Locates a gH/gL Binding Site on gD

Herpes simplex virus (HSV) requires fusion between the viral envelope and host membrane. Four glycoproteins, gD, gH/gL, and gB, are essential for this process. To initiate fusion, gD binds its receptor and undergoes a conformational change that hypothetically leads to activation of gH/gL, which in turn triggers the fusion protein gB to undergo rearrangements leading to membrane fusion. Our model predicts that gD must interact with both its receptor and gH/gL to promote fusion. In support of this, we have shown that gD is structurally divided into two "faces": one for the binding receptor and the other for its presumed interaction with gH/gL. However, until now, we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used surface plasmon resonance to show that the ectodomain of gH/gL binds directly to the ectodomain of gD when (i) gD is captured by certain anti-gD monoclonal antibodies (MAbs) that are bound to a biosensor chip, (ii) gD is bound to either one of its receptors on a chip, and (iii) gD is covalently bound to the chip surface. To localize the gH/gL binding site on gD, we used multiple anti-gD MAbs from six antigenic communities and determined which ones interfered with this interaction. MAbs from three separate communities block gD-gH/gL binding, and their epitopes encircle a geographical area on gD that we propose comprises the gH/gL binding domain. Together, our results show that gH/gL interacts directly with gD, supporting a role for this step in HSV entry.IMPORTANCE HSV entry is a multistep process that requires the actions of four glycoproteins, gD, gH/gL, and gB. Our current model predicts that gD must interact with both its receptor and gH/gL to promote viral entry. Although we know a great deal about how gD binds its receptors, until now we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used a highly sensitive surface plasmon resonance technique to clearly demonstrate that gD and gH/gL interact. Furthermore, using multiple MAbs with defined epitopes, we have delineated a domain on gD that is independent of that used for receptor binding and which likely represents the gH/gL interaction domain. Targeting this interaction to prevent fusion may enhance both therapeutic and vaccine strategies.

Antibody responses to crucial functional epitopes as a novel approach to assess immunogenicity of vaccine adjuvants

We are interested in developing a vaccine that prevents genital herpes. Adjuvants have a major impact on vaccine immunogenicity. We compared two adjuvants, an experimental Merck Sharp & Dohme lipid nanoparticle (LNP) adjuvant, LNP-2, with CpG oligonucleotide combined with alum for immunogenicity in mice when administered with herpes simplex virus type 2 (HSV-2) glycoproteins C, D and E (gC2, gD2, gE2). The immunogens are intended to produce neutralizing antibodies to gC2 and gD2, antibodies to gD2 and gE2 that block cell-to-cell spread, and antibodies to gE2 and gC2 that block immune evasion from antibody and complement, respectively. Overall, CpG/alum was better at producing serum and vaginal IgG binding antibodies, neutralizing antibodies, antibodies that block virus spread from cell-to-cell, and antibodies that block immune evasion domains on gC2. We used a novel high throughput biosensor assay to further assess differences in immunogenicity by mapping antibody responses to seven crucial epitopes on gD2 involved in virus entry or cell-to-cell spread. We found striking differences between CpG/alum and LNP-2. Mice immunized with gD2 CpG/alum produced higher titers of antibodies than LNP-2 to six of seven crucial epitopes and produced antibodies to more crucial epitopes than LNP-2. Measuring epitope-specific antibodies helped to define mechanisms by which CpG/alum outperformed LNP-2 and is a valuable technique to compare adjuvants.

Recent Advances in Surface Plasmon Resonance Imaging Sensors

The surface plasmon resonance (SPR) sensor is an important tool widely used for studying binding kinetics between biomolecular species. The SPR approach offers unique advantages in light of its real-time and label-free sensing capabilities. Until now, nearly all established SPR instrumentation schemes are based on single- or several-channel configurations. With the emergence of drug screening and investigation of biomolecular interactions on a massive scale these days for finding more effective treatments of diseases, there is a growing demand for the development of high-throughput 2-D SPR sensor arrays based on imaging. The so-called SPR imaging (SPRi) approach has been explored intensively in recent years. This review aims to provide an up-to-date and concise summary of recent advances in SPRi. The specific focuses are on practical instrumentation designs and their respective biosensing applications in relation to molecular sensing, healthcare testing, and environmental screening.

Using Antibodies and Mutants To Localize the Presumptive gH/gL Binding Site on Herpes Simplex Virus gD

HSV virus-cell and cell-cell fusion requires multiple interactions between four essential virion envelope glycoproteins, gD, gB, gH, and gL, and between gD and a cellular receptor, nectin-1 or herpesvirus entry mediator (HVEM). Current models suggest that binding of gD to receptors induces a conformational change that leads to activation of gH/gL and consequent triggering of the prefusion form of gB to promote membrane fusion. Since protein-protein interactions guide each step of fusion, identifying the sites of interaction may lead to the identification of potential therapeutic targets that block this process. We have previously identified two "faces" on gD: one for receptor binding and the other for its presumed interaction with gH/gL. We previously separated the gD monoclonal antibodies (MAbs) into five competition communities. MAbs from two communities (MC2 and MC5) neutralize virus infection and block cell-cell fusion but do not block receptor binding, suggesting that they block binding of gD to gH/gL. Using a combination of classical epitope mapping of gD mutants with fusion and entry assays, we identified two residues (R67 and P54) on the presumed gH/gL interaction face of gD that allowed for fusion and viral entry but were no longer sensitive to inhibition by MC2 or MC5, yet both were blocked by other MAbs. As neutralizing antibodies interfere with essential steps in the fusion pathway, our studies strongly suggest that these key residues block the interaction of gD with gH/gL.IMPORTANCE Virus entry and cell-cell fusion mediated by HSV require gD, gH/gL, gB, and a gD receptor. Neutralizing antibodies directed against any of these proteins bind to residues within key functional sites and interfere with an essential step in the fusion pathway. Thus, the epitopes of these MAbs identify critical, functional sites on their target proteins. Unlike many anti-gD MAbs, which block binding of gD to a cellular receptor, two, MC2 and MC5, block a separate, downstream step in the fusion pathway which is presumed to be the activation of the modulator of fusion, gH/gL. By combining epitope mapping of a panel of gD mutants with fusion and virus entry assays, we have identified residues that are critical in the binding and function of these two MAbs. This new information helps to define the site of the presumptive interaction of gD with gH/gL, of which we have limited knowledge.

Vaccine-induced antibodies to herpes simplex virus glycoprotein D epitopes involved in virus entry and cell-to-cell spread correlate with protection against genital disease in guinea pigs

Herpes simplex virus type 2 (HSV-2) glycoprotein D (gD2) subunit antigen is included in many preclinical candidate vaccines. The rationale for including gD2 is to produce antibodies that block crucial gD2 epitopes involved in virus entry and cell-to-cell spread. HSV-2 gD2 was the only antigen in the Herpevac Trial for Women that protected against HSV-1 genital infection but not HSV-2. In that trial, a correlation was detected between gD2 ELISA titers and protection against HSV-1, supporting the importance of antibodies. A possible explanation for the lack of protection against HSV-2 was that HSV-2 neutralization titers were low, four-fold lower than to HSV-1. Here, we evaluated neutralization titers and epitope-specific antibody responses to crucial gD2 epitopes involved in virus entry and cell-to-cell spread as correlates of immune protection against genital lesions in immunized guinea pigs. We detected a strong correlation between neutralizing antibodies and protection against genital disease. We used a high throughput biosensor competition assay to measure epitope-specific responses to seven crucial gD2 linear and conformational epitopes involved in virus entry and spread. Some animals produced antibodies to most crucial epitopes while others produced antibodies to few. The number of epitopes recognized by guinea pig immune serum correlated with protection against genital lesions. We confirmed the importance of antibodies to each crucial epitope using monoclonal antibody passive transfer that improved survival and reduced genital disease in mice after HSV-2 genital challenge. We re-evaluated our prior study of epitope-specific antibody responses in women in the Herpevac Trial. Humans produced antibodies that blocked significantly fewer crucial gD2 epitopes than guinea pigs, and antibody responses in humans to some linear epitopes were virtually absent. Neutralizing antibody titers and epitope-specific antibody responses are important immune parameters to evaluate in future Phase I/II prophylactic human vaccine trials that contain gD2 antigen.

Role of Microvesicles in the Spread of Herpes Simplex Virus 1 in Oligodendrocytic Cells

Herpes simplex virus 1 (HSV-1) is a neurotropic pathogen that can infect many types of cells and establishes latent infections in the neurons of sensory ganglia. In some cases, the virus spreads into the central nervous system, causing encephalitis or meningitis. Cells infected with several different types of viruses may secrete microvesicles (MVs) containing viral proteins and RNAs. In some instances, extracellular microvesicles harboring infectious virus have been found. Here we describe the features of shedding microvesicles released by the human oligodendroglial HOG cell line infected with HSV-1 and their participation in the viral cycle. Using transmission electron microscopy, we detected for the first time microvesicles containing HSV-1 virions. Interestingly, the Chinese hamster ovary (CHO) cell line, which is resistant to infection by free HSV-1 virions, was susceptible to HSV-1 infection after being exposed to virus-containing microvesicles. Therefore, our results indicate for the first time that MVs released by infected cells contain virions, are endocytosed by naive cells, and lead to a productive infection. Furthermore, infection of CHO cells was not completely neutralized when virus-containing microvesicles were preincubated with neutralizing anti-HSV-1 antibodies. The lack of complete neutralization and the ability of MVs to infect nectin-1/HVEM-negative CHO-K1 cells suggest a novel way for HSV-1 to spread to and enter target cells. Taken together, our results suggest that HSV-1 could spread through microvesicles to expand its tropism and that microvesicles could shield the virus from neutralizing antibodies as a possible mechanism to escape the host immune response.IMPORTANCE Herpes simplex virus 1 (HSV-1) is a neurotropic pathogen that can infect many types of cells and establishes latent infections in neurons. Extracellular vesicles are a heterogeneous group of membrane vesicles secreted by most cell types. Microvesicles, which are extracellular vesicles which derive from the shedding of the plasma membrane, isolated from the supernatant of HSV-1-infected HOG cells were analyzed to find out whether they were involved in the viral cycle. The importance of our investigation lies in the detection, for the first time, of microvesicles containing HSV-1 virions. In addition, virus-containing microvesicles were endocytosed into CHO-K1 cells and were able to actively infect these otherwise nonpermissive cells. Finally, the infection of CHO cells with these virus-containing microvesicles was not completely neutralized by anti-HSV-1 antibodies, suggesting that these extracellular vesicles might shield the virus from neutralizing antibodies as a possible mechanism of immune evasion.