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

While HSV-2 typically causes genital lesions, HSV-1 is increasingly the cause of genital herpes. In addition, neonatal HSV infections are associated with a high rate of mortality and HSV-2 may increase the risk for HIV or Zika infections, reinforcing the need to develop an effective vaccine. In the GSK Herpevac trial, doubly sero-negative women were vaccinated with a truncated form of gD2 [gD2(284t)], then examined for anti-gD serum titers and clinical manifestations of disease. Surprisingly, few vaccinees were protected against genital HSV-2 but 86% were protected from genital HSV-1. These observations suggest that subtle differences in gD structure might influence a protective response. To better understand the antigenic structure of gD and how it impacts a protective response, we previously utilized several key anti-gD monoclonal antibodies (mAbs) to dissect epitopes in vaccinee sera. Several correlations were observed but the methodology limited the number of sera and mAbs that could be tested. Here, we used array-based surface plasmon imaging (SPRi) to simultaneously measure a larger number of protein-protein interactions. We carried out cross-competition or “epitope binning” studies with 39 anti-gD mAbs and four soluble forms of gD, including a form [gD2(285t)] that resembles the Herpevac antigen. The results from these experiments allowed us to organize the mAbs into four epitope communities. Notably, relationships within and between communities differed depending on the form of gD, and off-rate analysis suggested differences in mAb-gD avidity depending on the gD serotype and length. Together, these results show that gD1 and gD2 differ in their structural topography. Consistent with the Herpevac results, several mAbs that bind both gD1 and gD2 neutralize only HSV-1. Thus, this technology provides new insights into the antigenic structure of gD and provides a rationale as to how vaccination with a gD2 subunit may lead to protection from HSV-1 infection.
Author summary Understanding the complex steps of herpes simplex virus (HSV) entry into susceptible cells is key for developing an effective vaccine to prevent clinical disease and control recurrences of this important human pathogen. For either serotype (HSV-1 or HSV-2), it is the interaction of glycoprotein D (gD) with receptor that initiates the cascade of events to begin infection. Importantly, gD stimulates high titers of virus-neutralizing antibodies that predict its importance in a protective response. Many HSV vaccines under development center around gD. In one recent clinical trial, subjects vaccinated with gD2 were protected against HSV-1 genital infection but not HSV-2, leading us to address how type specificity affects the antigenic topography of gD. Using a large panel of monoclonal antibodies (mAbs), we employed a new high-throughput, array-based approach to organize the mAbs into communities. Relationships within and between communities differed depending on the isotype and length of gD, suggesting significant differences in the epitope topography. We found that a subset of mAbs bound both gD1 and gD2 yet only neutralized HSV-1, possibly accounting for the anomalous results of the clinical trial. This new technology has proven to yield significant insights about how different regions of gD contribute to immune responses.