Date Published: May 23, 2018
Publisher: Public Library of Science
Author(s): Lauren M. Hook, Tina M. Cairns, Sita Awasthi, Benjamin D. Brooks, Noah T. Ditto, Roselyn J. Eisenberg, Gary H. Cohen, Harvey M. Friedman, Lindsey Hutt-Fletcher.
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.
Nearly one half-billion people worldwide are infected with herpes simplex virus type 2 (HSV-2) and another one-quarter billion have genital infection caused by herpes simplex virus type 1 (HSV-1) [1, 2]. The disease manifestations vary from asymptomatic genital infection to severe, recurrent ulcerative genital disease [3–5]. Genital herpes poses major risks to newborns delivered through an infected birth canal with an estimated 14,000 annual cases globally and a mortality of 60% if untreated [2, 6]. HSV-2 genital infection carries a three-fold increased risk for acquisition and transmission of HIV .
HSV-2 gD2 mediates virus receptor binding, activation of gH/gL and cell-to-cell spread, which are crucial viral functions [12, 37]. We hypothesized that an optimal gD2 vaccine is one that produces antibodies to multiple crucial epitopes involved in these functions. The high throughput biosensor technology provides a powerful new tool for measuring epitope-specific antibody responses to linear and conformational epitopes. We used the biosensor technology to demonstrate that some epitopes were immunogenic in almost all guinea pigs, while others were immunogenic in only a few. We determined in guinea pigs that the greater the number of crucial gD2 epitopes blocked by immunization, the better the protection was against genital lesions. We used MAb passive antibody studies in mice to demonstrate that MAbs to each of seven crucial gD2 epitopes provided significant, although partial protection against death and genital disease. Additional passive antibody studies in mice will be required to evaluate whether combinations of MAbs will provide maximum protection and to assess the extent of protection provided by the MAb IgG Fab domain compared with the Fc domain.