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

Abstract

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.

Author summary

Herpes simplex virus type 2 (HSV-2) glycoprotein D (gD2) mediates virus entry and cell-to-cell spread. We hypothesized that an effective gD2 vaccine needs to block these activities. Neutralizing titers, which assess blocking virus entry, correlated with protection against HSV-2 genital lesions in gD2-immunized guinea pigs. We used a high throughput biosensor assay to evaluate whether gD2-immunized guinea pigs produce antibodies that block the activities of gD2 epitopes involved in virus entry and cell-to-cell spread. We detected a strong correlation between the number of epitopes blocked by immune serum and protection against genital lesions. Passive transfer of monoclonal antibodies in mice infected intravaginally with HSV-2 confirmed the in vivo importance of antibodies to these epitopes. We re-evaluated our prior report of epitope-specific antibody responses in the gD2 Herpevac Trial, a trial that failed to protect subjects against HSV-2 genital infection. We detected antibody responses to significantly fewer epitopes in immunized humans than guinea pigs. We conclude that neutralizing antibodies and antibodies to multiple gD2 epitopes involved in virus entry and cell-to-cell spread represent important immune correlates of protection. Future human trials that contain gD2 as an immunogen need to assess these responses and correct deficiencies prior to embarking on large trials.