There are no effective vaccines or monoclonal antibodies available for the prevention or treatment of herpes simplex viral infections. This application addresses this global public health priority. We developed a novel single-cycle vaccine strain deleted in glycoprotein D (gD), designated DgD-2. DgD-2 completely protects mice from lethal challenges with clinical isolates of both HSV-1 and HSV-2. Protection is mediated by IgG2 antibodies that have little neutralizing activity but activate Fc gamma receptors to promote antibody-dependent cellular cytotoxicity (ADCC). The central role for ADCC in mediating protection is supported by the observations that immune serum from DgD-2 vaccinated mice completely protects naïve wild-type, but not Fcg receptor IV knockout (FcgRIV-/-) mice. FcgRIV is the primary receptor responsible for mediating ADCC in mice. We isolated a highly protective monoclonal antibody (mAb), BMPC-23, from DgD-2 vaccinated mice that exhibits potent FcgRIV activating activity and mapped its epitope to domain IV of viral glycoprotein B. Notably, in contrast to the response to DgD-2, the humoral response to acute HSV infection or to vaccination with gD subunit protein vaccines in both mice and humans is neutralizing with little or no ADCC. These observations suggest that gD may interfere with the elicitation of ADCC antibodies. We hypothesized that this novel immune evasion mechanism may be mediated by interactions between gD and the co-signaling molecule switch molecule herpes virus entry mediator (HVEM), which is expressed by most immune cells. When gD binds HVEM, the interactions between HVEM and its natural ligands (LIGHT, lymphotoxin-a, CD160 and BTLA) may be inhibited. Supporting our hypothesis, IgG2 ADCC responses were reduced and protection against HSV was lost when Hvem−/− (knockout) mice were vaccinated with DgD-2. This need for HVEM to elicit IgG2 ADCC responses may be generalizable since similar results were obtained with other vaccines. Based on these data, we propose that the enhanced ADCC response to DgD-2 compared to natural HSV infection or to gD protein vaccines reflects differences in viral targets recognized, IgG subclasses generated and ability to overcome gD-HVEM mediated immune evasion. We will isolate and characterize mAbs generated in response to DgD-2, primary and recurrent HSV infection in wild-type compared to Hvem−/− mice to determine how HVEM signaling contributes to the antigenic repertoire, subclass and antibody function. We will test the mAbs that are generated in response to DgD-2 vaccination or infection individually and in combination (with BMPC-23 and other combinations) for their ability to protect and treat acute or recurrent HSV-1 and HSV-2 disease following vaginal and skin (male and female) infection as well as in models of neonatal disease. Together, these studies will yield important new fundamental and translational knowledge applicable to the development of vaccines and mAbs that mediate ADCC...