Our objectives are to investigate immune evasion strategies used by herpes simplex virus types 1 and 2 (HSV-1 and -2), and to develop novel immunization approaches that diminish HSV-1 and -2 evasion. Our studies focus on HSV glycoproteins gC and gE. gC interferes with complement (C) activation, rendering C ineffective against HSV-1 infection. However, if a critical gC domain that binds complement component C3b is deleted, C is highly active, reducing HSV- 1 virulence ~100-fold. gE evades antibodies by binding the lgG Fc domain and blocking Fcmediated activities, including C activation and antibody-dependent cellular cytotoxicity. An HSV-1 gE mutant virus that is unable to evade antibody attack is ~100-fold less virulent in vivo than wild-type virus because of enhanced antibody activity. We prepared an HSV-1 gC-gE double mutant virus and demonstrated that the mutant virus is ~1,000-fold less virulent than wild-type virus because the mutant virus lacks stealth activities. However, antibodies produced during natural infection in humans fail to recognize gC and gE immune evasion domains, suggesting that these domains remain hidden from the host. Nevertheless, antibodies can detect evasion domains, since a gC monoclonal antibody blocks C3b binding to gC and has antiviral activity in vitro and in vivo. These findings support the relevance of gC and gE immune evasion in pathogenesis, and our proposal to block these activities. We will use murine and rabbit models to further define the importance of HSV-1 gC and gE immune evasion in infection, and to evaluate differences that have emerged comparing HSV-1 and HSV-2 evasion properties. We will define the mechanisms for these differences, and use animal models and knockout mice to examine whether gC and gE impair links between humoral and cellular limbs of innate immunity, and between innate and acquired immunity. We will attempt to expose hidden gC and gE domains by using baculovirus-expressed protein fragments, modifying N-linked carbohydrates, and immunizing with glycoproteins bound to their natural ligands with the purpose of uncovering hidden domains. We will evaluate gC and gE immunogens in animal models to determine whether the immunogens can elicit an immune response that blocks evasion domains and reduces virulence. Preventing viral stealth may greatly improve antibody and C activities in host defense, and may lead to new approaches for the next generation of successful vaccines.