In vivo immune evasion mediated by the herpes simplex virus type 1 immunoglobulin G Fc receptor

Herpes simplex virus (HSV) glycoproteins gE and gI form an immunoglobulin G (IgG) Fc receptor (Fc gamma R) that binds the Fe domain of human anti-HSV IgG and inhibits Fc-mediated immune functions in vitro, gE or gI deletion mutant viruses are avirulent, probably because gE and gI are also involved in cell-to-cell spread, In an effort to modify Fc gamma R activity without affecting other gE functions, we constructed a mutant virus, NS-gE(339), that has four amino acids inserted into gE within the domain homologous to mammalian IgG Fc gamma Rs. NS-gE(339) expresses gE and gI, is Fc gamma R-, and does not participate in antibody bipolar bridging since it does not block activities mediated by the Fc domain of anti-HSV IgG, In vivo studies were performed with mice because the HSV-1 Fc gamma R does not bind murine IgG; therefore, the absence of an Fc gamma R should not affect virulence in mice. NS-gE(339) causes disease at the skin inoculation site comparably to wild-type and rescued viruses, indicating that the Fc gamma R- mutant virus is pathogenic in animals. Mice were passively immunized with human anti-HSV IgG and then infected with mutant or wild-type virus. We postulated that the HSV-1 Fc gamma R should protect wild-type virus from antibody attack. Human anti-HSV IgG greatly reduced viral titers and disease severity in NS-gE(339)-infected animals while having little effect on,wild-type or rescued virus. We conclude that the HSV-1 Fc gamma R enables the virus to evade antibody attack in vivo, which likely explains why antibodies are relatively ineffective against HSV infection.