Abstract
Herpes simplex virus (HSV) glycoproteins gE and gI form an immunoglobulin G (IgG) Fc receptor (FcγR) that binds the Fc 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 FcγR activity without affecting other gE functions, we constructed a mutant virus, NS-gE339, that has four amino acids inserted into gE within the domain homologous to mammalian IgG FcγRs. NS-gE339 expresses gE and gI, is FcγR−, 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 FcγR does not bind murine IgG; therefore, the absence of an FcγR should not affect virulence in mice. NS-gE339 causes disease at the skin inoculation site comparably to wild-type and rescued viruses, indicating that the FcγR− 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 FcγR should protect wild-type virus from antibody attack. Human anti-HSV IgG greatly reduced viral titers and disease severity in NS-gE339-infected animals while having little effect on wild-type or rescued virus. We conclude that the HSV-1 FcγR enables the virus to evade antibody attack in vivo, which likely explains why antibodies are relatively ineffective against HSV infection.
Herpes simplex virus (HSV) establishes latency within sensory ganglia and periodically reactivates to produce recurrent infections. Latency is one mechanism used by HSV to evade immune attack, since during latency few if any viral proteins are produced and the virus remains hidden from the host. But how does the virus evade host immunity during recurrent infection? Virus can generally be recovered from lesions for several days after reactivation despite an already primed immune system.
HSV encodes at least 11 glycoproteins (48), several of which are essential for virus replication since they mediate virus entry or egress (30, 40, 53). Others are nonessential for replication in vitro yet are conserved in nature, suggesting an important role in vivo. Glycoproteins gE and gI are among the nonessential HSV glycoproteins. gE and gI form a hetero-oligomer complex that functions as a receptor for the Fc domain of immunoglobulin G (IgG) (5, 32, 33, 41). gE alone acts as a lower affinity IgG Fc receptor (FcγR), binding IgG aggregates but not IgG monomers, while the gE-gI complex acts as a higher-affinity FcγR, binding both IgG monomers and aggregates (6, 12).
IgG FcγRs are fairly widely distributed among human pathogens. Cells infected by HSV type 2 (HSV-2) (42), varicella-zoster virus (36), and cytomegalovirus (37) express virus-encoded IgG FcγRs. Certain protozoa (schistosomes and trypanosomes) (15, 50) and bacteria (for example, staphylococci [protein A] and streptococci [protein G]) (7, 47) also express IgG Fc binding proteins. Therefore, understanding the role of the HSV-1 FcγR in immune evasion may have broad implications for understanding microbial pathogenesis.
Initial studies of the HSV FcγR focused on its role in binding nonimmune IgG (1, 8, 11); however, the FcγR preferentially binds anti-HSV IgG by a process called antibody bipolar bridging (16, 51). This occurs when an HSV antibody molecule binds to its antigenic target by its Fab end and the Fc domain of the same molecule binds to the HSV-1 FcγR. In vitro studies indicate that the HSV FcγR inhibits complement-enhanced antibody neutralization (16), antibody-dependent cellular cytotoxicity (13), and attachment of granulocytes to the Fc domain of antibodies on HSV-infected cells (51). These results support a possible role for the FcγR in immune evasion and form the basis for studying the biologic relevance of the HSV-1 FcγR in vivo.
gE and gI play an important role in virus spread from cell to cell (2, 9, 10). This has created an obstacle to investigate the role of the HSV-1 FcγR in pathogenesis, since HSV-1 gE or gI null viruses are practically avirulent (2, 10, 43), probably because of their inability to spread. Therefore, to study the role of the FcγR in virulence it was necessary to develop HSV-1 mutant viruses that are deficient in Fc binding while retaining other gE and gI functions. Using this rationale, an HSV-1 mutant virus that has a four-amino-acid insert within the gE IgG Fc binding domain was generated (3, 14). This FcγR− virus remained intact for virus spread at the skin inoculation site in mice and caused disease comparable in severity to that caused by wild-type and marker-rescued viruses. In the presence of anti-HSV IgG, the FcγR− virus was significantly more susceptible to antibody attack than FcγR+ strains, indicating that the HSV FcγR promotes immune evasion in vivo.