Journal of General Virology (2010), 91, 2542–2552 DOI 10.1099/vir.0.023085-0 Correspondence Philip G. Stevenson pgs27@cam.ac.uk Received 26 April 2010 Accepted 31 May 2010 INTRODUCTION Herpesviruses are widespread pathogens that use immune evasion to establish persistent infectivity in immunocom- petent hosts (Yewdell & Hill, 2002). Most can be neutralized in vitro. However, herpesvirus carriers not only remain infectious, but can be superinfected (Gorman et al., 2006) and show little tendency to select viral antigenic variants. Therefore in the context of natural infection, virus neutralization works poorly. Understanding why is important, as this could potentially reveal new means of infection control. Studies of gammaherpesvirus neutralization have concen- trated on Epstein–Barr virus (EBV). Antibodies to gp350 can stop EBV infecting B cells (Thorley-Lawson & Poodry, 1982). However, the gp350-specific antibodies of EBV carriers do not render them non-infectious, and vaccina- tion with gp350 failed to reduce the incidence of EBV infection (Sokal et al., 2007). This could reflect gp350- independent host entry (Janz et al., 2000), which gp350- specific antibodies might even enhance (Turk et al., 2006). Neutralization studies of both EBV and the Kaposi’s sarcoma-associated herpesvirus (KSHV) have been ham- pered by the difficulty of infecting cells at more than a very low level. Thus, an anti-gH/gL mAb can block EBV infection of epithelial cells, an anti-gp42 mAb can block Supplementary figures are available with the online version of this paper. 2542 Vaccination with murid herpesvirus-4 glycoprotein B reduces viral lytic replication but does not induce detectable virion neutralization Janet S. May and Philip G. Stevenson Division of Virology, Department of Pathology, University of Cambridge, UK Herpesviruses characteristically disseminate from immune hosts. Therefore in the context of natural infection, antibody neutralizes them poorly. Murid herpesvirus-4 (MuHV-4) provides a tractable model with which to understand gammaherpesvirus neutralization. MuHV-4 virions blocked for cell binding by immune sera remain infectious for IgG-Fc receptor+ myeloid cells, so broadly neutralizing antibodies must target the virion fusion complex – glycoprotein B (gB) or gH/ gL. While gB-specific neutralizing antibodies are rare, its domains I+II (gB-N) contain at least one potent neutralization epitope. Here, we tested whether immunization with recombinant gB presenting this epitope could induce neutralizing antibodies in naive mice and protect them against MuHV-4 challenge. Immunizing with the full-length gB extracellular domain induced a strong gB-specific antibody response and reduced MuHV-4 lytic replication but did not induce detectable neutralization. gB-N alone, which more selectively displayed pre-fusion epitopes including neutralization epitopes, also failed to induce neutralizing responses, and while viral lytic replication was again reduced this depended completely on IgG Fc receptors. gB and gB-N also boosted neutralizing responses in only a minority of carrier mice. Therefore, it appears that neutralizing epitopes on gB are intrinsically difficult for the immune response to target. EBV infection of B cells (Li et al., 1995), and KSHV can be neutralized by sera raised against the whole virus (Dialyna et al., 2004), gH/gL (Naranatt et al., 2002), gB (Akula et al., 2001) or K8.1 (Sakamoto et al., 2010), but much of this neutralization has been weak (,threefold reduction in infection) in settings where infection may be suboptimal – for example, EBV infects epithelial cells poorly in vitro. Its quantitative significance is therefore hard to assess. In contrast to EBV and KSHV, murid herpesvirus-4 (MuHV-4) efficiently and productively infects a wide range of cell types (although in vitro B-cell infection remains problematic). It also infects mice. However, while antibody reduces MuHV-4 lytic replication in vivo (Stevenson et al., 1999a; Kim et al., 2002; Gangappa et al., 2002), such protection depends on IgG Fc receptor engagement rather than neutralization (Wright et al., 2009), perhaps because cell association allows neutralization-resistant virus spread (Stevenson et al., 2009). The likely key setting for MuHV-4 neutralization and its evasion – the transmission of cell-free virions between hosts – is not yet experimentally accessible. Nevertheless, MuHV-4 allows us to explore how gamma- herpesvirus neutralization might be improved through immunization. MuHV-4 infects fibroblasts and epithelial cells by first attaching to heparan sulfate via gp70 (Gillet et al., 2007a) or gH/gL (Gillet et al., 2008a). These interactions can be blocked by mAbs derived from virus carriers, and a block to 023085 G 2010 SGM Printed in Great Britain