Structure of the Kaposi’s sarcoma-associated herpesvirus gB in post-fusion conformation

Discovered in 1994 in lesions of an AIDS patient, Kaposi's sarcoma-associated herpesvirus (KSHV) is a member of the gammaherpesvirus subfamily of the Herpesviridae family, which contains a total of nine that infect humans. These viruses all contain a large envelope glycoprotein, glycoprotein B (gB), that is required for viral fusion with host cell membrane to initial infection. Although the atomic structures of five other human herpesviruses in their postfusion conformation and one in its prefusion conformation are known, the atomic structure of KSHV gB has not been reported. Here, we report the first structure of the KSHV gB ectodomain determined by single-particle cryogenic electron microscopy (cryoEM). Despite a similar global fold between herpesvirus gB, KSHV gB possesses local differences not shared by its relatives in other herpesviruses. The glycosylation sites of gB are arranged in belts down the symmetry axis with distinct localization compared to that of other herpesviruses, which occludes certain antibody binding sites. An extended glycan chain observed in domain I (DI), located proximal to the host membrane, may suggest its possible role in host cell attachment. Local flexibility of domain IV (DIV) governed by molecular hinges at its interdomain junctions identifies a means for enabling conformational change. A mutation in the domain III (DIII) central helix disrupts incorporation of gB into KSHV virions despite adoption of a canonical fold in vitro. Taken together, this study reveals mechanisms of structural variability of herpesvirus fusion protein gB and informs its folding and immunogenicity.IMPORTANCEIn 1994, a cancer-causing virus was discovered in lesions of AIDS patients, which was later named Kaposi's sarcoma-associated herpesvirus (KSHV). As the latest discovered human herpesvirus, KSHV has been classified into the gammaherpesvirus subfamily of the Herpesviridae. In this study, we have expressed KSHV gB and employed cryogenic electron microscopy (cryoEM) to determine its first structure. Importantly, our structure resolves some glycans beyond the first sugar moiety. These glycans are arranged in a pattern unique to KSHV, which impacts the antigenicity of KSHV gB. Our structure also reveals conformational flexibility caused by molecular hinges between domains that provide clues into the mechanism behind the drastic change between prefusion and postfusion states

Immunization of Mice with Virus-Like Vesicles of Kaposi Sarcoma-Associated Herpesvirus Reveals a Role for Antibodies Targeting ORF4 in Activating Complement-Mediated Neutralization.

Infection by Kaposi sarcoma-associated herpesvirus (KSHV) can cause severe consequences, such as cancers and lymphoproliferative diseases. Whole inactivated viruses (WIV) with chemically destroyed genetic materials have been used as antigens in several licensed vaccines. During KSHV productive replication, virus-like vesicles (VLVs) that lack capsids and viral genomes are generated along with virions. Here, we investigated the immunogenicity of KSHV VLVs produced from a viral mutant that was defective in capsid formation and DNA packaging. Mice immunized with adjuvanted VLVs generated KSHV-specific T cell and antibody responses. Neutralization of KSHV infection by the VLV immune serum was low but was markedly enhanced in the presence of the complement system. Complement-enhanced neutralization and complement deposition on KSHV-infected cells was dependent on antibodies targeting viral open reading frame 4 (ORF4). However, limited complement-mediated enhancement was detected in the sera of a small cohort of KSHV-infected humans which contained few neutralizing antibodies. Therefore, vaccination that induces antibody effector functions can potentially improve infection-induced humoral immunity. Overall, our study highlights a potential benefit of engaging complement-mediated antibody functions in future KSHV vaccine development. IMPORTANCE KSHV is a virus that can lead to cancer after infection. A vaccine that prevents KSHV infection or transmission would be helpful in preventing the development of these cancers. We investigated KSHV VLV as an immunogen for vaccination. We determined that antibodies targeting the viral protein ORF4 induced by VLV immunization could engage the complement system and neutralize viral infection. However, ORF4-specific antibodies were seldom detected in the sera of KSHV-infected humans. Moreover, these human sera did not potently trigger complement-mediated neutralization, indicating an improvement that immunization can confer. Our study suggests a new antibody-mediated mechanism to control KSHV infection and underscores the benefit of activating the complement system in a future KSHV vaccine

Immunization of Mice with Virus-Like Vesicles of Kaposi Sarcoma-Associated Herpesvirus Reveals a Role for Antibodies Targeting ORF4 in Activating Complement-Mediated Neutralization

KSHV is a virus that can lead to cancer after infection. A vaccine that prevents KSHV infection or transmission would be helpful in preventing the development of these cancers.</jats:p