Specificity and effector functions of non-neutralizing gB-specific monoclonal antibodies isolated from healthy individuals with human cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common congenital infection, and the leading nongenetic cause of sensorineural hearing loss (SNHL) in newborns globally. A gB subunit vaccine administered with adjuvent MF59 (gB/MF59) is the most efficacious tested to-date, achieving 50% efficacy in preventing infection of HCMV-seronegative mothers. We recently discovered that gB/MF59 vaccination elicited primarily non-neutralizing antibody responses, that HCMV strains acquired by vaccinees more often included strains with gB genotypes that are distinct from the vaccine antigen, and that protection against HCMV acquisition correlated with ability of vaccine-elicited antibodies to bind to membrane associated gB. Thus, we hypothesized that gB-specific non-neutralizing antibody binding breadth and function are dependent on their epitope and genotype specificity as well as their ability to interact with membrane-associated gB. Twenty-four gB-specific monoclonal antibodies (mAbs) isolated from naturally HCMV-infected individuals were mapped for gB domain specificity by binding antibody multiplex assay (BAMA) and for genotype preference binding to membrane-associated gB presented on transfected cells. We defined their non-neutralizing functions including antibody dependent cellular phagocytosis (ADCP) and antibody dependent cellular cytotoxicity (ADCC). The isolated gB-specific non-neutralizing mAbs were primarily specific for Domain II and linear antigenic domain 2 site 2 (AD2). We observed variability in mAb gB genotype binding preference, with increased binding to gB genotypes 2 and 4. Functional studies identified two gB-specific mAbs that facilitate ADCP and have binding specificities of AD2 and Domain II. This investigation provides novel understanding on the impact of gB domain specificity and antigenic variability on gB-specific non-neutralizing antibody responses. Importance HCMV is the most common congenital infection worldwide, but development of a successful vaccine remains elusive. gB-specific non-neutralizing mAbs, represent a distinct anti-HCMV Ab subset implicated in the protection against primary infection during numerous phase-II gB/MF59 vaccine trials. By studying non-neutralizing gB-specific mAbs from naturally infected individuals, this study provides novel characterization of binding site specificity, genotypic preference, and effector cell functions mediated by mAbs elicited in natural infection. We found that a panel of twenty-four gB-specific non-neutralizing mAbs bind across multiple regions of the gB protein, traditionally through to be targeted by neutralizing mAbs only, and bind differently to gB depending if the protein is soluble versus embedded in a membrane. This investigation provides novel insight into the gB-specific binding characteristics and effector cell functions mediated by non-neutralizing gB-specific mAbs elicited through natural infection, providing new endpoints for future vaccine development.