Pentameric complex of viral glycoprotein H is the primary target for potent neutralization by a human cytomegalovirus vaccine

Human cytomegalovirus (HCMV) can cause serious morbidity/mortality in transplant patients, and congenital HCMV infection can lead to birth defects. Developing an effective HCMV vaccine is a high medical priority. One of the challenges to the efforts has been our limited understanding of the viral antigens important for protective antibodies. Receptor-mediated viral entry to endothelial/epithelial cells requires a glycoprotein H (gH) complex comprising five viral proteins (gH, gL, UL128, UL130, and UL131). This gH complex is notably missing from HCMV laboratory strains as well as HCMV vaccines previously evaluated in the clinic. To support a unique vaccine concept based on the pentameric gH complex, we established a panel of 45 monoclonal antibodies (mAbs) from a rabbit immunized with an experimental vaccine virus in which the expression of the pentameric gH complex was restored. Over one-half (25 of 45) of the mAbs have neutralizing activity. Interestingly, affinity for an antibody to bind virions was not correlated with its ability to neutralize the virus. Genetic analysis of the 45 mAbs based on their heavy- and light-chain sequences identified at least 26 B-cell linage groups characterized by distinct binding or neutralizing properties. Moreover, neutralizing antibodies possessed longer complementarity-determining region 3 for both heavy and light chains than those with no neutralizing activity. Importantly, potent neutralizing mAbs reacted to the pentameric gH complex but not to gB. Thus, the pentameric gH complex is the primary target for antiviral antibodies by vaccination.Human cytomegalovirus (HCMV) is an important pathogen in transplant patients (1–5), and its infection can lead to invasive end-organ diseases, such as pneumonitis and hepatitis, as well as vascular pathology contributing to graft failure (4, 6, 7). HCMV is also the most common cause of in utero viral infections in North America and Europe, affecting 0.5–2% of newborns annually (8–10). Congenital HCMV infection can lead to symptomatic diseases at birth and also cause developmental disabilities in children (10, 11). Maternal seropositivity before conception protects against congenital transmission (12, 13), and both maternal humoral and cellular immunity are likely to contribute to the protection (14–16). Antibodies in particular are important for preventing congenital infection, serving as the first line of defense against maternal infection. It may also play a role in preventing transmission to the fetus, supported by the results of a small, nonrandomized study in pregnant women with primary HCMV infection, in which the passive immunity of monthly infusions of HCMV hyperimmune human IgG (HCMV-HIG) (200 mg/kg maternal weight) was ∼60% effective in protecting against congenital HCMV infection (17, 18). These studies suggest that it is feasible to develop a vaccine for preventing congenital HCMV infection and its sequelae. However, despite the fact that the Institute of Medicine has identified development of an effective vaccine for prevention of congenital HCMV as a top priority since 1999 (19), progress toward this goal has only been incremental (8, 20, 21). One of the hurdles to the efforts is our limited understanding of component of natural immunity associated with protection against HCMV infection.HCMV is a large, complex virus, with a genome capable of encoding >150 proteins (22–26). Because of the strict species specificity, options of animal models for HCMV research are limited (27). Thus, the functions of most HCMV antigens in viral infection in vivo and their roles as targets for host immunity are poorly understood. Furthermore, culture systems of single cell types have limitations for studying HCMV pathogenesis. Immunohistochemistry studies showed that HCMV can infect varieties of cells in vivo, including endothelial, epithelial cells, fibroblasts, and leukocytes (28–36). Many HCMV end-organ diseases, such as pneumonitis and gastroenteritis, are due to infection of the epithelial/endothelial cells in the affected organ (35–39). However, common laboratory strains, such as AD169 and Towne, were culture-adapted in fibroblast cells, with genomic mutations (22, 24, 40) and, more importantly, have lost their tropism to endothelial and epithelial cells, in contrast to pathogenic clinical isolates (32, 33, 41, 42).Loss of viral tropism to endothelial and epithelial cells was mapped to various mutations in the viral UL131-128 locus, and these mutations abrogated the expression of the pentameric glycoprotein H (gH) complex, composed of gH, gL, UL128, UL130, and UL131 proteins, a determinant for viral tropism to endothelial and epithelial cells (42–44). Because the pentameric gH complex is missing in common laboratory strains (42, 43), its importance in viral tropism, viral pathogenesis, and vaccine design was not fully appreciated until recently (42, 45). With this understanding, it is not surprising that Towne virus and recombinant glycoprotein B (gB) vaccines, although with ∼50% efficacy against primary infection in the clinic (46–49), induced poor neutralizing titers against viral infection of epithelial cells, in contrast to immune sera from HCMV-seropositive donors (50, 51). Thus, missing the pentameric gH complex is likely a deficiency in antigen composition for both vaccines (50). Studies of monoclonal antibodies (mAbs) isolated from HCMV-seropositive donors or polyclonal IgG enriched for antigen specificity supported the hypothesis that the pentameric gH complex, not gB, appears to be important for neutralizing activity in human subjects with natural infection (52).We recently described an experimental vaccine virus in which expression of the pentameric gH complex was restored (53). Unlike the parental AD169 virus and the recombinant gB vaccine, this virus can elicit high levels of neutralizing antibodies in rabbits and rhesus macaques (53). To support clinical development of this vaccine centered its concept on the pentameric gH complex, we established a comprehensive panel of 45 mAbs from a single rabbit that received vaccination. Of the 45 mAbs, 25 had neutralizing activity against viral entry in epithelial cells, including 11 elite neutralizers with ≥10-fold greater potency than HCMV-HIG. Biochemical analysis demonstrated that all elite neutralizers preferentially bound to the virus expressing the pentameric gH complex, and the majority of elite neutralizers (8 of 11) specifically recognized a recombinant form of the pentameric gH complex. Interestingly, binding affinity for intact virions was not correlated with neutralizing activity. Moreover, genetic analysis of the 45 mAbs based on their heavy- and light-chain sequences identified at least 26 B-cell linage groups characterized by distinct binding or neutralizing properties. In addition, neutralizing antibodies had longer complementarity-determining region 3 (CDR3) for both heavy and light chains than those of antibodies with no neutralizing activity. These data establish the importance of the pentameric gH complex as the primary target for potent neutralizing antibodies by vaccination, and support development of an experimental HCMV vaccine featuring the pentameric gH complex.