Recognition of a highly conserved glycoprotein B epitope by a bivalent antibody neutralizing HCMV at a post-attachment step

Computational design of prefusion-stabilized Herpesvirus gB trimers

In the absence of effective vaccines, human-infecting members of the Herpesvirus family cause considerable morbidity and mortality worldwide. Herpesvirus infection relies on receptor engagement by a gH/gL glycoprotein complex which induces large-scale conformational changes of the gB glycoprotein to mediate fusion of the viral and host membranes and infection. The instability of all herpesvirus gBs have hindered biochemical and functional studies, thereby limiting our understanding of the infection mechanisms of these pathogens and preventing vaccine design. Here, we computationally stabilized and structurally characterized the Epstein-Barr virus prefusion gB ectodomain trimer, providing an atomic-level description of this key therapeutic target. We show that this stabilization strategy is broadly applicable to other herpesvirus gB trimers and identified conformational intermediates supporting a previously unanticipated mechanism of gB-mediated fusion. These findings provide a blueprint to develop vaccine candidates for these pathogens with major public health burden.

Discrimination of primary and chronic cytomegalovirus infection based on humoral immune profiles in pregnancy

BACKGROUNDMost humans have been infected with cytomegalovirus (CMV) by midlife without clinical signs of disease. However, in settings in which the immune system is undeveloped or compromised, the virus is not adequately controlled and consequently presents a major infectious cause of both congenital disease during pregnancy as well as opportunistic infection in children and adults. With clear evidence that risk to the fetus varies with gestational age at the time of primary maternal infection, further research on humoral responses to primary CMV infection during pregnancy is needed.METHODSHere, systems serology tools were applied to characterize antibody responses to CMV infection in pregnant and nonpregnant women experiencing either primary or chronic infection.RESULTSWhereas strikingly different antibody profiles were observed depending on infection status, limited differences were associated with pregnancy status. Beyond known differences in IgM responses used clinically for identification of primary infection, distinctions observed in IgA and FcγR-binding antibodies and among antigen specificities accurately predicted infection status. Machine learning was used to define the transition from primary to chronic states and predict time since infection with high accuracy. Humoral responses diverged over time in an antigen-specific manner, with IgG3 responses toward tegument decreasing over time as typical of viral infections, while those directed to pentamer and glycoprotein B were lower during acute and greatest during chronic infection.CONCLUSIONIn sum, this work provides insights into the antibody response associated with CMV infection status in the context of pregnancy, revealing aspects of humoral immunity that have the potential to improve CMV diagnostics.FUNDINGCYMAF consortium and NIH NIAID.

Structure-based design of a soluble human cytomegalovirus glycoprotein B antigen stabilized in a prefusion-like conformation

Human cytomegalovirus (HCMV) glycoprotein B (gB) is a class III membrane fusion protein required for viral entry. HCMV vaccine candidates containing gB have demonstrated moderate clinical efficacy, but no HCMV vaccine has been approved. Here, we used structure-based design to identify and characterize amino acid substitutions that stabilize gB in its metastable prefusion conformation. One variant containing two engineered interprotomer disulfide bonds and two cavity-filling substitutions (gB-C7), displayed increased expression and thermostability. A 2.8 Å resolution cryoelectron microscopy structure shows that gB-C7 adopts a prefusion-like conformation, revealing additional structural elements at the membrane-distal apex. Unlike previous observations for several class I viral fusion proteins, mice immunized with postfusion or prefusion-stabilized forms of soluble gB protein displayed similar neutralizing antibody titers, here specifically against an HCMV laboratory strain on fibroblasts. Collectively, these results identify initial strategies to stabilize class III viral fusion proteins and provide tools to probe gB-directed antibody responses.

Subconfluent ARPE-19 Cells Display Mesenchymal Cell-State Characteristics and Behave like Fibroblasts, Rather Than Epithelial Cells, in Experimental HCMV Infection Studies

Human cytomegalovirus (HCMV) has a broad cellular tropism and epithelial cells are important physiological targets during infection. The retinal pigment epithelial cell line ARPE-19 has been used to model HCMV infection in epithelial cells for decades and remains a commonly used cell type for studying viral entry, replication, and the cellular response to infection. We previously found that ARPE-19 cells, despite being derived from an epithelial cell explant, express extremely low levels of canonical epithelial proteins, such as E-cadherin and EpCAM. Here, we perform comparative studies of ARPE-19 and additional epithelial cell lines with strong epithelial characteristics. We find that ARPE-19 cells cultured under subconfluent conditions resemble mesenchymal fibroblasts, rather than epithelial cells; this is consistent with previous studies showing that ARPE-19 cultures require extended periods of high confluency culture to maintain epithelial characteristics. By reanalyzing public gene expression data and using machine learning, we find evidence that ARPE-19 cultures maintained across many labs exhibit mesenchymal characteristics and that the majority of studies employing ARPE-19 use them in a mesenchymal state. Lastly, by performing experimental HCMV infections across mesenchymal and epithelial cell lines, we find that ARPE-19 cells behave like mesenchymal fibroblasts, producing logarithmic yields of cell-free infectious progeny, while cell lines with strong epithelial character exhibit an atypical infectious cycle and naturally restrict the production of cell-free progeny. Our work highlights important characteristics of the ARPE-19 cell line and suggests that subconfluent ARPE-19 cells may not be optimal for modeling epithelial infection with HCMV or other human viruses. It also suggests that HCMV biosynthesis and/or spread may occur quite differently in epithelial cells compared to mesenchymal cells. These differences could contribute to viral persistence or pathogenesis in epithelial tissues.

Vaccination with a replication-defective cytomegalovirus vaccine elicits a glycoprotein B-specific monoclonal antibody repertoire distinct from natural infection

Human Cytomegalovirus (HCMV) is the leading infectious congenital infection globally and the most common viral infection in transplant recipients, therefore identifying a vaccine for HCMV is a top priority. Humoral immunity is a correlate of protection for HCMV infection. The most effective vaccine tested to date, which achieved 50% reduction in acquisition of HCMV, was comprised of the glycoprotein B protein given with an oil-in-water emulsion adjuvant MF59. We characterize gB-specific monoclonal antibodies isolated from individuals vaccinated with a disabled infectious single cycle (DISC) CMV vaccine, V160, and compare these to the gB-specific monoclonal antibody repertoire isolated from naturally-infected individuals. We find that vaccination with V160 resulted in gB-specific antibodies that bound homogenously to gB expressed on the surface of a cell in contrast to antibodies isolated from natural infection which variably bound to cell-associated gB. Vaccination resulted in a similar breadth of gB-specific antibodies, with binding profile to gB genotypes 1-5 comparable to that of natural infection. Few gB-specific neutralizing antibodies were isolated from V160 vaccinees and fewer antibodies had identifiable gB antigenic domain specificity compared to that of naturally-infected individuals. We also show that glycosylation of gB residue N73 may shield binding of gB-specific antibodies.

A temperature-dependent virus-binding assay reveals the presence of neutralizing antibodies in human cytomegalovirus gB vaccine recipients' sera

Human cytomegalovirus (HCMV) remains an important cause of mortality in immune-compromised transplant patients and following congenital infection. Such is the burden, an effective vaccine strategy is considered to be of the highest priority. The most successful vaccines to date have focused on generating immune responses against glycoprotein B (gB) - a protein essential for HCMV fusion and entry. We have previously reported that an important component of the humoral immune response elicited by gB/MF59 vaccination of patients awaiting transplant is the induction of non-neutralizing antibodies that target cell-associated virus with little evidence of concomitant classical neutralizing antibodies. Here we report that a modified neutralization assay that promotes prolonged binding of HCMV to the cell surface reveals the presence of neutralizing antibodies in sera taken from gB-vaccinated patients that cannot be detected using standard assays. We go on to show that this is not a general feature of gB-neutralizing antibodies, suggesting that specific antibody responses induced by vaccination could be important. Although we can find no evidence that these neutralizing antibody responses are a correlate of protection in vivo in transplant recipients their identification demonstrates the utility of the approach in identifying these responses. We hypothesize that further characterization has the potential to aid the identification of functions within gB that are important during the entry process and could potentially improve future vaccine strategies directed against gB if they prove to be effective against HCMV at higher concentrations.

A single, improbable B cell receptor mutation confers potent neutralization against cytomegalovirus

Cytomegalovirus (CMV) is a leading cause of infant hearing loss and neurodevelopmental delay, but there are no clinically licensed vaccines to prevent infection, in part due to challenges eliciting neutralizing antibodies. One of the most well-studied targets for CMV vaccines is the viral fusogen glycoprotein B (gB), which is required for viral entry into host cells. Within gB, antigenic domain 2 site 1 (AD-2S1) is a target of potently neutralizing antibodies, but gB-based candidate vaccines have yet to elicit robust responses against this region. We mapped the genealogy of B cells encoding potently neutralizing anti-gB AD-2S1 antibodies from their inferred unmutated common ancestor (UCA) and characterized the binding and function of early lineage ancestors. Surprisingly, we found that a single amino acid heavy chain mutation A33N, which was an improbable mutation rarely generated by somatic hypermutation machinery, conferred broad CMV neutralization to the non-neutralizing UCA antibody. Structural studies revealed that this mutation mediated key contacts with the gB AD-2S1 epitope. Collectively, these results provide insight into potently neutralizing gB-directed antibody evolution in a single donor and lay a foundation for using this B cell-lineage directed approach for the design of next-generation CMV vaccines.

Neutralization Epitopes in Trimer and Pentamer Complexes Recognized by Potent Cytomegalovirus-Neutralizing Human Monoclonal Antibodies

Human cytomegalovirus (HCMV) infects 36% to almost 100% of adults and causes severe complications only in immunocompromised individuals. HCMV viral surface trimeric (gH/gL/gO) and pentameric (gH/gL/UL128/UL130/UL131A) complexes play important roles in HCMV infection and tropism. Here, we isolated and identified a total of four neutralizing monoclonal antibodies (MAbs) derived from HCMV-seropositive blood donors. Based on their reactivity to HCMV trimer and pentamer, these MAbs can be divided into two groups. MAbs PC0012, PC0014, and PC0035 in group 1 bind both trimer and pentamer and neutralize CMV by interfering with the postattachment steps of CMV entering into cells. These three antibodies recognize antigenic epitopes clustered in a similar area, which are overlapped by the epitope recognized by the known neutralizing antibody MSL-109. MAb PC0034 in group 2 binds only to pentamer and neutralizes CMV by blocking the binding of pentamer to cells. Epitope mapping using pentamer mutants showed that amino acid T94 of the subunit UL128 and K27 of UL131A on the pentamer are key epitope-associated residues recognized by PC0034. This study provides new evidence and insight information on the importance of the development of the CMV pentamer as a CMV vaccine. In addition, these newly identified potent CMV MAbs can be attractive candidates for development as antibody therapeutics for the prevention and treatment of HCMV infection. IMPORTANCE The majority of the global population is infected with HCMV, but severe complications occur only in immunocompromised individuals. In addition, CMV infection is a major cause of birth defects in newborns. Currently, there are still no approved prophylactic vaccines or therapeutic monoclonal antibodies (MAbs) for clinical use against HCMV infection. This study identified and characterized a panel of four neutralizing MAbs targeting the HCMV pentamer complex with specific aims to identify a key protein(s) and antigenic epitopes in the HCMV pentamer complex. The study also explored the mechanism by which these newly identified antibodies neutralize HCMV in order to design better HCMV vaccines focusing on the pentamer and to provide attractive candidates for the development of effective cocktail therapeutics for the prevention and treatment of HCMV infection.

Protective anti-gB neutralizing antibodies targeting two vulnerable sites for EBV-cell membrane fusion

Epstein-Barr virus (EBV) accounts for 200,000 new epithelial and B cell malignancy cases and 140,000 deaths annually. Glycoprotein B (gB) is the sole fusogen that is highly conserved and essential for all herpesvirus entry into target cells and thus, is attracting attention to identify potent antibodies to neutralize viral infection. Here, we discovered two anti-EBV gB neutralizing antibodies, 3A3 and 3A5, that effectively neutralized EBV infection of both B and epithelial cells. They also potently protected against EBV-induced lymphoproliferative disorders in humanized mice. Importantly, the 3A3 and 3A5 epitopes identified here represent the neutralizing antigenic sites to block EBV infection and membrane fusion. They are major targets of protective gB-specific neutralizing antibodies elicited by natural EBV infection in humans.

Epstein-Barr virus (EBV) infects more than 90% of the world’s adult population and accounts for a significant cancer burden of epithelial and B cell origins. Glycoprotein B (gB) is the primary fusogen essential for EBV entry into host cells. Here, we isolated two EBV gB-specific neutralizing antibodies, 3A3 and 3A5; both effectively neutralized the dual-tropic EBV infection of B and epithelial cells. In humanized mice, both antibodies showed effective protection from EBV-induced lymphoproliferative disorders. Cryoelectron microscopy analyses identified that 3A3 and 3A5 bind to nonoverlapping sites on domains D-II and D-IV, respectively. Structure-based mutagenesis revealed that 3A3 and 3A5 inhibit membrane fusion through different mechanisms involving the interference with gB-cell interaction and gB activation. Importantly, the 3A3 and 3A5 epitopes are major targets of protective gB-specific neutralizing antibodies elicited by natural EBV infection in humans, providing potential targets for antiviral therapies and vaccines.

Computational identification of HCV neutralizing antibodies with a common HCDR3 disulfide bond motif in the antibody repertoires of infected individuals

Despite recent success in hepatitis C virus (HCV) treatment using antivirals, an HCV vaccine is still needed to prevent reinfections in treated patients, to avert the emergence of drug-resistant strains, and to provide protection for people with no access to the antiviral therapeutics. The early production of broadly neutralizing antibodies (bNAbs) associates with HCV clearance. Several potent bNAbs bind a conserved HCV glycoprotein E2 epitope using an unusual heavy chain complementarity determining region 3 (HCDR3) containing an intra-loop disulfide bond. Isolation of additional structurally-homologous bNAbs would facilitate the recognition of key determinants of such bNAbs and guide rational vaccine design. Here we report the identification of new antibodies containing an HCDR3 disulfide bond motif using computational screening with the Rosetta software. Using the newly-discovered and already-known members of this antibody family, we review the required HCDR3 amino acid composition and propose determinants for the bent versus straight HCDR3 loop conformation observed in these antibodies.

Glycoprotein B Antibodies Completely Neutralize EBV Infection of B Cells

The Epstein-Barr virus (EBV) is the first reported oncogenic herpesvirus that establishes persistent infection in B lymphocytes in 95% of adults worldwide. Glycoprotein B (gB) plays a predominant role in the fusion of the viral envelope with the host cell membrane. Hence, it is of great significance to isolate gB-specific fusion-inhibiting neutralizing antibodies (NAbs). AMMO5 is the only gB NAb but fails to antagonize B-cell infection. It is essential to isolate potent NAbs that can completely block EBV infection of B cells. Using hybridoma technology and neutralization assay, we isolate two gB NAbs 8A9 and 8C12 that are capable of completely neutralizing B-cell infection in vitro. In addition, 8A9 shows cross-reactivity with rhesus lymphocryptovirus (rhLCV) gB. Competitive binding experiments demonstrate that 8A9 and 8C12 recognize novel epitopes that are different from the AMMO5 epitope. The epitopes of 8A9 and 8C12 are mapped to gB D-II, and the AMMO5 epitope is located precisely at gB aa 410-419. We find that 8A9 and 8C12 significantly inhibit gB-derived membrane fusion using a virus-free fusion assay. In summary, this study identifies two gB-specific NAbs that potently block EBV infection of B cells. Our work highlights the importance of gB D-II as a predominant neutralizing epitope, and aids in the rational design of therapeutics or vaccines based on gB.

Structural basis for HCMV Pentamer recognition by neuropilin 2 and neutralizing antibodies

Human cytomegalovirus (HCMV) encodes multiple surface glycoprotein complexes to infect a variety of cell types. The HCMV Pentamer, composed of gH, gL, UL128, UL130, and UL131A, enhances entry into epithelial, endothelial, and myeloid cells by interacting with the cell surface receptor neuropilin 2 (NRP2). Despite the critical nature of this interaction, the molecular determinants that govern NRP2 recognition remain unclear. Here, we describe the cryo-EM structure of NRP2 bound to Pentamer. The high-affinity interaction between these proteins is calcium dependent and differs from the canonical carboxyl-terminal arginine (CendR) binding that NRP2 typically uses. We also determine the structures of four neutralizing human antibodies bound to the HCMV Pentamer to define susceptible epitopes. Two of these antibodies compete with NRP2 binding, but the two most potent antibodies recognize a previously unidentified epitope that does not overlap the NRP2-binding site. Collectively, these findings provide a structural basis for HCMV tropism and antibody-mediated neutralization.

Recent progress in development of monoclonal antibodies against human cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause permanent childhood disabilities following in utero infection and life threatening diseases in immune-compromised individuals such as those post transplantation. Without an effective vaccine, small molecule antiviral drugs are routinely used in high-risk transplant recipients, but the effectiveness of which is limited by side effects and drug resistance. The potentials of antibody-based passive immune therapies alone or in combination with the small molecule antivirals to treat or prevent HCMV infection have been actively studied. In this review, we focus on the recent publications on identification and characterization of monoclonal antibodies that have the potential to be developed as anti-HCMV therapies. We review the progress in clinical evaluation of antibody-based therapies to prevent HCMV-associated diseases.

Prefusion structure of human cytomegalovirus glycoprotein B and structural basis for membrane fusion

Human cytomegalovirus (HCMV) causes congenital disease with long-term morbidity. HCMV glycoprotein B (gB) transitions irreversibly from a metastable prefusion to a stable postfusion conformation to fuse the viral envelope with a host cell membrane during entry. We stabilized prefusion gB on the virion with a fusion inhibitor and a chemical cross-linker, extracted and purified it, and then determined its structure to 3.6-Å resolution by electron cryomicroscopy. Our results revealed the structural rearrangements that mediate membrane fusion and details of the interactions among the fusion loops, the membrane-proximal region, transmembrane domain, and bound fusion inhibitor that stabilized gB in the prefusion state. The structure rationalizes known gB antigenic sites. By analogy to successful vaccine antigen engineering approaches for other viral pathogens, the high-resolution prefusion gB structure provides a basis to develop stabilized prefusion gB HCMV vaccine antigens.

Potent Bispecific Neutralizing Antibody Targeting Glycoprotein B and the gH/gL/pUL128/130/131 Complex of Human Cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause developmental disorders following congenital infection and life-threatening complications among transplant patients. Potent neutralizing monoclonal antibodies (MAbs) are promising drug candidates against HCMV infection. HCMV can infect a broad range of cell types. Therefore, single neutralizing antibodies targeting one HCMV glycoprotein often lack either potency or broad cell-type coverage. We previously characterized two human-derived HCMV neutralizing MAbs. One was the broadly neutralizing MAb 3-25, which targets the antigenic domain 2 of glycoprotein B (gB). The other was the highly potent MAb 2-18, which specifically recognizes the gH/gL/pUL128/130/131 complex (pentamer). To combine the strengths of gB- and pentamer-targeting MAbs, we developed an IgG-single-chain variable fragment (scFv) bispecific antibody by fusing the 2-18 scFv to the heavy-chain C terminus of MAb 3-25. The resulting bispecific antibody showed high-affinity binding to both gB and pentamer. Functionally, the bispecific antibody demonstrated a combined neutralization breadth and potency of the parental MAbs in multiple cell lines and inhibited postinfection viral spreading. Furthermore, the bispecific antibody was easily produced in CHO cells at a yield above 1 g/liter and showed a single-dose pharmacokinetic profile comparable to that of parental MAb 3-25 in rhesus macaques. Importantly, the bispecific antibody retained broadly and potent neutralizing activity after 21 days in circulation. Taken together, our research provides a proof-of-concept study for developing bispecific neutralizing antibody therapies against HCMV infection.