Zhihai Li

Identification of Broad-Genotype HPV L2 Neutralization Site for Pan-HPV Vaccine Development by a Cross-Neutralizing Antibody

Human Papillomavirus (HPV), a non-enveloped, double-stranded DNA virus, is responsible for 5% of human cancers. The HPV capsid consists of major and minor structural proteins, L1 and L2. L1 proteins form an icosahedral shell with building blocks of the pentameric capsomere, and one L2 molecule extends outward from the central hole of the capsid. Thus, L2 is concealed within L1 and only becomes exposed when the capsid interacts with host cells. The low antigenic variation of L2 means that this protein could offer a target for the development of a pan-HPV vaccine. Toward this goal, here we describe an anti-L2 monoclonal antibody, 14H6, which broadly neutralizes at least 11 types of HPV, covering types 6, 11, 16, 18, 31, 33, 35, 45, 52, 58 and 59, in pseudovirion—based cell neutralization assay. The mAb 14H6 recognizes a minimal linear epitope located on amino acids 21 to 30 of the L2 protein. Alanine scanning mutagenesis and sequence alignment identified several conserved residues (Cys22, Lys23, Thr27, Cys28 and Pro29) that are involved in the 14H6 binding with L2. The epitope was grafted to several scaffolding proteins, including HPV16 L1 virus-like particles, HBV 149 core antigen and CRM197. The resultant chimeric constructs were expressed in Escherichia coli and purified with high efficiency. Immunization with these pan-HPV vaccine candidates elicited high titers of the L2-specific antibody in mice and conferred robust (3-log) titers of cross-genotype neutralization, including against HPV11, 16, 18, 45, 52, 58 and 59. These findings will help in the development of an L2-based, pan-HPV vaccine.

Functional assessment and structural basis of antibody binding to human papillomavirus capsid.

Persistent high‐risk human papillomavirus (HPV) infection is linked to cervical cancer. Two prophylactic virus‐like particle (VLP)‐based vaccines have been marketed globally for nearly a decade. Here, we review the HPV pseudovirion (PsV)‐based assays for the functional assessment of the HPV neutralizing antibodies and the structural basis for these clinically relevant epitopes. The PsV‐based neutralization assay was developed to evaluate the efficacy of neutralization antibodies in sera elicited by vaccination or natural infection or to assess the functional characteristics of monoclonal antibodies. Different antibody binding modes were observed when an antibody was complexed with virions, PsVs or VLPs. The neutralizing epitopes are localized on surface loops of the L1 capsid protein, at various locations on the capsomere. Different neutralization antibodies exert their neutralizing function via different mechanisms. Some antibodies neutralize the virions by inducing conformational changes in the viral capsid, which can result in concealing the binding site for a cellular receptor like 1A1D‐2 against dengue virus, or inducing premature genome release like E18 against enterovirus 71. Higher‐resolution details on the epitope composition of HPV neutralizing antibodies would shed light on the structural basis of the highly efficacious vaccines and aid the design of next generation vaccines. In‐depth understanding of epitope composition would ensure the development of function‐indicating assays for the comparability exercise to support process improvement or process scale up. Elucidation of the structural elements of the type‐specific epitopes would enable rational design of cross‐type neutralization via epitope re‐engineering or epitope grafting in hybrid VLPs. Copyright

Bacterially expressed human papillomavirus type 6 and 11 bivalent vaccine: Characterization, antigenicity and immunogenicity

Human papillomavirus (HPV)-6 and HPV11 are the major etiological causes of condylomata acuminate. HPV neutralization by vaccine-elicited neutralizing antibodies can block viral infection and prevent subsequent disease. Currently, two commercially available HPV vaccines cover these two genotypes, expressed by Saccharomyces cerevisiae. Here we describe another HPV6/11 bivalent vaccine candidate derived from Escherichia coli. The soluble expression of N-terminally truncated L1 proteins was optimized to generate HPV6- and HPV11 L1-only virus-like particles (VLPs) as a scalable process. In a pilot scale, we used various biochemical, biophysical and immunochemical approaches to comprehensively characterize the scale and lot consistency of the vaccine candidate at 30L and 100L. Cryo-EM structure analysis showed that these VLPs form a T=7 icosahedral lattice, imitating the L1 capsid of the authentic HPV virion. This HPV6/11 bivalent vaccine confers a neutralization titer and antibody production profile in monkey that is comparable with the quadrivalent vaccine, Gardasil. This study demonstrates the robustness and scalability of a potential HPV6/11 bivalent vaccine using a prokaryotic system for vaccine production.

Characterization of an Escherichia coli-derived human papillomavirus type 16 and 18 bivalent vaccine

Human papillomavirus (HPV) types 16 and 18 account for approximately 70% of cervical cancer worldwide. Neutralizing HPV prophylactic vaccines offer significant benefit, as they block HPV infection and prevent subsequent disease. However, the three licensed HPV vaccines that cover these two genotypes were produced in eukaryotic cells, which is expensive, particularly for low-income countries where HPV is highest. Here, we report a new HPV16 and -18 bivalent candidate vaccine produced from Escherichia coli. We used two strategies of N-terminal truncation of HPV L1 proteins and soluble non-fusion expression to generate HPV16 and HPV18 L1-only virus-like particles (VLPs) in a scalable process. Through comprehensive characterization of the bivalent candidate vaccine, we confirm lot consistency in a pilot scale-up of 30L, 100L and 500L. Using cryo-EM 3D reconstruction, we found that HPV16 and -18VLPs present in a T=7 icosahedral arrangement, similar in shape and size to that of the native virions. This HPV16/18 bivalent vaccine shares comparable immunogenicity with the licensed vaccines. Overall, we show that the production of a HPV16/18 bivalent vaccine from an E. coli expression system is robust and scalable, with potentially good accessibility worldwide as a population-based immunization strategy.

Atomic structures of enterovirus D68 in complex with two monoclonal antibodies define distinct mechanisms of viral neutralization

Enterovirus D68 (EV-D68) undergoes structural transformation between mature, cell-entry intermediate (A-particle) and empty forms throughout its life cycle. Structural information for the various forms and antibody-bound capsids will facilitate the development of effective vaccines and therapeutics against EV-D68 infection, which causes childhood respiratory and paralytic diseases worldwide. Here, we report the structures of three EV-D68 capsid states representing the virus at major phases. We further describe two original monoclonal antibodies (15C5 and 11G1) with distinct structurally defined mechanisms for virus neutralization. 15C5 and 11G1 engage the capsid loci at icosahedral three-fold and five-fold axes, respectively. To block viral attachment, 15C5 binds three forms of capsids, and triggers mature virions to transform into A-particles, mimicking engagement by the functional receptor ICAM-5, whereas 11G1 exclusively recognizes the A-particle. Our data provide a structural and molecular explanation for the transition of picornavirus capsid conformations and demonstrate distinct mechanisms for antibody-mediated neutralization.Structures of three enterovirus D68 capsid states and two monoclonal antibodies provide a molecular explanation for the transition of picornavirus capsid conformations and reveal distinct mechanisms for viral neutralization.

N-terminal truncations on L1 proteins of human papillomaviruses promote their soluble expression in Escherichia coli and self-assembly in vitro

Human papillomavirus (HPV) is the causative agent in genital warts and nearly all cervical, anogenital, and oropharyngeal cancers. Nine HPV types (6, 11, 16, 18, 31, 33, 45, 52, and 58) are associated with about 90% of cervical cancers and 90% of genital warts. HPV neutralization by vaccine-elicited neutralizing antibodies can block viral infection and prevent HPV-associated diseases. However, there is only one commercially available HPV vaccine, Gardasil 9, produced from Saccharomyces cerevisiae that covers all nine types, raising the need for microbial production of broad-spectrum HPV vaccines. Here, we investigated whether N-terminal truncations of the major HPV capsid proteins L1, improve their soluble expression in Escherichia coli. We found that N-terminal truncations promoted the soluble expression of HPV 33 (truncated by 10 amino acids [aa]), 52 (15 aa), and 58 (10 aa). The resultant HPV L1 proteins were purified in pentamer form and extensively characterized with biochemical, biophysical, and immunochemical methods. The pentamers self-assembled into virus-like particles (VLPs) in vitro, and 3D cryo-EM reconstructions revealed that all formed T = 7 icosahedral particles having 50–60-nm diameters. Moreover, we formulated a nine-valent HPV vaccine candidate with aluminum adjuvant and L1 VLPs from four genotypes used in this study and five from previous work. Immunogenicity assays in mice and non-human primates indicated that this HPV nine-valent vaccine candidate elicits neutralizing antibody titers comparable to those induced by Gardasil 9. Our study provides a method for producing a nine-valent HPV vaccine in E. coli and may inform strategies for the soluble expression of other vaccine candidates. • N-terminal truncations promote the soluble expression of HPV L1 proteins in E. coli and their self-assembly of T = 7 icosahedral particle in vitro • An HPV 9-valent vaccine candidate was formulated with E. coli-derived HPV 6, 11, 16, 18, 31, 33, 45, 52, and 58 VLPs, and conferred comparable immunogenicity with Gardasil 9

Crystal Structures of Two Immune Complexes Identify Determinants for Viral Infectivity and Type-Specific Neutralization of Human Papillomavirus.

ABSTRACT Persistent, high-risk human papillomavirus (HPV) infection is the primary cause of cervical cancer. Neutralizing antibodies elicited by L1-only virus-like particles (VLPs) can block HPV infection; however, the lack of high-resolution structures has limited our understanding of the mode of virus infection and the requirement for type specificity at the molecular level. Here, we describe two antibodies, A12A3 and 28F10, that specifically bind to and neutralize HPV58 and HPV59, respectively, through two distinct binding stoichiometries. We show that the epitopes of A12A3 are clustered in the DE loops of two adjacent HPV58 L1 monomers, whereas 28F10 recognizes the HPV59 FG loop of a single monomer. Via structure-based mutagenesis and analysis of antibody binding, we further identified the residues HPV58 D154, S168, and N170 and HPV59 M267, Q270, E273, Y276, K278, and R283, which play critical roles in virus infection. By substituting these strategic epitope residues into other HPV genotypes, we could then redirect the type-specific binding of the antibodies to these genotypes, thus highlighting the importance of these specific residues, HPV58 R161, S168, and N308 and HPV59 Q270, E273, and D281. Overall, our findings provide molecular insights into potential structural determinants of HPV required for infectivity and type specificity. IMPORTANCE High-risk human papillomaviruses (HPVs) are considered the major causative pathogens of cancers that affect epithelial mucosa, such as cervical cancer. However, because of the lack of high-resolution structural information on the sites of neutralization, we have yet to determine the precise mode of HPV infection and how different types of HPV cause infection. Our crystal structures in this study have uncovered discrete binding stoichiometries for two different antibodies. We show that one A12A3 Fab binds to the center of one HPV58 pentamer, whereas five 28F10 Fabs bind along the top fringe of one HPV59 pentamer. Furthermore, through targeted epitope analysis, we show that 6 to 7 discontinuous residues of the L1 major capsid protein of HPV are determinants, at least in part, for virus infection and type specificity. This knowledge will help us to unravel the process of HPV infection and can potentially be used to drive the development of therapeutics that target neutralization-sensitive sites. IMPORTANCE High-risk human papillomaviruses (HPVs) are considered the major causative pathogens of cancers that affect epithelial mucosa, such as cervical cancer. However, because of the lack of high-resolution structural information on the sites of neutralization, we have yet to determine the precise mode of HPV infection and how different types of HPV cause infection. Our crystal structures in this study have uncovered discrete binding stoichiometries for two different antibodies. We show that one A12A3 Fab binds to the center of one HPV58 pentamer, whereas five 28F10 Fabs bind along the top fringe of one HPV59 pentamer. Furthermore, through targeted epitope analysis, we show that 6 to 7 discontinuous residues of the L1 major capsid protein of HPV are determinants, at least in part, for virus infection and type specificity. This knowledge will help us to unravel the process of HPV infection and can potentially be used to drive the development of therapeutics that target neutralization-sensitive sites.