Real time monitoring of antigenicity development of HBsAg virus-like particles (VLPs) during heat- and redox-treatment

Stability studies for the identification of critical process parameters for a pharmaceutical production of the Orf virus

A promising new vaccine platform is based on the Orf virus, a viral vector of the genus Parapoxvirus, which is currently being tested in phase I clinical trials. The application as a vaccine platform mandates a well-characterised, robust, and efficient production process. To identify critical process parameters in the production process affecting the virus' infectivity, the Orf virus was subjected to forced degradation studies, including thermal, pH, chemical, and mechanical stress conditions. The tests indicated a robust virus infectivity within a pH range of 5-7.4 and in the presence of the tested buffering substances (TRIS, HEPES, PBS). The ionic strength up to 0.5 M had no influence on the Orf virus' infectivity stability for NaCl and MgCl₂, while NH₄Cl destabilized significantly. Furthermore, short-term thermal stress of 2d up to 37 °C and repeated freeze-thaw cycles (20cycles) did not affect the virus' infectivity. The addition of recombinant human serum albumin was found to reduce virus inactivation. Last, the Orf virus showed a low shear sensitivity induced by peristaltic pumps and mixing, but was sensitive to ultrasonication. The isoelectric point of the applied Orf virus genotype D1707-V was determined at pH3.5. The broad picture of the Orf virus' infectivity stability against environmental parameters is an important contribution for the identification of critical process parameters for the production process, and supports the development of a stable pharmaceutical formulation. The work is specifically relevant for enveloped (large DNA) viruses, like the Orf virus and like most vectored vaccine approaches.

Development of a recombinant hepatitis B immunoglobulin derived from B cells collected from healthy individuals administered with hepatitis B virus vaccines: A feasibility study

BACKGROUND

In Japan, plasma with a high concentration of Hepatitis B Virus (HBV) antibodies for hepatitis B immunoglobulin (HBIG) is almost entirely imported. We aimed to produce recombinant HBIG by isolating immunoglobulin cDNAs against the HBV surface antigen (HBsAg).

STUDY DESIGN AND METHODS

B cells expressing HBsAg antibodies were obtained from blood center personnel who had been administered HB vaccine booster and then isolated by either an Epstein-Barr virus hybridoma or an antigen-specific memory B cell sorting method. Each cDNA of the heavy and light chains of the target antibody was cloned into an IgG₁ expression vector and transfected into Expi293F cells to produce a recombinant monoclonal antibody (mAb), which was screened by ELISA and in vitro HBV neutralizing assays. The cross-reactivity of the mAbs to normal human molecules was evaluated by ELISA and immunohistochemistry.

RESULTS

Antibody cDNAs were cloned from 11 hybridoma cell lines and 204 HBsAg-bound memory B cells. Three of the resulting recombinant mAbs showed stronger neutralizing activity in vitro than the currently used HBIG. All three bind to the conformational epitope(s) of HBsAg but not to human DNA or cells.

DISCUSSION

We successfully isolated HBV-neutralizing monoclonal antibodies from B cells collected from healthy plasma donors boosted against the HBV. To obtain an alternative source for HBIG, HBV-neutralizing monoclonal antibodies from B cells collected from healthy plasma donors boosted against the HBV may be useful.

Construction of atomic models of full hepatitis B vaccine particles at different stages of maturation

Hepatitis B, one of the world's most common liver infections, is caused by the Hepatitis B Virus (HBV). Via the infected cells, this virus generates non pathogen particles with similar surface structures as those found in the full virus. These particles are used in a recombinant form (HBsAg) to produce efficient vaccines. The atomic structure of the HBsAg particles is currently unsolved, and the only existing structural data for the full particle were obtained by electronic microscopy with a maximum resolution of 12 Å. As many vaccines, HBsAg is a complex bio-system. This complexity results from numerous sources of heterogeneity, and traditional bio-immuno-chemistry analytic tools are often limited in their ability to fully describe the molecular surface or the particle. For the Hepatitis B vaccine particle (HBsAg), no atomic data are available so far. In this study, we used the principal well-known elements of HBsAg structure to reconstitute and model the full HBsAg particle assembly at a molecular level (protein assembly, particle formation and maturation). Full HBsAg particle atomic models were built based on an exhaustive experimental data review, amino acid sequence analysis, iterative threading modeling, and molecular dynamic approaches.

Assessing virus like particles formation and r-HBsAg aggregation during large scale production of recombinant hepatitis B surface antigen from Pichia pastoris

The aggregation of recombinant proteins in the different stages of purification leads to the loss of a considerable portion of target protein and reduction in the process efficiency. As the active HBsAg used in Hepatitis B vaccine production is in the form of virus-like particle (VLP), therefore the time and stages at which the VLP assembling happened through the process would be important. The aim of this study was to explore the product aggregation during different stages of large scale production of rHBsAg in Pichia pastoris at production unit of the Pasteur Institute of Iran. Dynamic light scattering (DLS) and transmission electron microscopy (TEM), and also size exclusion-high-performance liquid chromatography (SE-HPLC) were carried out on samples taken from each downstream processes steps to determine the rate of VLPs formation as the desired product and the aggregated form at each stage of the purification. Based on the results, it was found that VLPs formation started at the acid precipitation stage and reached up to 80% at the thermal treatment stage. The ultrafiltration, ion exchange chromatography and immunoaffinity chromatography stages were disclosed to have the highest contribution in the formation of VLP (virus like particle) 22 nm.

Multifaceted characterization of recombinant protein-based vaccines: An immunochemical toolbox for epitope-specific analyses of the hepatitis E vaccine

The integrity of functional epitopes is a critical quality attribute for recombinant protein based vaccines since the presence of these native-like epitopes is the structural basis for vaccines to elicit functional antibodies. To demonstrate the quality and quantity of functional epitopes on vaccine antigens, a toolbox of assessing antigen characteristics is essential. Among the physicochemical, biophysical, immunochemical and in vivo potency analyses, the epitope-specific assays are most critical assessment of the antigen functionality. In this study, we used hepatitis E virus vaccine as an example to illustrated how the monoclonal antibody (mAb) based immunochemical assays were established for in-depth and multifaceted antigen characterization. A large panel of mAbs were developed and characterized using epitope clustering analysis. A subset of these mAbs recognizing non-overlapping epitopes were chosen to be used for assay development. Orthogonal methods, including surface plasma resonance-based BIAcore, solution competitive ELISA and sandwich ELISA, were developed for the antigenicity assessment. The sandwich ELISA with a pair of mAbs, recognizing two different epitopes, was used to assess the accelerated antigen stability, showing enhanced stability with adjuvant adsorption. Such a sandwich ELISA with robust performance has the potentials to be used for in vitro potency analysis to replace animal-based potency assay as product release test. In summary, using hepatitis E vaccine as an example, we demonstrated the importance and establishment of a mAb-based immunochemical toolbox for multifaceted antigen characterization. This is particularly important to demonstrate the successful reconstruction of the native-like and functional epitopes on a recombinant antigen post expression and purification. These epitope-specific and multifaceted assays serve as critical tools for process monitoring or lot consistency tests in support of vaccine development and manufacturing.

Escherichia coli-derived virus-like particles in vaccine development

Recombinant virus-like particle-based vaccines are composed of viral structural proteins and mimic authentic native viruses but are devoid of viral genetic materials. They are the active components in highly safe and effective vaccines for the prevention of infectious diseases. Several expression systems have been used for virus-like particle production, ranging from Escherichia coli to mammalian cell lines. The prokaryotic expression system, especially Escherichia coli, is the preferred expression host for producing vaccines for global use. Hecolin, the first licensed virus-like particle vaccine derived from Escherichia coli, has been demonstrated to possess good safety and high efficacy. In this review, we focus on Escherichia coli-derived virus-like particle based vaccines and vaccine candidates that are used for prevention (immunization against microbial pathogens) or disease treatment (directed against cancer or non-infectious diseases). The native-like spatial or higher-order structure is essential for the function of virus-like particles. Thus, the tool box for analyzing the key physicochemical, biochemical and functional attributes of purified virus-like particles will also be discussed. In summary, the Escherichia coli expression system has great potentials for producing a range of proteins with self-assembling properties to be used as vaccine antigens given the proper epitopes were preserved when compared to those in the native pathogens or disease-related target molecules.

MPLA-coated hepatitis B virus surface antigen (HBsAg) nanocapsules induce vigorous T cell responses in cord blood derived human T cells

Chronic hepatitis B virus (HBV) infection is the most prevalent serious liver infection in the world. A frequent route of infection represents mother-to-child transmission. Efficient control of HBV replication depends on antigen-specific cellular immune response mediated by dendritic cells (DCs). Aim of the present study was to evaluate optimized adjuvant combinations, efficiently maturing monocyte-derived neonatal and adult dendritic cells (moDCs). In addition, the potential of polymeric HBsAg-nanocapsules (HBsAg-NCs) was investigated regarding up-take by moDCs and the subsequent induction of specific T cell responses in a human co-culture model. Simultaneous stimulation of moDCs with MPLA and IFNγ induced up-regulation of CD80 and HLA-DR along with vigorous secretion of IL-12p70. MPLA-coating of HBsAg-NCs promoted NCs-uptake by moDCs. Finally, MPLA-HBsAg-NCs-pulsed moDCs with IFNγ increased T cell proliferation and induced antigen-specific IFNγ release by T cells. The herein presented vaccine approach provides a rational for neonatal and therapeutic immunization strategies against HBV.

Norovirus (NoV) specific protective immune responses induced by recombinant P dimer vaccine are enhanced by the mucosal adjuvant FlaB

Background

Noroviruses (NoVs) are a major cause of childhood gastroenteritis and foodborne diseases worldwide. Lack of appropriate animal models or cell-based culture systems makes the development and evaluation of NoV-specific vaccines a daunting task. VP1 is the major capsid protein of the NoVs that acts as a binding motif to human histo-blood group antigens (HBGAs) through its protruding 2 (P2) domain and can serve as a protective antigen candidate for vaccine development.

Methods

Recombinantly produced NoV specific P domain (Pd) vaccine was inoculated into groups of mice either alone or in conjugation with mucosal adjuvant FlaB, the flagellar protein from Vibrio vulnificus. Antigen specific humoral and cell mediated immune responses were assessed by enzyme linked immunosorbent assay (ELISA) or fluorescent activated cell sorting (FACS). A comparative analysis of various routes of vaccination viz. intranasal, sublingual and subcutaneous, was also done.

Results

In this study, we show that a recombinant Pd-vaccine administered through intranasal route induced a robust T_H2-dependent humoral immune response and that the combination of vaccine with FlaB significantly enhanced the antibody response. Interestingly, FlaB induced a mixed T_H1/T_H2 type of immune response with a significant induction of IgG1 as well as IgG2a antibodies. FlaB also induced strong IgA responses in serum and feces. FlaB mediated antibody responses were toll like receptor 5 (TLR5) dependent, since the FlaB adjuvanticity was lost in TLR5^−/− mice. Further, though the Pd-vaccine by itself failed to induce a cell mediated immune response, the Pd-FlaB combination stimulated a robust CD4⁺IFNγ⁺ and CD8⁺IFNγ⁺ T cell response in spleen and mesenteric lymph nodes. We also compared the adjuvant effects of FlaB with that of alum and complete Freund’s adjuvant (CFA). We found that subcutaneously inoculated FlaB induced more significant levels of IgG and IgA in both serum and feces compared to alum or CFA in respective samples.

Conclusion

We validate the use of TLR5 agonist as a strong mucosal adjuvant that would facilitate the development of NoV specific vaccines for humans and veterinary use. This study also highlights the importance of route of immunization in inducing the appropriate immune responses in mucosal compartments.

Lessons learned from successful human vaccines: Delineating key epitopes by dissecting the capsid proteins

Recombinant VLP-based vaccines have been successfully used against 3 diseases caused by viral infections: Hepatitis B, cervical cancer and hepatitis E. The VLP approach is attracting increasing attention in vaccine design and development for human and veterinary use. This review summarizes the clinically relevant epitopes on the VLP antigens in successful human vaccines. These virion-like epitopes, which can be delineated with molecular biology, cryo-electron microscopy and x-ray crystallographic methods, are the prerequisites for these efficacious vaccines to elicit functional antibodies. The critical epitopes and key factors influencing these epitopes are discussed for the HEV, HPV and HBV vaccines. A pentamer (for HPV) or a dimer (for HEV and HBV), rather than a monomer, is the basic building block harboring critical epitopes for the assembly of VLP antigen. The processing and formulation of VLP-based vaccines need to be developed to promote the formation and stabilization of these epitopes in the recombinant antigens. Delineating the critical epitopes is essential for antigen design in the early phase of vaccine development and for critical quality attribute analysis in the commercial phase of vaccine manufacturing.

The development of a recombinant hepatitis E vaccine HEV 239

Hepatitis E virus (HEV) infection is one of the main causes of acute hepatitis worldwide. A recombinant hepatitis E vaccine, HEV 239, has been licensed in China for immunizing adults of 16 y old and above. The vaccine antigen contains pORF2 aa 368 - 606 of the HEV genotype 1 expressed in E. coli. The quality of the vaccine is controlled through a combination of biophysical, biochemical and immunochemical methods. The vaccine is well tolerated in adults. The efficacy of the HEV 239 vaccine against symptomatic and asymptomatic infection had been proven to be high during a Phase III clinical trial and long-term follow up. The safety and efficacy of HEV 239 vaccine in certain high-risk populations remains to be further investigated.

Utilizing ELISA to monitor protein-protein interaction

Enzyme-linked immunosorbent assay (ELISA) is a commonly used method in analyzing biomolecular interactions. As a rapid, specific, and easy-to-operate method, ELISA has been used as a research tool as well as a widely adopted diagnostic method in clinical settings and for microbial testing in various industries. Inhibition ELISA is a one-site binding analysis method, which can monitor protein-protein interactions in solution as opposed to more commonly used sandwich ELISA in which the analyte capture step is required on a solid surface either through specific capture or through passive adsorption. Here, we introduce inhibition ELISA procedures, using a recombinant viral protein as an example, with emphasis on how inhibition ELISA could be used to probe subtle protein conformational changes in solution impacting protein-protein binding affinity. Inhibition ELISA is used to probe one binding site at a time for binding partners in solution with unrestricted conformation. The assay can be performed in a quantitative manner with a serially diluted analyte in solution for solution antigenicity or binding activity assessment.

Comparable quality attributes of hepatitis E vaccine antigen with and without adjuvant adsorption-dissolution treatment

Most vaccines require adjuvants for antigen stabilization and immune potentiation. Aluminum-based adjuvants are the most widely used adjuvants for human vaccines. Previous reports demonstrated the preservation of antigen conformation and other antigen characteristics after recovery from adjuvanted Hepatitis B and human papillomavirus vaccines. In this study, we used a combination of various physiochemical and immunochemical methods to analyze hepatitis E vaccine antigen quality attributes after recovery from adjuvants. All biochemical and biophysical methods showed similar characteristics of the p239 protein after recovery from adjuvanted vaccine formulation compared to the antigen in solution which never experienced adsorption/desorption process. Most importantly, we demonstrated full preservation of key antigen epitopes post-recovery from adjuvanted vaccine using a panel of murine monoclonal antibodies as exquisite probes. Antigenicity of p239 was probed with a panel of 9 mAbs using competition/blocking ELISA, surface plasmon resonance and sandwich ELISA methods. These multifaceted analyses demonstrated the preservation of antigen key epitopes and comparable protein thermal stability when adsorbed on adjuvants or of the recovered antigen post-dissolution treatment. A better understanding of the antigen conformation in adjuvanted vaccine will enhanced our knowledge of antigen-adjuvant interactions and facilitate an improved process control and development of stable vaccine formulation.

Vaccine instability in the cold chain: mechanisms, analysis and formulation strategies

Instability of vaccines often emerges as a key challenge during clinical development (lab to clinic) as well as commercial distribution (factory to patient). To yield stable, efficacious vaccine dosage forms for human use, successful formulation strategies must address a combination of interrelated topics including stabilization of antigens, selection of appropriate adjuvants, and development of stability-indicating analytical methods. This review covers key concepts in understanding the causes and mechanisms of vaccine instability including (1) the complex and delicate nature of antigen structures (e.g., viruses, proteins, carbohydrates, protein-carbohydrate conjugates, etc.), (2) use of adjuvants to further enhance immune responses, (3) development of physicochemical and biological assays to assess vaccine integrity and potency, and (4) stabilization strategies to protect vaccine antigens and adjuvants (and their interactions) during storage. Despite these challenges, vaccines can usually be sufficiently stabilized for use as medicines through a combination of formulation approaches combined with maintenance of an efficient cold chain (manufacturing, distribution, storage and administration). Several illustrative case studies are described regarding mechanisms of vaccine instability along with formulation approaches for stabilization within the vaccine cold chain. These include live, attenuated (measles, polio) and inactivated (influenza, polio) viral vaccines as well as recombinant protein (hepatitis B) vaccines.

Robust manufacturing and comprehensive characterization of recombinant hepatitis E virus-like particles in Hecolin(®)

The hepatitis E virus (HEV) vaccine, Hecolin(®), was licensed in China for the prevention of HEV infection and HEV-related diseases with demonstrated safety and efficacy [1,2]. The vaccine is composed of a truncated HEV capsid protein, p239, as the sole antigen encoded by open reading frame 2 and produced using Escherichia coli platform. The production of this virus-like particle (VLP) form of the antigen was successfully scaled up 50-fold from a bench scale to a manufacturing scale. Product consistency was demonstrated using a combination of biophysical, biochemical and immunochemical methods, which revealed comparable antigen characteristics among different batches. Particle size of the nanometer scale particulate antigen and presence of key epitopes on the particle surface are two prerequisites for an efficacious VLP-based vaccine. The particle size was monitored by several different methods, which showed diameters between 20 and 30nm for the p239 particles. The thermal stability and aggregation propensity of the antigen were assessed using differential scanning calorimetry and cloud point assay under heat stress conditions. Key epitopes on the particulate antigen were analyzed using a panel of murine anti-HEV monoclonal antibodies (mAbs). The immuno reactivity to the mAbs among the different antigen lots was highly consistent when analyzed quantitatively using a surface plasmon resonance technique. Using a sandwich ELISA to probe the integrity of two different epitopes in the antigen, the specific antigenicity of multiple batches was assessed to demonstrate consistency in these critical product attributes. Overall, our findings showed that the antigen production process is robust and scalable during the manufacturing of Hecolin(®).

Improved characteristics and protective efficacy in an animal model of E. coli-derived recombinant double-layered rotavirus virus-like particles

Live rotavirus vaccines that are effective in middle- and high-income countries have been found to be less immunogenic and effective in infants in resource-limited settings. The virus-like particle (VLP) approach is promising for rotavirus vaccine development, but challenges remain for VLP production at large scale. In this study, rotavirus capsid VP2 and VP6 proteins were expressed in Escherichia coli and were assembled with high efficiency into homogeneous single-layered VP6-VLPs or double-layered VP2/6-VLPs (dl2/6-VLPs) through a post-purification assembly process. The dl2/6-VLPs were observed to have better thermal stability and antigenicity. Although the immunogenicity of VP6 trimers, VP6-VLPs and dl2/6-VLPs was comparable, the efficacy of the dl2/6-VLPs to protect against rotavirus-induced diarrhea in pups was significantly higher than that of the trimeric VP6 or the VP6-VLPs when assessed using a mouse maternal antibody model. Taken together, the recombinant dl2/6-VLP antigen, which is highly analogous to rotavirus virion-derived double-layered particles, is a viable candidate for vaccine development and has the potential to be a parenterally administered safe and efficacious rotavirus vaccine.

Toward the development of monoclonal antibody-based assays to probe virion-like epitopes in hepatitis B vaccine antigen

Prophylactic vaccines against hepatitis B Virus (HBV) infection were produced in different expression systems under different processing conditions. Since the recombinant HBV surface antigen (HBsAg) in these vaccines is a cysteine-rich protein with 14 cysteines among a total of 226 amino acids, the epitopes are dependent on the formation of intra- and intermolecular disulfide bonds. A panel of 22 monoclonal antibodies (mAbs) were developed and evaluated with respect to their sensitivity to disulfide reduction treatment of recombinant HBsAg. Not surprisingly, different mAbs showed different degree of sensitivity to controlled HBsAg disulfide reduction. With a view to exploring the functionality of anti-HBsAg mAbs to be used in HBsAg quality analysis, in vitro neutralization activity for the mAbs was assessed. One of the mAbs tested, 5F11, which showed high sensitivity to the disulfide integrity in HBsAg, was shown also to be highly effective in neutralizing HBV in vitro. Conversely, 42B6, while exhibiting similar neutralization activity, showed comparable binding HBsAg with or without reduction treatment. Based on these mAb characteristics, a sandwich ELISA with 42B6 being the capture Ab and detection Ab was developed to quantify HBsAg (like a "mass" assay) during antigen bioprocessing or in vaccine products. In parallel, when 5F11 was used as the detection Ab (with the same capture Ab), the assay can be used to probe disulfide-dependent and virion-like epitopes in intermediates or final products of hepatitis B vaccine, serving as a surrogate marker for vaccine efficacy to elicit neutralizing antibodies. This approach enables the comparative epitope specific antigenicity analysis of HBsAg antigen preparations from different sources.

Virus-like particle-based human vaccines: quality assessment based on structural and functional properties

Human vaccines against three viruses use recombinant virus-like particles (VLPs) as the antigen: hepatitis B virus, human papillomavirus, and hepatitis E virus. VLPs are excellent prophylactic vaccine antigens because they are self-assembling bionanoparticles (20 to 60 nm in diameter) that expose multiple epitopes on their surface and faithfully mimic the native virions. Here we summarize the long journey of these vaccines from bench to patients. The physical properties and structural features of each recombinant VLP vaccine are described. With the recent licensure of Hecolin against hepatitis E virus adding a third disease indication to prophylactic VLP-based vaccines, we review how the crucial quality attributes of VLP-based human vaccines against all three disease indications were assessed, controlled, and improved during bioprocessing through an array of structural and functional analyses.

Toolbox for non-intrusive structural and functional analysis of recombinant VLP based vaccines: a case study with hepatitis B vaccine

Background

Fundamental to vaccine development, manufacturing consistency, and product stability is an understanding of the vaccine structure-activity relationship. With the virus-like particle (VLP) approach for recombinant vaccines gaining popularity, there is growing demand for tools that define their key characteristics. We assessed a suite of non-intrusive VLP epitope structure and function characterization tools by application to the Hepatitis B surface antigen (rHBsAg) VLP-based vaccine.

Methodology

The epitope-specific immune reactivity of rHBsAg epitopes to a given monoclonal antibody was monitored by surface plasmon resonance (SPR) and quantitatively analyzed on rHBsAg VLPs in-solution or bound to adjuvant with a competitive enzyme-linked immunosorbent assay (ELISA). The structure of recombinant rHBsAg particles was examined by cryo transmission electron microscopy (cryoTEM) and in-solution atomic force microscopy (AFM).

Principal Findings

SPR and competitive ELISA determined relative antigenicity in solution, in real time, with rapid turn-around, and without the need of dissolving the particulate aluminum based adjuvant. These methods demonstrated the nature of the clinically relevant epitopes of HBsAg as being responsive to heat and/or redox treatment. In-solution AFM and cryoTEM determined vaccine particle size distribution, shape, and morphology. Redox-treated rHBsAg enabled 3D reconstruction from CryoTEM images – confirming the previously proposed octahedral structure and the established lipid-to-protein ratio of HBsAg particles. Results from these non-intrusive biophysical and immunochemical analyses coalesced into a comprehensive understanding of rHBsAg vaccine epitope structure and function that was important for assuring the desired epitope formation, determinants for vaccine potency, and particle stability during vaccine design, development, and manufacturing.

Significance

Together, the methods presented here comprise a novel suite of non-intrusive VLP structural and functional characterization tools for recombinant vaccines. Key VLP structural features were defined and epitope-specific antigenicity was quantified while preserving epitope integrity and particle morphology. These tools should facilitate the development of other VLP-based vaccines.

Disassembly and reassembly of human papillomavirus virus-like particles produces more virion-like antibody reactivity

Background

Human papillomavirus (HPV) vaccines based on major capsid protein L1 are licensed in over 100 countries to prevent HPV infections. The yeast-derived recombinant quadrivalent HPV L1 vaccine, GARDASIL(R), has played an important role in reducing cancer and genital warts since its introduction in 2006. The L1 proteins self-assemble into virus-like particles (VLPs).

Results

VLPs were subjected to post-purification disassembly and reassembly (D/R) treatment during bioprocessing to improve VLP immunoreactivity and stability. The post-D/R HPV16 VLPs and their complex with H16.V5 neutralizing antibody Fab fragments were visualized by cryo electron microscopy, showing VLPs densely decorated with antibody. Along with structural improvements, post-D/R VLPs showed markedly higher antigenicity to conformational and neutralizing monoclonal antibodies (mAbs) H16.V5, H16.E70 and H263.A2, whereas binding to mAbs recognizing linear epitopes (H16.J4, H16.O7, and H16.H5) was greatly reduced.

Strikingly, post-D/R VLPs showed no detectable binding to H16.H5, indicating that the H16.H5 epitope is not accessible in fully assembled VLPs. An atomic homology model of the entire

HPV16 VLP was generated based on previously determined high-resolution structures of bovine papillomavirus and HPV16 L1 pentameric capsomeres.

Conclusions

D/R treatment of HPV16 L1 VLPs produces more homogeneous VLPs with more virion-like antibody reactivity. These effects can be attributed to a combination of more complete and regular assembly of the VLPs, better folding of L1, reduced non-specific disulfide-mediated aggregation and increased stability of the VLPs. Markedly different antigenicity of HPV16 VLPs was observed upon D/R treatment with a panel of monoclonal antibodies targeting neutralization sensitive epitopes. Multiple epitope-specific assays with a panel of mAbs with different properties and epitopes are required to gain a better understanding of the immunochemical properties of VLPs and to correlate the observed changes at the molecular level. Mapping of known antibody epitopes to the homology model explains the changes in antibody reactivity upon D/R. In particular, the H16.H5 epitope is partially occluded by intercapsomeric interactions involving the L1 C-terminal arm. The homology model allows a more precise mapping of antibody epitopes. This work provides a better understanding of VLPs in current vaccines and could guide the design of improved vaccines or therapeutics.

In-depth process understanding of RECOMBIVAX HB® maturation and potential epitope improvements with redox treatment: multifaceted biochemical and immunochemical characterization

Recombinant Hepatitis B surface antigen virus-like particles (VLPs) produced in yeast undergo spontaneous maturation during the vaccine production process, and the biophysical characteristics of the particles with respect to maturation were described in Zhao et al. (2006) [13]. Here we report additional biochemical and immunochemical characterization by various techniques, including the use of a panel of monoclonal antibodies (mAbs) that differ in their selectivity and conformation-sensitivity, for probing surface epitope structures. Crosslinking via interchain disulfide formation and binding of conformational specific antibodies in the mature particles were shown to be progressively enhanced. We show that redox-mediated VLP maturation is superior to heat-induced maturation in terms of generating VLPs which exhibit more complete crosslinking (>95%) and 2- to 3-fold higher antigenicity as defined by conformational antibodies. Therefore, the resulting VLPs from redox treatment resemble more closely their plasma-derived counterparts. The value of using multiple mAbs for probing surface epitopes was clearly demonstrated as different mAbs showed different degrees of sensitivity to the structural changes during HBsAg VLP maturation. The rapid, label-free technology of surface plasmon resonance performed at a single antigen concentration was shown to correlate well with a sandwich ELISA using parallel line analysis, currently implemented for product release and stability testing of RECOMBIVAX HB(®). Surface plasmon resonance offers both convenience and flexibility; multiple mAbs can be tested one at a time in the same set of experiments, providing a means to assess changes to individual epitopes. Taken together, these quantitative analytical tools enable more rapid, in-depth, and comprehensive process monitoring, process optimization, and assessment of product consistency and stability.