Hybrid Virus-like Particles Research Articles

Influenza virus-like particle-based hybrid vaccine containing RBD induces immunity against influenza and SARS-CoV-2 viruses

Abstract Several approaches have produced an effective vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, the influence of immunity against other vaccinations on the durability and efficacy of the immune response against SARS-CoV-2 is still unknown in settings where vaccines against other viruses need to be administered simultaneously. This will be an important factor in developing multivalent vaccines against seasonal viruses. We have developed a hybrid vaccine for SARS-CoV-2 and influenza viruses using influenza virus-like particles (VLP) incorporated with glycosylphosphatidylinositol (GPI)-anchored Spike RBD of SARS-CoV-2 fused to GM-CSF as an adjuvant. GPI-anchored fusion protein of GM-CSF and the SARS-CoV-2 S1 RBD was expressed in CHO-S cells, purified by immunoaffinity chromatography and incorporated onto influenza VLPs by protein transfer to make a hybrid VLP vaccine. The efficacy of the hybrid VLP vaccine was tested against both SARS-CoV-2 and influenza A/PR8 viruses in a mouse model. Our results show that the hybrid vaccine induced a strong antibody response and protected the mice from both influenza virus and mouse-adapted SARS-CoV-2 challenges, with vaccinated mice having less body weight loss and significantly lower lung viral titers compared to control mice. These results suggest that the hybrid vaccine is a promising candidate for preventing influenza A and SARS-CoV-2 infections. This work was supported by NIH/NIAID (SBIR Contract# 75N93019C00017 Amendment to Pack/Ramachandiran), and Intel Corporation for the Intel COVID-19 Global Technology Response Initiative grant.

In vitro refolding of the structural protein VP1 of parvovirus B19 produces virus-like particles with functional VP1 unique region

Virus-like particles (VLPs) from Parvovirus B19 (B19V) can be obtained by the self-assembly of the structural proteins VP1 and VP2. It is possible to produce B19V VLPs either from VP2 or a mixture of VP1 and VP2, through its heterologous expression in eukaryotic cells. The difference between VP1 and VP2 protein is a tract of 227 residues located at the N-terminal region of VP1, known as the VP1 unique region (VP1u). This region is critical for B19V infection, including tropism, cell internalization, and lysosomal scape through its phospholipase 2A activity. Herein, we report the in vitro self-assembly of VP1 to form VLPs. These species have phospholipase activity, suggesting that the phospholipase domain is correctly folded. Furthermore, VP1 and VP2 were co-assembled to produce hybrid VLPs which were able to bind and internalize in the non-permissive HepG2 cells, another evidence of the functionality of the in vitro refolded VP1u.

An Ultrastable Virus-Like Particle with a Carbon Dot Core and Expanded Sequence Plasticity.

Ordered bio-inorganic hybridization has evolved for the generation of high-performance materials in living organisms and inspires novel strategies to design artificial hybrid materials. Virus-like particles (VLPs) are attracting extensive interest as self-assembling systems and platforms in the fields of biotechnology and nanotechnology. However, as soft nanomaterials, their structural stability remains a general and fundamental problem in various applications. Here, an ultrastable VLP assembled from the major capsid protein (VP1) of simian virus 40 is reported, which contains a carbon dot (C-dot) core. Co-assembly of VP1 with C-dots led to homogeneous T= 1 VLPs with a fourfold increase in VLP yields. The resultant hybrid VLPs showed markedly enhanced structural stability and sequence plasticity. C-dots and a polyhistidine tag fused to the inner-protruding N-terminus of VP1 contributed synergistically to these enhancements, where extensive and strong noncovalent interactions on the C-dot/VP1 interfaces are responsible according to cryo-EM 3D reconstruction, molecular simulation, and affinity measurements. C-dot-enhanced ultrastable VLPs can serve as a new platform, enabling the fabrication of new architectures for bioimaging, theranostics, nanovaccines, etc. The hybridization strategy is simple and can easily be extended to other VLPs and protein nanoparticle systems.

Polyvalent Hybrid Virus-Like Nanoparticles with Displayed Heparin Antagonist Peptides.

The potential applications for nanomaterials continue to grow as new materials are developed and environmental and safety concerns are more adequately addressed. In particular, virus-like particles (VLPs) have myriad applications in medicine and biology, exploiting both the reliable, symmetric self-assembly mechanism and the ability to take advantage of surface functionalities that may be appropriately modified through mutation or bioconjugation. Herein we describe the design and application of hybrid VLPs for use as potent heparin antagonists, providing an alternative to the toxic heparin antidote protamine. A two-plasmid system was utilized to generate VLPs that contain both the wild-type coat protein and a second coat protein with either a C- or N-terminal cationic peptide extension (4-28 amino acids). Incorporation of the modified coat proteins varied from 8 to 31%, while activated partial thromboplastin time (APTT) assays revealed a range of the heparin antagonist activity. Notably, when examined on the basis of the quantity of peptide delivered due to the varied incorporation rates, it appeared that the VLPs largely followed a similar trend, with the quantity of peptide delivered more closely correlating with heparin antagonist activity. The particle with the highest incorporation rate and best antiheparin activity displayed the C-terminal peptide ARK2A2KA, which corresponds to the Cardin-Weintraub consensus sequence for binding to glycosaminoglycans. Analysis of this particle using heparin affinity chromatography with fraction collection revealed that particles eluting at higher salt concentration had a greater proportion of peptide incorporation. Preliminary dual polarization interferometry experiments further support a strong interaction between this particle and heparin.

Display of whole proteins on inner and outer surfaces of grapevine fanleaf virus-like particles.

SummaryVirus‐like particles (VLPs) derived from nonenveloped viruses result from the self‐assembly of capsid proteins (CPs). They generally show similar structural features to viral particles but are noninfectious and their inner cavity and outer surface can potentially be adapted to serve as nanocarriers of great biotechnological interest. While a VLP outer surface is generally amenable to chemical or genetic modifications, encaging a cargo within particles can be more complex and is often limited to small molecules or peptides. Examples where both inner cavity and outer surface have been used to simultaneously encapsulate and expose entire proteins remain scarce. Here, we describe the production of spherical VLPs exposing fluorescent proteins at either their outer surface or inner cavity as a result of the self‐assembly of a single genetically modified viral structural protein, the CP of grapevine fanleaf virus (GFLV). We found that the N‐ and C‐terminal ends of the GFLV CP allow the genetic fusion of proteins as large as 27 kDa and the plant‐based production of nucleic acid‐free VLPs. Remarkably, expression of N‐ or C‐terminal CP fusions resulted in the production of VLPs with recombinant proteins exposed to either the inner cavity or the outer surface, respectively, while coexpression of both fusion proteins led to the formation hybrid VLP, although rather inefficiently. Such properties are rather unique for a single viral structural protein and open new potential avenues for the design of safe and versatile nanocarriers, particularly for the targeted delivery of bioactive molecules.

A human monoclonal antibody against HPV16 recognizes an immunodominant and neutralizing epitope partially overlapping with that of H16.V5.

The presence of neutralizing epitopes in human papillomavirus (HPV) L1 virus-like particles (VLPs) is the structural basis of prophylactic vaccines. An anti-HPV16 neutralizing monoclonal antibody (N-mAb) 26D1 was isolated from a memory B cell of a human vaccinee. The pre-binding of heparan sulfate to VLPs inhibited the binding of both N-mAbs to the antigen, indicating that the epitopes are critical for viral cell attachment/entry. Hybrid VLP binding with surface loop swapping between types indicated the essential roles of the DE and FG loops for both 26D1 (DEa in particular) and H16.V5 binding. Specifically, Tyr135 and Val141 on the DEa loop were shown to be critical residues for 26D1 binding via site-directed mutagenesis. Partially overlap between the epitopes between 26D1 and H16.V5 was shown using pairwise epitope mapping, and their binding difference is demonstrated to be predominantly in DE loop region. In addition, 26D1 epitope is immunodominant epitope recognized by both antibodies elicited by the authentic virus from infected individuals and polyclonal antibodies from vaccinees. Overall, a partially overlapping but distinct neutralizing epitope from that of H16.V5 was identified using a human N-mAb, shedding lights to the antibody arrays as part of human immune response to vaccination and infection.

The use of hybrid virus-like particles to enhance the immunogenicity of a broadly protective HPV vaccine.

Virus-like particles (VLPs) can serve as a highly immunogenic vaccine platform for the multivalent display of epitopes from pathogens. We have used bacteriophage VLPs to develop vaccines that target a highly conserved epitope from the human papillomavirus (HPV) minor capsid protein, L2.VLPs displaying an L2-peptide from HPV16 elicit antibodies that broadly neutralize infection by HPV types associated with the development of cervical cancer. To broaden the cross-neutralization further, we have developed a strategy to display two different peptides on a single, hybrid VLP in a multivalent, highly immunogenic fashion. In general, hybrid VLPs elicited high-titer antibody responses against both targets, although in one case we observed an immunodominant response against only one of the displayed epitopes. Immunization with hybrid particles elicited antibodies that were able to neutralize heterologous HPV types at higher titers than those elicited by particles displaying one epitope alone, indicating that the hybrid VLP approach may be an effective technique to target epitopes that undergo antigenic variation.

A Plant-Produced Pfs25 VLP Malaria Vaccine Candidate Induces Persistent Transmission Blocking Antibodies against Plasmodium falciparum in Immunized Mice

Malaria transmission blocking vaccines (TBVs) are considered an effective means to control and eventually eliminate malaria. The Pfs25 protein, expressed predominantly on the surface of the sexual and sporogonic stages of Plasmodium falciparum including gametes, zygotes and ookinetes, is one of the primary targets for TBV. It has been demonstrated that plants are an effective, highly scalable system for the production of recombinant proteins, including virus-like particles (VLPs). We engineered VLPs (Pfs25-CP VLP) comprising Pfs25 fused to the Alfalfa mosaic virus coat protein (CP) and produced these non-enveloped hybrid VLPs in Nicotiana benthamiana plants using a Tobacco mosaic virus-based ‘launch’ vector. Purified Pfs25-CP VLPs were highly consistent in size (19.3±2.4 nm in diameter) with an estimated 20–30% incorporation of Pfs25 onto the VLP surface. Immunization of mice with one or two doses of Pfs25-CP VLPs plus Alhydrogel® induced serum antibodies with complete transmission blocking activity through the 6 month study period. These results support the evaluation of Pfs25-CP VLP as a potential TBV candidate and the feasibility of the ‘launch’ vector technology for the production of VLP-based recombinant vaccines against infectious diseases.

Reconstituted plant viral capsids can release genes to mammalian cells

The nucleocapsids of many plant viruses are significantly more robust and protective of their RNA contents than those of enveloped animal viruses. In particular, the capsid protein (CP) of the plant virus Cowpea Chlorotic Mottle Virus (CCMV) is of special interest because it has been shown to spontaneously package, with high efficiency, a large range of lengths and sequences of single-stranded RNA molecules. In this work we demonstrate that hybrid virus-like particles, assembled in vitro from CCMV CP and a heterologous RNA derived from a mammalian virus (Sindbis), are capable of releasing their RNA in the cytoplasm of mammalian cells. This result establishes the first step in the use of plant viral capsids as vectors for gene delivery and expression in mammalian cells. Furthermore, the CCMV capsid protects the packaged RNA against nuclease degradation and serves as a robust external scaffold with many possibilities for further functionalization and cell targeting.

Influence of minor displacements in loops of the porcine parvovirus VP2 capsid on virus-like particles assembly and the induction of antibody responses

An antigen-delivery system based on hybrid virus-like particles (VLPs) formed by the self-assembly of the capsid VP2 protein of porcine parvovirus (PPV) and expressing foreign peptides offers an alternative method for vaccination. In this study, the three-dimensional structure of the PPV capsid protein and surface loops deletion mutants were analyzed to define essential domains in PPV VP2 for the assembly of VLPs. Electron microscopic analysis and SDS-PAGE analysis confirmed the presence of abundant VLPs in a loop2 deletion mutant of expected size and appropriate morphology. Loop4 and loop2-loop4 deletion mutants, however, resulted in a lower number of particles and the morphology of the particles was not well preserved. Furthermore, the green fluorescent protein (gfp) gene was used as a model. GFP was observed at the same level in displacements mutants. However, GFP displacement mutants in loop2 construct allowed better adaptation for the fusion GFP to be further displayed on the surface of the capsid-like structure. Immunogenicity study showed that there is no obvious difference in mice inoculated with rAd-VP2(Δloop2), rAd-VP2(Δloop4), rAd-VP2(Δloop2-Δloop4), and PPV inactivated vaccine. The results suggested the possibility of inserting simultaneously B and T cell epitopes in the surface loop2 and the N-terminus. The combination of different types of epitopes (B, CD4+, and CD8+) in different positions of the PPV particles opens the way to the development of highly efficient vaccines, able to stimulate at the same time the different branches of the immune system.

Development of recombinant porcine parvovirus-like particles as an antigen carrier formed by the hybrid VP2 protein carrying immunoreactive epitope of porcine circovirus type 2

Virus-like particles (VLPs) are non-replicative vectors for delivery of heterologous epitopes and induction of immune responses. In this study, a self-assembled porcine parvovirus (PPV) VP2 capsid protein [PPV:VLP-(PCV2)] carrying immunoreactive epitopes, residues 165-200 from the Porcine circovirus 2 (PCV2) virus nucleoprotein was constructed. Immunogenicity study was carried out with hybrid VLPs derived from HEK-293 cells infected with recombinant adenovirus vectors. To our knowledge, this study presents the first demonstration that hybrid non-replicative PPV VLPs carrying PCV2 immunoreactive epitopes can induce stronger antibody responses against PCV2 than recombinant adenovirus of PCV2 ORF2, in the absence of any adjuvant. The hybrid VLPs [PPV:VLP-(PCV2)] might be a promising candidate vaccine for better prevention of the diseases associated with PCV2 as well as with co-infection by PCV2 and PPV.

Exploiting the Yeast L-A Viral Capsid for the In Vivo Assembly of Chimeric VLPs as Platform in Vaccine Development and Foreign Protein Expression

A novel expression system based on engineered variants of the yeast (Saccharomyces cerevisiae) dsRNA virus L-A was developed allowing the in vivo assembly of chimeric virus-like particles (VLPs) as a unique platform for a wide range of applications. We show that polypeptides fused to the viral capsid protein Gag self-assemble into isometric VLP chimeras carrying their cargo inside the capsid, thereby not only effectively preventing proteolytic degradation in the host cell cytosol, but also allowing the expression of a per se cytotoxic protein. Carboxyterminal extension of Gag by T cell epitopes from human cytomegalovirus pp65 resulted in the formation of hybrid VLPs that strongly activated antigen-specific CD8+ memory T cells ex vivo. Besides being a carrier for polypeptides inducing antigen-specific immune responses in vivo, VLP chimeras were also shown to be effective in the expression and purification of (i) a heterologous model protein (GFP), (ii) a per se toxic protein (K28 α-subunit), and (iii) a particle-associated and fully recyclable biotechnologically relevant enzyme (esterase A). Thus, yeast viral Gag represents a unique platform for the in vivo assembly of chimeric VLPs, equally attractive and useful in vaccine development and recombinant protein production.

Identification of Human Papillomavirus Type 16 L1 Surface Loops Required for Neutralization by Human Sera

The variable surface loops on human papillomavirus (HPV) virions required for type-specific neutralization by human sera remain poorly defined. To determine which loops are required for neutralization, a series of hybrid virus-like particles (VLPs) were used to adsorb neutralizing activity from HPV type 16 (HPV16)-reactive human sera before being tested in an HPV16 pseudovirion neutralization assay. The hybrid VLPs used were composed of L1 sequences of either HPV16 or HPV31, on which one or two regions were replaced with homologous sequences from the other type. The regions chosen for substitution were the five known loops that form surface epitopes recognized by monoclonal antibodies and two additional variable regions between residues 400 and 450. Pretreatment of human sera, previously found to react to HPV16 VLPs in enzyme-linked immunosorbent assays, with wild-type HPV16 VLPs and hybrid VLPs that retained the neutralizing epitopes reduced or eliminated the ability of sera to inhibit pseudovirus infection in vitro. Surprisingly, substitution of a single loop often ablated the ability of VLPs to adsorb neutralizing antibodies from human sera. However, for all sera tested, multiple surface loops were found to be important for neutralizing activity. Three regions, defined by loops DE, FG, and HI, were most frequently identified as being essential for binding by neutralizing antibodies. These observations are consistent with the existence of multiple neutralizing epitopes on the HPV virion surface.

Virus-like particles exhibit potential as a pan-filovirus vaccine for both Ebola and Marburg viral infections

A safe and effective pan-filovirus vaccine is highly desirable since the filoviruses Ebola virus (EBOV) and Marburg virus (MARV) cause highly lethal disease typified by unimpeded viral replication and severe hemorrhagic fever. Previously, we showed that expression of the homologous glycoprotein (GP) and matrix protein VP40 from a single filovirus, either EBOV or MARV, resulted in formation of wild-type virus-like particles (VLPs) in mammalian cells. When used as a vaccine, the wild-type VLPs protected from homologous filovirus challenge. The aim of this work was to generate a multi-agent vaccine that would simultaneously protect against multiple and diverse members of the Filoviridae family. Our initial approach was to construct hybrid VLPs containing heterologous viral proteins, of EBOV and MARV, and test the efficacy of the hybrid VLPs in a guinea pig model. Our data indicate that vaccination with GP was required and sufficient to protect against a homologous filovirus challenge, as heterologous wild-type VLPs or hybrid VLPs that did not contain the homologous GP failed to protect. Alternately, we vaccinated guinea pigs with a mixture of wild-type Ebola and Marburg VLPs. Vaccination with a single dose of the multivalent VLP vaccine elicited strong immune responses to both viruses and protected animals against EBOV and MARV challenge. This work provides a critical foundation towards the development of a pan-filovirus vaccine that is safe and effective for use in primates and humans.

Heparin-based ELISA reduces background reactivity in virus-like particle-based papillomavirus serology

The interaction between human papillomavirus (HPV) particles and cell surface heparan sulfate requires intact conformation of the HPV particles. Type-specific HPV serology is currently based on virus-like particles (VLPs) with intact conformation. Presence of incorrectly folded VLPs in VLP preparations is recognized as an important cause of cross-reactivity in HPV serology. Heparin-coated microtitre plates were evaluated for capturing conformationally correct VLPs and improving the type specificity of HPV serology. Hybrid VLPs between HPV16 and HPV11, which had been found to have significant reactivity with children's sera and a batch of HPV18 VLPs that had failed the quality control because of significant reactivity with sera from virginal women, were tested in parallel with heparin ELISA, ordinary ELISA and type-specific mAb capture ELISA. Control sera from children that had detectable reactivity with HPV16/11 hybrid VLPs in ordinary ELISA did not react in heparin-based ELISA, but some hybrid VLPs also had background reactivity in capture ELISAs. Control sera from virginal women that had some reactivity with a poor quality HPV18 VLP preparation in ordinary ELISA had no reactivity in heparin or capture ELISA, suggesting that certain VLP preparations expose cross-reactive epitopes that are not exposed on VLPs with heparin-binding ability. As the sensitivity was similar or only marginally affected by the use of heparin plates, use of heparin-coated plates may improve the type specificity of VLP-based ELISAs and reduce interassay variability attributable to variable quality of different VLP batches.

Recombinant dengue virus type 2 envelope/hepatitis B surface antigen hybrid protein expressed in Pichia pastoris can function as a bivalent immunogen

A truncated version of the dengue virus type 2 envelope protein (Den2E) encoding the first 395 amino acid (aa) residues, and Den2E fused in-frame with the full-length 226-aa hepatitis B surface antigen (Den2E-HBsAg) protein were expressed in the methylotrophic yeast, Pichia pastoris. Both the recombinant proteins showed evidence of the capacity to form high molecular weight aggregates. Electron microscopic analysis of the purified proteins showed that while Den2E displayed an amorphous morphology, Den2E-HBsAg existed as well-structured virus-like particles (VLPs). Using immuno-gold electron microscopy, these VLPs were demonstrated to contain both components of the Den2E-HBsAg hybrid protein. Seroanalysis showed that the hybrid VLPs could function in vivo as bivalent immunogens, which could elicit immune responses directed against both components of the hybrid protein, as evidenced by ELISA, immunoprecipitation and immunofluorescence data.

Hybrid papillomavirus L1 molecules assemble into virus-like particles that reconstitute conformational epitopes and induce neutralizing antibodies to distinct HPV types.

Human papillomavirus (HPV) hybrid virus-like particles (VLPs) were prepared using complementary regions of the major capsid L1 proteins of HPV-11 and -16. These hybrid L1 proteins were tested for assembly into VLPs, for presentation and mapping of conformational neutralizing epitopes, and as immunogens in rabbits and mice. Two small noncontiguous hypervariable regions of HPV-16 L1, when replaced into the HPV-11 L1 backbone, produced an assembly-positive hybrid L1 which was recognized by the type-specific, conformationally dependent HPV-16 neutralizing monoclonal antibody (N-MAb) H16.V5. Several new N-MAbs that were generated following immunization of mice with wild-type HPV-16 L1 VLPs also recognized this reconstructed VLP, demonstrating that these two hypervariable regions collectively constituted an immunodominant epitope. When a set of hybrid VLPs was tested as immunogens in rabbits, antibodies to both HPV-11 and -16 wild-type L1 VLPs were obtained. One of the hybrid VLPs containing hypervariable FG and HI loops of HPV-16 L1 replaced into an HPV-11 L1 background provoked neutralizing activity against both HPV-11 and HPV-16. In addition, conformationally dependent and type-specific MAbs to both HPV-11 and HPV-16 L1 VLP were obtained from mice immunized with hybrid L1 VLPs. These data indicated that hybrid L1 proteins can be constructed that retain VLP-assembly properties, retain type-specific conformational neutralizing epitopes, can map noncontiguous regions of L1 which constitute type-specific conformational neutralizing epitopes recognized by N-MAbs, and trigger polyclonal antibodies which can neutralize antigenically unrelated HPV types.

Effect of MOI ratio on the composition and yield of chimeric infectious bursal disease virus-like particles by baculovirus co-infection: deterministic predictions and experimental results.

Virus-like particles (VLPs) are empty particles consisting of virus capsid proteins that closely resemble native virus but are devoid of the native viral nucleic acids and therefore have attracted significant attention as noninfectious vaccines. A recombinant baculovirus, vIBD-7, which encodes the structural proteins (VP2, VP3, and VP4) of infectious bursal disease virus (IBDV), produces native IBD VLPs in infected Spodoptera frugiperda insect cells. Another baculovirus, vEDLH-22, encodes VP2 that is fused with a histidine affinity-tag (VP2H) at the C-terminus. By co-infection with these two baculoviruses, hybrid VLPs with histidine tags were formed and purified by immobilized metal affinity chromatography (Hu et al., 1999). Also, we demonstrated that varying the MOI ratio of these infecting viruses altered the extent of VP2H incorporated into the particles. A dynamic mathematical model that described baculovirus infection and VLP synthesis (Hu and Bentley, 2000) was adapted here for co-infection and validated by immunofluorescence labeling. It was shown to predict the VLP composition as a dynamic function of MOI. A constraint in the VP2H content incorporated into the particles was predicted and shown by experiments. Also, the MOI ratio of both infecting viruses was shown to be the major factor influencing the composition of the hybrid particles and an important factor in determining the overall yield. ELISA results confirmed that VP2H was exhibited to a varied extent on the outer surface of the particles. This model provides insight on the use of virus co-infection in virus-mediated recombinant protein expression systems and aids in the optimization of chimeric VLP synthesis.

CD4(+) T cells induced by virus-like particles expressing a major T cell epitope down-regulate IL-5 production in an ongoing immune response to Der p 1 independently of IFN-gamma production.

It has been previously demonstrated that hybrid Ty virus-like particles (VLP) prime effective CD8(+) and CD4(+) T cell responses. In this study, we investigated the effect of treating mice with Ty VLP carrying the immunodominant epitope of Der p 1 after sensitizing them to the group 1 allergen of the house dust mite Dermatophagoides pteronyssinus (Der p 1), under conditions that induce T(h)2 immunity. We show that i.p. treatment with the hybrid VLP abrogated allergen-specific IL-5 production and reduced allergen-specific cell proliferation. This suppression of the response was mediated by CD4(+) T cells and was not accompanied by an increase in IFN-gamma production.

Minor Displacements in the Insertion Site Provoke Major Differences in the Induction of Antibody Responses by Chimeric Parvovirus-like Particles

An antigen-delivery system based on hybrid virus-like particles (VLPs) formed by the self-assembly of the capsid VP2 protein of canine parvovirus (CPV) and expressing foreign peptides was investigated. In this report, we have studied the effects of inserting the poliovirus C3:B epitope in the four loops and the C terminus of the CPV VP2 on the particle structure and immunogenicity. Epitope insertions in the four loops allowed the recovery of capsids in all of the mutants. However, only insertions of the C3:B epitope in VP2 residue 225 of the loop 2 were able to elicit a significant anti-peptide antibody response, but not poliovirus-neutralizing antibodies, probably because residue 225 is located in an small depression of the surface. To fine modulate the insertion site in loop 2, a cassette-mutagenesis was carried out to insert the epitope in adjacent positions 226, 227, and 228. The epitope C3:B inserted into these positions was well recognized by the specific monoclonal antibody C3 by immunoelectron microscopy. BALB/c mice immunized with these chimeric C3:B CPV:VLPs were able to elicit an strong neutralizing antibody response (>3 log10 units) against poliovirus type 1 (Mahoney strain). Therefore, minor displacements in the insertion place cause dramatic changes in the accessibility of the epitope and the induction of antibody responses.