Melanoma vaccine candidates from chimeric hepatitis B core virus-like particles carrying a tumor-associated MAGE-3 epitope

Vaccination of melanoma patients with tumor-specific antigens recognized by cytotoxic T lymphocytes (CTLs) may produce significant tumor regressions. Here, we suggest a novel type of tumor vaccines, with well-studied CTL epitopes presented on highly immunogenic virus-like particle (VLP) carriers. Cancer-germline gene MAGE-3 encodes for an antigenic nonapeptide (MAGE-3(168-176) peptide) that is recognized by CTLs on human leukocyte antigen (HLA)-A1 and HLA-B35 molecules. A set of recombinant genes encoding hepatitis B virus core protein carrying MAGE-3 epitope was constructed and expressed in Escherichia coli cells. Variants that led to formation of chimeric VLPs in vivo were purified and analyzed for their DNA binding properties in vitro. VLPs exhibiting the most pronounced nucleic acid binding affinity were selected and loaded either with single-stranded DNA oligodeoxynucleotides rich in nonmethylated CG motifs, or with longer double-stranded DNA fragments. Packaged DNA was protected, at least partially, against the action of bacterial DNase. Such highly purified chimeric VLPs with entrapped immunomodulatory sequences could possibly be used as antitumor vaccines.

Monoclonal antibodies providing topological information on the duck hepatitis B virus core protein and avihepadnaviral nucleocapsid structure

The icosahedral capsid of duck hepatitis B virus (DHBV) is formed by a single core protein species (DHBc). DHBc is much larger than HBc from human HBV, and no high-resolution structure is available. In an accompanying study (M. Nassal, I. Leifer, I. Wingert, K. Dallmeier, S. Prinz, and J. Vorreiter, J. Virol. 81:13218-13229, 2007), we used extensive mutagenesis to derive a structural model for DHBc. For independent validation, we here mapped the epitopes of seven anti-DHBc monoclonal antibodies. Using numerous recombinant DHBc proteins and authentic nucleocapsids from different avihepadnaviruses as test antigens, plus a panel of complementary assays, particle-specific and exposed plus buried linear epitopes were revealed. These data fully support key features of the model.

The rheumatoid arthritis shared epitope increases cellular susceptibility to oxidative stress by antagonizing an adenosine-mediated anti-oxidative pathway

We have recently demonstrated that the rheumatoid arthritis (RA) shared epitope (SE) acts as a ligand that triggers nitric oxide (NO) signaling in opposite cells. Given the known pro-oxidative effect of NO and the proposed role of oxidative stress in the pathogenesis of RA, this study explores whether SE-triggered signaling can increase cellular oxidative stress. cAMP levels, adenylyl cyclase activity, and protein kinase A activity were measured using commercial kits. Generation of reactive oxygen species (ROS) was quantified using the fluorochrome dichlorofluorescein diacetate. Oxidative DNA damage was quantified using the single-cell electrophoresis technique. Here, we report that cells exposed to cell surface SE-positive HLA-DR (human leukocyte antigen-DR) molecules, to cell-free recombinant proteins genetically engineered to express the SE motif, or to SE-positive synthetic peptide showed diminished cAMP-dependent signaling, increased ROS levels, and higher vulnerability to oxidative DNA damage. Introduction of single amino acid substitutions into SE-positive peptides revealed a consensus five-amino acid sequence motif of Q/R-K/R-X-X-A that is necessary and sufficient for SE-triggered signaling. The pro-oxidative effect of the SE could be reversed by inhibiting NO production. We conclude that the SE acts as a signaling ligand that activates an NO-mediated pro-oxidative pathway. The potential contribution of this signaling aberration to RA pathogenesis is discussed.

Proteasomal degradation of core protein variants from chronic hepatitis B patients

The accumulation of complex hepatitis B virus (HBV) variants with internal in-frame deletions in the C gene in immunosuppressed renal transplant recipients is associated with a severe course of the infection leading to end-stage liver disease (ESLD). A set of six HBV C genes with internal in-frame deletions corresponding to the pattern of HBV population in immunosuppressed patients has been expressed in two different eukaryotic cell lines. Synthesis and proteasomal degradation of HBV core (HBc) protein variants were compared with those of the wild-type HBc. In all cases, the steady-state level of internally deleted HBc proteins, predominantly with longer deletions, were considerably lower and turnover was significantly higher in comparison with those of the wild-type HBc, since all deletion variants were degraded rapidly via the proteasome pathway. Involvement and consequences of the proteasomal degradation machinery in the HBc protein turnover during HBV infection with complex HBV variants in the immunosuppressed patients are discussed.

Hamster polyomavirus-derived virus-like particles are able to transfer in vitro encapsidated plasmid DNA to mammalian cells

The authentic major capsid protein 1 (VP1) of hamster polyomavirus (HaPyV) consists of 384 amino acid (aa) residues (42 kDa). Expression from an additional in-frame initiation codon located upstream from the authentic VP1 open reading frame (at position -4) might result in the synthesis of a 388 aa-long, amino-terminally extended VP1 (aa -4 to aa 384; VP1(ext)). In a plasmid-mediated Drosophila Schneider (S2) cell expression system, both VP1 derivatives as well as a VP1(ext) variant with an amino acid exchange of the authentic Met1Gly (VP1(ext-M1)) were expressed to a similar high level. Although all three proteins were detected in nuclear as well as cytoplasmic fractions, formation of virus-like particles (VLPs) was observed exclusively in the nucleus as confirmed by negative staining electron microscopy. The use of a tryptophan promoter-driven Escherichia coli expression system resulted in the efficient synthesis of VP1 and VP1(ext) and formation of VLPs. In addition, establishment of an in vitro disassembly/reassembly system allowed the encapsidation of plasmid DNA into VLPs. Encapsidated DNA was found to be protected against the action of DNase I. Mammalian COS-7 and CHO cells were transfected with HaPyV-VP1-VLPs carrying a plasmid encoding enhanced green fluorescent protein (eGFP). In both cell lines eGFP expression was detected indicating successful transfer of the plasmid into the cells, though at a still low level. Cesium chloride gradient centrifugation allowed the separation of VLPs with encapsidated DNA from "empty" VLPs, which might be useful for further optimization of transfection. Therefore, heterologously expressed HaPyV-VP1 may represent a promising alternative carrier for foreign DNA in gene transfer applications.

Preparation of hepatitis C virus structural and non-structural protein fragments and studies of their immunogenicity

Plasmids pQE-60 and pQE-30 containing 6 x His-tag sequence were used for expression of fragments of HCV structural and non-structural proteins in Escherichia coli (E. coli). The following fragments were used: core (1-98 aa), NS3 (202-482 aa), and tetramer of hypervariable region 1 (HVR1) of E2 protein. The constructed plasmids directed high levels of expression of HCV proteins in E. coli JM109. After purification by the metal-affinity chromatography on nickel-nitrilotriacetic acid (Ni-NTA) agarose, the His-tagged HCV proteins were used for immunization of BALB/c mice. All three proteins were able to induce high levels of specific antibodies and, in the case of the NS3 and HVR1 tetramer, also to mount vigorous cell-proliferating responses. High immunogenicity of the tested fragments of HCV proteins shows them as good candidates for inclusion into the future HCV vaccine preparations.

Recombinant virus-like particles as a carrier of B- and T-cell epitopes of hepatitis C virus (HCV)

The major aim of the project was the development of virus-like particles (VLP) displaying B- and T-cell epitopes of hepatitis C virus (HCV) proteins. To this end, hepatitis B virus core (HBc) particles were used as a carrier of HCV epitopes. Fragments of HCV genes encoding core (aa 98) and NS3 (aa 155) proteins were fused to the 3' terminus of the truncated HBV core gene. All recombinant plasmids led to relatively high levels of expression of chimeric proteins in E. coli, which resulted in the formation of complete "mature" VLP. Chimeric HBc/HCV VLPs were purified by combination of gel filtration and sucrose gradient centrifugation, and used for immunogenicity studies in mice. All variants of hybrid particles induced high humoral and cellular responses to HBcAg. Immunization with the HBc/HCV core particles led to relatively low antibody and T-cell proliferative responses to HCV core epitopes. The HBc/HCV NS3 particles were able to induce high levels of anti-NS3 antibodies in the absence of proliferative responses to HCV epitopes. Thus, the results of the current study have demonstrated the principal possibility of using VLP on the basis of HBcAg for creation of a new type of HCV-specific immunogen.

Signal sequences modulate the immunogenic performance of human hepatitis C virus E2 gene

Envelope protein E2 of human hepatitis C virus (HCV) is an attractive component of a prototype HCV vaccine. Delivered by DNA immunogens, E2 evokes specific immune response of Th1-type, failing to induce either considerable antibody production, or T-helper cell proliferation. We aimed at modulating the immunogenic performance of E2 gene by changing the mode of protein expression in eukaryotic cells. Plasmids were constructed encoding full-length E2 and nonstructural protein 1 (p7) fused to either 13 or 38 C-terminal amino acids (aa) of HCV E1 that contain second hydrophobic segment of E1 stop-transfer signal, or a complete E1 stop-transfer signal with duplicated second hydrophobic segment. Injected into BALB/c mice, E2/p7 genes induced potent antibody and T-helper cell response targeted against hypervariable region 1, aa 472-586 of E2, and a novel epitope at aa 774-796 of p7. Profile of cytokines secreted by proliferating mouse splenocytes stimulated in vitro with E2- and p7-derived peptides, indicated mixed Th1/Th2 type of immune response. Thus, the full-length E2 and p7 genes supplied in one cassette were both immunogenic. E2/p7 containing a complete E1 stop-transfer signal with prolonged membrane spanning domain was superior to the shorter E2/p7 version in terms of both antibody and cellular immunogenicity. Optimal performance of HCV E2 could thus be achieved without the aid of external/heterologous signals by easing, through modification of the E2 signal sequence, the release of E2 from the rough ER while retaining full-length E2 and p7 sequences. This finding may help to improve the Th2 performance of HCV envelope genes as prototype vaccines.

Assembly of bacteriophage Qbeta virus-like particles in yeast Saccharomyces cerevisiae and Pichia pastoris

Recombinant bacteriophage Qbeta coat protein (CP), which has been proposed as a promising carrier of foreign epitopes via their incorporation either by gene engineering techniques or by chemical coupling, efficiently self-assembles into virus-like particles (VLPs) when expressed in Escherichia coli. Here, we demonstrate expression and self-assembly of Qbeta CP in yeast Saccharomyces cerevisiae and Pichia pastoris. Production reached 3-4 mg/1g of wet cells for S. cerevisiae and 4-6 mg for P. pastoris, which was about 15-20% and 20-30% of the E. coli expression level, respectively. Qbeta VLPs were easily purified by size-exclusion chromatography in both cases and contained nucleic acid, shown by native agarose gel electrophoresis. The obtained particles were highly immunogenic in mice and the resulting sera recognized both E. coli- and yeast-derived Qbeta VLPs equally well.

Activation of nitric oxide signaling by the rheumatoid arthritis shared epitope

OBJECTIVE

Susceptibility to rheumatoid arthritis (RA) is closely associated with HLA-DRB1 alleles encoding a shared epitope (SE) in positions 70-74 of the HLA-DRbeta chain. The mechanistic basis for this association is unknown. Given the proposed pathogenic role of nitric oxide (NO) in RA, this study was undertaken to examine whether the SE can trigger NO signaling events.

METHODS

The intracellular levels of NO were measured with the fluorescent NO probe 4,5-diaminofluorescein diacetate and by the 2,3-diaminonaphthalene method. NO synthase activity was determined by measuring the rate of conversion of radioactive arginine to citrulline. Levels of cGMP were measured with a commercial enzyme-linked immunosorbent assay, and the cytolytic activity of T cells was measured using a standard (51)Cr release assay.

RESULTS

Lymphoblastoid B cell lines carrying SE-positive HLA-DR alleles displayed a higher rate of spontaneous NO production compared with SE-negative cells. L cell transfectants expressing SE-positive DR molecules on their surface also generated higher levels of NO. Tetrameric HLA-DR molecules containing a DRbeta-chain encoded by the SE-positive DRB1*0401 allele stimulated fibroblast cells to produce higher levels of NO compared with cells stimulated with a control HLA-DR tetramer. Multimeric hepatitis B core proteins engineered to express region 65-79 encoded by the DRB1*0401 allele, but not the same region encoded by the control allele DRB1*0402, stimulated NO production in fibroblasts. Similarly, synthetic 15-mer peptides corresponding to the region 65-79 encoded by SE-positive alleles triggered increased NO levels when incubated with class II major histocompatibility complex-negative cells. The signaling pathway was found to involve NO synthase activation, followed by increased production of cGMP. SE-triggered increased NO levels inhibited cytolytic elimination of target cells.

CONCLUSION

The SE can trigger NO-mediated signaling events in opposite cells, and may thereby contribute to RA pathogenesis.

Efficient homologous prime-boost strategies for T cell vaccination based on virus-like particles

Induction of high frequencies of specific T cells by vaccination requires prime-boost regimens. To reach optimal immune responses, it is necessary to use different vectors for priming and boosting as e.g. DNA vaccination followed by boosting with a recombinant viral vector. Here, we show that vaccines based on virus-like particles (VLP) displaying peptide epitopes are equally effective to induce CTL responses if used in a homologous or heterologous prime-boost setting. Strikingly, high frequencies (>20% of CD8(+) cells) of protective CTL could be induced and maintained by weekly injection of VLP. Thus, the use of VLP may avoid the requirement for complicated heterologous prime-boost regimens, facilitating the development of effective T cell-based vaccines.

Hepatitis B and C Virus Variants in Long-Term Immunosuppressed Renal Transplant Patients in Latvia

The incidence of genome variants of hepatitis B and hepatitis C viruses among 38 long-term (2-15 years) immunosuppressed patients after renal transplantation and 10 patients undergoing dialysis was investigated. Twelve patients had only HBV infection, 9 had only HCV infection and 14 were co-infected. Regions corresponding to the HBV X/EnII/BCP, preC/C, preS/S and to the HCV core were sequenced for molecular characterization of the HBV and HCV genomes. Fifty-seven percent of HBV DNA isolates belonged to genotype D and 42% to genotype A, whereas 77% of HCV RNA isolates belonged to genotype 1b and only 17% to genotype 3a. One sample (6%) was of genotype 2c. Detailed analysis of the above-mentioned HBV genome regions revealed the presence of nucleotide point mutations, which, in some cases, resulted in amino acid substitutions. The clinical significance of such mutations is discussed.

A hantavirus nucleocapsid protein segment exposed on hepatitis B virus core particles is highly immunogenic in mice when applied without adjuvants or in the presence of pre-existing anti-core antibodies

Hepatitis B virus (HBV) core particles carrying the amino-terminal 120 amino acids (aa) of the nucleocapsid (N) protein of the hantaviruses Dobrava, Hantaan or Puumala have been demonstrated to be highly immunogenic in mice when complexed with adjuvants. Here we demonstrate that even without adjuvant, these chimeric particles induced high-titered, and strongly cross-reactive N-specific antibody responses in BALB/c and C57BL/6 mice. The induced N-specific antibodies represented all IgG subclasses. Pre-existing core-specific antibodies did not abrogate the induction of an N-specific immune response by a hantavirus N insert presented on core particles. Therefore, chimeric core particles should represent promising vaccine candidates even for anti-core positive humans.

Translation of hepatitis B virus (HBV) surface proteins from the HBV pregenome and precore RNAs in Semliki Forest virus-driven expression

Hepatitis B virus (HBV) pregenome RNA (pgRNA) serves as a translation template for the HBV core (HBc) protein and viral polymerase (Pol). HBV precore RNA (pcRNA) directs the synthesis of the precore (preC) protein, a precursor of the hepatitis B e antigen (HBeAg). pgRNA and pcRNA were expressed in the Semliki Forest virus (SFV) expression system. Besides the HBc and preC proteins, there was revealed the synthesis of all three forms of HBV surface (HBs) proteins: long (LHBs), middle (MHBs) and short (SHBs), the start codons of which are located more than 1000 nt downstream of the HBc and preC start codons. Moreover, other HBV templates, such as 3′-truncated pgRNA lacking 3′ direct repeat and Pol mRNA, both carrying internally the HBs sequences, provided the synthesis of three HBs protein forms in the SFV-driven expression system. Maximal production of the HBs was provided by Pol mRNA, while HBc- and preC-producing templates showed relatively low internal translation of the HBs. These data allow the proposal of a ribosome leaky scanning model of internal translation initiation for HBs proteins. The putative functional role of such exceptional synthesis of the HBs proteins from the pgRNA and pcRNA templates in the natural HBV infection process needs further evaluation.

Mosaic hepatitis B virus core particles presenting the complete preS sequence of the viral envelope on their surface

The sequence of the preS domain of the hepatitis B virus (HBV, genotype D) envelope was inserted into the major immunodominant region (MIR) of the C-terminally truncated HBV core (HBc) protein. In Escherichia coli, the HBc–preS fusion protein was partially soluble and did not produce particles. Co-expression of the wild-type HBc as a helper protein along with the fusion protein led to the formation of mosaic HBc particles that exhibited HBc, preS1 and preS2 antigenicity. Two alternative combinations of medium- and high-copy plasmids were used for co-expression of fusion and helper proteins, in an attempt to improve mosaic particle production. However, the preS fusion content of the particles remained the same in both expression combinations. In a third co-expression in which the modified HBc helper lacked aa 76–85 in the MIR, the incorporation level of HBc–preS fusion into the particles was noticeably lower. Purified chimeric particles were immunogenic in mice.

An amino-terminal segment of hantavirus nucleocapsid protein presented on hepatitis B virus core particles induces a strong and highly cross-reactive antibody response in mice

Previously, we have demonstrated that hepatitis B virus (HBV) core particles tolerate the insertion of the amino-terminal 120 amino acids (aa) of the Puumala hantavirus nucleocapsid (N) protein. Here, we demonstrate that the insertion of 120 amino-terminal aa of N proteins from highly virulent Dobrava and Hantaan hantaviruses allows the formation of chimeric core particles. These particles expose the inserted foreign protein segments, at least in part, on their surface. Analysis by electron cryomicroscopy of chimeric particles harbouring the Puumala virus (PUUV) N segment revealed 90% T = 3 and 10% T = 4 shells. A map computed from T = 3 shells shows additional density splaying out from the tips of the spikes producing the effect of an extra shell of density at an outer radius compared with wild-type shells. The inserted Puumala virus N protein segment is flexibly linked to the core spikes and only partially icosahedrally ordered. Immunisation of mice of two different haplotypes (BALB/c and C57BL/6) with chimeric core particles induces a high-titered and highly cross-reactive N-specific antibody response in both mice strains.

Mosaic particles formed by wild-type hepatitis B virus core protein and its deletion variants consist of both homo- and heterodimers

Co-expression in Escherichia coli of wild-type (wt) hepatitis B virus core protein (HBc) and its naturally occurring variants with deletions at amino acid positions 77-93 or 86-93 leads to formation of mosaic particles, which consist of three dimer subunit compositions. These compositions are wt/variant HBc heterodimers and two types of homodimers, formed by wt HBc or the variant HBc themselves. Mosaic particles were found also when both HBc deletion variants 77-93 and 86-93 were co-expressed in E. coli. These findings are discussed in terms of their significance for hepatitis B virus pathogenesis and prospective use of mosaic particles in vaccine development.

Stop codon insertion restores the particle formation ability of hepatitis B virus core-hantavirus nucleocapsid protein fusions

In recent years, epitopes of various origin have been inserted into the core protein of hepatitis B virus (HBc), allowing the formation of chimeric HBc particles. Although the C-terminus of a C-terminally truncated HBc (HBc) tolerates the insertion of extended foreign sequences, the insertion capacity is still a limiting factor for the construction of multivalent vaccines. Previously, we described a new system to generate HBc mosaic particles based on a read-through mechanism in an Escherichia coli suppressor strain [J Gen Virol 1997;78:2049-2053]. Those mosaic particles allowed the insertion of a 114-amino acid (aa)-long segment of a Puumala hantavirus (PUUV) nucleocapsid (N) protein. To study the value and the potential limitations of the mosaic approach in more detail, we investigated the assembly capacity of 'non-mosaic' HBc fusion proteins and the corresponding mosaic constructs carrying 94, 213 and 433 aa of the hantaviral N protein. Whereas the fusion proteins carrying 94, 114, 213 or 433 aa were not assembled into HBc particles, or only at a low yield, the insertion of a stop codon-bearing linker restored the ability to form particles with 94, 114 and 213 foreign aa. The mosaic particles formed exhibited PUUV-N protein antigenicity. Immunization of BALB/c mice with these mosaic particles carrying PUUV-N protein aa 1-114, aa 1-213 and aa 340-433, respectively, induced HBc-specific antibodies, whereas PUUV-N protein-specific antibodies were detected only in mice immunized with particles carrying N-terminal aa 1-114 or aa 1-213 of the N protein. Both the anti-HBc and anti-PUUV antibody responses were IgG1 dominated. In conclusion, stop codon suppression allows the formation of mosaic core particles carrying large-sized and 'problematic', e.g. hydrophobic, hantavirus sequences.

Molecular epidemiology and immunology of hepatitis B virus infection - an update

Hepatitis B virus (HBV) continues to be one of the most important viral pathogens in humans. This review provides an update on the molecular epidemiology and immunology of HBV infection. DNA sequencing has allowed replacement of the initial serotypic classification of HBV strains by a more systematic genotype system that currently consists of 7 members (genotypes A-G). More recently, sequence analysis of virus isolates from many individual patients has revealed the occurrence of certain mutational hot spots in the genome, some of which appear to correlate with the patient's immunological and/or disease status; however, cause and effect are not always easily discernible. This holds particularly for the issue of whether virus variants exist that have, per se, an increased pathogenic potential; due to the scarcity of appropriate experimental in vivo models, such hypotheses are difficult to prove. Similarly, because of the compact organization of the HBV genome, almost every single mutation may have pleiotropic phenotypic effects. Nonetheless, there is accumulating evidence that at least some frequently observed mutations are causally related to viral escape from selective pressures, such as the presence of antibodies against dominant B cell epitopes, or drugs that inhibit the viral reverse transcriptase; possibly, this is also true for the cellular immune response. Therefore, despite the availability of an effective prophylactic vaccine, further extensive efforts are required to monitor the emergence of vaccination- and therapy-resistant HBV variants and to prevent their spread in the general population.

An outbreak of HBV and HCV infection in a paediatric oncology ward: epidemiological investigations and prevention of further spread

Hospital-acquired hepatitis B (HBV) and C virus (HCV) infections continue to occur despite increased awareness of this problem among the medical community. One hundred six patients were infected in a haematology oncology ward for children, over the time period 1996 to 2000. Serum samples from 45 such patients and 3 from infected medical personnel were used for nucleic acid amplification. HBV core, as well as HCV core and hypervariable region 1 (HVR1) nucleotide sequences, were analysed by phylogenetic tree analysis, in order to characterise the epidemiological pattern of viral transmission on the ward. Samples from 32 patients were positive for HBV-DNA or HCV-RNA by PCR. Ten patients were positive for both markers. Seventeen out of twenty-three HCV core gene sequences were found to be evolutionarily related and clustered separately from other local sequences in the phylogenetic tree, indicating nosocomial transmission. This was confirmed by analysis of HVR1 gene sequences. One nurse and one physician from the ward were HCV RNA positive, but their HCV sequences were not related evolutionarily to those of the patient cluster. Fifteen out of nineteen HBV core gene sequences were also clustered together and were positioned separately in the relevant tree. Epidemiological investigation excluded a common source infection and indicated that spread of infection was most likely due to inappropriate infection control measures on the ward. No obvious risk factors for transmission were identified during the retrospective survey in patients with related sequences, except use of multidose vials for saline and poor staff compliance with routine hand hygiene procedures. The preventive measures that were introduced reduced the incidence of infection significantly. No new cases of HBV infection and only three anti-HCV seroconversions occurred over a period of 19 months. The introduction and maintenance of strict prevention measures over a 2 year period, combined with HBV vaccination, reduced significantly the incidence of new HCV and HBV infections.

Chimeric bacteriophage fr virus-like particles harboring the immunodominant C-terminal region of hamster polyomavirus VP1 induce a strong VP1-specific antibody response in rabbits and mice

The late region of the hamster polyomavirus (HaPyV, former HaPV) genome encodes three structural proteins VP1, VP2, and VP3, where VP1 represents the major capsid protein of 384 amino acids. Screening of sera from HaPyV-infected papilloma-bearing and papilloma-free hamsters demonstrated the immunodominant features of all three capsid proteins. For both groups of hamsters in the C-terminal region of VP1 immunodominant B-cell epitopes were identified in the regions between amino acids 305 and 351 and amino acids 351 and 384. The high flexibility of the C-terminal region of VP1 was confirmed by the formation of chimeric virus-like particles based on the coat protein of the RNA bacteriophage fr which was previously found to tolerate only very short-sized foreign insertions. Phage fr coat protein-derived virus-like particles tolerated the N-terminal fusion of amino acids 333-384, 351-384, 351-374, and 364-384, respectively, of VP1. The induction of VP1-specific antibodies in rabbits and mice by immunization with chimeric virus-like particles harboring amino acids 333-384, 351-384, and 364-384, respectively, of VP1 suggested the immunodominant nature of the C-terminal region of VP1.

Priming of cytotoxic T cell responses to exogenous hepatitis B virus core antigen is B cell dependent

The hepatitis B virus (HBV) core antigen (HBcAg) has a unique ability to bind a high frequency of naive human and murine B cells. The role of HBcAg-binding naive B cells in the immunogenicity of HBcAg is not clear. The HBcAg-binding properties of naive B cells were characterized using HBcAg particles with mutated spike region (residues 76-85) sequences. Deletion of residues 76-85 (HBcDelta76-85) destroyed naive B cell binding, whereas deletion of residues 79-85 did not. HBcAg particles with an Ile instead of the natural Ala at position 80 did not bind naive B cells, whereas reversion of Ile80-->Ala restored B cell binding. Destroying the B cell-binding ability of HBcAg had a marginal effect on the overall B cell immunogenicity of the different particles, suggesting that they were equally efficient in priming T helper cells. Therefore, the importance of HBcAg-binding B cells is studied with relation to the priming of HBcAg-specific cytotoxic T cells (CTLs). The role of HBcAg-binding B cells in the priming of HBcAg-specific CTLs was evaluated by immunization with endogenous HBcAg (DNA immunization) and exogenous recombinant HBcAg particles. Endogenous HBcAg primed HBcAg-specific CTLs in wild-type and B cell-deficient mice, whereas exogenous HBcAg primed HBcAg-specific CTLs only in wild-type mice. Importantly, HBcDelta76-85 did not prime CTLs despite the presence of B cells. Thus, the ability of exogenous HBcAg particles to prime specific CTLs is B cell dependent, suggesting a possible role for HBcAg-binding B cells in HBV infections.

Regulation of IgG antibody responses by epitope density and CD21-mediated costimulation

Epitope density and organization have been shown to be important factors for B cell activation in many animal model systems. However, it has been difficult to separate the role of antigen organization from the role of local antigen concentrations because highly organized antigens are usually particulate whereas non-organized antigens are more soluble. Hence, highly organized and non-organized antigens may interact with different cell types and in different locations within lymphoid organs. In order to assess the role of antigen organization in regulating B cell responses, we immunized mice with highly repetitive virus-like particles, which exhibit different epitope densities covalently attached to them. Therefore, the same particulate structure was used to present identical epitopes that differed in their degree of organization. Induction of epitope-specific IgM titers, reflecting early B cell activation, were unaffected by the degree of epitope density. Furthermore, the absence of Th cells or CD21/CD35 did not reduce the IgM response. In contrast, the degree of organization was a critical factor influencing the magnitude of the epitope-specific IgG response. Moreover, the threshold for IgG responses was shifted in the absence of CD21/CD35, resulting in the requirement for higher epitope densities to allow efficient IgG responses. Thus, IgG but not IgM responses are regulated by epitope density and B cell costimulatory thresholds.

A molecular assembly system that renders antigens of choice highly repetitive for induction of protective B cell responses

Virus like particles (VLPs) are known to induce potent B cell responses in the absence of adjuvants. Moreover, epitope-specific antibody responses may be induced by VLPs that contain peptides inserted in their immunodominant regions. However, due to steric problems, the size of the peptides capable of being incorporated into VLPs while still permitting capsid assembly, is rather limited. While peptides genetically fused to either the N- or C-terminus of VLPs present fewer assembly problems, the immune responses obtained against such epitopes are often limited, most likely because the epitopes are not optimally exposed. In addition, such particles may be less stable in vivo. Here, we show that peptides and proteins engineered to contain a free cys can be chemically coupled to VLPs formed from the hepatitis B core antigen (HBcAg) containing a lys in the immuno-dominant region. By using this approach steric hindrance of capsid assembly is abrogated. Peptides or protein coupled to VLPs in an oriented fashion are shown to induce strong and protective B cell responses even against self-epitopes in the absence of adjuvants. This molecular assembly system may be used to induce strong B cell responses against most antigens.