Native display of complete foreign protein domains on the surface of hepatitis B virus capsids

Papillomavirus-like particle based vaccines: cervical cancer and beyond

Non-infectious human papillomavirus-like particles (VLP), composed of the L1 major capsid protein, are under active development as vaccines to prevent cervical cancer. They would presumably function primarily by generating virion-neutralising antibodies against the genital human papillomavirus (HPV) types that are the principal cause of most cervical cancers. Early phase clinical studies indicate that the VLP vaccines are well tolerated and able to consistently induce high titres of virus type-specific neutralising antibodies. Two types of second-generation VLP-based subunit vaccines with therapeutic implications, both related and unrelated to papillomavirus infection, are in preclinical development. One type seeks to induce cell-mediated immune responses, especially cytotoxic lymphocytes (CTL), against non-structural papillomavirus proteins, proteins of other viruses, or tumour associated antigens. The target antigen is incorporated into the VLPs as a fusion protein of L1 or the L2 minor capsid protein. In mouse models, this approach has generated potent CTL responses after low dose vaccination in the absence adjuvant. The second type of therapeutic VLP-based vaccine seeks to generate autoantibodies to self-antigens. The display of self polypeptides in the context of the highly ordered array of repetitive elements on the papillomavirus VLP surface abrogates the ability of the humoral immune system to functionally distinguish between foreign and self. High titre and high avidity auto-reactive IgG antibodies have been induced to both soluble (TNF-alpha) and cell surface (CCR5) central self-antigens. Vaccines based on this approach could potentially be effective alternatives to monoclonal antibody (mAb)-based therapies for a variety of disease targets.

Peptide display on live MS2 phage: restrictions at the RNA genome level

The potential of the RNA phage MS2 to accommodate extra amino acids in its major coat protein has been examined. Accordingly, a pentapeptide was encoded in the genome as an N-terminal extension. In the MS2 crystal structure, this part of the coat protein forms a loop that extends from the outer surface of the icosahedral virion. At the RNA level, the insert forms a large loop at the top of an existing hairpin. This study shows that it is possible to maintain inserts in the coat protein of live phages. However, not all inserts were genetically stable. Some suffer deletions, while others underwent adaptation by base substitutions. Whether or not an insert is stable appears to be determined by the choice of the nucleic acid sequence used to encode the extra peptide. This effect was not caused by differential translation, because coat-protein synthesis was equal in wild-type and mutants. We conclude that the stability of the insert depends on the structure of the large RNA hairpin loop, as demonstrated by the fact that a single substitution can convert an unstable loop into a stable one.

Hepatitis B virus core antigen binds and activates naive human B cells in vivo: studies with a human PBL-NOD/SCID mouse model

The hepatitis B virus (HBV) core (HBc) antigen (HBcAg) is a highly immunogenic subviral particle. Studies with mice have shown that HBcAg can bind and activate B cells in a T-cell-independent fashion. By using a human peripheral blood leukocyte (hu-PBL)-Nod/LtSz-Prkdc(scid)/Prkdc(scid) (NOD/SCID) mouse model, we show here that HBcAg also activates human B cells in vivo in a T-cell-independent way. HBcAg was capable of inducing the secretion of HBcAg-binding human immunoglobulin M (IgM) in naive human B cells derived from adult human and neonatal (cord blood) donors when these hu-PBL were transferred directly into the spleens of optimally conditioned NOD/SCID mice. No such responses were found in chimeric mice that were given hu-PBL plus HBV e antigen or hu-PBL plus phosphate-buffered saline. In addition, HBcAg activated purified human B cells to produce anti-HBc IgM in the chimeric mice, thus providing evidence that HBcAg behaves as a T-cell-independent antigen in humans. However, HBcAg-activated hu-PBL from naive donors were unable to switch from IgM to IgG production, even after a booster dose of HBcAg. Production of HBcAg-specific IgG could only be induced when hu-PBL from subjects who had recovered from or had an ongoing chronic HBV infection were transferred into NOD/SCID mice. Our data suggest that humans also have a population of naive B cells that can bind HBcAg and is subsequently activated to produce HBcAg-binding IgM.

Individual rotavirus-like particles containing 120 molecules of fluorescent protein are visible in living cells

Rotaviruses are large, complex icosahedral particles consisting of three concentric capsid layers. When the innermost capsid protein VP2 is expressed in the baculovirus-insect cell system it assembles as core-like particles. The amino terminus region of VP2 is dispensable for assembly of virus-like particles (VLP). Coexpression of VP2 and VP6 produces double layered VLP. We hypothesized that the amino end of VP2 could be extended without altering the auto assembly properties of VP2. Using the green fluorescent protein (GFP) or the DsRed protein as model inserts we have shown that the chimeric protein GFP (or DsRed)-VP2 auto assembles perfectly well and forms fluorescent VLP (GFP-VLP2/6 or DsRed-VLP2/6) when coexpressed with VP6. The presence of GFP inside the core does not prevent the assembly of the outer capsid layer proteins VP7 and VP4 to give VLP2/6/7/4. Cryo-electron microscopy of purified GFP-VLP2/6 showed that GFP molecules are located at the 5-fold vertices of the core. It is possible to visualize a single fluorescent VLP in living cells by confocal fluorescent microscopy. In vitro VLP2/6 did not enter into permissive cells or in dendritic cells. In contrast, fluorescent VLP2/6/7/4 entered the cells and then the fluorescence signal disappear rapidly. Presented data indicate that fluorescent VLP are interesting tools to follow in real time the entry process of rotavirus and that chimeric VLP could be envisaged as "nanoboxes" carrying macromolecules to living cells.

Significant interference with hepatitis B virus replication by a core-nuclease fusion protein

Hepatitis B virus (HBV), a small DNA containing virus that replicates via reverse transcription, causes acute and chronic B-type hepatitis in humans. The limited success of current therapies for chronic infection has prompted exploration of alternative strategies. Capsid-targeted viral inactivation is a conceptually powerful approach that exploits virion structural proteins to target a degradative enzyme specifically into viral particles. Its principal feasibility has been demonstrated in retroviral model systems but not yet for a medically relevant virus outside the retrovirus family. Recently, we found that C proximal fusion to the HBV capsid protein of the Ca(2+)-dependent nuclease (SN) from Staphylococcus aureus yields a chimeric protein, coreSN, that in Escherichia coli coassembles with the wild-type capsid protein into particles with internal SN domains. Here we show that, in HBV co-transfected human hepatoma cells, less than 1 coreSN protein per 10 wild-type core protein subunits reduced titers of enveloped DNA containing virions by more than 95%. The antiviral effect depends on both an enzymatically active SN and on the core domain. CoreSN does not block assembly of RNA containing nucleocapsids but interferes with proper synthesis of viral DNA inside the capsid, or leads to rapid DNA degradation. Our data suggest an intracellular nuclease activation that, owing to the characteristics of HBV morphogenesis, is nonetheless highly virus specific. HBV may therefore be particularly vulnerable to the capsid-targeted viral inactivation approach.

Puumala (PUU) hantavirus strain differences and insertion positions in the hepatitis B virus core antigen influence B-cell immunogenicity and protective potential of core-derived particles

Hepatitis B virus (HBV) core-derived chimeric particles carrying a Puumala (PUU) hantavirus (strain Vranica/Hällnäs) nucleocapsid (N) protein sequence (aa 1-45), alternatively inserted at three distinct positions (N-, C-terminus, or the internal region), and mosaic particles consisting of HBV core as well as core/PUU (Vranica/Hällnäs) N (aa 1-45) readthrough protein were generated. Chimeric particles carrying the insert at the N-terminus or the internal region of core induced some protective immune response in bank voles (Clethrionomys glareolus) against a subsequent PUU virus (strain Kazan) challenge; 40-50% of the animals showed markers of protection. In contrast, internal insertion of PUU strain CG18-20 N (aa 1-45) into the HBV core caused a highly protective immune response in the bank vole model. Immunizations with particles carrying aa 75-119 of PUU (CG18-20) N at the C-terminus of core verified the presence of a second, minor protective region in the N protein. A strong PUU N-specific antibody response was detected not only in bank voles immunized with chimeric particles containing internal and N-terminal fusions of PUU N protein but also in animals immunized with the corresponding mosaic particles. Except for the exclusive occurrence of antibodies directed against aa 231-240 of N in non-protected animals post virus challenge, there was no additional obvious difference in the epitope-specificity of N-specific antibodies from immunized animals prior and post virus challenge.

Packaging of up to 240 subunits of a 17 kDa nuclease into the interior of recombinant hepatitis B virus capsids

The icosahedral nucleocapsid of hepatitis B virus (HBV) consists of multiple subunits of a single 183 amino acids (aa) core protein encasing the viral genome. However, recombinant core protein alone also forms capsid-like particles. We have recently shown that a 238 aa protein centrally inserted into the core protein can be displayed on the particle surface. Here we demonstrate that replacement of the C-terminal basic domain by the 17 kDa Staphylococcus aureus nuclease also yields particles but that in these the foreign domains are located in the interior. The packaged nuclease is enzymatically active, and the chimeric protein forms mosaic particles with the wild-type core protein. Hence the HBV capsid is useful as a molecular platform which, dependent on the fusion site, allows foreign protein domains to either be packaged into or be exposed on the exterior of the particle. These results are of relevance for the use of the HBV capsid as a vaccine carrier, and as a target for antiviral therapy.

Structure of a molluscan hemocyanin didecamer (HtH1 from Haliotis tuberculata) at 12 A resolution by cryoelectron microscopy

A 12 A resolution three-dimensional density map of the Haliotis tuberculata hemocyanin type 1 (HtH1) didecamer has been obtained by cryoelectron microscopy of unstained molecules and angular reconstitution. The dyad symmetry of the 8 MDa D5 HtH1 didecamer, formed by the pairing of two asymmetric 4 MDa ring-like C5 decamers, is emphasised. The major and minor surface helical grooves of the didecamer are well defined, in agreement with earlier data on molluscan hemocyanins. The location of the obliquely orientated repeating unit, a subunit dimer, within the decamer has been defined. Following interactive extraction of this dimer, several new structural features of the dimer and of the subunit have now emerged with improved detail. The subunit dimer possesses pseudo 2-fold symmetry, resulting from the steric arrangement of the wall domains/functional units (FUs-abcdef) of the two subunits. The arc and collar FUs (g and h) depart from this inherent 2-fold symmetry and are thereby responsible for the asymmetry of the C5 decamer, with the internalised collar/arc complex at one edge of the decamer. The FU heterodimers forming the wall morphological units have a hollow centre, and thus create a series of repeating channels that extend within the wall through all three tiers of the decamer. The connections between the wall and the arc are defined with improved clarity, and evidence is provided to indicate that the arc and collar FU pairs have a homodimeric composition (gg and hh, respectively). Two possibilities for the subunit path within the subunit dimer are presented, which correlate with the available structural, immunolabelling and protease cleavage data from HtH1 and other molluscan hemocyanins.

Polyhistidine-tagged hepatitis B core particles as carriers of HIV-1/gp120 epitopes of different HIV-1 subtypes

The hepatitis B core antigen is a widely accepted carrier particle to enhance the immunogenicity of foreign epitopes. From electron cryomicroscopy, the immunodominant region between amino acid positions 79 to 81 is known to protrude from the surface of the shells. It can be replaced by heterologous sequences without interfering with the particle-forming capacity in many cases. Here we have introduced various V3 sequences of the envelope protein of different subtypes (A, B, O) of HIV-1/gp120 in order to enhance their immunogenicity and broaden the immune response against the virus. To improve purification efficiency and solubility of the E. coli-expressed hybrids, six histidine residues were fused to amino acid 156. An adjustable purification scheme was utilised including denaturation, Ni(2+)-NTA affinity chromatography and particle renaturation under high salt conditions, resulting in highly pure antigen preparations. The hybrids reacted specifically with sera of HIV-1-infected patients. They further induced an autologous, subtype-specific anti-HIV-1 antibody response superior to that of Keyhole limpet-haemocyanine-coupled peptides.

Engineering cowpea mosaic virus RNA-2 into a vector to express heterologous proteins in plants

A series of new cowpea mosaic virus (CPMV) RNA-2-based expression vectors were designed. The jellyfish green fluorescent protein (GFP) was introduced between the movement protein (MP) and the large (L) coat protein or downstream of the small (S) coat protein. Release of the GFP inserted between the MP and L proteins was achieved by creating artificial processing sites each side of the insert, either by duplicating the MP-L cleavage site or by introducing a sequence encoding the foot-and-mouth disease virus (FMDV) 2A catalytic peptide. Eight amino acids derived from the C-terminus of the MP and 14-19 amino acids from the N-terminus of the L coat protein were necessary for efficient processing of the artificial Gln/Met sites. Insertion of the FMDV 2A sequence at the C-terminus of the GFP resulted in a genetically stable construct, which produced particles containing about 10 GFP-2A-L fusion proteins. Immunocapture experiments indicated that some of the GFP is present on the virion surface. Direct fusion of GFP to the C-terminus of the S coat protein resulted in a virus which was barely viable. However, when the sequence of GFP was linked to the C-terminus by an active FMDV 2A sequence, a highly infectious construct was obtained.

Methods for reconstructing density maps of "single" particles from cryoelectron micrographs to subnanometer resolution

Recently the resolution attainable in density maps calculated from cryo-electron micrographs of free-standing virus capsids has advanced to resolutions below 1 nm. This represents a significant milestone in that resolutions of this order potentially allow direct visualization of individual elements of protein secondary structure (i.e., alpha-helices), in addition to the shapes and connectivity of subdomains. We describe here a computational strategy for structural analyses at this level of detail: its principal innovation is a procedure for correcting the contrast transfer function of the electron microscope. Also important is the practice of combining data from pairs of differently defocused micrographs to improve the signal-to-noise ratio of the images, thereby allowing more precise determinations of the particles' orientations and origins and contributing to higher resolution reconstructions. These procedures proved instrumental in our analysis of the capsid of hepatitis B virus at 9-A resolution (Conway et al., 1997, Nature 386, 91-94). Finally, we discuss the prospects for achieving comparable resolutions for isolated macromolecular complexes with lower symmetry or no symmetry and for further extension of the resolution.

Analysis of macromolecular structure and dynamics by electron cryo-microscopy

Electron cryo-microscopy has yielded a wealth of detailed new information on structures of biological macromolecules ranging from alphabeta-tubulin at 3.7 A resolution to hepatitis B virus at 7.4 A resolution, as well as a number of membrane proteins at 6-8 A resolution. Much of this progress was made possible by recent advances in instrumentation and image processing techniques.

Insertional gene fusion technology

The classical 'end to end' gene fusion technique has widely been used for monitoring gene expression, biological screening and purification of recombinant proteins. Recent progress with the 'insertional' gene fusion approach, on the other hand, has demonstrated that this technique can be utilized for membrane protein topology analysis, display of randomized protein libraries and design of biosensor proteins. In this review, we describe examples of insertional gene fusion and compare the old and new gene fusion techniques.