In the middle 80s, recombinant hepatitis B virus cores (HBc) gave onset to icosahedral virus-like particles (VLPs) as a basic class of non-infectious carriers of foreign immunological epitopes. The recombinant HBc particles were used to display immunodominant epitopes of hepatitis B, C, and E virus, human rhinovirus, papillomavirus, hantavirus, and influenza virus, human and simian immunodeficiency virus, bovine and feline leukemia virus, foot-and-mouth disease virus, murine cytomegalovirus and poliovirus, and other virus proteins, as well as of some bacterial and protozoan protein epitopes. Practical applicability of the HBc particles as carriers was enabled by their ability to high level synthesis and correct self-assembly in heterologous expression systems. The interest in the HBc VLPs was reinforced by the resolution of their fine structure by electron cryomicroscopy and X-ray crystallography, which revealed an unusual α-helical organization of dimeric units of HBc shells, alternative packing into icosahedrons with T = 3 and T = 4 symmetry, and the existence of long protruding spikes. The tips of the latter seem to be the optimal targets for the display of foreign sequences up to 238 amino acid residues in length. Combination of numerous experimental data on epitope display with the precise structural information enables a knowledge-based design of diagnostic, and vaccine and gene therapy tools on the basis of the HBc particles.

Virus-like particle (VLP) technology is a promising approach for the construction of novel vaccines, diagnostic tools, and gene therapy vectors. Initially, VLPs were primarily derived from non-enveloped icosahedral or helical viruses and proved to be viable vaccine candidates due to their effective presentation of epitopes in a native conformation. VLP technology has also been used to prepare chimeric VLPs decorated with genetically fused or chemically coupled epitope stretches selected from immunologically defined target proteins. However, structural constraints associated with the rigid geometrical architecture of icosahedral or helical VLPs pose challenges for the expression and presentation of large epitopes. Complex VLPs derived from non-symmetric enveloped viruses are increasingly being used to incorporate large epitopes and even full-length foreign proteins. Pleomorphic VLPs derived from influenza or other enveloped viruses can accommodate multiple full-length and/or chimeric proteins that can be rationally designed for multifunctional purposes, including multivalent vaccines. Therefore, a second generation of VLP carriers is represented by complex particles reconstructed from natural or chimeric structural proteins derived from complex enveloped viruses. Further development of safe and efficient VLP nanotechnology may require a rational combination of both approaches.

Because it exhibits a remarkable capability to accept mutational intervention and undergo correct folding and selfassembly in all viable prokaryotic and eukaryotic expression systems, hepatitis B core (HBc) protein has been favored over other proposed particulate carriers. Structurally, the unusual K K-helical organization of HBc dimeric units allows introduction of foreign peptide sequences into several areas of HBc shells, including their most protruding spikes. Progress toward full resolution of the spatial structure as well as accumulation of chimeric HBc-based structures has brought closer the knowledge-based design of future vaccines, gene therapy tools and other artificial particulate objects.z 1999 Federation of European Biochemical Societies.

HBV core (HBc) particle is one of the most intensively studied particulate carriers for the insertion of foreign peptide sequences. Recombinant HBc protein expressed from the cloned gene undergoes the correct folding in a large variety of bacterial, yeast, insect and mammalian cells. Unique assembly properties and shape of 30/34-nm HBc particles allow substantial insertions into their primary structure without loss of their capsid-forming ability. N- and C-terminal regions, as well as the immunodominant loop in the middle of the molecule are widely accepted as targets for the introduction of foreign epitopes, ensuring retention and even enhancement of the original immunological activity of inserted sequences. Special sets of display vectors have been constructed on the basis of the cloned HBc gene. Epitope sequences of viral (BLV, FeLV, FMDV, HBV, HCV, HIV-1, HRV2, MCMV, PV-1, SIV) and nonviral (human chorionic gonadotropin) origin have been studied as model display moieties.