Table 2.
Main characteristics (indented text) of viral vectors (bold type) used in (or considered for) human studies15–17
| Retroviruses (diploid positive-strand RNA viruses that replicate through a DNA intermediate) |
|---|
| Only infect proliferating cells, and transfer their genetic information into the genome of the target cell, leading to a risk of insertional mutagenesis. Thus, their utilization as vectors in humans (e.g. lentiviral vectors derived from HIV-1) appears to be limited. Engineered to be replication defective and thus their production involves packaging cell lines expressing viral proteins necessary for their propagation. Prototypic oncoretroviruses such as the Moloney murine leukaemia virus, have been used in humans as gene delivery vectors not only for gene therapy, but also to genetically modify autologous cancer cells in order to create cellular cancer vaccines. Amphotropic retroviruses bind to a ubiquitous phosphate transporter expressed on most dividing mammalian cells. Cell specificity can be modified using ligand-directed targeting of viruses with a modified env gene15 |
| Poxviruses: vaccinia, MVA, NYVAC, ALVAC, fowlpox (large, enveloped viruses containing a linear double-stranded DNA genome) |
| Only DNA-containing viruses known to replicate within the cytoplasm of infected cells. Can accommodate large size inserts (30 kb), allowing for the engineering of recombinant viruses expressing multiple foreign gene products. The most frequently used poxvirus vectors, have been attenuated strains such as the Wyeth or Copenhagen vaccinia strains, MVA (modified vaccinia Ankara strain), NYVAC (derived from the Copenhagen strain by further deletion of 18 open reading frames encoding molecules implicated in pathogenicity and host-range regulatory functions). These vectors have been shown to be safe in immunocompromised macaques and in human phase I/II clinical trials. There were also able to elicit both humoral and cellular (lymphoproliferative and CTL) responses against antigens such as HIV env or gag, P. falciparum antigens, HPV16 E6/E7, tumour associated antigens (CEA, Muc1, gp100), etc. |
| Avipox viruses, such as the canarypox vector ALVAC (derived from the Kanapox strain) or the fowlpox virus, do not replicate in human cells. They can, nevertheless, be produced in fairly high yields using primary chicken embryo fibroblasts. These vectors have an excellent safety profile. Also, they elicit less anti-vector immunity than attenuated vaccinia strains and thus can be used for boosting several times15. No high-affinity receptor for poxviruses has been identified yet, even if the myxoma virus, a poxvirus that induces a lethal systemic disease in rabbits, is using chemokine receptors (i.e. CCR1, CCR5, CXCR4) to infect leukocytes |
| Adenovirus (medium size, icosahedral viruses containing a double-stranded linear DNA genome) |
| Replicate in the nucleus of infected cells without integration of viral DNA into the host genome The majority of adult people (i.e. ≥ 85%) have strong (or pre-existing) immune responses, both humoral and cellular, against the most common adenovirus serotypes thereby precluding multiple administrations. First-generation adenovirus-based vectors, based on deletion of the E1A and E1B genes and more recently vectors carrying a double E1 + E4 deletion, have been developed. These replication-defective adenoviral vectors can accommodate up to 7.5 kb of foreign genes and can be generated and amplified to high titres (1012 infectious units/ml) in 293 cells. Adenoviruses bind efficiently to both replicating and non-replicating epithelial cells following attachment of the viral fibre knob to a common receptor for coxsackie B viruses and adenovirus 2 and 5 (CAR)15 |
| Adeno-associated viral vector (single-stranded DNA virus) |
| AAV is not associated with any known human disease. It needs a helper virus (e.g. adenovirus or herpes simplex virus) to replicate, has a limited insert capacity (i.e. 4.5 kb) and is difficult to produce in large quantities15. It can stably integrate its genome in infected cells as double-stranded DNA in a site-specific manner in a locus on human chromosome 19. AAV can infect a wide range of cell types including non-dividing cells. Membrane-associated heparan sulphate proteoglycan was shown to be a receptor for AAV2 |
| Herpes simplex virus (enveloped virus containing a double-stranded DNA) |
| Replication-incompetent viruses were obtained following deletion of nine HSV-1 immediate early genes (including ICP4), allowing reduced vector cytotoxicity while allowing for the expression of multiple transgenes. In another approach, disabled infectious single-cycle herpes simplex (DISC-HSV) lacking the gene for the essential glycoprotein H (gH) vectors have been engineered, and tested in humans, both as a vaccine against HSV disease and as vehicles for cancer immunotherapy15 |
| Alphaviruses: Semliki Forest virus, Venezuelan equine encephalitis virus, (capped and polyadenylated single-stranded positive sense RNA of approximately 12 kb, surrounded by an icosahedral capsid protein shell) |
| Virus particles carrying replicons based on the viral replicase. This replicase copies the genome into negative strands which are templates for new positive-strand genomes and utilizes a highly active internal promoter to transcribe a sub-genomic mRNA encoding a foreign gene product. The vector replicons lack the virus structural protein genes and, therefore, are incapable of generating virus particles and causing a productive infection. Replicon-based vaccines produced in the form of either RNA, DNA, or infectious particles have been successfully used to elicit broad immune responses in animals, including primates. Alphaviruses are interesting vectors because of their high level of replication and gene expression and their ability to infect a variety of cell types. Furthermore, these suicidal vectors induce cell death and the release of apoptotic bodies that are efficiently taken up by antigen-presenting cells, thus enhancing immune cross-priming15 |
| Virus-like particles, from human papilloma viruses, rotaviruses, parvoviruses (self-assembling non-replicative capsid proteins) |
| VLPs based on L1 from HPV16 and HPV11 have been tested in humans as candidate vaccines against papilloma viruses associated with cervical cancer, or genital warts, respectively. Non-infectious papilloma virus-like particles bind and activate dendritic cells, leading to the induction of Th1 immune responses against inserted T-cell epitopes derived from the E7 antigen16. VLPs based on the VP2/VP6/VP7 capsid proteins (used alone or in combination) from rotavirus can be produced in SF9 insect cells. Such VLPs can be used as a vaccine against human or bovine rotaviruses, but also as a vehicle to express heterologous polypeptides (up to 600 amino acids long). The determination of the 3-D structure of VP6 had led to the identification of external loops which can be manipulated for insertion or substitution by heterologous polypeptides. VLPs from porcine parvoviruses can present heterologous T-cell epitopes and induce CTLs in mouse models. Following uptake by dendritic cells, VLPs induce DC maturation17 |