From lethal virus to life-saving vaccine: developing inactivated vaccines for pandemic influenza

Cross-species transfer of viruses: implications for the use of viral vectors in biomedical research, gene therapy and as live-virus vaccines

All living organisms are continuously exposed to a plethora of viruses. In general, viruses tend to be restricted to the natural host species which they infect. From time to time viruses cross the host-range barrier expanding their host range. However, in very rare cases cross-species transfer is followed by the establishment and persistence of a virus in the new host species, which may result in disease. Recent examples of viruses that have crossed the species barrier from animal reservoirs to humans are hantavirus, haemorrhagic fever viruses, arboviruses, Nipah and Hendra viruses, avian influenza virus (AI), monkeypox virus, and the SARS-associated coronavirus (SARS-CoV). The opportunities for cross-species transfer of mammalian viruses have increased in recent years due to increased contact between humans and animal reservoirs. However, it is difficult to predict when such events will take place since the viral adaptation that is needed to accomplish this is multifactorial and stochastic. Against this background the intensified use of viruses and their genetically modified variants as viral gene transfer vectors for biomedical research, experimental gene therapy and for live-vector vaccines is a cause for concern. This review addresses a number of potential risk factors and their implications for activities with viral vectors from the perspective of cross-species transfer of viruses in nature, with emphasis on the occurrence of host-range mutants resulting from either cell culture or tropism engineering. The issues are raised with the intention to assist in risk assessments for activities with vector viruses.

Cloning of the chicken RNA polymerase I promoter and use for reverse genetics of influenza A viruses in avian cells

Reverse genetics techniques to rescue influenza viruses have thus far been based on the use of a human polymerase I (PolI) promoter to direct the synthesis of the eight viral RNAs. They can only be used on cells from primate origin due to the species specificity of the PolI promoter. Here we report the cloning of the chicken PolI promoter sequence and the generation of recombinant influenza virus upon transfection of bidirectional PolI/PolII plasmids in avian cells. Potential contributions of this new reverse genetics system in the fields of influenza virus research and influenza vaccine production are discussed.

Expression and purification of an influenza hemagglutinin--one step closer to a recombinant protein-based influenza vaccine

Numerous human infections with avian influenza viruses in Asia in recent years have raised the concern that the next influenza pandemic is imminent. The most effective way to combat influenza is through the vaccination of the public. However, a minimum of 3–6 months is needed to develop an influenza vaccine using the traditional egg-based vaccine approach. The influenza hemagglutinin protein (HA), the active ingredient in the current vaccine, can be expressed in insect cells using the baculovirus expression vector system and purified rapidly. An influenza vaccine based on such a recombinant antigen allows a more timely response to a potential influenza pandemic. Here, we report an innovative monitoring assay for recombinant HA (rHA) expression and a rapid purification process. Various biochemical analyses indicate that the purified rHA is properly folded and biologically active.

An improved reverse genetics system for influenza A virus generation and its implications for vaccine production

The generation of vaccines for highly pathogenic avian influenza viruses, including those of the H5N1 subtype, relies on reverse genetics, which allows the production of influenza viruses from cloned cDNA. In the future, reverse genetics will likely be the method of choice for the generation of conventional influenza vaccine strains because gene reassortment by more traditional methods is cumbersome. Established systems for the artificial generation of influenza A viruses require transfection of cells with the eight to 12 plasmids that provide the eight influenza viral RNAs as well as the polymerase and nucleoproteins of the virus. However, cell lines appropriate for human vaccine production (e.g., Vero cells) cannot be transfected with high efficiencies. To overcome these problems, we established a reverse genetics system in which the eight RNA polymerase I transcription cassettes for viral RNA synthesis are combined on one plasmid. Similarly, two cassettes encoding the hemagglutinin and neuraminidase segments and six cassettes encoding the remaining proteins were combined. We also combined three RNA polymerase II transcription cassettes for the expression of the polymerase subunits. By combining these cassettes, we reduced the number of plasmids required for virus generation significantly and produced influenza A virus in Vero cells with higher efficiency than with the traditional 12 plasmid system. This new system is thus suitable for influenza virus vaccine production and may be applicable to other reverse genetics systems that rely on the introduction of several plasmids into eukaryotic cells.

Avian influenza A (H5N1)

Since their reemergence in 2003, highly pathogenic avian influenza A (H5N1) viruses have reached endemic levels among poultry in several southeast Asian countries and have caused a still increasing number of more than 100 reported human infections with high mortality. These developments have ignited global fears of an imminent influenza pandemic. The current knowledge of the virology, clinical spectrum, diagnosis and treatment of human influenza H5N1 virus infections is reviewed herein.

A review of vaccine research and development: human acute respiratory infections

Worldwide, acute respiratory infections (ARIs) constitute the leading cause of acute illnesses, being responsible for nearly 4 million deaths every year, mostly in young children and infants in developing countries. The main infectious agents responsible for ARIs include influenza virus, respiratory syncytial virus (RSV), parainfluenza virus type 3 (PIV-3), Streptococcus pneumoniae and Haemophilus influenzae. While effective vaccines against influenza, H. influenzae type b (Hib) and S. pneumoniae infections have been available for several years, no vaccine is available at present against illnesses caused by RSV, PIV-3, metapneumovirus or any of the three novel coronaviruses. In addition, the threat constituted by the multiple outbreaks of avian influenza during the last few years is urgently calling for the development of new influenza vaccines with broader spectrum of efficacy, which could provide immunity against an avian influenza virus pandemic. This article reviews the state of the art in vaccine R&D against ARIs and attempts to address these basic public health questions.

Influenza: lessons from past pandemics, warnings from current incidents

Recent outbreaks of highly pathogenic avian influenza A virus infections (H5 and H7 subtypes) in poultry and in humans (through direct contact with infected birds) have had important economic repercussions and have raised concerns that a new influenza pandemic will occur in the near future. The eradication of pathogenic avian influenza viruses seems to be the most effective way to prevent influenza pandemics, although this strategy has not proven successful so far. Here, we review the molecular factors that contribute to the emergence of pandemic strains.

Newer respiratory virus infections: human metapneumovirus, avian influenza virus, and human coronaviruses

PURPOSE OF REVIEW

Recently, several previously unrecognized respiratory viral pathogens have been identified and several influenza A virus subtypes, previously known to infect poultry and wild birds, were transmitted to humans. Here we review the recent literature on these respiratory viruses.

RECENT FINDINGS

Human metapneumovirus has now been detected worldwide, causing severe respiratory tract illnesses primarily in very young, elderly and immunocompromised individuals. Animal models and reverse genetic techniques were designed for human metapneumovirus, and the first vaccine candidates have been developed. Considerable genetic and antigenic diversity was observed for human metapneumovirus, but the implication of this diversity for vaccine development and virus epidemiology requires further study. Two previously unrecognized human coronaviruses were discovered in 2004 in The Netherlands and Hong Kong. Their clinical impact and epidemiology are largely unknown and warrant further investigation. Several influenza A virus subtypes were transmitted from birds to humans, and these viruses continue to constitute a pandemic threat. The clinical symptoms associated with these zoonotic transmissions range from mild respiratory illnesses and conjunctivitis to pneumonia and acute respiratory distress syndrome, sometimes resulting in death. More basic research into virus ecology and evolution and development of effective vaccines and antiviral strategies are required to limit the impact of influenza A virus zoonoses and the threat of an influenza pandemic.

SUMMARY

Previously unknown and emerging respiratory viruses are an important threat to human health. Development of virus diagnostic tests, antiviral strategies, and vaccines for each of these pathogens is crucial to limit their impact.

Vaccines: past, present and future

The vaccines developed over the first two hundred years since Jenner's lifetime have accomplished striking reductions of infection and disease wherever applied. Pasteur's early approaches to vaccine development, attenuation and inactivation, are even now the two poles of vaccine technology. Today, purification of microbial elements, genetic engineering and improved knowledge of immune protection allow direct creation of attenuated mutants, expression of vaccine proteins in live vectors, purification and even synthesis of microbial antigens, and induction of a variety of immune responses through manipulation of DNA, RNA, proteins and polysaccharides. Both noninfectious and infectious diseases are now within the realm of vaccinology. The profusion of new vaccines enables new populations to be targeted for vaccination, and requires the development of routes of administration additional to injection. With all this come new problems in the production, regulation and distribution of vaccines.