Protection against lethal lymphocytic choriomeningitis virus (LCMV) infection by immunization of mice with an influenza virus containing an LCMV epitope recognized by cytotoxic T lymphocytes

Recombinant influenza A viruses as vaccine vectors

INTRODUCTION

Various viruses, including poxviruses, adenoviruses and vesicular stomatitis virus, have been considered as vaccine vectors for the delivery of antigens of interest in the development of vaccines against newly emerging pathogens.

AREAS COVERED

Here, we review results that have been obtained with influenza A viruses (IAV) as vaccine vectors. With the advent of reverse genetics technology, IAV-based recombinant vaccine candidates have been constructed that induce protective immunity to a variety of different pathogens of interest, including West Nile virus, Plasmodium falciparum and respiratory syncytial virus. The various cloning strategies to produce effective and attenuated, safe to use IAV-based viral vectors are discussed.

EXPERT COMMENTARY

It was concluded that IAV-based vector system has several advantages and holds promise for further development.

Replication-incompetent rabies virus vector harboring glycoprotein gene of lymphocytic choriomeningitis virus (LCMV) protects mice from LCMV challenge

BACKGROUND

Lymphocytic choriomeningitis virus (LCMV) causes a variety of diseases, including asymptomatic infections, meningitis, and congenital infections in the fetus of infected mother. The development of a safe and effective vaccine against LCMV is imperative. This study aims to develop a new candidate vaccine against LCMV using a recombinant replication-incompetent rabies virus (RV) vector.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we have generated a recombinant deficient RV expressing the LCMV glycoprotein precursor (GPC) (RVΔP-LCMV/GPC) which is lacking the RV-P gene. RVΔP-LCMV/GPC is able to propagate only in cells expressing the RV-P protein. In contrast, the LCMV-GPC can be expressed in general cells, which do not express RV-P protein. The ability of RVΔP-LCMV/GPC to protect mice from LCMV infection and induce cellular immunity was assessed. Mice inoculated intraperitoneally with RVΔP-LCMV/GPC showed higher survival rates (88.2%) than those inoculated with the parental recombinant RV-P gene-deficient RV (RVΔP) (7.7%) following a LCMV challenge. Neutralizing antibody (NAb) against LCMV was not induced, even in the sera of surviving mice. CD8+ T-cell depletion significantly reduced the survival rates of RVΔP-LCMV/GPC-inoculated mice after the LCMV challenge. These results suggest that CD8+ T cells play a major role in the observed protection against LCMV. In contrast, NAbs against RV were strongly induced in sera of mice inoculated with either RVΔP-LCMV/GPC or RVΔP. In safety tests, suckling mice inoculated intracerebrally with RVΔP-LCMV/GPC showed no symptoms.

CONCLUSIONS/SIGNIFICANCE

These results show RVΔP-LCMV/GPC might be a promising candidate vaccine with dual efficacy, protecting against both RV and LCMV.

Induction of lung CD8+ T cell responses by consecutive inoculations of a poly(I:C) influenza vaccine

The cytotoxic T lymphocyte (CTL) response plays a key role in host recovery from influenza virus infection and in subsequent immunity. Compared to natural infection with influenza virus, however, intranasal vaccination with adjuvant-combined inactivated vaccine elicits only moderate CTL responses. Here we demonstrate that 5 days of consecutive, intranasal vaccination with a combination of inactivated influenza vaccine and poly(I:C) elicits a strong CTL response in the lung. Antigen-captured respiratory DCs did efficiently migrate from the lung to the mediastinal lymph node (mLN) after the 5 day series of inoculations with vaccine and poly(I:C). Importantly, formalin-inactivated whole virus vaccine and poly(I:C) adjuvant have synergic effects on consecutive vaccinations to elicit a strong CTL response in the lung. Although the CTL response was less effective against heterologous influenza virus, we show for the first time that intranasal administration of inactivated influenza virus vaccine and poly(I:C) for 5 consecutive days can elicit high levels of influenza virus-specific CD8⁺ T cells in the lung.

Designing optimal HIV-vaccine T-cell responses

PURPOSE OF REVIEW

T cells can efficaciously control HIV replication, and it has been hypothesized that inducing those responses before exposure occurs may prevent HIV infection. However, conventional attempts to generate protective CD8 T-cell responses against HIV have generally failed. Based on current knowledge from chronic HIV infection and previous vaccine trials, this review details optimal CD8 and CD4 T-cell response design that may confer protection from HIV infection.

RECENT FINDINGS

The failure of two vaccines geared toward inducing T-cell response (STEP trial and HVTN505/Phambili) as well as the modest protection of the RV144 that mainly demonstrated nonneutralizing antibodies to be a correlate of protection have rattled the idea that a pure T-cell-based vaccine may induce protection. Moreover, in the recent years, CD4 T cells, and in particular the T follicular helper cell subset, received attention as a critical component for T-cell-inducing and antibody-inducing vaccines.

SUMMARY

It is apparent that all vaccines depend for their efficacy on a cellular component either to directly kill virally infected cells or to provide important helper signals for the development of efficacious B-cell responses. Recent vaccine trials have had a major impact on the field and are guiding new approaches for HIV vaccine design.

Recombinant influenza virus carrying the respiratory syncytial virus (RSV) F85-93 CTL epitope reduces RSV replication in mice

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in infants worldwide. Despite decades of research, there is still no registered vaccine available for this major pathogen. We investigated the protective efficacy of a recombinant influenza virus, PR8/NA-F(85-93), that carries the RSV CD8(+) T cell epitope F(85-93) in its neuraminidase stalk. F(85-93)-specific cytotoxic T lymphocytes (CTLs) were induced in mice after a single intranasal immunization with PR8/NA-F(85-93) virus, and these CTLs provided a significant reduction in the lung viral load upon a subsequent challenge with RSV. To avoid influenza-induced morbidity, we treated mice with matrix protein 2 (M2e)-specific monoclonal antibodies before PR8/NA-F(85-93) virus infection. Treatment with anti-M2e antibodies reduced the infiltration of immune cells in the lungs upon PR8/NA-F(85-93) infection, whereas the formation of inducible bronchus-associated lymphoid tissue was not affected. Moreover, this treatment prevented body weight loss yet still permitted the induction of RSV F-specific T cell responses and significantly reduced RSV replication upon challenge. These results demonstrate that it is possible to take advantage of the infection-permissive protection of M2e-specific antibodies against influenza A virus to induce heterologous CD8(+) T cell-mediated immunity by an influenza A virus vector expressing the RSV F(85-93) epitope.

Engineering influenza viral vectors

The influenza virus is a respiratory pathogen with a negative-sense, segmented RNA genome. Construction of recombinant influenza viruses in the laboratory was reported starting in the 1980s. Within a short period of time, pioneer researchers had devised methods that made it possible to construct influenza viral vectors from cDNA plasmid systems. Herein, we discuss the evolution of influenza virus reverse genetics, from helper virus-dependent systems, to helper virus-independent 17-plasmid systems, and all the way to 3- and 1- plasmid systems. Successes in the modification of different gene segments for various applications, including vaccine and gene therapies are highlighted.

Viruses - from pathogens to vaccine carriers

Vaccination is mankind's greatest public health success story. By now vaccines to many of the viruses that once caused fatal childhood diseases are routinely used throughout the world. Traditional methods of vaccine development through inactivation or attenuation of viruses have failed for some of the most deadly human pathogens, necessitating new approaches. Genetic modification of viruses not only allows for their attenuation but also for incorporation of sequences from other viruses, turning one pathogen into a vaccine carrier for another. Recombinant viruses have pros and cons as vaccine carriers, as discussed below using vectors based on adenovirus, herpesvirus, flavivirus, and rhabdovirus as examples.

Recombinant influenza viruses as delivery vectors for hepatis B virus epitopes

Purpose

Neuraminidase (NA) of influenza virus contains stalk region that shows a great deal of variability in both amino acid sequence and length. In this paper, we investigated generation of recombinant influenza viruses that had hepatitis B virus (HBV) B cell epitopes in the NA stalk region as a dual vaccine candidate.

Materials and Methods

We used the WSH-HK reassortant helper virus for rescue of recombinant influenza virus containing HBV epitopes and reverse genetic protocol based on the use of micrococcal nuclease-treated virus cores for reconstitution of ribonucleoproteins.

Results

We successfully generated a chimeric influenza viruses which contained 22 amino acid peptides in the stalk region derived from the surface and pre-surface protein HBV. The growth kinetics of the recombinant viruses was investigated after infection of Madin-Darby canine kidney (MDCK) and Madin-Darby bovine kidney (MDBK) cells and the rIV-BVPreS virus showed higher titer than other viruses in MDCK cells. We also confirmed the presence of HBV epitopes in the chimeric viruses by enzyme-linked immunosorbent assay (ELISA) using anti-HBV polyclonal antibody. When the ratio of recombinant virus verse wild type virus was calculated by ELISA, recombinant viruses exhibited 2 fold higher values than the wild type virus.

Conclusion

These results suggest that chimeric influenza virus which contained foreign antigens can be used as dual vaccine against both HBV and influenza viruses.

A recombinant influenza A virus expressing domain III of West Nile virus induces protective immune responses against influenza and West Nile virus

West Nile virus (WNV) continues to circulate in the USA and forms a threat to the rest of the Western hemisphere. Since methods for the treatment of WNV infections are not available, there is a need for the development of safe and effective vaccines. Here, we describe the construction of a recombinant influenza virus expressing domain III of the WNV glycoprotein E (Flu-NA-DIII) and its evaluation as a WNV vaccine candidate in a mouse model. FLU-NA-DIII-vaccinated mice were protected from severe body weight loss and mortality caused by WNV infection, whereas control mice succumbed to the infection. In addition, it was shown that one subcutaneous immunization with 10⁵ TCID₅₀ Flu-NA-DIII provided 100% protection against challenge. Adoptive transfer experiments demonstrated that protection was mediated by antibodies and CD4+T cells. Furthermore, mice vaccinated with FLU-NA-DIII developed protective influenza virus-specific antibody titers. It was concluded that this vector system might be an attractive platform for the development of bivalent WNV-influenza vaccines.

A highly optimized DNA vaccine confers complete protective immunity against high-dose lethal lymphocytic choriomeningitis virus challenge

Protection against infection is the hallmark of immunity and the basis of effective vaccination. For a variety of reasons there is a great demand to develop new, safer and more effective vaccine platforms. In this regard, while 'first-generation' DNA vaccines were poorly immunogenic, new genetic 'optimization' strategies and the application of in vivo electroporation (EP) have dramatically boosted their potency. We developed a highly optimized plasmid DNA vaccine that expresses the lymphocytic choriomeningitis virus (LCMV) nucleocapsid protein (NP) and evaluated it using the LCMV challenge model, a gold standard for studying infection and immunity. When administered intramuscularly with EP, robust NP-specific cellular and humoral immune responses were elicited, the magnitudes of which approached those following acute LCMV infection. Furthermore, these responses were capable of providing 100% protection against a high-dose, normally lethal virus challenge. This is the first non-infectious vaccine conferring complete protective immunity up to 8 weeks after vaccination and demonstrates the potential of 'next-generation' DNA vaccines.

The effects of preexisting immunity to influenza on responses to influenza vectors in mice

The use of viral vectors as vaccine candidates has shown promise against a number of pathogens. However, preexisting immunity to these vectors is a concern that must be addressed when deciding which viruses are suitable for use. A number of properties, including the existence of antigenically distinct subtypes, make influenza viruses attractive candidates for use as viral vectors. Here, we evaluate the ability of influenza viral vectors containing inserts of foreign pathogens to elicit antibody and CD8(+) T cell responses against these foreign antigens in the presence of preexisting immunity to influenza virus in mice. Specifically, responses to an H3N1-based vector expressing a 90 amino acid polypeptide derived from the protective antigen (PA) of Bacillus anthracis or an H1N1-based vector containing a CD8(+) T cell epitope from the glycoprotein (GP) of lymphocytic choriomeningitis virus were evaluated following infections with either homosubtypic or heterosubtypic influenza viruses. We found that mice previously infected with influenza viruses, even those expressing HA and NA proteins of completely different subtypes, were severely compromised in their ability to mount an immune response against the inserted epitopes. This inhibition was demonstrated to be mediated by CD8(+) T cells, which recognize multiple strains of influenza viruses. These CD8(+) T cells were further shown to protect mice from a lethal challenge by a heterologous influenza subtype. The implication of these data for the use of influenza virus vectors and influenza vaccination in general are discussed.

A live bivalent influenza vaccine based on a H9N2 virus strain

The purpose of this study was to construct an H9N2 virus-based bivalent live vaccine expressing the protective antigen of a different subtype of influenza virus. Reverse genetics was used to generate an influenza virus containing nine gene segments derived from the A/Chicken/Jiangsu/11/2002 (H9N2) strain, including independent M1 and M2 matrix gene segments. A recombinant virus expressing the H1N1 HA1 hemagglutinin protein was produced on this framework by substituting the extracellular domain of the H9N2 M2 gene with the H1N1 HA1 fragment from A/PR/8/34 (PR8, H1N1). The resulting hybrid virus H9N2-PR8/HA1 was genetically stable and of low pathogenicity. Intra-nasal immunization of BALB/c mice with H9N2-PR8/HA1 virus induced both anti-H9N2 virus and anti-PR8 HA antibodies and conferred protection to mice against lethal challenge (40x LD(50)) with either H1N1 or H9N2 viruses. This study provides a new influenza H9N2 virus model for the expression and/or delivery of foreign antigens.

Cutting edge: local recall responses by memory T cells newly recruited to peripheral nonlymphoid tissues

Infection results in the formation of a circulating effector memory T cell population able to enter peripheral tissues either in the steady state or in response to localized infection. As a consequence, recall is thought to result from a phased response first involving those T cells already at the site of infection followed by the infiltration of memory cells from the wider circulation. We have recently reported that tissue-resident T cells can undergo stimulation and proliferation in response to local infection. In this study, we examine the proliferation of memory T cells newly recruited from the circulation. Our results show that although recruitment of circulating memory cells is nonspecific in nature, there is preferential proliferation of specific T cells within infected tissues. Thus, expansion represents a means of local Ag-specific enrichment of T cells recruited from a circulating memory pool of mixed specificities.

A seven-segmented influenza A virus expressing the influenza C virus glycoprotein HEF

Influenza viruses are classified into three types: A, B, and C. The genomes of A- and B-type influenza viruses consist of eight RNA segments, whereas influenza C viruses only have seven RNAs. Both A and B influenza viruses contain two major surface glycoproteins: the hemagglutinin (HA) and the neuraminidase (NA). Influenza C viruses have only one major surface glycoprotein, HEF (hemagglutinin-esterase fusion). By using reverse genetics, we generated two seven-segmented chimeric influenza viruses. Each possesses six RNA segments from influenza virus A/Puerto Rico/8/34 (PB2, PB1, PA, NP, M, and NS); the seventh RNA segment encodes either the influenza virus C/Johannesburg/1/66 HEF full-length protein or a chimeric protein HEF-Ecto, which consists of the HEF ectodomain and the HA transmembrane and cytoplasmic regions. To facilitate packaging of the heterologous segment, both the HEF and HEF-Ecto coding regions are flanked by HA packaging sequences. When introduced as an eighth segment with the NA packaging sequences, both viruses are able to stably express a green fluorescent protein (GFP) gene, indicating a potential use for these viruses as vaccine vectors to carry foreign antigens. Finally, we show that incorporation of a GFP RNA segment enhances the growth of seven-segmented viruses, indicating that efficient influenza A viral RNA packaging requires the presence of eight RNA segments. These results support a selective mechanism of viral RNA recruitment to the budding site.

Evolutionarily conserved protein sequences of influenza a viruses, avian and human, as vaccine targets

Background

Influenza A viruses generate an extreme genetic diversity through point mutation and gene segment exchange, resulting in many new strains that emerge from the animal reservoirs, among which was the recent highly pathogenic H5N1 virus. This genetic diversity also endows these viruses with a dynamic adaptability to their habitats, one result being the rapid selection of genomic variants that resist the immune responses of infected hosts. With the possibility of an influenza A pandemic, a critical need is a vaccine that will recognize and protect against any influenza A pathogen. One feasible approach is a vaccine containing conserved immunogenic protein sequences that represent the genotypic diversity of all current and future avian and human influenza viruses as an alternative to current vaccines that address only the known circulating virus strains.

Methodology/Principal Findings

Methodologies for large-scale analysis of the evolutionary variability of the influenza A virus proteins recorded in public databases were developed and used to elucidate the amino acid sequence diversity and conservation of 36,343 sequences of the 11 viral proteins of the recorded virus isolates of the past 30 years. Technologies were also applied to identify the conserved amino acid sequences from isolates of the past decade, and to evaluate the predicted human lymphocyte antigen (HLA) supertype-restricted class I and II T-cell epitopes of the conserved sequences. Fifty-five (55) sequences of 9 or more amino acids of the polymerases (PB2, PB1, and PA), nucleoprotein (NP), and matrix 1 (M1) proteins were completely conserved in at least 80%, many in 95 to 100%, of the avian and human influenza A virus isolates despite the marked evolutionary variability of the viruses. Almost all (50) of these conserved sequences contained putative supertype HLA class I or class II epitopes as predicted by 4 peptide-HLA binding algorithms. Additionally, data of the Immune Epitope Database (IEDB) include 29 experimentally identified HLA class I and II T-cell epitopes present in 14 of the conserved sequences.

Conclusions/Significance

This study of all reported influenza A virus protein sequences, avian and human, has identified 55 highly conserved sequences, most of which are predicted to have immune relevance as T-cell epitopes. This is a necessary first step in the design and analysis of a polyepitope, pan-influenza A vaccine. In addition to the application described herein, these technologies can be applied to other pathogens and to other therapeutic modalities designed to attack DNA, RNA, or protein sequences critical to pathogen function.

Attachment of infectious influenza A viruses of various subtypes to live mammalian and avian cells as measured by flow cytometry

At present there is much interest in the cell tropism and host range of influenza viruses, especially those of the H5N1 subtype. We wished to develop a method that would enable investigation of attachment of infectious virus through the interaction of the hemagglutinin molecule and live mammalian and avian cells and the subsequent infection of these cells. To this end, influenza viruses of various HA subtypes were constructed that either carry the green fluorescent protein (GFP) instead of the neuraminidase protein, or that express GFP in the cytoplasm of infected cells. The HA genes were derived from influenza viruses A/PR/8/34 (H1N1), A/Netherlands/178/95 (H3N2) and A/Vietnam/1194/04 (H5N1). Using these pairs of viruses, attachment and post-attachment events in the virus replication cycle can be distinguished. In general, the expression of NeuAc(alpha2-3)Gal or NeuAc(alpha2-6)Gal receptors on the cells tested corresponded with the attachment of the viruses that were studied with respect to predicted receptor specificity. Virus attachment was not always predictive for efficient infection of the cells.

Recombinant influenza virus vectors

Newly optimized reverse genetics techniques have allowed influenza researchers to generate recombinant influenza viruses expressing mutant viral proteins, as well as foreign proteins. Approaches include the insertion of noninfluenza epitopes and polypeptides into viral glycoproteins, foreign open reading frames as additional segments, and the fusion of independent proteins into viral genes encoding glycoproteins or the nonstructural protein 1. These genetically engineered viruses have been demonstrated to be good viral vectors for mounting B- and T-cell responses and are attractive candidates for vaccine development. As the molecular biology of influenza viral infection is more fully understood, influenza vectors can be concurrently manipulated to produce designed chimeric viruses, unveiling the possibility of a prosperous future with cheap, effective and safe vaccines against different human diseases.

Antigen-specific and non-specific CD4+ T cell recruitment and proliferation during influenza infection

To track epitope-specific CD4(+) T cells at a single-cell level during influenza infection, the MHC class II-restricted OVA(323-339) epitope was engineered into the neuraminidase stalk of influenza/A/WSN, creating a surrogate viral antigen. The recombinant virus, influenza A/WSN/OVA(II), replicated well, was cleared normally, and stimulated both wild-type and DO11.10 or OT-II TCR transgenic OVA-specific CD4(+) T cells. OVA-specific CD4 T cells proliferated during infection only when the OVA epitope was present. However, previously primed (but not naive) transgenic CD4(+) T cells were recruited to the infected lung both in the presence and absence of the OVA(323-339) epitope. These data show that, when primed, CD4(+) T cells may traffic to the lung in the absence of antigen, but do not proliferate. These results also document a useful tool for the study of CD4 T cells in influenza infection.

Chimeric influenza virus hemagglutinin proteins containing large domains of the Bacillus anthracis protective antigen: protein characterization, incorporation into infectious influenza viruses, and antigenicity

Large polypeptides of the Bacillus anthracis protective antigen (PA) were inserted into an influenza A virus hemagglutinin glycoprotein (HA), and the chimeric proteins were functionally characterized and incorporated into infectious influenza viruses. PA domain 1', the region responsible for binding to the other toxin components, the lethal factor and edema factor, and domain 4, the receptor binding domain (RBD), were inserted at the C-terminal flank of the HA signal peptide and incorporated into the HA1 subunit of HA. The chimeric proteins, designated as LEF/HA (90 amino acid insertion) and RBD/HA (140 amino acid insertion), were initially analyzed following expression using recombinant vaccinia viruses. Both chimeric proteins were shown to display functional phenotypes similar to that of the wild-type HA. They transport to the cell surface, can be cleaved into the HA1 and HA2 subunits by trypsin to activate membrane fusion potential, are able to undergo the low-pH-induced conformational changes required for fusion, and are capable of inducing the fusion process. We were also able to generate recombinant influenza viruses containing the chimeric RBD/HA and LEF/HA genes, and the inserted PA domains were maintained in the HA gene segments following several passages in MDCK cells or embryonated chicken eggs. Furthermore, DNA immunization of mice with plasmids that express the chimeric RBD/HA and LEF/HA proteins, and the recombinant viruses containing them, induced antibody responses against both the HA and PA components of the protein. These approaches may provide useful tools for vaccines against anthrax and other diseases.

Consequences of immunodominant epitope deletion for minor influenza virus-specific CD8+-T-cell responses

The extent to which CD8+ T cells specific for other antigens expand to compensate for the mutational loss of the prominent DbNP366 and DbPA224 epitopes has been investigated using H1N1 and H3N2 influenza A viruses modified by reverse genetics. Significantly increased numbers of CD8+ KbPB1(703)+, CD8+ KbNS2(114)+, and CD8+ DbPB1-F2(62)+ T cells were found in the spleen and in the inflammatory population recovered by bronchoalveolar lavage from mice that were first given the -NP-PA H1N1 virus intraperitoneally and then challenged intranasally with the homologous H3N2 virus. The effect was less consistent when this prime-boost protocol was reversed. Also, though the quality of the response measured by cytokine staining showed some evidence of modification when these minor CD8+-T-cell populations were forced to play a more prominent part, the effects were relatively small and no consistent pattern emerged. The magnitude of the enhanced clonal expansion following secondary challenge suggested that the prime-boost with the -NP-PA viruses gave a response overall that was little different in magnitude from that following comparable exposure to the unmanipulated viruses. This was indeed shown to be the case when the total response was measured by ELISPOT analysis with virus-infected cells as stimulators. More surprisingly, the same effect was seen following primary challenge, though individual analysis of the CD8+ KbPB1(703)+, CD8+ KbNS2(114)+, and CD8+ DbPB1-F2(62)+ sets gave no indication of compensatory expansion. A possible explanation is that novel, as yet undetected epitopes emerge following primary exposure to the -NP-PA deletion viruses. These findings have implications for both natural infections and vaccines.

The virus-immunity ecosystem

The ecology of pathogenic viruses can be considered both in the context of survival in the macro-environments of nature, the theme pursued generally by epidemiologists, and in the micro-environments of the infected host. The long-lived, complex, higher vertebrates have evolved specialized, adaptive immune systems designed to minimise the consequences of such parasitism. Through evolutionary time, the differential selective pressures exerted variously by the need for virus and host survival have shaped both the "one-host" viruses and vertebrate immunity. With the development of vaccines to protect us from many of our most familiar parasites, the most dangerous pathogens threatening us now tend to be those "emerging", or adventitious, infectious agents that sporadically enter human populations from avian or other wild-life reservoirs. Such incursions must, of course, have been happening through the millenia, and are likely to have led to the extraordinary diversity of recognition molecules, the breadth in effector functions, and the persistent memory that distinguishes the vertebrate, adaptive immune system from the innate response mechanisms that operate more widely through animal biology. Both are important to contemporary humans and, particularly in the period immediately following infection, we still rely heavily on an immediate response capacity, elements of which are shared with much simpler, and more primitive organisms. Perhaps we will now move forward to develop useful therapies that exploit, or mimic, such responses. At this stage, however, most of our hopes for minimizing the threat posed by viruses still focus on the manipulation of the more precisely targeted, adaptive immune system.

Influenza B virus requires BM2 protein for replication

The BM2 protein of influenza B virus functions as an ion channel, which is suggested to be important for virus uncoating in endosomes of virus-infected cells. Because direct support for this function is lacking, whether BM2 plays an essential role in the viral life cycle remains unknown. We therefore attempted to generate BM2 knockout viruses by reverse genetics. Mutant viruses possessing M segments with the mutated initiation codon of BM2 protein at the stop-start pentanucleotide were viable and still expressed BM2. The introduction of multiple stop codons and a one-nucleotide deletion downstream of the stop-start pentanucleotide, in addition to disablement of the BM2 initiation codon, failed to generate viable mutant viruses, but the mutant M segments still expressed proteins that reacted with the BM2 peptide antiserum. To completely abolish BM2 expression, we generated a mutant M gene whose BM2 open reading frame was deleted. Although this mutant was not able to replicate in normal MDCK cells, it did replicate in a cell line that we established which constitutively expresses BM2. Furthermore, a virus possessing the mutant M gene lacking the BM2 open reading frame and a mutant NA gene containing the BM2 open reading frame instead of the NA open reading frame underwent multiple cycles of replication in MDCK cells, with exogenous sialidase used to supplement the deleted viral sialidase activity. These findings demonstrate that the BM2 protein is essential for influenza B virus replication.

Characterization of a neuraminidase-deficient influenza a virus as a potential gene delivery vector and a live vaccine

We recently identified a packaging signal in the neuraminidase (NA) viral RNA (vRNA) segment of an influenza A virus, allowing us to produce a mutant virus [GFP(NA)-Flu] that lacks most of the NA open reading frame but contains instead the gene encoding green fluorescent protein (GFP). To exploit the expanding knowledge of vRNA packaging signals to establish influenza virus vectors for the expression of foreign genes, we studied the replicative properties of this virus in cell culture and mice. Compared to wild-type virus, GFP(NA)-Flu was highly attenuated in normal cultured cells but was able to grow to a titer of >10(6) PFU/ml in a mutant cell line expressing reduced levels of sialic acid on the cell surface. GFP expression from this virus was stable even after five passages in the latter cells. In intranasally infected mice, GFP was detected in the epithelial cells of nasal mucosa, bronchioles, and alveoli for up to 4 days postinfection. We attribute the attenuated growth of GFP(NA)-Flu to virion aggregation at the surface of bronchiolar epithelia. In studies to test the potential of this mutant as a live attenuated influenza vaccine, all mice vaccinated with >/==" BORDER="0">10(5) PFU of GFP(NA)-Flu survived when challenged with lethal doses of the parent virus. These results suggest that influenza virus could be a useful vector for expressing foreign genes and that a sialidase-deficient virus may offer an alternative to the live influenza vaccines recently approved for human use.

Mucosal and systemic immune responses to a human immunodeficiency virus type 1 epitope induced upon vaginal infection with a recombinant influenza A virus

The humoral and cellular immune responses in the genital mucosa likely play an important role in the prevention of sexually transmitted infections, including infection with human immunodeficiency virus type 1 (HIV-1). Here we show that vaginal infection of progesterone-treated BALB/c mice with a recombinant influenza virus bearing the immunodominant P18IIIB cytotoxic T-lymphocyte (CTL) epitope of the gp160 envelope protein from an HIV-1 IIIB isolate (P18IIIB; RIQRGPGRAFVTIGK) can induce a specific immune response in regional mucosal lymph nodes, as well as in a systemic site (the spleen). A single inoculation of mice with the recombinant influenza virus induced long-lasting (at least 5 months) antigen-specific CTL memory detectable as a rapid recall of effector CTLs upon vaginal infection with recombinant vaccinia virus expressing HIV-1 IIIB envelope gene products. Long-term antigen-specific CTL memory was also induced and maintained in distant mucosal tissues when mice were intranasally immunized with the recombinant influenza virus. These results indicate that mucosal immunization and, in particular, local vaginal immunization with recombinant influenza virus can provide strong, durable immune responses in the female genital tract of mice.

Toward novel vaccines and therapies based on negative-strand RNA viruses

The study of negative-strand RNA viruses has suggested new strategies to produce more attenuated viruses. Reverse genetics has allowed the implementation of the strategies, and new or improved monovalent vaccines are being developed. In addition, recombinant viruses expressing foreign proteins or epitopes have been produced with the aim of developing multivalent vaccines capable of stimulating humoral and cellular immune responses against more than one pathogen. Finally, recombinant viruses that selectively enter cells expressing tumor markers or the HIV envelope protein have been engineered and shown to lyse target cells. Preclinical and clinical trials of improved and multivalent vaccines and therapeutic (oncolytic) viruses are ongoing.

Selective incorporation of influenza virus RNA segments into virions

The genome of influenza A virus is comprised of eight viral RNA (vRNA) segments. Although the products of all eight vRNA segments must be present for viral replication, little is known about the mechanism(s) responsible for incorporation of these segments into virions. Two models have been proposed for the generation of infectious virions containing eight vRNA segments. The random-incorporation model assumes a common structural feature in all the vRNAs, enabling any combination of vRNAs to be incorporated randomly into virions. The selective-incorporation model predicts the presence of specific structures in each vRNA segment, leading to the incorporation of a set of eight vRNA segments into virions. Here we demonstrate that eight different vRNA segments must be present for efficient virion formation and that sequences within the coding region of (and thus unique to) the neuraminidase vRNA possess a signal that drives incorporation of this segment into virions. These findings indicate a unique contribution from individual vRNA segments and thus suggest a selective (rather than random) mechanism of vRNA recruitment into virions. The neuraminidase vRNA incorporation signal and others yet to be identified should provide attractive targets for the attenuation of influenza viruses in vaccine production and the design of new antiviral drugs.

The early expression of glycoprotein B from herpes simplex virus can be detected by antigen-specific CD8+ T cells

The immune response to cutaneous herpes simplex virus type 1 (HSV-1) infection begins with remarkable rapidity. Activation of specific cytotoxic T lymphocytes (CTL) begins within hours of infection, even though the response within the draining lymph nodes peaks nearly 5 days later. HSV gene products are classified into three main groups, alpha, beta, and gamma, based on their kinetics and requirements for expression. In C57BL/6 mice, the immunodominant epitope from HSV is derived from glycoprotein B (gB(498-505)). While gB is considered a gamma or "late" gene product, previous reports have indicated that some level of gene expression may occur soon after infection. Using brefeldin A as a specific inhibitor of viral antigen presentation to major histocompatibility complex class I-restricted CTL, we have formally addressed the timing of gB peptide expression in an immunologically relevant manner following infection. Presentation of gB peptide detected by T-cell activation was first observed within 2 h of infection. Comparison with another viral epitope expressed early during infection, HSV-1 ribonucleotide reductase, demonstrated that gB is presented with the same kinetics as this classical early-gene product. Moreover, this rapidity of gB expression was further illustrated via rapid priming of naïve transgenic CD8(+) T cells in vivo after HSV-1 infection of mice. These results establish that gB is expressed rapidly following HSV-1 infection, at levels capable of effectively stimulating CD8(+) T cells.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

Induction of specific CD8+ memory T cells and long lasting protection following immunization with Salmonella typhimurium expressing a lymphocytic choriomeningitis MHC class I-restricted epitope

Numerous studies have shown the potential of Salmonella typhimurium as a vector for delivery of heterologous proteins for vaccination against other pathogens. Earlier studies showed that the inefficient elicitation of MHC class I-restricted responses could limit the use of S. typhimurium as a heterologous antigen delivery vector for vaccination. We recently developed an approach to overcome this limitation by using a bacterial-encoded specialized protein secretion system, termed type III, to deliver proteins into the class I antigen presenting pathways. Thus, peptides of interest fused to proteins bearing the type III secretion signal, which can elicit protective CTL responses. Because protective immunity is usually assessed a few weeks after vaccination, there is a paucity of information regarding duration of protective immunity induced by this system. We show here that mice immunized orally with S. typhimurium vectors expressing a MHC class I-restricted epitope of the lymphocytic choriomeningitis virus (LCMV) nucleoprotein developed specific antiviral CTL responses. CD8+ T cells were found to be necessary for this CTL activity against targets presenting the LCMV epitope. The survival of mice challenged with lethal doses of LCMV 60 or 135 days after vaccination was as complete as the survival of mice challenged 2 weeks after immunization with the same vectors. By demonstrating their ability to induce prolonged protective immunity after oral delivery, S. typhimurium vectors have met an essential requirement in support of their development as vectors for heterologous vaccination.

Postexposure vaccination massively increases the prevalence of gamma-herpesvirus-specific CD8+ T cells but confers minimal survival advantage on CD4-deficient mice

Mice that lack CD4(+) T cells remain clinically normal for more than 60 days after respiratory challenge with the murine gamma-herpesvirus 68 (gammaHV-68), then develop symptoms of a progressive wasting disease. The gammaHV-68-specific CD8(+) T cells that persist in these I-A(b-/-) mice are unable to prevent continued, but relatively low level, virus replication. Postexposure challenge with recombinant vaccinia viruses expressing gammaHV-68 lytic cycle epitopes massively increased the magnitude of the gammaHV-68-specific CD8(+) population detectable by staining with tetrameric complexes of MHC class I glycoprotein + peptide, or by interferon-gamma production subsequent to in vitro restimulation with peptide. The boosting effect was comparable for gammaHV-68-infected I-A(b-/-) and I-A(b+/+) mice within 7 days of challenge, and took more than 110 days to return to prevaccination levels in the I-A(b+/+) controls. Although the life-span of the I-A(b-/-) mice was significantly increased, there was no effect on long-term survival. A further boost with a recombinant influenza A virus failed to improve the situation. Onset of weight loss was associated with a decline in gammaHV-68-specific CD8(+) T cell numbers, though it is not clear whether this was a cause or an effect of the underlying pathology. Even very high levels of virus-specific CD8(+) T cells thus provide only transient protection against the uniformly lethal consequences of gammaHV-68 infection under conditions of CD4(+) T cell deficiency.

Transient bicistronic vRNA segments for indirect selection of recombinant influenza viruses

The 5'- and 3'-terminal regions of influenza vRNA molecules are known to constitute the promoter structure upon association with viral RNA polymerase in an activated complementary conformation. An inherent requirement for their location at the very ends of the vRNA molecules always has been implied because of that natural structure, but this study demonstrates that one or both of the promoter sequences may be relocated into vRNA-internal positions and still retain their polymerase-binding function. External extensions of vRNA molecules employed include either single-stranded RNA sequences </=750 nucleotides in length or complementary, and hence potentially double-stranded sequences, or promoter duplications. 5' RACE analyses of internally promoted cRNA and mRNA molecules prove initiation to occur at exactly the 3' standard template position 1, as defined by the regular promoter structure. Thereby any template extensions are lost from the resulting RNA molecules and progeny virions. These observations have been used to construct bicistronic vRNAs with an additional 3'-promoter sequence located between the two reading frames. During propagation, these spontaneously give rise to monocistronic vRNAs upon internal initiation reactions. Accordingly designed bicistronic vRNAs can be employed for indirectly selecting any foreign gene encoded in the resulting monocistronic vRNA for incorporation into recombinant influenza viruses.

A gamma-herpesvirus sneaks through a CD8(+) T cell response primed to a lytic-phase epitope

To determine whether established CD8(+) T cell memory to an epitope prominent during the replicative phase of a gamma-herpesvirus infection protects against subsequent challenge, mice were primed with a recombinant vaccinia virus expressing the p56 peptide and then boosted by intranasal exposure to an influenza A virus incorporating p56 in the neuraminidase protein. Clonally expanded populations of functional, p56-specific CD8(+) T cells were present at high frequency in both the lung and the lymphoid tissue 1 month later, immediately before respiratory challenge with gammaHV-68. This prime-and-boost regime led to a massive reduction of productive gammaHV-68 infection in the respiratory tract and, initially, to much lower levels of latency in both the regional lymph nodes and the spleen. The CD8(+) T cell response to another epitope (p79) was diminished, there was less evidence of B cell activation, and the onset of the CD4(+) T cell-dependent splenomegaly was delayed. Within 3-4 weeks of the gammaHV-68 challenge, however, the extent of latent infection in the lymph nodes and spleen was equivalent, and both groups developed the prominent infectious mononucleosis-like syndrome that is characteristic of this infection. The reverse protocol (influenza then vaccinia) seemed to be slightly less effective. Even though immune CD8(+) T cells may be present at the time and site of virus challenge, establishing a high level of CD8(+) T cell memory to lytic-phase epitopes alone does not protect against the longer-term consequences of this gammaHV infection.

Immunization with a single major histocompatibility complex class I-restricted cytotoxic T-lymphocyte recognition epitope of herpes simplex virus type 2 confers protective immunity

We have evaluated the potential of conferring protective immunity to herpes simplex virus type 2 (HSV-2) by selectively inducing an HSV-specific CD8(+) cytotoxic T-lymphocyte (CTL) response directed against a single major histocompatibility complex class I-restricted CTL recognition epitope. We generated a recombinant vaccinia virus (rVV-ES-gB498-505) which expresses the H-2Kb-restricted, HSV-1/2-cross-reactive CTL recognition epitope, HSV glycoprotein B residues 498 to 505 (SSIEFARL) (gB498-505), fused to the adenovirus type 5 E3/19K endoplasmic reticulum insertion sequence (ES). Mucosal immunization of C57BL/6 mice with this recombinant vaccinia virus induced both a primary CTL response in the draining lymph nodes and a splenic memory CTL response directed against HSV gB498-505. To determine the ability of the gB498-505-specific memory CTL response to provide protection from HSV infection, immunized mice were challenged with a lethal dose of HSV-2 strain 186 by the intranasal (i.n.) route. Development of the gB498-505-specific CTL response conferred resistance in 60 to 75% of mice challenged with a lethal dose of HSV-2 and significantly reduced the levels of infectious virus in the brains and trigeminal ganglia of challenged mice. Finally, i.n. immunization of C57BL/6 mice with either a recombinant influenza virus or a recombinant vaccinia virus expressing HSV gB498-505 without the ES was also demonstrated to induce an HSV-specific CTL response and provide protection from HSV infection. This finding confirms that the induction of an HSV-specific CTL response directed against a single epitope is sufficient for conferring protective immunity to HSV. Our findings support the role of CD8(+) T cells in the control of HSV infection of the central nervous system and suggest the potential importance of eliciting HSV-specific mucosal CD8(+) CTL in HSV vaccine design.

Negative-strand RNA viruses: applications to biotechnology

The establishment of reverse-genetics techniques to manipulate the genome of negative-strand RNA viruses has contributed enormously to a better understanding of the replication mechanisms and pathogenicity of this group of viruses. The generation of recombinant viruses bearing specific mutations in the coding and noncoding regions of their genomic RNAs now allows the functions in the replicative cycle of specific RNA regions and protein domains of these viruses to be studied. In addition, recombinant negative-strand RNA viruses can now be designed to have specific properties that make them attractive biotechnological tools.

DNA vaccination with cytokine fusion constructs biases the immune response to ovalbumin

DNA vaccination may work through direct transfection of antigen presenting cells (APC), or by secretion of the encoded protein by muscle or skin cells for uptake by APC. If cytokines are attached to the antigen, they may influence APC or responding T cells to drive the response toward a Th1 or Th2 direction, and/or potentiate it in an antigen-specific manner. To test this concept, expression vectors were constructed containing the ovalbumin (OVA) gene either alone, or linked to cytokine genes including GM-CSF, IFN-gamma, IL-2, IL-4, IL-12, or a sequence encoding nine amino acids of IL-1 beta. These constructs expressed OVA-cytokine fusion proteins in vitro which retained cytokine bioactivity. C57BL/6 mice were injected intramuscularly with the DNA constructs. Little if any OVA-specific antibody was produced in response to any of the DNA constructs, except for OVA-IL-4. However, lymphocytes from BALB/c mice vaccinated with OVA-IL-12 and OVA-IL-1 beta constructs produced more IFN-gamma and less IL-4 during in vitro restimulation assays than did other groups. All constructs elicited OVA-specific cytotoxic responses which were maintained or even increased over 16 weeks. The OVA-IL-12 and OVA-IL-1 beta peptide constructs elicited the strongest cytotoxic responses at 2 weeks postinjection. Cytotoxic responses were seen in all animals, even those lacking OVA-specific Ab, and were not related to Ab level. These studies indicate that the humoral, cytokine, and cytotoxic responses to DNA vaccination can be effectively altered by certain cytokine fusion constructs.

Recombinant parvovirus-like particles as an antigen carrier: a novel nonreplicative exogenous antigen to elicit protective antiviral cytotoxic T cells

To develop a strategy that promotes efficient antiviral immunity, hybrid virus-like particles (VLP) were prepared by self-assembly of the modified porcine parvovirus VP2 capsid protein carrying a CD8(+) T cell epitope from the lymphocytic choriomeningitis virus nucleoprotein. Immunization of mice with these hybrid pseudoparticles, without adjuvant, induced strong cytotoxic T lymphocyte (CTL) responses against both peptide-coated- or virus-infected-target cells. This CD8(+) class I-restricted cytotoxic activity persisted in vivo for at least 9 months. Furthermore, the hybrid parvovirus-like particles were able to induce a complete protection of mice against a lethal lymphocytic choriomeningitis virus infection. To our knowledge, this study represents the first demonstration that hybrid nonreplicative VLP carrying a single viral CTL epitope can induce protection against a viral lethal challenge, in the absence of any adjuvant. These recombinant particles containing a single type of protein are easily produced by the baculovirus expression system and, therefore, represent a promising and safe strategy to induce strong CTL responses for the elimination of virus-infected cells.

Negative-strand RNA viruses: genetic engineering and applications

The negative-strand RNA viruses are a broad group of animal viruses that comprise several important human pathogens, including influenza, measles, mumps, rabies, respiratory syncytial, Ebola, and hantaviruses. The development of new strategies to genetically manipulate the genomes of negative-strand RNA viruses has provided us with new tools to study the structure-function relationships of the viral components and their contributions to the pathogenicity of these viruses. It is also now possible to envision rational approaches--based on genetic engineering techniques--to design live attenuated vaccines against some of these viral agents. In addition, the use of different negative-strand RNA viruses as vectors to efficiently express foreign polypeptides has also become feasible, and these novel vectors have potential applications in disease prevention as well as in gene therapy.

Mucosal model of immunization against human immunodeficiency virus type 1 with a chimeric influenza virus

Previously, we constructed a chimeric influenza virus that expresses the highly conserved amino acid sequence ELDKWA of gp41 of human immunodeficiency virus type 1 (HIV-1). Antisera elicited in mice by infection with this chimeric virus showed neutralizing activity against distantly related HIV-1 isolates (T. Muster, R. Guinea, A. Trkola, M. Purtscher, A. Klima, F. Steindl, P. Palese, and H. Katinger, J. Virol. 68:4031-4034, 1994). In the present study, we demonstrated that intranasal immunizations with this chimeric virus are also able to induce a humoral immune response at the mucosal level. The immunized mice had ELDKWA-specific immunoglobulins A in respiratory, intestinal, and vaginal secretions. Sustained levels of these secretory immunoglobulins A were detectable for more than 1 year after immunization. The results show that influenza virus can be used to efficiently induce secretory antibodies against antigens from foreign pathogens. Since long-lasting mucosal immunity in the genital and intestinal tracts might be essential for protective immunity against HIV-1, influenza virus appears to be a promising vector for HIV-1-derived immunogens.

The cytoplasmic tail of the neuraminidase protein of influenza A virus does not play an important role in the packaging of this protein into viral envelopes

We have rescued a transfectant influenza virus, NA/TAIL(-), whose neuraminidase (NA) protein lacks the predicted cytoplasmic tail. The virus was attenuated (one log10 reduction) both in tissue culture and in mouse lungs. Attenuation correlated with a 50% reduction of the level of NA in infected cells and levels of incorporation of the tail-less NA protein into viral particles paralleled that in infected cells. This result indicates that the signal for packaging of the NA protein into the viral envelope is not located in its cytoplasmic domain.

Attenuated Mengo virus: a new vector for live recombinant vaccines

Several features make Mengo virus an excellent candidate for use as a vaccine vector. The virus has a wide host range, including rodents, pigs, monkeys, and most likely humans, and expresses its genome exclusively in the cytoplasm of the infected cell. Stable attenuated strains exist which are deleted for part of the 5' noncoding region of the genome. Here we report an attenuated Mengo virus recombinant, vLCMG4, that encodes an immunodominant cytotoxic T-lymphocyte epitope of the lymphocytic choriomeningitis virus (LCMV) nucleo-protein. vLCMG4 induced protective immunity against lethal LCMV infection after a single, low-dose immunization in BALB/c mice and elicited an LCMV-specific CD8+ cytotoxic T lymphocyte response. This demonstrates the potential of recombinant Mengo virus vaccines to confer protection against infectious diseases by the induction of cellular immune responses.

Polymorphism within the herpes simplex virus (HSV) ribonucleotide reductase large subunit (ICP6) confers type specificity for recognition by HSV type 1-specific cytotoxic T lymphocytes

A panel of herpes simplex virus type 1 (HSV-1)-specific, CD8+, major histocompatibility complex class I (H-2Kb)-restricted cytotoxic T-lymphocyte (CTL) clones was derived from HSV-1-immunized C57BL/6 (H-2b) mice in order to identify the HSV-1 CTL recognition epitope(s) which confers type specificity. HSV-1 x HSV-2 intertypic recombinants were used to narrow the region encoding potential CTL recognition epitopes to within 0.51 to 0.58 map units of the HSV-1 genome. Using an inhibitor of viral DNA synthesis and an ICP6 deletion mutant, the large subunit of ribonucleotide reductase (ICP6, RR1) was identified as a target protein for these type-specific CTL. Potential CTL recognition epitopes within RR1 were located on the basis of the peptide motif predicted to bind to the MHC class I H-2Kb molecule. A peptide corresponding to residues 822 to 829 of RR1 was shown to confer susceptibility on H-2Kb-expressing target cells to lysis by the type 1-specific CTL. On the basis of a comparison of the HSV-1 RR1 epitope (residues 822 to 829) with the homologous sequence of HSV-2 RR1 (residues 828 to 836) and by the use of amino acid substitutions within synthetic peptides, we identified HSV-1 residue 828 as being largely responsible for the type specificity exhibited by HSV-1-specific CTL. This HSV-1 RR1 epitope, when expressed in recombinant simian virus 40 large T antigen in primary C57BL/6 cells, was recognized by the HSV-1 RR1-specific CTL clones. These results indicate that an early HSV protein with enzymatic activity provides a target for HSV-specific CTL and that type specificity is dictated largely by a single amino acid.

CD8+ T-cell memory to viruses

Recent experiments show that laboratory mice infected once with an influenza A virus or with the murine parainfluenza type 1 virus, called the Sendai virus, have enhanced numbers of cytotoxic T-lymphocyte precursors ( > 20x background) for life. Neither virus persists at the genome level, and the mice are maintained under conditions where there is no possibility of re-infection. These observations are highly relevant to any understanding of CD8+ cell memory and suggest that the clonal burst size associated with the primary challenge is a key determining factor.