Sequential addition of temperature-sensitive missense mutations into the PB2 gene of influenza A transfectant viruses can effect an increase in temperature sensitivity and attenuation and permits the rational design of a genetically engineered live influenza A virus vaccine

Comparative study of the biological properties of influenza А virus mutants obtained by site-specific mutagenesis and the live influenza reassortant vaccine variant

The aim of study was to carry out comparative investigation of biological properties of site-specific mutants of Influenza A virus and variant of live cold-adapted (CA) influenza reassortant vaccine. Materials and methods. The genetic stability of site-specific mutants (SSM) of the A/WSN/33 (H1N1) strain with ts (temperature sensitive)-mutations in polymerase genes was studied using a stress-test in MadinDarby Canine Kidney (MDCK) culture. A comparative study of immunogenicity of U2 and M26 mutants with the high genetic stability and the CA-reassortant with similar surface proteins was carried out. The increase in the antibody titer was investigated using enzyme-linked immunosorbent assay and the reaction of delayed hemagglutination. Ability of the studied viruses to induce type 1 interferon in A549 cells was determined using real-time polymerase chain reaction (real-time PCR). Results. It was shown that U2 and M26 mutants, which have 3 ts-mutations or more in polymerase genes have high genetic stability. It was found that U2 and M26 mutants induced a higher antibody titers than the CA reassortant in mice following the intranasal immunization. The ability of site-specific mutants and CA reassortant to induce type 1 interferon was also investigated. Mutants U2 and M26 increased the level of interferon to a greater extent than the CA-reassortant. Conclusion. The data obtained indicate that SSM U2 and M26 with 3 ts-mutations or more in the genome have a significant level of genetic stability. Mutants U2 and M26 have a higher immunogenicity and a higher ability to induce interferon in comparison with the CA reassortant. These facts allow us to conclude that SSM of the influenza virus with a set of mutations in polymerase genes can be considered as promising candidates for live influenza vaccines.

Stimulus-responsive viral vectors for controlled delivery of therapeutics

Virus-based therapies have gained momentum as the next generation of treatments for a variety of serious diseases. In order to make these therapies more controllable, stimulus-responsive viral vectors capable of sensing and responding to specific environmental inputs are currently being developed. A number of viruses naturally respond to endogenous stimuli, such as pH, redox, and proteases, which are present at different concentrations in diseases and at different organ and organelle sites. Additionally, rather than relying on natural viral properties, efforts are underway to engineer viruses to respond to endogenous stimuli in new ways as well as to exogenous stimuli, such as temperature, magnetic field, and optical light. Viruses with stimulus-responsive capabilities, either nature-evolved or human-engineered, will be reviewed to capture the current state of the field. Stimulus-responsive viral vector design considerations as well as gaps in current research efforts will be identified.

NS1 Protein Amino Acid Changes D189N and V194I Affect Interferon Responses, Thermosensitivity, and Virulence of Circulating H3N2 Human Influenza A Viruses

Influenza virus NS1 protein is a nonstructural, multifunctional protein that counteracts host innate immune responses, modulating virus pathogenesis. NS1 protein variability in subjects infected with H3N2 influenza A viruses (IAVs) during the 2010/2011 season was analyzed, and amino acid changes in residues 86, 189, and 194 were found. The consequences of these mutations for the NS1-mediated inhibition of IFN responses and the pathogenesis of the virus were evaluated, showing that NS1 mutations D189N and V194I impaired the ability of the NS1 protein to inhibit general gene expression, most probably because these mutations decreased the binding of NS1 to the cleavage and polyadenylation specificity factor 30 (CPSF30). A recombinant A/Puerto Rico/8/34 (PR8) H1N1 virus encoding the H3N2 NS1-D189N protein was slightly attenuated, whereas the virus encoding the H3N2 NS1-V194I protein was further attenuated in mice. The higher attenuation of this virus could not be explained by differences in the ability of the two NS1 proteins to counteract host innate immune responses, indicating that another factor must be responsible. In fact, we showed that the virus encoding the H3N2 NS1-V194I protein demonstrated a temperature-sensitive (ts) phenotype, providing a most likely explanation for the stronger attenuation observed. As far as we know, this is the first description of a mutation in NS1 residue 194 conferring a ts phenotype. These studies are relevant in order to identify new residues important for NS1 functions and in human influenza virus surveillance to assess mutations affecting the pathogenicity of circulating viruses.IMPORTANCE Influenza viral infections represent a serious public health problem, with influenza virus causing a contagious respiratory disease that is most effectively prevented through vaccination. The multifunctional nonstructural protein 1 (NS1) is the main viral factor counteracting the host antiviral response. Therefore, influenza virus surveillance to identify new mutations in the NS1 protein affecting the pathogenicity of the circulating viruses is highly important. In this work, we evaluated amino acid variability in the NS1 proteins from H3N2 human seasonal viruses and the effect of the mutations on innate immune responses and virus pathogenesis. NS1 mutations D189N and V194I impaired the ability of the NS1 protein to inhibit general gene expression, and recombinant viruses harboring these mutations were attenuated in a mouse model of influenza infection. Interestingly, a virus encoding the H3N2 NS1-V194I protein demonstrated a temperature-sensitive phenotype, further attenuating the virus in vivo.

The L, M, and S Segments of Rift Valley Fever Virus MP-12 Vaccine Independently Contribute to a Temperature-Sensitive Phenotype

Rift Valley fever (RVF) is endemic to Africa, and the mosquito-borne disease is characterized by "abortion storms" in ruminants and by hemorrhagic fever, encephalitis, and blindness in humans. Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus) has a tripartite negative-stranded RNA genome (L, M, and S segments). A live-attenuated vaccine for RVF, the MP-12 vaccine, is conditionally licensed for veterinary use in the United States. MP-12 is fully attenuated by the combination of the partially attenuated L, M, and S segments. Temperature sensitivity (ts) limits viral replication at a restrictive temperature and may be involved with viral attenuation. In this study, we aimed to characterize the ts mutations for MP-12. The MP-12 vaccine showed restricted replication at 38°C and replication shutoff (100-fold or greater reduction in virus titer compared to that at 37°C) at 39°C in Vero and MRC-5 cells. Using rZH501 reassortants with either the MP-12 L, M, or S segment, we found that all three segments encode a temperature-sensitive phenotype. However, the ts phenotype of the S segment was weaker than that of the M or L segment. We identified Gn-Y259H, Gc-R1182G, L-V172A, and L-M1244I as major ts mutations for MP-12. The ts mutations in the L segment decreased viral RNA synthesis, while those in the M segment delayed progeny production from infected cells. We also found that a lack of NSs and/or 78kD/NSm protein expression minimally affected the ts phenotype. Our study revealed that MP-12 is a unique vaccine carrying ts mutations in the L, M, and S segments.

IMPORTANCE

Rift Valley fever (RVF) is a mosquito-borne viral disease endemic to Africa, characterized by high rates of abortion in ruminants and severe diseases in humans. Vaccination is important to prevent the spread of disease, and a live-attenuated MP-12 vaccine is currently the only vaccine with a conditional license in the United States. This study determined the temperature sensitivity (ts) of MP-12 vaccine to understand virologic characteristics. Our study revealed that MP-12 vaccine contains ts mutations independently in the L, M, and S segments and that MP-12 displays a restrictive replication at 38°C.

Molecular basis of live-attenuated influenza virus

Human influenza is a seasonal disease associated with significant morbidity and mortality. The most effective means for controlling infection and thereby reducing morbidity and mortality is vaccination with a three inactivated influenza virus strains mixture, or by intranasal administration of a group of three different live attenuated influenza vaccine strains. Comparing to the inactivated vaccine, the attenuated live viruses allow better elicitation of a long-lasting and broader immune (humoral and cellular) response that represents a naturally occurring transient infection. The cold-adapted (ca) influenza A/AA/6/60 (H2N2) (AA ca) virus is the backbone for the live attenuated trivalent seasonal influenza vaccine licensed in the United States. Similarly, the influenza A components of live-attenuated vaccines used in Russia have been prepared as reassortants of the cold-adapted (ca) H2N2 viruses, A/Leningrad/134/17/57-ca (Len/17) and A/Leningrad/134/47/57-ca (Len/47) along with virulent epidemic strains. However, the mechanism of temperature-sensitive attenuation is largely elusive. To understand how modification at genetic level of influenza virus would result in attenuation of human influenza virus A/PR/8/34 (H1N1,A/PR8), we investigated the involvement of key mutations in the PB1 and/or PB2 genes in attenuation of influenza virus in vitro and in vivo. We have demonstrated that a few of residues in PB1 and PB2 are critical for the phenotypes of live attenuated, temperature sensitive influenza viruses by minigenome assay and real-time PCR. The information of these mutation loci could be used for elucidation of mechanism of temperature-sensitive attenuation and as a new strategy for influenza vaccine development.

Role of CD8(+) T-cell immunity in influenza infection: potential use in future vaccine development

Continued circulation of the highly pathogenic avian H5N1 influenza A virus has many people worried that an influenza pandemic is imminent. Compounding this is the realization that H5N1 vaccines based on current influenza vaccine technology (designed to generate protective antibody responses) may be suboptimal at providing protection. As a consequence, there is recent interest in vaccine strategies that elicit cellular immunity, particularly the cytotoxic T lymphocyte response, in an effort to provide protection against a potential pandemic. A major issue is the lack of information about the precise role that these 'hitmen' of the immune system have in protecting against both pandemic and seasonal influenza. We need to know more about how the induction and maintenance of cytotoxic T lymphocytes after influenza infection can impact protection from further infection. The challenge is then to use this information in the design of vaccines that will protect against pandemic influenza and will help optimize CD8(+) killer T-cell responses in other infections.

Engineering temperature sensitive live attenuated influenza vaccines from emerging viruses

The licensed live attenuated influenza A vaccine (LAIV) in the United States is created by making a reassortant containing six internal genes from a cold-adapted master donor strain (ca A/AA/6/60) and two surface glycoprotein genes from a circulating/emerging strain (e.g., A/CA/7/09 for the 2009/2010 H1N1 pandemic). Technologies to rapidly create recombinant viruses directly from patient specimens were used to engineer alternative LAIV candidates that have genomes composed entirely of vRNAs from pandemic or seasonal strains. Multiple mutations involved in the temperature-sensitive (ts) phenotype of the ca A/AA/6/60 master donor strain were introduced into a 2009 H1N1 pandemic strain rA/New York/1682/2009 (rNY1682-WT) to create rNY1682-TS1, and additional mutations identified in other ts viruses were added to rNY1682-TS1 to create rNY1682-TS2. Both rNY1682-TS1 and rNY1682-TS2 replicated efficiently at 30°C and 33°C. However, rNY1682-TS1 was partially restricted, and rNY1682-TS2 was completely restricted at 39°C. Additionally, engineering the TS1 or TS2 mutations into a distantly related human seasonal H1N1 influenza A virus also resulted pronounced restriction of replication in vitro. Clinical symptoms and virus replication in the lungs of mice showed that although rNY1682-TS2 and the licensed FluMist(®)-H1N1pdm LAIV that was used to combat the 2009/2010 pandemic were similarly attenuated, the rNY1682-TS2 was more protective upon challenge with a virulent mutant of pandemic H1N1 virus or a heterologous H1N1 (A/PR/8/1934) virus. This study demonstrates that engineering key temperature sensitive mutations (PB1-K391E, D581G, A661T; PB2-P112S, N265S, N556D, Y658H) into the genomes of influenza A viruses attenuates divergent human virus lineages and provides an alternative strategy for the generation of LAIVs.

Efficacy of live attenuated vaccines against 2009 pandemic H1N1 influenza in ferrets

The advent of the H1N1 influenza pandemic (pH1N1) in 2009 triggered the rapid production of pandemic influenza vaccines, since seasonal influenza vaccines were expected and demonstrated not to provide significant cross-protection against the newly emerged pandemic virus. To increase vaccine production capacity and further evaluate the effectiveness of different candidate pandemic influenza vaccines, the World Health Organization stimulated the evaluation of different vaccination concepts including the use of live attenuated influenza vaccines (LAIVs). Therefore, we have immunized ferrets intranasally with a single dose of pH1N1-LAIV from different manufacturers. They all induced adequate serum HI antibody titers in the ferrets and protected them against intratracheal wild-type pH1N1 virus challenge: pH1N1 virus replication in the upper respiratory tract and lungs was reduced and no disease signs or severe broncho-interstitial pneumonia were observed in any of the vaccinated ferrets. These data together with the relatively efficient production process emphasize the potential of the LAIV concept for pandemic preparedness.

Human body temperature and new approaches to constructing temperature-sensitive bacterial vaccines

Many of the live human and animal vaccines that are currently in use are attenuated by virtue of their temperature-sensitive (TS) replication. These vaccines are able to function because they can take advantage of sites in mammalian bodies that are cooler than the core temperature, where TS vaccines fail to replicate. In this article, we discuss the distribution of temperature in the human body, and relate how the temperature differential can be exploited for designing and using TS vaccines. We also examine how one of the coolest organs of the body, the skin, contains antigen-processing cells that can be targeted to provoke the desired immune response from a TS vaccine. We describe traditional approaches to making TS vaccines, and highlight new information and technologies that are being used to create a new generation of engineered TS vaccines. We pay particular attention to the recently described technology of substituting essential genes from Arctic bacteria for their homologues in mammalian pathogens as a way of creating TS vaccines.

Priming with cold-adapted influenza A does not prevent infection but elicits long-lived protection against supralethal challenge with heterosubtypic virus

We show in this study several novel features of T cell-based heterosubtypic immunity against the influenza A virus in mice. First, T cell-mediated heterosubtypic protection against lethal challenge can be generated by a very low priming dose. Second, it becomes effective within 5-6 days. Third, it provides protection against a very high dose challenge for >70 days. Also novel is the finding that strong, long-lasting, heterosubtypic protection can be elicited by priming with attenuated cold-adapted strains. We demonstrate that priming does not prevent infection of the lungs following challenge, but leads to earlier clearance of the virus and 100% survival after otherwise lethal challenge. Protection is dependent on CD8 T cells, and we show that CD4 and CD8 T cells reactive to conserved epitopes of the core proteins of the challenge virus are present after priming. Our results suggest that intranasal vaccination with cold-adapted, attenuated live virus has the potential to provide effective emergency protection against emerging influenza strains for several months.

Differential activities of alpha/beta IFN subtypes against influenza virus in vivo and enhancement of specific immune responses in DNA vaccinated mice expressing haemagglutinin and nucleoprotein

Vaccines are urgently needed to elicit immunity to different influenza virus strains. DNA vaccines can elicit partial protective immunity, however their efficacy requires improvement. We assessed the capacity of individual type I IFN multigene family members as subtype transgenes to abrogate influenza virus replication in a vaccination/challenge mouse model. Differences in antiviral efficacy were found among the subtypes with IFNA5 and IFNA6 being most effective, while IFNA1 was the least effective in reducing lung virus replication. Mice vaccinated with combinatorial HA/IFNA6 or NP/IFNA6 showed reduced lung viral titres, clinical score, body weight loss, and pulmonary tissue damage compared to IFNA6, HA, or NP viral vaccination alone. In addition, IFNA6 increased IgG2a titres with upregulation of IFN-gamma response in the respiratory tract. We conclude that IFN-alpha 6 has antiviral and immunomodulatory effects, which improve efficacy of DNA vaccines for enhanced control of influenza.

The phosphoprotein of attenuated measles AIK-C vaccine strain contributes to its temperature-sensitive phenotype

Measles AIK-C vaccine strain exhibits temperature-sensitivity (ts). To identify the structural proteins, which contribute to the ts property of AIK-C virus, reverse genetics was used. MV-minigenome RNA was replicated at 32.5, 37, and 39 degrees C when the plasmids expressing N, P, and L proteins of the Edmonston strain (the parental strain of AIK-C) were used, whereas the minigenome RNA replicated only at 32.5 degrees C but did not at 37 degrees C and higher temperature when N, P, and L protein expression plasmids of the AIK-C strain were used. A series of minigenome experiments revealed that the amino acid substitution of leucine at position 439 of the P protein by proline (P439-Pro) contributes to the ts phenotype of AIK-C. Four recombinant viruses having various P genes were rescued from the modified AIK-C genome cDNA and ts-characteristics were compared in Vero cells by plaque formation assay. The results showed that the P439-Pro of AIK-C virus played a key role in the ts phenotype, but the other substitutions in the P gene might have an accessory function in the expression of the phenotype.

Severe acute respiratory syndrome coronavirus infection of golden Syrian hamsters

Small animal models are needed in order to evaluate the efficacy of candidate vaccines and antivirals directed against the severe acute respiratory syndrome coronavirus (SARS CoV). We investigated the ability of SARS CoV to infect 5-week-old Golden Syrian hamsters. When administered intranasally, SARS CoV replicates to high titers in the lungs and nasal turbinates. Peak replication in the lower respiratory tract was noted on day 2 postinfection (p.i.) and was cleared by day 7 p.i. Low levels of virus were present in the nasal turbinates of a few hamsters at 14 days p.i. Viral replication in epithelial cells of the respiratory tract was accompanied by cellular necrosis early in infection, followed by an inflammatory response coincident with viral clearance, focal consolidation in pulmonary tissue, and eventual pulmonary tissue repair. Despite high levels of virus replication and associated pathology in the respiratory tract, the hamsters showed no evidence of disease. Neutralizing antibodies were detected in sera at day 7 p.i., and mean titers at day 28 p.i. exceeded 1:400. Hamsters challenged with SARS CoV at day 28 p.i. were completely protected from virus replication and accompanying pathology in the respiratory tract. Comparing these data to the mouse model, SARS CoV replicates to a higher titer and for a longer duration in the respiratory tract of hamsters and is accompanied by significant pathology that is absent in mice. Viremia and extrapulmonary spread of SARS CoV to liver and spleen, which are seen in hamsters, were not detected in mice. The hamster, therefore, is superior to the mouse as a model for the evaluation of antiviral agents and candidate vaccines against SARS CoV replication.

Influenza vaccines generated by reverse genetics

Influenza viruses cause annual epidemics and occasional pandemics of acute respiratory disease. Vaccination is the primary means to prevent and control the disease. However, influenza viruses undergo continual antigenic variation, which requires the annual reformulation of trivalent influenza vaccines, making influenza unique among pathogens for which vaccines have been developed. The segmented nature of the influenza virus genome allows for the traditional reassortment between two viruses in a coinfected cell. This technique has long been used to generate strains for the preparation of either inactivated or live attenuated influenza vaccines. Recent advancements in reverse genetics techniques now make it possible to generate influenza viruses entirely from cloned plasmid DNA by cotransfection of appropriate cells with 8 or 12 plasmids encoding the influenza virion sense RNA and/or mRNA. Once regulatory issues have been addressed, this technology will enable the routine and rapid generation of strains for either inactivated or live attenuated influenza vaccine. In addition, the technology offers the potential for new vaccine strategies based on the generation of genetically engineered donors attenuated through directed mutation of one or more internal genes. Reverse genetics techniques are also proving to be important for the development of pandemic influenza vaccines, because the technology provides a means to modify genes to remove virulence determinants found in highly pathogenic avian strains. The future of influenza prevention and control lies in the application of this powerful technology for the generation of safe and more effective influenza vaccines.

Imparting temperature sensitivity and attenuation in ferrets to A/Puerto Rico/8/34 influenza virus by transferring the genetic signature for temperature sensitivity from cold-adapted A/Ann Arbor/6/60

The four temperature-sensitive (ts) loci identified in the PB1 and PB2 gene segments of cold-adapted A/Ann Arbor/6/60 influenza virus, the master donor virus for influenza A virus (MDV-A) FluMist vaccines, were introduced into a divergent A/Puerto Rico/8/34 influenza virus strain. Recombinant A/Puerto Rico/8/34 virus with these four introduced ts loci exhibited both ts and att phenotypes similar to those of MDV-A, which could be used as a donor virus for manufacturing large quantities of inactivated influenza virus vaccine against potential pandemic strains.

Patterns of epistasis in RNA viruses: a review of the evidence from vaccine design

Epistasis results when the fitness effects of a mutation change depending on the presence or absence of other mutations in the genome. The predictions of many influential evolutionary hypotheses are determined by the existence and form of epistasis. One rich source of data on the interactions among deleterious mutations that has gone untapped by evolutionary biologists is the literature on the design of live, attenuated vaccine viruses. Rational vaccine design depends upon the measurement of individual and combined effects of deleterious mutations. In the current study, we have reviewed data from 29 vaccine-oriented studies using 14 different RNA viruses. Our analyses indicate that (1) no consistent tendency towards a particular form of epistasis exists across RNA viruses and (2) significant interactions among groups of mutations within individual viruses occur but are not common. RNA viruses are significant pathogens of human disease, and are tractable model systems for evolutionary studies--we discuss the relevance of our findings in both contexts.

Multiple amino acid residues confer temperature sensitivity to human influenza virus vaccine strains (FluMist) derived from cold-adapted A/Ann Arbor/6/60

FluMist influenza A vaccine strains contain the PB1, PB2, PA, NP, M, and NS gene segments of ca A/AA/6/60, the master donor virus-A strain. These gene segments impart the characteristic cold-adapted (ca), attenuated (att), and temperature-sensitive (ts) phenotypes to the vaccine strains. A plasmid-based reverse genetics system was used to create a series of recombinant hybrids between the isogenic non-ts wt A/Ann Arbor/6/60 and MDV-A strains to characterize the genetic basis of the ts phenotype, a critical, genetically stable, biological trait that contributes to the attenuation and safety of FluMist vaccines. PB1, PB2, and NP derived from MDV-A each expressed determinants of temperature sensitivity and the combination of all three gene segments was synergistic, resulting in expression of the characteristic MDV-A ts phenotype. Site-directed mutagenesis analysis mapped the MDV-A ts phenotype to the following four major loci: PB1(1195) (K391E), PB1(1766) (E581G), PB2(821) (N265S), and NP(146) (D34G). In addition, PB1(2005) (A661T) also contributed to the ts phenotype. The identification of multiple genetic loci that control the MDV-A ts phenotype provides a molecular basis for the observed genetic stability of FluMist vaccines.

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.

Preparation of vaccines against H5N1 influenza

In response to the pandemic warning provided by the highly pathogenic H5N1 influenza virus infections in Hong Kong, there were world-wide attempts to develop vaccines. Three strategies were followed and although each was associated with some success, there were also some problems. Pre-clinical vaccine efficacy results are presented from one such strategy, that of using an apathogenic H5N3 avian strain for vaccine production.

Residue 627 of PB2 is a determinant of cold sensitivity in RNA replication of avian influenza viruses

Human influenza A viruses replicate in the upper respiratory tract at a temperature of about 33 degrees C, whereas avian viruses replicate in the intestinal tract at a temperature close to 41 degrees C. In the present study, we analyzed the influence of low temperature (33 degrees C) on RNA replication of avian and human viruses in cultured cells. The kinetics of replication of the NP segment were similar at 33 and 37 degrees C for the human A/Puerto-Rico/8/34 and A/Sydney/5/97 viruses, whereas replication was delayed at 33 degrees C compared to 37 degrees C for the avian A/FPV/Rostock/34 and A/Mallard/NY/6750/78 viruses. Making use of a genetic system for the in vivo reconstitution of functional ribonucleoproteins, we observed that the polymerase complexes derived from avian viruses but not human viruses exhibited cold sensitivity in mammalian cells, which was determined mostly by residue 627 of PB2. Our results suggest that a reduced ability of the polymerase complex of avian viruses to ensure replication of the viral genome at 33 degrees C could contribute to their inability to grow efficiently in humans.

Emergence of H3N2 reassortant influenza A viruses in North American pigs

In late summer through early winter of 1998, there were several outbreaks of respiratory disease in the swine herds of North Carolina, Texas, Minnesota and Iowa. Four viral isolates from outbreaks in different states were analyzed, both antigenically and genetically. All of the isolates were identified as H3N2 influenza viruses with antigenic profiles similar to those of recent human H3 strains. Genotyping and phylogenetic analysis demonstrated that the four swine viruses had emerged through two different pathways. The North Carolina isolate is the product of genetic reassortment between human and swine influenza viruses, while the others arose from reassortment of human, swine and avian viral genes. The hemagglutinin genes of the four isolates were all derived from the human H3N2 virus circulating in 1995. It remains to be determined if either of these recently emerged viruses will become established in the pigs in North America and whether they will become an economic burden.

Reduced levels of neuraminidase of influenza A viruses correlate with attenuated phenotypes in mice

We have previously obtained four transfectant influenza A viruses containing neuraminidase (NA) genes with mutated base pairs in the conserved double-stranded RNA region of the viral promoter by using a ribonucleoprotein transfection system. Two mutant viruses (D2 and D1/2) which share a C-G-->A-U mutation at positions 11 and 12 of the 3' and 5' ends, respectively, of the NA gene, showed an approximate 10-fold reduction of NA-specific mRNA and protein levels (Fodor et al., Journal of Virology 72, 6283-6290, 1998). These viruses have now allowed us to determine the effects of decreased NA levels on virus pathogenicity. Both D2 and D1/2 viruses were highly attenuated in mice, and their replication in mouse lungs was highly compromised as compared with wild-type influenza A/WSN/33 virus. The results highlight the importance of the level of NA activity in the biological cycle and virulence of influenza viruses. Importantly, mice immunized by a single intranasal administration of 10(3) infectious units of D2 or D1/2 viruses were protected against challenge with a lethal dose of wild-type influenza virus. Attenuation of influenza viruses by mutations resulting in the decreased expression of a viral protein represents a novel strategy which could be considered for the generation of live attenuated influenza virus vaccines.

Influenza vaccines: present and future

Immunization is the most feasible method for preventing influenza. Vaccination against influenza is recommended for everyone 65 years of age and older and for persons less than 65 years of age who are at risk for developing complications of influenza. Immune correlates of protection have been established, and a global network is in place to monitor the appearance and circulation of antigenic variants of influenza viruses, as well as the appearance of novel subtypes of influenza A. Antigenic and genetic analyses of circulating viruses and testing of serum from vaccine recipients guide vaccine composition updates. The efficacy of influenza vaccines depends in part on the closeness of the antigenic match between the vaccine strain and the epidemic strain. Currently licensed influenza vaccines are trivalent, formalin-inactivated, egg-derived vaccines; their efficacy ranges from 70 to 90% in young, healthy populations when there is a close antigenic match between vaccine strains and epidemic strains. Development of intranasally administered alternative vaccines and improvement of the existing vaccine are areas of active research. A trivalent, ca live vaccine is the most promising LAIV candidate. In a field trial, efficacy rates of LAIV in young children were 96% against influenza A (H3N2) and 91% against influenza B. However, few data are available to compare this formulation of the trivalent ca live vaccine with the trivalent, inactivated vaccine. Influenza vaccine recommendations will most likely be revised on licensure of LAIV; each vaccine may offer distinct advantages in specific populations.

Mutation of neuraminidase cysteine residues yields temperature-sensitive influenza viruses

The influenza virus neuraminidase (NA) is a tetrameric, virus surface glycoprotein possessing receptor-destroying activity. This enzyme facilitates viral release and is a target of anti-influenza virus drugs. The NA structure has been extensively studied, and the locations of disulfide bonds within the NA monomers have been identified. Because mutation of cysteine residues in other systems has resulted in temperature-sensitive (ts) proteins, we asked whether mutation of cysteine residues in the influenza virus NA would yield ts mutants. The ability to rationally design tight and stable ts mutations could facilitate the creation of efficient helper viruses for influenza virus reverse genetics experiments. We generated a series of cysteine-to-glycine mutants in the influenza A/WSN/33 virus NA. These were assayed for neuraminidase activity in a transient expression system, and active mutants were rescued into infectious virus by using established reverse genetics techniques. Mutation of two cysteines not involved in intrasubunit disulfide bonds, C49 and C146, had modest effects on enzymatic activity and on viral replication. Mutation of two cysteines, C303 and C320, which participate in a single disulfide bond located in the beta5L0,1 loop, produced ts enzymes. Additionally, the C303G and C320G transfectant viruses were found to be attenuated and ts. Because both the C303G and C320G viruses exhibited stable ts phenotypes, they were tested as helper viruses in reverse genetics experiments. Efficiently rescued were an N1 neuraminidase from an avian H5N1 virus, an N2 neuraminidase from a human H3N2 virus, and an N7 neuraminidase from an H7N7 equine virus. Thus, these cysteine-to-glycine NA mutants allow the rescue of a variety of wild-type and mutant NAs into influenza virus.

Two separate sequences of PB2 subunit constitute the RNA cap-binding site of influenza virus RNA polymerase

BACKGROUND

Influenza virus RNA polymerase with the subunit composition of PB1-PB2-PA is a unique multifunctional enzyme with the activities of both synthesis and cleavage of RNA, and is involved in both transcription and replication of the RNA genome. Transcription is initiated by using capped RNA fragments, which are generated after cleavage of host cell mRNA by the RNA polymerase-associated capped RNA endonuclease. To identify the RNA cap 1-binding site on the RNA polymerase, viral ribonucleoprotein (RNP) cores were subjected to UV-crosslinking with RNA which was labelled with 32P only at the cap-1 structure.

RESULTS

After SDS-PAGE of UV-crosslinked cores, 32P was found to be associated only with the PB2 subunit (759 amino acid residues). The labelled PB2 was subjected, together with PB2 expressed in E. coli, to limited digestion with V8 protease. Analysis of the amino terminal sequences of some isolated fragments with the crosslinked cap-1 indicated that two separate sequences within the PB2 were involved in RNA cap-1 binding, one (N-site) at the N-terminal proximal region approximately between amino acid residues 242-282 downstream from the PB1 subunit-binding site and the other (C-site) between residues 538-577 including the cap-binding motifs. Two lines of evidence support the prediction of the involvement of two separate PB2 sequences on the RNA cap-binding: (i) cross-linking of the capped RNA on to expressed and isolated PB2 fragments, each containing either the N-site or the C-site; and (ii) competition of capped RNA-binding to PB2 by both of the N- and C-terminal PB2 fragments. Taking together, we propose that two separate sequences within PB2 constitute the capped RNA-binding site of the RNA polymerase.

CONCLUSION

Two separate sequences, one N-(242-282) and the other C-terminal (538-577) proximal segments of PB2 subunit, constitute the RNA cap-binding site of the influenza virus RNA polymerase.

Attenuation of the recombinant human parainfluenza virus type 3 cp45 candidate vaccine virus is augmented by importation of the respiratory syncytial virus cpts530 L polymerase mutation

A phenylalanine to leucine mutation at position 521 in the L polymerase of cpts530, a live-attenuated respiratory syncytial virus (RSV) cold-passaged (cp), temperature-sensitive (ts) candidate vaccine, specifies the ts and attenuation (att) phenotypes. Sequence alignment of this region in the L proteins of several distantly related paramyxoviruses revealed that this phenylalanine is conserved. Using reverse genetics, the analogous phenylalanine at position 456 in the L protein of wild-type PIV3 was mutagenized to leucine (F456L). The resulting virus, designated r456(L), was ts (40 degrees C shut-off temperature of plaque formation), and its replication in the upper, but not the lower, respiratory tract of hamsters was 10-fold reduced compared with that of the recombinant wild-type PIV3 (rwt). Thus the phenylalanine to leucine mutation specified a similar level of temperature sensitivity and attenuation in two distantly related paramyxoviruses. We next sought to determine whether the addition of this mutation to the L protein of two rPIV3 candidate vaccine viruses, one bearing the three cp45 ts missense mutations in the L protein (rcp45(L)) and the other bearing all 15 cp45 mutations (rcp45), would further attenuate the viruses in vivo. Each rcp45 derivative to which the F456L mutation was added exhibited an increased level of temperature sensitivity. Furthermore rcp45(L)-456 and rcp45-456 were 100- to 1000-fold more restricted in replication in hamsters than their rcp45(L) and rcp45 parents. Despite the high level of restriction of replication in hamsters, immunization with rcp45-456 induced a moderate level of resistance to replication of PIV3 challenge virus. In contrast to the highly restricted replication observed in hamsters, rcp45-456 was only fivefold more restricted in the respiratory tract of chimpanzees than rcp45 and induced a comparable, moderate to high level of PIV3-specific serum antibodies. rcp45 and rcp45-456 viruses isolated from chimpanzees throughout the 2-week course of replication maintained the level of temperature sensitivity of their respective input viruses, illustrating their phenotypic stability. Thus the acquisition of the F456L mutation by the cp45 virus resulted in a small, incremental increase in its level of attenuation, indicating its possible usefulness in the fine tuning of the level of attenuation of the cp45 vaccine candidate. The ability to transfer mutations identified in heterologous paramyxoviruses, which in this case represent different subfamilies, greatly enhances our ability to rapidly develop novel parainfluenza virus candidate vaccines.

Addition of a missense mutation present in the L gene of respiratory syncytial virus (RSV) cpts530/1030 to RSV vaccine candidate cpts248/404 increases its attenuation and temperature sensitivity

Respiratory syncytial virus (RSV) cpts530/1030 is an attenuated, temperature-sensitive subgroup A vaccine candidate derived previously from cold-passaged RSV (cpRSV) by two sequential rounds of chemical mutagenesis and biological selection. Here, cpts530/1030 was shown to be highly attenuated in the upper and lower respiratory tracts of seronegative chimpanzees. However, evaluation in seropositive children showed that it retains sufficient replicative capacity and virulence to preclude its direct use as a live attenuated vaccine. Nucleotide sequence analysis of the genome of cpts530/1030 showed that it had acquired two nucleotide substitutions (compared to its cpts530 parent), both of which were in the L gene: a silent mutation at nucleotide position 8821 (amino acid 108) and a missense mutation at nucleotide position 12458 resulting in a tyrosine-to-asparagine change at amino acid 1321, herein referred to as the 1030 mutation. It also contained the previously identified 530 missense mutation at nucleotide 10060 in the L gene. The genetic basis of attenuation of cpts530/1030 was defined by the introduction of the 530 and 1030 mutations into a cDNA clone of cpRSV, from which recombinant RSV was derived and analyzed to determine the contribution of each mutation to the temperature sensitivity (ts) and attenuation (att) phenotypes of cpts530/1030. The 530 mutation, derived from cpts530, was previously shown to be responsible for the ts and att phenotypes of that virus. In the present study, the 1030 mutation was shown to be responsible for the increased temperature sensitivity of cpts530/1030. In addition, the 1030 mutation was shown to be responsible for the increased level of attenuation of cpts530/1030 in the upper and lower respiratory tracts of mice. The 530 and 1030 mutations were additive in their effects on the ts and att phenotypes. It was possible to introduce the 1030 mutation, but not the 530 mutation, into an attenuated vaccine candidate with residual reactogenicity in very young infants, namely, cpts248/404, by use of reverse genetics. The inability to introduce the 530 mutation into the cpts248/404 virus was shown to be due to its incompatibility with the 248 missense mutation at the level of L protein function. The resulting rA2cp248/404/1030 mutant virus was more temperature sensitive and more attenuated than the cpts248/404 parent virus, making it a promising new RSV vaccine candidate created by use of reverse genetics to improve upon an existing vaccine virus.

Identification of mutations contributing to the temperature-sensitive, cold-adapted, and attenuation phenotypes of the live-attenuated cold-passage 45 (cp45) human parainfluenza virus 3 candidate vaccine

The live-attenuated human parainfluenza virus 3 (PIV3) cold-passage 45 (cp45) candidate vaccine was shown previously to be safe, immunogenic, and phenotypically stable in seronegative human infants. Previous findings indicated that each of the three amino acid substitutions in the L polymerase protein of cp45 independently confers the temperature-sensitive (ts) and attenuation (att) phenotypes but not the cold-adaptation (ca) phenotype (29). cp45 contains 12 additional potentially important point mutations in other proteins (N, C, M, F, and hemagglutinin-neuraminidase [HN]) or in cis-acting sequences (the leader region and the transcription gene start [GS] signal of the N gene), and their contribution to these phenotypes was undefined. To further characterize the genetic basis for the ts, ca, and att phenotypes of this promising vaccine candidate, we constructed, using a reverse genetics system, a recombinant cp45 virus that contained all 15 cp45-specific mutations mentioned above, and found that it was essentially indistinguishable from the biologically derived cp45 on the basis of plaque size, level of temperature sensitivity, cold adaptation, level of replication in the upper and lower respiratory tract of hamsters, and ability to protect hamsters from subsequent wild-type PIV3 challenge. We then constructed recombinant viruses containing the cp45 mutations in individual proteins as well as several combinations of mutations. Analysis of these recombinant viruses revealed that multiple cp45 mutations distributed throughout the genome contribute to the ts, ca, and att phenotypes. In addition to the mutations in the L gene, at least one other mutation in the 3' N region (i.e., including the leader, N GS, and N coding changes) contributes to the ts phenotype. A recombinant virus containing all the cp45 mutations except those in L was more ts than cp45, illustrating the complex nature of this phenotype. The ca phenotype of cp45 also is a complex composite phenotype, reflecting contributions of at least three separate genetic elements, namely, mutations within the 3' N region, the L protein, and the C-M-F-HN region. The att phenotype is a composite of both ts and non-ts mutations. Attenuating ts mutations are located in the L protein, and non-ts attenuating mutations are located in the C and F proteins. The presence of multiple ts and non-ts attenuating mutations in cp45 likely contributes to the high level of attenuation and phenotypic stability of this promising vaccine candidate.

A single nucleotide substitution in the transcription start signal of the M2 gene of respiratory syncytial virus vaccine candidate cpts248/404 is the major determinant of the temperature-sensitive and attenuation phenotypes

Respiratory syncytial virus (RSV) cpts248/404 is a live-attenuated, temperature-sensitive (ts) vaccine candidate derived from cole-passaged cpRSV by two rounds of chemical mutagenesis and biological selection. Previous sequence analysis showed that these two steps introduced three single nucleotide substitutions into the cpRSV parent. Two of these occurred with the coding sequence for the L protein, and each resulted in a single amino acid substitution: Gin-831-Leu (248 mutation) and Asp-1183-Glu (404-L mutation). The third mutation resulted in a nucleotide substitution at position 9 of the c/s-acting gene start signal of the M2 gene (404-M2 mutation). In the present study, the genetic basis of attenuation of cpts248/404 was defined by the introduction of each of these mutations (singly or in combination) into a full-length cDNA clone of cpRSV. Recombinant RSV derived from each mutant cDNA was analyzed to determine the contribution of each mutation to the ts and attenuation phenotypes of the virus. This analysis showed that the 248 mutation specifies a significant reduction of plaque formation at 38 degrees and is responsible for an intermediate level of attenuation in mice. In contrast, the 404-L mutation did not contribute to the ts or attenuation phenotype alone or in combination with other mutations and is thus an incidental change. unexpectedly, the 404-M2 mutation alone specified complete restriction of plaque formation at 37 degrees C an a high level of attenuation in mice. This indicates that the level of temperature sensitivity and attenuation of cpts248/404 can be attributed primarily to the 404-M2 mutation. Thus the cpts248/404 virus contains a set of ts and non-ts attenuating mutations, which likely accounts for its genetic stability. The recombinant version of this virus, rA2cp248/404, was phenotypically indistinguishable from cpts248/404 and represents a background into which additional mutations can be introduced as needed to obtain the desired level of attenuation for successful immunization of the very young human infant.

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.

Three amino acid substitutions in the L protein of the human parainfluenza virus type 3 cp45 live attenuated vaccine candidate contribute to its temperature-sensitive and attenuation phenotypes

Studies were initiated to define the genetic basis of the temperature-sensitive (ts), cold adaptation (ca), and attenuation (att) phenotypes of the human parainfluenza virus type 3 (PIV3) cp45 live attenuated vaccine candidate. Genetic data had previously suggested that the L polymerase protein of cp45, which contains three amino acid substitutions at positions 942, 992, and 1558, contributed to its temperature sensitivity (R. Ray, M. S. Galinski, B. R. Heminway, K. Meyer, F. K. Newman, and R. B. Belshe, J. Virol. 70:580-584, 1996; A. Stokes, E. L. Tierney, C. M. Sarris, B. R. Murphy, and S. L. Hall, Virus Res. 30:43-52, 1993). To study the individual and aggregate contributions that these amino acid substitutions make to the ts, att, and ca phenotypes of cp45, seven PIV3 recombinant viruses (three single, three double, and one triple mutant) representing all possible combinations of the three amino acid substitutions were recovered from full-length antigenomic cDNA and analyzed for their ts, att, and ca phenotypes. None of the seven mutant recombinant PIVs was cold adapted. The substitutions at L protein amino acid positions 992 and 1558 each specified a 105-fold reduction in plaque formation in cell culture at 40 degrees C, whereas the substitution at position 942 specified a 300-fold reduction. Thus, each of the three mutations contributes individually to the ts phenotype. The triple recombinant which possesses an L protein with all three mutations was almost as temperature sensitive as cp45, indicating that these mutations are the major contributors to the ts phenotype of cp45. The three individual mutations in the L protein each contributed to restricted replication in the upper or lower respiratory tract of hamsters, and this likely contributes to the observed stability of the ts and att phenotypes of cp45 during replication in vivo. Importantly, the recombinant virus possessing L protein with all three mutations was as restricted in replication as was the cp45 mutant in both the upper and lower respiratory tracts of hamsters, indicating that the L gene of the cp45 virus is a major attenuating component of this candidate vaccine.

Mapping of the mutations present in the genome of the Rift Valley fever virus attenuated MP12 strain and their putative role in attenuation

The MP12 attenuated strain of Rift Valley fever virus was obtained by 12 serial passages of a virulent isolate ZH548 in the presence of 5-fluorouracil (Caplen et al., 1985. Mutagen-directed attenuation of Rift Valley fever virus as a method for vaccine development. J. Gen. Virol., 66, 2271-2277). The comparison of the M segment of the two strains has already been reported by Takehara et al. (Takehara et al., 1989. Identification of mutations in the M RNA of a candidate vaccine strain of Rift Valley fever virus. Virology 169, 452-457). We have completed the comparison and found that altogether a total of nine, 12 and four nucleotides were changed in the L, M and S segments of the two strains, respectively. Three mutations induced amino acid changes in the L protein but none of them was located in the recognized motifs conserved among RNA dependent polymerases. In the S segment, a single change modified an amino acid in the NSs protein and in the M segment, seven of the mutations resulted in amino acid changes in each of the four encoded G1, G2, 14 kDa and 78 kDa proteins. Characterization of the MP12 virus indicated that determinants for attenuation were present in each segment and that they were introduced progressively during the 12 passages in the presence of the mutagen (Saluzzo and Smith, 1990. Use of reassortant viruses to map attenuating and temperature-sensitive mutations of the Rift Valley fever virus MP-12 vaccine. Vaccine 8, 369-375). Passages 4 and 7-9 were found to be essential for introduction of temperature-sensitive lesions and attenuation. In an attempt to correlate some of the mutations with the attenuated or temperature-sensitive phenotypes, we determined by sequencing the passage level at which the different mutations appeared. This work should help to address the question of the role of the viral gene products in Rift Valley fever pathogenesis.

An influenza A live attenuated reassortant virus possessing three temperature-sensitive mutations in the PB2 polymerase gene rapidly loses temperature sensitivity following replication in hamsters

The purpose of the present study was to produce an influenza A H2N2 donor virus from which an attenuating PB2 gene bearing three discrete temperature sensitive (ts) mutations could be readily transferred to currently epidemic influenza A H1N1 and H3N2 viruses via genetic reassortment. An influenza A transfectant virus was first produced that contained site-directed ts mutations at amino acids 112, 265, and 556 in the PB2 gene of influenza A/AA/60 virus origin in a background of the other seven RNA segments from the influenza A/LA/87 (H3N2) virus. The A/LA/87 PB2 ts transfectant virus (clone 22B1) was mated with the A/AA/60 (H2N2) wild type virus, and six H2N2 ts reassortants were obtained. One reassortant virus, clone 25A1, possessed the triple ts PB2 gene in the context of all seven other genes of homologous A/AA/60 origin. Isolation of this reassortant permitted an examination of the contribution of the ts mutations present in a triple ts PB2 transfectant virus to its attenuation and phenotypic stability independent from an effect of the A/AA/60-A/LA/87 gene constellation on attenuation. It was found that the A/AA/60 triple ts reassortant virus was less ts, less attenuated, and less phenotypically stable than the A/LA/87 triple ts transfectant virus from which it was derived. The A/AA/60 reassortant possessing the PB2 gene containing three introduced ts mutations underwent rapid and significant loss of its temperature sensitivity following replication in the lungs of immunocompetent hamsters. This indicated that the A/AA/60-A/LA/87 gene constellation contributed significantly to the overall level of temperature-sensitivity, attenuation, and stability of the A/LA/87 triple ts transfectant virus. It is likely that the instability of the ts phenotype exhibited by the A/AA/60 triple ts reassortant virus would not be acceptable for a vaccine to be used in humans. The implications of these findings for the usefulness of ts mutations as the sole attenuating mutation in influenza virus vaccines is discussed.

Biochemical characterizations of two temperature-sensitive and attenuated strains of respiratory syncytial virus subgroup B

Cold-adapted, temperature-sensitive (ts), attenuated strains of respiratory syncytial virus have been developed from a B subgroup clinical isolate for potential use as vaccine candidates. The replication of two B subgroup ts mutant viruses (2B33F and 2B20L) at the permissive and nonpermissive temperatures have been compared with that of the parental 2B virus to establish differences that may account for their ts and/or attenuated phenotypes. We have shown that the ts restriction at 39 degrees C in the replication of the two mutant viruses in tissue culture occurs at a step after virus adsorption but before or including initiation of virus-specific mRNA transcription. At the permissive temperature of 32 degrees C a 12- to 24-h delay in the accumulation of mRNA for both mutant viruses in comparison to that of the parental 2B virus was exhibited. This effect was mirrored by equivalent delays in viral protein synthesis and production of infectious virus. By 36 h postinfection both mutants had produced levels of viral mRNA, protein, and infectious virus that were similar to those of the parent virus at 32 degrees C. ts+ revertant viruses derived from both mutants have also reverted in their viral mRNA, protein, and infectious virus production kinetics at 32 degrees C to rates more like those exhibited by the parental 2B virus. This suggests a positive correlation between the ts step in the replication of the mutant viruses and the initial delay in mRNA production that occurs at the permissive temperature.

Genetically engineered live attenuated influenza A virus vaccine candidates

We have generated new influenza A virus live attenuated vaccine candidates by site-directed mutagenesis and reverse genetics. By mutating specific amino acids in the PB2 polymerase subunit, two temperature-sensitive (ts) attenuated viruses were obtained. Both candidates have 38 degrees C shutoff temperatures in MDCK cells, are attenuated in the respiratory tracts of mice and ferrets, and have very low reactogenicity in ferrets. Infection of mice or ferrets with either mutant conferred significant protection from challenge with the homologous wild-type virus. Three tests for genetic stability were used to assess the propensity for reversion to virulence: 14 days of replication in nude mice, growth at 37 degrees C in tissue culture, and serial passage in ferrets. One candidate, which contains mutations intended to reduce the ability of PB2 to bind to cap structures, was stable in all three assays, whereas the second candidate, which contains mutations found only in other ts strains of influenza virus, lost its ts phenotype in the last two assays. This approach has therefore enabled the creation of live attenuated influenza A virus vaccine candidates suitable for human testing.

Temperature sensitive mutants of influenza A virus generated by reverse genetics and clustered charged to alanine mutagenesis

Temperature sensitive (ts) mutants of influenza A virus have the potential to serve as live attenuated (att) virus vaccines. Previously, ts mutants were isolated by chemical mutagenesis or arose spontaneously, and most likely contained point mutations in one or more genes. While sufficiently attenuated, even the most genetically stable of these viruses was found to revert to a more virulent form in a seronegative vaccinee. Recently developed technology, however, allows the introduction of engineered mutations into the genome of influenza A and B viruses, permitting the rational design of attenuated mutants with the potential for increased genetic stability. To accomplish this goal, we have introduced ts mutations into the PB2 gene of A/Los Angeles/2/87 (H3N2) and rescued the mutated genes into infectious viruses. We have used clustered charged to alanine mutagenesis (substitution of alanine for charged amino acid residues which are present in clusters) of the PB2 gene to generate novel ts mutants. Viruses containing such ts PB2 genes were attenuated in mice and ferrets. This approach has thus yielded several vaccine candidates with ts and attenuated characteristics in animal models. Combination of these mutations with each other or with other ts mutations may lead to a high level of genetic stability.