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

Human parainfluenza virus 3 vaccine candidates attenuated by codon-pair deoptimization are immunogenic and protective in hamsters

Human parainfluenza virus type 3 (HPIV3) is a major pediatric respiratory pathogen lacking available vaccines or antiviral drugs. We generated live-attenuated HPIV3 vaccine candidates by codon-pair deoptimization (CPD). HPIV3 open reading frames (ORFs) encoding the nucleoprotein (N), phosphoprotein (P), matrix (M), fusion (F), hemagglutinin-neuraminidase (HN), and polymerase (L) were modified singly or in combination to generate 12 viruses designated Min-N, Min-P, Min-M, Min-FHN, Min-L, Min-NP, Min-NPM, Min-NPL, Min-PM, Min-PFHN, Min-MFHN, and Min-PMFHN. CPD of N or L severely reduced growth in vitro and was not further evaluated. CPD of P or M was associated with increased and decreased interferon (IFN) response in vitro, respectively, but had little effect on virus replication. In Vero cells, CPD of F and HN delayed virus replication, but final titers were comparable to wild-type (wt) HPIV3. In human lung epithelial A549 cells, CPD F and HN induced a stronger IFN response, viral titers were reduced 100-fold, and the expression of F and HN proteins was significantly reduced without affecting N or P or the relative packaging of proteins into virions. Following intranasal infection in hamsters, replication in the nasal turbinates and lungs tended to be the most reduced for viruses bearing CPD F and HN, with maximum reductions of approximately 10-fold. Despite decreased in vivo replication (and lower expression of CPD F and HN in vitro), all viruses induced titers of serum HPIV3-neutralizing antibodies similar to wt and provided complete protection against HPIV3 challenge. In summary, CPD of HPIV3 yielded promising vaccine candidates suitable for further development.

Human parainfluenza virus type 3 expressing the respiratory syncytial virus pre-fusion F protein modified for virion packaging yields protective intranasal vaccine candidates

Human respiratory syncytial virus (RSV) and parainfluenza virus type 3 (HPIV3) are among the most common viral causes of childhood bronchiolitis and pneumonia worldwide, and lack effective antiviral drugs or vaccines. Recombinant (r) HPIV3 was modified to express the RSV fusion (F) glycoprotein, the major RSV neutralization and protective antigen, providing a live intranasal bivalent HPIV3/RSV vaccine candidate. This extends previous studies using a chimeric bovine-human PIV3 vector (rB/HPIV3). One advantage is that rHPIV3 expresses all of the HPIV3 antigens compared to only two for rB/HPIV3. In addition, the use of rHPIV3 as vector should avoid excessive attenuation following addition of the modified RSV F gene, which may occur with rB/HPIV3. To enhance its immunogenicity, RSV F was modified (i) to increase the stability of the prefusion (pre-F) conformation and (ii) by replacement of its transmembrane (TM) and cytoplasmic tail (CT) domains with those of HPIV3 F (H3TMCT) to increase incorporation in the vector virion. RSV F (+/- H3TMCT) was expressed from the first (F/preN) or the second (F/N-P) gene position of rHPIV3. The H3TMCT modification dramatically increased packaging of RSV F into the vector virion and, in hamsters, resulted in significant increases in the titer of high-quality serum RSV-neutralizing antibodies, in addition to the increase conferred by pre-F stabilization. Only F-H3TMCT/preN replication was significantly attenuated in the nasal turbinates by the RSV F insert. F-H3TMCT/preN, F/N-P, and F-H3TMCT/N-P provided complete protection against wt RSV challenge. F-H3TMCT/N-P exhibited the most stable and highest expression of RSV F, providing impetus for its further development.

The Role of Human Parainfluenza Virus Infections in the Immunopathology of the Respiratory Tract

Viral infections are leading causes of both upper and lower airway acute illness in all age groups of healthy persons, and have also been implicated in the acute exacerbations of chronic respiratory disorders like asthma and COPD. Human rhinovirus, respiratory syncytial virus, influenza virus and coronavirus have been considered as the most important respiratory pathogens and relatively little attention has been paid to the role of parainfluenza viruses (hPIVs). Human parainfluenza viruses are single-stranded RNA viruses belonging to the paramyxovirus family that may evoke lower respiratory infections in infants, children and immunocompromised individuals. Among non-immune compromised adults, hPIV infection typically causes mild disease manifested as upper respiratory tract symptoms and is infrequently associated with severe croup or pneumonia. Moreover, hPIV infection may be associated with viral exacerbations of chronic airway diseases, asthma or COPD or chronic rhinosinusitis. In this review, we summarized the basic epidemiology and immunology of hPIVs and addressed the more recent data implicating the role of parainfluenza viruses in the exacerbation of chronic airway disorders.

Mechanisms of temperature sensitivity of attenuated Urabe mumps virus

Temperature sensitivity is a phenotype often associated with attenuation of viruses. Previously, we purified several mumps variants from an incompletely attenuated Urabe strain live attenuated vaccine. Here we characterize one isolate that is sensitive to growth at high temperature. This virus was attenuated in a small animal model of mumps virulence, and we identified unique coding substitutions in the hemagglutinin-neuraminidase (HN), the viral polymerase (L) gene, and a non-coding substitution close to the anti-genome promoter sequences. At the non-permissive temperature, transcription of viral mRNAs and production of the replication intermediate were reduced compared to events at the permissive temperature and to a non-ts virulent Urabe virus. As well, synthesis of viral proteins was also reduced at the higher temperature. While the actual sequence substitutions in the ts virus were unique, the pattern of substitutions in HN, L and genome end sequences is similar to another attenuated Urabe virus previously described by us.

Constraints on the Genetic and Antigenic Variability of Measles Virus

Antigenic drift and genetic variation are significantly constrained in measles virus (MeV). Genetic stability of MeV is exceptionally high, both in the lab and in the field, and few regions of the genome allow for rapid genetic change. The regions of the genome that are more tolerant of mutations (i.e., the untranslated regions and certain domains within the N, C, V, P, and M proteins) indicate genetic plasticity or structural flexibility in the encoded proteins. Our analysis reveals that strong constraints in the envelope proteins (F and H) allow for a single serotype despite known antigenic differences among its 24 genotypes. This review describes some of the many variables that limit the evolutionary rate of MeV. The high genomic stability of MeV appears to be a shared property of the Paramyxovirinae, suggesting a common mechanism that biologically restricts the rate of mutation.

Small Animal Models for Human Metapneumovirus: Cotton Rat is More Permissive than Hamster and Mouse

Human metapneumovirus (hMPV) is the second most prevalent causative agent of pediatric respiratory infections worldwide. Currently, there are no vaccines or antiviral drugs against this virus. One of the major hurdles in hMPV research is the difficulty to identify a robust small animal model to accurately evaluate the efficacy and safety of vaccines and therapeutics. In this study, we compared the replication and pathogenesis of hMPV in BALB/c mice, Syrian golden hamsters, and cotton rats. It was found that BALB/c mice are not permissive for hMPV infection despite the use of a high dose (6.5 log₁₀ PFU) of virus for intranasal inoculation. In hamsters, hMPV replicated efficiently in nasal turbinates but demonstrated only limited replication in lungs. In cotton rats, hMPV replicated efficiently in both nasal turbinate and lung when intranasally administered with three different doses (4, 5, and 6 log₁₀ PFU) of hMPV. Lungs of cotton rats infected by hMPV developed interstitial pneumonia with mononuclear cells infiltrates and increased lumen exudation. By immunohistochemistry, viral antigens were detected at the luminal surfaces of the bronchial epithelial cells in lungs. Vaccination of cotton rats with hMPV completely protected upper and lower respiratory tract from wildtype challenge. The immunization also elicited elevated serum neutralizing antibody. Collectively, these results demonstrated that cotton rat is a robust small animal model for hMPV infection.

Development of oligomannose-coated liposome-based nasal vaccine against human parainfluenza virus type 3

Human parainfluenza viruses (HPIVs) are the etiologic agents of lower respiratory infections and pneumonia in infants, young children and immunocompromised hosts. The overarching goal for the prevention of HPIV infection is the development of an effective vaccine against HPIVs. In the present study, we investigated the effectiveness of oligomannose-coated liposomes (OMLs) as an antigen-delivery system in combination with a synthetic double-stranded RNA analog for the induction of mucosal and systematic immunity against HPIV3. Full-length hemagglutinin-neuraminidase (HN) protein was synthesized using the wheat germ cell-free protein production system and then encapsulated into OML to serve as the antigen. Intranasal administration of the HN-filling OML (OML-HN) with the synthetic double-stranded RNA adjuvant, polyriboinosinic-polyribocytidylic acid [poly(I:C)] generated significant viral-specific systemic and mucosal immune responses as evidenced by the prominent induction of serum IgG and nasal wash IgA, respectively. On the other hand, no significant immune responses were observed in mice immunized with OML-HN without the adjuvant. Furthermore, serum from mice immunized with OML-HN plus poly(I:C) significantly suppressed viral infection in cell culture model. Our results provide the first evidence that intranasal co-administration of OML-encapsulated HN with the poly(I:C) adjuvant augments the viral-specific immunity against HPIV3.

Safety and infectivity of two doses of live-attenuated recombinant cold-passaged human parainfluenza type 3 virus vaccine rHPIV3cp45 in HPIV3-seronegative young children

BACKGROUND

Human parainfluenza virus type 3 (HPIV3) is a common cause of upper and lower respiratory tract illness in infants and young children. Live-attenuated cold-adapted HPIV3 vaccines have been evaluated in infants but a suitable interval for administration of a second dose of vaccine has not been defined.

METHODS

HPIV3-seronegative children between the ages of 6 and 36 months were randomized 2:1 in a blinded study to receive two doses of 10⁵ TCID₅₀ (50% tissue culture infectious dose) of live-attenuated, recombinant cold-passaged human PIV3 vaccine (rHPIV3cp45) or placebo 6 months apart. Serum antibody levels were assessed prior to and approximately 4-6 weeks after each dose. Vaccine virus infectivity, defined as detection of vaccine-HPIV3 in nasal wash and/or a≥4-fold rise in serum antibody titer, and reactogenicity were assessed on days 3, 7, and 14 following immunization.

RESULTS

Forty HPIV3-seronegative children (median age 13 months; range 6-35 months) were enrolled; 27 (68%) received vaccine and 13 (32%) received placebo. Infectivity was detected in 25 (96%) of 26 evaluable vaccinees following doses 1 and 9 of 26 subject (35%) following dose 2. Among those who shed virus, the median duration of viral shedding was 12 days (range 6-15 days) after dose 1 and 6 days (range 3-8 days) after dose 2, with a mean peak log₁₀ viral titer of 3.4 PFU/mL (SD: 1.0) after dose 1 compared to 1.5 PFU/mL (SD: 0.92) after dose 2. Overall, reactogenicity was mild, with no difference in rates of fever and upper respiratory infection symptoms between vaccine and placebo groups.

CONCLUSION

rHPIV3cp45 was immunogenic and well-tolerated in seronegative young children. A second dose administered 6 months after the initial dose was restricted in those previously infected with vaccine virus; however, the second dose boosted antibody responses and induced antibody responses in two previously uninfected children.

Progress in the development of human parainfluenza virus vaccines

In children under 5 years of age, human parainfluenza viruses (HPIVs) as a group are the second most common etiology of acute respiratory illness leading to hospitalization, surpassed only by respiratory syncytial virus but ahead of influenza viruses. Using reverse genetics systems for HPIV serotypes 1, 2 and 3 (HPIV1, 2 and 3), several live-attenuated HPIVs have been generated and evaluated as intranasal vaccines in adults and in children. Two vaccines against HPIV3 were found to be well tolerated, infectious and immunogenic in Phase I trials in HPIV3-seronegative infants and children and should progress to proof-of-concept trials. Vaccines against HPIV1 and HPIV2 are less advanced and have just entered pediatric trials.

The cDNA-derived investigational human parainfluenza virus type 3 vaccine rcp45 is well tolerated, infectious, and immunogenic in infants and young children

BACKGROUND

Human parainfluenza virus type 3 (HPIV3) is an important yet underappreciated cause of lower respiratory tract illness in children, and a licensed vaccine is not yet available.

METHODS

A live-attenuated investigational HPIV3 vaccine virus designated rcp45 was derived from cDNA by using reverse genetics. rcp45 is genetically similar to the biologically derived cp45 vaccine virus and contains all of the known attenuating mutations of cp45, but has the advantage of a short, well-characterized passage history. We evaluated the tolerability, infectivity, and immunogenicity of 2 intranasal doses of rcp45 administered 4 to 10 weeks apart in a placebo-controlled, double-blind trial. A total of 45 infants and children between 6 and 36 months of age participated in this study. Tolerability and antibody responses to vaccine or placebo were assessed in all recipients. Infectivity was assessed by quantitation of vaccine virus shedding in a subset of vaccinated children.

RESULTS

rcp45 was well tolerated and highly infectious in HPIV3-seronegative children. A second dose of vaccine administered 4 to 10 weeks after the first dose was restricted in replication and did not boost serum antibody responses. The stability of 9 cp45 mutations, including the 6 major attenuating mutations, was examined and confirmed for viral isolates from 10 children.

CONCLUSIONS

The level of attenuation and immunogenicity of cDNA-derived rcp45 is comparable to what was previously observed with the biologically derived cp45 vaccine, and preliminary data suggest that the attenuating mutations in this vaccine virus are genetically stable. Continued clinical development of rcp45 is warranted.

Progress in respiratory virus vaccine development

Viral respiratory infections cause significant morbidity and mortality in infants and young children as well as in at-risk adults and the elderly. Although many viral pathogens are capable of causing respiratory disease, vaccine development has to focus on a limited number of pathogens, such as those that commonly cause serious lower respiratory illness (LRI). Whereas influenza virus vaccines have been available for some time (see the review by Clark and Lynch in this issue), vaccines against other medically important viruses such as respiratory syncytial virus (RSV), the parainfluenza viruses (PIVs), and metapneumovirus (MPVs) are not available. This review aims to provide a brief update on investigational vaccines against RSV, the PIVs, and MPV that have been evaluated in clinical trials or are currently in clinical development.

A randomized, double-blind, placebo-controlled, phase 1/2a study of the safety and immunogenicity of a live, attenuated human parainfluenza virus type 3 vaccine in healthy infants

OBJECTIVE

To evaluate the safety, tolerability, immunogenicity, and viral shedding profiles of a recombinant, live, attenuated human parainfluenza virus type 3 (HPIV3) vaccine, rHPIV3cp45, in healthy HPIV3-seronegative infants 6 to <12 months of age.

METHODS

In this double-blind, multicenter study, subjects were randomized 2:1 to receive a 10(5)TCID(50) dose of rHPIV3cp45 (n=20) or placebo (n=10) at enrollment and at 2 and 4 months after the first dose. Blood for evaluation of antibody to HPIV3 was collected at baseline and approximately 1 month after each dose. Solicited adverse events (SEs) and unsolicited adverse events (AEs) were collected on days 0-28 after each dose. Nasal wash samples for vaccine virus shedding were collected 3 times after each dose (7-10, 12-18, and 28-34 days post dose) and at unscheduled illness visits. Subjects were followed for 180 days after the last dose.

RESULTS

Vaccine virus was shed by 85% of vaccine recipients after dose 1, by 1 subject after dose 2, and was not shed by any subject after dose 3. The highest titer of shed virus was detected on day 7 after dose 1. The attenuation phenotype and the genotype of the vaccine virus were stable in shed virus. Seroresponse (≥ 4-fold rise in HPIV3 antibody from baseline) occurred in 61% of subjects after dose 1 and in 77% after dose 3. Either seroresponse or shedding occurred in 95% of vaccine subjects. Adverse events were similar in vaccine and placebo recipients.

CONCLUSION

The safety, shedding, and immunogenicity profiles of rHPIV3cp45 in HPIV3-seronegative infants 6 to <12 months of age support further development of this vaccine.

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.

Genetic characterization of Chinese measles vaccines by analysis of complete genomic sequences

The complete genomic sequences of two Chinese measles vaccine viruses, Shanghai-191 (S-191) and Changchun-47 (C-47), were determined and compared to the sequences of other measles vaccine strains as well as the prototype measles strain, Edmonston wild-type (Edwt). Compared to Edwt, S-191 and C-47 had 49 and 43 nucleotide changes, respectively. These differences were found at 52 nucleotide positions that were not found in other vaccine strains. Phylogenetic analysis of the all of the available genomic sequences for measles vaccines showed that S-191 and C-47 were most closely related to the Leningrad-4 strain. S-191 and C-47 shared conserved vaccine virus-specific amino acid changes in the phosphoprotein (P), V, C, matrix (M), and hemagglutinin (H) that could represent important targets for future studies aimed at understanding the molecular basis of attenuation. In addition, S-191 and C-47 had several unique amino acid changes including 13 positions that differed from Edwt. This is the first comparison of the complete genomic sequences of Chinese measles vaccines to the sequences of other vaccine strains.

Claire M, Elkins R, Collins PL, Murphy BR, Schaap-Nutt A. Recombinant human parainfluenza virus type 2 vaccine candidates containing a 3' genomic promoter mutation and L polymerase mutations are attenuated and protective in non-human primates

Previously, we identified several attenuating mutations in the L polymerase protein of human parainfluenza virus type 2 (HPIV2) and genetically stabilized those mutations using reverse genetics [Nolan SM, Surman S, Amaro-Carambot E, Collins PL, Murphy BR, Skiadopoulos MH. Live-attenuated intranasal parainfluenza virus type 2 vaccine candidates developed by reverse genetics containing L polymerase protein mutations imported from heterologous paramyxoviruses. Vaccine 2005;39(23):4765-74]. Here we describe the discovery of an attenuating mutation at nucleotide 15 (15(T-->C)) in the 3' genomic promoter that was also present in the previously characterized mutants. We evaluated the properties of this promoter mutation alone and in various combinations with the L polymerase mutations. Amino acid substitutions at L protein positions 460 (460A or 460P) or 948 (948L), or deletion of amino acids 1724 and 1725 (Delta1724), each conferred a temperature sensitivity (ts) phenotype whereas the 15(T-->C) mutation did not. The 460A and 948L mutations each contributed to restricted replication in the lower respiratory tract of African green monkeys, but the Delta1724 mutation increased attenuation only in certain combinations with other mutations. We constructed two highly attenuated viruses, rV94(15C)/460A/948L and rV94(15C)/948L/Delta1724, that were immunogenic and protective against challenge with wild-type HPIV2 in African green monkeys and, therefore, appear to be suitable for evaluation in humans.

Successful topical respiratory tract immunization of primates against Ebola virus

Ebola virus causes outbreaks of severe viral hemorrhagic fever with high mortality in humans. The virus is highly contagious and can be transmitted by contact and by the aerosol route. These features make Ebola virus a potential weapon for bioterrorism and biological warfare. Therefore, a vaccine that induces both systemic and local immune responses in the respiratory tract would be highly beneficial. We evaluated a common pediatric respiratory pathogen, human parainfluenza virus type 3 (HPIV3), as a vaccine vector against Ebola virus. HPIV3 recombinants expressing the Ebola virus (Zaire species) surface glycoprotein (GP) alone or in combination with the nucleocapsid protein NP or with the cytokine adjuvant granulocyte-macrophage colony-stimulating factor were administered by the respiratory route to rhesus monkeys--in which HPIV3 infection is mild and asymptomatic--and were evaluated for immunogenicity and protective efficacy against a highly lethal intraperitoneal challenge with Ebola virus. A single immunization with any construct expressing GP was moderately immunogenic against Ebola virus and protected 88% of the animals against severe hemorrhagic fever and death caused by Ebola virus. Two doses were highly immunogenic, and all of the animals survived challenge and were free of signs of disease and of detectable Ebola virus challenge virus. These data illustrate the feasibility of immunization via the respiratory tract against the hemorrhagic fever caused by Ebola virus. To our knowledge, this is the first study in which topical immunization through respiratory tract achieved prevention of a viral hemorrhagic fever infection in a primate model.

An improved method for the recovery of recombinant paramyxovirus vaccine candidates suitable for use in human clinical trials

We describe a method for the generation of clinical grade, live-attenuated vaccines in Vero cells entirely from cDNA plasmids. The entire electroporation procedure can be completed in less than 15 minutes and this is a significant improvement over previous lipid or electroporation based transfection techniques that also involve a heat-shock step. Importantly, the virus preparations can be generated with a minimal use of animal product derived materials, an important consideration for a vaccine candidate that is to be tested in humans. Since it is likely that all live-attenuated parainfluenza virus and pneumovirus vaccines in the future will be generated using reverse genetics, this simplified method provides guidance on how this can be achieved.

A serum-free Vero production platform for a chimeric virus vaccine candidate

MedImmune Vaccines has engineered a live, attenuated chimeric virus that could prevent infections caused by parainfluenza virus type 3 (PIV3) and respiratory syncytial virus (RSV), causative agents of acute respiratory diseases in infants and young children. The work here details the development of a serum-free Vero cell culture production platform for this virus vaccine candidate. Efforts to identify critical process parameters and optimize culture conditions increased infectious virus titers by approximately 2 log(10) TCID(50)/ml over the original serum-free process. In particular, the addition of a chemically defined lipid concentrate to the pre-infection medium along with the shift to a lower post-infection cultivation temperature increased virus titers by almost 100-fold. This improved serum-free process achieved comparable virus titers to the serum-supplemented process, and demonstrated consistent results upon scale-up: Vero cultures in roller bottles, spinner flasks and bioreactors reproducibly generated maximum infectious virus titers of 8 log(10) TCID(50)/ml.

The genomic termini of wild-type and vaccine strains of measles virus

The genomic termini from 18 strains of measles virus (MV) including wild-type MVs from the pre-vaccine period, recent wild-type isolates and various vaccine strains were sequenced. The first 25 nucleotides of the 3' terminus and last 52 nucleotides of the 5' terminus were conserved in all of the viruses examined. Nucleotides 26 and 42 of the 3' leader were A and G, respectively, in all genotype A viruses except Edmonston wild-type (Ed-WT). All non-genotype A viruses and Ed-WT had U in both positions. No consistent substitution pattern was found in the 5' trailer region of the genome. The nucleotide substitutions at positions 26 and 42 in the 3' leader region were introduced into a MV-CAT mini-genome to test for their effect on the production of reporter protein in both a vaccinia T7-driven, plasmid-based replication assay as well as in a helper virus system. Regardless of the source of the polymerase proteins or the natural leader sequence of the helper viruses, the mini-genome 26A42G produced more CAT protein than 26U42U. The nucleotide substitution at 26 had the greatest effect on CAT production. These results indicated that naturally occurring nucleotide variations in the 3' leader region can affect the levels of reporter protein synthesis, and presumably affected the level of replication of the virus.

Transmissibility, infectivity and immunogenicity of a live human parainfluenza type 3 virus vaccine (HPIV3cp45) among susceptible infants and toddlers

BACKGROUND

This study examined the transmissibility between young children of an intranasally administered live attenuated human parainfluenza virus type 3 (HPIV3)-cp45 vaccine candidate.

METHODS

Eighty subjects were enrolled in playgroups among whom there was at least one infected vaccinee in close contact with a seronegative placebo recipient over 21 days without a confounding infection with wtHPIV3. Following vaccination viral cultures were obtained on nine occasions to detect shedding and transmission of HPIV3cp45. Serum antibody titers were measured before and 7 weeks after vaccination.

RESULTS

No child fulfilled the criteria for transmission of HPIV3cp45 giving a risk of transmission of 0.04 (95% CI 0.01-0.19), hence establishing that HPIV3cp45 is less infectious than wtHPIV3 and risk of transmission is not a limitation to further clinical development of this vaccine candidate.

A tick-borne Langat virus mutant that is temperature sensitive and host range restricted in neuroblastoma cells and lacks neuroinvasiveness for immunodeficient mice

Langat virus (LGT), the naturally attenuated member of the tick-borne encephalitis virus (TBEV) complex, was tested extensively in clinical trials as a live TBEV vaccine and was found to induce a protective, durable immune response; however, it retained a low residual neuroinvasiveness in mice and humans. In order to ablate or reduce this property, LGT mutants that produced a small plaque size or temperature-sensitive (ts) phenotype in Vero cells were generated using 5-fluorouracil. One of these ts mutants, clone E5-104, exhibited a more than 10(3)-fold reduction in replication at the permissive temperature in both mouse and human neuroblastoma cells and lacked detectable neuroinvasiveness for highly sensitive immunodeficient mice. The E5-104 mutant possessed five amino acid substitutions in the structural protein E and one change in each of the nonstructural proteins NS3 and NS5. Using reverse genetics, we demonstrated that a Lys(46)-->Glu substitution in NS3 as well as a single Lys(315)-->Glu change in E significantly impaired the growth of LGT in neuroblastoma cells and reduced its peripheral neurovirulence for SCID mice. This study and our previous experience with chimeric flaviviruses indicated that a decrease in viral replication in neuroblastoma cells might serve as a predictor of in vivo attenuation of the neurotropic flaviviruses. The combination of seven mutations identified in the nonneuroinvasive E5-104 mutant provided a useful foundation for further development of a live attenuated TBEV vaccine. An evaluation of the complete sequence of virus recovered from brain of SCID mice inoculated with LGT mutants identified sites in the LGT genome that promoted neurovirulence/neuroinvasiveness.

Rational attenuation of a morbillivirus by modulating the activity of the RNA-dependent RNA polymerase

Negative-strand RNA viruses encode a single RNA-dependent RNA polymerase (RdRp) which transcribes and replicates the genome. The open reading frame encoding the RdRp from a virulent wild-type strain of rinderpest virus (RPV) was inserted into an expression plasmid. Sequences encoding enhanced green fluorescent protein (EGFP) were inserted into a variable hinge of the RdRp. The resulting polymerase was autofluorescent, and its activity in the replication/transcription of a synthetic minigenome was reduced. We investigated the potential of using this approach to rationally attenuate a virus by inserting the DNA sequences encoding the modified RdRp into a full-length anti-genome plasmid from which a virulent virus (rRPV(KO)) can be rescued. A recombinant virus, rRPV(KO)L-RRegfpR, which grew at an indistinguishable rate and to an identical titer as rRPV(KO) in vitro, was rescued. Fluorescently tagged polymerase was visible in large cytoplasmic inclusions and beneath the cell membrane. Subcutaneous injection of 10(4) TCID(50) of the rRPV(KO) parental recombinant virus into cattle leads to severe disease symptoms (leukopenia/diarrhea and pyrexia) and death by 9 days postinfection. Animals infected with rRPV(KO)L-RRegfpR exhibited transient leukopenia and mild pyrexia, and the only noticeable clinical signs were moderate reddening of one eye and a slight ocular-nasal discharge. Viruses that expressed the modified polymerase were isolated from peripheral blood lymphocytes and eye swabs. This demonstrates that a virulent morbillivirus can be attenuated in a single step solely by modulating RdRp activity and that there is not necessarily a correlation between virus growth in vitro and in vivo.

Live-attenuated intranasal parainfluenza virus type 2 vaccine candidates developed by reverse genetics containing L polymerase protein mutations imported from heterologous paramyxoviruses

Live-attenuated recombinant human parainfluenza virus type 2 (rHPIV2) vaccine candidates were created using reverse genetics by importing known attenuating mutations in the L polymerase protein from heterologous paramyxoviruses into the homologous sites of the HPIV2 L protein. Four recombinants (rF460L, rY948H, rL1566I, and rS1724I) were recovered and three were attenuated for replication in hamsters. The genetic stability of the imported mutations at three of the four sites was enhanced by use of alternative codons or by deletion of a pair of amino acids. rHPIV2s bearing these modified mutations exhibited enhanced attenuation. The genetically stabilized mutations conferring a high level of attenuation will be useful in generating a live-attenuated virus vaccine for HPIV2.

Human parainfluenza virus type I (HPIV1) vaccine candidates designed by reverse genetics are attenuated and efficacious in African green monkeys

A set of recombinant, live attenuated human parainfluenza virus type 1 (rHPIV1) vaccine candidates was evaluated for attenuation, immunogenicity, and protective efficacy in African green monkeys (AGMs). Temperature sensitive (ts) and non-ts attenuating (att) mutations in the P/C and L genes were introduced individually or in various combinations into rHPIV1, including the C(R84G) and HN(T553A) mutations identified in the present work and the C(F170S), L(Y942A), and L(L992C) mutations identified previously. The rHPIV1 vaccine candidates exhibited a spectrum of attenuation in AGMs. One genetically and phenotypically stable vaccine candidate, rC(R84G/F170S)L(Y942A/L992C), was attenuated and efficacious in AGMs and is a promising live attenuated intranasal HPIV1 vaccine candidate suitable for clinical evaluation.

Recombinant Sendai viruses with L1618V mutation in their L polymerase protein establish persistent infection, but not temperature sensitivity

The Sendai virus pi strain (SeVpi) isolated from cells persistently infected with SeV shows mainly two phenotypes: (1) temperature sensitivity and (2) an ability of establishing persistent infection (steady state). Three amino acid substitutions are found in the Lpi protein and are located at aa 1088, 1618, and 1664. Recombinant SeV(Lpi) (rSeV(Lpi)) having all these substitutions is temperature sensitive and is capable of establishing persistent infection (steady state). rSeVs carrying the fragment containing L1618V show both phenotypes. rSeV(L1618V), in which leucine at aa 1618 is replaced with valine, has the ability of establishing persistent infection, but is not a temperature-sensitive mutant, indicating that the ability of a virus to establish persistent infection can be separated from temperature sensitivity. The amino acid change at 1618(L-->V) coexisting with aa 1169 threonine is required for acquirement of a temperature-sensitive phenotype. Three amino acid substitutions are also found in the Ppi protein, but rSeV(Ppi) does not show these phenotypes.

The two major human metapneumovirus genetic lineages are highly related antigenically, and the fusion (F) protein is a major contributor to this antigenic relatedness

The growth properties and antigenic relatedness of the CAN98-75 (CAN75) and the CAN97-83 (CAN83) human metapneumovirus (HMPV) strains, which represent the two distinct HMPV genetic lineages and exhibit 5 and 63% amino acid divergence in the fusion (F) and attachment (G) proteins, respectively, were investigated in vitro and in rodents and nonhuman primates. Both strains replicated to high titers (> or =6.0 log(10)) in the upper respiratory tract of hamsters and to moderate titers (> or =3.6 log(10)) in the lower respiratory tract. The two lineages exhibited 48% antigenic relatedness based on reciprocal cross-neutralization assay with postinfection hamster sera, and infection with each strain provided a high level of resistance to reinfection with the homologous or heterologous strain. Hamsters immunized with a recombinant human parainfluenza virus type 1 expressing the fusion F protein of the CAN83 strain developed a serum antibody response that efficiently neutralized virus from both lineages and were protected from challenge with either HMPV strain. This result indicates that the HMPV F protein is a major antigenic determinant that mediates extensive cross-lineage neutralization and protection. Both HMPV strains replicated to low titers in the upper and lower respiratory tracts of rhesus macaques but induced high levels of HMPV-neutralizing antibodies in serum effective against both lineages. The level of HMPV replication in chimpanzees was moderately higher, and infected animals developed mild colds. HMPV replicated the most efficiently in the respiratory tracts of African green monkeys, and the infected animals developed a high level of HMPV serum-neutralizing antibodies (1:500 to 1:1,000) effective against both lineages. Reciprocal cross-neutralization assays in which postinfection sera from all three primate species were used indicated that CAN75 and CAN83 are 64 to 99% related antigenically. HMPV-infected chimpanzees and African green monkeys were highly protected from challenge with the heterologous HMPV strain. Taken together, the results from hamsters and nonhuman primates support the conclusion that the two HMPV genetic lineages are highly related antigenically and are not distinct antigenic subtypes or subgroups as defined by reciprocal cross-neutralization in vitro.

Identification of small-animal and primate models for evaluation of vaccine candidates for human metapneumovirus (hMPV) and implications for hMPV vaccine design

Human metapneumovirus (hMPV), a recently identified paramyxovirus, is the causative agent of respiratory tract disease in young children. Epidemiological studies have established the presence of hMPV in retrospective as well as current clinical samples in Europe, USA, Canada, Hong Kong and Australia. The hMPV disease incidence rate varied from 7 to 12 %. This rate of disease attack places hMPV in severity between respiratory syncytial virus and human parainfluenza virus type 3, two common respiratory pathogens of young children, the elderly and immunosuppressed individuals. To evaluate the effectiveness and safety of future hMPV antiviral drugs, therapeutic and prophylactic monoclonal antibodies (mAbs), and vaccine candidates, it was necessary to identify small-animal and primate models that efficiently supported hMPV replication in the respiratory tract and produced neutralizing serum antibodies, commonly a clinical correlate of protection in humans. In this study, various rodents (mice, cotton rats, hamsters and ferrets) and two primate species, rhesus macaques and African green monkeys (AGMs), were evaluated for hMPV replication in the respiratory tract. The results showed that hamsters, ferrets and AGMs supported hMPV replication efficiently and produced high levels of hMPV-neutralizing antibody titres. Hamsters vaccinated with subgroup A hMPV were protected from challenge with subgroup A or subgroup B hMPV, which has implications for hMPV vaccine design. Although these animal models do not mimic human hMPV disease signs, they will nevertheless be invaluable for the future evaluation of hMPV antivirals, mAbs and vaccines.

Codon substitution mutations at two positions in the L polymerase protein of human parainfluenza virus type 1 yield viruses with a spectrum of attenuation in vivo and increased phenotypic stability in vitro

The Y942H and L992F temperature-sensitive (ts) and attenuating amino acid substitution mutations, previously identified in the L polymerase of the HPIV3cp45 vaccine candidate, were introduced into homologous positions of the L polymerase of recombinant human parainfluenza virus type 1 (rHPIV1). In rHPIV1, the Y942H mutation specified the ts phenotype in vitro and the attenuation (att) phenotype in hamsters, whereas the L992F mutation specified neither phenotype. Each of these codon mutations was generated by a single nucleotide substitution and therefore had the potential to readily revert to a codon specifying the wild-type amino acid residue. We introduced alternative amino acid assignments at codon 942 or 992 as a strategy to increase genetic stability and to generate mutants that exhibit a range of attenuation. Twenty-three recombinants with codon substitutions at position 942 or 992 of the L protein were viable. One highly ts and att mutant, the Y942A virus, which had a difference of three nucleotides from the codon encoding a wild-type tyrosine, also possessed a high level of genetic and phenotypic stability upon serial passage in vitro at restrictive temperatures compared to that of the parent Y942H virus, which possessed a single nucleotide substitution. We obtained mutants with substitutions at position 992 that, in contrast to the L992F virus, possessed the ts and att phenotypes. These findings identify the use of alternative codon substitution mutations as a method that can be used to generate candidate vaccine viruses with increased genetic stability and/or a modified level of attenuation.

Generation of recombinant human parainfluenza virus type 1 vaccine candidates by importation of temperature-sensitive and attenuating mutations from heterologous paramyxoviruses

Human parainfluenza virus type 1 (HPIV1) is a significant cause of respiratory tract disease in infants and young children for which a vaccine is needed. In the present study, we sought to attenuate HPIV1 by the importation of one or more known attenuating point mutations from heterologous paramyxoviruses into homologous sites in HPIV1. The introduced mutations were derived from three attenuated paramyxoviruses: (i) HPIV3cp45, a live-attenuated HPIV3 vaccine candidate containing multiple attenuating mutations; (ii) the respiratory syncytial virus cpts530 with an attenuating mutation in the L polymerase protein; and (iii) a murine PIV1 (MPIV1) attenuated by a mutation in the accessory C protein. Recombinant HPIV1 (rHPIV1) mutants bearing a single imported mutation in C, any of three different mutations in L, or a pair of mutations in F exhibited a 100-fold or greater reduction in replication in the upper or lower respiratory tract of hamsters. Both temperature-sensitive (ts) (mutations in the L and F proteins) and non-ts (the mutation in the C protein) attenuating mutations were identified. rHPIV1 mutants containing a combination of mutations in L were generated that were more attenuated than viruses bearing the individual mutations, showing that the systematic accretion of mutations can yield progressive increases in attenuation. Hamsters immunized with rHPIV1 mutants bearing one or two mutations developed neutralizing antibodies and were resistant to challenge with wild-type HPIV1. Thus, importation of attenuating mutations from heterologous viruses is an effective means for rapidly identifying mutations that attenuate HPIV1 and for generating live-attenuated HPIV1 vaccine candidates.

Role of fusion protein cleavage site in the virulence of Newcastle disease virus

Newcastle disease virus (NDV) causes a highly contagious and economically important disease in poultry. Viral determinants of NDV virulence are not completely understood. The amino acid sequence at the protease cleavage site of the fusion (F) protein has been postulated as a major determinant of NDV virulence. In this study, we have examined the role of F protein cleavage site sequence in NDV virulence using reverse genetics technology. The sequence G-R-Q-G-R present at the cleavage site of the F protein of avirulent strain LaSota was mutated to R-R-Q-K-R, which is present in the F cleavage site of neurovirulent strain Beaudette C (BC). The resultant mutated LaSota V.F. virus did not require exogenous protease for infectivity in cell culture, indicating that the F protein was cleaved by intracellular proteases. The virulence of the mutant and parental viruses was evaluated in vivo by intracerebral pathogenicity index (ICPI) and intravenous pathogenicity index (IVPI) tests in chickens. Our results showed that the modification of the F protein cleavage site resulted in a dramatic increase in virulence from an ICPI value of 0.00 for LaSota to a value of 1.12 for LaSota V.F. However, the ICPI value of LaSota V.F. was lower than that of BC, which had a value of 1.58. Interestingly, the IVPI tests showed values of 0.00 for both LaSota and LaSota V.F. viruses, compared to the IVPI value of 1.45 of BC. In vitro characteristics of the viruses were also studied. Our results demonstrate that the efficiency of cleavage of the F protein plays an important role if the NDV is delivered directly into the brains of chicks, but there could be other viral factors that probably affect peripheral replication, viremia, or entry into the central nervous system.

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.

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.

Progress in the development of respiratory syncytial virus and parainfluenza virus vaccines

Respiratory syncytial virus (RSV) and human parainfluenza viruses (hPIVs) are leading causes of viral lower respiratory tract illness in children and in high-risk adult populations. Despite decades of research, licensed vaccines for RSV and hPIVs do not exist. Recently, however, genetically engineered live attenuated RSV and hPIV candidate vaccines have been generated, several of which are already being evaluated in clinical trials. Recombinant technology allows candidate vaccines to be "fine-tuned" in response to clinical data, which should hasten the development of vaccines against these important respiratory pathogens.

Effects of human metapneumovirus and respiratory syncytial virus antigen insertion in two 3' proximal genome positions of bovine/human parainfluenza virus type 3 on virus replication and immunogenicity

A live attenuated bovine parainfluenza virus type 3 (PIV3), harboring the fusion (F) and hemagglutinin-neuraminidase (HN) genes of human PIV3, was used as a virus vector to express surface glycoproteins derived from two human pathogens, human metapneumovirus (hMPV) and respiratory syncytial virus (RSV). RSV and hMPV are both paramyxoviruses that cause respiratory disease in young children, the elderly, and immunocompromised individuals. RSV has been known for decades to cause acute lower respiratory tract infections in young children, which often result in hospitalization, while hMPV has only been recently identified as a novel human respiratory pathogen. In this study, the ability of bovine/human PIV3 to express three different foreign transmembrane surface glycoproteins and to induce a protective immune response was evaluated. The RNA-dependent RNA polymerase of paramyxoviruses binds to a single site at the 3' end of the viral RNA genome to initiate transcription of viral genes. The genome position of the viral gene determines its level of gene expression. The promoter-proximal gene is transcribed with the highest frequency, and each downstream gene is transcribed less often due to attenuation of transcription at each gene junction. This feature of paramyxoviruses was exploited using the PIV3 vector by inserting the foreign viral genes at the 3' terminus, at position 1 or 2, of the viral RNA genome. These locations were expected to yield high levels of foreign viral protein expression stimulating a protective immune response. The immunogenicity and protection results obtained with a hamster model showed that bovine/human PIV3 can be employed to generate bivalent PIV3/RSV or PIV3/hMPV vaccine candidates that will be further evaluated for safety and efficacy in primates.

Sequence analysis of the Washington/1964 strain of human parainfluenza virus type 1 (HPIV1) and recovery and characterization of wild-type recombinant HPIV1 produced by reverse genetics

A complete consensus sequence was determined for the genomic RNA of human parainfluenza virus type 1 (HPIV1) strain Washington/20993/1964 (HPIV1 WASH/64), a clinical isolate that previously was shown to be virulent in adults. The sequence exhibited a high degree of relatedness to both Sendai virus, a PIV1 virus recovered from mice, and human PIV3 (HPIV3) with regard to cis-acting regulatory regions and protein-coding sequences. This consensus sequence was used to generate a full-length antigenomic cDNA and to recover a recombinant wild-type HPIV1 (rHPIV1). Interestingly, the rHPIV1 could be rescued from full-length antigenomic rHPIV1 cDNA using HPIV3 support plasmids, HPIV1 support plasmids, or a mixture thereof. The replication of rHPIV1 in vitro and in the respiratory tract of hamsters was similar to that of its biologically derived parent virus. The similar biological properties of rHPIV1 and HPIV1 WASH/64 in vitro and in vivo, together with the previous demonstration of the virulence of this specific isolate in humans, authenticates the rHPIV1 sequence as that of a wild-type virus. This rHPIV1 can now be used to study the biological properties of HPIV1 and as a substrate to introduce attenuating mutations for the generation of live-attenuated HPIV1 vaccine candidates.

A live human parainfluenza type 3 virus vaccine is attenuated and immunogenic in young infants

BACKGROUND

Parainfluenza type 3 virus (PIV-3) infections cause lower respiratory tract illness in children throughout the world. A licensed PIV-3 vaccine is not yet available.

METHODS

A live attenuated cold-adapted (ca) and temperature-sensitive (ts) PIV-3 vaccine, designated cp-45, was evaluated sequentially in open label studies in 20 adults and in placebo-controlled, double blind studies in 24 PIV-3-seropositive children, 52 PIV-3-seronegative infants and children and 49 infants 1 to 2 months old. A single dose of this intranasal vaccine was evaluated in adults [106 plaque-forming units (pfu)] and seropositive children, and 104 and 105 pfu were evaluated in seronegative children. In the infant study, two 104 pfu doses of vaccine were administered at 1- or 3-month intervals. Safety, infectivity, immunogenicity and phenotypic stability of the vaccine were evaluated in all cohorts.

RESULTS

The cp-45 vaccine was well-tolerated in all age groups and infected 94% of vaccinated seronegative children and 94% of vaccinated infants. Although immunization with the first dose of cp-45 diminished the replication of a second dose in all infants, those immunized after 3 months shed vaccine virus more frequently than those immunized after 1 month (62% vs. 24%, respectively). Antibody responses to PIV-3 were readily detected in seronegative children with a variety of assays; however, the IgA response to the viral hemagglutinin-neuraminidase was the best measure of immunogenicity in young infants. Of 109 vaccine virus specimens recovered from nasal washes, 98 were ts and 11 were temperature-sensitive intermediate (tsi) viruses, with pinpoint plaques visible at 40 degrees C. tsi viruses appeared transiently at the time of peak viral replication, represented a very small proportion of the total virus shed and were not associated with changes in clinical status. ca revertants were not detected.

CONCLUSIONS

The cp-45 vaccine is appropriately attenuated and immunogenic in infants as young as 1 month of age. Further development of this vaccine is warranted.

Determinants of the host range restriction of replication of bovine parainfluenza virus type 3 in rhesus monkeys are polygenic

The Kansas strain of bovine parainfluenza virus type 3 (BPIV3) is 100- to 1,000-fold restricted in replication in the respiratory tracts of nonhuman primates compared to human PIV3 (HPIV3), an important pathogen of infants and young children. BPIV3 is also restricted in replication in human infants and children, yet it is immunogenic and is currently being evaluated in clinical trials as a vaccine candidate to protect against illness caused by HPIV3. We have examined the genetic basis for the host range attenuation phenotype of BPIV3 by exchanging each open reading frame (ORF) of a recombinant wild-type HPIV3 with the analogous ORF from BPIV3, with the caveats that the multiple ORFs of the P gene were exchanged as a single unit and that the HN and F genes were exchanged as a single unit. Recombinant chimeric bovine-human PIV3s were recovered from cDNA, and the levels of viral replication in vitro and in the respiratory tract of rhesus monkeys were determined. Recombinant chimeric HPIV3s bearing the BPIV3 N or P ORF were highly attenuated in the upper and lower respiratory tracts of monkeys, whereas those bearing the BPIV3 M or L ORF or the F and HN genes were only moderately attenuated. This indicates that the genetic determinants of the host range restriction of replication of BPIV3 for primates are polygenic, with the major determinants being the N and P ORFs. Monkeys immunized with these bovine-human chimeric viruses, including the more highly attenuated ones, developed higher levels of HPIV3 hemagglutination-inhibiting serum antibodies than did monkeys immunized with BPIV3 and were protected from challenge with wild-type HPIV3. Furthermore, host range determinants could be combined with attenuating point mutations to achieve an increased level of attenuation. Thus, chimeric recombinant bovine-human PIV3 viruses that manifest different levels of attenuation in rhesus monkeys are available for evaluation as vaccine candidates to protect infants from the severe lower respiratory tract disease caused by HPIV3.

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.

Evaluation of the replication and immunogenicity of recombinant human parainfluenza virus type 3 vectors expressing up to three foreign glycoproteins

The level of replication and immunogenicity of recombinant parainfluenza virus type 3 (rHPIV3) bearing one, two, or three gene insertions expressing foreign protective antigens was examined. cDNA-derived recombinant HPIV3s bearing genes encoding the open reading frames (ORFs) of the hemagglutinin-neuraminidase (HN) of HPIV1, the HN of HPIV2, or the hemagglutinin (HA) of measles virus replicated efficiently in vitro, including the largest recombinant, which had three gene unit insertions and which was almost 23 kb in length, 50% longer than unmodified HPIV3. Several viruses were recovered from cDNAs whose genome length was not a multiple of six nucleotides and these contained nucleotide insertions that corrected the length to be a multiple of 6, confirming that the "rule of six" applies to HPIV3. Using a hemagglutination inhibition assay, we determined that the HPIV1 HN expressed by recombinant HPIV3 was incorporated into HPIV3 virions, whereas using this assay incorporation of the HPIV2 HN could not be detected. HPIV3 virions bearing HPIV1 HN were not neutralized by HPIV1 antiserum but were readily neutralized by antibodies to the HPIV3 HN or fusion protein (F). Viruses with inserts were restricted for replication in the respiratory tract of hamsters, and the level of restriction was a function of the total number of genes inserted, the nature of the insert, and the position of the inserted gene in the gene order. A single insert of HPIV2 HN or measles virus HA reduced the in vivo replication of rHPIV3 up to 25-fold, whereas the HPIV1 HN insert decreased replication almost 1000-fold. This indicates that the HPIV1 HN insert has an attenuating effect in addition to that of the extra gene insert itself, presumably because it is incorporated into the virus particle. Viruses containing two inserts were generally more attenuated than those with a single insert, and viruses with three inserts were over-attenuated for replication in hamsters. Inserts between the N and P genes were slightly more attenuating than those between the P and the M genes. A recombinant HPIV3 bearing both the HPIV1 and the HPIV2 HN genes (r1HN 2HN) was attenuated, immunogenic, and protected immunized hamsters from challenge with HPIV1, HPIV2, and HPIV3. Thus, it is possible to use a single HPIV vector expressing two foreign gene inserts to protect infants and young children from the severe lower respiratory tract disease caused by the three major human PIV pathogens.

The recombinant chimeric human parainfluenza virus type 1 vaccine candidate, rHPIV3-1cp45, is attenuated, immunogenic, and protective in African green monkeys

A recombinant live-attenuated chimeric human parainfluenza virus type 1 (HPIV1) candidate vaccine was previously generated by replacing the fusion (F) and hemagglutinin-neuraminidase (HN) glycoprotein open reading frames (ORFs) of the HPIV3 candidate vaccine, rHPIV3cp45, with those of wild-type HPIV1. Previously, this recombinant chimeric virus, designated rHPIV3-1cp45, exhibited a greater level of the temperature sensitivity of replication in vitro and a greater level of attenuation of replication in the respiratory tract of immunized hamsters when compared to its HPIV3cp45 parent virus. In the present study, rHPIV3-1cp45 was evaluated for its level of attenuation and efficacy in African green monkeys (Cercopithecus aethiops), a primate in which both HPIV1 and HPIV3 wild-type viruses replicate efficiently. The rHPIV3-1cp45 candidate vaccine was as restricted in replication in the upper and lower respiratory tract as its thoroughly characterized rHPIV3cp45 parent indicating that the attenuating mutations present in the rHPIV3cp45 backbone specified an appropriate level of attenuation of rHPIV3-1cp45 for primates. The level to which rHPIV3-1cp45 replicated in the respiratory tract of African green monkeys was also sufficient to induce a strong immune response to HPIV1 and provided protection against challenge with wild-type HPIV1. These results provide a basis for further evaluation of this HPIV1 candidate vaccine in humans.

Expression of a foreign gene by recombinant canine distemper virus recovered from cloned DNAs

A canine distemper virus (CDV) genomic cDNA clone and expression plasmids required to establish a CDV rescue system were generated from a laboratory-adapted strain of the Onderstepoort vaccine virus. In addition, a CDV minireplicon was prepared and used in transient expression studies performed to identify optimal virus rescue conditions. Results from the transient expression experiments indicated that minireplicon-encoded reporter gene activity was increased when transfected cell cultures were maintained at 32 rather than 37 degrees C, and when the cellular stress response was induced by heat shock. Applying these findings to rescue of recombinant CDV (rCDV) resulted in efficient recovery of virus after transfected HEp2 or A549 cells were co-cultured with Vero cell monolayers. Nucleotide sequence determination and analysis of restriction site polymorphisms confirmed that rescued virus was rCDV. A rCDV strain also was engineered that contained the luciferase gene inserted between the P and M genes; this virus directed high levels of luciferase expression in infected cells.

Complete genome comparison of porcine reproductive and respiratory syndrome virus parental and attenuated strains

Two full-length porcine reproductive and respiratory syndrome virus (PRRSV) genomes, strain VR-2332 and its cell culture passaged descendent RespPRRS vaccine strain, were compared and analyzed in order to identify possible sites of attenuation. Of the 41 nucleotide changes, 12 resulted in conservative changes and 18 produced non-conservative changes. The results suggest that key amino acids in ORF1 may contribute to the phenotype of RespPRRS, which includes increased growth rate on MA-104 cells and decreased virulence in swine. The results provide a genetic basis for future manipulation of a PRRSV reverse genetics system.

Construction of a live-attenuated bivalent vaccine virus against human parainfluenza virus (PIV) types 1 and 2 using a recombinant PIV3 backbone

PIV1 and PIV2 are important agents of pediatric respiratory tract disease. We are developing live-attenuated vaccines against these viruses. We earlier constructed a PIV3/PIV1 antigenic chimeric virus, designated rPIV3-1, in which the hemagglutinin-neuraminidase (HN) and fusion (F) proteins of wild type rPIV3 were replaced by their PIV1 counterparts. In the present study, rPIV3-1 was used as a vector to express the HN protein of PIV2 to generate a single virus capable of inducing immunity to both PIV1 and PIV2. The PIV2 HN open reading frame was expressed from an extra gene cassette, under the control of PIV3 cis-acting transcription signals, inserted between the F and HN genes of rPIV3-1. The recombinant derivative, designated rPIV3-1.2HN, was readily recovered and exhibited a level of temperature sensitivity and in vitro growth similar to that of its parental virus. The rPIV3-1.2HN virus was restricted in replication in both the upper and lower respiratory tracts of hamsters compared with rPIV3-1, identifying an attenuating effect of the PIV2 HN insert in hamsters. rPIV3-1.2HN elicited serum antibodies to both PIV1 and PIV2 and induced resistance against challenge with wild type PIV1 or PIV2. Thus, rPIV3-1.2HN, a virus attenuated solely by the insertion of the PIV2 HN gene, functioned as a live attenuated bivalent vaccine candidate against both PIV1 and PIV2.