The temperature-sensitive (ts) phenotype of a cold-passaged (cp) live attenuated respiratory syncytial virus vaccine candidate, designated cpts530, results from a single amino acid substitution in the L protein.

cpts530, a candidate live-virus vaccine, is an attenuated strain of human respiratory syncytial virus (RSV). It was derived by subjecting a cold-passaged (cp) strain of RSV to a single round of chemical mutagenesis. cpts530 is a temperature-sensitive (ts) mutant that is attenuated in mice and chimpanzees, and its ts phenotype exhibits a high level of stability during replication in both species. In the present study, the complete nucleotide sequence of cpts530 RSV was determined. The five mutations known to be present in the parent cpRSV were retained in its cpts530 derivative, and one additional nucleotide change was identified at nucleotide (nt) 10060, which resulted in a phenylalanine-to-leucine change at amino acid 521 in the large polymerase (L) protein. To determine if this single amino acid substitution was indeed responsible for the ts phenotype of cpts530, it was introduced alone or in combination with the cp mutations into the full-length cDNA clone of the wild-type A2 RSV. Analysis of infectious viruses recovered from mutant cDNAs indicated that this single mutation specified complete restriction of plaque formation of recombinant cp530 in HEp-2 cell monolayer cultures at 40 degrees C, and the level of temperature sensitivity was not influenced by the presence of the five cpRSV mutations. These findings identify the phenylalanine-to-leucine change at amino acid 521 in the L protein as the mutation that specifies the ts phenotype of cpts530. Furthermore, these findings illustrate the feasibility of using the cDNA-based recovery system to analyze and construct defined attenuated vaccine viruses.     

Recombinant respiratory syncytial viruses lacking the C-terminal third of the attachment (G) protein are immunogenic and attenuated in vivo and in vitro

The design of attenuated vaccines for respiratory syncytial virus (RSV) historically focused on viruses made sensitive to physiologic temperature through point mutations in the genome. These prototype vaccines were not suitable for human infants primarily because of insufficient attenuation, genetic instability, and reversion to a less-attenuated phenotype. We therefore sought to construct novel attenuated viruses with less potential for reversion through genetic alteration of the attachment G protein. Complete deletion of G protein was previously shown to result in RSV strains overly attenuated for replication in mice. Using reverse genetics, recombinant RSV (rRSV) strains were engineered with truncations at amino acid 118, 174, 193, or 213 and respectively designated rA2cpDeltaG118, rA2cpDeltaG174, rA2cpDeltaG193, and rA2cpDeltaG213. All rA2cpDeltaG strains were attenuated for growth in vitro and in the respiratory tracts of BALB/c mice but not restricted for growth at 37 degrees C. The mutations did not significantly affect nascent genome synthesis in human lung epithelial (A549) cells, but infectious rA2cpDeltaG virus shed into the culture medium was dramatically diminished. Hence, the data suggested that a site within the C-terminal 85 amino acids of G protein is important for efficient genome packaging or budding of RSV from the infected cell. Vaccination with the rA2cpDeltaG strains also generated efficacious immune responses in mice that were similar to those elicited by the temperature-sensitive cpts248/404 strain previously tested in human infants. Collectively, the data indicate that the rA2cpDeltaG strains are immunogenic, not likely to revert to the less-attenuated phenotype, and thus candidates for further development as vaccines against RSV.     

Recombinant respiratory syncytial virus bearing a deletion of either the NS2 or SH gene is attenuated in chimpanzees

The NS2 and SH genes of respiratory syncytial virus (RSV) have been separately deleted from a recombinant wild-type RSV strain, A2 (M. N. Teng and P. L. Collins, J. Virol. 73:466-473, 1998; A. Bukreyev et al., J. Virol. 71:8973-8982, 1997; and this study). The resulting viruses, designated rA2DeltaNS2 and rA2DeltaSH, were administered to chimpanzees to evaluate their levels of attenuation and immunogenicity. Recombinant virus rA2DeltaNS2 replicated to moderate levels in the upper respiratory tract, was highly attenuated in the lower respiratory tract, and induced significant resistance to challenge with wild-type RSV. The replication of rA2DeltaSH virus was only moderately reduced in the lower, but not the upper, respiratory tract. However, chimpanzees infected with either virus developed significantly less rhinorrhea than those infected with wild-type RSV. These findings demonstrate that a recombinant RSV mutant lacking either the NS2 or SH gene is attenuated and indicate that these deletions may be useful as attenuating mutations in new, live recombinant RSV vaccine candidates for both pediatric and elderly populations. The DeltaSH mutation was incorporated into a recombinant form of the cpts248/404 vaccine candidate, was evaluated for safety in seronegative chimpanzees, and can now be evaluated as a vaccine for humans.     

Recombinant Respiratory Syncytial Virus (RSV) Bearing a Set of Mutations from Cold-Passaged RSV Is Attenuated in Chimpanzees

A set of five missense mutations previously identified by nucleotide sequence analysis of subgroup A cold-passaged (cp) respiratory syncytial virus (RSV) has been introduced into a recombinant wild-type strain of RSV. This recombinant virus, designated rA2cp, appears to replicate less efficiently in the upper and lower respiratory tracts of seronegative chimpanzees than either biologically derived or recombinant wild-type RSV. Infection with rA2cp also resulted in significantly less rhinorrhea and cough than infection with wild-type RSV. These findings confirm the role of the cp mutations in attenuation of RSV and identify their usefulness for inclusion in future live attenuated recombinant RSV vaccine candidates.     

人呼吸道合胞病毒活疫苗研究进展

人呼吸道合胞病毒是引起婴幼儿支气管炎和肺炎的主要原因,也可导致免疫缺陷病人及老年人群显著发病和死亡.人呼吸道合胞病毒疫苗已被世界卫生组织(World Health Organization,WHO)列为全球最优先发展的疫苗之一.经过50多年的研究,尤其是随着重组技术和反向遗传学的出现,对RSV疫苗的研究取得了重要进展,...     

Characterization of recombinant respiratory syncytial viruses with the region responsible for type 2 T-cell responses and pulmonary eosinophilia deleted from the attachment (G) protein

It is essential that preventative vaccines for respiratory syncytial virus (RSV) elicit balanced T-cell responses. Immune responses dominated by type 2 T cells against RSV antigens are believed to cause exaggerated respiratory tract disease and may also contribute to unwanted inflammation in the airways that predisposes infants to wheeze through adolescence. Here we report on the construction and characterization of recombinant RSV (rRSV) strains with amino acids 151 to 221 or 178 to 219 of the attachment (G) glycoprotein deleted (rA2cpDeltaG150-222 or rA2cpDeltaG177-220, respectively). The central ectodomain was chosen for modification because a peptide spanning amino acids 149 to 200 of G protein has recently been shown to prime several strains of na?ve inbred mice for polarized type 2 T-cell responses, and peripheral blood T cells from most human donors recognize epitopes within this region. Quantitative PCR demonstrated that synthesis of nascent rRSV genomes in human lung epithelial cell lines was similar to that for the parent virus (cp-RSV). Plaque assays further indicated that rRSV replication was not sensitive to 37 degrees C, but pinpoint morphology was observed at 39 degrees C. Both rRSV strains replicated in the respiratory tracts of BALB/c mice and elicited serum neutralization and anti-F-protein immunoglobulin G titers that were equivalent to those elicited by cp-RSV and contributed to a 3.9-log(10)-unit reduction in RSV A2 levels 4 days after challenge. Importantly, pulmonary eosinophilia was significantly diminished in BALB/c mice primed with native G protein and challenged with either rA2cpDeltaG150-222 or rA2cpDeltaG177-220. These findings are important for the development of attenuated RSV vaccines.     

Virally delivered cytokines alter the immune response to future lung infections

Respiratory syncytial virus (RSV) is an important cause of infant morbidity and mortality worldwide and is increasingly recognized to have a role in the development and exacerbation of chronic lung diseases. There is no effective vaccine, and we reasoned that it might be possible to skew the immune system towards beneficial nonpathogenic responses by selectively priming protective T-cell subsets. We therefore tested recombinant RSV (rRSV) candidates expressing prototypic murine Th1 (gamma interferon [IFN-γ]) or Th2 (interleukin-4 [IL-4]) cytokines, with detailed monitoring of responses to subsequent infections with RSV or (as a control) influenza A virus. Although priming with either recombinant vector reduced viral load during RSV challenge, enhanced weight loss and enhanced pulmonary influx of RSV-specific CD8⁺ T cells were observed after challenge in mice primed with rRSV/IFN-γ. By contrast, rRSV/IL-4-primed mice were protected against weight loss during secondary challenge but showed airway eosinophilia. When rRSV/IL-4-primed mice were challenged with influenza virus, weight loss was attenuated but was again accompanied by marked airway eosinophilia. Thus, immunization directed toward enhancement of Th1 responses reduces viral load but is not necessarily protective against disease. Counter to expectation, Th2-biased responses were more beneficial but also influenced the pathological effects of heterologous viral challenge.     

Respiratory syncytial virus (RSV) G glycoprotein is not necessary for vaccine-enhanced disease induced by immunization with formalin-inactivated RSV

Following respiratory syncytial virus (RSV) challenge, mice immunized with RSV G or with formalin-inactivated RSV (FI-RSV) exhibit severe disease associated with type 2 cytokine production and pulmonary eosinophilia. This has led to the proposal that the presence of RSV G is the factor in FI-RSV that induces disease-enhancing T-cell responses. Therefore, we evaluated the role of RSV G and its immunodominant region in the induction of aberrant immune responses during FI-RSV immunization. BALB/c mice were immunized with FI preparations of wild-type (wt) RSV or recombinant RSV (rRSV) containing deletions of (i) the entire G gene, (ii) the region of the G gene encoding amino acids 187 to 197 of the immunodominant region, or (iii) the entire SH gene. After challenge, illness, RSV titers, cytokine levels, and pulmonary eosinophilia were measured. Peak RSV titers postchallenge were significantly greater in mice immunized with FI preparations of the deletion viruses than in those immunized with FI-rRSV wt, suggesting that the absence of G or SH in FI-RSV reduced its protective efficacy. Deletion of G or its epitope did not reduce illness, cytokine production, or eosinophilia relative to that in mice immunized with FI-rRSV wt. While cytokine levels and eosinophilia were similar, illness was reduced in mice immunized with SH-deleted FI-RSV. These data suggest that G-specific immune responses may be important for vaccine-induced protection and are not solely the basis for FI-RSV vaccine-enhanced illness. These data suggest that the method of RSV antigen delivery, rather than the protein composition, influences the phenotype of the induced immune responses and that RSV G should not necessarily be excluded from potential vaccine strategies.     

Replacement of the F and G proteins of respiratory syncytial virus (RSV) subgroup A with those of subgroup B generates chimeric live attenuated RSV subgroup B vaccine candidates

Human respiratory syncytial virus (RSV) exists as two antigenic subgroups, A and B, both of which should be represented in a vaccine. The F and G glycoproteins are the major neutralization and protective antigens, and the G protein in particular is highly divergent between the subgroups. The existing system for reverse genetics is based on the A2 strain of RSV subgroup A, and most efforts to develop a live attenuated RSV vaccine have focused on strain A2 or other subgroup A viruses. In the present study, the development of a live attenuated subgroup B component was expedited by the replacement of the F and G glycoproteins of recombinant A2 virus with their counterparts from the RSV subgroup B strain B1. This gene replacement was initially done for wild-type (wt) recombinant A2 virus to create a wt AB chimeric virus and then for a series of A2 derivatives which contain various combinations of A2-derived attenuating mutations located in genes other than F and G. The wt AB virus replicated in cell culture with an efficiency which was comparable to that of the wt A2 and B1 parents. AB viruses containing temperature-sensitive mutations in the A2 background exhibited levels of temperature sensitivity in vitro which were similar to those of A2 viruses bearing the same mutations. In chimpanzees, the replication of the wt AB chimera was intermediate between that of the A2 and B1 wt viruses and was accompanied by moderate rhinorrhea, as previously seen in this species. An AB chimeric virus, rABcp248/404/1030, which was constructed to contain a mixture of attenuating mutations derived from two different biologically attenuated A2 viruses, was highly attenuated in both the upper and lower respiratory tracts of chimpanzees. This attenuated AB chimeric virus was immunogenic and conferred a high level of resistance on chimpanzees to challenge with wt AB virus. The rABcp248/404/1030 chimeric virus is a promising vaccine candidate for RSV subgroup B and will be evaluated next in humans. Furthermore, these results suggest that additional attenuating mutations derived from strain A2 can be inserted into the A2 background of the recombinant chimeric AB virus as necessary to modify the attenuation phenotype in a reasonably predictable manner to achieve an optimal balance between attenuation and immunogenicity in a virus bearing the subgroup B antigenic determinants.     

Recombinant respiratory syncytial virus that does not express the NS1 or M2-2 protein is highly attenuated and immunogenic in chimpanzees

Mutant recombinant respiratory syncytial viruses (RSV) which cannot express the NS1 and M2-2 proteins, designated rA2DeltaNS1 and rA2DeltaM2-2, respectively, were evaluated as live-attenuated RSV vaccines. The rA2DeltaNS1 virus contains a large deletion that should have the advantageous property of genetic stability during replication in vitro and in vivo. In vitro, rA2DeltaNS1 replicated approximately 10-fold less well than wild-type recombinant RSV (rA2), while rA2DeltaM2-2 had delayed growth kinetics but reached a final titer similar to that of rA2. Each virus was administered to the respiratory tracts of RSV-seronegative chimpanzees to assess replication, immunogenicity, and protective efficacy. The rA2DeltaNS1 and rA2DeltaM2-2 viruses were 2,200- to 55,000-fold restricted in replication in the upper and lower respiratory tracts but induced a level of RSV-neutralizing antibody in serum that was only slightly reduced compared to the level induced by wild-type RSV. The replication of wild-type RSV in immunized chimpanzees after challenge was reduced more than 10,000-fold at each site. Importantly, rA2DeltaNS1 and rA2DeltaM2-2 were 10-fold more restricted in replication in the upper respiratory tract than was the cpts248/404 virus, a vaccine candidate that retained mild reactogenicity in the upper respiratory tracts of 1-month-old infants. Thus, either rA2DeltaNS1 or rA2DeltaM2-2 might be appropriately attenuated for this age group, which is the major target population for an RSV vaccine. In addition, these results show that neither NS1 nor M2-2 is essential for RSV replication in vivo, although each is important for efficient replication.     

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.     

Increased genetic and phenotypic stability of a promising live-attenuated respiratory syncytial virus vaccine candidate by reverse genetics

Human respiratory syncytial virus (RSV) is the most important viral cause of serious pediatric respiratory illness worldwide. Currently, the most promising live-attenuated vaccine candidate is a temperature-sensitive (ts) cDNA-derived virus named rA2cp248/404/1030ΔSH, in reference to its set of attenuating mutations. In a previous clinical study, more than one-third of postvaccination nasal wash isolates exhibited partial loss of the ts phenotype. Most of this instability appeared to be due to reversion at a missense point mutation called 1030. This 1030 mutation is a single-nucleotide tyrosine-to-asparagine substitution at position 1321 (Y1321N) of the polymerase L protein that contributes to the ts and attenuation phenotypes of the vaccine candidate. The goals of the present study were to identify a reversion-resistant codon at position 1321 conferring a comparable level of attenuation and to use this to develop a genetically stable version of the vaccine virus. We modified wild-type (wt) RSV to insert each of the 20 possible amino acids at position 1321; 19 viruses were recoverable. We also investigated small deletions at or near this position, but these viruses were not recoverable. Phenotypic analysis identified alternative attenuating amino acids for position 1321. Several of these amino acids were predicted, based on the genetic code, to be refractory to deattenuation. Classical genetics, using temperature stress tests in vitro combined with nucleotide sequencing, confirmed this stability but identified a second site with a compensatory mutation at position 1313. It was possible to stabilize the 1313 site as well, providing a stable 1030 mutation. Further stress tests identified additional incidental mutations, but these did not reverse the ts/attenuation phenotype. An improved version of the vaccine candidate virus was constructed and validated in vitro by temperature stress tests and in vivo by evaluation of attenuation in seronegative chimpanzees. In addition to developing an improved version of this promising live-attenuated RSV vaccine candidate, this study demonstrated the propensity of an RNA virus to escape from attenuation but also showed that, through systematic analysis, genetics can be used to cut off the routes of escape.     

西藏地区儿童急性呼吸道感染中呼吸道合胞病毒的初步研究

本研究为了解西藏地区儿童急性呼吸道感染中呼吸道合胞病毒(Respiratory syncytial virus,RSV)及基因型别。首先采用直接免疫荧光法检测2011年4～7月西藏自治区人民医院儿科病房因急性呼吸道感染住院患儿的鼻咽分泌物标本中7种常见的呼吸道病毒及人类偏肺病毒(Human metapneumovirus,hMPV)的抗原。然后对RSV抗原阳性的标本分别提取RNA,用逆转录-巢式聚合酶链反应法(Nest-PCR)确定RSV型别,同时用实时荧光PCR(Real-Time PCR)方法进行验证。再通过对G蛋白基因PCR扩增产物序列测定确定RSV的基因型。通过与GenBank中不同地区RSV分离株的G蛋白基因序列比对,了解西藏地区RSV G蛋白的结构特点及变异情况。结果表明,从167例标本中检测出呼吸道病毒抗原阳性的为65例,总阳性率为38.9%(65/167),其中RSV 45例,占阳性标本的69.2%(45/65),对其中42例RSV阳性标本进行了PCR分型,其中40例为A亚型,2例为B亚型。对7株A亚型RSV G蛋白基因PCR产物测序结果显示,全部为GA2基因型。西藏RSV与RSV原型株A2株核苷酸的同源性为90.7%～91.8%,氨基酸的同源性只有86.5%～87.2%。氨基酸的变异主要集中在胞外区一个高度保守序列的两端。7株西藏A亚型RSV G蛋白的核苷酸序列与GenBank中不同的RSV分离株相比同源性为90.7%～91.8%。西藏地区2011年春季小儿急性呼吸道感染的病毒病原主要为呼吸道合胞病毒,A亚型是2011年西藏地区的流行优势型别,其G蛋白胞外区基因具有较高的变异性。     

Viral RNA N6-methyladenosine modification modulates both innate and adaptive immune responses of human respiratory syncytial virus

Human respiratory syncytial virus (RSV) is the leading cause of respiratory tract infections in humans. A well-known challenge in the development of a live attenuated RSV vaccine is that interferon (IFN)-mediated antiviral responses are strongly suppressed by RSV nonstructural proteins which, in turn, dampens the subsequent adaptive immune responses. Here, we discovered a novel strategy to enhance innate and adaptive immunity to RSV infection. Specifically, we found that recombinant RSVs deficient in viral RNA N⁶-methyladenosine (m⁶A) and RSV grown in m⁶A methyltransferase (METTL3)-knockdown cells induce higher expression of RIG-I, bind more efficiently to RIG-I, and enhance RIG-I ubiquitination and IRF3 phosphorylation compared to wild-type virion RNA, leading to enhanced type I IFN production. Importantly, these m⁶A-deficient RSV mutants also induce a stronger IFN response in vivo, are significantly attenuated, induce higher neutralizing antibody and T cell immune responses in mice and provide complete protection against RSV challenge in cotton rats. Collectively, our results demonstrate that inhibition of RSV RNA m⁶A methylation enhances innate immune responses which in turn promote adaptive immunity.     

Nonstructural proteins 1 and 2 of respiratory syncytial virus suppress maturation of human dendritic cells

Human respiratory syncytial virus (RSV) is the most important agent of serious pediatric respiratory tract disease worldwide. One of the main characteristics of RSV is that it readily reinfects and causes disease throughout life without the need for significant antigenic change. The virus encodes nonstructural protein 1 (NS1) and NS2, which are known to suppress type I interferon (IFN) production and signaling. In the present study, we monitored the maturation of human monocyte-derived myeloid dendritic cells (DC) following inoculation with recombinant RSVs bearing deletions of the NS1 and/or NS2 proteins and expressing enhanced green fluorescent protein. Deletion of the NS1 protein resulted in increased expression of cell surface markers of DC maturation and an increase in the expression of multiple cytokines and chemokines. This effect was enhanced somewhat by further deletion of the NS2 protein, although deletion of NS2 alone did not have a significant effect. The upregulation was largely inhibited by pretreatment with a blocking antibody against the type I IFN receptor, suggesting that suppression of DC maturation by NS1/2 is, at least in part, a result of IFN antagonism mediated by these proteins. Therefore, this study identified another effect of the NS1 and NS2 proteins. The observed suppression of DC maturation may result in decreased antigen presentation and T-lymphocyte activation, leading to incomplete and/or weak immune responses that might contribute to RSV reinfection.     

Recombinant respiratory syncytial virus lacking secreted glycoprotein G is attenuated, non-pathogenic but induces protective immunity

Respiratory syncytial virus (RSV) causes intense pulmonary inflammatory responses in some infected infants. The surface attachment protein 'G' of RSV has membrane-bound and secreted forms and shows homology to the CX3C chemokine fractalkine. Using recombinant techniques, we generated replication-competent recombinant clonal RSV expressing normal G proteins ('rRSV') or only the membrane-bound form of G ('Gmem rRSV'). Both recombinants grew well in HEp-2 cells, but after primary intranasal infection in mice, pulmonary Gmem rRSV replication was reduced tenfold compared to parental or rRSV; moreover, CCL2 and CCL5 production was greatly reduced and no apparent disease or pulmonary cellular infiltration was observed. However, Gmem rRSV-infected mice developed good antibody responses and were fully protected against subsequent intranasal challenge with parental virus. Even in mice sensitized to G by cutaneous infection with recombinant vaccinia expressing G, intranasal challenge with Gmem rRSV caused insignificant disease. We conclude that secreted G is a key viral product assisting virus replication in vivo, enhancing CCL2 and CCL5 production and promoting illness. Engineered RSV mutants lacking the ability to secrete G are thus promising vaccine candidates.     

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.