Evaluation of a live attenuated bovine parainfluenza type 3 vaccine in two- to six-month-old infants

A chimeric human-bovine parainfluenza virus type 3 expressing measles virus hemagglutinin is attenuated for replication but is still immunogenic in rhesus monkeys

The chimeric recombinant virus rHPIV3-N(B), a version of human parainfluenza virus type 3 (HPIV3) that is attenuated due to the presence of the bovine PIV3 nucleocapsid (N) protein open reading frame (ORF) in place of the HPIV3 ORF, was modified to encode the measles virus hemagglutinin (HA) inserted as an additional, supernumerary gene between the HPIV3 P and M genes. This recombinant, designated rHPIV3-N(B)HA, replicated like its attenuated rHPIV3-N(B) parent virus in vitro and in the upper and lower respiratory tracts of rhesus monkeys, indicating that the insertion of the measles virus HA did not further attenuate rHPIV3-N(B) in vitro or in vivo. Monkeys immunized with rHPIV3-N(B)HA developed a vigorous immune response to both measles virus and HPIV3, with serum antibody titers to both measles virus (neutralizing antibody) and HPIV3 (hemagglutination inhibiting antibody) of over 1:500. An attenuated HPIV3 expressing a major protective antigen of measles virus provides a method for immunization against measles by the intranasal route, a route that has been shown with HPIV3 and respiratory syncytial virus vaccines to be relatively refractory to the neutralizing and immunosuppressive effects of maternally derived virus-specific serum antibodies. It should now be possible to induce a protective immune response against measles virus in 6-month-old infants, an age group that in developing areas of the world is not responsive to the current measles virus vaccine.

Active immunization in the United States: developments over the past decade

The Centers for Disease Control and Prevention has identified immunization as the most important public health advance of the 20th century. The purpose of this article is to review the changes that have taken place in active immunization in the United States over the past decade. Since 1990, new vaccines have become available to prevent five infectious diseases: varicella, rotavirus, hepatitis A, Lyme disease, and Japanese encephalitis virus infection. Improved vaccines have been developed to prevent Haemophilus influenzae type b, pneumococcus, pertussis, rabies, and typhoid infections. Immunization strategies for the prevention of hepatitis B, measles, meningococcal infections, and poliomyelitis have changed as a result of the changing epidemiology of these diseases. Combination vaccines are being developed to facilitate the delivery of multiple antigens, and improved vaccines are under development for cholera, influenza, and meningococcal disease. Major advances in molecular biology have enabled scientists to devise new approaches to the development of vaccines against diseases ranging from respiratory viral to enteric bacterial infections that continue to plague the world's population.

A single amino acid substitution in the viral polymerase creates a temperature-sensitive and attenuated recombinant bovine parainfluenza virus type 3

Bovine parainfluenza virus type 3 (bPIV3) is under development as a live virus vaccine vector. The RNA genome of a recombinant bPIV3 harbored four nucleotide changes, one of which resulted in a mutation of the viral polymerase (A. A. Haller et al., 2000, J. Virol. 74, 11626-11635). The contribution of this conservative amino acid substitution (I1103V) in the polymerase to the temperature-sensitive and attenuation phenotypes of r-bPIV3 was investigated by creating a new virus, r-bPIV3(I), that expressed the wild-type polymerase. r-bPIV3(I) was not temperature-sensitive for growth in vitro and the replication of r-bPIV3(I) was no longer restricted in hamsters. The effect of the amino acid substitution in the polymerase was also studied in a chimeric bovine/human PIV3, a virus that displayed temperature-sensitive and attenuated phenotypes (A. A. Haller et al., 2000, J. Virol. 74, 11626-11635). It was not clear whether these defects were due to the impaired polymerase or the replacement of the bPIV3 surface glycoproteins with those of hPIV3. The results showed that the altered polymerase was indeed responsible for the temperature-sensitive phenotype of bovine/human PIV3 but did not appear to play a role in the attenuation phenotype.

Vaccine prevention of acute otitis media

The incidence of acute otitis media (AOM) in infants and young children has increased dramatically in recent years in the United States. AOM often follows upper respiratory tract infections due to pathogens such as respiratory syncytial virus (RSV), influenza virus, and parainfluenza virus (PIV). These viruses cause eustachian tube dysfunction that is critical to the pathogenesis of AOM. Vaccines against these viruses would likely reduce the incidence of AOM. In three previous studies, influenza virus vaccines reduced the incidence of AOM by 30% to 36%. Vaccines to prevent infections with RSV and PIV type 3 are undergoing clinical testing at this time. Streptococcus pneumoniae, nontypeable Haemophilus influenzae (NTHi), and Moraxella catarrhalis are the three most common AOM pathogens. Heptavalent pneumococcal conjugate vaccine is effective in preventing invasive disease and AOM caused by serotypes contained in the vaccine. Vaccine candidates for NTHi and M. catarrhalis are under development.

Recombinant bovine/human parainfluenza virus type 3 (B/HPIV3) expressing the respiratory syncytial virus (RSV) G and F proteins can be used to achieve simultaneous mucosal immunization against RSV and HPIV3

Recombinant bovine/human parainfluenza virus type 3 (rB/HPIV3), a recombinant bovine PIV3 (rBPIV3) in which the F and HN genes were replaced with their HPIV3 counterparts, was used to express the major protective antigens of respiratory syncytial virus (RSV) in order to create a bivalent mucosal vaccine against RSV and HPIV3. The attenuation of rB/HPIV3 is provided by the host range restriction of the BPIV3 backbone in primates. RSV G and F open reading frames (ORFs) were placed under the control of PIV3 transcription signals and inserted individually into the rB/HPIV3 genome in the promoter-proximal position preceding the nucleocapsid protein gene. The recombinant PIV3 expressing the RSV G ORF (rB/HPIV3-G1) was not restricted in its replication in vitro, whereas the virus expressing the RSV F ORF (rB/HPIV3-F1) was eightfold restricted compared to its rB/HPIV3 parent. Both viruses replicated efficiently in the respiratory tract of hamsters, and each induced RSV serum antibody titers similar to those induced by RSV infection and anti-HPIV3 titers similar to those induced by HPIV3 infection. Immunization of hamsters with rB/HPIV3-G1, rB/HPIV3-F1, or a combination of both viruses resulted in a high level of resistance to challenge with RSV or HPIV3 28 days later. These results describe a vaccine strategy that obviates the technical challenges associated with a live attenuated RSV vaccine, providing, against the two leading viral agents of pediatric respiratory tract disease, a bivalent vaccine whose attenuation phenotype is based on the extensive host range sequence differences of BPIV3.

Sequence determination and molecular analysis of two strains of bovine parainfluenza virus type 3 that are attenuated for primates

The Kansas/15626/84 (Ka) and Shipping Fever (SF) strains of bovine parainfluenza virus type 3 (BPIV3) replicate less efficiently than human PIV3 (HPIV3) in the upper and lower respiratory tract of rhesus monkeys, and BPIV3 Ka is also highly attenuated in humans and is in clinical trials as a candidate vaccine against HPIV3. To initiate an investigation of the genetic basis of the observed attenuation phenotype of BPIV3 in primates, the complete genomic sequences of Ka and SF genomes were determined and compared to those of BPIV3 strain 910N and two HPIV3 strains, JS and Wash/47885/57. There is a high degree of identity between the five PIV3 viruses in their 55 nucleotide (nt) leader (83.6%) and 44 nt trailer (93.2%) sequences. The five viruses display amino acid sequence identity ranging from 58.6% for the phosphoprotein to 89.7% for the matrix protein. Interestingly, the majority of amino acid residues found to be variable at a given position in a five-way protein alignment are nonetheless identical within the viruses of either host species (BPIV3 or HPIV3). These host-specific residues might be products of distinct selective pressures on BPIV3 and HPIV3 during evolution in their respective hosts. These host-specific sequences likely include ones which are responsible for the host range differences, such as the efficient growth of BPIV3 in bovines compared to its restricted growth in primates. It should now be possible using the techniques of reverse genetics to import sequences from BPIV3 into HPIV3 and identify those nt or protein sequences which attenuate HPIV3 for primates. This information should be useful in understanding virus-host interactions and in the development of vaccines to protect against HPIV3-induced disease.

Expression of the surface glycoproteins of human parainfluenza virus type 3 by bovine parainfluenza virus type 3, a novel attenuated virus vaccine vector

Bovine parainfluenza virus type 3 (bPIV3) is being evaluated as an intranasal vaccine for protection against human PIV3 (hPIV3). In young infants, the bPIV3 vaccine appears to be infectious, attenuated, immunogenic, and genetically stable, which are desirable characteristics for an RNA virus vector. To test the potential of the bPIV3 vaccine strain as a vector, an infectious DNA clone of bPIV3 was assembled and recombinant bPIV3 (r-bPIV3) was rescued. r-bPIV3 displayed a temperature-sensitive phenotype for growth in tissue culture at 39 degrees C and was attenuated in the lungs of Syrian golden hamsters. In order to test whether r-bPIV3 could serve as a vector, the fusion and hemagglutinin-neuraminidase genes of bPIV3 were replaced with those of hPIV3. The resulting bovine/human PIV3 was temperature sensitive for growth in Vero cells at 37 degrees C. The replication of bovine/human PIV3 was also restricted in the lungs of hamsters, albeit not as severely as was observed for r-bPIV3. Despite the attenuation phenotypes observed for r-bPIV3 and bovine/human PIV3, both of these viruses protected hamsters completely upon challenge with hPIV3. In summary, bPIV3 was shown to function as a virus vector that may be especially suitable for vaccination of infants and children against PIV3 and other viruses.

Bovine parainfluenza virus type 3 (BPIV3) fusion and hemagglutinin-neuraminidase glycoproteins make an important contribution to the restricted replication of BPIV3 in primates

This study examines the contribution of the fusion (F) and hemagglutinin-neuraminidase (HN) glycoprotein genes of bovine parainfluenza virus type 3 (BPIV3) to its restricted replication in the respiratory tract of nonhuman primates. A chimeric recombinant human parainfluenza type 3 virus (HPIV3) containing BPIV3 F and HN glycoprotein genes in place of its own and the reciprocal recombinant consisting of BPIV3 bearing the HPIV3 F and HN genes (rBPIV3-F(H)HN(H)) were generated to assess the effect of glycoprotein substitution on replication of HPIV3 and BPIV3 in the upper and lower respiratory tract of rhesus monkeys. The chimeric viruses were readily recovered and replicated in simian LLC-MK2 cells to a level comparable to that of their parental viruses, suggesting that the heterologous glycoproteins were compatible with the PIV3 internal proteins. HPIV3 bearing the BPIV3 F and HN genes was restricted in replication in rhesus monkeys to a level similar to that of its BPIV3 parent virus, indicating that the glycoprotein genes of BPIV3 are major determinants of its host range restriction of replication in rhesus monkeys. rBPIV3-F(H)HN(H) replicated in rhesus monkeys to a level intermediate between that of HPIV3 and BPIV3. This observation indicates that the F and HN genes make a significant contribution to the overall attenuation of BPIV3 for rhesus monkeys. Furthermore, it shows that BPIV3 sequences outside the F and HN region also contribute to the attenuation phenotype in primates, a finding consistent with the previous demonstration that the nucleoprotein coding sequence of BPIV3 is a determinant of its attenuation for primates. Despite its restricted replication in the respiratory tract of rhesus monkeys, rBPIV3-F(H)HN(H) conferred a level of protection against challenge with HPIV3 that was indistinguishable from that induced by previous infection with wild-type HPIV3. The usefulness of rBPIV3-F(H)HN(H) as a vaccine candidate against HPIV3 and as a vector for other viral antigens is discussed.

Human parainfluenza virus type 3 (PIV3) expressing the hemagglutinin protein of measles virus provides a potential method for immunization against measles virus and PIV3 in early infancy

Recombinant human parainfluenza virus type 3 (PIV3) was used as a vector to express the major protective antigen of measles virus, the hemagglutinin (HA) glycoprotein, in order to create a bivalent PIV3-measles virus that can be administered intranasally. The measles virus HA open reading frame (ORF) was inserted as an additional transcriptional unit into the N-P, P-M, or HA-neuraminidase (HN)-L gene junction of wild-type PIV3 or into the N-P or P-M gene junction of an attenuated derivative of PIV3, termed rcp45L. The recombinant PIV3 (rPIV3) viruses bearing the HA inserts replicated more slowly in vitro than their parental viruses but reached comparable peak titers of >/=10(7.5) 50% tissue culture infective doses per ml. Each of the wild-type or cold-passaged 45L (cp45L) PIV3(HA) chimeric viruses replicated 5- to 10-fold less well than its respective parent virus in the upper respiratory tract of hamsters. Thus, insertion of the approximately 2-kb ORF itself conferred attenuation, and this attenuation was additive to that conferred by the cp45L mutations. The attenuated cp45L PIV3(HA) recombinants induced a high level of resistance to replication of PIV3 challenge virus in hamsters and induced very high levels of measles virus neutralizing antibodies (>1:8,000) that are well in excess of those known to be protective in humans. rPIV3s expressing the HA gene in the N-P or P-M junction induced about 400-fold more measles virus-neutralizing antibody than did the rPIV3 with the HA gene in the HN-L junction, indicating that the N-P or P-M junction appears to be the preferred insertion site. Previous studies indicated that the PIV3 cp45 virus, a more attenuated version of rcp45L, replicates efficiently in the respiratory tract of monkeys and is immunogenic and protective even when administered in the presence of very high titers of passively transferred PIV3 antibodies (A. P. Durbin, C. J. Cho, W. R. Elkins, L. S. Wyatt, B. Moss, and B. R. Murphy, J. Infect. Dis. 179:1345-1351, 1999). This suggests that this intranasally administered PIV3(HA) chimeric virus can be used to immunize infants with maternally acquired measles virus antibodies in whom the current parenterally administered live measles virus vaccine is ineffective.

Replacement of the ectodomains of the hemagglutinin-neuraminidase and fusion glycoproteins of recombinant parainfluenza virus type 3 (PIV3) with their counterparts from PIV2 yields attenuated PIV2 vaccine candidates

We sought to develop a live attenuated parainfluenza virus type 2 (PIV2) vaccine strain for use in infants and young children, using reverse genetic techniques that previously were used to rapidly produce a live attenuated PIV1 vaccine candidate. The PIV1 vaccine candidate, designated rPIV3-1cp45, was generated by substituting the full-length HN and F proteins of PIV1 for those of PIV3 in the attenuated cp45 PIV3 vaccine candidate (T. Tao et al., J. Virol. 72:2955-2961, 1998; M. H. Skiadopoulos et al., Vaccine 18:503-510, 1999). However, using the same strategy, we failed to recover recombinant chimeric PIV3-PIV2 isolate carrying the full-length PIV2 glycoproteins in a wild-type PIV3 backbone. Viable PIV3-PIV2 chimeras were recovered when chimeric HN and F open reading frames (ORFs) rather than complete PIV2 F and HN ORFs were used to construct the full-length cDNA. The recovered viruses, designated rPIV3-2CT, in which the PIV2 ectodomain and transmembrane domain were fused to the PIV3 cytoplasmic domain, and rPIV3-2TM, in which the PIV2 ectodomain was fused to the PIV3 transmembrane and cytoplasmic tail domain, possessed similar in vitro and in vivo phenotypes. Thus, it appeared that only the cytoplasmic tail of the HN or F glycoprotein of PIV3 was required for successful recovery of PIV3-PIV2 chimeras. Although rPIV3-2CT and rPIV3-2TM replicated efficiently in vitro, they were moderately to highly attenuated for replication in the respiratory tracts of hamsters, African green monkeys (AGMs), and chimpanzees. This unexpected finding indicated that chimerization of the HN and F proteins of PIV2 and PIV3 itself specified an attenuation phenotype in vivo. Despite this attenuation, these viruses were highly immunogenic and protective against challenge with wild-type PIV2 in hamsters and AGMs, and they represent promising candidates for clinical evaluation as a vaccine against PIV2. These chimeric viruses were further attenuated by the addition of 12 mutations of PIV3cp45 which lie outside of the HN and F genes. The attenuating effects of these mutations were additive with that of the chimerization, and thus inclusion of all or some of the cp45 mutations provides a means to further attenuate the PIV3-PIV2 chimeric vaccine candidates if necessary.

Immunization of pediatric solid organ transplant candidates and recipients

Organ transplantation has evolved from an experimental procedure to an accepted treatment for otherwise irreversible or congenital disorders. The immunosuppression necessary to prevent rejection enhances the severity of many infectious diseases and may potentially attenuate the response to vaccines designed to prevent disease. In spite of the frequency and severity of infectious diseases in organ transplant recipients, many children are not fully vaccinated before transplantation. The safety and efficacy of many of the currently available vaccines for solid organ transplant recipients have not been evaluated. We review the currently available data on immunization safety and efficacy, discuss experimental vaccines, and outline strategies to avoid vaccine-preventable diseases in pediatric organ transplant recipients.

A recombinant human parainfluenza virus type 3 (PIV3) in which the nucleocapsid N protein has been replaced by that of bovine PIV3 is attenuated in primates

The shipping fever (SF) and Kansas (Ka) strains of bovine parainfluenza virus type 3 (BPIV3) are restricted in their replication in rhesus monkeys 100- to 1,000-fold compared to human parainfluenza virus type 3 (HPIV3), and the Ka strain also was shown to be attenuated in humans. To initiate an investigation of the genetic basis of the attenuation of BPIV3 in primates, we produced viable chimeric HPIV3 recombinants containing the nucleoprotein (N) open reading frame (ORF) from either BPIV3 Ka or SF in place of the HPIV3 N ORF. These chimeric recombinants were designated cKa-N and cSF-N, respectively. Remarkably, cKa-N and cSF-N grew to titers comparable to those of their HPIV3 and BPIV3 parents in LLC-MK2 monkey kidney and Madin-Darby bovine kidney cells. Thus, the heterologous nature of the N protein did not impede replication in vitro. However, cKa-N and cSF-N were each restricted in replication in rhesus monkeys to a similar extent as Ka and SF, respectively. This identified the BPIV3 N protein as a determinant of the host range restriction of BPIV3 in primates. These chimeras thus combine the antigenic determinants of HPIV3 with the host range restriction and attenuation phenotype of BPIV3. Despite their restricted replication in rhesus monkeys, the chimeric viruses induced a level of resistance to HPIV3 challenge in these animals which was indistinguishable from that conferred by immunization with HPIV3. The infectivity, attenuation, and immunogenicity of these BPIV3/HPIV3 chimeras suggest that the modified Jennerian approach described in the present report represents a novel method to design vaccines to protect against HPIV3-induced disease in humans.

A live attenuated recombinant chimeric parainfluenza virus (PIV) candidate vaccine containing the hemagglutinin-neuraminidase and fusion glycoproteins of PIV1 and the remaining proteins from PIV3 induces resistance to PIV1 even in animals immune to PIV3

Using a reverse genetics system for PIV3, we previously recovered recombinant chimeric PIV3-PIV1 virus bearing the major protective antigens of PIV1, the hemaglutinin-neuraminidase and fusion proteins, on a background of PIV3 genes bearing temperature sensitive (ts) and attenuating mutations in the L gene. Immunization of hamsters with this virus, designated rPIV3-1.cp45L, induced a high level of resistance to replication of wild type (wt) PIV1 and, surprisingly, also induced a moderate amount of restriction of the replication of PIV3 challenge virus. This suggested that some immunity is conferred by the internal PIV3 proteins shared by the two viruses. In the present study, we found that the immunity to PIV3 conferred by infection with rPIV3-1.cp45L is short-lived and completely disappeared four months after immunization, whereas resistance to replication of PIV3 induced by prior infection with PIV3 remains high even after an interval of four months. Since a live attenuated PIV1 vaccine such as the chimeric rPIV3-1.cp45L virus will likely be given to infants after a live attenuated PIV3 vaccine in a sequential immunization schedule, we examined the immunogenicity and efficacy of rPIV3-1.cp45L against PIV1 challenge in animals with and without prior immunity to PIV3. rPIV3-1.cp45L efficiently infected hamsters previously infected with wt or attenuated PIV3, but there was approximately a five-fold reduction in replication of rPIV3-1. cp45L virus in the PIV3-immune animals. This reduction in replication of rPIV3-1.cp45L in PIV3-immune animals was accompanied by a significant decrease in efficacy against PIV1 challenge. However, rPIV3-1.cp45L immunization of PIV3-immune animals induced a vigorous serum antibody response to PIV1 and reduced replication of PIV1 challenge virus 1000-fold in the lower respiratory tract and 25 to 200-fold in the upper respiratory tract. This study demonstrated that the recombinant chimeric rPIV3-1.cp45L candidate vaccine can induce immunity to PIV1 even in animals immune to PIV3. This establishes the feasibility of employing a sequential immunization schedule in which a recombinant chimeric rPIV3-1.cp45L vaccine is given following a live attenuated PIV3 vaccine.

Mutations in the C, D, and V open reading frames of human parainfluenza virus type 3 attenuate replication in rodents and primates

Human parainfluenza virus type 3 (HPIV3) is a single-stranded negative-sense RNA virus belonging to the Respirovirus genus of the Paramyxoviridae family in the order Mononegavirales. The P gene encodes at least four proteins, including the C protein, which is expressed from an open reading frame (ORF) that overlaps the P ORF, and the D protein, which is encoded when the P ORF is fused to the D ORF by transcriptional editing. The P mRNA also contains a third ORF for the V protein, although it is unclear how or whether this ORF is accessed. We have used recombinant DNA technology to recover five mutant viruses that either interrupt or alter the C, D, and V ORFs. In one mutant virus, rC-KO, expression of the C protein was abrogated by changing the start codon from methionine to threonine and introducing two stop codons at amino acid positions 7 and 26 of the C ORF. In a second mutant virus, rF164S, a point mutation was introduced into the C ORF changing amino acid position 164 from phenylalanine (F) to serine (S), which corresponds to the F170S mutation described in the C protein of Sendai virus (Itoh et al., J. Gen. Virol. 78, 3207-3215). rC-KO was significantly attenuated in vitro and in vivo (rodents and primates), whereas rF164S was attenuated only in vivo. Interestingly, the rF164S mutant was more attenuated in the upper than in the lower respiratory tract of hamsters and monkeys. This pattern is the converse of that seen with temperature-sensitive attenuating mutations, and thus inclusion of this novel mutation in a recombinant live-attenuated vaccine candidate might prove useful in reducing residual virulence in the upper respiratory tract. Both rC-KO and rF164S conferred protection against challenge with wild-type HPIV3. In three other viruses, the D and V ORFs were interrupted singly or in combination. Although interruption of the D and V ORFs individually did not affect virus replication in vitro or in vivo, interruption of both together attenuated replication in vivo. These results indicate that the C, D, and V proteins of HPIV3 each has a role in virus replication in vitro, in vivo, or both, and define mutations that might be useful for the development of a vaccine against HPIV3.

A live attenuated chimeric recombinant parainfluenza virus (PIV) encoding the internal proteins of PIV type 3 and the surface glycoproteins of PIV type 1 induces complete resistance to PIV1 challenge and partial resistance to PIV3 challenge

The recovery of wild type and attenuated human parainfluenza type 3 (PIV3) recombinant viruses has made possible a new strategy to rapidly generate a live-attenuated vaccine virus fof PIV1. We previously replaced the coding sequences for the hemagglutinin-neuraminidase (HN) and fusion (F) proteins of PIV3 with those of PIV1 in the PIV3 antigenomic cDNA. This was used to recover a fully-viable, recombinant chimeric PIV3-PIV1 virus, termed rPIV3-1, which bears the major protective antigens of PIV1 and is wild type-like with regard to growth in cell culture and in hamsters [Tao T, Durbin AP, Whitehead SS, Davoodi F, Collins PL, Murphy BR. Recovery of a fully viable chimeric human parainfluenza virus (PIV) type 3 in which the hemagglutinin-neuraminidase and fusion glycoprotein have been replaced by those of PIV type 1. J Virol 1998;72:2955-2961]. Here we report the recovery of a derivative of rPIV3-1 carrying the three temperature-sensitive and attenuating amino acid coding changes found in the L gene of the live-attenuated cp45 PIV3 candidate vaccine virus. This virus, termed rPIV3-1.cp45L, is temperature-sensitive with a shut-off temperature of 38 degrees C, which is similar to that of the recombinant rPIV3cp45L, which possesses the same three mutations. rPIV3-1.cp45L is attenuated in the respiratory tract of hamsters to the same extent as rPIV3cp45L. Infection of hamsters with rPIV3-1.cp45L generated a moderate level of hemagglutination-inhibiting antibodies against wild type PIV1 and induced complete resistance to challenge with wild type PIV1. This demonstrates that this novel attenuated chimeric virus is capable of inducing a highly effective immune response against PIV1. It confirms previous observations that the surface glycoproteins of parainfluenza viruses are sufficient to induce a high level of resistance to homologous virus challenge. Unexpectedly, infection with recombinant chimeric virus rPIV3-1.cp45L or rPIV3-1, each bearing the surface glycoprotein genes of PIV1 and the internal genes of PIV3, also induced a moderate level of resistance to replication of wild type PIV3 challenge virus. This indicates that the internal genes of PIV3 can independently induce protective immunity against PIV3 in rodents, albeit a lower level of resistance than that induced by the surface glycoproteins. Thus, a reverse genetics system for PIV3 has been used successfully to produce a live attenuated PIV1 vaccine candidate that is attenuated and protective in experimental infection in hamsters.

Immune responses of infants to infection with respiratory viruses and live attenuated respiratory virus candidate vaccines

Respiratory viruses such as respiratory syncytial virus (RSV), the parainfluenza viruses (PIV), and the influenza viruses cause severe lower respiratory tract diseases in infants and children throughout the world. Experimental live attenuated vaccines for each of these viruses are being developed for intranasal administration in the first weeks or months of life. A variety of promising RSV, PIV-3, and influenza virus vaccine strains have been developed by classical biological methods, evaluated extensively in preclinical and clinical studies, and shown to be attenuated and genetically stable. The ongoing clinical evaluation of these vaccine candidates, coupled with recent major advances in the ability to develop genetically engineered viruses with specified mutations, may allow the rapid development of respiratory virus strains that possess ideal levels of replicative capacity and genetic stability in vivo. A major remaining obstacle to successful immunization of infants against respiratory virus associated disease may be the relatively poor immune response of very young infants to primary virus infection. This paper reviews the immune correlates of protection against disease caused by these viruses, immune responses of infants to naturally-acquired infection, and immune responses of infants to experimental infection with candidate vaccine viruses.

Recovery of infectious human parainfluenza virus type 3 from cDNA

Infectious HPIV3 was produced by the intracellular coexpression of four plasmid-borne cDNAs. These separately encoded a complete HPIV3 genome (negative-sense), the HPIV3 nucleocapsid protein N, the phosphoprotein P, and the polymerase protein L. The cDNA-encoded HPIV3 genome differed from the JS wildtype (wt) strain of HPIV3 used in its construction by seven point mutations: four of these are silent mutations in the HN or L gene coding regions that serve as markers of a cDNA-derived virus, two were introduced to create an amino acid substitution that ablates an epitope recognized by the HN-specific monoclonal neutralizing antibody 423/6, and the remaining point mutation results in an incidental amino acid substitution in the HN protein at amino acid position 263. The four plasmids were transfected into HEp-2 cell monolayers and their expression was driven by T7 RNA polymerase supplied by a vaccinia virus recombinant. The titer of virus present in the harvested transfection supernatant was low (<5 PFU/ml), and the recovered recombinant virus (rJS) retained each of the seven mutations present in the cDNA from which it was derived. Despite the introduced and incidental mutations, rJS retained the wt phenotypes as regards replication at elevated temperature in vitro and efficient replication in the upper and lower respiratory tract of hamsters. rJS was also recovered from a cDNA encoding a complete antigenome (positive-sense) with slightly greater efficiency than from the negative-sense construct. The ability to generate infectious HPIV3 from cDNA should greatly enhance our ability to develop new live-attenuated parainfluenza virus vaccines, including chimeric PIV1 and PIV2 vaccines, and to understand the genetic basis of attenuation of PIV3 candidate vaccines.