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Vaccination approaches to combat human metapneumovirus lower respiratory tract infections
Journal of Clinical Virology 41 (2008) 49–52
Vaccination approaches to combat human metapneumovirus lower respiratory tract infections Sander Herfst, Ron A.M. Fouchier ∗ Department of Virology, Erasmus Medical Center, Rotterdam, The Netherlands Received 19 October 2007; accepted 19 October 2007
Abstract Human metapneumovirus (hMPV) was discovered in 2001 as a causative agent of respiratory disease in young children, immunocompromised individuals and the elderly. Clinical signs of hMPV infection range from mild respiratory illness to bronchiolitis and pneumonia. Two main genetic lineages of hMPV that circulate worldwide were found to be antigenically different, but antibodies against the F protein, the major determinant of protection, were shown to be cross-protective. Since the discovery of hMPV in 2001, several research groups have developed vaccine candidates that may be used to protect different risk groups against hMPV-induced respiratory disease. The studies in rodent and non-human primate models look promising, but none of the vaccine candidates has been tested yet in human volunteers. Here we give an overview of the immunogenicity and protective efficacy of a variety of live attenuated, virus vectored, inactivated virus and subunit vaccines that have been tested in animal models. © 2007 Elsevier B.V. All rights reserved. Keywords: Metapneumovirus; Immunization; Protective efficacy; Vaccine; Animals
The human metapneumovirus (hMPV) was first isolated from respiratory specimens obtained from children in The Netherlands hospitalized for acute respiratory tract illness (RTI) (van den Hoogen et al., 2001). Based on genomic organization, hMPV was classified as the first mammalian member of the Paramyxoviridae family, subfamily Pneumovirinae, genus Metapneumovirus. Clinical manifestations of hMPV infections are similar to those caused by respiratory syncytial virus (RSV), ranging from mild respiratory illness to bronchiolitis and pneumonia (van den Hoogen et al., 2003; Williams et al., 2006). Phylogenetic analysis of fusion (F) and attachment (G) genes of a large number of hMPV isolates revealed the existence of two main genetic virus lineages each divided into at least two sublineages. The two main lineages were found to be antigenically distinct in virusneutralization assays with ferret sera (van den Hoogen et al., 2004). However, the F protein is highly conserved between ∗ Corresponding author at: Deparment of Virology, Erasmus Medical Center, P.O. Box 2040, 3000 CA, Rotterdam, The Netherlands. Tel.: +31 10 4088066; fax: +31 10 4089485. E-mail address: [email protected] (R.A.M. Fouchier).
1386-6532/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcv.2007.10.022
the two major lineages, and antibodies induced against the F protein correlate with protection in animal models (Tang et al., 2005; Skiadopoulos et al., 2006). Since hMPV-associated RTI has been demonstrated in young children, individuals with underlying disease and the elderly, a variety of vaccination strategies may be required to prevent hMPV respiratory tract infections in the community. Live attenuated viruses may be useful to prevent infection in young na¨ıve children, while inactivated viruses or subunit vaccines may be useful to boost existing immune responses in immunocompromised individuals and the elderly. A major drawback for using inactivated vaccines is the experience with a formalininactivated (FI-) RSV vaccine in the 1960s. Immunization of na¨ıve children with this vaccine induced enhanced disease upon subsequent infection (Kim et al., 1969). The upper respiratory tract (URT) of cotton rats immunized with FIhMPV was almost completely protected against infection, but an increase in lung pathology combined with a change in cytokine profiles was observed (Table 1) (Yim et al., 2007). Thus, alternative vaccine candidates for hMPV may be required. Here we give an overview of approaches explored to protect mammals against hMPV infections.
50
S. Herfst, R.A.M. Fouchier / Journal of Clinical Virology 41 (2008) 49–52
1. Live attenuated vaccines
1.2. Deletion mutants
Live attenuated vaccines (LAVs) have the advantage of mimicking a natural infection and may provide protection against subsequent infections without inducing enhanced disease. The development of reverse genetics systems for hMPV provides a powerful tool to design LAV (Biacchesi et al., 2004a; Herfst et al., 2004). A number of strategies for attenuating RSV have been explored, which may be helpful for the development of LAV against hMPV.
Recombinant hMPVs lacking the small hydrophobic (SH), attachment (G) or second ORF of the M2 (M22) proteins have been described to replicate efficiently in vitro, while being attenuated in vivo (Table 1) (Biacchesi et al., 2004b; Buchholz et al., 2005). After initial experiments in hamsters, replication kinetics, immunogenicity and protective efficacy of these viruses were studied in AGMs (Biacchesi et al., 2005). Replication of G and M2-2 viruses was reduced 6- and 160-fold in the URT and 3200- and 4000-fold in the LRT respectively, whereas viral titers for the SH virus were only slightly lower as compared to wild type (wt) hMPV. Upon challenge infection, only trace amounts of virus were detected in the URT, and virus shedding in the LRT was virtually undetectable.
1.1. Vectored vaccines The first described LAV was a chimeric bovine parainfluenza virus type 3 (PIV3), harboring the F and hemagglutinin-neuraminidase (HN) genes of human PIV3 (Table 1). This b/hPIV3 virus was used as a vector to express the F protein of hMPV, from position 2 of the genome (b/hPIV3/hMPV F2) (Tang et al., 2003). Immunization of hamsters and African green monkeys (AGMs) with b/hPIV3/hMPV F2 induced both PIV3 and hMPVspecific neutralizing antibodies that protected against PIV3 and hMPV challenge infection (Tang et al., 2003, 2005). In AGMs, no hMPV replication was observed in the lower respiratory tract (LRT) after challenge infection and the virus titer in the URT was more than 100-fold reduced. Additional studies in rhesus monkeys demonstrated that the b/hPIV3/hMPV F2 virus replicated to the same extent as a recombinant bovine PIV3 (rbPIV3), that was previously found to be attenuated and safe in human infants (Karron et al., 1995; Tang et al., 2005).
1.3. Chimeric viruses Chimeric viruses have been generated by replacing the nucleoprotein (N) or phosphoprotein (P) proteins of hMPV by their counterparts of avian metapneumovirus type C (aMPV-C) (Table 1) (Pham et al., 2005). aMPV-C is the closest known relative of hMPV and causes respiratory illnesses in poultry. In hamsters, high levels of protective neutralizing antibodies were induced after intranasal infection with such chimeric metapneumoviruses, although virus titers in the lungs and nasal turbinates were approximately 100-fold reduced compared to wild type hMPV at 3 days post infection (dpi). At 5 dpi, there was only a small difference between viral titers of the chimeras and wt hMPV. In AGMs, the N-chimera replicated to ∼10-fold lower titers in the lower respiratory
Table 1 Vaccine
Animal model
Outcome
Refs.
FI-hMPV
Cotton rats
Yim et al. (2007)
B/hPIV3 expressing hMPVF
Hamsters, AGM
HMPV deletion mutants
Hamsters, AGM
Chimeric hMPV/aMPV-C
Hamsters, AGM
Soluble F protein
Hamsters, cotton rats
Soluble F protein and F DNA vaccine
Cotton rats
Almost complete protection in lungs, but dramatic increase in lung pathology b/hPIV3/ hMPV F2 was sufficiently attenuated in rhesus monkeys. Immunization of AGMs resulted in complete protection of the LRT and virus titers in the URT were >100-fold reduced G and M2-2 viruses were attenuated in AGMs. After challenge, virus shedding in the LRT was virtually undetectable The P-chimera was attenuated 100- and 1000-fold in the URT and LRT of AGMs. Protective efficacy is comparable with wt hMPV. Immunization with adjuvanted soluble F protein induced complete protection of the LRT against heterologous challenge infection Viral replication after challenge infection was 10-fold reduced in the LRT. No significant reduction was observed in the URT compared to non-immunized animals
Tang et al. (2003, 2005)
Biacchesi et al. (2004b, 2005); Buchholz et al. (2005) Pham et al. (2005)
Herfst et al. (2007)
Cseke et al., 2007
FI: formalin-inactivated, hMPV: human metapneumovirus, b/hPIV3: bovine/human parainfluenzavirus type 3, AGM: African green monkeys, F: fusion protein, LRT: lower respiratory tract, URT: upper respiratory tract, G: attachment protein, M2-2: second ORF of M2, P: phosphoprotein, wt: wild type.
S. Herfst, R.A.M. Fouchier / Journal of Clinical Virology 41 (2008) 49–52
tract, whereas the P-chimera was reduced 100–1000-fold in the upper and lower respiratory tract. The immunogenicity and protective efficacy of both chimeras were comparable with wt hMPV.
2. Subunit vaccines We have recently evaluated the antigenicity, immunogenicity and cross-protective efficacy of a soluble hMPV F (Fsol) subunit vaccine in Syrian golden hamsters (Table 1) (Herfst et al., 2007). Two immunizations with 10 g of adjuvanted Fsol resulted in high neutralizing antibody titers. Two different Fsol proteins, representing the two main genetic lineages of hMPV, were used to immunize different groups of animals. After homologous or heterologous virus challenge infection, the LRT of animals in both immunized groups were completely protected against infection. Furthermore, viral titers in the URT were reduced significantly. No difference in protective efficacy was observed for two different adjuvants, but unadjuvanted Fsol vaccines were found to be inferior. The protective efficacy of a soluble F protein of hMPV lacking the transmembrane domain (FTM) has also been tested in cotton rats (Cseke et al., 2007). In this study, three different prime/boost protocols were compared for their ability to induce protection: two immunizations with 100 g of plasmid DNA encoding hMPV-F, two immunizations with 25 g adjuvanted FTM protein, and a primary immunization with DNA plasmid followed by a boost-immunization with adjuvanted FTM. The highest serum neutralizing antibody titers were induced by the FTM protein. In these animals, virus replication was significantly reduced in the URT and more than 1500-fold reduced in the LRT after homologous virus challenge infection.
3. Discussion Experiences with inactivated virus or purified protein vaccines for RSV demonstrated that caution is warranted when exploring these vaccines. Immunization of na¨ıve individuals with such vaccines can prime for enhanced disease upon subsequent exposure to virus infection. Studies in cotton rats revealed that immunization with FI-hMPV-induced enhanced pathology in the lungs of animals after subsequent infection with hMPV (Yim et al., 2007). A variety of alternative vaccination strategies have been explored to protect the different risk groups from hMPV-induced respiratory illness. LAV may be useful in young children to prime the hMPV-specific immune response, while inactivated or subunit vaccines may be useful to boost the immune response in individuals that have previously been exposed to hMPV. Cynomolgus macaques that had been inoculated three times with hMPV within 10 weeks, were not protected against challenge infection 8 months after the last inoculation (van den Hoogen et al., 2007). This indicates
51
that transient protective immunity is induced after wt hMPV infection, so an efficient candidate vaccine for hMPV should ideally be more immunogenic and protective than natural hMPV infection. For immunocompromised individuals and the elderly, F subunit vaccines seem to be promising for boosting the hMPV-specific immune response. Such vaccines might have potential in particular risk groups as they could be administered together with the annual influenza vaccine. A variety of LAVs, including chimeric viruses, vectored vaccines and deletion mutants, look promising for priming of the immune response, for instance in young children. However, the balance between a satisfactory degree of attenuation and a satisfactory level of immunogenicity may be difficult to obtain. The classical method to attenuate viruses by cold passaging virus at low temperatures to induce cold adaptation and temperature sensitivity has not been described yet. We are currently exploring the possibilities for such vaccines for hMPV.
References Biacchesi S, Skiadopoulos MH, Tran KC, Murphy BR, Collins PL, Buchholz UJ. Recovery of human metapneumovirus from cDNA: optimization of growth in vitro and expression of additional genes. Virology 2004a;321:247–59. Biacchesi S, Skiadopoulos MH, Yang L, Lamirande EW, Tran KC, Murphy BR, et al. Recombinant human metapneumovirus lacking the small hydrophobic SH and/or attachment G glycoprotein: deletion of G yields a promising vaccine candidate. J Virol 2004b;78:12877– 87. Biacchesi S, Pham QN, Skiadopoulos MH, Murphy BR, Collins PL, Buchholz UJ. Infection of nonhuman primates with recombinant human metapneumovirus lacking the SH, G, or M2-2 protein categorizes each as a nonessential accessory protein and identifies vaccine candidates. J Virol 2005;79:12608–13. Buchholz UJ, Biacchesi S, Pham QN, Tran KC, Yang L, Luongo CL, et al. Deletion of M2 gene open reading frames 1 and 2 of human metapneumovirus: effects on RNA synthesis, attenuation, and immunogenicity. J Virol 2005;79:6588–97. Cseke G, Wright DW, Tollefson SJ, Johnson JE, Crowe Jr JE, Williams JV. Human metapneumovirus fusion protein vaccines that are immunogenic and protective in cotton rats. J Virol 2007;81:698–707. Herfst S, de Graaf M, Schickli JH, Tang RS, Kaur J, Yang CF, et al. Recovery of human metapneumovirus genetic lineages A and B from cloned cDNA. J Virol 2004;78:8264–70. Herfst S, de Graaf M, Schrauwen EJ, Ulbrandt N, Barnes AS, Senthil K, et al. Immunization of Syrian golden hamsters with F subunit vaccine of human metapneumovirus induces protection against challenge with homologous or heterologous strains. J Gen Virol 2007;88:2702–9. Karron RA, Wright PF, Hall SL, Makhene M, Thompson J, Burns BA, et al. A live attenuated bovine parainfluenza virus type 3 vaccine is safe, infectious, immunogenic, and phenotypically stable in infants and children. J Infect Dis 1995;171:1107–14. Kim HW, Canchola JG, Brandt CD, Pyles G, Chanock RM, Jensen K, et al. Respiratory syncytial virus disease in infants despite prior administration of antigenic inactivated vaccine. Am J Epidemiol 1969;89:422–34. Pham QN, Biacchesi S, Skiadopoulos MH, Murphy BR, Collins PL, Buchholz UJ. Chimeric recombinant human metapneumoviruses with the nucleoprotein or phosphoprotein open reading frame replaced by that of avian metapneumovirus exhibit improved growth in vitro and attenuation in vivo. J Virol 2005;79:15114–22.
52
S. Herfst, R.A.M. Fouchier / Journal of Clinical Virology 41 (2008) 49–52
Skiadopoulos MH, Biacchesi S, Buchholz UJ, Amaro-Carambot E, Surman SR, Collins PL, et al. Individual contributions of the human metapneumovirus F, G, and SH surface glycoproteins to the induction of neutralizing antibodies and protective immunity. Virology 2006;345:492–501. Tang RS, Schickli JH, MacPhail M, Fernandes F, Bicha L, Spaete J, et al. 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. J Virol 2003;77:10819–28. Tang RS, Mahmood K, Macphail M, Guzzetta JM, Haller AA, Liu H, et al. A host-range restricted parainfluenza virus type 3 (PIV3) expressing the human metapneumovirus (hMPV) fusion protein elicits protective immunity in African green monkeys. Vaccine 2005;23:1657–67. van den Hoogen BG, de Jong JC, Groen J, Kuiken T, de Groot R, Fouchier RA, et al. A newly discovered human pneumovirus isolated from young children with respiratory tract disease. Nat Med 2001;7:719–24.
van den Hoogen BG, Doornum GJJv, Fockens JC, Cornelissen JJ, Beyer WEP, Groot Rd, et al. Prevalence and clinical symptoms of human metapneumovirus infection in hospitalized patients. J Infect Dis 2003:188. van den Hoogen BG, Herfst S, Sprong L, Cane PA, Forleo E, de Swart RL, et al. Antigenic and genetic variability of human metapneumoviruses. Emerg Infect Dis 2004;10:658–66. van den Hoogen BG, Herfst S, de Graaf M, Sprong L, van Lavieren R, van Amerongen G, et al. Experimental infection of macaques with human metapneumovirus induces transient protective immunity. J Gen Virol 2007;88:1251–9. Williams JV, Wang CK, Yang CF, Tollefson SJ, House FS, Heck JM, et al. The role of human metapneumovirus in upper respiratory tract infections in children: a 20-year experience. J Infect Dis 2006;193:387–95. Yim KC, Cragin RP, Boukhvalova MS, Blanco JC, Hamlin ME, Boivin G, et al. Human metapneumovirus: enhanced pulmonary disease in cotton rats immunized with formalin-inactivated virus vaccine and challenged. Vaccine 2007;25:5034–40.
Vaccination approaches to combat human metapneumovirus lower respiratory tract infections Sander Herfst, Ron A.M. Fouchier ∗ Department of Virology, Erasmus Medical Center, Rotterdam, The Netherlands Received 19 October 2007; accepted 19 October 2007
Abstract Human metapneumovirus (hMPV) was discovered in 2001 as a causative agent of respiratory disease in young children, immunocompromised individuals and the elderly. Clinical signs of hMPV infection range from mild respiratory illness to bronchiolitis and pneumonia. Two main genetic lineages of hMPV that circulate worldwide were found to be antigenically different, but antibodies against the F protein, the major determinant of protection, were shown to be cross-protective. Since the discovery of hMPV in 2001, several research groups have developed vaccine candidates that may be used to protect different risk groups against hMPV-induced respiratory disease. The studies in rodent and non-human primate models look promising, but none of the vaccine candidates has been tested yet in human volunteers. Here we give an overview of the immunogenicity and protective efficacy of a variety of live attenuated, virus vectored, inactivated virus and subunit vaccines that have been tested in animal models. © 2007 Elsevier B.V. All rights reserved. Keywords: Metapneumovirus; Immunization; Protective efficacy; Vaccine; Animals
The human metapneumovirus (hMPV) was first isolated from respiratory specimens obtained from children in The Netherlands hospitalized for acute respiratory tract illness (RTI) (van den Hoogen et al., 2001). Based on genomic organization, hMPV was classified as the first mammalian member of the Paramyxoviridae family, subfamily Pneumovirinae, genus Metapneumovirus. Clinical manifestations of hMPV infections are similar to those caused by respiratory syncytial virus (RSV), ranging from mild respiratory illness to bronchiolitis and pneumonia (van den Hoogen et al., 2003; Williams et al., 2006). Phylogenetic analysis of fusion (F) and attachment (G) genes of a large number of hMPV isolates revealed the existence of two main genetic virus lineages each divided into at least two sublineages. The two main lineages were found to be antigenically distinct in virusneutralization assays with ferret sera (van den Hoogen et al., 2004). However, the F protein is highly conserved between ∗ Corresponding author at: Deparment of Virology, Erasmus Medical Center, P.O. Box 2040, 3000 CA, Rotterdam, The Netherlands. Tel.: +31 10 4088066; fax: +31 10 4089485. E-mail address: [email protected] (R.A.M. Fouchier).
1386-6532/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcv.2007.10.022
the two major lineages, and antibodies induced against the F protein correlate with protection in animal models (Tang et al., 2005; Skiadopoulos et al., 2006). Since hMPV-associated RTI has been demonstrated in young children, individuals with underlying disease and the elderly, a variety of vaccination strategies may be required to prevent hMPV respiratory tract infections in the community. Live attenuated viruses may be useful to prevent infection in young na¨ıve children, while inactivated viruses or subunit vaccines may be useful to boost existing immune responses in immunocompromised individuals and the elderly. A major drawback for using inactivated vaccines is the experience with a formalininactivated (FI-) RSV vaccine in the 1960s. Immunization of na¨ıve children with this vaccine induced enhanced disease upon subsequent infection (Kim et al., 1969). The upper respiratory tract (URT) of cotton rats immunized with FIhMPV was almost completely protected against infection, but an increase in lung pathology combined with a change in cytokine profiles was observed (Table 1) (Yim et al., 2007). Thus, alternative vaccine candidates for hMPV may be required. Here we give an overview of approaches explored to protect mammals against hMPV infections.
50
S. Herfst, R.A.M. Fouchier / Journal of Clinical Virology 41 (2008) 49–52
1. Live attenuated vaccines
1.2. Deletion mutants
Live attenuated vaccines (LAVs) have the advantage of mimicking a natural infection and may provide protection against subsequent infections without inducing enhanced disease. The development of reverse genetics systems for hMPV provides a powerful tool to design LAV (Biacchesi et al., 2004a; Herfst et al., 2004). A number of strategies for attenuating RSV have been explored, which may be helpful for the development of LAV against hMPV.
Recombinant hMPVs lacking the small hydrophobic (SH), attachment (G) or second ORF of the M2 (M22) proteins have been described to replicate efficiently in vitro, while being attenuated in vivo (Table 1) (Biacchesi et al., 2004b; Buchholz et al., 2005). After initial experiments in hamsters, replication kinetics, immunogenicity and protective efficacy of these viruses were studied in AGMs (Biacchesi et al., 2005). Replication of G and M2-2 viruses was reduced 6- and 160-fold in the URT and 3200- and 4000-fold in the LRT respectively, whereas viral titers for the SH virus were only slightly lower as compared to wild type (wt) hMPV. Upon challenge infection, only trace amounts of virus were detected in the URT, and virus shedding in the LRT was virtually undetectable.
1.1. Vectored vaccines The first described LAV was a chimeric bovine parainfluenza virus type 3 (PIV3), harboring the F and hemagglutinin-neuraminidase (HN) genes of human PIV3 (Table 1). This b/hPIV3 virus was used as a vector to express the F protein of hMPV, from position 2 of the genome (b/hPIV3/hMPV F2) (Tang et al., 2003). Immunization of hamsters and African green monkeys (AGMs) with b/hPIV3/hMPV F2 induced both PIV3 and hMPVspecific neutralizing antibodies that protected against PIV3 and hMPV challenge infection (Tang et al., 2003, 2005). In AGMs, no hMPV replication was observed in the lower respiratory tract (LRT) after challenge infection and the virus titer in the URT was more than 100-fold reduced. Additional studies in rhesus monkeys demonstrated that the b/hPIV3/hMPV F2 virus replicated to the same extent as a recombinant bovine PIV3 (rbPIV3), that was previously found to be attenuated and safe in human infants (Karron et al., 1995; Tang et al., 2005).
1.3. Chimeric viruses Chimeric viruses have been generated by replacing the nucleoprotein (N) or phosphoprotein (P) proteins of hMPV by their counterparts of avian metapneumovirus type C (aMPV-C) (Table 1) (Pham et al., 2005). aMPV-C is the closest known relative of hMPV and causes respiratory illnesses in poultry. In hamsters, high levels of protective neutralizing antibodies were induced after intranasal infection with such chimeric metapneumoviruses, although virus titers in the lungs and nasal turbinates were approximately 100-fold reduced compared to wild type hMPV at 3 days post infection (dpi). At 5 dpi, there was only a small difference between viral titers of the chimeras and wt hMPV. In AGMs, the N-chimera replicated to ∼10-fold lower titers in the lower respiratory
Table 1 Vaccine
Animal model
Outcome
Refs.
FI-hMPV
Cotton rats
Yim et al. (2007)
B/hPIV3 expressing hMPVF
Hamsters, AGM
HMPV deletion mutants
Hamsters, AGM
Chimeric hMPV/aMPV-C
Hamsters, AGM
Soluble F protein
Hamsters, cotton rats
Soluble F protein and F DNA vaccine
Cotton rats
Almost complete protection in lungs, but dramatic increase in lung pathology b/hPIV3/ hMPV F2 was sufficiently attenuated in rhesus monkeys. Immunization of AGMs resulted in complete protection of the LRT and virus titers in the URT were >100-fold reduced G and M2-2 viruses were attenuated in AGMs. After challenge, virus shedding in the LRT was virtually undetectable The P-chimera was attenuated 100- and 1000-fold in the URT and LRT of AGMs. Protective efficacy is comparable with wt hMPV. Immunization with adjuvanted soluble F protein induced complete protection of the LRT against heterologous challenge infection Viral replication after challenge infection was 10-fold reduced in the LRT. No significant reduction was observed in the URT compared to non-immunized animals
Tang et al. (2003, 2005)
Biacchesi et al. (2004b, 2005); Buchholz et al. (2005) Pham et al. (2005)
Herfst et al. (2007)
Cseke et al., 2007
FI: formalin-inactivated, hMPV: human metapneumovirus, b/hPIV3: bovine/human parainfluenzavirus type 3, AGM: African green monkeys, F: fusion protein, LRT: lower respiratory tract, URT: upper respiratory tract, G: attachment protein, M2-2: second ORF of M2, P: phosphoprotein, wt: wild type.
S. Herfst, R.A.M. Fouchier / Journal of Clinical Virology 41 (2008) 49–52
tract, whereas the P-chimera was reduced 100–1000-fold in the upper and lower respiratory tract. The immunogenicity and protective efficacy of both chimeras were comparable with wt hMPV.
2. Subunit vaccines We have recently evaluated the antigenicity, immunogenicity and cross-protective efficacy of a soluble hMPV F (Fsol) subunit vaccine in Syrian golden hamsters (Table 1) (Herfst et al., 2007). Two immunizations with 10 g of adjuvanted Fsol resulted in high neutralizing antibody titers. Two different Fsol proteins, representing the two main genetic lineages of hMPV, were used to immunize different groups of animals. After homologous or heterologous virus challenge infection, the LRT of animals in both immunized groups were completely protected against infection. Furthermore, viral titers in the URT were reduced significantly. No difference in protective efficacy was observed for two different adjuvants, but unadjuvanted Fsol vaccines were found to be inferior. The protective efficacy of a soluble F protein of hMPV lacking the transmembrane domain (FTM) has also been tested in cotton rats (Cseke et al., 2007). In this study, three different prime/boost protocols were compared for their ability to induce protection: two immunizations with 100 g of plasmid DNA encoding hMPV-F, two immunizations with 25 g adjuvanted FTM protein, and a primary immunization with DNA plasmid followed by a boost-immunization with adjuvanted FTM. The highest serum neutralizing antibody titers were induced by the FTM protein. In these animals, virus replication was significantly reduced in the URT and more than 1500-fold reduced in the LRT after homologous virus challenge infection.
3. Discussion Experiences with inactivated virus or purified protein vaccines for RSV demonstrated that caution is warranted when exploring these vaccines. Immunization of na¨ıve individuals with such vaccines can prime for enhanced disease upon subsequent exposure to virus infection. Studies in cotton rats revealed that immunization with FI-hMPV-induced enhanced pathology in the lungs of animals after subsequent infection with hMPV (Yim et al., 2007). A variety of alternative vaccination strategies have been explored to protect the different risk groups from hMPV-induced respiratory illness. LAV may be useful in young children to prime the hMPV-specific immune response, while inactivated or subunit vaccines may be useful to boost the immune response in individuals that have previously been exposed to hMPV. Cynomolgus macaques that had been inoculated three times with hMPV within 10 weeks, were not protected against challenge infection 8 months after the last inoculation (van den Hoogen et al., 2007). This indicates
51
that transient protective immunity is induced after wt hMPV infection, so an efficient candidate vaccine for hMPV should ideally be more immunogenic and protective than natural hMPV infection. For immunocompromised individuals and the elderly, F subunit vaccines seem to be promising for boosting the hMPV-specific immune response. Such vaccines might have potential in particular risk groups as they could be administered together with the annual influenza vaccine. A variety of LAVs, including chimeric viruses, vectored vaccines and deletion mutants, look promising for priming of the immune response, for instance in young children. However, the balance between a satisfactory degree of attenuation and a satisfactory level of immunogenicity may be difficult to obtain. The classical method to attenuate viruses by cold passaging virus at low temperatures to induce cold adaptation and temperature sensitivity has not been described yet. We are currently exploring the possibilities for such vaccines for hMPV.
References Biacchesi S, Skiadopoulos MH, Tran KC, Murphy BR, Collins PL, Buchholz UJ. Recovery of human metapneumovirus from cDNA: optimization of growth in vitro and expression of additional genes. Virology 2004a;321:247–59. Biacchesi S, Skiadopoulos MH, Yang L, Lamirande EW, Tran KC, Murphy BR, et al. Recombinant human metapneumovirus lacking the small hydrophobic SH and/or attachment G glycoprotein: deletion of G yields a promising vaccine candidate. J Virol 2004b;78:12877– 87. Biacchesi S, Pham QN, Skiadopoulos MH, Murphy BR, Collins PL, Buchholz UJ. Infection of nonhuman primates with recombinant human metapneumovirus lacking the SH, G, or M2-2 protein categorizes each as a nonessential accessory protein and identifies vaccine candidates. J Virol 2005;79:12608–13. Buchholz UJ, Biacchesi S, Pham QN, Tran KC, Yang L, Luongo CL, et al. Deletion of M2 gene open reading frames 1 and 2 of human metapneumovirus: effects on RNA synthesis, attenuation, and immunogenicity. J Virol 2005;79:6588–97. Cseke G, Wright DW, Tollefson SJ, Johnson JE, Crowe Jr JE, Williams JV. Human metapneumovirus fusion protein vaccines that are immunogenic and protective in cotton rats. J Virol 2007;81:698–707. Herfst S, de Graaf M, Schickli JH, Tang RS, Kaur J, Yang CF, et al. Recovery of human metapneumovirus genetic lineages A and B from cloned cDNA. J Virol 2004;78:8264–70. Herfst S, de Graaf M, Schrauwen EJ, Ulbrandt N, Barnes AS, Senthil K, et al. Immunization of Syrian golden hamsters with F subunit vaccine of human metapneumovirus induces protection against challenge with homologous or heterologous strains. J Gen Virol 2007;88:2702–9. Karron RA, Wright PF, Hall SL, Makhene M, Thompson J, Burns BA, et al. A live attenuated bovine parainfluenza virus type 3 vaccine is safe, infectious, immunogenic, and phenotypically stable in infants and children. J Infect Dis 1995;171:1107–14. Kim HW, Canchola JG, Brandt CD, Pyles G, Chanock RM, Jensen K, et al. Respiratory syncytial virus disease in infants despite prior administration of antigenic inactivated vaccine. Am J Epidemiol 1969;89:422–34. Pham QN, Biacchesi S, Skiadopoulos MH, Murphy BR, Collins PL, Buchholz UJ. Chimeric recombinant human metapneumoviruses with the nucleoprotein or phosphoprotein open reading frame replaced by that of avian metapneumovirus exhibit improved growth in vitro and attenuation in vivo. J Virol 2005;79:15114–22.
52
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Skiadopoulos MH, Biacchesi S, Buchholz UJ, Amaro-Carambot E, Surman SR, Collins PL, et al. Individual contributions of the human metapneumovirus F, G, and SH surface glycoproteins to the induction of neutralizing antibodies and protective immunity. Virology 2006;345:492–501. Tang RS, Schickli JH, MacPhail M, Fernandes F, Bicha L, Spaete J, et al. 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. J Virol 2003;77:10819–28. Tang RS, Mahmood K, Macphail M, Guzzetta JM, Haller AA, Liu H, et al. A host-range restricted parainfluenza virus type 3 (PIV3) expressing the human metapneumovirus (hMPV) fusion protein elicits protective immunity in African green monkeys. Vaccine 2005;23:1657–67. van den Hoogen BG, de Jong JC, Groen J, Kuiken T, de Groot R, Fouchier RA, et al. A newly discovered human pneumovirus isolated from young children with respiratory tract disease. Nat Med 2001;7:719–24.
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