Editorial
Live attenuated vaccine: the first clinically approved dengue vaccine? Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
Expert Rev. Vaccines 13(2), 185–188 (2014)
Katja Fink Author for correspondence: Singapore Immunology Network, Agency for Science, Technology and Research, 8A Biomedical Grove, Singapore Tel.: +65 6407 0414 Fax: +65 6464 2056 [email protected]. edu.sg
Pei-Yong Shi Novartis Institute for Tropical Diseases, Singapore
Dengue virus (DENV) is the most prevalent mosquito-borne viral pathogen in humans. There are 390 million human infections each year, with 96 million infections exhibiting disease symptoms. Currently, there is no clinically approved vaccine and antiviral for DENV. The four serotypes of DENV (DENV-1, -2, -3 and -4) have 25–40% variation at the amino acid level. Such variation has posed challenges for the development of a tetravalent vaccine and therapeutics. Sanofi Pasteur’s dengue vaccine candidate CYD: learning & moving on
CYD is a live-attenuated dengue vaccine candidate. It is a chimera of a yellow fever vaccine strain in which the structural genes (prM and E) were exchanged with dengue virus DENV-1, -2, -3 or -4 sequences. When Sanofi Pasteur started a clinical trial in Thailand to test the CYD candidate for efficacy in 2009 [1], a dengue vaccine seemed within reach. Never before had a dengue vaccine reached such an advanced stage of clinical testing. The vaccine trial has ignited the interest of government health agencies and got funding bodies on the stage again, and it also spurred development of alternative vaccine candidates by other companies and academia, preparing to be second on market. Clearly, even if CYD’s overall efficacy was disappointing, Sanofi Pasteur’s clinical trial has helped dengue vaccine development immensely. Besides the realization how massive an undertaking a dengue vaccine clinical trial is, the key piece of education was the following: the weak link in the current chain of tests to assess dengue vaccine candidates is the neutralization assay, and the problem is the absence of anything superior. There is much ongoing effort to find out the mechanisms responsible for the failure, but it could be a difficult task because of the limitations of currently used assays and animal models.
From antibody transfer experiments in mice and monkeys, from natural infections and epidemiological evidence, we know that serotype-specific antibodies are necessary for long-term protection, at least after a single infection. For example, a DENV-1 infection would not protect against DENV-2, -3 and -4 several months later. To be protective, antibodies must circulate in the blood in sufficient titers at the time point of infection. Dengue is an acute viral infection with a rapid increase in viral load. Pre-existing antibodies present at the time of infection are therefore required to reduce the initial viral load or to even provide sterile protection, which means to completely bind and inactivate the virus load, to avoid further multiplication of the virus. The term ‘protective antibodies’ is used in this context to differentiate them from ‘neutralizing antibodies’ as defined by the neutralization assay, which are, as mentioned before, not necessarily protective. The lack of a consistent correlation between cross-neutralization in vitro and cross-protection in vivo is not only observed for polyclonal serum, but also for monoclonal antibodies [2]. However, there is an exception to the rule that serotype-specific antibodies are required for protection: protection can also be achieved in the presence of high titers of cross-neutralizing antibodies for several months after infection [3]. Protection across all four serotypes requires, besides
KEYWORDS: antibodies • dengue • live-attenuated • T cells • vaccine
www.expert-reviews.com
10.1586/14760584.2014.870888
Ó 2014 Informa UK Ltd
ISSN 1476-0584
185
Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
Editorial
Fink & Shi
T cells, which are discussed later, a not yet defined (and difficult to define, given the lack of a standardized assay) titer of cross-neutralizing antibodies. Interestingly, cross-neutralizing antibodies in CYD-immunized individuals were apparently not sufficiently cross-protective, which shows that we do not yet understand which are the mechanisms at play that induce the cross-protection observed after multiple natural infections [4,5], even though recent data from mice suggest an important role of T cells [6]. What differentiates CYD from wild-type DENV strains is its composition: only the E protein and prM sequences are derived from DENV, whereas the rest of the chimeric virus genome is derived from yellow fever. While it has not been demonstrated to which extent T cells are required for generation of serotypespecific protective and cross-neutralizing antibodies, the T cell response in CYD reacts largely against yellow fever nonstructural (NS) proteins and might thus not optimally support anamnestic antibody responses to natural DENV infection. NS proteins, particularly NS3 and NS5, contain key CD8+ Tcell epitopes. CD4+ T cells, which serve the role of maintaining B- and T-cell memory and of boosting antibodies during repeated infection, are capsid, NS1 and E protein-specific after natural DENV infection [7]. The limited knowledge about the functional role of T cells in DENV infection comes mostly from the IFN-/- receptor deficient mouse model. Studies in this mouse model showed that T cells were not required for the antibody response during a primary infection. However, the induction of CD4+ T cells using peptide vaccination reduced viral load upon challenge [8]. The study did not address whether the enhanced viral clearance was due to cytotoxic activity of Th cells and direct killing of infected cells, or due to a boost in antibody titers. Furthermore, depletion of CD8+ T cells in mice resulted in increased viral titers after infection, suggesting that CD8+ T cells are an important component of the anti-viral response [9]. A key question that has not been studied yet is the requirement of serotype-specific CD4+ and CD8+ T cells for long-term serotype-specific protection. Alternative live-attenuated DENV vaccine approaches in development
What can we learn from alternative attenuated strains, and can they do better than CYD? Several approaches are in development: DENVax from Takeda (formerly Inviragen) and 30-DENV constructs (also referred to as live-attenuated trachoma vaccine [LATV]) from the National Institute of Allergy and Infectious Diseases have been tested in clinical tools and were found to be safe and immunogenic [10,11]. DENVax is based on the PDK53 DENV-2 backbone (a DENV-2 strain attenuated by serial passaging on PDK cells) and E and prM structural proteins were exchanged in the DENV-2 infectious cDNA clone to create the other serotypes. LATV constructs are DENV-1, -3 and -4 sequences containing a 30 nucleotide deletion in the 3´ untranslated region, while the DENV-2 prM and E were cloned into the DENV-4 backbone to achieve 186
tetravalency for the structural proteins prM and E. The limitation of tetravalent DENVax and LATV seems to be the interference between serotypes in vivo, possibly due to different virulence and immunogenicity, resulting in imbalanced quantity of serotype-specific antibody titers. The imbalance in serotype-specific structural and non-structural NS proteins could potentially have an impact on tetravalent protection, but since there is no in vitro assay or animal model to accurately predict protection, the only way to find out will be efficacy trials in humans. Other approaches in early development have recently been published and could be interesting alternatives: a new chimeric approach using JEV vaccine SA14-14-2 as a backbone [12], and DENV strains containing methyltransferase mutations for attenuation. The methyltransferase mutants are non-chimeric viruses and showed protection in mice and rhesus macaques, and the tetravalent formulation is currently being studied [13]. Methyltransferase mutants, DENVax and LATV generate a DENV-specific T-cell response because their backbones encode DENV non-structural proteins of at least one serotype instead of yellow fever non-structural proteins as in the CYD constructs. T-cell responses and cross-neutralizing antibodies are optimally induced by live-attenuated vaccines. Do these features make them superior? Are cross-neutralizing antibodies desirable after all? Probably yes, considering that genetic diversity is immense and that DENV evolves slowly, but constantly. A strain or clade-specific protection might be outdated very soon and be limited to one geographic area where the specific strains or clades occur. The potential benefit of crossneutralizing antibodies generated by live-attenuated vaccines is, however, based on the assumption that they can achieve the same functional cross-protection that is observed after two or more subsequent natural infections in endemic countries. Functional cross-protection is, again, difficult to measure because it does not necessarily concur with cross-neutralization measured in the neutralization assay. Potential limitations of live-attenuated vaccines
While there are clear advantages, some concerns are associated with live-attenuated vaccines: reversion of attenuated strains to wild type and induction of antibody-dependent enhancement, which can be the other side of the coin of cross-reactive antibodies. Fortunately, the clinical trials done so far did not show an increased risk of severe disease in individuals with preexisting immunity. Vaccination of naı¨ve individuals and individuals having received a heterologous live-attenuated DENV strain before showed similar degrees of adverse effects [14]. Monitoring vaccinated individuals during more than 8 years showed that they were not exposed to a higher risk of developing severe forms of dengue [15]. Nevertheless, these studies involved only small numbers of individuals, and it will be wise to follow vaccinated cohorts for a number of years, particularly in countries with relatively low dengue prevalence, such as Singapore. Even if the number of severe cases among vaccinated persons compared with those with natural pre-existing Expert Rev. Vaccines 13(2), (2014)
Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
Live attenuated vaccine
immunity were just marginally increased, this would have implications for the acceptance of a DENV vaccine into vaccination programs. Balanced tetravalent titers and an adequate re-vaccination program to keep antibody concentrations at protective levels should, however, be able to minimize the risk associated with pre-existing immunity. Reversion of mutated viruses to the wild-type, nonattenuated form is a potential risk that is addressed by introducing multiple mutations in attenuated vaccine strains. For example, reversion was not observed in animal models infected with methyltransferase mutants [13] and during manufacturing of DENVax [16]. Another potential problem of live-attenuated vaccines could be the immunogenicity of the fusion loop and potentially other conserved sequences in the E protein. The fusion loop is conserved across all four serotypes, leading to a massive boost in antibody titers during heterologous infections. In humans with a history of multiple infections, the polyclonal pool of serum antibodies is largely dominated by fusion-loop-specific antibodies [17], and this overwhelming dominance possibly competes with the serotype-specific response. Hence, when individuals with pre-existing immunity are vaccinated, it is likely that fusion-loop-specific antibodies will be boosted over-proportionally. Approaches to genetically modify viruses to reduce crossreactivity are very encouraging, as has been shown recently using modified virus-like particles [18]. However, conserved regions are naturally critical for the virus structure, and manipulations in the conserved sequences usually strongly attenuate or kill the virus [19]. Recombinant protein vaccines or virus-like particles References 1.
2.
3.
4.
Sabchareon A, Wallace D, Lang J, et al. Protective efficacy of the recombinant, live-attenuated, CYD tetravalent dengue vaccine in Thai schoolchildren: a randomised, controlled phase 2b trial. Lancet 2012;380(9853):1559-67 Xu M, Hadinoto V, Appanna R, et al. Plasmablasts generated during repeated dengue infection are virus glycoprotein-specific and bind to multiple virus serotypes. J Immunol 2012;189(12): 5877-85
Financial & competing interests disclosure
The authors are inventors in a pending patent related to this subject matter. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending or royalties. No writing assistance was utilized in the production of this manuscript.
Olkowski S, Forshey BM, Morrison AC, et al. Reduced risk of disease during postsecondary dengue virus infections. J Infect Dis 2013;208(6):1026-33
6.
Zellweger RM, Miller R, Eddy WE, et al. Role of humoral versus cellular responses induced by a protective dengue vaccine candidate. PLoS Pathog 2013;9(10):e1003723
7.
Rivino L, Kumaran EA, Jovanovic V, et al. Differential targeting of viral components by CD4+ versus CD8+ T lymphocytes in dengue virus infection. J Virol 2013;87(5): 2693-706
8.
Endy TP, Anderson KB, Nisalak A, et al. Determinants of inapparent and symptomatic dengue infection in a prospective study of primary school children in Kamphaeng Phet, Thailand. PLoS Negl Trop Dis 2011;5(3):e975
9.
www.expert-reviews.com
might provide more flexibility toward cross-reactivity-reduced constructs. Prime-boost scenarios involving live-attenuated prime and protein boost might therefore be a valuable option to shape serotype-specific antibody titers. As Dr Scott Halstead put it in a recent review, dengue vaccine development is an ‘empirical process’ [20], and it seems quite obvious that we will only be able to move quickly toward a protective dengue vaccine by doing more clinical trials in humans. Since it is still unclear which epitopes are critical for short- and long-term protection, a live-attenuated vaccine is probably the ‘easier’ approach because it imitates a natural infection. With their proven safety and superior immunogenicity, particularly with regards to T cell responses, it is likely that a live-attenuated candidate will be the first dengue vaccine, and, at least at the current stage of developments, DENVax and LATV seem superior to CYD. Nevertheless, subunit or inactivated formulations could be useful in prime-boost scenarios, and could be adjuvenated with novel innate immune triggers to improve the magnitude and longevity of the response.
5.
Anderson KB, Gibbons RV, Cummings DA, et al. A shorter time interval between first and second dengue infections is associated with protection from clinical illness in a school-based cohort in Thailand. J Infect Dis 2013; Epub ahead of print
10.
Editorial
Yauch LE, Prestwood TR, May MM, et al. CD4+ T cells are not required for the induction of dengue virus-specific CD8+ T cell or antibody responses but contribute to protection after vaccination. J Immunol 2010;185(9):5405-16 Yauch LE, Zellweger RM, Kotturi MF, et al. A protective role for dengue virus-specific CD8+ T cells. J Immunol 2009;182(8):4865-73 Durbin AP, Kirkpatricka BD, Pierce KK, et al. A single dose of any of four different live attenuated tetravalent dengue vaccines is
safe and immunogenic in flavivirus-naive adults: a randomized, double blind clinical trial. J Infect Dis 2013;207(6):957-65 11.
Osorio JE, Huang CY, Kinney RM, Stinchcomb DT. Development of DENVax: a chimeric dengue-2 PDK-53based tetravalent vaccine for protection against dengue fever. Vaccine 2011; 29(42):7251-60
12.
Li XF, Deng Y-Q, Yang H-Q, et al. A chimeric dengue virus vaccine using Japanese encephalitis virus vaccine strain SA14-14-2 as backbone is immunogenic and protective against either parental virus in mice and nonhuman primates. J Virol 2013;87(24):13694-705
13.
Zu¨st R, Dong H, Li XF, et al. Rational design of a live attenuated dengue vaccine: 2´-O-methyltransferase mutants are highly attenuated and immunogenic in mice and macaques. PLoS Pathog 2013;9(8): e1003521
14.
Durbin AP, Schmidt A, Elwood D, et al. Heterotypic dengue infection with live attenuated monotypic dengue virus vaccines: implications for vaccination of populations
187
Editorial
Fink & Shi
Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
in areas where dengue is endemic. J Infect Dis 2011;203(3):327-34 15.
Thomas SJ, Endy TP. Critical issues in dengue vaccine development. Curr Opin Infect Dis 2011;24(5):442-50
16.
Huang CY, Kinney RM, Livengood JA, et al. Genetic and phenotypic characterization of manufacturing seeds for a tetravalent dengue vaccine (DENVax). PLoS Negl Trop Dis 2013;7(5):e2243
188
17.
18.
Lai CY, Tsai WY, Lin SR, et al. Antibodies to envelope glycoprotein of dengue virus during the natural course of infection are predominantly cross-reactive and recognize epitopes containing highly conserved residues at the fusion loop of domain II. J Virol 2008;82(13):6631-43 Hughes HR, Crill WD, Chang GJ. Manipulation of immunodominant dengue virus E protein epitopes reduces potential
antibody-dependent enhancement. Virol J 2012;9:115 19.
Christian EA, Kahle KM, Mattia K, et al. Atomic-level functional model of dengue virus Envelope protein infectivity. Proc Natl Acad Sci USA 2013;110(46):18662-7
20.
Halstead SB. Identifying protective dengue vaccines: guide to mastering an empirical process. Vaccine 2013;31(41):4501-7
Expert Rev. Vaccines 13(2), (2014)
Live attenuated vaccine: the first clinically approved dengue vaccine? Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
Expert Rev. Vaccines 13(2), 185–188 (2014)
Katja Fink Author for correspondence: Singapore Immunology Network, Agency for Science, Technology and Research, 8A Biomedical Grove, Singapore Tel.: +65 6407 0414 Fax: +65 6464 2056 [email protected]. edu.sg
Pei-Yong Shi Novartis Institute for Tropical Diseases, Singapore
Dengue virus (DENV) is the most prevalent mosquito-borne viral pathogen in humans. There are 390 million human infections each year, with 96 million infections exhibiting disease symptoms. Currently, there is no clinically approved vaccine and antiviral for DENV. The four serotypes of DENV (DENV-1, -2, -3 and -4) have 25–40% variation at the amino acid level. Such variation has posed challenges for the development of a tetravalent vaccine and therapeutics. Sanofi Pasteur’s dengue vaccine candidate CYD: learning & moving on
CYD is a live-attenuated dengue vaccine candidate. It is a chimera of a yellow fever vaccine strain in which the structural genes (prM and E) were exchanged with dengue virus DENV-1, -2, -3 or -4 sequences. When Sanofi Pasteur started a clinical trial in Thailand to test the CYD candidate for efficacy in 2009 [1], a dengue vaccine seemed within reach. Never before had a dengue vaccine reached such an advanced stage of clinical testing. The vaccine trial has ignited the interest of government health agencies and got funding bodies on the stage again, and it also spurred development of alternative vaccine candidates by other companies and academia, preparing to be second on market. Clearly, even if CYD’s overall efficacy was disappointing, Sanofi Pasteur’s clinical trial has helped dengue vaccine development immensely. Besides the realization how massive an undertaking a dengue vaccine clinical trial is, the key piece of education was the following: the weak link in the current chain of tests to assess dengue vaccine candidates is the neutralization assay, and the problem is the absence of anything superior. There is much ongoing effort to find out the mechanisms responsible for the failure, but it could be a difficult task because of the limitations of currently used assays and animal models.
From antibody transfer experiments in mice and monkeys, from natural infections and epidemiological evidence, we know that serotype-specific antibodies are necessary for long-term protection, at least after a single infection. For example, a DENV-1 infection would not protect against DENV-2, -3 and -4 several months later. To be protective, antibodies must circulate in the blood in sufficient titers at the time point of infection. Dengue is an acute viral infection with a rapid increase in viral load. Pre-existing antibodies present at the time of infection are therefore required to reduce the initial viral load or to even provide sterile protection, which means to completely bind and inactivate the virus load, to avoid further multiplication of the virus. The term ‘protective antibodies’ is used in this context to differentiate them from ‘neutralizing antibodies’ as defined by the neutralization assay, which are, as mentioned before, not necessarily protective. The lack of a consistent correlation between cross-neutralization in vitro and cross-protection in vivo is not only observed for polyclonal serum, but also for monoclonal antibodies [2]. However, there is an exception to the rule that serotype-specific antibodies are required for protection: protection can also be achieved in the presence of high titers of cross-neutralizing antibodies for several months after infection [3]. Protection across all four serotypes requires, besides
KEYWORDS: antibodies • dengue • live-attenuated • T cells • vaccine
www.expert-reviews.com
10.1586/14760584.2014.870888
Ó 2014 Informa UK Ltd
ISSN 1476-0584
185
Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
Editorial
Fink & Shi
T cells, which are discussed later, a not yet defined (and difficult to define, given the lack of a standardized assay) titer of cross-neutralizing antibodies. Interestingly, cross-neutralizing antibodies in CYD-immunized individuals were apparently not sufficiently cross-protective, which shows that we do not yet understand which are the mechanisms at play that induce the cross-protection observed after multiple natural infections [4,5], even though recent data from mice suggest an important role of T cells [6]. What differentiates CYD from wild-type DENV strains is its composition: only the E protein and prM sequences are derived from DENV, whereas the rest of the chimeric virus genome is derived from yellow fever. While it has not been demonstrated to which extent T cells are required for generation of serotypespecific protective and cross-neutralizing antibodies, the T cell response in CYD reacts largely against yellow fever nonstructural (NS) proteins and might thus not optimally support anamnestic antibody responses to natural DENV infection. NS proteins, particularly NS3 and NS5, contain key CD8+ Tcell epitopes. CD4+ T cells, which serve the role of maintaining B- and T-cell memory and of boosting antibodies during repeated infection, are capsid, NS1 and E protein-specific after natural DENV infection [7]. The limited knowledge about the functional role of T cells in DENV infection comes mostly from the IFN-/- receptor deficient mouse model. Studies in this mouse model showed that T cells were not required for the antibody response during a primary infection. However, the induction of CD4+ T cells using peptide vaccination reduced viral load upon challenge [8]. The study did not address whether the enhanced viral clearance was due to cytotoxic activity of Th cells and direct killing of infected cells, or due to a boost in antibody titers. Furthermore, depletion of CD8+ T cells in mice resulted in increased viral titers after infection, suggesting that CD8+ T cells are an important component of the anti-viral response [9]. A key question that has not been studied yet is the requirement of serotype-specific CD4+ and CD8+ T cells for long-term serotype-specific protection. Alternative live-attenuated DENV vaccine approaches in development
What can we learn from alternative attenuated strains, and can they do better than CYD? Several approaches are in development: DENVax from Takeda (formerly Inviragen) and 30-DENV constructs (also referred to as live-attenuated trachoma vaccine [LATV]) from the National Institute of Allergy and Infectious Diseases have been tested in clinical tools and were found to be safe and immunogenic [10,11]. DENVax is based on the PDK53 DENV-2 backbone (a DENV-2 strain attenuated by serial passaging on PDK cells) and E and prM structural proteins were exchanged in the DENV-2 infectious cDNA clone to create the other serotypes. LATV constructs are DENV-1, -3 and -4 sequences containing a 30 nucleotide deletion in the 3´ untranslated region, while the DENV-2 prM and E were cloned into the DENV-4 backbone to achieve 186
tetravalency for the structural proteins prM and E. The limitation of tetravalent DENVax and LATV seems to be the interference between serotypes in vivo, possibly due to different virulence and immunogenicity, resulting in imbalanced quantity of serotype-specific antibody titers. The imbalance in serotype-specific structural and non-structural NS proteins could potentially have an impact on tetravalent protection, but since there is no in vitro assay or animal model to accurately predict protection, the only way to find out will be efficacy trials in humans. Other approaches in early development have recently been published and could be interesting alternatives: a new chimeric approach using JEV vaccine SA14-14-2 as a backbone [12], and DENV strains containing methyltransferase mutations for attenuation. The methyltransferase mutants are non-chimeric viruses and showed protection in mice and rhesus macaques, and the tetravalent formulation is currently being studied [13]. Methyltransferase mutants, DENVax and LATV generate a DENV-specific T-cell response because their backbones encode DENV non-structural proteins of at least one serotype instead of yellow fever non-structural proteins as in the CYD constructs. T-cell responses and cross-neutralizing antibodies are optimally induced by live-attenuated vaccines. Do these features make them superior? Are cross-neutralizing antibodies desirable after all? Probably yes, considering that genetic diversity is immense and that DENV evolves slowly, but constantly. A strain or clade-specific protection might be outdated very soon and be limited to one geographic area where the specific strains or clades occur. The potential benefit of crossneutralizing antibodies generated by live-attenuated vaccines is, however, based on the assumption that they can achieve the same functional cross-protection that is observed after two or more subsequent natural infections in endemic countries. Functional cross-protection is, again, difficult to measure because it does not necessarily concur with cross-neutralization measured in the neutralization assay. Potential limitations of live-attenuated vaccines
While there are clear advantages, some concerns are associated with live-attenuated vaccines: reversion of attenuated strains to wild type and induction of antibody-dependent enhancement, which can be the other side of the coin of cross-reactive antibodies. Fortunately, the clinical trials done so far did not show an increased risk of severe disease in individuals with preexisting immunity. Vaccination of naı¨ve individuals and individuals having received a heterologous live-attenuated DENV strain before showed similar degrees of adverse effects [14]. Monitoring vaccinated individuals during more than 8 years showed that they were not exposed to a higher risk of developing severe forms of dengue [15]. Nevertheless, these studies involved only small numbers of individuals, and it will be wise to follow vaccinated cohorts for a number of years, particularly in countries with relatively low dengue prevalence, such as Singapore. Even if the number of severe cases among vaccinated persons compared with those with natural pre-existing Expert Rev. Vaccines 13(2), (2014)
Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
Live attenuated vaccine
immunity were just marginally increased, this would have implications for the acceptance of a DENV vaccine into vaccination programs. Balanced tetravalent titers and an adequate re-vaccination program to keep antibody concentrations at protective levels should, however, be able to minimize the risk associated with pre-existing immunity. Reversion of mutated viruses to the wild-type, nonattenuated form is a potential risk that is addressed by introducing multiple mutations in attenuated vaccine strains. For example, reversion was not observed in animal models infected with methyltransferase mutants [13] and during manufacturing of DENVax [16]. Another potential problem of live-attenuated vaccines could be the immunogenicity of the fusion loop and potentially other conserved sequences in the E protein. The fusion loop is conserved across all four serotypes, leading to a massive boost in antibody titers during heterologous infections. In humans with a history of multiple infections, the polyclonal pool of serum antibodies is largely dominated by fusion-loop-specific antibodies [17], and this overwhelming dominance possibly competes with the serotype-specific response. Hence, when individuals with pre-existing immunity are vaccinated, it is likely that fusion-loop-specific antibodies will be boosted over-proportionally. Approaches to genetically modify viruses to reduce crossreactivity are very encouraging, as has been shown recently using modified virus-like particles [18]. However, conserved regions are naturally critical for the virus structure, and manipulations in the conserved sequences usually strongly attenuate or kill the virus [19]. Recombinant protein vaccines or virus-like particles References 1.
2.
3.
4.
Sabchareon A, Wallace D, Lang J, et al. Protective efficacy of the recombinant, live-attenuated, CYD tetravalent dengue vaccine in Thai schoolchildren: a randomised, controlled phase 2b trial. Lancet 2012;380(9853):1559-67 Xu M, Hadinoto V, Appanna R, et al. Plasmablasts generated during repeated dengue infection are virus glycoprotein-specific and bind to multiple virus serotypes. J Immunol 2012;189(12): 5877-85
Financial & competing interests disclosure
The authors are inventors in a pending patent related to this subject matter. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending or royalties. No writing assistance was utilized in the production of this manuscript.
Olkowski S, Forshey BM, Morrison AC, et al. Reduced risk of disease during postsecondary dengue virus infections. J Infect Dis 2013;208(6):1026-33
6.
Zellweger RM, Miller R, Eddy WE, et al. Role of humoral versus cellular responses induced by a protective dengue vaccine candidate. PLoS Pathog 2013;9(10):e1003723
7.
Rivino L, Kumaran EA, Jovanovic V, et al. Differential targeting of viral components by CD4+ versus CD8+ T lymphocytes in dengue virus infection. J Virol 2013;87(5): 2693-706
8.
Endy TP, Anderson KB, Nisalak A, et al. Determinants of inapparent and symptomatic dengue infection in a prospective study of primary school children in Kamphaeng Phet, Thailand. PLoS Negl Trop Dis 2011;5(3):e975
9.
www.expert-reviews.com
might provide more flexibility toward cross-reactivity-reduced constructs. Prime-boost scenarios involving live-attenuated prime and protein boost might therefore be a valuable option to shape serotype-specific antibody titers. As Dr Scott Halstead put it in a recent review, dengue vaccine development is an ‘empirical process’ [20], and it seems quite obvious that we will only be able to move quickly toward a protective dengue vaccine by doing more clinical trials in humans. Since it is still unclear which epitopes are critical for short- and long-term protection, a live-attenuated vaccine is probably the ‘easier’ approach because it imitates a natural infection. With their proven safety and superior immunogenicity, particularly with regards to T cell responses, it is likely that a live-attenuated candidate will be the first dengue vaccine, and, at least at the current stage of developments, DENVax and LATV seem superior to CYD. Nevertheless, subunit or inactivated formulations could be useful in prime-boost scenarios, and could be adjuvenated with novel innate immune triggers to improve the magnitude and longevity of the response.
5.
Anderson KB, Gibbons RV, Cummings DA, et al. A shorter time interval between first and second dengue infections is associated with protection from clinical illness in a school-based cohort in Thailand. J Infect Dis 2013; Epub ahead of print
10.
Editorial
Yauch LE, Prestwood TR, May MM, et al. CD4+ T cells are not required for the induction of dengue virus-specific CD8+ T cell or antibody responses but contribute to protection after vaccination. J Immunol 2010;185(9):5405-16 Yauch LE, Zellweger RM, Kotturi MF, et al. A protective role for dengue virus-specific CD8+ T cells. J Immunol 2009;182(8):4865-73 Durbin AP, Kirkpatricka BD, Pierce KK, et al. A single dose of any of four different live attenuated tetravalent dengue vaccines is
safe and immunogenic in flavivirus-naive adults: a randomized, double blind clinical trial. J Infect Dis 2013;207(6):957-65 11.
Osorio JE, Huang CY, Kinney RM, Stinchcomb DT. Development of DENVax: a chimeric dengue-2 PDK-53based tetravalent vaccine for protection against dengue fever. Vaccine 2011; 29(42):7251-60
12.
Li XF, Deng Y-Q, Yang H-Q, et al. A chimeric dengue virus vaccine using Japanese encephalitis virus vaccine strain SA14-14-2 as backbone is immunogenic and protective against either parental virus in mice and nonhuman primates. J Virol 2013;87(24):13694-705
13.
Zu¨st R, Dong H, Li XF, et al. Rational design of a live attenuated dengue vaccine: 2´-O-methyltransferase mutants are highly attenuated and immunogenic in mice and macaques. PLoS Pathog 2013;9(8): e1003521
14.
Durbin AP, Schmidt A, Elwood D, et al. Heterotypic dengue infection with live attenuated monotypic dengue virus vaccines: implications for vaccination of populations
187
Editorial
Fink & Shi
Expert Review of Vaccines Downloaded from informahealthcare.com by SUNY Health Sciences Centre at Brooklyn on 03/30/15 For personal use only.
in areas where dengue is endemic. J Infect Dis 2011;203(3):327-34 15.
Thomas SJ, Endy TP. Critical issues in dengue vaccine development. Curr Opin Infect Dis 2011;24(5):442-50
16.
Huang CY, Kinney RM, Livengood JA, et al. Genetic and phenotypic characterization of manufacturing seeds for a tetravalent dengue vaccine (DENVax). PLoS Negl Trop Dis 2013;7(5):e2243
188
17.
18.
Lai CY, Tsai WY, Lin SR, et al. Antibodies to envelope glycoprotein of dengue virus during the natural course of infection are predominantly cross-reactive and recognize epitopes containing highly conserved residues at the fusion loop of domain II. J Virol 2008;82(13):6631-43 Hughes HR, Crill WD, Chang GJ. Manipulation of immunodominant dengue virus E protein epitopes reduces potential
antibody-dependent enhancement. Virol J 2012;9:115 19.
Christian EA, Kahle KM, Mattia K, et al. Atomic-level functional model of dengue virus Envelope protein infectivity. Proc Natl Acad Sci USA 2013;110(46):18662-7
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