Immunology and evolvement of the adenovirus prime, MVA boost Ebola virus vaccine

Available online at www.sciencedirect.com

ScienceDirect Immunology and evolvement of the adenovirus prime, MVA boost Ebola virus vaccine Yan Zhou and Nancy J Sullivan The 2014 Ebola virus outbreak caused an order of magnitude more deaths in a single outbreak than all previous known outbreaks combined, affecting both local and international public health, and threatening the security and economic stability of the countries in West Africa directly confronting the outbreak. The severity of the epidemic lead to a global response to assist with patient care, outbreak control, and deployment of vaccines. The latter was possible due to the long history of basic and clinical research aimed at identifying a safe and effective vaccine to protect against Ebola virus infection. This review highlights the immunology, development, and progress of vaccines based on replication-defective adenovirus vectors, culminating in the successful launch of the first Phase III trial of an Ebola virus vaccine.

appearance of symptoms, and leaving only a small window of opportunity for immunologic clearance. In addition, vaccine induction of antibodies targeting irrelevant viral proteins or antigenic sites that do not play a central role in virus entry could thwart vaccine efficacy. Alternatively, rapid dysregulation by the virus of host immune responses and mortality within days or weeks could outpace plasma cell production of antibodies [1]. Finally, the replication strategy of Ebola virus to encode a secreted form of the glycoprotein (GP) as a potential immune decoy, and to use macropinocytosis and an intracellular host cell receptor for entry could misdirect antibody targeting or impede access to crucial epitopes on the GP.

Address Biodefense Research Section, Vaccine Research Center, National Institute for Allergy and Infectious Disease, National Institutes of Health, Bethesda, MD 20814, USA

Gene-based vaccination demonstrates protection of macaques

Corresponding author: Sullivan, Nancy J ([email protected])

Current Opinion in Immunology 2015, 35:131–136 This review comes from a themed issue on Vaccines Edited by Rafi Ahmed and John R Mascola

http://dx.doi.org/10.1016/j.coi.2015.06.006 0952-7915/Published by Elsevier Ltd.

To overcome potential limitations of antibody-based vaccines strategies, genetic vaccination was employed as a means to stimulate both humoral and cellular immunity (Figure 1). While early generation plasmid DNA vaccines showed promise in small animal models for the induction of both T-lymphocytes and antibodies that were associated with enhanced survival, immune responses in macaques were insufficient to protect against infectious Ebola virus challenge [2,3] (unpublished data). This species difference in vaccine efficacy may relate to fundamental mechanistic differences in infection or immune protection for each animal model, or could simply reflect more subtle requirements for the level or balance of specific immune responses. While not predictive of protection in primates, rodent models were crucial for identifying functional genetic vector systems, targeting immunogenic virus proteins, and defining potential immune correlates of protection [2,4–6].

Introduction The aim of viral vaccines is the generation of sterilizing immunity achieved through the induction of neutralizing antibodies that block virus attachment or membrane fusion. Indeed, the efficacy of most licensed vaccines is assessed by, and correlates well with, antibody titers in recipients. Therefore, early approaches for a vaccine against Ebola virus utilized technologies proven to generate potent virus-directed antibody responses, such as killed virus preparations that contain a mixture of inactivated virus proteins, or individual laboratory synthesized recombinant proteins. The failure of these approaches to protect nonhuman primates against Ebola infection despite the presence of high titer antibodies might be explained by the aggressive nature of the virus, with lethality occurring within days to a week after the www.sciencedirect.com

Gene-based vaccination strategies based on plasmid DNA or viral vectors achieved a conceptual advance in the 1990s when it was observed that heterologous primeboosting, using one modality for the priming vaccination and different antigen platform for the boost, induced much higher responses than homologous prime-boost [7]. For Ebola vaccination of macaques, DNA plasmid vector priming followed by boosting with replicationdefective adenovirus vectors containing the GP and nucleoprotein genes of Ebola virus (EBOV) yielded an order of magnitude higher antibody responses compared to either modality alone (administered singly or as a homologous prime-boost), and this strategy demonstrated that vaccine protection of primates against EBOV is possible [8]. Macaques primed three times with DNA encoding Current Opinion in Immunology 2015, 35:131–136

132 Vaccines

Figure 1

1. Muscle

Lymph node

Gene expression

3.

PRR-mediated signaling

phagocytosis

Gene expression

IFN-α/β

MHC class I antigen presentation

2. migration Antigen processing

MHC class I antigen crosspresentation

rAd

CD8

GP MHC class II antigen presentation

Class I epitope Class II epitope IFN-α/β MHC class I MHC class II

Bcell

B cell receptor

CD8 Help

Help CD4

T cell receptor DC-SIGN Current Opinion in Immunology

Induction of immune responses after inoculation with rAd-based Ebola vaccine vectors. (1) Recombinant adenoviral vector (rAd) transduces muscle cells in intramuscular vaccination and expresses EBOV GP. Cell death associated with GP cytotoxicity may enhance antigen crosspresentation through phagocytosis of apoptotic cells by dendritic cells (DCs). Little is known about innate responses in muscle and how they shape adaptive responses. (2) GP-specific B cells, CD4+ and CD8+ T cells are primed in secondary lymph node. Cognate B cell encounters GP antigen captured by recently migrated DCs [33]. CD8+ T-cell response depends on antigen cross-presentation. Primed CD4+ T cells provide help for optimal humoral and cellular responses. (3) rAd transduces DCs and induces type I IFN expression through pattern recognition receptor (PRR)mediated signaling pathway. Type I IFN facilitates DC maturation, enhances CD8+ T-cell effector function, but also can limits GP expression and initial expansion of CD8+ T-cell responses. Weak Type I IFN response against rAd is associated with high CD8+ T-cell immunity [34].

the GPs from the four Ebola species known at that time, boosted with rAd5 vectors and challenged three months later with a lethal dose of EBOV were uniformly protected against infection. The same strategy later provided the first demonstration of protection against an Ebola species not contained within the vaccine, Bundibugyo [9]. Current Opinion in Immunology 2015, 35:131–136

In this case boosting occurred a full year after DNA priming, illustrating the power of this platform for breadth and durability, in addition to potency. The above properties fulfilled the scope of efficacy requirements considered important for most vaccines. However, for Ebola, it is desirable to have a fast-acting vaccine that could be www.sciencedirect.com

ChAd3/MVA Ebola vaccine Zhou and Sullivan 133

considered for use during an outbreak to generate rapid immunity in individuals at high risk of exposure such as health care workers or community members living in an outbreak area. To this end, a single inoculation with rAd5GP was demonstrated to achieve uniform protection in macaques exposed to a lethal dose of EBOV one month after vaccination [10], and provided a framework for vaccine testing as new vector platforms were developed [11–15].

ELISA IgG as immunologic correlate of vaccine protection Since human efficacy trials may not be feasible for Ebola, licensure of a vaccine will probably proceed using an alternative regulatory pathway based on the demonstration of protective efficacy in macaques. This requires the definition of immune correlates of protection in macaques that can be used for bridging to human immune responses measured in clinical trials. Protection with the DNA/rAd5 vaccine associated with the presence of GP-specific binding antibodies in the serum of vaccinated macaques, and this association was maintained for the single-shot rAd5GP vaccine [16]. These vaccines showed promise in Phase I clinical trials where 100% of the vaccinated subjects developed anti-GP antibodies after immunization [17]. However, an Achilles heel in the Ad5 platform was revealed in subjects who had previously been exposed to adenovirus type 5 in the context of natural infection; ‘pre-existing immunity’ against the vaccine vector reduced potency of the Ebola vaccine for induction of anti-GP antibodies [18]. Likewise, vaccine-induced protection against EBOV challenge was lost in macaques with pre-existing Ad5 immunity. Interestingly, these macaques developed post-vaccination anti-GP antibodies, the preliminary immune correlate, indicative of protection. However, CD8+ T cell responses were low or absent in Ad5-immune macaques, suggesting an importance for T-cell immunity in protection. Consistent with a role for T cells in rAd vaccine protective immunity, experiments in macaques demonstrated that depletion of CD3+ or CD8+ cells eliminated vaccineinduced protection against Ebola infection. Moreover, passive transfer of high concentration polyclonal IgG purified from DNA/rAd-vaccinate macaques into naı¨ve animals failed to protect the animals against Ebola infection [19]. In addition, heterologous protection against Ebola Bundibugyo using an Ebola Zaire/Sudan vaccine was associated with the presence of cross-species specific T-cells and absence of cross reactive anti-GP antibodies [9]. Taken together the results in animal studies suggest that serum anti-GP antibodies induced after rAd5 vaccination represent a non-mechanistic immune correlate of protection that, while probably playing a role in immune protection, are a quantitative surrogate marker for the totality of immune responses, humoral and cellular, required for efficient and complete clearance of EBOV. www.sciencedirect.com

Role for T cells in vaccine protection Given the participation of T cells in rAd-based protection, an immune correlate of survival related to cellular immunity should be possible to identify. The first indication of a T-cell correlate of survival was revealed in studies evaluating a rAd5 vaccine containing a GP insert modified to eliminate in vitro cytopathicity of GP, and assessing the contribution of the NP insert to protection [16]. All vaccine groups induced serum anti-GP antibodies, but only groups who generated CD8+ T-cell responses were protected against lethal EBOV infection. Subsequent studies testing rare serotype rAd vectors to circumvent Ad5 immunity illustrated that the magnitude of CD8+ T-cell responses was not always predictive of vaccine efficacy. Macaques inoculated with a single shot of rAd35 and rAd26 Ebola vaccines at the uniformly protective rAd5 vaccine dose, 1010 particles, generated serum anti-GP antibodies as well as CD4+ and CD8+ Tcell responses comparable to rAd5, yet failed to protect animals against EBOV infection [20]. Although this was a setback in Ebola vaccine development, the failure of these vectors revealed a previously unappreciated correlate of protection in T-cell quality. Measuring populations of T cells producing combinations of IFN-g, IL-2, or TNF at the single-cell level, allows enumeration of distinct functional T-cell subsets. Previous studies suggested that CD8+ T lymphocytes simultaneously secreting TNF and IFN-g have enhanced cytolytic activity [21]. Interrogation of the cytokine secretion patterns among antigen-specific T-cells after Ebola vaccination showed a striking difference between rAd5 and the rare serotype (rAd26 or rAd35) vaccines was that the CD8+ Tcell response induced by the protective (rAd5) vaccine comprised primarily TNF/IFN-g double positive, cytolytic effector cells, but this population was underrepresented after vaccination by non-protective vectors (Figure 2). The requirement for rapidly available and highly functional effector cells is logical for protection against Ebola, which kills its host within just days after illness appears. Although the difference between rare serotype vectors and rAd5 was at first perplexing, the fact that the former utilize a different cellular receptor (CD46) than rAd5 (CAR), suggests that differential in vivo targeting, particularly of dendritic cell subsets [22], of the vectors could drive the difference in T-cell quality resulting from vaccination.

Identification of vectors that circumvent antivector immunity Therefore, Ad genetic grouping (receptor preference), anti-GP antibody titers, and CD8+ T-cell quality formed the basis for selection of rAd vectors to replace previous vaccines that yielded suboptimal protection in the face of anti-vector immunity and/or inferior T-cell responses. Presumed rare human adenoviruses displayed higher international seroprevalence than originally thought, so Current Opinion in Immunology 2015, 35:131–136

134 Vaccines

Figure 2

Table 1

GP ELISA Titer (EC90)

Protection (%)

Immune responses and protective efficacy associated with rAd vectors.

105

100

50

Vector 104

rAd5 ChAd3 ChAd63 rAd35 rAd26

103

102

0

rAd26 rAd35 rAd5 3+

rAd26 rAd35

3+

rAd5 3+

2+ 1+

rAd26

2+

rAd35

rAd5

50

Cytokine+/CD8+

100 100 25 17 0

high high low low low

CD8+ T-cell response Magnitude

Quality

high high low high high

good good intermediate poor poor

Data in the table refer to responses associated with bivalent rAd vaccines (Zaire plus Sudan GP) administered to macaques. In each case immune responses were measured one-week prior to infectious challenge with EBOV.

1+

2+ 1+

Protection (%) GP ELISA titer

40 30

vector demonstrated a new complexity of immune correlates of protection for rAd-based Ebola vaccines, illustrating the importance of both the magnitude and quality of CD8+ T-cell responses, along with anti-GP antibody titers for prediction of protection against Ebola infection across vector types. However, it should be noted that for a given protective vector platform (rAd5 or ChAd3), antiGP antibody titers are a robust predictor of survival.

20 10

IFNγ IL2 TNF

+ + + 3+

+ +

+ +

+ + -

+

+ -

+ -

1+

2+

Current Opinion in Immunology

Levels of protection and immune responses associated with adenovirus vectors. rAd vectors from different genetic serotypes are shown [19,20]. Protection refers to the survival rate in macaques who received 1010 particles of rAd vaccine by intramuscular injection and were challenged with a target dose of 1000PFU EBOV Kikwit. GP ELISA titer is expressed as the reciprocal serum dilution achieving the 90% effective concentration of GP-specific immunoglobulin 3–4 weeks after vaccination. Pie charts represent CD8+ T-cell responses measured by intracellular cytokine staining after stimulation of vaccinated macaque PBMC with GP peptides. Pie slices represent the proportions of CD8+ T cells expressing, 1 (blue), 2 (green), or 3 (yellow) cytokines simultaneously. Bars show the breakdown of all possible combinations of cytokine secretion on a per cell base.

alternative nonhuman adenoviruses were isolated from chimpanzees (ChAd) and modified for use as vaccine vectors [22]. Analysis of Ebola vaccine vectors constructed using ChAd3 that segregates genetically with Ad5 displayed robust antibody titers and high proportions of TNF/IFN-g double positive CD8+ T-cell quality remarkably similar to rAd5 in vaccinated macaques (Table 1) [23]. ChAd63, chosen in part because of its use of the CAR receptor, displayed favorable CD8+ T-cell quality but lower magnitude antibody and cellular responses. Reduced protection of macaques using this Current Opinion in Immunology 2015, 35:131–136

Durable protective memory using prime-boost vaccination After the initial demonstration of rAd5 vaccine protection in macaques against Ebola exposure administered one month after vaccination, several vaccine platforms were developed as either single-shot or prime-boost vaccines using one-month vaccine-challenge model. However, it is expected that licensed vaccines will induce immunologic memory that past years, not weeks or months. The singleinoculation ChAd3 vaccine provided 100% protective efficacy shortly after vaccination and 50% protection when macaques were exposed to Ebola virus nearly one year after vaccination. By the standards for vaccine efficacy against most pathogens this would be considered a successful vaccine. However, given the high lethality of Ebola and the fact that human efficacy cannot be tested directly, there is an unspoken expectation that a prerequisite for the advanced development of an Ebola vaccine is uniform protection of macaques. Therefore, the original prime-boost strategy was revisited in an effort to extend the duration of uniform immunologic protection to one year, but with a different vector combination than DNA/ rAd5. A rationale was developed to utilize a priming vaccination that provides rapid protection and for this purpose ChAd3 was chosen since a single DNA inoculation does not protect against Ebola (unpublished data). Then, a modified vaccinia Ankara (MVA) booster was chosen due to the ability of pox vectors to induce CD4 T cell responses required for long term immunologic memory. Both ChAd3-GP and ChAd63-GP primed antibody and T-cell responses in macaques that were boosted several-fold by MVA-GP administered 8 weeks after www.sciencedirect.com

ChAd3/MVA Ebola vaccine Zhou and Sullivan 135

priming, with ChAd3/MVA yielding somewhat higher anti-GP antibody and CD8+ T-cell responses than ChAd63/MVA [23] (unpublished data). As expected, MVA boosting generated a large population of cells secreting IFN-g, IL-2, and TNF simultaneously, reflecting long-term memory potential, and this was seen with both priming vectors. A key difference observed between the priming vectors was that ChAd3 generated a higher proportion of TNF/IFN-g double positive CD8+ T cells than ChAd63 after both the prime and the boost, and this difference was maintained through the long term memory time points. This observed difference was essential for defining correlates of durable vaccination protection against Ebola, and showed that long-term protection requires both effector (TNF/IFN-g double positive) and memory (IL-2/TNF/IFN-g triple positive) CD8+ T cells. More importantly, these studies demonstrated that high magnitude immune responses measured after boosting do not predict long-term protection, and the Tcell quality induced by the priming vector is crucial for the induction of durable protection.

will support ultimate licensure of an Ebola vaccine for human use.

Acknowledgements The authors thank Dr. Julie Ledgerwood for critical reading of the manuscript and Ms. Brenda Hartman for graphics. This work was supported by the Intramural Research Program of the Vaccine Research Center, NIAID, National Institutes of Health. NJS declares intellectual property on adenovirus-based Ebola vaccines.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Misasi J, Sullivan NJ: Camouflage and misdirection: the full-on assault of ebola virus disease. Cell 2014, 159:477-486.

2.

Xu L, Sanchez A, Yang Z, Zaki SR, Nabel EG, Nichol ST, Nabel GJ: Immunization for Ebola virus infection. Nat Med 1998, 4:37-42.

3.

Vanderzanden L, Bray M, Fuller D, Roberts T, Custer D, Spik K, Jahrling P, Huggins J, Schmaljohn A, Schmaljohn C: DNA vaccines expressing either the GP or NP genes of Ebola virus protect mice from lethal challenge. Virology 1998, 246:134-144.

4.

Gupta M, Greer P, Mahanty S, Shieh WJ, Zaki SR, Ahmed R, Rollin PE: CD8-mediated protection against Ebola virus infection is perforin dependent. J Immunol 2005, 174:41984202.

5.

Wong G, Richardson JS, Pillet S, Patel A, Qiu X, Alimonti J, Hogan J, Zhang Y, Takada A, Feldmann H et al.: Immune parameters correlate with protection against ebola virus infection in rodents and nonhuman primates. Sci Transl Med 2012, 4 158ra146.

6.

Choi JH, Schafer SC, Zhang L, Juelich T, Freiberg AN, Croyle MA: Modeling pre-existing immunity to adenovirus in rodents: immunological requirements for successful development of a recombinant adenovirus serotype 5-based ebola vaccine. Mol Pharm 2013, 10:3342-3355.

7.

Letvin NL, Montefiori DC, Yasutomi Y, Perry HC, Davies ME, Lekutis C, Alroy M, Freed DC, Lord CI, Handt LK et al.: Potent, protective anti-HIV immune responses generated by bimodal HIV envelope DNA plus protein vaccination. Proc Natl Acad Sci U S A 1997, 94:9378-9383.

8.

Sullivan NJ, Sanchez A, Rollin PE, Yang ZY, Nabel GJ: Development of a preventive vaccine for Ebola virus infection in primates. Nature 2000, 408:605-609.

9.

Hensley LE, Mulangu S, Asiedu C, Johnson J, Honko AN, Stanley D, Fabozzi G, Nichol ST, Ksiazek TG, Rollin PE et al.: Demonstration of cross-protective vaccine immunity against an emerging pathogenic Ebolavirus species. PLoS Pathog 2010, 6:e1000904.

Phase III studies of an Ebola vaccine The 2014 West Africa Ebola epidemic stands apart from prior outbreaks in that it was not controlled by traditional approaches that employed quarantine of cases and contact tracing. Enhanced clinical care alone can greatly reduce mortality from Ebola infection [24,25], but the rapid increase in case numbers, reduced physician to patient ratio, and international spread of cases highlighted the need for vaccines and treatments. The ChAd3/MVA Ebola vaccine is the only platform thus far to demonstrate durable protection in macaques and, combined due to the ability of the ChAd3-GP priming inoculation to generate rapid protection, was advanced for Phase I testing in 2013, before there was evidence of an outbreak. In 2014, the vaccine was demonstrated to be safe and immunogenic in humans at vaccine doses that are easily manufactured. Importantly, the priming vaccine (ChAd3-GP) generated antibody responses in 20/20 subjects [26], and the antiGP titers were in the range observed to provide protection against EBOV challenge in macaques, thus providing a strong basis for licensure using an accelerated approval pathway [27]. The studies in macaques demonstrating efficacy, the definition of an anti-GP antibody immune correlate of survival, and the long history of Phase I clinical trials using rAd Ebola vaccines [18,28,29,30] demonstrating vaccine safety were pivotal for launching Phase III Ebola vaccine efficacy trials in West Africa during the 2014 Ebola outbreak. While the declining numbers of cases may not permit direct measures of vaccine efficacy in human subjects [31,32], the current trials along with numerous ongoing and international Phase I and II trials using the ChAd3 Ebola vaccine (with or without MVA boosting) www.sciencedirect.com

10. Sullivan NJ, Geisbert TW, Geisbert JB, Xu L, Yang ZY, Roederer M, Koup RA, Jahrling PB, Nabel GJ: Accelerated vaccination for Ebola virus haemorrhagic fever in non-human primates. Nature 2003, 424:681-684. 11. Jones SM, Feldmann H, Stroher U, Geisbert JB, Fernando L, Grolla A, Klenk HD, Sullivan NJ, Volchkov VE, Fritz EA et al.: Live attenuated recombinant vaccine protects nonhuman primates against Ebola and Marburg viruses. Nat Med 2005, 11:786-790. 12. Warfield KL, Swenson DL, Olinger GG, Kalina WV, Aman MJ, Bavari S: Ebola virus-like particle-based vaccine protects nonhuman primates against lethal Ebola virus challenge. J Infect Dis 2007, 196(Suppl 2):S430-S437. 13. Geisbert TW, Geisbert JB, Leung A, Daddario-DiCaprio KM, Hensley LE, Grolla A, Feldmann H: Single-injection vaccine protects nonhuman primates against infection with marburg virus and three species of ebola virus. J Virol 2009, 83: 7296-7304. Current Opinion in Immunology 2015, 35:131–136

136 Vaccines

14. Richardson JS, Pillet S, Bello AJ, Kobinger GP: Airway delivery of an adenovirus-based Ebola virus vaccine bypasses existing immunity to homologous adenovirus in nonhuman primates. J Virol 2013, 87:3668-3677. 15. Marzi A, Halfmann P, Hill-Batorski L, Feldmann F, Shupert WL, Neumann G, Feldmann H, Kawaoka Y: Vaccines. An Ebola whole-virus vaccine is protective in nonhuman primates. Science 2015, 348:439-442. 16. Sullivan NJ, Geisbert TW, Geisbert JB, Shedlock DJ, Xu L, Lamoreaux L, Custers JH, Popernack PM, Yang ZY, Pau MG et al.: Immune protection of nonhuman primates against Ebola virus with single low-dose adenovirus vectors encoding modified GPs. PLoS Med 2006, 3:e177. 17. Martin JE, Sullivan NJ, Enama ME, Gordon IJ, Roederer M, Koup RA, Bailer RT, Chakrabarti BK, Bailey MA, Gomez PL et al.: A DNA vaccine for Ebola virus is safe and immunogenic in a phase I clinical trial. Clin Vaccine Immunol 2006, 13:1267-1277. 18. Ledgerwood JE, Costner P, Desai N, Holman L, Enama ME, Yamshchikov G, Mulangu S, Hu Z, Andrews CA, Sheets RA et al.: A replication defective recombinant Ad5 vaccine expressing Ebola virus GP is safe and immunogenic in healthy adults. Vaccine 2010, 29:304-313. 19. Sullivan NJ, Hensley L, Asiedu C, Geisbert TW, Stanley D, Johnson J, Honko A, Olinger G, Bailey M, Geisbert JB et al.: CD8+ cellular immunity mediates rAd5 vaccine protection against Ebola virus infection of nonhuman primates. Nat Med 2011, 17:1128-1131. 20. Geisbert TW, Bailey M, Hensley L, Asiedu C, Geisbert J, Stanley D, Honko A, Johnson J, Mulangu S, Pau MG et al.: Recombinant adenovirus serotype 26 (Ad26) and Ad35 vaccine vectors bypass immunity to Ad5 and protect nonhuman primates against ebolavirus challenge. J Virol 2011, 85:4222-4233. 21. Lichterfeld M, Yu XG, Waring MT, Mui SK, Johnston MN, Cohen D, Addo MM, Zaunders J, Alter G, Pae E et al.: HIV-1-specific cytotoxicity is preferentially mediated by a subset of CD8(+) T cells producing both interferon-gamma and tumor necrosis factor-alpha. Blood 2004, 104:487-494. 22. Colloca S, Barnes E, Folgori A, Ammendola V, Capone S, Cirillo A, Siani L, Naddeo M, Grazioli F, Esposito ML et al.: Vaccine vectors derived from a large collection of simian adenoviruses induce potent cellular immunity across multiple species. Sci Transl Med 2012, 4 115ra112. 23. Stanley DA, Honko AN, Asiedu C, Trefry JC, Lau-Kilby AW,  Johnson JC, Hensley L, Ammendola V, Abbate A, Grazioli F et al.: Chimpanzee adenovirus vaccine generates acute and durable protective immunity against ebolavirus challenge. Nat Med 2014, 20:1126-1129. This study identified chimpanzee-derived adenovirus 3 (ChAd3) as a substitute for human adenovirus vectors, inducing rapid, uniform acute protection against lethal EBOV challenge in macaques, and circumventing pre-existing immunity. Furthermore, for the first time, uniform durable protection was achieved in macaques primed by ChAd3 and boosted with MVA. 24. Wong KK, Perdue CL, Malia J, Kenney JL, Peng S, Gwathney JK, Raczniak GA: Supportive care of the first two Ebola virus disease patients at the Monrovia Medical Unit. Clin Infect Dis 2015. Advanced access published June 14th, 2015. 25. Liddell AM, Davey RT Jr, Mehta AK, Varkey JB, Kraft CS, Tseggay GK, Badidi O, Faust AC, Brown KV, Suffredini AF et al.:

Current Opinion in Immunology 2015, 35:131–136

Characteristics and clinical management of a cluster of 3 patients with ebola virus disease, including the first domestically acquired cases in the United States. Ann Intern Med 2015, 163:81-90. 26. Ledgerwood JE, DeZure AD, Stanley DA, Novik L, Enama ME,  Berkowitz NM, Hu Z, Joshi G, Ploquin A, Sitar S et al.: Chimpanzee adenovirus vector Ebola vaccine — preliminary report. N Engl J Med 2014 http://dx.doi.org/10.1056/ NEJMoa1410863. Published online Nov 26. This first in human clinical trial demonstrated that the ChAd3 vaccine against EBOV was safe and immunogenic in all subjects in the Phase I clinical trial. 27. Krause PR, Cavaleri M, Coleman G, Gruber MF: Approaches to demonstration of Ebola virus vaccine efficacy. Lancet Infect Dis 2015, 15:627-629. 28. Kibuuka H, Berkowitz NM, Millard M, Enama ME, Tindikahwa A,  Sekiziyivu AB, Costner P, Sitar S, Glover D, Hu Z et al.: Safety and immunogenicity of Ebola virus and Marburg virus glycoprotein DNA vaccines assessed separately and concomitantly in healthy Ugandan adults: a phase 1b, randomised, doubleblind, placebo-controlled clinical trial. Lancet 2015, 385: 1545-1554. This was the first Ebola vaccine clinical trial performed in an African population. The study demonstrated that DNA vaccines were well tolerated, and elicited antigen-specific humoral and cellular immune responses. 29. Sarwar UN, Costner P, Enama ME, Berkowitz N, Hu Z, Hendel CS, Sitar S, Plummer S, Mulangu S, Bailer RT et al.: Safety and immunogenicity of DNA vaccines encoding Ebolavirus and Marburgvirus wild-type glycoproteins in a phase I clinical trial. J Infect Dis 2015, 211:549-557. 30. Rampling T, Ewer K, Bowyer G, Wright D, Imoukhuede EB, Payne R, Hartnell F, Gibani M, Bliss C, Minhinnick A et al.: A monovalent chimpanzee adenovirus Ebola vaccine — preliminary report. N Engl J Med 2015 http://dx.doi.org/10.1056/ NEJMoa1411627. Published online Jan 28. 31. Bellan SE, Pulliam JR, Pearson CA, Champredon D, Fox SJ, Skrip L, Galvani AP, Gambhir M, Lopman BA, Porco TC et al.: Statistical power and validity of Ebola vaccine trials in Sierra Leone: a simulation study of trial design and analysis. Lancet Infect Dis 2015, 15:703-710. 32. Lanini S, Zumla A, Ioannidis JP, Caro AD, Krishna S, Gostin L, Girardi E, Pletschette M, Strada G, Baritussio A et al.: Are adaptive randomised trials or non-randomised studies the best way to address the Ebola outbreak in west Africa? Lancet Infect Dis 2015, 15:738-745. 33. Gonzalez SF, Degn SE, Pitcher LA, Woodruff M, Heesters BA, Carroll MC: Trafficking of B cell antigen in lymph nodes. Annu Rev Immunol 2011, 29:215-233. 34. Quinn KM, Zak DE, Costa A, Yamamoto A, Kastenmuller K, Hill BJ,  Lynn GM, Darrah PA, Lindsay RW, Wang L et al.: Antigen expression determines adenoviral vaccine potency independent of IFN and STING signaling. J Clin Invest 2015, 125:1129-1146. Using a system biology approach, this study demonstrated that type I IFN and STING signaling were the major innate pathways induced by rAds, which limit Ag expression. The amount and duration of Ag is the best predictor of protective CD8+ T-cell immunity. This study provides fundamental criteria for selecting adenoviral serotypes in clinical development of rAd vaccines.

www.sciencedirect.com