Division of Experimental Virology, Department of Biomedicine, University of Basel, Switzerland
Vaccines represent one of the most successful chapters in the history of medicine. Over the past decades, the advent of recombinant cDNA technology has enabled the biomedical community to genetically engineer viruses for vaccine delivery purposes. As a starting point, this review evaluates the unmet medical needs, which drive scientists and industry to exploit such fundamentally new technology for human vaccination.
The author discusses the molecular functioning, production and safety profile of replication-competent and -deficient viral vector systems, representing two fundamentally distinct classes of “genetic vaccines”. Building upon this knowledge, he dissects the immunological mechanisms rendering immune responses to viral vectors qualitatively and quantitatively distinct from those elicited by non-live vaccination approaches. These mechanisms comprise (1) the vectors’ innate immune recognition by the host cell, (2) potent priming of CD8+ cytotoxic T cells as a result of dendritic cell targeting and endogenous protein synthesis, (3) conformational antigen display for protective antibody induction as well as (4) prolonged availability of substantial quantities of antigen. Deduced from these features, preferential indications for virally vectored vaccines are discussed, taking into consideration specific medical needs as well as risk-benefit assessments of replicating vector systems. The limitations and challenges in virally vectored vaccination must also be given careful consideration. Pre-existing and vaccination-induced anti-vector immunity can interfere with vaccine immunogenicity and prime-boost vaccination, respectively. Additionally, the requirement for eukaryotic production systems imposes technological as well as regulatory hurdles. Existing strategies to overcome these challenges are outlined.
With the recent licensure of the first virally vectored vaccine this review seems timely to herald the introduction of virally vectored vaccines into daily medical practice.
Keywords: virus, vector, vaccine
The need for new vaccine delivery technology
Inactivated vaccines such as the Salk polio virus vaccine and subunit vaccines as represented by the hepatitis B virus (HBV) vaccine have a long history of clinical success in preventing infectious diseases [1, 2]. These “non-live” approaches often combine a favourable safety profile with limited reactogenicity, warranting broad acceptance in population-wide vaccination campaigns. Acceptable costs of goods and product stability provide additional practical arguments in favour of these approaches. Conversely, virally vectored vaccines, neither replicating nor replication-deficient, cannot reasonably claim to combine all of these advantages. One may therefore be wondering why academic researchers and the vaccine industry keep developing and refining virally vectored vaccine delivery strategies, despite the complexity, which is inherent to “live”, “genetic” products? Here, I intend to summarise the conceptual grounds and medical needs, which together form a strong rationale to pursue virally vectored vaccine approaches.
It is a commonly held notion that for disease indications that can efficiently be prevented by non-live vaccines, the corresponding products have already been brought to the market in the course of the 20th century. This “rule of thumb” in the perception of the “vaccine opportunity landscape” results predominantly from vaccine failures in highly challenging disease indications such as human immunodeficiency virus / acquired immunodeficiency syndrome (HIV/AIDS) [3, 4]. Missed opportunities for more non-live vaccines may, however, readily be identified amongst emerging diseases such as Chikungunya or Zika virus [5–7], for which medical needs and/or commercial opportunities have long provided insufficient incentive to drive serious vaccine development efforts. Moreover, the clinical efficacy of the human papilloma virus vaccine (for the prevention of cervical cancer), which was introduced to the clinic in the 21st century only, illustrates that some new vaccines of global impact and based on non-live vaccine technology remain to be developed . Very encouraging recent phase III data on a subunit herpes zoster vaccine support this notion .
Still, the above rule of thumb analysis of vaccine opportunities is not overly pessimistic. Contemporary vaccine makers are indeed confronted with numerous challenging indications. Besides HIV/AIDS, hepatitis C, malaria and tuberculosis, which are most commonly cited in this context, other pressing “leftovers” of the 20th century vaccine makers comprise respiratory syncytial virus (RSV), cytomegalovirus (CMV) and the causative agents of haemorrhagic fever such as Ebola, Nipah and Lassa virus. It is in the context of these diseases that virally vectored vaccines find their primary application and represent strong development candidates.
Two main classes of virally vectored vaccines
Virally vectored vaccines can be categorised according to several criteria, such as the type or family of virus used for vectorisation, the medical target indication and population, prophylactic or therapeutic use, and so forth. For the purpose of this review, I propose to categorise virally vectored vaccines into two main classes, based on their ability (or the lack thereof) to replicate. “Replicating” will refer to the capacity of a viral vector to form infectious progeny and thereby establish a propagating infection in the vaccinee’s cells (see below). Conversely, the sole ability to express viral genes and amplify the viral genome intracellularly would not qualify as “replicating”. The term “vectored” will be used in a fairly restricted manner here. It will refer to the delivery of a piece of genetic information (viral, bacterial, parasitic or other), originating from a different species from that of the viral backbone used for delivery. Accordingly, live-attenuated vaccines, including the cold-adapted influenza virus vaccine, will not be considered as “vectored”.
Replication-deficient viral vectors
Viral vectors, both replication-competent and -deficient, are viral particles that can infect or transduce target cells in a vaccinee to deliver and express their genetic information. Whereas the term “infection” requires that viral replication takes place inside a target cell, the mere introduction of genetic information is referred to as “transduction”. Accordingly, viral vectors infecting or transducing target cells both represent “genetic vaccines”. In marked contrast to wild type or live-attenuated viruses, however, replication-deficient viral vectors fail to produce new infectious progeny particles when infecting or transducing the cell of a vaccinee. Still, and despite the fact that viruses do not represent living organisms, replication-deficient viral vectors are commonly classified as “live” vaccines, since the immunogenic principle of “genetic vaccines” consists in de novo expression of vaccine antigen(s) in the vaccinee’s cells. The antigen, which should induce a protective immune response, is not necessarily contained in the formulated particle. Antigen expression is a “live” process occurring in vivo. Thus, strictly speaking, “replication-deficient” viral vector systems are able to replicate their genetic information in the vaccinee’s cells but are unable to propagate infectivity in the vaccinee, and thus can also be referred to as “propagation-deficient”. For the purpose of this review, virus-like particles and other forms of non-live viral derivatives, which merely constitute virally adjuvanted protein, will not be counted amongst replication-deficient viral vectors.
Replication-deficient viral vectors commonly require a specific cellular substrate for production (production cells), which, unlike the cells of a vaccinee, permit the formation of infectious progeny. Most commonly, this cell substrate is made to express one or multiple viral protein(s), which are necessary for the assembly of infectious viral particles (fig. 1). Such expression can be obtained by transient transfection or by stable integration of genetic elements into the production cell line. Thereby, the cell line complements in trans one or multiple viral gene products, which have been deleted from the viral genome, converting the virus into a replication-deficient vector. Accordingly, the vector is replication-deficient because formation of infectious particles depends on trans-complementation of one (or multiple) gene product(s), and these are missing in the vaccinee’s cells. This mechanistic principle applies to most types of replication-deficient vectors such as those of the alphavirus , adenovirus  and arenavirus-based systems . Recombinant versions of modified vaccinia virus Ankara (MVA) should also be counted amongst the replication-deficient viral vectors. Unlike the aforementioned vector types, however, MVA only is replication-deficient in mammalian cells such as those of the vaccinee. MVA replicates in chicken embryonic fibroblasts as a production cell substrate, and trans-complementation of a viral gene product is not needed .
The lack of in vivo replication after administration to a vaccinee has important implications for clinical exploitation. If viral reversion is excluded [14, 15], the lack of replication itself warrants for a basic level of patient safety. Although adverse events in vectored gene delivery can be unpredictable , overwhelming infection and resulting disease represent a primary concern surrounding all virus-based products and are excluded if a vector is replication-deficient. This facilitates clinical trials and eventually also licensure for human use. After all, safety is first priority, particularly for vaccines to be used in a healthy population that is not at imminent risk of vaccine-preventable severe disease. For the development of vaccines in disease indications with the aforementioned characteristics, replication-deficient viral vector systems are thus usually preferred over replicating ones.
Replication-competent viral vectors
Analogously to replication-deficient viral vectors, replication-competent ones are genetic vaccines and require eukaryotic cell substrates for production. By definition, however, they do not depend on complementation, rendering any genetic modification of production cells unnecessary.
Essentially, replication-competent viral vectors are produced on Good Manufacturing Practice (GMP)-grade eukaryotic cell lines, analogously to the widely used live-attenuated measles, mumps and rubella vaccines. These processes have been industrial routine for long and will not be the subject of this review. Accordingly, live-attenuated human vaccines can represent attractive opportunities for vectorisation, one reason being that production processes are already established, warranting reasonably low costs of goods.
Safety considerations are in the spotlight of every clinical development programme relying on replicating viral vector technology. Potential concerns and regulatory limitations can emerge from two independent angles: patient safety and environmental safety. It depends on each specific vector and its context of use whether spread to other individuals or into the environment represents a possibility and/or a risk. The natural host range and transmission routes of the vectorised virus will help to establish an environmental risk assessment . The fact that these vectors are genetically modified organisms must also be given consideration, and implies additional legislation. Patient safety is dictated by the hazard profile of the parental virus in combination with any potentially attenuating modifications introduced. At times, the process of genetic modification for vectorisation, i.e., the introduction of a vaccine antigen, can by itself result in some level of viral attenuation . In other instances, specific attenuating mutations or deletions are made, with the goal of improving patient safety [19, 20]. The use of animal viruses, which are known or expected not to cause disease in humans, can represent an additional attractive option . One of the most commonly pursued strategies consists, however, in the exploitation of clinically validated live-attenuated vaccines as vectors. Besides established production processes (see above) and well-documented immunogenicity, the clinical safety profile of live-attenuated vaccines such as those against yellow fever, measles or polio represent a strong incentive to their use as vectors [22–24]. Depending on the clinical context of use, however, fairly human-pathogenic viruses such as replicating cytomegalovirus are also being considered for vaccine delivery purposes ( see below).
Mechanisms underlying the exquisite immunogenicity of virally vectored vaccines
Virally vectored vaccines are self-adjuvanted
The mammalian immune system has evolved to efficiently recognise viral intruders, and to mount potent innate and adaptive responses to infection. Accordingly, multiple innate sensors and pathways that coordinate immunity to viruses have been identified (fig. 2). Depending on their nature and genome replication strategy, diverse pattern recognition receptors are involved. Nucleic acid sensors include Toll-like receptor (TLR) 3, TLR7, TLR8 and TLR9 in the endosome, as well as cytosolic recognition molecules such as RIG-I-like receptors RIG-I (retinoic acid inducible gene I), MDA5 (melanoma differentiation-associated gene 5) [26–30] and DNA sensors such as cGAS (cyclic GMP-AMP synthase) ([31–34], reviewed in ). Upon binding of viral genomes, these receptors signal through the NFκB (nuclear factor “kappa-light-chain-enhancer” of activated B-cells) and mitogen-activated protein (MAP) kinase pathways, resulting in the induction of pro-inflammatory cytokines and chemokines. Additionally, interferon regulatory factor (IRF) 3 and IRF7 signalling leads to type I interferon (IFN-I) induction (reviewed in ). The primary evolutionary purpose of these pathways consists in antimicrobial defence, with the IFN-I-induced “antiviral state” as a paradigm . Additionally, these virus-induced transcriptional programmes activate infected antigen-presenting cells, which instruct the adaptive immune system to mount potent antibody and helper T cell type 1 (Th1)-biased cellular responses including CD8+ T cells. Accordingly, virus-induced inflammation is optimally suited to induce potent vaccine responses, and virally vectored vaccines are generally “self-adjuvanted”. The meaning of this term refers to the finding that adjuvant activity is “inbuilt”, with no need for supplementation. Attenuation and replication deficiency can profoundly impact or virtually abrogate systemic inflammation, thus preventing, or at least substantially reducing, the subjective feeling of disease that would result from natural infection. At the cellular level and in the tissue microcompartments, to which live vaccines are delivered and drained, the cellular pathways of innate activation can, however, operate fairly normally. In select cases, attenuation can even unleash immunostimulatory properties not evident with the wild type virus. Vaccinia virus is a classic example: the virus expresses an IFN-I decoy receptor  and antagonists of intracellular IFN-I induction. In contrast, modified vaccinia virus Ankara (MVA), a replication-deficient vaccinia virus variant with a large genome deletion, has lost these virulence factors and thus cannot suppress the host IFN-I response . This and other examples demonstrate that the replication capacity and level of attenuation cannot readily predict the self-adjuvanting abilities of live virus vaccines.
Viral particles: nature’s delivery system for cytotoxic T cell priming
Cytotoxic CD8+ T cells (CTLs) represent an important correlate of protection against many types of intracellular pathogens, as well as against tumours . Accordingly, robust CTL immunity is a key element that should be supplied for potent protection against a range of pathogens (see below), and CTL immunity is most potently induced by viruses. As “nature’s own gene delivery system”, viruses exploit the host cell’s translational machinery for gene expression (fig. 2). Gene products are synthesised in the cytosol of infected cells and peptidic fragments thereof, through the proteasome and the transporter associated with antigen (TAP) complex, have direct access to the host major histocompatibility complex (MHC) class I peptide loading machinery. This process is referred to as “direct presentation” . It contrasts with non-live vaccines, which rely exclusively upon crosspresentation, a process whereby phagocytic uptake delivers peptide fragments onto MHC class I complexes of professional antigen presenting cells (APCs) . Accordingly, potent activation, expansion and differentiation of CTL responses (CTL induction) by live viral vaccines is, at least in part, due to expression of vectored antigens in professional APCs such as dendritic cells. Additionally, if direct presentation occurs, the very APCs, which present antigenic peptide, are functionally activated by the viral infection . Simultaneous functional activation will augment, if not critically decide, the ability of APCs to prime potent CTL responses . Interestingly, dendritic cells can apparently be infected or at least transduced by a very broad range of viruses, even by some, such as hepatitis B, that otherwise exhibit a fairly restricted cell type- and tissue-tropism (, reviewed in ). Additionally, viral vectors can, of course, also exploit the crosspresentation machinery . In conjunction with the self-adjuvanted inflammatory milieu, all of these mechanisms synergise to potentiate CTL induction by virally vectored vaccines.
Conformational antigen display on vector-infected host cell membranes
For challenging antibody targets such as viral envelope proteins, correct three-dimensional folding and mammalian glycosylation patterns are often essential to display conformational epitopes for protective antibody induction. Full-length protein expression and display on host cell membranes can also be of importance to warrant intact epitope display . Membranous antigens, not only on virions but also on APCs, represent an efficient source for antigen recognition and uptake by B cells [48, 49]. Live viral vaccines direct conformational antigen expression on the cell surface of infected APCs and thus are optimally suited to meet these requirements. Conversely, the incorporation of vaccine antigens into virions does not seem a prerequisite for antibody induction. Even non-enveloped vaccine vectors can induce antibody responses against conformational antigens that are membrane-anchored . Accordingly, the “genetic vaccines” concept applies also to antibody induction.
Sustained supply of significant amounts of antigen
Virally vectored vaccines have the potential to express vaccine antigen in significant quantities and over prolonged periods of time, which represents an important basis for optimal immune stimulation . Whereas this applies first and foremost to replicating live vaccines , also the potency of immune responses to replication-deficient vector systems can be governed by the amount of antigen expressed in the vaccinee . The principle of genetic vaccines warrants that the persistence of vaccine antigen is not a simple function to the antigen’s in vivo half-life. Protracted production of vaccine antigen enables its supply to the ensuing immune response for critical periods of time.
Areas of preferential application for virally vectored vaccines
From these considerations one can deduce several circumstances and conditions, in which virally vectored vaccines find preferential use. Three shall be discussed here.
When classical vaccine technology falls short in efficacy
HIV represents a classic example where subunit vaccines have failed to provide clinical efficacy [3, 4]. In addition, pilot studies in nonhuman primates, which aimed at testing live-attenuated vaccines based on viral genome mutations or deletions, evidenced a rapid reversion to virulence . For tuberculosis, a live-attenuated vaccine (bacille Calmette Guérin, BCG) exists, but does not afford consistent protection in adult populations . Several candidate vaccines against CMV have been tested but did not provide the desired level of protection . A partially protective malaria subunit vaccine is about to enter clinical use in highly endemic areas , but formulations of higher efficacy should be sought. For these and similar indications, virally vectored delivery technologies represent attractive strategies owing to the mechanistic considerations outlined above in the section “Mechanisms underlying the exquisite immunogenicity of virally vectored vaccines”.
When CD8+ T cell immunity is key.
Control of primary infection with HIV and hepatitis C virus critically depends on CD8 T cell responses [58–61]. Also for Mycobacterium tuberculosis, an intracellular parasite, a role for CD8+ T cells has been firmly established in recent years , and the control of certain protozoa such as liver stage plasmodium can be T cell-dependent .
When inactivated vaccines augment disease severity upon infection
Respiratory syncytial virus (RSV) represents the classic example where an inactivated vaccine has led to disease enhancement rather than protection . Underlying mechanisms consist in insufficient potency and/or durability of immune responses, in combination with the type of immune response induced. Dengue virus bears the potential for disease enhancement if immunity does not cover all four serotypes and/or if antibody levels drop below critical thresholds [65, 66]. An inactivated measles virus vaccine has led to disease enhancement analogously to the RSV vaccine . The currently used live-attenuated vaccine against measles virus may eventually have to be replaced by an alternative product in order to eradicate the disease.
Indications for replicating vaccine vectors with imperfect safety or reactogenicity profiles
Both patient safety and environmental safety considerations argue rather in favour of replication-deficient viral vectors and against replicating ones. Depending on virus type, degree of attenuation and administered dose, the spread of replication-competent viral vectors can cause significant reactogenicity, together with the potential for severe adverse events even at sites that are distant from vaccine administration . In certain circumstances, however, these drawbacks may be well in balance with advantages such as augmented immunogenicity. The latter can result from higher levels and more prolonged vaccine antigen expression or from desirable effects of inflammation (see below). The following situations come to mind:
Prophylactic vaccination in epidemic outbreaks with high lethality: In an Ebola outbreak, the risk-benefit evaluation of vaccination speaks clearly in favour of using a reactogenic vaccine, provided no equally effective product with a better safety/reactogenicity profile is available [21, 50, 68, 69].
When a vaccine must induce rapid protection: Postexposure prophylaxis after exposure to a deadly pathogen requires that immunity be installed before the pathogen has replicated to critical thresholds [70, 71]. Under these circumstances, vaccine safety and reactogenicity are of lesser importance than efficacy, unless the expected risks are comparable to the one imposed by the pathogen itself.
When a single vaccine dose must induce durable protection: Vaccination and notably pathogen eradication campaigns in resource-limited circumstances and/or in remote locations can render repeated booster vaccinations logistically challenging, if not impossible. Lassa virus vaccination in rural areas of West Africa may represent such a case. Lassa virus seroprevalence rates approach 50% , such that the risk – benefit assessment and logistical constraints may justify the use of replicating virally vectored vaccines with an imperfect safety profile .
Endemic diseases with substantial public health impact: The first Dengue vaccine (CYD-TDV, see also section below “Challenges and limitations in virally vectored vaccination”), a vectorised form of the live-attenuated Yellow fever vaccine virus, has been licensed in a handful of highly Dengue-endemic countries [74, 75]. This is noteworthy for two reasons: First, the Yellow fever vaccine can, albeit very rarely, lead to severe disease [76, 77]. Secondly, the World Health Organization recommends vaccination while acknowledging the possibility that vaccination may increase the risk for severe Dengue illness in select groups of vaccinees . This concern is, however, outweighed by the strong evidence that the vaccine will reduce Dengue illness at the population level. Similar considerations might apply to a hypothetical HIV vaccine to be used in highly endemic areas. The cumulated lifetime infection risk of an individual might outweigh significant side effects of vaccination. Accordingly, even replicating recombinant cytomegalovirus, a life-long persistent infection with considerable disease potential in humans, has been proposed as a vector for an HIV vaccine .
Therapy of persistently infected or terminally diseased individuals: The aforementioned CMV-based vaccine candidate is given particularly serious consideration for therapeutic application in persistently HIV infected individuals . Analogously, a replication-competent herpes simplex virus-based oncolytic product has been licensed for the treatment of cancer . With the clinical introduction of immune checkpoint inhibitors in cancers, active immunisation also approaches [81–83]. Replicating virally vectored vaccines clearly have their place in this rapidly evolving field , and virus-induced inflammation – in prophylactic vaccination an unwanted side-effect – may even benefit tumour control .
Challenges and limitations in virally vectored vaccination
Recombinant DNA technology, which allows for the engineering of viral genomes as live vaccine delivery systems, has been available for more than three decades [86, 87]. Given the numerous advantages of these technologies, as outlined, the paucity of human products on the market may seem surprising. The currently first and only one, the tetravalent dengue vaccine CYD-TDV [74, 75], is a recombinant, replicating live vaccine for the prevention of severe Dengue fever and has been licensed in a handful of highly Dengue-endemic countries areas over the past two years. The broad pipeline of products, both replicating and replication-deficient (reviewed in ), that are fairly advanced in clinical testing, raises hope that many more products will enter the market in the coming years. Clinical development costs in the range of several hundred million dollars are common to most prophylactic vaccines, and GMP production, release testing and regulatory approval are particularly onerous for virally vectored products. Moreover, certain limitations and challenges are inherently linked to virally vectored vaccine approaches. Considering that some of these have considerably slowed down the clinical translation of virally vectored vaccine technology, two of them will be summarised below.
From the standpoint of immunogenicity and efficacy, one limitation of viral vaccine delivery systems consists in the interference resulting from so-called “anti-vector immunity”. The same phenomenon can manifest in essentially two ways:
Pre-existing anti-vector immunity
Individuals who are immune to the wild type parent virus, from which a viral vaccine vector is derived, tend to respond poorly to vaccination. This phenomenon was prominently noted in HIV vaccine trials relying on adenovirus 5- (Ad5-) based vectors. When compared to Ad5-seronegative individuals, study participants with antibody immunity to Ad5 responded poorly to vaccination [89, 90]. The underlying mechanism consists in immunological interference by so-called “anti-vector immunity”, which can comprise humoral as well as cellular mechanisms [12, 91, 92]. Adoptive serum transfer experiments in animals demonstrated that infection with Ad5, which in certain areas of the globe is fairly common, elicits “interfering antibodies” that virtually abrogate the immunogenicity of Ad5-based vectors . Supposedly they operated by neutralising the incoming viral vector particles, i.e., rendering them non-infectious and triggering their degradation, the primary purpose antibodies serve in nature to prevent viral re-infection .
Vaccination-induced anti-vector immunity
Viral vectors induce not only immune responses against the vectored antigen (i.e., the antigen of the pathogen, against which immunity should be induced) but result also in immunity to the vector backbone itself – anti-vector immunity. Most vaccines require, however, booster administrations in order to elicit optimal immune protection. Moreover, regular refresher vaccinations are often needed to maintain immunity over the years. The induction of anti-vector immunity upon primary vaccination, most prominently represented by interfering antibodies, can, however, dramatically curtail the effectiveness of booster immunizations.
Vaccine makers can draw a number of conclusions from these observations. First, it seems advantageous to exploit viruses with low seroprevalence in the human population, thereby avoiding pre-existing anti-vector immunity [50, 94]. To minimise detrimental effects of vaccination-induced anti-vector immunity on prime-boost vaccination, one should preferentially vectorise viruses that fail to elicit significant amounts of interfering antibodies [12, 95]. Alternatively, strategies can be developed to render vaccine vector particles less susceptible to antibody neutralisation [92, 96]. A combination of these approaches should, in principle, overcome the limitations imposed by pre-existing or vaccination-induced anti-vector immunity, allowing us to leverage the full potential of virally vectored delivery systems in homologous prime-boost vaccination. An additional strategy to overcome vaccination-induced anti-vector immunity, which has become fairly popular in recent years, consists in heterologous prime-boost combinations. Responses to adenoviral vector primary immunisation are, for example, commonly boosted by alternative viral delivery vehicles such as poxvirus-based or rhabdovirus-based ones in so-called “heterologous prime-boost” vaccination [50, 97, 98]. The combination of two active principles in one vaccine product introduces, however, an additional layer of complexity notably with respect to vaccine production, licensure and administration in the field.
Complexity of production
By nature, mammalian viruses generally require eukaryotic and often mammalian cells for production. Although large-scale GMP bioreactor work has become an industrial routine over the past decades, it can still represent a major cost driver. The balance between the vaccine’s effective dose and production yields per culture volume (including purification processes, see below) is usually indicative of the commercial viability of a vaccine vector in a given disease programme. Replication-deficient vector systems tend to show more marked dose dependency than replication-deficient ones and, accordingly, production yields are often a more prominent bottleneck. Purification processes pose additional challenges since viral infectivity, the mechanistic principle of live vaccine activity, must be preserved. This excludes virus inactivation steps, which are a routine for most mammalian cell culture products such as monoclonal antibodies. To exclude the presence of adventitious infectious agents in vaccine batches , extensive testing of cell lines and products is required, narrowing the range of eligible cell lines to a select few commonly available ones. New immortalised cell lines could be generated, in principle, but the effort and time required for their generation from primary human tissue, their selection based on favourable biological characteristics, their characterisation and eventual regulatory acceptance are substantial, and success is uncertain. Accordingly, the process of generating a new cell line is dissuasive for most vaccine programmes, and a small number of rather old but well-characterised cell lines dominate the field. As a consequence, cell line development has been somewhat neglected in the recent past. Ultimately, the ongoing revolution in mammalian genome engineering  may thus represent a game changer also to virally vectored vaccination.
Viruses represent nature’s gene delivery system, and thus are optimally suited to deliver genetic information also for the purpose of vaccination. Additionally, our immune system was trained in evolution to fight viral intruders, having led to a range of mechanisms that specifically potentiate immune responses against antigens encountered in a viral infection context. These conceptual considerations underscore the utility of viruses as delivery vehicles of genetic vaccines. Preclinical and clinical experience supports these mechanistic considerations, positioning virally vectored delivery systems amongst the most promising technologies to fight infectious diseases and cancers. Given the considerable unmet medical needs in both aforementioned domains of medicine, the likelihood is that virally vectored vaccines will become integrated into daily medical routine in the not too far future.
I wish to thank Paul-Henri Lambert and Claire-Anne Siegrist for insightful discussions and mentoring in vaccine-related topics.
At the time of writing, the Experimental Virology laboratory was supported by research contributions from the Swiss National Science Foundation, the EU 7th framework programme (FP7, European Research Council), the EU Horizon2020 program through the Swiss Staatssekretariat für Bildung, Forschung und Innovation (SBFI), the Gemeinnützige Klaus Tschira Stiftung gGmbH, and it maintained a research collaboration with Hookipa Biotech AG.
Daniel Pinschewer is a founder, shareholder and consultant to Hookipa Biotech AG commercialising arenavirus-based vector technology. He is listed as an inventor on patents describing arenavirus-based vector technology.
Prof. Daniel D. Pinschewer, MD, Division of Experimental Virology, Department of Biomedicine, Haus Petersplatz, University of Basel, Petersplatz 10, CH-4009 Basel, Switzerland., Daniel.Pinschewer[at]unibas.ch
1 WHO. Hepatitis B fact sheet. http://wwwwhoint/mediacentre/factsheets/fs204/en/. 2015;(Updated July 2016).
2 WHO. Poliomyelitis fact sheet. http://wwwwhoint/mediacentre/factsheets/fs114/en/. 2015;(Updated July 2016).
3 Flynn NM, Forthal DN, Harro CD, Judson FN, Mayer KH, Para MF; rgp120 HIV Vaccine Study Group. Placebo-controlled phase 3 trial of a recombinant glycoprotein 120 vaccine to prevent HIV-1 infection. J Infect Dis. 2005;191(5):654–65. http://dx.doi.org/10.1086/428404 PubMed
4 Pitisuttithum P, Gilbert P, Gurwith M, Heyward W, Martin M, van Griensven F, et al.; Bangkok Vaccine Evaluation Group. Randomized, double-blind, placebo-controlled efficacy trial of a bivalent recombinant glycoprotein 120 HIV-1 vaccine among injection drug users in Bangkok, Thailand. J Infect Dis. 2006;194(12):1661–71. http://dx.doi.org/10.1086/508748 PubMed
5 Erasmus JH, Auguste AJ, Kaelber JT, Luo H, Rossi SL, Fenton K, et al.A chikungunya fever vaccine utilizing an insect-specific virus platform. Nat Med. 2017;23(2):192–9. http://dx.doi.org/10.1038/nm.4253 PubMed
6 Akahata W, Yang ZY, Andersen H, Sun S, Holdaway HA, Kong WP, et al.A virus-like particle vaccine for epidemic Chikungunya virus protects nonhuman primates against infection. Nat Med. 2010;16(3):334–8. http://dx.doi.org/10.1038/nm.2105 PubMed
8 Harper DM, Franco EL, Wheeler CM, Moscicki AB, Romanowski B, Roteli-Martins CM, et al.; HPV Vaccine Study group. Sustained efficacy up to 4.5 years of a bivalent L1 virus-like particle vaccine against human papillomavirus types 16 and 18: follow-up from a randomised control trial. Lancet. 2006;367(9518):1247–55. http://dx.doi.org/10.1016/S0140-6736(06)68439-0 PubMed
9 Lal H, Cunningham AL, Godeaux O, Chlibek R, Diez-Domingo J, Hwang SJ, et al.; ZOE-50 Study Group. Efficacy of an adjuvanted herpes zoster subunit vaccine in older adults. N Engl J Med. 2015;372(22):2087–96. http://dx.doi.org/10.1056/NEJMoa1501184 PubMed
12 Flatz L, Hegazy AN, Bergthaler A, Verschoor A, Claus C, Fernandez M, et al.Development of replication-defective lymphocytic choriomeningitis virus vectors for the induction of potent CD8+ T cell immunity. Nat Med. 2010;16(3):339–45. http://dx.doi.org/10.1038/nm.2104 PubMed
13 Volz A, Sutter G. Modified Vaccinia Virus Ankara: History, Value in Basic Research, and Current Perspectives for Vaccine Development. Adv Virus Res. 2017;97:187–243. http://dx.doi.org/10.1016/bs.aivir.2016.07.001 PubMed
14 Hyvärinen A, Yongabi F, Mäkinen K, Wahlfors J, Pellinen R. Recombination of replicon and helper RNAs and the emergence of propagation-competent vectors upon Sindbis virus vector production. Int J Mol Med. 2013;32(2):410–22. doi:. http://dx.doi.org/10.3892/ijmm.2013.1395 PubMed
15 Bett AJ, Prevec L, Graham FL. Packaging capacity and stability of human adenovirus type 5 vectors. J Virol. 1993;67(10):5911–21. PubMed
17 Baldo A, Galanis E, Tangy F, Herman P. Biosafety considerations for attenuated measles virus vectors used in virotherapy and vaccination. Hum Vaccin Immunother. 2016;12(5):1102–16. http://dx.doi.org/10.1080/21645515.2015.1122146 PubMed
18 Buller RM, Smith GL, Cremer K, Notkins AL, Moss B. Decreased virulence of recombinant vaccinia virus expression vectors is associated with a thymidine kinase-negative phenotype. Nature. 1985;317(6040):813–5. Published online October 06, 1985. http://dx.doi.org/10.1038/317813a0 PubMed
19 Brun J, McManus D, Lefebvre C, Hu K, Falls T, Atkins H, et al.Identification of genetically modified Maraba virus as an oncolytic rhabdovirus. Mol Ther. 2010;18(8):1440–9. http://dx.doi.org/10.1038/mt.2010.103 PubMed
20 Mineta T, Rabkin SD, Yazaki T, Hunter WD, Martuza RL. Attenuated multi-mutated herpes simplex virus-1 for the treatment of malignant gliomas. Nat Med. 1995;1(9):938–43. http://dx.doi.org/10.1038/nm0995-938 PubMed
21 Agnandji ST, Huttner A, Zinser ME, Njuguna P, Dahlke C, Fernandes JF, et al.Phase 1 Trials of rVSV Ebola Vaccine in Africa and Europe. N Engl J Med. 2016;374(17):1647–60. http://dx.doi.org/10.1056/NEJMoa1502924 PubMed
22 Mandl S, Sigal LJ, Rock KL, Andino R. Poliovirus vaccine vectors elicit antigen-specific cytotoxic T cells and protect mice against lethal challenge with malignant melanoma cells expressing a model antigen. Proc Natl Acad Sci USA. 1998;95(14):8216–21. http://dx.doi.org/10.1073/pnas.95.14.8216 PubMed
23 Monath TP, Seligman SJ, Robertson JS, Guy B, Hayes EB, Condit RC, et al.; Brighton Collaboration Viral Vector Vaccines Safety Working Group (V3SWG). Live virus vaccines based on a yellow fever vaccine backbone: standardized template with key considerations for a risk/benefit assessment. Vaccine. 2015;33(1):62–72. http://dx.doi.org/10.1016/j.vaccine.2014.10.004 PubMed
25 Hansen SG, Vieville C, Whizin N, Coyne-Johnson L, Siess DC, Drummond DD, et al.Effector memory T cell responses are associated with protection of rhesus monkeys from mucosal simian immunodeficiency virus challenge. Nat Med. 2009;15(3):293–9. http://dx.doi.org/10.1038/nm.1935 PubMed
26 Querec T, Bennouna S, Alkan S, Laouar Y, Gorden K, Flavell R, et al.Yellow fever vaccine YF-17D activates multiple dendritic cell subsets via TLR2, 7, 8, and 9 to stimulate polyvalent immunity. J Exp Med. 2006;203(2):413–24. http://dx.doi.org/10.1084/jem.20051720 PubMed
27 Gitlin L, Barchet W, Gilfillan S, Cella M, Beutler B, Flavell RA, et al.Essential role of mda-5 in type I IFN responses to polyriboinosinic:polyribocytidylic acid and encephalomyocarditis picornavirus. Proc Natl Acad Sci USA. 2006;103(22):8459–64. http://dx.doi.org/10.1073/pnas.0603082103 PubMed
28 Kato H, Takeuchi O, Sato S, Yoneyama M, Yamamoto M, Matsui K, et al.Differential roles of MDA5 and RIG-I helicases in the recognition of RNA viruses. Nature. 2006;441(7089):101–5. http://dx.doi.org/10.1038/nature04734 PubMed
30 Pichlmair A, Schulz O, Tan CP, Näslund TI, Liljeström P, Weber F, et al.RIG-I-mediated antiviral responses to single-stranded RNA bearing 5′-phosphates. Science. 2006;314(5801):997–1001. http://dx.doi.org/10.1126/science.1132998 PubMed
31 Ablasser A, Goldeck M, Cavlar T, Deimling T, Witte G, Röhl I, et al.cGAS produces a 2′-5′-linked cyclic dinucleotide second messenger that activates STING. Nature. 2013;498(7454):380–4. http://dx.doi.org/10.1038/nature12306 PubMed
32 Li XD, Wu J, Gao D, Wang H, Sun L, Chen ZJ. Pivotal roles of cGAS-cGAMP signaling in antiviral defense and immune adjuvant effects. Science. 2013;341(6152):1390–4. http://dx.doi.org/10.1126/science.1244040 PubMed
33 Gao D, Wu J, Wu YT, Du F, Aroh C, Yan N, et al.Cyclic GMP-AMP synthase is an innate immune sensor of HIV and other retroviruses. Science. 2013;341(6148):903–6. http://dx.doi.org/10.1126/science.1240933 PubMed
34 Gao P, Ascano M, Wu Y, Barchet W, Gaffney BL, Zillinger T, et al.Cyclic [G(2′,5′)pA(3′,5′)p] is the metazoan second messenger produced by DNA-activated cyclic GMP-AMP synthase. Cell. 2013;153(5):1094–107. http://dx.doi.org/10.1016/j.cell.2013.04.046 PubMed
38 Symons JA, Alcamí A, Smith GL. Vaccinia virus encodes a soluble type I interferon receptor of novel structure and broad species specificity. Cell. 1995;81(4):551–60. http://dx.doi.org/10.1016/0092-8674(95)90076-4 PubMed
39 Waibler Z, Anzaghe M, Frenz T, Schwantes A, Pöhlmann C, Ludwig H, et al.Vaccinia virus-mediated inhibition of type I interferon responses is a multifactorial process involving the soluble type I interferon receptor B18 and intracellular components. J Virol. 2009;83(4):1563–71. http://dx.doi.org/10.1128/JVI.01617-08 PubMed
40 Schreiber RD, Old LJ, Smyth MJ. Cancer immunoediting: integrating immunity’s roles in cancer suppression and promotion. Science. 2011;331(6024):1565–70. http://dx.doi.org/10.1126/science.1203486 PubMed
41 Neefjes J, Jongsma ML, Paul P, Bakke O. Towards a systems understanding of MHC class I and MHC class II antigen presentation. Nat Rev Immunol. 2011;11(12):823–36. doi:. http://dx.doi.org/10.1038/nri3084 PubMed
42 Rock KL, Shen L. Cross-presentation: underlying mechanisms and role in immune surveillance. Immunol Rev. 2005;207(1):166–83. http://dx.doi.org/10.1111/j.0105-2896.2005.00301.x PubMed
43 Probst HC, Lagnel J, Kollias G, van den Broek M. Inducible transgenic mice reveal resting dendritic cells as potent inducers of CD8+ T cell tolerance. Immunity. 2003;18(5):713–20. http://dx.doi.org/10.1016/S1074-7613(03)00120-1 PubMed
44 Arima S, Akbar SM, Michitaka K, Horiike N, Nuriya H, Kohara M, et al.Impaired function of antigen-presenting dendritic cells in patients with chronic hepatitis B: localization of HBV DNA and HBV RNA in blood DC by in situ hybridization. Int J Mol Med. 2003;11(2):169–74. PubMed
46 Bernhard CA, Ried C, Kochanek S, Brocker T. CD169+ macrophages are sufficient for priming of CTLs with specificities left out by cross-priming dendritic cells. Proc Natl Acad Sci USA. 2015;112(17):5461–6. http://dx.doi.org/10.1073/pnas.1423356112 PubMed
47 Rujas E, Caaveiro JM, Partida-Hanon A, Gulzar N, Morante K, Apellániz B, et al.Structural basis for broad neutralization of HIV-1 through the molecular recognition of 10E8 helical epitope at the membrane interface. Sci Rep. 2016;6(1):38177. http://dx.doi.org/10.1038/srep38177 PubMed
49 Sanderson NS, Zimmermann M, Eilinger L, Gubser C, Schaeren-Wiemers N, Lindberg RL, et al.Cocapture of cognate and bystander antigens can activate autoreactive B cells. Proc Natl Acad Sci USA. 2017;114(4):734–9. http://dx.doi.org/10.1073/pnas.1614472114 PubMed
50 Ewer K, Rampling T, Venkatraman N, Bowyer G, Wright D, Lambe T, et al.A Monovalent Chimpanzee Adenovirus Ebola Vaccine Boosted with MVA. N Engl J Med. 2016;374(17):1635–46. http://dx.doi.org/10.1056/NEJMoa1411627 PubMed
52 Akondy RS, Johnson PL, Nakaya HI, Edupuganti S, Mulligan MJ, Lawson B, et al.Initial viral load determines the magnitude of the human CD8 T cell response to yellow fever vaccination. Proc Natl Acad Sci USA. 2015;112(10):3050–5. http://dx.doi.org/10.1073/pnas.1500475112 PubMed
53 Quinn KM, Zak DE, Costa A, Yamamoto A, Kastenmuller K, Hill BJ, et al.Antigen expression determines adenoviral vaccine potency independent of IFN and STING signaling. J Clin Invest. 2015;125(3):1129–46. http://dx.doi.org/10.1172/JCI78280 PubMed
54 Sawai ET, Hamza MS, Ye M, Shaw KE, Luciw PA. Pathogenic conversion of live attenuated simian immunodeficiency virus vaccines is associated with expression of truncated Nef. J Virol. 2000;74(4):2038–45. http://dx.doi.org/10.1128/JVI.74.4.2038-2045.2000 PubMed
55 Moliva JI, Turner J, Torrelles JB. Prospects in Mycobacterium bovis Bacille Calmette et Guérin (BCG) vaccine diversity and delivery: why does BCG fail to protect against tuberculosis?Vaccine. 2015;33(39):5035–41. http://dx.doi.org/10.1016/j.vaccine.2015.08.033 PubMed
57 Agnandji ST, Lell B, Soulanoudjingar SS, Fernandes JF, Abossolo BP, Conzelmann C, et al., RTS,S Clinical Trials Partnership. First results of phase 3 trial of RTS,S/AS01 malaria vaccine in African children. N Engl J Med. 2011;365(20):1863–75. http://dx.doi.org/10.1056/NEJMoa1102287 PubMed
58 Carrington M, Nelson GW, Martin MP, Kissner T, Vlahov D, Goedert JJ, et al.HLA and HIV-1: heterozygote advantage and B*35-Cw*04 disadvantage. Science. 1999;283(5408):1748–52. http://dx.doi.org/10.1126/science.283.5408.1748 PubMed
59 Migueles SA, Sabbaghian MS, Shupert WL, Bettinotti MP, Marincola FM, Martino L, et al.HLA B*5701 is highly associated with restriction of virus replication in a subgroup of HIV-infected long term nonprogressors. Proc Natl Acad Sci USA. 2000;97(6):2709–14. http://dx.doi.org/10.1073/pnas.050567397 PubMed
60 Neumann-Haefelin C, McKiernan S, Ward S, Viazov S, Spangenberg HC, Killinger T, et al.Dominant influence of an HLA-B27 restricted CD8+ T cell response in mediating HCV clearance and evolution. Hepatology. 2006;43(3):563–72. Published online February 24, 2006. http://dx.doi.org/10.1002/hep.21049 PubMed
61 Neumann-Haefelin C, Timm J, Schmidt J, Kersting N, Fitzmaurice K, Oniangue-Ndza C, et al.Protective effect of human leukocyte antigen B27 in hepatitis C virus infection requires the presence of a genotype-specific immunodominant CD8+ T-cell epitope. Hepatology. 2010;51(1):54–62. Published online December 25, 2009. http://dx.doi.org/10.1002/hep.23275 PubMed
62 Chen CY, Huang D, Wang RC, Shen L, Zeng G, Yao S, et al.A critical role for CD8 T cells in a nonhuman primate model of tuberculosis. PLoS Pathog. 2009;5(4):e1000392. http://dx.doi.org/10.1371/journal.ppat.1000392 PubMed
63 Fernandez-Ruiz D, Ng WY, Holz LE, Ma JZ, Zaid A, Wong YC, et al.Liver-Resident Memory CD8(+) T Cells Form a Front-Line Defense against Malaria Liver-Stage Infection. Immunity. 2016;45(4):889–902. http://dx.doi.org/10.1016/j.immuni.2016.08.011 PubMed
64 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(4):422–34. http://dx.doi.org/10.1093/oxfordjournals.aje.a120955 PubMed
66 Murphy BR, Whitehead SS. Immune response to dengue virus and prospects for a vaccine. Annu Rev Immunol. 2011;29(1):587–619. http://dx.doi.org/10.1146/annurev-immunol-031210-101315 PubMed
67 Fulginiti VA, Eller JJ, Downie AW, Kempe CH. Altered reactivity to measles virus. Atypical measles in children previously immunized with inactivated measles virus vaccines. JAMA. 1967;202(12):1075–80. http://dx.doi.org/10.1001/jama.1967.03130250057008 PubMed
68 Huttner A, Dayer JA, Yerly S, Combescure C, Auderset F, Desmeules J, et al.; VSV-Ebola Consortium. The effect of dose on the safety and immunogenicity of the VSV Ebola candidate vaccine: a randomised double-blind, placebo-controlled phase 1/2 trial. Lancet Infect Dis. 2015;15(10):1156–66. http://dx.doi.org/10.1016/S1473-3099(15)00154-1 PubMed
69 Henao-Restrepo AM, Longini IM, Egger M, Dean NE, Edmunds WJ, Camacho A, et al.Efficacy and effectiveness of an rVSV-vectored vaccine expressing Ebola surface glycoprotein: interim results from the Guinea ring vaccination cluster-randomised trial. Lancet. 2015;386(9996):857–66. http://dx.doi.org/10.1016/S0140-6736(15)61117-5 PubMed
70 Günther S, Feldmann H, Geisbert TW, Hensley LE, Rollin PE, Nichol ST, et al.Management of accidental exposure to Ebola virus in the biosafety level 4 laboratory, Hamburg, Germany. J Infect Dis. 2011;204(Suppl 3):S785–90. http://dx.doi.org/10.1093/infdis/jir298 PubMed
71 Feldmann H, Jones SM, Daddario-DiCaprio KM, Geisbert JB, Ströher U, Grolla A, et al.Effective post-exposure treatment of Ebola infection. PLoS Pathog. 2007;3(1):e2. http://dx.doi.org/10.1371/journal.ppat.0030002 PubMed
72 McCormick JB, Webb PA, Krebs JW, Johnson KM, Smith ES. A prospective study of the epidemiology and ecology of Lassa fever. J Infect Dis. 1987;155(3):437–44. http://dx.doi.org/10.1093/infdis/155.3.437 PubMed
74 Capeding MR, Tran NH, Hadinegoro SR, Ismail HI, Chotpitayasunondh T, Chua MN, et al.; CYD14 Study Group. Clinical efficacy and safety of a novel tetravalent dengue vaccine in healthy children in Asia: a phase 3, randomised, observer-masked, placebo-controlled trial. Lancet. 2014;384(9951):1358–65. http://dx.doi.org/10.1016/S0140-6736(14)61060-6 PubMed
75 Villar L, Dayan GH, Arredondo-García JL, Rivera DM, Cunha R, Deseda C, et al.; CYD15 Study Group. Efficacy of a tetravalent dengue vaccine in children in Latin America. N Engl J Med. 2015;372(2):113–23. http://dx.doi.org/10.1056/NEJMoa1411037 PubMed
76 Martin M, Tsai TF, Cropp B, Chang GJ, Holmes DA, Tseng J, et al.Fever and multisystem organ failure associated with 17D-204 yellow fever vaccination: a report of four cases. Lancet. 2001;358(9276):98–104. http://dx.doi.org/10.1016/S0140-6736(01)05327-2 PubMed
77 Vasconcelos PF, Luna EJ, Galler R, Silva LJ, Coimbra TL, Barros VL, et al.; Brazilian Yellow Fever Vaccine Evaluation Group. Serious adverse events associated with yellow fever 17DD vaccine in Brazil: a report of two cases. Lancet. 2001;358(9276):91–7. http://dx.doi.org/10.1016/S0140-6736(01)05326-0 PubMed
78 WHO. Dengue vaccine: WHO position paper – July 2016. http://wwwwhoint/wer/2016/wer9130pdf?ua=1. 2016.
79 Hansen SG, Piatak M, Ventura AB, Hughes CM, Gilbride RM, Ford JC, et al.Immune clearance of highly pathogenic SIV infection. Nature. 2013;502(7469):100–4. http://dx.doi.org/10.1038/nature12519 PubMed
80 Andtbacka RH, Kaufman HL, Collichio F, Amatruda T, Senzer N, Chesney J, et al.Talimogene Laherparepvec Improves Durable Response Rate in Patients With Advanced Melanoma. J Clin Oncol. 2015;33(25):2780–8. http://dx.doi.org/10.1200/JCO.2014.58.3377 PubMed
81 Madan RA, Mohebtash M, Arlen PM, Vergati M, Rauckhorst M, Steinberg SM, et al.Ipilimumab and a poxviral vaccine targeting prostate-specific antigen in metastatic castration-resistant prostate cancer: a phase 1 dose-escalation trial. Lancet Oncol. 2012;13(5):501–8. http://dx.doi.org/10.1016/S1470-2045(12)70006-2 PubMed
82 van den Eertwegh AJ, Versluis J, van den Berg HP, Santegoets SJ, van Moorselaar RJ, van der Sluis TM, et al.Combined immunotherapy with granulocyte-macrophage colony-stimulating factor-transduced allogeneic prostate cancer cells and ipilimumab in patients with metastatic castration-resistant prostate cancer: a phase 1 dose-escalation trial. Lancet Oncol. 2012;13(5):509–17. http://dx.doi.org/10.1016/S1470-2045(12)70007-4 PubMed
83 Hodi FS, O’Day SJ, McDermott DF, Weber RW, Sosman JA, Haanen JB, et al.Improved survival with ipilimumab in patients with metastatic melanoma. N Engl J Med. 2010;363(8):711–23. http://dx.doi.org/10.1056/NEJMoa1003466 PubMed
84 Kantoff PW, Schuetz TJ, Blumenstein BA, Glode LM, Bilhartz DL, Wyand M, et al.Overall survival analysis of a phase II randomized controlled trial of a Poxviral-based PSA-targeted immunotherapy in metastatic castration-resistant prostate cancer. J Clin Oncol. 2010;28(7):1099–105. http://dx.doi.org/10.1200/JCO.2009.25.0597 PubMed
85 Zamarin D, Holmgaard RB, Subudhi SK, Park JS, Mansour M, Palese P, et al.Localized oncolytic virotherapy overcomes systemic tumor resistance to immune checkpoint blockade immunotherapy. Sci Transl Med. 2014;6(226):226ra32. http://dx.doi.org/10.1126/scitranslmed.3008095 PubMed
89 Quirk EK, Mogg R, Brown DD, Lally MA, Mehrotra DV, DiNubile MJ, et al.HIV seroconversion without infection after receipt of adenovirus-vectored HIV type 1 vaccine. Clin Infect Dis. 2008;47(12):1593–9. http://dx.doi.org/10.1086/593313 PubMed
90 McElrath MJ, De Rosa SC, Moodie Z, Dubey S, Kierstead L, Janes H, et al.; Step Study Protocol Team. HIV-1 vaccine-induced immunity in the test-of-concept Step Study: a case-cohort analysis. Lancet. 2008;372(9653):1894–905. http://dx.doi.org/10.1016/S0140-6736(08)61592-5 PubMed
91 Schirmbeck R, Reimann J, Kochanek S, Kreppel F. The immunogenicity of adenovirus vectors limits the multispecificity of CD8 T-cell responses to vector-encoded transgenic antigens. Mol Ther. 2008;16(9):1609–16. http://dx.doi.org/10.1038/mt.2008.141 PubMed
92 Roberts DM, Nanda A, Havenga MJ, Abbink P, Lynch DM, Ewald BA, et al.Hexon-chimaeric adenovirus serotype 5 vectors circumvent pre-existing anti-vector immunity. Nature. 2006;441(7090):239–43. http://dx.doi.org/10.1038/nature04721 PubMed
93 Sumida SM, Truitt DM, Lemckert AA, Vogels R, Custers JH, Addo MM, et al.Neutralizing antibodies to adenovirus serotype 5 vaccine vectors are directed primarily against the adenovirus hexon protein. J Immunol. 2005;174(11):7179–85. http://dx.doi.org/10.4049/jimmunol.174.11.7179 PubMed
94 Barouch DH, Pau MG, Custers JH, Koudstaal W, Kostense S, Havenga MJ, et al.Immunogenicity of recombinant adenovirus serotype 35 vaccine in the presence of pre-existing anti-Ad5 immunity. J Immunol. 2004;172(10):6290–7. http://dx.doi.org/10.4049/jimmunol.172.10.6290 PubMed
95 Penaloza MacMaster P, Shields JL, Alayo QA, Cabral C, Jimenez J, Mondesir J, et al.Development of novel replication-defective lymphocytic choriomeningitis virus vectors expressing SIV antigens. Vaccine. 2017;35(1):1–9. http://dx.doi.org/10.1016/j.vaccine.2016.11.063 PubMed
96 Kreppel F, Kochanek S. Modification of adenovirus gene transfer vectors with synthetic polymers: a scientific review and technical guide. Mol Ther. 2008;16(1):16–29. http://dx.doi.org/10.1038/sj.mt.6300321 PubMed
98 Flatz L, Cheng C, Wang L, Foulds KE, Ko SY, Kong WP, et al.Gene-based vaccination with a mismatched envelope protects against simian immunodeficiency virus infection in nonhuman primates. J Virol. 2012;86(15):7760–70. http://dx.doi.org/10.1128/JVI.00599-12 PubMed
99 Victoria JG, Wang C, Jones MS, Jaing C, McLoughlin K, Gardner S, et al.Viral nucleic acids in live-attenuated vaccines: detection of minority variants and an adventitious virus. J Virol. 2010;84(12):6033–40. http://dx.doi.org/10.1128/JVI.02690-09 PubMed
Published under the copyright license CC BY-NC-SA: This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format for noncommercial purposes only, and only so long as attribution is given to the creator. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.