Immunity is a result of coevolution between the immune system and pathogens. Microbes have evolved multiple strategies to evade, confuse or inhibit the immune defense mechanism in order to proliferate in a biological shelter. The immune system must counter microbial invasions by eliciting protective immunity without inducing autoimmune disorders that could dampen species survival. Vaccination activates the immune system by mimicking a microbial invasion without the harmful, sometimes fatal, consequences of an infection. Vaccine design can potentially exploit pathogens’ weaknesses, thus tipping the scales of coevolution toward defeating pathogens at their own game.
Few medical interventions have been as cost effective as vaccination in fostering universal benefit. The demand for vaccines has been escalating as the world population grows, new germs with pandemic potential emerge and some insidious pathogens are developed into potential bioweapons for the intent of massive economic and/or human destruction. Despite the success and importance of vaccines, it is acknowledged that vaccination carries unknown risks and sometimes can cause harm, ranging from tetanus vaccine-induced inflammation to the rare occurrence of paralytic polio induced by the Sabin oral polio vaccine (OPV) Citation[1]. Vague concerns over vaccine-associated risks of cancer, heart diseases, auto-immune illnesses and neurological disorders are also not completely eliminated.
Currently licensed vaccines (killed or inactivated, whole-cell, subunit protein or live-attenuated) are usually mixed with adjuvants and inoculated principally by needle injections. Only a small number of licensed vaccines are administered through nasal spray (e.g., live-attenuated influenza virus vaccine) or oral ingestion (e.g., OPV). The needle–syringe, introduced into the medical community more than 100 years ago, poses a series of health and environmental problems through intentional or inadvertent reuse, needlestick injury and waste disposal. Under the current vaccination schedule in the USA, children under the age of 5 years may receive more than 20 vaccine shots. Needle injections have to be performed by trained medical personnel who may be in short supply during a crisis. The clinical visits also incur medical and societal costs, and the vaccination rights for many impoverished people in developing countries have been denied due to a lack of adequate access to qualified medical services. Moreover, aichmophobia (the fear of sharp pointed objects) can interfere with vaccine coverage to some degree. Evolution may have fostered aichmophobia because, aside from the associated pain, penetration by a sharp foreign object usually resulted in prolonged trauma or death before antibiotics were introduced as a medical intervention. It is conceivable that most humans and animals may have developed psychological resistance to skin penetration, which translates into a marked dislike of needle injections.
The problems associated with needle injections can erode any population-wide benefits derived from vaccination. When parents weigh the odds of infection against the odds of vaccination on healthy children, a theoretical risk would be converted to a real risk if vaccination is delayed or refused, not only for the unvaccinated individual but also for the population. The potential eradication or control of many infectious diseases is currently jeopardized by a staggering disparity in worldwide vaccine coverage. When a pandemic occurs, vaccination of only a small fraction of the population can be likened to spraying a garden hose on a forest fire.
Other concerns over injectable vaccines include the aluminum adjuvants, thimerosal and induction of weak mucosal immunity Citation[2–4]. An adjuvant is an immune-response-boosting additive that operates at the interface between the immune system and the administered vaccine. The only widely used adjuvant in most injectable vaccines for humans is an aluminum salt. It improves antibody production but has little effect on enhancing cellular immune responses Citation[5]. The aluminum adjuvants may act as an antigen depot by slowly releasing adsorbed antigens, causing the immune system to react more potently and persistently to the vaccine Citation[6]. Although aluminum hydroxide was believed to be cleared quickly from the body, recent evidence shows otherwise and that macrophagic myofasciitis lesions, a muscle ailment, may be related to intramuscular injection of aluminum hydroxide-containing vaccines Citation[7]. Thimerosal, a mercury compound used to prevent vaccines from becoming contaminated by bacteria or fungi, has been associated with development of autism in young children, even though compelling evidence is lacking Citation[8]. Overall, a causal connection between vaccination and long-term chronic side effects is very difficult to confirm. Vaccines could be made safer if aluminum salts, thimerosal and other artificial ingredients are eliminated from vaccine formulas, regardless of their full impact on health.
It is conceivable that vaccine coverage would be boosted when vaccines containing only natural components and administered through natural pathways are developed in compliance with evolutionary medicine. Such vaccines would allow the public to have better confidence in vaccines’ components that are familiar to the immune system and have been time tested during evolution, particularly when they are administered needle free.
Vectored vaccines with a high compliance rate to evolutionary medicine have been developed in the laboratory and may eventually replace conventional vaccines. A vaccine vector usually refers to a bioengineered benign microbe that delivers a pathogen-derived antigen or the antigen gene into a person or animal to introduce highly specific immune interventions based on well-defined antigens that can be the focus of specific immune reactivity. Development of vectored vaccines has been motivated by the potential economic and safety advantages, as well as patient comfort, attainable through mass production at relatively low input costs and mass administration by non-medical personnel during a crisis. In contrast to live-attenuated vaccines (e.g., OPV) that occasionally may revert to a virulent pathogen Citation[1], it is not possible for a benign vectored vaccine to gain virulence because only a fraction of the pathogen’s genome is encoded by the vector. Vectored vaccines with antigens presented in the context of nonreplicating delivery vectors can be administered noninvasively without pain, fear and perceivable tissue damage. It is conceivable that a vaccine in compliance with evolutionary medicine may boost vaccine coverage worldwide.
Vectored vaccines can be administered in a natural manner into the outer layer of mucosa (e.g., by nasal spray or oral ingestion) or skin (e.g., by topical application of a patch), and subsequently broadcast a signal to mobilize the immune repertoire toward a beneficial immune protection. Mucocutaneous surfaces are covered by an epithelium, which exerts the role of a physical barrier, limiting the penetration of microbes, while ensuring that microbes that penetrate the epithelium are captured, killed and remembered by the immune system through antigen presentation. It is conceivable that the superficial layer along the mucocutaneous interface must be more immunocompetent than deep tissues (e.g., dermis or muscle) because immune cells have to be deployed as sentries at epithelial borders to defend against the frequent onslaught of microbial invaders. This logic has been corro-borated by the finding that antigen genes inoculated into stratum granulosum (the outermost layer of viable skin underneath the stratum corneum) were more effective in eliciting an immune response than their counterparts delivered into deeper tissues, such as dermis or muscle Citation[9,10]. It has also been shown that intranasal administration of an adenovirus-vectored TB vaccine was more effective in eliciting protective immunity than intramuscular injection of the same vaccine against pulmonary TB Citation[4], although the superiority of a nasal vaccine against a respiratory pathogen may be interpreted as a result of efficient antigen expression in the immunocompetent superficial layer of mucosa in conjunction with induction of a more focused mucosal immune response when vaccines are administered into the respiratory tract through the same route as that of a pathogen. Needle-free alternatives also hold the promise of dissolving psychological resistance to vaccination. If syringe needles are destined to the same fate as typewriters, this trend would be favored by the public since all of us may have aichmophobia to some degree.
It has been demonstrated that humans and animals can be effectively immunized by intranasal administration of adenovirus-vectored vaccines Citation[4,10–12] or topical application of adenovirus- and Escherichia coli-vectored epicutaneous vaccines Citation[10,11,13,14] without including any artificial adjuvants or thimerosal in the vaccine formulas. Conceivably, the immune system must have memory to epitopes presented by common microbes, such as adenovirus or Escherichia coli, since coevolution with these microbes has progressed for long time periods. Because the immune system must have learned how to react to these microbes during coevolution and humans have survived the natural selection, some of the microbes’ components, or even the whole particle, may serve as natural adjuvants without the unknown risks associated with man-made adjuvants. At least one of the adenovirus components, hexon, was highly immunogenic and conferred adjuvant activity to exogenous antigens Citation[15]. This natural adjuvant component in adenovirus vectors may facilitate efficient immunization following expression of a small number of antigen molecules in the outer layer of mucosa. The hypothesis that microbe-derived vectors may contain endogenous natural adjuvants in support of antigenicity of a vectored vaccine was further corroborated by the demonstration that topical application of E. coli vectors overproducing tetanus toxin C-fragment (tetC) could protect animals against tetanus in a single-dose regimen without involving any exogenous adjuvants Citation[13], whereas topical application of purified tetC protein was ineffective unless an artificial adjuvant was added Citation[16]. Noninvasive immunization by topical application of intact E. coli vectors overproducing pathogen-derived antigens eliminates the time-consuming and deleterious requirement for biochemical purification of antigens, the risks of man-made adjuvants and the intrinsic problems associated with needle injections. It is amenable to large-scale, rapid, low-cost production, distribution and administration, by way of an existing biological pathway. These findings provide a new quality of vaccination that is not attainable through the use of conventional vaccines.
Although E. coli-vectored vaccines can be mass produced rapidly at low costs, epicutaneous vaccines may not be able to elicit a robust mucosal immune response against respiratory pathogens, similar to the weak mucosal immunity induced by injectable vaccines Citation[2–4]. It would be unsafe to administer E. coli vectors as a nasal vaccine carrier since the airway is not the natural portal for E. coli to enter the body. In addition, intranasal application of E. coli enterotoxin as a nasal adjuvant (an approach not in compliance with evolutionary medicine) has been associated with the induction of Bell’s palsy in humans Citation[17]. To mitigate diseases caused by respiratory pathogens, such as influenza, an adenovirus-vectored nasal vaccine holds promise as a critical tool that can be generated rapidly in response to the emergence of new virus strains, mass produced at low costs and mass administered through nasal spray. Seroconversion has been induced in human volunteers following intranasal administration of an adenovirus-vectored influenza vaccine at a dose of 5 × 108 viral particles without any appreciable side effects Citation[11]. In contrast to gene therapy, pre-existing immunity to adeno-virus did not interfere with the potency of adenovirus-vectored nasal vaccines on many occasions Citation[10–12,18], probably due to both the high efficiency of gene delivery and antigen presentation in the respiratory tract and the low threshold for triggering an immune reaction. Adenovirus-vectored vaccines can be manufactured in the well-characterized PER.C6 packaging cell line in serum-free suspension bioreactors Citation[19] and purified inexpensively by column chromatography Citation[20]. A single 500-l cellbag on a wave bioreactor is capable of producing millions of doses of these vaccines in a few days Citation[21], an obvious advantage over the current method for manufacturing influenza virus vaccines using embryonated chicken eggs Citation[21]. Development of a stable liquid formulation enables adenovirus-vectored vaccines to be stored for long periods without the need for freezing Citation[22].
Although a large number of vectors have been tested as vaccine carriers, it is crucial that no genetically modified organisms capable of replication are introduced into the ecosystem once vectored vaccines are released into the field. The replication-competent adenovirus-free adenovirus vectors produced in PER.C6 cells Citation[23] are endowed with a high compliance rate and forego many of the potential safety concerns related to replication-associated biological hazards. Unlike live-attenuated influenza virus vaccines, which may exchange genetic material with wild influenza viruses through reassortment, resulting in the generation of harmful new strains Citation[24], the DNA genome of an adenovirus is unable to reassort with the RNA genome of an influenza virus. E. coli-vectored epicutaneous vaccines can also be made nonreplicating by γ-irradiation of E. coli particles without appreciably losing potency Citation[13]. Inhalation of adenovirus or cutaneous contact with E. coli is part of evolution. Noninvasive administration of nonreplicating adenovirus or E. coli particles to their natural portal sites amplifies the safety margin of this mode of vaccination.
Overall, this class of vaccines in compliance with evolutionary medicine provides the foundation for mitigating disease outbreaks and bioterrorist attacks in a simple, rapid, effective, economical, painless, and safe manner.
Acknowledgements
The authors thank D Grove for critical reading of the manuscript and the NIH and the US Navy for their support.
Financial & competing interests disclosure
DC Tang and KV Kampen acted as principal investigators for various government grants and contracts; both are shareholders of Vaxin Inc. and inventors on patents pertaining to vectored vaccines. 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 apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
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