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Implementation

Future Directions In Vaccines: The Payoffs Of Basic Research

Abstract

 
Vaccine development has historically relied on approaches such as live attenuated, subunit, and whole-cell vaccine designs to present antigens to the immune system. These strategies are no longer nimble enough to rapidly address public health threats, particularly emerging infectious diseases. New vaccines will require a strong scientific base partnered with the leveraging of emerging and enabling technologies so that candidate vaccines can be developed more rapidly and with the greatest chance of proving effective. This paper focuses on new strategies, technologies, and immunologic research that will provide important opportunities for the development of new and improved vaccines.

How does induction of the immune response occur? The disease-causing organisms contain proteins called antigens, which stimulate the immune response. The resulting immune response is multifold and includes the synthesis of proteins called antibodies or the stimulation of specific immune cells, or both. Antibodies bind to the disease-causing organisms and lead to their eventual destruction, while specific immune cells bind to infected cells and destroy them. In addition, memory cells are produced in an immune response. These are cells that remain in the bloodstream, sometimes for the life span of the host, ready to mount a quick protective immune response against subsequent infections with the agent that induced their production. If such an infection were to occur, the memory cells would respond so quickly that the resulting immune response could inactivate the disease-causing agents, and symptoms would be prevented. This response is often so rapid that clinical signs of infection do not develop.

Making Vaccines: Standard And New Approaches

Top Making Vaccines: Standard And... Postgenomic Approaches:... New Approaches To Delivery Policy Implications NOTES

 
A live, virulent organism cannot be used as a vaccine because it would induce the very disease it is being used to prevent. Therefore, the first step in making a vaccine is to isolate or create an organism—or part of one—that is unable to cause full-blown disease but that still retains the antigens responsible for inducing the host’s immune response. This is done in many ways (Exhibit 1).

New vaccines are also likely to exploit genomics and high-throughput screening approaches that are based on computational methods. These methods will allow for development of rationally based approaches that select potential antigens more effectively and precisely. In addition, future vaccines will use these new tools to get around the challenges of the remaining infectious diseases.² These challenges include the inherent ability of many viruses to change (antigenic variation), as is seen with HIV and influenza; the need to develop vaccines that rely on cell-based immunity for protection for infections such as tuberculosis; and tools for addressing a pathogen’s ability to outsmart the immune system—immune evasion strategies, such as seen with hepatitis C.³

Impact of new immune concepts. Research on the immune system has helped identify new ways of fighting infections and is helping define the mechanisms needed for successful immunization. Most currently licensed vaccines protect by producing neutralizing antibodies, made by the B cells of the immune system. One of the advantages of stimulating this arm of the immune system is that it can be easily measured. Researchers believe that vaccines against many of the infections that are of highest priority (HIV, TB, and malaria) will need to have the other arm of the immune system—the cellular component, or T cells—pulled into action.⁴ For the first time in sixty years, new TB vaccines are in clinical trials.⁵

Basic research in immunology has provided a greater appreciation for the importance of the innate immune system and has given researchers new insights into how to rally this component of protection. Once thought of as nonessential, the innate immune response is now recognized as having an important role in the body’s response to infection. The innate immune system is a conserved, inborn response, which leads to a generic but immediate response. When infections are recognized, the innate systems triggers an inflammatory response to halt the further spread of the pathogen. By contrast, adaptive immunity takes longer to rally and clear pathogens from the body, but it has the ability to be remembered for future encounters. Toll-like receptors (TLRs) have recently emerged as a key component of the innate immune system and are capable of detecting microbial infection and triggering antimicrobial host defense responses. In addition, TLRs control multiple dendritic cell functions and activate signals that are critically involved in the initiation of adaptive immune responses.⁶

The new understanding of the innate system may also allow researchers to develop more broadly protective vaccines and to expand the number of effective adjuvants.⁷ For example, particular DNA sequences (CpG motifs), which are abundant in bacterial but not in mammalian DNA, have been shown to activate and stimulate TLRs. Researchers have developed synthetic CpG motifs that mimic the action of bacterial DNA and are using them as adjuvants to boost the immune response.⁸ Similarly, the concept of needing more than one arm of the immune system to effectively combat many of the diseases for which vaccines do not yet exist has led to the consideration of the combination vaccines. Prior to this, the words "combination vaccine" had been restricted to the mixing a several vaccines against a series of diseases for ease of administration (for example, the DTaP vaccine is a mixture of vaccines used to prevent diphtheria, tetanus, and pertussis). The newer concept of combination vaccine, which also includes the "prime boost" approach, has its roots in maximizing the immune response to a single disease. The need to maximize both B and T cell responses in this disease has led to early studies that pioneered the combination concept of "prime boost." In brief, this approach is an attempt to start the immune system rolling by using vaccine A ("prime") and then at the right time either directing or expanding the immune response by giving vaccine B ("boost"). A Phase III trial is under way to assess the use of a vectored vaccine (canarypox vector containing the HIV sequences for the envelope portion of the virus as well as the gag and pol genes) as the prime, followed by the gp120 protein as the boost. Previous studies on the canarypox vectored vaccine had demonstrated that the T cell responses that were elicited by this vaccine were able to recognize many versions of HIV, unlike the narrow antibody response seen when the gp120 vaccine was given alone.⁹ By providing a breadth of T cell type response using the "prime" and honing in on the predominant virus by generating specific antibodies using the "boost" vaccine, it is hoped that viruses that escape destruction by means of the circulating antibodies will be noticed by the broader range of T cells circulating.¹⁰

Power of genomics. Genomics is the identification and study of genes and how they create the blueprints for an organism. Different proteins perform different duties: structural proteins compose the framework of a cell, toxins attack and damage cells, and enzymes direct the hundreds of different chemical reactions a microbe requires for survival. The power of the genome is that it contains a complete, coded list of all of the proteins a pathogen makes. Researchers use functional genomics to scour that list and determine what role each protein plays. These tools are helping researchers identify and fine-tune the targets most appropriate for use in developing candidate vaccines. Additionally, genomics allows researchers to work directly with the pathogen and eliminates the need to first grow it in cells.

The payoffs have already begun. Genomics was applied to the development of a meningococcal vaccine. Using the sequence of the bacteria that causes this infection, researchers identified 600 potential antigens. More than half were purified and studied in mouse models. Twenty-nine were found to induce antibodies and are now in clinical testing.¹¹ TB researchers have sequenced the genome, have identified new targets for vaccine development, and are analyzing the function of more than 400 proteins. The first new TB candidate vaccines in more than sixty years moved into human clinical trials in 2004, all of this accomplished in less than six years.¹² Malaria vaccination is also benefiting from genomics. In 2001 both the genomes for the mosquito vector that transmits malaria and the malaria pathogen were sequenced.¹³ Together with the human genome data, this information is allowing researchers to eavesdrop on immunologic and genetic conversations between a vector, pathogen, and host throughout the course of infection.¹⁴

Postgenomic Approaches: Exploiting The Immune Response

Top Making Vaccines: Standard And... Postgenomic Approaches:... New Approaches To Delivery Policy Implications NOTES

 
Just as recombinant DNA technology has allowed researchers to improve vaccines by altering the way the antigen is "seen" by the immune system, so too genomics and bioinformatics are providing insights into how the immune system can be manipulated. Some groups are using genomics tools to understand the molecular signature of the host response during the entire course of infection.¹⁵ By taking advantage of the complete sequence of a pathogen and computer technology, one can now identify antigens from the genome, based on patterns of genetic activity throughout the host-pathogen interaction. Gene- and protein-based microarrays are being used to detect pathogen signals and to characterize host-gene responses to various stages of infection. Microarray technology uses genetic material extracted from the pathogen during specific conditions. Microarrays are a ruler for measuring gene expression at different points in infection and thus provide a powerful tool for rapidly assessing thousands of genes and proteins. It is conceivable that such eavesdropping will allow scientists to identify correlates of protection—the holy grail of vaccine development.

In the coming decades, genomics, proteomics (an understanding of individual proteins), and bioinformatics will be the underpinnings of vaccine research and development. The explosion of knowledge that has occurred in the fields of genomics and immunology is also leading to new concepts and approaches to developing vaccines. Many of the newer approaches depart from the conventional wisdom of simply replicating the immune response seen upon natural infection. The goal is to use rational vaccine design and direct the body’s immunological response. Two approaches that have matured are de novo construction of genetic platforms, and combination use of either conventional or nonconventional vaccines.

De novo construction builds upon the remarkable knowledge gained from functional genomic studies. For example, a genetic platform can be built that then adds a specific protein or genetic sequences to elicit a response to that particular pathogen. These include the following: (1) Entry and replication targets: These approaches use specific genetic sequences from the pathogen that are important for entry and replication to a certain subset of cells found in the immune system. This approach ensures that a particular component of the immune system (for example, B or T cell; TH1 or TH2) is activated and dominates the response. (2) Cellular enzyme targets: Some approaches use cellular enzymes, which are critical to prolonging the life of cells in which the sequence is being expressed. Such approaches assure that that the pathogen is maximally expressed in the engineered vaccine. (3) Imprinting targets: Some sequences, when expressed on the surface of a cell containing the vaccine, will marshal the energies of the "first responding" immune cells toward that cell, thus assuring maximal recognition and "imprinting" the future of these immune cells. (4) Directed uptake targets: Certain sequences can preferentially direct the uptake of a vaccine into a narrow subset of cells. This allows for lower doses and targeted cell entry. (5) Replication targets: These sequences enhance the replication power of the vaccine construct, thus assuring maximal use of each genome.

An example of the progress with a platform approach is from the development of malaria vaccines. A malaria candidate vaccine, RTS, S/AS02A, uses several techniques to boost the immune system’s fight against the malaria parasite. Its designers engineered a hybrid protein that combines a protein fragment from the parasite with a piece of a protein from the hepatitis B virus. The malaria protein is a promising target because it is present on the parasite’s surface when it is first injected into the bloodstream by the bite of an infected mosquito. The hepatitis B protein is included because it is particularly effective at prompting an immune response. The vaccine also contains a powerful new adjuvant, developed by GSK Biologicals, which increases the body’s production of antibodies and T cells.¹⁶

In addition to pure genetic construction, de novo construction may also include a protein core upon which certain well-defined amino acid/peptide sequences are selectively incorporated. For example, the hepatitis B core (that is, the noninfectious shell of the hepatitis virus) naturally reassembles into particles that are easily recognized and aggressively processed by the immune system. The result is that high levels of antibodies are produced when the immune system encounters these core proteins. These particles have been considered as platforms in the development of vaccines against a wide range of other diseases, from the common cold to malaria. More recently, the recognition that antibodies are often not sufficient to quell an infection has led to other modification of such platforms to include, for example, insertion of sequences that are preferentially recognized by the T cell branch of the immune system in addition to the B cell (antibody-forming) branch. In theory, this approach would allow for the qualitative (and possibly quantitative) development of immune responses of both the B and T cell types.

New Approaches To Delivery

Top Making Vaccines: Standard And... Postgenomic Approaches:... New Approaches To Delivery Policy Implications NOTES

 
Historically, most licensed vaccines have been delivered by needle injection (parenteral delivery). Issues of practicality and preference have stimulated consideration for alternative modes of delivery ever since the licensure of the oral Sabin polio vaccine. For example, an influenza virus vaccine, FluMist, represents many years of development and testing of a nasal delivery system capable of delivering live, attenuated influenza virus vaccine in a particulate form. The issues of practicality (for example, needle disposal, storage, and mass immunization strategies, which has accelerated in the wake of biodefense concerns) and preference (children now receive a minimum of seven vaccines, which typically translates into twenty-three injections, before the age of six) have resulted in research focused on delivery systems using the mucosal or skin routes.

The mucosal delivery approach has a well-developed base upon which to build a vaccine delivery system. This includes our knowledge of the importance of local (IgA) antibodies for infections that are mucosally transmitted. Vaccines being developed and tested for delivery via mucosal routes (such as oral, nasal, or vaginal) have tended to be focused on pathogens for which these routes are the natural routes of infection. In general, vaccines that have been developed using this alternative approach have stimulated a breadth rather than a depth of immune responses. Nasal influenza vaccines, for example, stimulate a faster, broader, and longer-lasting range of immune responses when compared with the traditional inactivated influenza vaccine. They have not, however, stimulated high levels of antibody responses when compared with the parentally administered inactivated vaccines. To address this issue, nasal vaccines are being developed as broad-based nasal delivery "adjuvant" systems intended to improve the depth of the immune response.

Research on delivery systems for mucosal vaccines has involved a large number of approaches, including the development of more-efficient spray pumps and improved particulate formulations. In general, the goal is to develop ways to maximize both the depth and the breadth of the immune response (for example, uniform particle size to assure delivery to a specific location within the respiratory tree and construction of a delivery vehicle that can deepen or direct the immune response) while considering the issues of practicality (for example, build upon new delivery systems for already developed/proven vaccines) and consumer preference (for example, type of nasal spray). Several companies are involved in the design of nasal delivery technology that can maximize the delivery and immune response of a variety of vaccine constructs (such as live, inactivated, or DNA-based products) and formulations (liquid or powder). One example is the use of a crystalline polymer that can be formulated in a well-defined size with extended release properties when administered intranasally. Systems such as this tend to promote a broader immune response than that seen when a comparable antigen is delivered parenterally. Efficacy trials, although limited to certain pathogens and some animal models, have suggested that this approach may be feasible.

Cochleates. One of the best-studied delivery systems in this category is the cochleate delivery system. Cochleates are multilayered phospholipid structures capable of encasing a variety of products. Cochleates have been developed and tested with a wide variety of vaccine candidates (protein and DNA) and different routes of administration (oral, intranasal, intraocular, and parenteral). Of particular interest in oral delivery systems, cochleates are also resistant to rapid degradation, which occurs following oral administration of an "unprotected" protein. The entrapped vaccine is released only when the cochleate naturally degrades. Other oral delivery/packaging systems continue to evolve, one of which involves a naturally occurring microbial material, which allows for the evasion of enzymatic activities found in the gastrointestinal tract. Bacterial ghosts, consisting of the outer membranes of bacteria, are also being considered as carriers of vaccines. These ghosts, which are void of the internal bacterial components and are therefore not infectious, still retain their ability to elicit an immune response. Studies have demonstrated that this natural "adjuvant" property tends to increase immune responses to the antigens when delivered in this manner.

Attenuated bacteria. Attenuated bacteria, containing specific genes that express vaccine antigens of interest, are also being considered as a delivery vehicle. One of the best-studied approaches has been the use of attenuated Salmonella typhi. Although this has proved to be a much more difficult system to develop, early clinical studies have demonstrated that the vector itself is safe and easily tolerated. As with other oral delivery vehicles under investigation, this system offers ease of administration, broadening of the immune repertoire to include mucosal responses, and the ability to package multiple antigens, all at a relatively low cost.

Edible vaccines. A novel oral delivery system is under development in the form of edible vaccines. Edible vaccines were initially developed by genetically inserting a defined gene that expressed the antigen of interest into a potato, growing the potato "vaccine," and providing it for raw consumption.¹⁷ In addition to its being a novel and well-understood delivery system (eating food), the engineering of plants could provide unlimited propagation and sizable amounts of vaccine product in a relatively simple "factory."¹⁸ More recently, this technology has focused on alternative food systems including tomatoes, bananas, and alfalfa as the vaccine platforms—specifically, dehydrated products such as alfalfa tablets and tomatoes encapsulated into gelatin capsules.¹⁹ Both of these products/antigens retain their stability and immunogenicity properties under these conditions.

Needle-less approaches. Several versions of needle-less delivery systems are also under development to introduce antigens via the intradermal, subcutaneous, or parenteral route. Originally developed in response to the practical needs of mass immunization campaigns, rather than the reduction of pain, these systems have continued to evolve to address consumers’ interest. One of the older approaches, the jet gun, had been used with various levels of success in large immunization campaigns. This needle-less system, which used high pressure to penetrate the skin, was associated with possible contamination of bloodborne pathogens. Newer and safer systems are being developed with the intention of providing even more ease in clinical application. Conventional vaccines now must be administered by trained technicians and often have low acceptance rates by patients because of pain and general discomfort. The use of transcutaneous immunizations represents a novel approach for the painless delivery of vaccines through the skin. Such an approach can also provide for the timed release of a vaccine antigen, a prolonged shelf life, and the possibility of self-administration. Some examples of self-administered systems include the "patch," "wipe and go," and self-administered microneedles. The patch system has been under development for several years.²⁰ Skin immunization made possible by cholera toxin has been modified over this period to include the use of both the vaccine antigen and adjuvants delivered as a combination patch or the use of an immunostimulating patch in conjunction with a parenterally delivered vaccine antigen.²¹ This modification has taken place as the importance of adjuvants in attaining a robust immune response using this method has become clearer.

Policy Implications

Top Making Vaccines: Standard And... Postgenomic Approaches:... New Approaches To Delivery Policy Implications NOTES

 
Although great scientific opportunities are possible in vaccine research because of these new tools and information, the fragility of the current global vaccine enterprise is troubling and suggests that there will be limits in how this new science can be translated into actual products for use in preventing disease. Several reports have drawn attention to the diminishing number of manufacturers making vaccines and suggest that the industry has gotten out of the business of making vaccines for reasons having no easy or quick solutions.²² Vaccines are undervalued in modern society. We talk about how expensive vaccines are, when in reality they provide enormous value for the cost. The cost-effectiveness of immunizations is very well documented and is higher than that of virtually any other preventive or therapeutic health activity. Despite this, vaccines are viewed as a risky business. Our society holds vaccines to a higher safety standard since they are generally used on healthy people. The risks and unpredictability associated with working with biological organisms are considerably more challenging than those entailed with working with an inert drug. Most importantly, the limited profit margin of vaccines, particularly when compared with drugs for chronic conditions, makes them a business of diminishing returns. Taken together, these issues suggest that vaccine research will have little impact in improving global health if a system for translating and implementing the benefits of this research is not made available.

Since 1985 there has been increasing recognition of the fragility of the U.S. vaccine infrastructure. The Institute of Medicine (IOM) has raised concerns about the impact of high research and development (R&D) costs, risk of litigation, and limited sales on this infrastructure for two decades.²³ In 2003 the National Vaccine Advisory Committee (NVAC) outlined steps to shore up the U.S. vaccine supply.²⁴ In particular, the authors suggested that more could be done to help industry deal with the costly and difficult regulatory landscape required to bring new and effective vaccines to market, which includes validation of not just the product but also the process. The American Enterprise Institute for Public Policy Research (AEI) has also provided recommendations for policies that would address impediments to the vaccine industry. It urges policies to encourage widespread adoption of new technologies, such as cell culture, and the manufacturing tools and processes to evaluate these technologies, noting that without incentives, it is prohibitively expensive to compete in the established vaccine market.²⁵

Clearly, these issues have global implications. Several Sabin Vaccine Colloquium reports have discussed policy issues with getting orphan vaccines to the developing world.²⁶ One report cautions that in addition to the technical, legal, and regulatory issues discussed above, there are other hurdles for global vaccines, including ensuring an adequate and sustained infrastructure for implementation and delivery of new vaccines.²⁷ Considerable progress has been made in obtaining resources from private organizations such as the Bill and Melinda Gates Foundation. The Global Alliance for Vaccines and Immunization (GAVI), the Vaccine Fund, and PATH are helping address these obstacles. Because of such partnerships, a rotavirus vaccine is now licensed in Mexico, with licensure pending in an additional twenty countries.

There also has been heightened awareness by policymakers regarding the importance of vaccines for global health and security. AIDS, malaria, and TB have demonstrated to the world the importance of public health in economic development. In the post-9/11 era, the threat of bioterrorism has reminded Americans of the value of vaccines as public health tools.

As we look to the decade ahead, the payoffs from basic research will continue to invigorate vaccine development. Although the new technologies described in this paper and the improved understanding of the immune system are likely to have profound impacts on the science, it remains to be seen whether innovation can be translated into public health approaches that can be implemented by the global community. Technology will come with a financial cost in terms of both up-front investment in research and the downstream cost of financing once the vaccines are licensed. Intellectual property issues may pose additional challenges, particularly for platform approaches. Despite these challenges, we believe that the global social returns in improved health that will be achieved with these new tools far outweigh the costs.

Editor's Notes

 
Sarah Landry is associate director of policy and program operations, National Vaccine Program Office, U.S. Department of Health and Human Services, in Washington, D.C. Carole Heilman (cheilman{at}niaid.nih.gov) directs the Division of Microbiology and Infectious Diseases at the National Institute of Allergy and Infectious Diseases, National Institutes of Health, in Bethesda, Maryland.

Sarah Landry’s husband is an employee of MedImmune Inc.

NOTES

Top Making Vaccines: Standard And... Postgenomic Approaches:... New Approaches To Delivery Policy Implications NOTES

 

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