The thought of birth defects caused by rubella, rows of iron lungs housing children crippled by polio, or the horrific sound of a baby struggling with whooping cough can still evoke dread among people who have seen firsthand the damage inflicted by these and other vaccine-preventable diseases. Fortunately, those scourges are virtually unknown to modern generations that have had access to vaccines all their lives.

For more than 200 years vaccines have proved to be one of the most successful, lifesaving and economical methods of preventing infectious disease, second only to the sanitization of water. Vaccines have spared millions of people from early death or crippling illnesses and made the global eradication of smallpox in 1979 possible. Health experts now pledge to eliminate polio, measles and perhaps one day even malaria—although, as we shall see, a malaria vaccine will require novel approaches to immunization to be successful.

Broadly speaking, the premise of vaccination is that exposure to a small sample of a disease-causing microorganism teaches the human immune system to recognize it and prepare to fight it off the next time it is encountered. But classical vaccines do not always work in all people, nor can they guard against all illnesses. Some populations, such as the elderly, may have immune systems too weak to respond sufficiently to traditional vaccines. And certain disease-causing organisms have been able to evade the kinds of immune defenses evoked by vaccines—malaria, tuberculosis and AIDS are examples of illnesses that vaccines still cannot reliably ward off. The principles of vaccination could also be extended to a host of other afflictions, such as cancer, allergy or Alzheimer’s disease, but these applications would require provoking the immune system to respond to something it would normally recognize only weakly or not at all.

In all these situations, immune system stimulators that boost the body’s ability to recognize and respond to a vaccine could make the difference. Such immunity-stimulating substances are often called adjuvants, from the Latin adjuvare, meaning “to help.” Some have been known for more than a century and used to enhance vaccines and cancer therapies. Like the mechanisms underlying vaccines themselves, however, the exact details of how adjuvants interact with immune cells were poorly understood until very recently. Tremendous advances in immunology, especially over the past decade, have provided new insights into how adjuvants produce their effects and opened avenues for designing vaccines tailored precisely to the population to be protected and the pathogen to protect against. With these new tools, vaccines that were once impossible to create are now in development, and existing vaccines are becoming more efficient and effective.

Mimicking Infection to Avert It
Many natural infections have at least one benefit in that a bout of illness confers lifelong immunity against the causative pathogen. An ideal vaccine would also offer such lasting protection, preferably with a single dose, and perhaps even protect against related threats, such as all members of the ever evolving family of human flu viruses. To achieve those goals, a vaccine must engage multiple cellular actors in the immune system, the same ones stimulated during a real infection.

When a wild pathogen enters the body for the first time, it immediately encounters cells of the innate immune system that are constantly patrolling for invaders. These sentries include macrophages and dendritic cells, which engulf and destroy pathogens as well as infected body cells. The guard cells then break down the material they have ingested and display samples of the intruder’s components—known as antigens—so that members of the adaptive immune system, T and B cells, can become familiar with the pathogen’s appearance. At the same time, the antigen-presenting cells release signaling chemicals called cytokines that induce inflammation and alert T and B cells to the emergency.

Once a population of T and B cells adapted to the specific pathogen matures, the B cells release antibody molecules, and killer T cells seek out and destroy cells that have already been colonized by the invader. It takes a few days for interactions with antigen-presenting cells to create these tailored T and B cells, but a subset of them can remain in the body as “memory” cells—sometimes for decades—ready to squelch any attempted reinfection by the same organism. Vaccines replicate this process by introducing a whole pathogen or fragments of it that will be recognized as a foreign invader. Not all vaccines succeed in generating a full immune response, but some pathogens can be stopped by antibodies alone, so killer T cells are not needed for protection.

The nature of the pathogen and how it causes illness are among vaccine designers’ considerations when choosing what type of antigen to use. The material administered in a standard vaccine may be live but weakened (“attenuated”) bacteria or viruses; killed or inactivated versions of the whole organism; or purified proteins derived from the original pathogen. Each choice has advantages and drawbacks.

Live attenuated vaccines reproduce very slowly in the body, but because they do reproduce and thus continue to present antigen to the immune system, they can trigger a robust and long-lasting immune response. Because of their inherently infectious nature, however, attenuated vaccines cannot be used in individuals with weakened immune systems, which may become overwhelmed. The danger of certain live viruses mutating and reverting to a virulent form also makes attenuated vaccines too dangerous to use for deadly pathogens such as HIV.

More common are vaccines consisting of whole virus particles that have been “killed” using a method such as heating. The particles cannot replicate, but the virus proteins are still relatively intact and easily recognized by immune cells, although periodic booster shots are required to reinforce the immune response.

A third form is the subunit vaccine, which presents antigen to the immune system without introducing microorganisms, whole or otherwise. This antigen can be isolated from the pathogen itself, or it may be manufactured through recombinant genetic engineering. Because subunit vaccines contain only part of the pathogen, however, they do not always trigger the danger signals required to stimulate the optimum immune response.

In recent years scientists have come to recognize the critical role played by antigen-presenting cells, particularly the dendritic cells, in assessing the level of threat posed by a pathogen and determining the necessary response. When dendritic cells become loaded with antigen at the site of infection or at the site of a vaccine injection, they mature and migrate to neighboring lymph nodes, where they begin the signaling and interactions that elicit protective B and T cell responses. Without the danger indicators unique to whole microorganisms, dendritic cells fail to mature and migrate properly, and subunit vaccines often require an adjuvant to provide the red flag that stimulates dendritic cells to action.

Most vaccines used in the U.S. already do contain one of the oldest adjuvants, alum, which is a shorthand term for members of a chemical family of aluminum salts. Although alum has been used in human vaccines since the 1930s and has proved its usefulness in many current vaccines, it is insufficient as a helper in vaccines against diseases that require more than antibody protection to be effective.

Various pathogens that can cause life-threatening infections such as HIV, hepatitis C virus, Mycobacterium tuberculosis and Plasmodium parasites (the source of malaria) can evade antibodies, and an effective vaccine against these pathogens would need to stimulate robust T cell responses. Indeed, efforts to fight these very challenging organisms spurred a revived interest in vaccine adjuvants while driving breakthroughs in understanding the immune system, which in turn has led to better adjuvants.

Adjuvant Revival
Even as French chemist Louis Pasteur was confronting a rabid bulldog to extract saliva for the first rabies vaccine in the 1880s, a New York bone surgeon was unwittingly inventing a technique for boosting overall immune response that can be considered the first use of adjuvants. William B. Coley of the New York Cancer Hospital was intrigued by reports of tumors shrinking or disappearing entirely in cancer patients who became infected by a particular strain of Streptococcus bacteria, S. pyogenes. On the hunch that the patients’ immune reactions to the bacteria were enhancing their ability to fight off the tumors, he began a series of experiments in 1881, administering the live bacteria and later infusions of killed bacteria to cancer patients. These treatments, which came to be known as “Coley’s Toxins,” achieved some impressive remissions, although exactly how they worked would long remain a mystery.

Early 20th-century researchers nonetheless extended the idea that bacteria and other substances could improve natural human immune responses. French veterinarian Gaston Ramon and English immunologist Alexander T. Glenny experimented with substances as varied as tapioca and aluminum hydroxide to boost the effectiveness of diphtheria and tetanus vaccines given to animals. During the 1930s other scientists found that suspending antigens in emulsions of oil and water could enhance vaccine potency, and bacterial extracts such as lipopolysaccharide (LPS), a component of some bacterial cell walls, continued to be explored. Many of these additives had the desired effects, but too often adverse reactions, such as excessive inflammation, made the approach unpredictable.

Interest in adjuvant research faded as a result, until the 1980s when the arrival of a new viral challenge called for every imaginable tactic to be deployed. HIV proved to be far beyond the reach of classical vaccination methods. The virus selectively attacked T cells, effectively disabling the adaptive immune system, and it morphed so continually that antibodies could never keep up with it. Vaccine researchers working with recombinant HIV proteins had to find ways to boost the immune system’s recognition of the antigens, leading them to try combinations of known adjuvants as well as refining those to fashion new ones.

Perhaps the biggest breakthrough for adjuvant research came in 1997, however, with the discovery that specialized pattern-recognition receptors on and within dendritic cells are devoted to recognizing fundamental parts of microorganisms, such as the protein flagellin that is found in the tails of many different bacteria. These pathogen-detecting receptors provide the danger signal that spurs dendritic cells to action as well as information about what type of threat is present. Among these newly discovered cellular keys, a group known as the Toll-like receptors (TLRs) seemed most important for driving the dendritic cells’ behavior [see “Immunity’s Early-Warning System,” by Luke A. J. O’Neill; Scientific American, January 2005].

To date, 10 functional Toll-like receptors have been identified, and each recognizes a different basic motif of viruses or bacteria. TLR-4 recognizes LPS, for instance, whereas TLR-7 registers the single-stranded RNA typical of some viruses. After these discoveries, it became clear that microbial extracts acted as immune-stimulating adjuvants because they provided a danger signal to dendritic cells via TLRs. The revelation of these mechanisms meant that a vaccine designer could use one or a combination of adjuvants to target specific TLRs. 

Next-Generation Vaccines
The research initiated in the 1980s and 1990s sought to identify and evaluate natural adjuvants, as well as synthetic or modified ones, that might be used to modulate the immune response to specific pathogens or in certain populations. These ingredients include the traditional adjuvants, such as alum, and oil-in-water emulsions, such as MF59 and AS03, both approved in Europe for use in certain influenza vaccines. More broadly, adjuvants can also be any chemical compounds that improve the quantity and quality of immune  responses by acting on dendritic or other immune cell types.

Experimentation and advances in immunology have allowed scientists to eliminate elements that caused unwanted toxicity in earlier adjuvants and to mix and match adjuvant substances so that their collective action is optimized to elicit the desired immune response. One novel adjuvant, monophosphoryl lipid A (MPL), for example, was produced by detoxifying and further purifying one of the lipids from the LPS molecule, yielding an adjuvant with TLR-4-stimulating properties but without the unwanted toxicity. It has been incorporated into several vaccines that are already on the market or in late-stage clinical testing with encouraging results.

Among these is an experimental vaccine against malaria that one of us (Garçon) helped to develop as head of GlaxoSmithKline Biologicals’s vaccine adjuvant center. Caused by protozoan parasites of the genus Plasmodium, malaria is a serious disease that kills more than a million people a year, mostly children under the age of five. These parasites are able to hide within cells, evading immune mechanisms. They also change form several times over the course of their life cycles, making it difficult to find an antigen that will serve as an effective vaccine target in all stages of infection. Eliciting both antibody- and T cell–mediated immunity to protect against these parasites by preventing them from entering cells and by destroying cells that are already infected is important. These goals, in turn, required adjuvants that go beyond alum.

Taking all these factors into account, our group developed a vaccine based on an antigen we call RTS,S, which joins a recombinant partial protein present on the parasite’s surface before it enters the human host’s blood cells and during early cell infection, and links it to a hepatitis B surface antigen to further stimulate immune recognition. This compound molecule is then administered with an adjuvant mixture consisting of an oil-in-water emulsion, MPL and QS21, a plant derivative used since the 1930s in veterinary medicine. After optimizing the formulation, we and our collaborators at the Walter Reed Army Institute of Research tried the vaccine in small human tests involving volunteers willing to stick their arms into a box of malaria-carrying mosquitoes and be bitten at least five times. Six out of seven vaccine recipients were protected from infection, whereas recipients of a version containing alum were not.

Real-life conditions with continued exposure to the parasite are the ultimate test, and larger trials conducted in the Gambia among adults demonstrated 71 percent of recipients to be protected from infection during nine weeks of follow-up. Later trials in children in malaria-endemic areas of Mozambique showed that three doses protected 30 percent of the kids from infection, and the group’s incidence of severe disease over six months was reduced by nearly 60 percent. An improved version of this vaccine containing liposomes is nearing the end of late-stage (phase 3) clinical testing in infants. As  the first vaccine ever to show significant rates of protection against malaria infection and severe illness, it fosters great hope for contributing to controlling the disease.

The success of this vaccine illustrates the potential for rational vaccine design combining antigen and adjuvants to produce the desired immune response—both in making new vaccines and in improving old ones. Many existing vaccines that are generally successful may not be safe or effective in certain parts of the population, including the people who need them most. Seasonal influenza is an example: the elderly and infants are most vulnerable to lethal flu infections because infant immune systems are not fully developed, and immune responses also decline with age. Only about half of people older than 65 who receive a standard flu vaccine will develop sufficient antibodies to prevent infection.

In contrast, an experimental seasonal influenza vaccine containing the oil-in-water emulsion AS03 yielded protective antibody levels in 90.5 percent of recipients 65 or older. Because adjuvants boost immune cell recognition of antigens, they can also be used to make effective vaccines with less antigen. This consideration becomes especially important in the case of a pandemic requiring a potentially huge population to be vaccinated quickly. Another experimental AS03 vaccine, this one against the avian H5N1 flu strain, elicited protective antibody responses using just a third the amount of antigen in a typical seasonal flu vaccine.

These examples illustrate the kinds of new vaccines that are close to widespread human use because the revival and development of adjuvants in the 1980s and 1990s are bearing fruit now. Scientists’ realization during that era that the pattern-recognition abilities of dendritic cells are a critical link between the innate and adaptive immune systems has also permitted the design of new types of adjuvants. This work is in earlier stages but has the potential to create an arsenal of adjuvant components from which vaccine designers can pick and choose to build vaccines with unprecedented precision.

New Generation of Adjuvants
Along with advances in immunology and molecular biology, materials science has provided many new methods for achieving adjuvant effects. Liposomal carriers are already employed to encapsulate drugs and other substances, delivering their contents to a target tissue in the body while protecting them from degradation. When used to carry vaccine antigens, they offer similar protection, creating a depot that allows for extended exposure of the antigen to immune cells. Variations on this principle are seen in polymer antigen cages made, for example, both from natural polysaccharides, such as those found in bacterial cell walls, and from synthetic polyesters. These materials have the added benefit of incorporating natural or added immune-stimulating chemicals that can trigger desirable immune cell signaling.

As the language of immune cells has been deciphered, scientists have come to realize that the early signaling by dendritic cells to sound an alarm also directs the nature of the response depending on the type of threat at hand. Thus, a vaccine designer can theoretically tailor adjuvant combinations to summon an immune response that emphasizes antibody production or one that preferentially stimulates certain subsets of T cells. Indeed, signaling molecules themselves are among the substances being tried experimentally as adjuvants. A class of cytokines known as interleukins (IL) has long been used to enhance immunity in cancer and AIDS treatments, but interleukins are naturally produced by dendritic cells, and the cells’ mixture of signals can determine which immune cells respond—for instance, IL-4, IL-5 and IL-6 enhance killer T cell production, whereas IL-2 and IL-12 will favor antibody responses.   

Similar effects can be achieved through combinations of TLR activators. Various TLRs recognize microbial products, and one of them, TLR-4, also recognizes molecules released by the body under stress, known as heat-shock proteins. Some combinations of TLR activators with non-TLR adjuvants, such as oil emulsions, show particularly strong synergy in activating dendritic cells and may prove useful in some of the most challenging vaccine applications.

Among these is cancer, an unusual vaccine target because instead of being a foreign invader, cancer cells arise from the victim’s own body. As a result, the immune system does mount some response to tumor cells, but it is rarely adequate to fight off the cancer. Attempts to create therapeutic vaccines to stimulate an immune reaction to tumor cells have met with disappointing results; however, the right combination of adjuvants may make a difference. A variety of experimental cancer vaccines employing different adjuvant combinations have produced promising results.

One of these, now in late-stage clinical trials, combines an antigen (Mage-A3) that is highly specific to certain tumor cells with AS15, an adjuvant mixture of stable liposomes, MPL and QS-21, as well as a bacterial component called CpG. In trials among patients with non-small-cell lung cancer, 96 percent of those receiving the vaccine showed a strong Mage-A3 antibody response and indications that desirable interleukin signaling had been triggered. Almost a third experienced stabilization or regression of their tumors. Another current trial is deploying CpG along with chemotherapy and radiation therapy against several types of cancer. CpG is a distinctive bacterial DNA motif that is recognized by TLR-9 and spurs dendritic cell activation of strong T cell responses. Thus, its use as an adjuvant echoes William Coley’s long-ago bacterial treatments for cancer patients. Fittingly, the company created to develop CpG as an adjuvant was named Coley Pharmaceuticals.

The various adjuvant systems we have described are pushing the limits of disease prevention through vaccination and bringing great hope in areas of unmet medical need. Early clinical tests of CpG added to ragweed antigen have shown promise as a vaccine against hay fever. Adjuvants’ ability to induce immune defenses that recognize related strains of flu offers the possibility of creating more broadly protective flu vaccines. And for the first time, people whose immune systems are compromised by disease or chemotherapy may have access to vaccines that are able to evoke immune protection. Adjuvants may not be the answer to all the shortcomings of the modern vaccine arsenal, but they will surely provide part of the solution.

Modulating the immune system is delicate work, of course, and an ongoing critical assessment of vaccine safety and transparent dissemination of accurate information about next-generation vaccines and adjuvants is essential. A detailed understanding of the mode of action of the adjuvants incorporated into new vaccines is guiding their development and will direct their use and monitoring. Encouragingly, the most advanced adjuvanted preventive vaccines have not shown any signs of problems that warrant concern, but developers must remain vigilant.

As this field continues to progress, vaccines will better serve specific subpopulations and target diseases in a rational manner that elicits the optimum immune protection, while balancing safety and efficacy. This is vaccine development of the future. And that future is nearly here.

Note: This article was originally printed with the title, "Boosting Vaccine Power."