Adjuvants are compounds that can enhance the specific immune response against co-inoculated antigens. Pure recombinant or synthetic antigens used in modern day vaccines are generally far less immunogenic than older style live or killed whole organism vaccines, which is the major reason leading to the need for improved and more powerful adjuvants for use in vaccine production.
Adjuvants can be used for purposes to:
- Enhance the immunogenicity of highly purified or recombinant antigens
- Reduce the amount of antigen or the number of immunizations needed for protective immunity
- Improve the efficacy of vaccines in newborns, the elderly or immuno-compromised persons
- Serve as antigen delivery systems for the uptake of antigens by the mucosa13, 14, 15.
Types of Vaccine Adjuvants
Adjuvants have been used in commercially available vaccines and tested in clinical trials.
- Pathogen Components
Using naturally occurring parts of pathogens as adjuvants can help trigger early non-specific, or innate, immune responses to vaccines. These adjuvants target various receptors inside or on the surface of innate immune cells. The innate immune system influences adaptive immune responses, which provide long-lasting protection against the pathogen that the vaccine targets. Examples of this type of adjuvants include Monophosphoryl Lipid A, Poly(I:C), CpG DNA Adjuvants and Emulsions.
- Particulate Adjuvants
Particulate adjuvants can form very small particles that can stimulate the immune system and also may enhance delivery of antigen to immune cells. Examples of particulate adjuvants include Alum, the most commonly used vaccine adjuvant consists of aluminum salts that are not soluble in water; and Virosomes, particles that resemble viruses but are noninfectious.
Cytokines have the ability to coordinate immune responses and some cytokines have been evaluated as vaccine adjuvants. For example, scientists have conducted animal studies to evaluate interleukin 12 (IL-12) as an adjuvant in vaccines against various bacterial and viral infections. Results have showed that IL-12 may increase protective immunity to some respiratory pathogens.
- Combination Adjuvants
Combinations of adjuvants have ability to elicit multiple protective immune responses. Adjuvants that have a modest effect when used alone may induce a more potent immune response when used together. Combination adjuvant research is in the early stages and scientists still work hard to identify how adjuvants can be combined to elicit immune responses that are useful for a given antigen.