In immunology, an adjuvant is a substance that potentiates and/or modulates the immune responses to an antigen to improve them. The word "adjuvant" comes from the Latin word adiuvare, meaning to help or aid. "An immunologic adjuvant is defined as any substance that acts to accelerate, prolong, or enhance antigen-specific immune responses when used in combination with specific vaccine antigens."
In the early days of vaccine manufacture, significant variations in the efficacy of different batches of the same vaccine were correctly assumed to be caused by contamination of the reaction vessels. However, it was soon found that more scrupulous cleaning actually seemed to reduce the effectiveness of the vaccines, and that some contaminants actually enhanced the immune response.
Adjuvants in immunology are often used to modify or augment the effects of a vaccine by stimulating the immune system to respond to the vaccine more vigorously, and thus providing increased immunity to a particular disease. Adjuvants accomplish this task by mimicking specific sets of evolutionarily conserved molecules, so called PAMPs, which include liposomes, lipopolysaccharide (LPS), molecular cages for antigen, components of bacterial cell walls, and endocytosed nucleic acids such as double-stranded RNA (dsRNA), single-stranded DNA (ssDNA), and unmethylated CpG dinucleotide-containing DNA. Because immune systems have evolved to recognize these specific antigenic moieties, the presence of an adjuvant in conjunction with the vaccine can greatly increase the innate immune response to the antigen by augmenting the activities of dendritic cells (DCs), lymphocytes, and macrophages by mimicking a natural infection.
There are many adjuvants, some of which are inorganic (such as alum), that also carry the potential to augment immunogenicity. Two common salts include aluminium phosphate and aluminium hydroxide. Aluminium salts are the most commonly-used adjuvants in human vaccines. Their adjuvant activity was described in 1926.
The precise mechanism of alum action remains unclear but some insights have been gained. For instance, alum can trigger dendritic cells (DC) and other immune cells to secrete interleukin-1β (IL-1β), an immune signal that promotes antibody production. Alum adheres to the cell’s plasma membrane and rearranges certain lipids there. Spurred into action, the DC picks up the antigen and speeds to a lymph node, where it sticks tightly to a helper T cell and presumably induces an immune response. A second mechanism depends on alum killing immune cells at the injection site although researchers aren’t sure exactly how alum kills these cells. It has been speculated that the dying cells release DNA which serves as an immune alarm. Some studies found that DNA from dying cells causes them to adhere more tightly to helper T cells which ultimately leads to an increased release of antibodies by B cells. No matter what the mechanism is, alum is not a perfect adjuvant because it does not work with all antigens (e.g. malaria and tuberculosis).
Freund's complete adjuvant is a solution of inactivated Mycobacterium tuberculosis in mineral oil developed in 1930. It is not safe enough for human use. A version without the bacteria, that is only oil in water, is known as Freund's incomplete adjuvant. It helps vaccines release antigens for a longer time. Despite the side effects, its potential benefit has led to a few clinical trials.
Squalene is a naturally-occurring organic compound that is used in human and animal vaccines. Squalene is an oil, made up of carbon and hydrogen atoms, produced by plants and is present in many foods. Squalene is also produced by the human liver and present in human sebum. MF59 is an oil-in-water emulsion of squalene adjuvant used in some human vaccines. Over 22 million doses of a vaccine with squalene have been administered with no safety concerns.
Monophosphoryl lipid A (MPL), a detoxified version of Salmonella minnesota lipopolysaccharide, interacts with TLR4 to enhance immune response. It is a part of the Shingrix vaccine approved in 2017.
Adaptive immune response
In order to understand the links between the innate immune response and the adaptive immune response to help substantiate an adjuvant function in enhancing adaptive immune responses to the specific antigen of a vaccine, the following points should be considered:
- Innate immune response cells such as dendritic cells (DCs) engulf pathogens through a process called phagocytosis.
- DCs then migrate to the lymph nodes where T cells (adaptive immune cells) wait for signals to trigger their activation.
- In the lymph nodes, DCs mince the engulfed pathogen and then express the pathogen clippings as antigen on their cell surface by coupling them to a special receptor known as a major histocompatibility complex (MHC).
- T cells can then recognize these clippings and undergo a cellular transformation resulting in their own activation.
- γδ T cells possess characteristics of both the innate and adaptive immune responses.
- Macrophages can also activate T cells in a similar approach (but do not do so naturally).
This process carried out by both DCs and macrophages is termed antigen presentation and represents a physical link between the innate and adaptive immune responses.
Upon activation, mast cells release heparin and histamine to effectively increase trafficking to and seal off the site of infection to allow immune cells of both systems to clear the area of pathogens. In addition, mast cells also release chemokines which result in the positive chemotaxis of other immune cells of both the innate and adaptive immune responses to the infected area.
Due to the variety of mechanisms and links between the innate and adaptive immune response, an adjuvant-enhanced innate immune response results in an enhanced adaptive immune response. Specifically, adjuvants may exert their immune-enhancing effects according to five immune-functional activities.
- First, adjuvants may help in the translocation of antigens to the lymph nodes where they can be recognized by T cells. This will ultimately lead to greater T cell activity resulting in a heightened clearance of pathogen throughout the organism.
- Second, adjuvants may provide physical protection to antigens which grants the antigen a prolonged delivery. This means that the organism will be exposed to the antigen for a longer duration, making the immune system more robust as it makes use of the additional time by upregulating the production of B and T cells needed for greater immunological memory in the adaptive immune response.
- Third, adjuvants may help to increase the capacity to cause local reactions at the injection site (during vaccination), inducing greater release of danger signals by chemokine releasing cells such as helper T cells and mast cells.
- Fourth, they may induce the release of inflammatory cytokines which helps to not only recruit B and T cells at sites of infection but also to increase transcriptional events leading to a net increase of immune cells as a whole.
- Finally, adjuvants are believed to increase the innate immune response to antigen by interacting with pattern recognition receptors (PRRs) on or within accessory cells.
The ability of the immune system to recognize molecules that are broadly shared by pathogens is, in part, due to the presence of immune receptors called Toll-Like Receptors (TLRs) that are expressed on the membranes of leukocytes including dendritic cells, macrophages, natural killer cells, cells of the adaptive immunity (T and B lymphocytes) and non-immune cells (epithelial and endothelial cells, and fibroblasts).
The binding of ligands – either in the form of adjuvant used in vaccinations or in the form of invasive moieties during times of natural infection – TLR marks the key molecular events that ultimately lead to innate immune responses and the development of antigen-specific acquired immunity.
As of 2016, several TLR ligands were in clinical development or being tested in animal models as potential adjuvants.
Aluminium salts used in many human vaccines are generally regarded as safe.
Aluminum adjuvants have caused motor neuron death in mice when injected directly onto the spine at the scruff of the neck, and oil-water suspensions have been reported to increase the risk of autoimmune disease in mice. Squalene has caused rheumatoid arthritis in rats already prone to arthritis.
In cats, vaccine-associated sarcoma (VAS) occurs at a rate of between 1 and 10 per 10,000 injections. In 1993, a causal relationship between VAS and administration of aluminum adjuvated rabies and FeLV vaccines was established through epidemiologic methods, and in 1996 the Vaccine-Associated Feline Sarcoma Task Force was formed to address the problem. However, evidence conflicts on whether types of vaccines, manufacturers or factors have been associated with sarcomas.
As of 2006[update], the premise that TLR signaling acts as the key node in antigen-mediated inflammatory responses has been in question as researchers have observed antigen-mediated inflammatory responses in leukocytes in the absence of TLR signaling. One researcher found that in the absence of MyD88 and Trif (essential adapter proteins in TLR signaling), they were still able to induce inflammatory responses, increase T cell activation and generate greater B cell abundancy using conventional adjuvants (alum, Freund’s complete adjuvant, Freund’s incomplete adjuvant, and monophosphoryl-lipid A/trehalose dicorynomycolate (Ribi's adjuvant)).
These observations suggest that although TLR activation can lead to increases in antibody responses, TLR activation is not required to induce enhanced innate and adaptive responses to antigens.
Investigating the mechanisms which underlie TLR signaling has been significant in understanding why adjuvants used during vaccinations are so important in augmenting adaptive immune responses to specific antigens. However, with the knowledge that TLR activation is not required for the immune-enhancing effects caused by common adjuvants, we can conclude that there are, in all likelihood, other receptors besides TLRs that have not yet been characterized, opening the door to future research.
- Recommendations for Use and Alternatives to Freund's Complete Adjuvant. University of Iowa
- Vaxjo: Comprehensive vaccine adjuvant database.
- "Guideline on Adjuvants in Vaccines for Human Use" (PDF). The European Medicines Agency. Retrieved 8 May 2013.
- Sasaki S, Okuda K (2000). "The Use of Conventional Immunologic Adjuvants in DNA Vaccine Preparations". In Lowrie DB, Whalen RG (eds.). DNA Vaccines: Methods and Protocols. Humana Press. ISBN 978-0-89603-580-5.
- Travis K (January 2007). "Deciphering Immunology's Dirty Secret". The Scientist.
- Gavin AL, Hoebe K, Duong B, Ota T, Martin C, Beutler B, Nemazee D (December 2006). "Adjuvant-enhanced antibody responses in the absence of toll-like receptor signaling". Science. 314 (5807): 1936–8. Bibcode:2006Sci...314.1936G. doi:10.1126/science.1135299. PMC 1868398. PMID 17185603.
- Majde JA, ed. (1987). Immunopharmacology of infectious diseases: vaccine adjuvants and modulators of non-specific resistance. Progress in leukocyte biology. 6. Alan R. Liss, Inc. ISBN 978-0-8451-4105-2.
- "Immunization schedule in India 2016". Superbabyonline. Retrieved 5 May 2016.
- Clements CJ, Griffiths E (May 2002). "The global impact of vaccines containing aluminium adjuvants". Vaccine. 20 Suppl 3 ( ): S24–33. doi:10.1016/s0264-410x(02)00168-8. PMID 12184361.
- Glenny A, Pope C, Waddington H, Wallace U (1926). "The antigenic value of toxoid precipitated by potassium alum". J Pathol Bacteriol. 29: 38–45.
- Apostólico Jde, S; Lunardelli, VA; Coirada, FC; Boscardin, SB; Rosa, DS (2016). "Adjuvants: Classification, Modus Operandi, and Licensing". Journal of Immunology Research. 2016: 1459394. doi:10.1155/2016/1459394. PMC 4870346. PMID 27274998.
- Leslie M (July 2013). "Solution to vaccine mystery starts to crystallize". Science. 341 (6141): 26–7. doi:10.1126/science.341.6141.26. PMID 23828925.
- Podda, Audino; Rappuoli, Rino; Donnelly, John; O'Hagan, Derek; Palla, Emanuela; Henriksson, Thomas; Hora, Maninder; Bugarini, Roberto; Fragapane, Elena (2006-09-01). "Vaccines with the MF59 Adjuvant Do Not Stimulate Antibody Responses against Squalene". Clin. Vaccine Immunol. 13 (9): 1010–1013. doi:10.1128/CVI.00191-06. ISSN 1556-679X. PMC 1563566. PMID 16960112.
- "WHO | Squalene-based adjuvants in vaccines". WHO. Retrieved 2019-01-10.
- Alving, Carl R.; Beck, Zoltan; Matyas, Gary R.; Rao, Mangala (June 2016). "Liposomal adjuvants for human vaccines". Expert Opinion on Drug Delivery. 13 (6): 807–816. doi:10.1517/17425247.2016.1151871. ISSN 1744-7593. PMID 26866300.
- "SHINGRIX package insert" (PDF). Food and Drug Administration. Retrieved 7 April 2019.
- "SHINGRIX package insert" (PDF). Food and Drug Administration. Retrieved 7 April 2019.
- Bousso P, Robey E (June 2003). "Dynamics of CD8+ T cell priming by dendritic cells in intact lymph nodes". Nature Immunology. 4 (6): 579–85. doi:10.1038/ni928. PMID 12730692.
- Mempel TR, Henrickson SE, Von Andrian UH (January 2004). "T-cell priming by dendritic cells in lymph nodes occurs in three distinct phases". Nature. 427 (6970): 154–9. Bibcode:2004Natur.427..154M. doi:10.1038/nature02238. PMID 14712275.
- Gaboury JP, Johnston B, Niu XF, Kubes P (January 1995). "Mechanisms underlying acute mast cell-induced leukocyte rolling and adhesion in vivo". Journal of Immunology. 154 (2): 804–13. PMID 7814884.
- Kashiwakura J, Yokoi H, Saito H, Okayama Y (October 2004). "T cell proliferation by direct cross-talk between OX40 ligand on human mast cells and OX40 on human T cells: comparison of gene expression profiles between human tonsillar and lung-cultured mast cells". Journal of Immunology. 173 (8): 5247–57. doi:10.4049/jimmunol.173.8.5247. PMID 15470070.
- Schijns VE (August 2000). "Immunological concepts of vaccine adjuvant activity". Current Opinion in Immunology. 12 (4): 456–63. doi:10.1016/S0952-7915(00)00120-5. PMID 10899018.
- Delneste Y, Beauvillain C, Jeannin P (January 2007). "[Innate immunity: structure and function of TLRs]". Médecine/Sciences. 23 (1): 67–73. doi:10.1051/medsci/200723167. PMID 17212934.
- Takeda K, Akira S (January 2005). "Toll-like receptors in innate immunity". International Immunology. 17 (1): 1–14. doi:10.1093/intimm/dxh186. PMID 15585605.
- Medzhitov R, Preston-Hurlburt P, Janeway CA (July 1997). "A human homologue of the Drosophila Toll protein signals activation of adaptive immunity". Nature. 388 (6640): 394–7. doi:10.1038/41131. PMID 9237759.
- Toussi DN, Massari P Immune Adjuvant Effect of Molecularly-defined Toll-Like Receptor Ligands. Vaccines (Basel). 2014 Apr 25;2(2):323-53. PMID 26344622 PMC 4494261/
- Baylor NW, Egan W, Richman P (May 2002). "Aluminum salts in vaccines--US perspective". Vaccine. 20 Suppl 3 (Suppl 3): S18–23. doi:10.1016/S0264-410X(02)00166-4. PMID 12184360.
- Petrik MS, Wong MC, Tabata RC, Garry RF, Shaw CA (2007). "Aluminum adjuvant linked to Gulf War illness induces motor neuron death in mice". Neuromolecular Medicine. 9 (1): 83–100. doi:10.1385/NMM:9:1:83. PMID 17114826.
- Satoh M, Kuroda Y, Yoshida H, Behney KM, Mizutani A, Akaogi J, Nacionales DC, Lorenson TD, Rosenbauer RJ, Reeves WH (August 2003). "Induction of lupus autoantibodies by adjuvants". Journal of Autoimmunity. 21 (1): 1–9. doi:10.1016/S0896-8411(03)00083-0. PMID 12892730.
- Carlson BC, Jansson AM, Larsson A, Bucht A, Lorentzen JC (June 2000). "The endogenous adjuvant squalene can induce a chronic T-cell-mediated arthritis in rats". The American Journal of Pathology. 156 (6): 2057–65. doi:10.1016/S0002-9440(10)65077-8. PMC 1850095. PMID 10854227. Archived from the original on 2003-11-21.
- Richards JR, Elston TH, Ford RB, Gaskell RM, Hartmann K, Hurley KF, Lappin MR, Levy JK, Rodan I, Scherk M, Schultz RD, Sparkes AH (November 2006). "The 2006 American Association of Feline Practitioners Feline Vaccine Advisory Panel report". Journal of the American Veterinary Medical Association. 229 (9): 1405–41. doi:10.2460/javma.229.9.1405. PMID 17078805.
- Kirpensteijn J (October 2006). "Feline injection site-associated sarcoma: Is it a reason to critically evaluate our vaccination policies?". Veterinary Microbiology. 117 (1): 59–65. doi:10.1016/j.vetmic.2006.04.010. PMID 16769184.
- Wickelgren I (December 2006). "Immunology. Mouse studies question importance of toll-like receptors to vaccines". Science. 314 (5807): 1859–60. doi:10.1126/science.314.5807.1859a. PMID 17185572.