In immunology, an adjuvant is a component that potentiates the immune responses to an antigen and/or modulates it towards the desired immune responses. 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."
Adjuvants have been whimsically called the dirty little secret of vaccines in the scientific community. This dates from the early days of commercial vaccine manufacture, when significant variations in the effectiveness of different batches of the same vaccine were observed, correctly assumed to be due to contamination of the reaction vessels. However, it was soon found that more scrupulous attention to cleanliness actually seemed to reduce the effectiveness of the vaccines, and that the contaminants – "dirt" – actually enhanced the immune response. There are many known adjuvants in widespread use, including oils, aluminium salts, and virosomes.
- 1 Overview
- 2 Adaptive immune response
- 3 Toll-like receptors
- 4 Medical complications
- 5 Controversy
- 6 Vaccine adjuvant database
- 7 See also
- 8 External links
- 9 References
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. These are the most common adjuvants in human vaccines.
The precise mechanism of alum action remains unclear but a few 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). Recently in February 2015, a link was claimed between the brain translocation of alum particles to a Trojan horse mechanism that is seen in infectious particles like HIV, that obeys to CCL2, signaling the major inflammatory monocyte chemoattractant. CCL2 expression in glial cells is increased in epilepsy, brain ischemia, and Alzheimer’s disease. A study claims that Docosahexaenoic acid (DHA) may help to prevent aluminum induced biochemical and morphological alteration in the cerebellum.
While aluminium salts are popularly used in human vaccines, the organic compound squalene is also used (e.g. AS03). However, organic adjuvants are more commonly used in animal vaccines.
Another market-approved adjuvant and carrier system are virosomes. During the last two decades, a variety of technologies have been investigated to improve the widely used adjuvants based on aluminium salts. These salts are unfavorable, since they develop their effect by inducing local inflammation, which is also the basis for the extended side-effect pattern of this adjuvant. In contrast, the adjuvant capabilities of virosomes are independent of any inflammatory reaction. Virosomes contain a membrane-bound hemagglutinin and neuraminidase derived from the influenza virus, and serve to amplify fusogenic activity and therefore facilitate the uptake into antigen presenting cells (APC) and induce a natural antigen-processing pathway. The delivery of the antigen by virosomes to the immune system in a way that mimics a natural path may be a reason why virosome-based vaccines stand out due to their excellent safety profile.
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 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 and other cells. TLRs were first discovered in drosophila, and are membrane bound pattern recognition receptors (PRRs) responsible for detecting most (although certainly not all) antigen-mediated infections. In fact, some studies have shown that in the absence of TLR, leukocytes become unresponsive (no inflammatory responses) to some microbial components such as LPS. There are at least thirteen different forms of TLR, each with its own characteristic ligand. Prevailing TLR ligands described to date (all of which elicit adjuvant effects) include many evolutionarily conserved molecules such as LPS, lipoproteins, lipopeptides, flagellin, double-stranded RNA, unmethylated CpG islands and various other forms of DNA and RNA classically released by bacteria and viruses.
The binding of ligand - either in the form of adjuvant used in vaccinations or in the form of invasive moieties during times of natural infection - to the TLR marks the key molecular events that ultimately lead to innate immune responses and the development of antigen-specific acquired immunity. The very fact that TLR activation leads to adaptive immune responses to foreign entities explains why so many adjuvants used today in vaccinations are developed to mimic TLR ligands. So far, single ligands have been used as vaccine adjuvants. However, studies in 2006 and 2011 suggest that the combination of more than one adjuvant with either an interferon or an interleukin could produce a synergistic enhancement of immune response.
Upon activation, TLRs recruit adapter proteins (proteins that mediate other protein-protein interactions) within the cytosol of the immune cell in order to propagation the antigen-induced signal transduction pathway. To date, four adapter proteins have been well-characterized. These proteins are known as MyD88, Trif, Tram and TIRAP (also called Mal). These recruited proteins are then responsible for the subsequent activation of other downstream proteins, including protein kinases (IKKi, IRAK1, IRAK4, and TBK1) that further amplify the signal and ultimately lead to the upregulation or suppression of genes that orchestrate inflammatory responses and other transcriptional events. Some of these events lead to cytokine production, proliferation, and survival, while others lead to greater adaptive immunity. The high sensitivity of TLR for microbial ligands is what makes adjuvants that mimic TLR ligands such a prime candidate for enhancing the overall effects of antigen specific vaccinations on immunological memory.
Finally, the expression of TLRs is vast as they are found on the cell membranes of innate immune cells (DCs, macrophages, natural killer cells), cells of the adaptive immunity (T and B lymphocytes) and non immune cells (epithelial and endothelial cells, fibroblasts).
This further substantiates the importance of administering vaccines with adjuvants in the form of TLR ligands, as they will be capable of eliciting their positive effects across the entire spectrum of innate and adaptive immunity. Nevertheless, there are adjuvants whose immune-stimulatory function completely bypasses the TLR signaling pathway. While all TLR ligands are adjuvants, not all adjuvants are TLR ligands.
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, vaccinations have been linked to sarcomas, at a rate of between 1 and 10 per 10,000 injections. No specific types of vaccines,[dubious ] manufacturers or factors have been associated with sarcomas.
In 1993, a causal relationship between VAS and administration of aluminum adjuvanted 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.
Recently, 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.
Vaccine adjuvant database
Vaxjo is a newly published, web-based vaccine adjuvant database. Vaxjo curates, stores, and analyzes vaccine adjuvants and their usages in vaccine development. Basic information of a vaccine adjuvant stored in Vaxjo includes: adjuvant name, components, structure, appearance, storage, preparation, function, safety, and vaccines that use this adjuvant. All information stored in Vaxjo is well curated and cited from reliable references. Currently over 100 vaccine adjuvants have been annotated in Vaxjo. These adjuvants have been used in over 380 vaccines against over 81 pathogens, cancers, or allergies.
- 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.
- DNA Vaccines: Methods and Protocols, D.B. Lowrie and R.G. Whalen, Humana Press, 2000. ISBN 978-0-89603-580-5.
- The Use of Conventional Immunologic Adjuvants in DNA Vaccine Preparations, by Shin Sasaki and Kenji Okuda. In D.B. Lowrie and R.G. Whalen (editors), DNA Vaccines: Methods and Protocols, Humana Press, 2000. ISBN 978-0-89603-580-5.
- The Scientist "Deciphering Immunology's Dirty Secret." (subscription required)
- Gavin A, Hoebe K, Duong B, Ota T, Martin C, Beutler B, Nemazee D (2006). "Adjuvant-enhanced antibody responses occur without Toll-like receptor signaling". Science 314 (5807): 1936–8. Bibcode:2006Sci...314.1936G. doi:10.1126/science.1135299. PMC 1868398. PMID 17185603.
- Majde JA. 1987. Progress in leukocyte biology. Alan R. Liss, Inc. vol. 6.
- Clements C, Griffiths E (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, and Wallace U. 1926. The antigenic value of toxoid precipitated by potassium alum. J Pathol Bacteriol. 29: 38-45.
- Leslie, M. (2013) Solution to Vaccine Mystery Starts to Crystallize" Science 341: 26-27
- CCL2#Clinical importance
- "Safety, immunogenicity, and kinetics of the immune response to a single dose of virosome-formulated hepatitis A vaccine in Thais," Yong Poovorawana, Apiradee Theamboonlersa, Saowani Chumdermpadetsuka, Reinhard Glückb and Stanley J. Cryz, Jr., Vaccine, Volume 13, Issue 10, 1995, Pages 891-893.
- "Immunogenicity and adverse effects of inactivated virosome versus alum-adsorbed hepatitis A vaccine: a randomized controlled trial," Benedikt R. Holzer*, Christoph Hatz†, Dagmar Schmidt-Sissolak†, Reinhard Glück‡, Beat Althaus‡ and Matthias Egger, Vaccine, Volume 14, Issue 10, July 1996, Pages 982-986.
- "Adjuvant activity of immunopotentiating reconstituted influenza virosomes (IRIVs)," Reinhard Glück, Vaccine, Volume 17, Issues 13-14, January 1999, Pages 1782-1787.
- "Immunologic adjuvants for modern vaccine formulations," F. R. Vogel, Annals of the New York Academy of Sciences, Vol 754, Issue 1, 1995, pp. 153-160
- Ghochikyan A, Mkrtichyan M, Petrushina I, Movsesyan N, Karapetyan A, Cribbs D, Agadjanyan M (2006). "Prototype Alzheimer's disease epitope vaccine induced strong Th2-type anti-Aβ antibody response with Alum to Quil A adjuvant switch". Vaccine 24 (13): 2275–82. doi:10.1016/j.vaccine.2005.11.039. PMC 2081151. PMID 16368167.
- Bousso P, Robey E (2003). "Dynamics of CD8+ T cell priming by dendritic cells in intact lymph nodes". Nat Immunol 4 (6): 579–85. doi:10.1038/ni928. PMID 12730692.
- Mempel T, Henrickson S, Von Andrian U (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 J, Johnston B, Niu X, Kubes P (1995). "Mechanisms underlying acute mast cell-induced leukocyte rolling and adhesion in vivo". J Immunol 154 (2): 804–13. PMID 7814884.
- Kashiwakura J, Yokoi H, Saito H, Okayama Y (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". J Immunol 173 (8): 5247–57. doi:10.4049/jimmunol.173.8.5247. PMID 15470070.
- Schijns V (2000). "Immunological concepts of vaccine adjuvant activity". Curr Opin Immunol 12 (4): 456–63. doi:10.1016/S0952-7915(00)00120-5. PMID 10899018.
- Lemaitre B, Nicolas E, Michaut L, Reichhart J, Hoffmann J (1996). "The dorsoventral regulatory gene cassette spätzle/Toll/cactus controls the potent antifungal response in Drosophila adults". Cell 86 (6): 973–83. doi:10.1016/S0092-8674(00)80172-5. PMID 8808632.
- Beutler B (2004). "Inferences, questions and possibilities in Toll-like receptor signalling". Nature 430 (6996): 257–63. Bibcode:2004Natur.430..257B. doi:10.1038/nature02761. PMID 15241424.
- Poltorak A, He X, Smirnova I, Liu M, Van Huffel C, Du X, Birdwell D, Alejos E, Silva M, Galanos C, Freudenberg M, Ricciardi-Castagnoli P, Layton B, Beutler B (1998). "Defective LPS signaling in C3H/HeJ and C57BL/10ScCr mice: mutations in Tlr4 gene". Science 282 (5396): 2085–8. Bibcode:1998Sci...282.2085P. doi:10.1126/science.282.5396.2085. PMID 9851930.
- Bültmann B, Finger H, Heymer B, Schachenmayr W, Hof H, Haferkamp O (1975). "Adjuvancy of streptococcal nucleic acids". Z Immunitatsforsch Exp Klin Immunol 148 (5): 425–30. PMID 127450.
- Capanna SL, Kong Y (1974). "Further studies on the prevention of tolerance induction by poly A:U". Immunology 27 (4): 647–653. PMC 1445716. PMID 4140149.
- Nauciel C, Fleck J, Martin J, Mock M, Nguyen-Huy H (1974). "Adjuvant activity of bacterial peptidoglycans on the production of delayed hypersensitivity and on antibody response". Eur J Immunol 4 (5): 352–6. doi:10.1002/eji.1830040509. PMID 4604064.
- Schmidtke JR, Johnson A (1971). "Regulation of the immune system by synthetic polynucleotides. I. Characteristics of adjuvant action on antibody synthesis". J. Immunol 106 (5): 1191–1200. PMID 4102398.
- Youmans AS, Youmans G (1969). "Factors Affecting Immunogenic Activity of Mycobacterial Ribosomal and Ribonucleic Acid Preparations". J. Bacteriol 99 (1): 42–50. PMC 249964. PMID 4979447.
- Youmans G, Youmans A (1969). "Allergenicity of Mycobacterial Ribosomal and Ribonucleic Acid Preparations in Mice and Guinea Pigs". J Bacteriol 97 (1): 134–9. PMC 249562. PMID 4236903.
- Takeda, Kiyoshi; Akira, Shizuo (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 C (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.
- Maria Wysocka1, Noor Dawany, Bernice Benoit, Andrew V. Kossenkov, Andrea B. Troxel, Joel M. Gelfand, Michael kelly Sell1, Louise C. Showe, & Alain H. Rook (2011). "Synergistic enhancement of cellular immune responses by the novel Toll receptor 7/8 agonist 3M-007 and interferon-γ: implications for therapy of cutaneous T-cell lymphoma". Leuk Lymphoma 52 (10): 1970–1979. doi:10.3109/10428194.2011.582202. PMID 21942329.
- Ghosh TK, Mickelson DJ, Fink J, Solberg JC, Inglefield JR, Hook D, Gupta SK, Gibson S, Alkan SS. (2006). "Toll-like receptor (TLR) 2-9 agonists-induced cytokines and chemokines: I. Comparison with T cell receptor-induced responses". Cellular Immunology 243 (1): 48–57. doi:10.1016/j.cellimm.2006.12.002. PMID 17250816.
- Berczi I, Bertok L, Bereznai T (1966). "Comparative studies on the toxicity of Escherichia coli lipopolysaccharide endotoxin in various animal species". Can. J. Microbiol 12 (5): 1070–1071. doi:10.1139/m66-143. PMID 5339644.
- Yamamoto, M.; Sato, S.; Hemmi, H.; Hoshino, K.; Kaisho, T.; Sanjo, H.; Takeuchi, O.; Sugiyama, M.; Okabe, M.; et al. (2003). "Role of adapter TRIF in the MyD88-independent Toll-like receptor signaling pathway". Science 301 (5633): 640–643. Bibcode:2003Sci...301..640Y. doi:10.1126/science.1087262. PMID 12855817.
- Yamamoto M, Sato S, Hemmi H, Sanjo H, Uematsu S, Kaisho T, Hoshino K, Takeuchi O, Kobayashi M, Fujita T, Takeda K, Akira S (2002). "Essential role for TIRAP in activation of the signalling cascade shared by TLR2 and TLR4". Nature 420 (6913): 324–9. Bibcode:2002Natur.420..324Y. doi:10.1038/nature01182. PMID 12447441.
- Yamamoto M, Sato S, Hemmi H, Uematsu S, Hoshino K, Kaisho T, Takeuchi O, Takeda K, Akira S (2003). "TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway". Nat Immunol 4 (11): 1144–50. doi:10.1038/ni986. PMID 14556004.
- Delneste Y, Beauvillain C, Jeannin P (2007). "Innate immunity: structure and function of TLRs". Med Sci (Paris) 23 (1): 67–73. doi:10.1051/medsci/200723167. PMID 17212934.
- Baylor N, Egan W, Richman P (2002). "Aluminum salts in vaccines--US perspective". Vaccine 20 (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 Med 9 (1): 83–100. doi:10.1385/NMM:9:1:83. PMID 17114826.
- Satoh, M; et al. (2003). "Induction of lupus autoantibodies by adjuvants". J Autoimmun 21 (1): 1–9. doi:10.1016/S0896-8411(03)00083-0. PMID 12892730.
- Carlson, BC; Jansson AM; Larsson A; Bucht A; Lorentzen JC (2000). "The Endogenous Adjuvant Squalene Can Induce a Chronic T-Cell-Mediated Arthritis in Rats". American Journal of Pathology 156 (2057–2065): 2057–65. doi:10.1016/S0002-9440(10)65077-8. PMC 1850095. PMID 10854227.
- Kirpensteijn, J (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.
- Richards J, Elston T, Ford R, Gaskell R, Hartmann K, Hurley K, Lappin M, Levy J, Rodan I, Scherk M, Schultz R, Sparkes A (2006). "The 2006 American Association of Feline Practitioners Feline Vaccine Advisory Panel report". J Am Vet Med Assoc 229 (9): 1405–41. doi:10.2460/javma.229.9.1405. PMID 17078805.
- Wickelgren I (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.
- Sayers, S; Ulysse, G; Xiang, Z; He, Y (2012). "Vaxjo: a web-based vaccine adjuvant database and its application for analysis of vaccine adjuvants and their uses in vaccine development.". Journal of biomedicine & biotechnology 2012: 831486. doi:10.1155/2012/831486. PMC 3312338. PMID 22505817.