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.
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.
Inorganic adjuvants 
Aluminium salts 
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.
Organic adjuvants 
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.
Experimental adjuvants 
Adjuvants and the 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.
Adjuvants and toll-like receptors 
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 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.
Medical complications 
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.
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