Inimmunology, anadjuvant is a substance that increases or modulates the immune response to avaccine.[1] The word "adjuvant" comes from the Latin wordadiuvare, 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 vaccineantigens."[2]
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 toreduce the effectiveness of the vaccines, and some contaminants actually enhanced the immune response.
There are many known adjuvants in widespread use, includingpotassium alum, various plant and animal derived oils andvirosomes.[3]
Combinations:Freund's complete adjuvant, Freund's incomplete adjuvant,[7] AS01 (combining MPL and Quillaia saponins),[11]Matrix-M (combining Quillaia saponins and two types of fat)
Small molecules: TLR7/8 agonists (imidazoquinolines,[12] imidazopyrimidines[13])
Physical methods: radiofrequency heating (only tested in mice),[14] microneedles (only tested in mice)[15]
There are many adjuvants, some of which areinorganic, that carry the potential to augmentimmunogenicity.[16][17]Alum was the first aluminiumsalt used for this purpose, but has been almost completely replaced byaluminium hydroxide andaluminium phosphate for commercial vaccines.[18] Aluminium salts are the most commonly used adjuvants in human vaccines. Their adjuvant activity was described in 1926.[19]
The precise mechanism of aluminium salts remains unclear but some insights have been gained. It was formerly thought that they function as delivery systems by generating depots that trap antigens at the injection site, providing a slow release that continues to stimulate the immune system.[20] However, studies have shown that surgical removal of these depots had no impact on the magnitude ofIgG1 response.[21]
Alum can triggerdendritic cells and other immune cells to secreteInterleukin 1 beta (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 dendritic cells pick up the antigen and speed to lymph nodes, where they stick tightly to ahelper T cell and presumably induce 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 byB 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).[22] However, recent research indicates that alum formulated in ananoparticle form rather than microparticles can broaden the utility of alum adjuvants and promote stronger adjuvant effects.[23]
Freund's complete adjuvant is a solution of inactivatedMycobacterium 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.[19]
Squalene is a naturally occurring organic compound 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 as a precursor tocholesterol and is present in humansebum.[24]MF59 is an oil-in-water emulsion of squalene adjuvant used in some human vaccines. As of 2021, over 22 million doses of one vaccine with squalene, FLUAD, have been administered with no severe adverse effects reported.[25]AS03 is another squalene-containing adjuvant.[26] In addition, squalene-based O/W emulsions have also been shown to stably incorporate small molecule TLR7/8 adjuvants (e.g. PVP-037) and lead to enhanced adjuvanticity via synergism.[13]
The combination of QS-21, cholesterol and MPL forms the adjuvant AS01[11] which is used in theShingrix vaccine approved in 2017,[29] as well as in the approvedmalaria vaccineMosquirix.[11]
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:
Dendritic cells then migrate to thelymph nodes whereT cells (adaptive immune cells) wait for signals to trigger their activation.[30]
In the lymph nodes, dendritic cells mince the engulfed pathogen and then express the pathogen clippings as antigen on theircell surface by coupling them to a special receptor known as amajor histocompatibility complex.
T cells can then recognize these clippings and undergo acellular transformation resulting in their own activation.[31]
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 dendritic cells and macrophages is termedantigen presentation and represents a physical link between the innate and adaptive immune responses.
Upon activation,mast cells releaseheparin andhistamine to effectively increase trafficking to and seal off the site ofinfection to allow immune cells of both systems to clear the area of pathogens. In addition, mast cells also releasechemokines which result in the positivechemotaxis of other immune cells of both the innate and adaptive immune responses to the infected area.[32][33]
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.[34]
First, adjuvants may help in the translocation ofantigens to thelymph nodes where they can be recognized byT cells. This will ultimately lead to greater T cell activity resulting in a heightenedclearance ofpathogen throughout theorganism.
Second, adjuvants may provide physical protection to antigens which grants the antigen a prolonged delivery. This means the organism will be exposed to the antigen for a longer duration, making the immune system morerobust as it makes use of the additional time by upregulating the production of B and T cells needed for greater immunologicalmemory in the adaptive immune response.
Third, adjuvants may help to increase the capacity to cause local reactions at theinjection site (during vaccination), inducing greater release of danger signals bychemokine releasing cells such ashelper 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 ofinfection but also to increasetranscriptional events leading to a net increase ofimmune cells as a whole.
Finally, adjuvants are believed to increase the innate immune response to antigen by interacting withpattern recognition receptors (PRRs) on or within accessory cells.[11]
The binding ofligands – either in the form of adjuvant used invaccinations or in the form of invasive moieties during times of natural infection – to TLRs mark the keymolecular events that ultimately lead to innate immune responses and the development of antigen-specific acquired immunity.[36][37]
As of 2016, several TLR ligands were in clinical development or being tested in animal models as potential adjuvants.[38]
Aluminium salts used in many human vaccines are regarded as safe byFood and Drug Administration.[39] Although there are studies suggesting the role of aluminium, especially injected highlybioavailable antigen-aluminium complexes inAlzheimer's disease development,[40] most researchers do not support a causal connection with aluminium.[41] Adjuvants may make vaccines tooreactogenic, which often leads tofever. This is often an expected outcome upon vaccination and is usually controlled by oralparacetamol if necessary.
An increased number ofnarcolepsy (a chronic neurological disorder) cases in children and adolescents was observed inScandinavian and other European countries after vaccinations to address theH1N1 "swine flu"pandemic in 2009. Narcolepsy has previously been associated withHLA-subtype DQB1*602, which has led to the prediction that it is an autoimmune process. After a series of epidemiological investigations, researchers found that the higherincidence correlated with the use of AS03-adjuvanted influenza vaccine (Pandemrix). Those vaccinated with Pandemrix have almost a twelve-times higher risk of developing the disease.[42][43] The adjuvant of the vaccine containedvitamin E that was no more than a day's normal dietary intake. Vitamin E increaseshypocretin-specific fragments that bind to DQB1*602 in cell culture experiments, leading to the hypothesis that autoimmunity may arise in genetically susceptible individuals,[44] but there is no clinical data to support this hypothesis. The third AS03 ingredient ispolysorbate 80.[26] Polysorbate80 is also found in both theOxford–AstraZeneca andJanssenCOVID-19 vaccines.[45][46]
Aluminium adjuvants have caused motor neuron death in mice[47] when injected directly onto the spine at the scruff of the neck, and oil–water suspensions have been reported to increase the risk ofautoimmune disease in mice.[48] Squalene has causedrheumatoid arthritis in rats already prone to arthritis.[49]
In cats,vaccine-associated sarcoma (VAS) occurs at a rate of 1–10 per 10,000 injections. In 1993, acausal relationship between VAS and administration of aluminium adjuvated rabies and FeLV vaccines was established throughepidemiologic methods, and in 1996 the Vaccine-Associated Feline Sarcoma Task Force was formed to address the problem.[50] However, evidence conflicts on whether types of vaccines, manufacturers or factors have been associated with sarcomas.[51]
As of 2006[update], the premise that TLR signaling acts as the key node in antigen-mediatedinflammatory responses has been in question asresearchers have observed antigen-mediated inflammatory responses inleukocytes in the absence of TLR signaling.[4][52] Oneresearcher found that in the absence ofMyD88 andTrif (essentialadapter proteins in TLR signaling), they were still able to induce inflammatory responses, increase T cell activation and generate greaterB cell abundancy using conventional adjuvants (alum, Freund's complete adjuvant, Freund's incomplete adjuvant, and monophosphoryl-lipid A/trehalose dicorynomycolate (Ribi's adjuvant)).[4]
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 themechanisms which underlie TLR signaling has been significant in understanding why adjuvants used during vaccinations are so important in augmenting adaptive immune responses to specificantigens. 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.
Discussing the safety of squalene as an adjuvant in 2006, the World Health Organisation stated "follow-up to detect any vaccine-related adverse events will need to be performed."[55] No such followup has been published by the WHO.
Subsequently, the American National Center for Biotechnology Information published an article discussing the comparative safety of vaccine adjuvants which stated that "the biggest remaining challenge in the adjuvant field is to decipher the potential relationship between adjuvants and rare vaccine adverse reactions, such as narcolepsy, macrophagic myofasciitis or Alzheimer's disease."[56]
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^Masoudi S, Ploen D, Kunz K, Hildt E (May 2014). "The adjuvant component α-tocopherol triggers via modulation of Nrf2 the expression and turnover of hypocretin in vitro and its implication to the development of narcolepsy".Vaccine.32 (25):2980–2988.doi:10.1016/j.vaccine.2014.03.085.PMID24721530.
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