immunology Immunology is a branch of medicineImmunology for Medical Students, Roderick Nairn, Matthew Helbert, Mosby, 2007 and biology that covers the medical study of immune systems in humans, animals, plants and sapient species. In such we can see ther ...

, an adjuvant is a substance that increases or modulates the immune response to a vaccine. 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

antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

s." 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 some contaminants actually enhanced the immune response. There are many known adjuvants in widespread use, including aluminium salts, oils and virosomes.

Overview

Adjuvants in

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

pathogen-associated molecular pattern Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. A vast arra ...

s, which include liposomes,

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer me ...

, molecular cages for

s, components of bacterial

cell walls A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

, and endocytosed nucleic acids such as RNA,

double-stranded RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohyd ...

, single-stranded DNA, and unmethylated CpG dinucleotide-containing DNA. Because immune systems have evolved to recognize these specific

antigenic In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

moieties, the presence of an adjuvant in conjunction with the vaccine can greatly increase the

innate immune response The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the ...

to the antigen by augmenting the activities of dendritic cells, lymphocytes, and macrophages by mimicking a natural infection.

Types

*Inorganic compounds: potassium alum,

aluminium hydroxide Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic and ...

aluminium phosphate Aluminium phosphate is a chemical compound. In nature it occurs as the mineral berlinite. Many synthetic forms of aluminium phosphate are known. They have framework structures similar to zeolites and some are used as catalysts, ion-exchangers o ...

, calcium phosphate hydroxide *Oils: paraffin oil, propolis (only in

preclinical studies In drug development, preclinical development, also termed preclinical studies or nonclinical studies, is a stage of research that begins before clinical trials (testing in humans) and during which important feasibility, iterative testing and drug ...

), Adjuvant 65 (based on peanut oil. Adjuvant 65 was tested in influenza vaccines in the 1970s, but was never released commercially.) *Bacterial products: killed bacteria of the species '' Bordetella pertussis'', '' Mycobacterium bovis'',

toxoid A toxoid is an inactivated toxin (usually an exotoxin) whose toxicity has been suppressed either by chemical (formalin) or heat treatment, while other properties, typically immunogenicity, are maintained. Toxins are secreted by bacteria, whereas ...

s *Plant saponins from '' Quillaja'' (see Quil A), soybean, ''

Polygala senega ''Polygala senega'' is a species of flowering plant in the milkwort family, Polygalaceae. It is native to North America, where it is distributed in southern Canada and the central and eastern United States.IL-1, IL-2, IL-12 *Combinations: Freund's complete adjuvant, Freund's incomplete adjuvant *Other organic substances: squalene

Inorganic adjuvants

Aluminium salts

There are many adjuvants, some of which are

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...

, that carry the potential to augment immunogenicity.

Alum An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the ...

was the first aluminium salt used for this purpose, but has been almost completely replaced by

and

for commercial vaccines. Aluminium salts are the most commonly-used adjuvants in human vaccines. Their adjuvant activity was described in 1926. 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. However, studies have shown that surgical removal of these depots had no impact on the magnitude of IgG1 response. Alum can trigger dendritic cells and other immune cells to secrete Interleukin 1 beta (IL1β), 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 a helper 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 by

B cells B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

. 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). However, recent research indicates that alum formulated in a nanoparticle form rather than microparticles can broaden the utility of alum adjuvants and promote stronger adjuvant effects.

Organic adjuvants

Freund's complete adjuvant is a solution of inactivated ''

Mycobacterium tuberculosis ''Mycobacterium tuberculosis'' (M. tb) is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, ''M. tuberculosis'' has an unusual, waxy coating on its ...

'' 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 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 to cholesterol and is present in human sebum.

MF59 MF59 is an immunologic adjuvant that uses squalene. It is Novartis' proprietary adjuvant that is added to influenza vaccines to help stimulate the human body's immune response through production of CD4 memory cells. MF59 is the first oil-in-water ...

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.

AS03 AS03 (for "Adjuvant System 03") is the trade name for a squalene-based immunologic adjuvant used in various vaccine products by GlaxoSmithKline (GSK). It is used, for example, in GSK's A/H1N1 pandemic flu vaccine Pandemrix. It is also in Arepanrix ...

is another squalene-containing adjuvant.Pandemrix – Summary of product characteristics
, European Medicines Agency websi
European Medicines Agency website
/ref> The plant extract

QS-21 QS-21 is a purified plant extract used as a vaccine adjuvant. It is derived from the soap bark tree (''Quillaja saponaria''), which is native to the countries of Chile, Peru, and Bolivia. The crude drug (''Quillajae cortex'') is imported from Pe ...

is a liposome made up of plant saponins from '' Quillaja saponaria'', the soap bark tree. It is a part of the Shingrix vaccine approved in 2017. Monophosphoryl lipid A (MPL), a detoxified version of '' Salmonella minnesota''

, interacts with the receptor TLR4 to enhance immune response. It is also a part of the Shingrix vaccine.

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 engulf

pathogens In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...

through a process called phagocytosis. *Dendritic cells then migrate to the

lymph nodes A lymph node, or lymph gland, is a kidney-shaped organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that includ ...

where T cells (adaptive immune cells) wait for signals to trigger their activation. *In the lymph nodes, dendritic cells mince the engulfed pathogen and then express the pathogen clippings as antigen on their

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

surface by coupling them to a special receptor known as a major histocompatibility complex. *T cells can then recognize these clippings and undergo a

cellular transformation In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane(s). For transformation to ta ...

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 dendritic cells and macrophages is termed

antigen presentation Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment, now ...

and represents a physical link between the innate and adaptive immune responses. Upon activation,

mast cells A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a par ...

release

heparin Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. Since heparins depend on the activity of antithrombin, they are considered anticoagulants. Specifically it is also used in the treatm ...

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 Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In additi ...

which result in the positive

chemotaxis Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemi ...

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 In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

to the

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 the organism will be exposed to the antigen for a longer duration, making the immune system more

robust Robustness is the property of being strong and healthy in constitution. When it is transposed into a system, it refers to the ability of tolerating perturbations that might affect the system’s functional body. In the same line ''robustness'' ca ...

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 Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In additio ...

releasing cells such as helper T cells and

. *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 Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules calle ...

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.

Toll-like receptors

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 Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

and

endothelial cells The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vesse ...

, and

fibroblasts A fibroblast is a type of biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework ( stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells o ...

). The binding of ligandseither in the form of adjuvant used in

vaccinations Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulatin ...

or in the form of invasive moieties during times of natural infectionTLR 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.

Medical complications

Humans

Aluminium salts used in many human vaccines are regarded as safe by Food and Drug Administration, Although there are studies suggesting the role of aluminium, especially injected highly

bioavailable In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. By definition, when a medication is administered intravenously, its bioavailability is 100%. Ho ...

antigen-aluminium complexes when used as adjuvant, in

Alzheimer's disease Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As ...

development, the majority of researchers do not support a causal connection with aluminium. Adjuvants may make vaccines too reactogenic, which often leads to fever. This is often an expected outcome upon vaccination and is usually controlled in infants by over-the-counter medication if necessary. An increased number of

narcolepsy Narcolepsy is a long-term neurological disorder that involves a decreased ability to regulate sleep–wake cycles. Symptoms often include periods of excessive daytime sleepiness and brief involuntary sleep episodes. About 70% of those affect ...

(a chronic sleep disorder) cases in children and adolescents was observed in Scandinavian and other European countries after vaccinations to address the H1N1 “swine flu” pandemic in 2009. Narcolepsy has previously been associated with HLA-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 higher incidence correlated with the use of AS03-adjuvanted influenza vaccine (

Pandemrix Pandemrix is an influenza vaccine for influenza pandemics, such as the 2009 flu pandemic. The vaccine was developed by GlaxoSmithKline (GSK) and patented in September 2006. The vaccine was one of the H1N1 vaccines approved for use by the Europ ...

). Those vaccinated with Pandemrix have almost a twelve-times higher risk of developing the disease. The adjuvant of the vaccine contained

vitamin E Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. Vitamin E deficiency, which is rare and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vit ...

that was no more than a day's normal dietary intake. Vitamin E increases

hypocretin Orexin (), also known as hypocretin, is a neuropeptide that regulates arousal, wakefulness, and appetite. The most common form of narcolepsy, type 1, in which the individual experiences brief losses of muscle tone ("drop attacks" or cataplexy), ...

-specific fragments that bind to DQB1*602 in cell culture experiments, leading to the hypothesis that autoimmunity may arise in genetically susceptible individuals, but there is no clinical data to support this hypothesis. The third AS03 ingredient is polysorbate 80. Polysorbate80 is also found in both the Oxford–AstraZeneca and Janssen

COVID-19 vaccine A COVID19 vaccine is a vaccine intended to provide acquired immunity against severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2), the virus that causes coronavirus disease 2019 ( COVID19). Prior to the COVID19 pandemic, an es ...

Animals

Aluminium 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 An autoimmune disease is a condition arising from an abnormal immune response to a functioning body part. At least 80 types of autoimmune diseases have been identified, with some evidence suggesting that there may be more than 100 types. Nearly a ...

in mice. Squalene has caused rheumatoid arthritis in rats already prone to arthritis. In cats,

vaccine-associated sarcoma A vaccine-associated sarcoma (VAS) or feline injection-site sarcoma (FISS) is a type of malignant tumor found in cats (and often, dogs and ferrets) which has been linked to certain vaccines. VAS has become a concern for veterinarians and cat ow ...

(VAS) occurs at a rate of 1–10 per ten thousand injections. In 1993, a

causal Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the ca ...

relationship between VAS and administration of aluminium adjuvated rabies and FeLV vaccines was established through

epidemiologic Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evidenc ...

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.

Controversy

TLR signalling

, 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 Research is " creative and systematic work undertaken to increase the stock of knowledge". It involves the collection, organization and analysis of evidence to increase understanding of a topic, characterized by a particular attentiveness ...

found that in the absence of MyD88 and Trif (essential

adapter An adapter or adaptor is a device that converts attributes of one electrical device or system to those of an otherwise incompatible device or system. Some modify power or signal attributes, while others merely adapt the physical form of one co ...

proteins in TLR signaling), they were still able to induce inflammatory responses, increase T cell activation and generate greater

B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

abundancy using conventional adjuvants (

alum An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the ...

, 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

. 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.

Safety

Reports after the first Gulf War linked anthrax vaccine adjuvants to Gulf War syndrome in American and British troops. The US Defence Department strongly denied the claims. 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." 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."

References

External links

Adjuvant therapyAnimal researchVaxjo database
{{Authority control Adjuvants Immunology