Immunologic adjuvant
Encyclopedia
In immunology
, an adjuvant is an agent that may stimulate the immune system
and increase the response to a vaccine
, without having any specific antigen
ic effect in itself. 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 virosome
s, although precisely how they work is still not entirely understood.
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
. Furthermore, because adjuvants are attenuated beyond any function of virulence
, they pose little or no independent threat to a host organism.
), 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.
is also used. However, organic adjuvants are more commonly used in animal vaccines.
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.
and phosphate adjuvants are being tested on humans.
The compound QS21
is under investigation as a possible immunological adjuvant
as is Novartis' (formerly Chiron) MF59
.
that takes place between the innate immune response and the adaptive immune response with the onset of infection
is considered it becomes difficult to separate the two systems.
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:
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, a recent study has observed that adjuvants may exert their immune-enhancing effects according to five immune-functional activities.
s that are broadly shared by pathogen
s is, in part, due to the presence of special Immune receptor
s called TLRs that are expressed on leukocyte membrane
s. 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.
It is believed that 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 certainly adjuvants whose immune-stimulatory function completely bypasses the putative requisite for TLR signaling. In short, all TLR ligands are adjuvants but not all adjuvants are TLR ligands.
in mice. Squalene has caused rheumatoid arthritis
in rats already prone to arthritis.
In cats, vaccinations have been linked to sarcoma
s, at a rate of between 1 and 10 per 10,000 injections. No specific types of vaccines, manufacturers or factors have been associated with sarcomas.
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
.
Immunology
Immunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. It deals with the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders ; the...
, an adjuvant is an agent that may stimulate the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
and increase the response to a vaccine
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
, without having any specific antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
ic effect in itself. 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 virosome
Virosome
A virosome is a drug or vaccine delivery mechanism consisting of unilamellar phospholipid bilayer vesicle incorporating virus derived proteins to allow the virosomes to fuse with target cells...
s, although precisely how they work is still not entirely understood.
Overview
Adjuvants in immunologyImmunology
Immunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. It deals with the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders ; the...
are often used to modify or augment the effects of a vaccine
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
by stimulating the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
to respond to the vaccine more vigorously, and thus providing increased immunity to a particular disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
. Adjuvants accomplish this task by mimicking specific sets of evolutionarily conserved molecules, so called PAMPs, which include liposomes, lipopolysaccharide
Lipopolysaccharide
Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
(LPS), molecular cages for antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
, components of bacterial cell walls, and endocytosed nucleic acids such as double-stranded RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
(dsRNA), single-stranded DNA (ssDNA), and unmethylated CpG
CpG
CpG can be:*CpG site - methylated sequences of DNA significant in gene regulation*CpG Oligodeoxynucleotide - unmethylated sequences of DNA that have immunostimulatory properties*CpG island - regions of DNA that contain several CpG sites...
dinucleotide-containing DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. 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
Natural
Natural is an adjective that refers to Nature.Natural may refer too:In science and mathematics:* Natural transformation, category theory in mathematics* Natural foods...
infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
. Furthermore, because adjuvants are attenuated beyond any function of virulence
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
, they pose little or no independent threat to a host organism.
Aluminium salts
There are many adjuvants, some of which are inorganic (such as alumAlum
Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with the formula KAl2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB2.12H2O.-Chemical properties:Alums are...
), that also carry the potential to augment immunogenicity
Immunogenicity
Immunogenicity is the ability of a particular substance, such as an antigen or epitope, to provoke an immune response in the body of a human or animal.- Immunogenicity :The ability to induce humoral and/or cell-mediated immune responses....
. Two common salts include aluminium phosphate
Aluminium phosphate
Aluminium phosphate is a chemical compound. It is used in cake mixes and in some baking powders as a leavening agent to help baked goods rise. Medicinally it is used as adsorbent for toxoid.-Uses:...
and aluminium hydroxide
Aluminium hydroxide
Aluminium hydroxide, Al3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO, and aluminium oxide, Al2O3,...
. These are the most common adjuvants in human vaccines.
Organic adjuvants
While Aluminium salts are popularly used in human vaccines, the organic compound SqualeneSqualene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
is also used. However, organic adjuvants are more commonly used in animal vaccines.
Virosomes
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 hemagglutininHemagglutinin
Influenza hemagglutinin or haemagglutinin is a type of hemagglutinin found on the surface of the influenza viruses. It is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected...
and neuraminidase
Neuraminidase
Neuraminidase enzymes are glycoside hydrolase enzymes that cleave the glycosidic linkages of neuraminic acids. Neuraminidase enzymes are a large family, found in a range of organisms. The most commonly known neuraminidase is the viral neuraminidase, a drug target for the prevention of the spread...
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
An increasing number of vaccines with squaleneSqualene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
and phosphate adjuvants are being tested on humans.
The compound QS21
QS21
QS-21 is a purified plant extract that enhances the ability of the immune system to respond to vaccine antigens. It is derived from the Soap bark tree , which is native to the country of Chile....
is under investigation as a possible immunological adjuvant
as is Novartis' (formerly Chiron) MF59
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 influenza vaccine adjuvant to be commercialized in...
.
Adjuvants and the adaptive immune response
One misconception concerning adjuvant function is that an adjuvant-enhanced innate immune response should affect only the transient reaction of the innate immune response and not the more long-lived effects of the adaptive immune response. Although it may appear fitting to separate the two systems, it is however important to realize the interconnected nature of the two systems. When the amount of communicationCommunication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
that takes place between the innate immune response and the adaptive immune response with the onset of infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
is considered it becomes difficult to separate the two systems.
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 CellDendritic cellDendritic cells are immune cells forming part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system. That is, dendritic cells function as antigen-presenting cells...
s (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 cellCell (biology)The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
surface by coupling them to a special receptor known as a major histocompatibility complexMajor histocompatibility complexMajor histocompatibility complex is a cell surface molecule encoded by a large gene family in all vertebrates. MHC molecules mediate interactions of leukocytes, also called white blood cells , which are immune cells, with other leukocytes or body cells...
(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
Antigen presentation
Antigen presentation is a process in the body's immune system by which macrophages, dendritic cells and other cell types capture antigens and then enable their recognition by T-cells....
and represents a physical link between the innate and adaptive immune responses.
Upon activation, mast cells release heparin
Heparin
Heparin , also known as unfractionated heparin, a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule...
and histamine
Histamine
Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...
to effectively increase trafficking to and seal off the site of infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
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
Chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
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, a recent study has observed that adjuvants may exert their immune-enhancing effects according to five immune-functional activities.
- First, it was found that adjuvants all 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 clearanceClearance (medicine)In medicine, the clearance is a measurement of the renal excretion ability. Although clearance may also involve other organs than the kidney, it is almost synonymous with renal clearance or renal plasma clearance. Each substance has a specific clearance that depends on its filtration characteristics...
of pathogenPathogenA pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
throughout the organismOrganismIn biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
. - Second, adjuvants 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 memoryMemoryIn psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
in the adaptive immune response. - Third, adjuvants help to increase the capacity to cause local reactions at the injectionInjection (medicine)An injection is an infusion method of putting fluid into the body, usually with a hollow needle and a syringe which is pierced through the skin to a sufficient depth for the material to be forced into the body...
site (during vaccination), inducing greater release of danger signals by chemokineChemokineChemokines are a family of small cytokines, or proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines...
releasing cells such as helper T cells and mast cells. - Fourth, they induce the release of inflammatory cytokines which helps to not only recruit B and T cells at sites of infectionInfectionAn infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
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), specifically Toll-like receptors (TLRs), on accessory cells.
Adjuvants and toll-like receptors
The ability of immune system to recognize moleculeMolecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s that are broadly shared by pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s is, in part, due to the presence of special Immune receptor
Immune receptor
An immune receptor is a receptor, usually on a cell membrane, which binds to a substance and causes a response in the immune system.-Types:...
s called TLRs that are expressed on leukocyte membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s. TLRs were first discovered in drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, and are membrane bound pattern recognition receptors (PRRs
PRRS
PRRS can refer to:*Porcine Reproductive and Respiratory Syndrome Virus*Radical Socialist Republican Party...
) responsible for detecting most (although certainly not all) antigen-mediated infections.
In fact, some studies have shown that in the absence of TLR
TLR
The three letter abbreviation TLR can mean* Toll-like receptors, proteins constituting an important part of the immune system* Tonic labyrinthine reflex, a primitive reflex found in newborn humans* Twin-lens reflex camera, a photo camera with two lenses...
, leukocytes become unresponsive (no inflammatory responses) to some microbial components such as LPS
LPS
LPS can stand for more than one thing:* Lipopolysaccharide * Liters per second* Low Pressure Sodium vapor lamps* Leases per second, a speed measure for DHCP servers* Leica Photogrammetry Suite, official name was changed to "LPS" in 2008...
. There are at least thirteen different forms of TLR, each with its own characteristic ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
. Prevailing TLR ligands described to date (all of which elicit adjuvant effects) include many evolutionarily conserved molecules such as LPS, lipoproteins, lipopeptides, flagellin
Flagellin
Flagellin is a protein that arranges itself in a hollow cylinder to form the filament in bacterial flagellum. It has a mass of about 30,000 to 60,000 daltons...
, double-stranded RNA, unmethylated CpG islands and various other forms of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
classically released by bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and viruses.
The binding of ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
- 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.
It is believed that upon activation, TLRs recruit adapter proteins (proteins that mediate other protein-protein interactions) within the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
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
Myd88
Myeloid differentiation primary response gene is a protein that, in humans, is encoded by the MYD88 gene.-Function:In mice, MyD88 is a universal adapter protein as it is used by all TLRs to activate the transcription factor NF-κB. Mal is necessary to recruit Myd88 to TLR 2 and TLR 4, and MyD88...
, Trif
Trif
TIR-domain-containing adapter-inducing interferon-β is an adapter in responding to activation of toll-like receptors . It mediates the rather delayed cascade of two TLR-associated signaling cascades, where the other one is dependent upon a MyD88 adapter.Toll-like receptors recognize specific...
, Tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...
and Tirap
Tirap
Tirap can refer to:* Tirap district, district in Arunachal Pradesh* Tirap River, river that flows in Tirap district* Tirap, Kenya* TIRAP, toll-interleukin 1 receptor domain containing adaptor protein...
(also called Mal). These recruited proteins are then responsible for the subsequent activation of other downstream
Downstream (bioprocess)
The downstream part of a bioprocess refers to the part where the cell mass from the upstream are processed to meet purity and quality requirements. Downstream processing is usually divided into three main sections, a capture section, a purification section and a polishing section....
proteins, including protein kinases (IKKi, IRAK1
IRAK1
Interleukin-1 receptor-associated kinase 1 is an enzyme that in humans is encoded by the IRAK1 gene.-Interactions:IRAK1 has been shown to interact with Myd88, IKBKG, IKK2, TRAF6, CHUK and Ubiquitin C.-Further reading:...
, IRAK4, and TBK1) that further amplify the signal and ultimately lead to the upregulation or suppression of genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
that orchestrate inflammatory
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
responses and other transcriptional events. Some of these events lead to cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
production, proliferation
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...
, and survival, while others lead to greater adaptive immunity. The high sensitivity
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called 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 certainly adjuvants whose immune-stimulatory function completely bypasses the putative requisite for TLR signaling. In short, all TLR ligands are adjuvants but not all adjuvants are TLR ligands.
Animals
Aluminum adjuvants have caused motor neuron death in mice and oil-water suspensions have been reported to increase the risk of autoimmune diseaseAutoimmune disease
Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
in mice. Squalene has caused rheumatoid arthritis
Rheumatoid arthritis
Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...
in rats already prone to arthritis.
In cats, vaccinations have been linked to sarcoma
Sarcoma
A sarcoma is a cancer that arises from transformed cells in one of a number of tissues that develop from embryonic mesoderm. Thus, sarcomas include tumors of bone, cartilage, fat, muscle, vascular, and hematopoietic tissues...
s, at a rate of between 1 and 10 per 10,000 injections. No specific types of vaccines, manufacturers or factors have been associated with sarcomas.
Controversy
Recently, the premise that TLR signaling acts as the key node in antigen-mediated inflammatoryInflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
responses has been in question as researchers have observed antigen-mediated inflammatory responses in leukocytes in the absence of TLR signaling. One researcher
Researcher
A researcher is somebody who performs research, the search for knowledge or in general any systematic investigation to establish facts. Researchers can work in academic, industrial, government, or private institutions.-Examples of research institutions:...
found that in the absence of MyD88
Myd88
Myeloid differentiation primary response gene is a protein that, in humans, is encoded by the MYD88 gene.-Function:In mice, MyD88 is a universal adapter protein as it is used by all TLRs to activate the transcription factor NF-κB. Mal is necessary to recruit Myd88 to TLR 2 and TLR 4, and MyD88...
and Trif
Trif
TIR-domain-containing adapter-inducing interferon-β is an adapter in responding to activation of toll-like receptors . It mediates the rather delayed cascade of two TLR-associated signaling cascades, where the other one is dependent upon a MyD88 adapter.Toll-like receptors recognize specific...
(essential adapter
Adapter
An adapter or adaptor is a person that adapts or a device that converts attributes of one device or system to those of an otherwise incompatible device or system.Some adapters may only affect physical attributes:...
proteins in TLR signaling), they were still able to induce inflammatory responses, increase T cell activation and generate greater B cell
B cell
B cells are lymphocytes that play a large role in the humoral immune response . The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells and eventually develop into memory B cells after activation by antigen interaction...
abundancy using conventional adjuvants (alum
Alum
Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with the formula KAl2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB2.12H2O.-Chemical properties:Alums are...
, Freund’s complete adjuvant
Freund's adjuvant
Freund's adjuvant is a solution of antigen emulsified in mineral oil and used as an immunopotentiator . The complete form, Freund's Complete Adjuvant, is composed of inactivated and dried mycobacteria , whereas the incomplete form lacks the mycobacterial components...
, Freund’s incomplete adjuvant
Freund's adjuvant
Freund's adjuvant is a solution of antigen emulsified in mineral oil and used as an immunopotentiator . The complete form, Freund's Complete Adjuvant, is composed of inactivated and dried mycobacteria , whereas the incomplete form lacks the mycobacterial components...
, 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
Mechanism (biology)
In biology --and in science in general-- a mechanism is a complex object or, more generally, a process that produces a regular phenomenon. For example, natural selection is one of the mechanisms of biological evolution, other being genetic drift, biased mutation, and gene flow; competition,...
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
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
.
External links
- Recommendations for Use and Alternatives to Freund's Complete Adjuvant. University of IowaUniversity of IowaThe University of Iowa is a public state-supported research university located in Iowa City, Iowa, United States. It is the oldest public university in the state. The university is organized into eleven colleges granting undergraduate, graduate, and professional degrees...