Today we discuss Aluminum-containing adjuvants. Let me give you examples of
Aluminum-containing adjuvants:
E.g.; aluminum phosphate, aluminum hydroxide and alum-precipitated vaccines
Mechanism of action
- Formation of a repository or depot of antigen in tissues.
- Direct effect on APCs (eosinophil or macrophage activation.)-increased immune response to DNA vaccines when mixed with aluminum phosphate adjuvants.
- Direct activation of dendritic cells.
Aluminum hydroxide has been found to be a more potent adjuvant than aluminum phosphate.
Aluminum hydroxide showed higher adsorption of tetanus toxoid and diphtheria toxoid than aluminum phosphate at room temperature at a pH of 6.0
Adsorption mechanisms: The main mechanisms by which aluminum-containing adjuvants adsorb antigens are:
electrostatic attraction – based on isoelectric point
hydrophobic forces – tested by ethylene glycol
ligand exchange – occurs with phosphorylated antigens
DISADVANTAGES:
Although widely approved and effective as adjuvants, alum and other aluminum salts do have some issues since they require relatively large quantities of antigen which requires repeated dosing, are nonbioadhesive, cannot elicit cell-mediated Immunity, and require constant refrigeration.
In addition, alums are not effective by the mucosal route as adjuvants and there are concerns relating to the production of IgF when alum is used. These are becoming more relevant as vaccines for use in tropical underdeveloped nations become of increasing concern movement toward heat-stable, single-dose vaccines and this may be achieved by using microparticle formulated vaccines.
• Granulomas are common when alum is administered via the subcutaneous or intradermal rather than intramuscular route.
• Other side effects of alum are increased IgE production, allergenicity and potential neurotoxicity.
• High aluminium levels in the body predominately affect the brain and bone tissues causing fatal neurological syndrome and dialysis-associated dementia.
• Aluminium intoxication is also potentially linked to amyotrophic lateral sclerosis and Alzheimer’s disease.
• Alternatively, the salts of calcium, iron and zirconium have also been used to adsorb antigens. In particular, calcium phosphate has been used for diphtheria tetanus-pertussis vaccines (DPT).
MF 59:a oil-in-water emulsion :
MF59 is a low-oil-content o/w emulsion.
The oil used for MF59 is squalene, naturally occurring biodegradable and biocompatible oil.
MF59 contains 2 nonionic surfactants, Tween 80 and Span 85.
Citrate buffer is also used in MF59 to stabilize pH.
Composition of MF59:
0.5% Tween 80 – water-soluble surfactant
0.5% Span 85 – oil -soluble surfactant
4.3% squalene oil
Water for injection
10 nM Na-citrate buffer.
Mechanism of action of MF59: The emulsion acts as a direct delivery system and was responsible for promoting the uptake of antigen into antigen- presenting cells (APCs).
A direct effect on cytokine levels in vivo has been observed.
Recent studies have confirmed the ability of MF59 to have a direct effect on immune cells, triggering the release of chemokines and other factors responsible for recruitment and maturation of immune cells.
Squalene is the main component of MF59 emulsion.
Freund’s adjuvant:
Freund, in 1937, demonstrated the adjuvant effect of mineral (paraffin) oil mixed with killed Mycobacteria, referred to as Freund’s complete adjuvant (FCA). The water-in-oil emulsion without Mycobacteria, known as Freund’s incomplete adjuvant (FIA), has been used in a number of veterinary vaccines.
The mode of action of FIA was attributed to depot formation at the site of injection and slow release of the antigen with stimulation of antibody-producing plasma cells. Injection of FIA and antigen at separate sites did not increase the immune esponse. The antigen must be trapped within water droplets (aqueous phase) in the lipid emulsion for augmentation of the immune response.
FIA was used in humans, particularly with influenza and killed poliomyelitis vaccines enhancing their immunogenicity. FIA is not currently used in humans because of the side effects such as local reactions at the site of injection (granuloma and cyst formation), oil-induced neoplasmas in mice.
Microorganism – derived adjuvants:
Some components of mycobacterial cells may also have immunostimulatory action. Synthetic and semisynthetic derivatives have been tested, including muramyl di- and tripeptides, MDP and MTP. MDP, –acetyl-muramyl-L-alanyl-D-isoglutamine, is a small glycopeptide which appears to represent the smallest structure essential for mycobacterial adjuvanticity. However, synthetic MDP and some other analogues have the ability to enhance nonspecific resistance against diverse microbial infections and are capable of conferring resistance against a wide variety of pathogens, including influenza, herpes simplex, vaccinia, and Sendai virus. A purified monophosphoryl A in an emulsion has beenevaluated clinically although it does not appear to have progressed to the market place.
Bacterial or fungal substances constitute a productive source of potential adjuvants.
Bacterial cell wall peptidoglycan or LPS enhances the immune response.
This adjuvant activity is mediated through activation of Toll-like receptors (TLRs).
Different species of bacteria used as a source of adjuvants include Mycobacterium spp., Corynebacterium parvum, C. granulosum, Bordetella pertussis and Neisseria meningitidis.
The adjuvants obtained from microorganisms are:
a) muramyl dipeptide (MDP) , muramyl tripeptide ( MTP)
b) lipid A
c) trehalose dimycolate (TDM).
The major adjuvant activity of these bacteria is mediated by N-acetyl muramyl-l-alanyl-d-isoglutamine, also called muramyl dipeptide (MDP).
In saline, MDP mainly enhances humoral immunity, whilst when incorporated into liposomes or mixed with glycerol it induces strong cellular immunity. Compounds with adjuvant activity derived from MDP include treonyl –MDP.
Another important group of compounds derived from the cell wall of Gram-negative bacteria is LPS. The major structural element of LPS responsible for their adjuvant effect is lipid A.
In low acid conditions, lipid A can be hydrolyzed to obtain monophosphoryl lipid A (MPL), a compound which retains the adjuvant activity of lipid A with reduced toxicity.
Another extract from bacterial walls is trehalose dimycolate(TDM), an adjuvant which simulates both humoral and cellular responses.