D ILUIR A LA PERSONA

The goal of vaccination is to induce protective immunity but in some cases, this can only be achieved by addition of adjuvants (Table 1.1). Adjuvants are used in vaccines to enhance and modulate the immunogenicity of a vaccine antigen. Many licensed vaccines consist of whole or inactivated pathogens that can promote a strong protective immune response since they contain naturally occurring adjuvants. However, there is a move toward vaccines based on purified antigens (subunit vaccines) which although safer are generally more tolerogenic with limited immunogenicity. Therefore, adjuvants are added to enhance immune responses towards the antigens and induce strong and long-lasting protection against infection.

There have already been some very successful subunit vaccines developed for use in humans, with alum adjuvanted vaccines being particularly successful for enhancing humoral responses although this adjuvant is a poor inducer of T cell responses. Therefore, new vaccines need to target not only a robust antibody response but also a strong T helper and cytotoxic T lymphocyte (CTL) response.

Only six new adjuvants have been used in licensed vaccines in the last 20 years (Table 1.2). Alum was first licensed in 1926 after it was demonstrated to induce increased antibody titres in guinea pigs when injected with soluble toxoid. Since then, both the USA and Europe have licensed other vaccine adjuvants. These include the squalene-based oil-in-water emulsion MF59, consisting of the naturally occurring squalene oil and of non-ionic surfactants Tween

80 and Span 85 emulsified in uniform particles of ∼160 nm in size. MF59 has been used in

influenza vaccines (FLUAD™) (345) and currently, is in clinical trials for vaccines against infectious diseases such as herpes simplex virus (HSV), HBV, and HIV (Table 1.2). Another squalene-based oil-in-water emulsion, namely adjuvant system 03 (AS03), has been used in the pandemic H5N1 influenza vaccine (Q-Pan H5N1) and the H1N1 influenza vaccine Pandemrix. AS03 has been shown to promote higher antigen-specific antibody responses compared with aluminium hydroxide (346). Virosomes are reconstituted membranes of an enveloped virus, consisting of a spherical phospholipid cell membrane bilayer carrying antigen either bound to the surface or encapsulated within the lumen. A combination of aluminium hydroxide and monophospholipid (MPL) A, an LPS analogue, (AS04) was approved for use in vaccines against hepatitis B virus (HBV) and human papilloma virus (HPV) (Table 1.2). Finally, AS01 is a liposome-based adjuvant formulation, comprised of MPL and

42

the saponin QS-21 (347). The malaria vaccine, RTS,S, consists of a portion of the

circumsporozoite protein of Plasmodium falciparum fused to hepatitis B surface antigen virus-

like particles and AS01.

Despite the use of adjuvants clinically (and the many adjuvants under development) their mechanisms of action have yet to be fully elucidated. However, research conducted over the past decade is revealing the mechanisms responsible (Fig 1.10). There are several types of adjuvants with differing modes of action. These include mineral salts, oil emulsions, immune stimulating complexes (ISCOM), bacterial derivatives (TLR agonists), carbohydrate

adjuvants, liposomes and virosomes, cytokines (IL-1, IFN-γ, IL-12 and GM-CSF), virus like particles and polymeric micro and nano particle adjuvants.

43

Table 1.1: potential benefits of using new adjuvants

Benefit Example

Reduce antigen dose Increase global antigen supply in case of an epidemic and increase safety profile of vaccines

Broader antibody response and increased functional antibody titres

Needed for complex pathogens which display antigenic drift and strain variation (Malaria, HIV and Influenza)

More rapid response to pathogens

Reduce number of doses needed, important for pandemic flu outbreaks

Effective T cell responses Important for vaccines against pathogens that are controlled by cellular immune responses, including those causing malaria, tuberculosis and HIV

Vaccines for elderly, young and immunocompromised

Overcome immune senescence

44

Table 1.2: Licensed vaccine adjuvants

Adjuvant Composition Description Manufacturer Licensed vaccine for use in humans Alum Mineral salt-

aluminium hydroxide aluminium phosphate

Improves humoral immune responses and antigen stability. Antigens are adsorbed to the surface.

Various Pertussis, Tetanus, Diphtheria Haemophilus influenzae type b Pneumococcus Hepatitis A and B Papilloma virus MF59 Oil in water emulsion

Improves humoral and cell- mediated immunity. Increases APC recruitment and activation.

Promotes antigen depot, delivery, uptake and migration of cells to lymph nodes.

Novartis/GSK Influenza (FLUAD)

AS03 Oil in water emulsion

Improves humoral and cell- mediated immunity Promotes cytokine

production and recruitment of innate cells.

GSK Pandemic H1N1

influenza vaccine Pandemrix and Arepanrix

Virosomes Liposome Carrier capabilities increases uptake and presentation by APCs Improves humoral and cell- mediated immunity.

Berna Biotech Influenza (Inflexal V)

Hepatitis A virus (Epaxal)

AS04 alum co-absorbed with TLR4 agonist monophosphoryl lipid A (MPL)

Improves humoral and cell- mediated immunity Enhances DC maturation and presentation. GSK Cervical cancer vaccine Cervarix against Papilloma virus

AS01 MPL and QS21 Enhances antibody titres, Th1 type immunity and CD8 T cell-mediated immunity.

GSK Malaria vaccine,

45

Figure 1.10: Proposed mechanisms of adjuvant actions

Several mechanisms have been proposed through which adjuvants mediate their activity. Particulate vaccines can form a depot at the site of injection, slowly realising antigen. Particulate vaccines can also modulate cytokine and chemokine release at the site of injection leading to immune cell recruitment (APCs, NK, neutrophils, monocytes and eosinophils) which

also secrete chemokines and attract other immune cells (A).Inflammasome activation has also

been implicated as a mechanism for some adjuvants. Activation of the inflammasome leads to

the production of the proinflammatory cytokines IL-1β and IL-18 (B). Many adjuvants can act

as ligands for PRRs on the cell surface (TLRs and CLRs) and intracellular receptors (NLR and RLRs) that activate an innate immune response. Receptor signalling can then activate transcription factors that induce the production of cytokines and chemokines that help direct a particular immune response, such as a Th1 or Th2 type response, as well as influence the

immune cells that are recruited to the site of injection (C). Some adjuvants can influence

maturation of APCs, increasing antigen processing, presentation and migration to draining lymph nodes. APCs then interact with antigen-specific B and T cells to activate the adaptive

immune response (D). Other mechanisms include improving targeting and delivery to facilitate

transport of antigens to APCs (D). Taken from (348).

A. B.

C.

46