A perfectly potent partnership – MPL and Aluminium Salt

Published: 2019/12/05 Number of words: 1485

A novel approach to prophylactic and therapeutic adjuvanticity; combined stimulation of TLR4 and optimal activation of APC’s enhances the robustness of ASO4 adjuvanted vaccine responses.

For successful vaccination, the vaccine must be able to induce a hardy innate and adaptive response to create long lived plasma cells and memory B cells. So upon a second meeting with the same antigen, the body is primed to make a robust response to clear the pathogen, by induction of persistent serum antibodies.6 The innate immune system is triggered by PAMP’s activating PRR’s, such as TLRs and C-type lectins. The fundamental principle of vaccination is based on selectively activating these PRRs, which have effects on the adaptive response, to achieve a seroprotective antibody titer, whilst also conferring a functionally protective memory cell population.3

However the vaccine must also be safe, it must have a low reactogenicity but a high immunogenicity. Many safer attenuated vaccine antigens often have low immunogenicity, which is why adjuvants are required to boost their effect.4 An adjuvant is a molecule that boosts the potency and longevity of specific immune responses to an antigen, whilst causing minimal toxicity or long lived effects. There are many types of adjuvant available which have different immunomodulatory effects, meaning they can polarise either Th1 or Th2 response.5

ASO4 is a novel generation of adjuvants licensed for human use, consisting of MPL and aluminium salt. MPL, one half of the potent partnership in ASO4 is a detoxified derivative of LPS and signals through TLR4 to induce a proinflammatory Th1 immune skew. Stimulation of TLR4 activates NF-Kß transcription, subsequent expression of proinflammatory cytokines TNF-α and IL-6, which are major players in innate activation. These cytokines also enhance the adaptive response by repressing Treg activity whilst stimulating the maturation of APC’s. MPL also promotes antigen specific CD4+ T cell IFN-γ production.4 The other component of ASO4, Aluminium salt, is a safe and classical adjuvant which activates antigen presenting cells to present more efficiently. It works by an important depot effect of delayed antigen release and inducing IL-4 production, skewing towards a Th2 response. It also induces secretion of chemokines CCL2, CCL3, CCL4, CXCL8, so has a clear role in monocyte migration and differentiation.2 There is some data that suggests aluminium salt is able to stimulate the inflammasome via Nlrp3, but not strongly enough to induce transcription of IL-1ß.­9

Didierlaurent et al demonstrated the combination of ASO4 and HPV enhanced the immunogenicity of the vaccine, through the mechanism of acutely activating the innate response in a spatially localised manner, and prolonging the longevity of cytokine response at the I.M injection site. They showed MPL to be the principle driver of APC proliferation and activation, restricted to the draining lymph nodes. ASO4 was able to directly stimulate human DC’s, monocytes and subsequent production of TNF-a and IL-6, and upregulate surface expression of co-stimulatory molecules CD83/CD86.8 FACs analysis data shows ASO4 induced a significant surge of migrating APC’s loaded with antigen to enter the draining lymph node with the enhanced capability to directly stimulate T cells.­­­8 Using a transgenic mouse model with a luciferase reporter gene, I.M injection of MPL or AS04 alone led to localised and transient NF-Kß activation, a cytokine which is responsible for immune activation and involved in pathogen clearance.8 10 Co-localisation of ASO4 with antigen is required for optimal immune activation, as Didierlaurent et al demonstrated by vaccination of C57BL/6 mice at different time points with ASO4-HPV or MPL-HPV.8 The transient and localised nature of the innate stimulation of ASO4-HPV demonstrates a favourable clinical safety profile, which looks extremely positive for future applications. Aluminium salt also plays an integral role in stabilisation of MPL and VPL antigens within the vaccine, along with a spatially confining depot effect.8 Their data strongly supports the role of ASO4 adjuvant, in effecting a persistent vaccine induced immune response of high quality.

Another interesting study supports the work of Didierlaurent et al, by demonstrating ASO4 enhances humoral and memory B cellular immunity in combination with HPV16/18 L1 VLP antigen.7

In other work, MPL alone has been found to activate local follicular Th cells expressing TCR of high affinity to antigenic peptides.11 12 Further work is required to categorise the generation of follicular Th cells in the draining lymph nodes and determine the affinity of their TCR repertoire post vaccination with HPV-ASO4. To test the hypothesis that the superior adjuvant activity of ASO4 is due to Th follicular cells with high affinity TCR’s. Another element of further investigation is the contribution of induction of IL-1ß secretion through MPL stimulation of TLR4, via the activation of caspase-8 as recently reported by Maelfait et al.13

Ultimately the perfect adjuvant would be one tailored to each specific antigen to create the optimal immune response, whilst maintaining safety, efficacy and immunogenicity. It’s important to select the perfect combination of PRR stimulators to induce the optimal polarisation and protective immunity. The current trend of designing safer specific vaccines utilising genetic engineering does inevitably produce antigens that lack the original contaminants responsible for their immunogenicity.5 Elucidation of innate and adaptive immunity receptors and their mechanisms of function, combined with structurally well-defined adjuvants, would enable the rational design of novel polarising adjuvants. The effect of adjuvants interacting with TLR’s or co-stimulatory receptors can be either synergistic or cumulative, which allows for opportunities to design adjuvant combinations that can yield specific responses. This method could be applied to many other areas of vaccination, such as CTL activation against virally infected cells. Adjuvants such as ASO4 will thus play a pivotal role in eliciting and enhancing the appropriate immune response. It’s encouraging to recognise the ongoing pipeline of development of ASO4 and other similar adjuvants, to make more effective and well tolerated adjuvant systems.


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