Aluminium-based adjuvants (ABA) are the predominant adjuvants used in human vaccinations. While a consensus is yet to be reached on the aetiology of the biological activities of ABA several studies have identified shape, crystallinity and size as critical factors affecting their adjuvanticity. In spite of recent advances, the fate of ABA following their administration remains unclear. Few if any studies have demonstrated the unequivocal presence of intracellular ABA. Herein we demonstrate for the first time the unequivocal identification of ABA within a monocytic T helper 1 (THP-1) cell line, using lumogallion as a fluorescent molecular probe for aluminium. Use of these new methods revealed that particulate ABA was only found in the cell cytoplasm. Transmission electron microscopy revealed that ABA were contained within vesicle-like structures of approximately 0.5–1 μm in diameter.

Aluminium adjuvants remain the most widely used and effective adjuvants in vaccination and immunotherapy. Herein, the particle size distribution (PSD) of aluminium oxyhydroxide and aluminium hydroxyphosphate adjuvants was elucidated in attempt to correlate these properties with the biological responses observed post vaccination. Heightened solubility and potentially the generation of Al3+ in the lysosomal environment were positively correlated with an increase in cell mortality in vitro, potentially generating a greater inflammatory response at the site of simulated injection. The cellular uptake of aluminium based adjuvants (ABAs) used in clinically approved vaccinations are compared to a commonly used experimental ABA, in an in vitro THP-1 cell model. Using lumogallion as a direct-fluorescent molecular probe for aluminium, complemented with transmission electron microscopy provides further insight into the morphology of internalised particulates, driven by the physicochemical variations of the ABAs investigated. We demonstrate that not all aluminium adjuvants are equal neither in terms of their physical properties nor their biological reactivity and potential toxicities both at the injection site and beyond. High loading of aluminium oxyhydroxide in the cytoplasm of THP-1 cells without immediate cytotoxicity might predispose this form of aluminium adjuvant to its subsequent transport throughout the body including access to the brain.

Aluminium salts are by far the most commonly used adjuvants in vaccines. There are only two aluminium salts which are used in clinically-approved vaccines, Alhydrogel® and AdjuPhos®, while the novel aluminium adjuvant used in Gardasil® is a sulphated version of the latter. We have investigated the physicochemical properties of these two aluminium adjuvants and specifically in milieus approximating to both vaccine vehicles and the composition of injection sites. Additionally we have used a monocytic cell line to establish the relationship between their physicochemical properties and their internalisation and cytotoxicity. We emphasise that aluminium adjuvants used in clinically approved vaccines are chemically and biologically dissimilar with concomitantly potentially distinct roles in vaccine-related adverse events.

The physicochemical properties of aluminum salts are key determinants of their resultant adjuvanticity in vivo when administered as part of a vaccine. While there are links between particle size and the efficacy of the immune response, the limited literature directly characterizing the PSD of aluminum adjuvants has stymied the elucidation of such a relationship for these materials. Hence, this comparative study was undertaken to monitor the PSD of aluminum adjuvants throughout the process of vaccine formulation using DLS. A significant proportion of the stock suspensions was highly agglomerated (>9 μm) and Alhydrogel® exhibited the smallest median size (2677 ± 120 nm) in comparison to Adju-Phos® or Imject alum® (7152 ± 308 and 7294 ± 146 nm respectively) despite its large polydispersity index (PDI). Dilution of these materials induced some degree of disaggregation within all samples with Adju-Phos® being the most significantly affected. The presence of BSA caused the median size of Alhydrogel® to increase but these trends were not evident when model vaccines were formulated with either Adju-Phos® or Imject alum®. Nevertheless, Alhydrogel® and Adju-Phos® exhibited comparable median sizes in the presence of this protein (4194 ± 466 and 4850 ± 501 nm respectively) with Imject alum® being considerably smaller (2155 ± 485 nm). These results suggest that the PSD of aluminum adjuvants is greatly influenced by dilution and the degree of protein adsorption experienced within the vaccine itself. The size of the resultant antigen-adjuvant complex may be important for its immunological recognition and subsequent clearance from the injection site.

Alzheimer’s disease is a debilitating neurodegenerative condition that progressively causes synaptic loss and major neuronal damage. Immunotherapy utilising Aβ as an active immunogen or via passive treatment utilising antibodies raised to amyloid have shown therapeutic promise. The migratory properties of peripheral blood-borne monocytes and their ability to enter the central nervous system, suggests a beneficial role in mediating tissue damage and neuroinflammation. However, the intrinsic phagocytic properties of such cells have pre-disposed them to internalise misfolded amyloidogenic peptides that could act as seeds capable of nucleating amyloid formation in the brain. Mechanisms governing the cellular fate of amyloid therefore, may prove to be key in the development of future vaccination regimes. Herein, we have developed unequivocal and direct conformation-sensitive fluorescent molecular probes that reveal the intracytoplasmic and intranuclear persistence of amyloid in a monocytic T helper 1 (THP-1) cell line. Use of the pathogenic Aβ42 species as a model antigen in simulated vaccine formulations suggested differing mechanisms of cellular internalisation, in which fibrillar amyloid evaded lysosomal capture, even when co-deposited on particulate adjuvant materials. Taken collectively, direct fluorescent labelling of antigen-adjuvant complexes may serve as critical tools in understanding subsequent immunopotentiation in vaccines directed against amyloidosis and wider dementia.