Low concentrations of aluminum hydroxide adjuvant, forming limited size aggregates, selectively induce cerebral aluminum increase and long-term neurotoxicity in mouse

International audienceBackgroundAluminium hydroxide (alum) has long been added as an adjuvant of vaccines. It consists of nanoparticles forming aggregates. Unexpectedly long-lasting biopersistence of alum aggregates were found within immune cells of patients with chronic fatigue, cognitive dysfunction, myalgias and dysimmunity [1], [2]. We documented in mice slow translocation of alum aggregates captured by monocyte-lineage cells from the injected muscle to brain [3], [4], [5]. Herein, brain function and aluminium (Al) concentration were examined long after injections.MethodsAlhydrogel® was injected in TA muscle in adult female CD1 mice at 3 doses ranging from 133 to 800 μg Al/kg. Eight validated tests were used to evaluate cognitive and motor performances 180 days after injection. Brains were collected for Al level determination and Iba-1 immunohistochemistry.ResultsA most unusual neuro-toxicological pattern limited to lower doses of alum was observed. Neurobehavioral changes, including decreased activity levels and altered anxiety-like behaviour, were documented in animals exposed to the two lowest doses (133 and 200 μg Al/kg) but not at the highest dose (800 μg Al/kg), compared to controls. Consistently, cerebral Al levels were increased in animals exposed to the lowest doses. Microglial cell increase was found in amygdala of the 200 μg Al/kg group. Interestingly, the injected suspensions corresponding to the two lowest doses contained much smaller aggregates (1.50–1.75 μm) compared to the highest dose (4.70 μm).ConclusionAlum particles injected in muscle may induce neurotoxic effects and Al cerebral accumulation six months after injection in mice. Neurotoxic effects are restricted to low concentration suspensions forming small particle aggregates. Such bacteria-sized aggregates are known to be selectively captured by monocyte-lineage cells. This study strongly suggests that, in contrast to “the dose makes the poison” paradigm of classical toxicology, alum toxicology obeys the specific rules of small particle toxicology, thus deserving in depth revaluation. (This study was supported by ANSM)