Ageing is associated with an increased neuroinflammation and a deteriorating brain function. Gastrointestinal microbiota has been implicated as key mediators in the communication between the gut and the brain [1], and regulating microglia function and maturation [2]. A dysregulation of this communication may be critical in mediating age-related decline in brain physiology. Thus, targeting the gut microbiota by prebiotics, non-digestible fibres fermented by colonic bacteria, may alter microglia activation state and brain function in ageing.

The aim of this study was to determine if administration of prebiotics affect age-related changes in gut microbiota profile, stress and immune function.

Male young adult (8 weeks) and middle-aged C57BL/6 mice (10 months) received chow enriched with 10% oligofructose-enriched inulin (FOS-Inulin: mixture of 92±2% Inulin and 8±2% Fructooligosaccharide, Orafti ® Synergy1; Beneo/Belgium) or control chow (CTR) for 12 weeks. After 3 weeks of diet intervention the effects on stress and immunity were assessed. Microglia activation and peripheral immune cell activation were investigated by flow cytometry. Caecal samples were collected for 16S bacterial rRNA sequencing and short-chain fatty acid analysis. Parametric data was analysed by one-way ANOVA post-hoc Tukey, non-parametric data by Kruskal-Wallis post-hoc Dunn’s.

FOS-Inulin profoundly altered the caecal gut microbiota in young adult and middle-aged mice with significant changes of the gut microbiota profile on the family (Verrucomicrobiaceae, Kruskal-Wallis p=0.0002, Prevotellaceae p=0.0015, Rikenallaceae p=0.0002) and genus level (Akkermansia p=0.0006, Alistipes p=0.0004, Bacteroides, p=0.0003). Moreover, beta-diversity analysis suggests that FOS-Inulin drives gut microbiota changes reversing the aging microbiota phenotype towards a young adult phenotype. This was associated with an altered caecal short chain fatty acid profile and an increased caecum size in young adult and middle-aged mice (one-way ANOVA F= F (3, 35) = 23.53, p<0.0001; FOS-Inulin adult vs. CTR adult p<0.0001 (2.79±0.1 vs. 1.56±0.21 %bodyweight), FOS-Inulin middle-aged vs. CTR middle-aged p<0.0001 (2.57±0.09 vs. 1.41±0.14 % bodyweight). Specifically, the short-chain fatty acid propionate was increased in FOS-Inulin treated middle-aged mice (one-way ANOVA F (3, 35) = 3.542, p=0.0243; 8.18±0.66 vs. 5.6±0.25 umol/g wet mass). FOS-Inulin counteracted stress-induced peripheral immune cell activation in middle-aged mice (one-way ANOVA F (3, 33) = 5.082, p=0.0053; 0.49±0.28 vs. 3.17±1.06 % change of MHCII+-neutrophils compared to baseline), suggesting an immunoregulatory effect of prebiotics on immune cell priming in response to stress in ageing. Moreover, FOS-Inulin counteracted age-associated monocyte infiltration into the brain (LY6Chi-monocytes, one-way ANOVA F (3, 39) = 10.28, p<0.0001; 4.01±1.04 vs. 10.33±3.05 % of CNS monocytes) and reversed the age-induced increase in a fraction of activated microglia (LY6C+ microglia, one-way ANOVA F (3, 43) = 9,615, p<0.0001; 0.44±0.06 vs. 1.73±0.52 % of microglia).

Targeting the microbiome with prebiotics altered the gut microbiota profile in young adult, but also in middle-aged mice. This was concomitant with effects on stress-induced peripheral immune cell priming and microglia activation. The regulatory effect of prebiotics on monocyte infiltration into the brain and accompanied regulation of age-related microglia activation highlight a potential pathway by which prebiotics can modulate the peripheral immune response and alter neuroinflammation in ageing. Our data highlight a novel strategy for the amelioration of age-related neuroinflammatory pathologies and brain function.