Background/Objectives: Neuroinflammation, a hallmark of Alzheimer’s disease (AD), is characterized by elevated levels of inflammatory signaling molecules, including cytokines and eicosanoids, as well as increased microglial reactivity, and is augmented by gut microbiota dysbiosis via the gut–brain axis. We conducted a pilot experiment to elucidate the anti-inflammatory effects of dietary omega-3 polyunsaturated fatty acid (ω-3 PUFA) eicosapentaenoic acid (EPA) on the gut microbiota and neuroinflammation. Methods: Female APP/PS1 mice (TG) and non-transgenic littermates (WT), 13–14 months old, were fed a diet supplemented with 0.3% EPA or control chow for 3 weeks. The gut microbiota composition, hippocampal and plasma eicosanoids levels, platelet activation, and microglial phagocytosis, as well as the brain and retinal genes and protein expression, were analyzed. Results: EPA supplementation decreased the percentage of Bacteroidetes and increased bacteria of the phylum Firmicutes in APP/PS1 and WT mice. Inflammatory lipid mediators were elevated in the hippocampus of the TG mice, accompanied by a reduction in the endocannabinoid docosahexaenoyl ethanolamide (DHEA). Dietary EPA did not affect hippocampal lipid mediators, but reduced the levels of arachidonic-derived 5-HETE and N-arachidonoylethanolamine (AEA) in WT plasma. Moreover, EPA supplementation decreased major histocompatibility complex class II (MHCII) gene expression in the retina in both genotypes, and MHCII+ cells in the hippocampus of TG mice. Conclusions: This pilot study showed that short-term EPA supplementation shaped the gut microbiota by increasing butyrate-producing bacteria of the Firmicutes phylum and decreasing Gram-negative LPS-producing bacteria of the Bacteroidetes phylum, and downregulated the inflammatory microglial marker MHCII in two distinct regions of the central nervous system (CNS). Further investigation is needed to determine whether EPA-mediated effects on the microbiome and microglial MHCII have beneficial long-term effects on AD pathology and cognition.

Alzheimer’s disease (AD) progression has been associated with the presence of brain-resident CD8 ⁺ T cells, and recent studies suggest a potential role of the CXCL16-CXCR6 axis in their recruitment to the brain. Here, we examined publicly available single-cell RNA sequencing datasets revealing that in the mouse brain, the receptor Cxcr6 is mainly expressed by CD8 ⁺ T cells, while the expression of its ligand Cxcl16 is predominantly observed in microglial cells. We found higher levels of Cxcl16 and Cxcr6 expression in APP/PS1 compared to wild-type mice. Furthermore, in vitro experiments using immortalized and primary murine cells suggested that Cxcl16 expression is driven by Aβ pathology. In contrast to our expectations, no changes in the number of Cxcr6 ⁺ CD8 ⁺ cells was evident in the brains of microglia-depleted APP/PS1 mice, treated with the CSF1R antagonist PLX5622. This was related to an increased compensatory Cxcl16 expression by depletion-resistant microglia or by other brain-resident myeloid cells. Although we demonstrated a strong association between microglial Cxcl16 and AD pathology, PLX5622-sensitive microglia are dispensable in the recruitment of Cxcr6 ⁺ CD8 ⁺ T cells to the brain of APP/PS1 mice. Future in vivo analysis will help to dissect the mechanism of CD8 ⁺ T cell recruitment to the brain.

CD8⁺ T cells infiltrate the brain parenchyma of patients with Alzheimer’s disease (AD), where they localize near microglia and neurons. Although recent studies have revealed that they can clonally expand and possess a unique phenotype, their contribution to disease pathogenesis remains poorly understood. To investigate the role of these cells in AD, we first performed exploratory immunopeptidome profiling in aged transgenic AD (APP/PS1) mice to identify potential major histocompatibility complex class I-presented antigens that might drive CD8⁺ T cell recruitment. Surprisingly, no peptides were found to be overrepresented in transgenic compared with control brains, suggesting the absence of a dominant antigenic signature. We next characterized brain-infiltrating CD8⁺ T cells by flow cytometry and found that, in aged APP/PS1 mice, these cells exhibit a tissue-resident phenotype distinct from their circulating counterparts. To assess their functional relevance, we induced short-term CD8⁺ T cell depletion in two-year-old APP/PS1 mice using anti-CD8 antibody injections over three days and subsequently analyzed brain pathology and gene expression. CD8⁺ T cell depletion effectively reduced CD8⁺ T cell numbers in both blood and brain. Although amyloid plaque burden remained unchanged, female mice showed reduced LAMP1 ⁺ dystrophic neurites surrounding plaques and decreased phosphorylated tau deposition. These effects were accompanied by a modest increase in microglial abundance and elevated hippocampal Il1b expression. Collectively, these findings indicate that brain-resident CD8⁺ T cells contribute to specific aspects of AD-related neurodegeneration in a potential sex-dependent manner (although further experiments are needed), supporting a previously underappreciated role for CD8⁺ T cells in AD progression.