Recent evidence has highlighted the role of the oral microbiome in the inflammatory response and pathogenesis of Alzheimer's disease (AD). The oral pathogen Porphyromonas gingivalis (P. gingivalis) and its gingipain virulence factors may be involved in the development of AD. Concurrently, a decline in the antimicrobial protein lactoferrin has been observed in AD saliva. In this study, we analyzed whether the presence of P. gingivalis and gingipain were associated with salivary lactoferrin and inflammatory markers in patients with AD, mild cognitive impairment (MCI) and age-matched controls. Salivary presence of P. gingivalis was higher in MCI and AD patients and exhibited a negative correlation with global cognitive function. We further showed a marked reduction in salivary lactoferrin with AD progression. Salivary/plasma levels of pro-inflammatory and anti-inflammatory cytokines were significantly higher and lower, respectively, in MCI and AD patients compared to controls. Specifically, salivary levels of Interleukin-6 (IL-6) and IL-23 were higher in MCI and AD patients compared to controls, while the IL-1 receptor antagonist (IL-1ra) and IP-10 were reduced in both saliva and plasma in AD patients. Moreover, stromal cell derived factor 1α (SDF1α)/CXCL12, macrophage inflammatory protein-1β (MIP-1β), and vascular endothelial growth factor (VEGF) salivary and plasma levels were reduced in AD patients. Correlation analyses revealed an inverse relationship between salivary lactoferrin and bacterial load and cytokine concentrations. Our findings suggest that AD is associated with alterations to the oral environment, as revealed by higher P. gingivalis presence, lower lactoferrin levels and elevated pro-inflammatory cytokines.

Lactoferrin is present in brain cells and is known to protect against injury through anti-inflammatory and immunomodulatory processes. Furthermore, it has also been found within amyloid plaques in Alzheimer’s disease (AD), leading to the hypothesis that its presence in the AD brain may be detrimental. However, the identity of the brain cells that produce lactoferrin and how its expression changes in AD remain unclear. In this study, we examined lactoferrin expression and localization in 3- and 12-month-old male and female APP/PS1 mice. Using a microglial enrichment protocol and confocal imaging, we identified microglia as an important source of brain lactoferrin, and demonstrated that its expression is modulated by age, sex, and amyloid burden. In female mice, microglial lactoferrin expression was increased at both the mRNA and protein levels in 3-month-old APP/PS1 mice compared with age-matched wild-type controls. Conversely, lactoferrin transcript in the enrichment fraction, and protein localization were also reduced in microglia from 12-month-old female APP/PS1 mice, coinciding with increased amyloid burden. We further demonstrated that gene expression of lactoferrin follows daily variations under a light–dark cycle, similar to established microglial markers such as Iba1 and P2ry12, with alterations in this rhythmic pattern in APP/PS1 mice occurring predominantly in females. In addition, our findings suggest that neurons may internalize lactoferrin via LRP1, whose expression is downregulated by amyloid burden. This Aβ-dependent relationship was further corroborated in both microglial and neuronal in vitro models. Collectively, these findings identify microglia as an important source of lactoferrin in the brain. We suggest that sex- and age-dependent alterations in microglial lactoferrin expression may contribute to the sexual dimorphism observed in AD, possibly through alterations in LRP1-mediated neuronal uptake.