It has been noticed in recent years that the unfavorable effects of the gut microbiota could exhaust host vigor and life, yet knowledge and theory are just beginning to be established. Increasing documentation suggests that the microbiota–gut–brain axis not only impacts brain cognition and psychiatric symptoms but also precipitates neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and multiple sclerosis (MS). How the blood–brain barrier (BBB), a machinery protecting the central nervous system (CNS) from the systemic circulation, allows the risky factors derived from the gut to be translocated into the brain seems paradoxical. For the unique anatomical, histological, and immunological properties underpinning its permeable dynamics, the BBB has been regarded as a biomarker associated with neural pathogenesis. The BBB permeability of mice and rats caused by GM dysbiosis raises the question of how the GM and its metabolites change BBB permeability and causes the brain pathophysiology of neuroinflammation and neurodegeneration (NF&ND) and brain aging, a pivotal multidisciplinary field tightly associated with immune and chronic systemic inflammation. If not all, gut microbiota-induced systemic chronic inflammation (GM-SCI) mainly refers to excessive gut inflammation caused by gut mucosal immunity dysregulation, which is often influenced by dietary components and age, is produced at the interface of the intestinal barrier (IB) or exacerbated after IB disruption, initiates various common chronic diseases along its dispersal routes, and eventually impairs BBB integrity to cause NF&ND and brain aging. To illustrate the immune roles of the BBB in pathophysiology affected by inflammatory or “leaky” IB resulting from GM and their metabolites, we reviewed the selected publications, including the role of the BBB as the immune barrier, systemic chronic inflammation and inflammation influences on BBB permeability, NF&ND, and brain aging. To add depth to the bridging role of systemic chronic inflammation, a plausible mechanism indispensable for BBB corruption was highlighted; namely, BBB maintenance cues are affected by inflammatory cytokines, which may help to understand how GM and its metabolites play a major role in NF&ND and aging.

The immune system of centenarians remains active and young to prevent cancer and infections. Aging is associated with inflammaging, a persistent low-grade inflammatory state in which CD4+ T cells play a role. However, there are few studies that have been done on the CD4+ T cell subsets in centenarians. Herein, the changes in CD4+ T cell subsets were investigated in centenarians. It was found that with aging, the old adults had higher levels of proinflammatory cytokines and lower levels of anti-inflammatory cytokines in plasma. The levels of CRP, IL-12, TNF-α, IFN-γ, IL-6 and IL-10 were further increased in centenarians compared to old adults. While the levels of IL-17A, IL-1β, IL-23 and TGF-β in centenarians were closer to those in young adults. The total CD4+, CD8+, Th17 and Treg cells from peripheral blood mononuclear cells (PBMCs) were similar among the three groups. It was observed that the ratio of Th17/Treg cells was elevated in old adults compared to young adults. The ratio was not further elevated in centenarians but rather decreased. In addition, the ex vivo PBMCs differentiation assay showed that increased Th17 cells in centenarians tended to secrete fewer proinflammatory cytokines, while decreased Treg cells in centenarians were prone to secrete more anti-inflammatory cytokines. These observations suggested centenarians alleviated inflammaging by decreasing the ratio of Th17/Treg cells and changing them into anti-inflammatory secretory phenotypes, which provided a novel mechanism for anti-aging research.