Sex differences in T cell immune responses, gut permeability and outcome after ischemic stroke in aged mice.

INTRODUCTION: Stroke is a disease that presents with well-known sex differences. While women account for more stroke deaths, recent data show that after adjusting for age and pre-stroke functional status, mortality is higher in men. Immune responses are key determinants of stroke outcome and may differ by sex. This study examined sex differences in central and peripheral T cell immune responses, systemic effects on gut permeability and microbiota diversity and behavioral outcomes after stroke in aged mice. We hypothesized that there are sex differences in the immune response to stroke in aged animals. METHODS: C57BL/6CR mice (20-22 months) were subjected to 60 min middle cerebral artery occlusion, or sham surgery. T cells were quantified in brain and blood at 3, 7 and 15 days (d) post-stroke by flow cytometry. Peripheral effects on gut permeability and microbiota diversity, as well as neurological function were assessed up to 14 d, and at 21 d (cognitive function) post-stroke. Brain glial fibrillary acidic protein (GFAP) expression was evaluated at 42 d post-stroke. RESULTS AND DISCUSSION: Mortality (50% vs 14%, p < 0.05) and hemorrhagic transformation (44% vs 0%) were significantly higher in males than in females. No difference in infarct size at 3d were observed. Peripherally, stroke induced greater gut permeability of FITC-dextran in males at d3 (p < 0.05), and non-reversible alterations in microbiota diversity in males. Following the sub-acute phase, both sexes demonstrated a time-dependent increase of CD4+ and CD8+ T cells in the brain, with significantly higher levels of CD8+ T cells and Regulatory T cells in males at d15 (p < 0.01). Aged males demonstrated greater neurological deficits up to d5 and impaired sensorimotor function up to d15 when assessed by the corner asymmetry test (p < 0.001 and p < 0.01, respectively). A trend in greater cognitive decline was observed at d21 in males. Increased GFAP expression in the ischemic hemisphere, indicating astroglial activation and gliosis, was demonstrated in both males and females 42d post-stroke. Our findings indicate that despite a similar initial ischemic brain injury, aged male mice experience greater peripheral effects on the gut and ongoing central neuroinflammation past the sub-acute phase after stroke.

Aging exacerbates neutrophil pathogenicity in ischemic stroke.

Ischemic stroke is major cause of disability and mortality worldwide, and aging is strong risk factor for poor post-stroke outcome. Neutrophils traffic rapidly to the brain following ischemic stroke, and recent evidence has suggested that aging may alter neutrophil function after tissue injury. In this study, we hypothesize that aging enhances the pro-inflammatory function of neutrophils, directly contributing to the poorer outcomes seen in aging patients. We utilized demographic data and biological specimens from ischemic stroke patients and an experimental mouse model to determine the correlation between age, neutrophil function and stroke outcomes. In ischemic stroke patients, age was associated with increased mortality and morbidity and higher levels of neutrophil-activating cytokines. In mice, aged animals had higher stroke mortality and morbidity, higher levels of neutrophil-activating cytokines and enhanced generation of neutrophil reactive oxygen species compared to young mice. Finally, depletion of neutrophils via a specific monoclonal antibody after ischemic stroke led to long-term benefits in functional outcome in aged male and female animals, with no benefit observed in young. These results demonstrate that aging is associated with augmented neutrophil pathogenicity in ischemic stroke, and that neutrophil-targeted therapies may confer greater benefit in aged subjects.

Age-related changes in the gut microbiota influence systemic inflammation and stroke outcome.

OBJECTIVE: Chronic systemic inflammation contributes to the pathogenesis of many age-related diseases. Although not well understood, alterations in the gut microbiota, or dysbiosis, may be responsible for age-related inflammation. METHODS: Using stroke as a disease model, we tested the hypothesis that a youthful microbiota, when established in aged mice, produces positive outcomes following ischemic stroke. Conversely, an aged microbiota, when established in young mice, produces negative outcomes after stroke. Young and aged male mice had either a young or an aged microbiota established by fecal transplant gavage (FTG). Mice were subjected to ischemic stroke (middle cerebral artery occlusion; MCAO) or sham surgery. During the subsequent weeks, mice underwent behavioral testing and fecal samples were collected for 16S ribosomal RNA analysis of bacterial content. RESULTS: We found that the microbiota is altered after experimental stroke in young mice and resembles the biome of uninjured aged mice. In aged mice, the ratio of Firmicutes to Bacteroidetes (F:B), two main bacterial phyla in gut microbiota, increased ∼9-fold (p < 0.001) compared to young. This increased F:B ratio in aged mice is indicative of dysbiosis. Altering the microbiota in young by fecal gavage to resemble that of aged mice (∼6-fold increase in F:B ratio, p < 0.001) increased mortality following MCAO, decreased performance in behavioral testing, and increased cytokine levels. Conversely, altering the microbiota in aged to resemble that of young (∼9-fold decrease in F:B ratio, p < 0.001) increased survival and improved recovery following MCAO. INTERPRETATION: Aged biome increased the levels of systemic proinflammatory cytokines. We conclude that the gut microbiota can be modified to positively impact outcomes from age-related diseases. Ann Neurol 2018;83:23-36.

Sex Differences in Adipose Tissue CD8+ T Cells and Regulatory T Cells in Middle-Aged Mice.

The prevalence of cardiovascular disease has increased among middle-aged women in the United States, yet has declined in middle-aged men. In experimental stroke, middle-aged females have larger strokes and greater inflammation than age-matched males or younger females. The mechanism underlying this shift from an "ischemia-protected" to an "ischemia-sensitive" phenotype in aging females is unknown. One potential factor is an age-related increase in systemic factors that induce inflammation. Increased abdominal fat deposition is seen in women during middle age. Adipose tissue plays a key role in obesity-induced systemic inflammation, including increased pro-inflammatory cytokines. We hypothesized that age and sex differences in adipose immune cells promote an augmented pro-inflammatory milieu in middle-aged females driven by a balance shift between pro-inflammatory and anti-inflammatory T cells. Abdominal adipose tissue immune cells from young (3-4 months) and middle-aged (15-16 months) male and female C57BL/6J mice were analyzed by flow cytometry. Plasma triglyceride (TG), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) levels were determined with colorimetric assays. Middle-aged mice had higher adipose tissue mass compared to young mice. Lipid profiling showed no sex differences in TG and LDL, but middle-aged females had lower HDL (0.84 ± 0.07 µg/µl) than middle-aged males (1.35 ± 0.06 µg/µl). Flow cytometry data demonstrated an age-associated increase in adipose tissue CD8+ T cells that was augmented by female sex, with middle-aged females having a higher percentage of CD8+ cells (34.4 ± 3.2% of CD3+ T cells) than middle-aged males (24.4 ± 2.2%). This increase in CD8+ T-cell proportion was adipose tissue-specific, as this change was not observed in blood. Middle-aged females had higher numbers of activated (CD69+) CD8+ T cells than males. In addition, female CD8+ T cells produced higher levels of IFN-γ, TNF-α, and granzyme B ex vivo, and females had higher adipose levels of IFN-γ, RANTES and MIP-1ß than middle-aged males. In parallel, females had lower levels of regulatory T cells (Tregs), an anti-inflammatory T-cell subtype, compared to age-matched males. In conclusion, middle-aged females have a detrimental combination of elevated pro-inflammatory T cells and decreased anti-inflammatory Tregs in adipose tissue, which may promote a pro-inflammatory milieu and contribute to increased cardiovascular disease burden in aging females.

Sex differences in neuroinflammation and neuroprotection in ischemic stroke.

Stroke is not only a leading cause of mortality and morbidity worldwide it also disproportionally affects women. There are currently over 500,000 more women stroke survivors in the US than men, and elderly women bear the brunt of stroke-related disability. Stroke has dropped to the fifth leading cause of death in men, but remains the third in women. This review discusses sex differences in common stroke risk factors, the efficacy of stroke prevention therapies, acute treatment responses, and post-stroke recovery in clinical populations. Women have an increased lifetime risk of stroke compared to men, largely due to a steep increase in stroke incidence in older postmenopausal women, yet most basic science studies continue to only evaluate young male animals. Women also have an increased lifetime prevalence of many common stroke risk factors, including hypertension and atrial fibrillation, as well as abdominal obesity and metabolic syndrome. None of these age-related risk factors have been well modeled in the laboratory. Evidence from the bench has implicated genetic and epigenetic factors, differential activation of cell-death programs, cell-cell signaling pathways, and systemic immune responses as contributors to sex differences in ischemic stroke. The most recent basic scientific findings have been summarized in this review, with an emphasis on factors that differ between males and females that are pertinent to stroke outcomes. Identification and understanding of the underlying biological factors that contribute to sex differences will be critical to the development of translational targets to improve the treatment of women after stroke. © 2016 Wiley Periodicals, Inc.