The effect of LDL-C on T-cell metabolism and its implications for cerebrovascular function
Date
2023
Authors
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Publisher
University of Delaware
Abstract
INTRODUCTION: Alzheimer’s disease (AD) mortality is the fifth leading cause of death in adults 65 years and older. Aging is the greatest risk factor for AD, and is associated with an increase in chronic, sterile, low-grade inflammation termed “inflammaging”. Inflammaging is caused, in part, by dysregulation of the immune system. Adaptive immune cells called T-lymphocytes (T-cells) are primary contributors to inflammaging. T-cells become dysregulated with aging because of impaired mitochondrial function, oxidative stress, and repetitive activation, which shift T-cells towards a pro-inflammatory or senescent state. T-cells have also recently been recognized as important regulators of cerebrovascular function in animals, and impaired cerebrovascular function is a hallmark of AD. However, whether T-cell function relates to human cerebrovascular function is not known. Further, the cause of T-cell dysregulation is not entirely understood. We and others have shown that higher endogenous low-density lipoprotein cholesterol (LDL-C) is related to lower immune cell mitochondrial function and treating T-cells with exogenous LDL-C impaired ATP production. Additionally, animal models have shown that LDL-C can act as a “neo-antigen”, resulting in the activation of T-cells. OBJECTIVES: Therefore, the objectives of this study were to (1) determine whether T-cell mitochondrial respiration is predictive of cerebrovascular function in middle-aged humans and (2) determine whether treatment with a high concentration of LDL-C impairs mitochondrial respiration, induces activation and senescence, and increases intracellular cytokine production and mitochondrial oxidative stress, compared to treatment with a low concentration of LDL-C. HYPOTHESES: (1) We hypothesized that higher T-cell mitochondrial respiration would be predictive of greater cerebrovascular function and that higher T-cell glycolysis would be predictive of lower cerebrovascular function and (2) treatment with a high concentration of exogenous LDL-C would lower mitochondrial respiration, increase activation and glycolysis, pro-inflammatory cytokine production, and mitochondrial oxidative stress, and induce senescence in CD4+ and CD8+ T-cells from middle-aged adults. METHODS: Twenty middle-aged adults were recruited to assess the relation between mitochondrial respiration and cerebrovascular function, and eighteen middle-aged adults were recruited to determine the effect of exogenous LDL-C on T-cell function. T-cells were separated from peripheral blood mononuclear cells using magnetic bead separation. For the first objective, mitochondrial respiration was assessed by measuring oxygen consumption rate (OCR), and glycolysis was assessed by measuring extracellular acidification rate using extracellular flux analysis. Cerebrovascular function was assessed by measuring the change in blood flow velocity in the middle-cerebral artery (MCAv) during a 30-second breath-hold. The breath-hold index (BHI) was calculated as the main outcome measure for cerebrovascular reactivity. For the second objective, CD4+ and CD8+ T-cells were treated with a high and low physiological concentration of exogenous LDL-C for 20-hours. Mitochondrial respiration and extracellular acidification rate (ECAR) were measured using extracellular flux analysis, and all other measures of T-cell function were measured using flow cytometry. RESULTS: We found that higher CD8+ basal was predictive of lower MCAv BHI (β=-2.20, R2=0.29, P=0.015), likely by modulating the sympathetic pressor response to the breath-hold. No measure of CD4+ T-cell mitochondrial respiration or ECAR were predictive of MCAv BHI. We also found that treatment with a high concentration of LDL-C impaired CD4+ T-cell basal (low: 0.75 ± 0.34 vs. high: 0.65 ± 0.35 pMol/min/10,000 cells, p=0.022), and impaired CD4+ and CD8+ T-cell ATP-linked OCR (CD4+ low: 0.84 ± 0.31 vs. CD4+ high: 0.64 ± 0.33 pMol/min/10,000 cells, p=<0.0001; & CD8+ low:0.85 ± 0.35 vs. CD8+ high: 0.64 ± 0.14 pMol/min/10,000 cells, p=0.0024). Treatment with high LDL-C also increased glycolysis in CD8+ T-cells (low: 0.20 ± 0.072 vs. high: 0.23 ± 0.089 mpH/min/10,000 cells, p=0.025). Additionally, treatment with high LDL-C induced activation (CD4+ low: 6.78 ± 5.77 vs. CD4+ high: 19.16 ± 20.51%, p=0.0093; & CD8+ low: 12.75 ± 11.73 vs. CD8+ high: 27.91 ± 26.87%, p=0.0009) and senescence (CD4+ low: 1.04 ± 1.40 vs. CD4+ high: 2.40 ± 2.52%, p=0.0021; & CD8+ low: 13.01 ± 7.78 vs. CD8+ high: 14.86 ± 8.10%, p=0.030) and increased the production of intracellular cytokines, such as IL-6 (CD4+ low: 72.49 ± 21.47 vs. CD4+ high: 77.19 ± 22.42%, p=0.0092; & CD8+ low: 74.48 ± 25.54 vs. CD8+ high: 78.72 ± 26.45%, p=0.0044) and mitochondrial oxidative stress (CD4+ low: 11.23 ± 7.12 vs. CD4+ high: 36.04 ± 35.43%, p=0.0026; & CD8+ low: 4.90 ± 5.35 vs. CD8+ high: 20.17 ± 22.19%, p=0.0024) in both CD4+ and CD8+ T-cells, compared to the treatment with low LDL-C. CONCLUSION: Higher CD8+ T-cell glycolysis was associated with lower cerebrovascular function in humans. We also found that treatment with a high concentration of LDL-C impaired mitochondrial respiration and shifted T-cells towards glycolytic metabolism. Additionally, high LDL-C induced activation and senescence and the production of pro-inflammatory cytokine and mitochondrial oxidative stress. To prevent increases in T-cell inflammation and oxidative stress and preserve mitochondrial function it is important to maintain low concentrations of LDL-C with aging. Therapeutics targeting T-cells may provide novel treatments or preventative strategies for the development of age-related inflammatory diseases, such as AD.
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Keywords
Cerebrovascular function, Alzheimer's disease, T-cells, Mononuclear cells, Oxygen consumption rate