Defective Signaling in the JAK-STAT Pathway Tracks with Chronic Inflammation and Cardiovascular Risk in Aging Humans

Abstract

Chronic inflammation, a decline in immune responsiveness, and reduced cardiovascular function are all associated with aging, but the relationships among these phenomena remain unclear. Here, we longitudinally profiled a total of 84 signaling conditions in 91 young and older adults and observed an age-related reduction in cytokine responsiveness within four immune cell lineages, most prominently T cells. The phenotype can be partially explained by elevated baseline levels of phosphorylated STAT (pSTAT) proteins and a different response capacity of naive versus memory T cell subsets to interleukin 6 (IL-6), interferon α (IFN-α), and, to a lesser extent, IL-21 and IFN-γ. Baseline pSTAT levels tracked with circulating levels of C-reactive protein (CRP), and we derived a cytokine response score that negatively correlates with measures of cardiovascular disease, specifically diastolic dysfunction and atherosclerotic burden, outperforming CRP. Thus, we identified an immunological link between inflammation, decreased cell responsiveness in the JAK-STAT pathway, and cardiovascular aging. Targeting chronic inflammation may ameliorate this deficiency in cellular responsiveness and improve cardiovascular function.

Trial registration: ClinicalTrials.gov NCT01827462.

Keywords: aging; cardiovascular; cytokine responses; immune monitoring; immune signaling; immunosenescence; inflammaging; systems immunology.

Figure 1. Phosphorylation of STATs in Response to Cytokine Stimulation Decreases with Age

(A) Diagram of study design. Young and old adults were profiled repeatedly at yearly intervals for up to 3 years using multiplex immune assays. In the fourth year of the study, 40 older adult participants volunteered to undergo comprehensive cardiovascular monitoring. (B) Heatmap of 72 cytokine stimulation assays measured across 29 individuals of different ages. Rows are assays color-coded by the condition (cell subset, cytokine, and pSTAT combination). Heatmap cells are colored according to the normalized fold change Z scores. Age-associated differences at a q ≤ 0.15 are marked on the right (red, decrease with age; purple, increase; gray, no significant difference). Inset of CD8 T cells stimulated by IL-6 and assayed for change in pSTAT3 abundance illustrates the reduced fold change response of some older adults. (C) Individual age-associated cytokine response assays summarized by cell subset, stimuli or pSTAT protein. Reduction in responses is often systemic, with an individual exhibiting multiple dysfunctions. Inset shows a Z score summarization across individuals of all 72 assays. Older adults show reduced responses compared to young adults and high within-group variability. (D) Reduced cellular response in old adults is independent of stimulus concentration. The entire 72-condition cytokine response assay was repeated at five different doses per stimuli for two young (orange) and old (blue) adults. Shown is an example histogram (top) and line plot visualization (bottom) of the fold change cytokine response as a function of cytokine stimuli concentration. A fold change of 1 signifies no change from baseline.

Figure 2. Population Stability of Age-Associated Differences in Cellular Response

(A) Heatmap of 60 cytokine stimulations and 12 baseline measured over 3 years in partially overlapping individuals of different ages. Each row represents a single condition and is color-coded by the FDR (q-value) of the assay in a given cohort. A Fisher’s combined probability test, weighted for sample size each year, was performed on the multiple hypothesis adjusted p values of all assays in a year and is shown on the left. Red denotes a Fisher’s combined probability p value < 0.05. (B) The intra-class correlation (ICC), a measure of within-subject stability, of age-associated cytokine response assays was found to be moderate to low for individual cytokine response assays when measured across older adults enrolled in the study for 3 years. Bar length denotes ICC magnitude, whereas bar width denotes measured (not normalized) fold change of response and color corresponds to the mean percent reduction in response of old compared to the young. (C) The ratio of ICC values between old adults and all study participants is shown for all 15 ICC positive cytokine response assays. Bar color corresponds to the mean percent reduction in response of old versus young adults. A negative correlation exists between the cytokine response assay ICC score and the mean percent reduction in cytokine response (Spearman’s rho = −0.49, p = 0.058). Older adults tend toward increased stability (i.e., higher ICC), except in those assays in which they show a large reduction in response compared to the young.

Figure 3. Reduced Responses to Cytokine Stimulation Are Primarily due to Elevation of Basal Levels of Phosphorylated STAT Proteins and Differences in Naive and Memory Cell Responses

(A) The mean coefficient of association across three study years of immune measures found associated with the CRS (using a Fisher’s combined p value < 0.05 and a q-value < 5% in participants for least 2 of 3 years). (B and C) Boxplots showing naive and memory CD4⁺ and CD8⁺ T cell cytokine responses of T cell member assays in the CRS (B) and baseline pSTAT levels (C) compared within an individual and between the young and old age groups. Zero (red dotted line) denotes no difference between naive and memory fold-change responses. Asterisks at the bottom denote significance of a paired within-individual naive memory comparison and those above boxplots denote between-group comparisons by Wilcoxon test (***p < 0.005, **p < 0.01, *p < 0.05).

Figure 4. Baseline Phosphoprotein Levels and Cytokine Response Are Correlated with Inflammation

(A) Stratification of older adults based on standard levels of clinical risk for CRP (<1 mg/mL versus >3 mg/L for CRP) show significant differences in sum baseline cellular pSTAT levels (p = 0.01 by Wilcoxon test). (B) A reciprocal, but not statistically significant, trend is observed for the CRS. Error bars depict SE.

Figure 5. Reduced Signaling Responses Are Associated with Diastolic Dysfunction and Atherosclerotic Burden

(A–C) The CRS is strongly associated with markers of diastolic function. (A) A positive linear correlation is observed between age adjusted e′, a sensitive measure of ventricular relaxation, and the age adjusted CRS (r = 0.62, p = 10⁻⁴). Age adjusted CRSs are significantly reduced in (B) individuals with increased E to e′ ratio (>9), a measure of increased ventricular filling pressure (p = 0.02) and in (C) individuals with a high left atrium volume index (>34), a measure of atrial remodeling (p = 0.02). (D) Individuals with evidence of atherosclerotic burden (brown, orange, and red for minimal-inclusive, sub-clinical, and clinical subgroups) exhibit reduced CRS compared to those with healthy (green) cardiovascular aging (p = 0.02). (E) Older adults exhibiting high central arterial stiffness as assessed by age-adjusted pulse wave velocity, a clinically validated measure (high > 10), show a reduced CRS (p = 0.058). (F) ROC curve analysis comparing sensitivity and specificity of atherosclerosis burden classification by the CRS (as computed from year 2 or 3 data), the sum of pSTAT baseline data, and CRP in year 2. Both year 3 and year 2 CRS show improved classifier performance compared to that of CRP (AUC = 0.7, 0.65, 0.54, and 0.54, respectively for CRS year 3, CRS, sum of pSTAT baseline levels, and CRP).

Figure 6. A Model Associating Elevated Baseline pSTAT Levels with Cellular Unresponsiveness and Chronic Pro-inflammation

Proposed model for elevation of baseline pSTAT levels and its association with hallmark phenomenon of immunosenescence, an increased pro-inflammatory environment, unresponsive cells, and a clinical impact on immune response.