Peripheral levels of C-reactive protein, tumor necrosis factor-α, interleukin-6, and interleukin-1β across the mood spectrum in bipolar disorder: A meta-analysis of mean differences and variability

Abstract Importance It is unclear whether differences exist in the magnitude and variability of pro-inflammatory mediators in the different phases of bipolar disorder (BD) and among subjects with BD, as compared to healthy controls. Objective To run a comparative meta-analysis of C-Reactive Protein (CRP), IL-1, IL-6, TNF-α in BD vs healthy controls, measuring mean and variability effects on all subjects. Sensitivity analyses include disease activity. Data sources Systematic review of observational studies in PubMed and PsycInfo up to February 2nd, 2020. Study selection Case-control studies reporting inflammatory mediators' levels in BD and controls. Data extraction and synthesis Summary distribution measures of circulating CRP, IL-1β, IL-6, TNF-α in participants with BD and control groups were extracted. Random-effects multivariate meta-analyses were conducted based on individual study/mediator effect sizes (Hedge’s g). Main outcomes and measures Co-primary outcomes were inflammatory mediators' levels (Hedge’s g) and variability (coefficient of variance ratio (CVR)) differences between participants with BD across the mood spectrum and controls. Results Out of the initial 729 papers, 72 were assessed and then excluded after full-text review, and ultimately 53 studies were included in the systematic review, while 49 were included in the meta-analysis. The mean age was 36.96 (SD: 9.29) years, and the mean female percentage was 56.31 (SD: 16.61). CRP (g = 0.70, 95% CI 0.31–1.09, k = 37, BD = 2,215 vs HC = 3,750), IL-6 (g = 0.81, 95% CI 0.46–1.16, k = 45, BD = 1,956 vs HC = 4,106), TNF-α (g = 0.49, 95% CI 0.19–0.78, k = 49, BD = 2,231 vs HC = 3,017) were elevated in subjects with BD vs HC, but not IL-1β (g = -0.28, 95% CI −0.68–0.12, k = 4, BD = 87 vs HC = 66). When considering euthymic, depressive, and manic episodes separately, CRP and TNF-α were elevated in both depressive and manic episodes, but not in euthymia, while IL-6 remained elevated regardless of the disease state. No difference in CVR emerged for CRP, IL-1β, and TNF-α, while a lower CVR was observed for IL-6. When considering disease phases, CVR was higher in BD than in HCs for CRP during depressive episodes, lower for IL-6 during euthymia, and higher during manic episodes for CRP, IL-6, and TNF-α. Sensitivity analyses after excluding outliers identified with funnel plot visual inspection, low-quality studies, and considering only studies matched per body mass index confirmed the main results. Meta-regression showed that age (IL-6, TNF-α), gender (CRP), duration of illness (CRP) moderated elevated individual inflammatory levels. Conclusions and relevance Peripheral pro-inflammatory marker elevations were confirmed in BD. CRP and TNF-α could represent state markers, as they were only elevated during mood episodes, while IL-6 appeared to be a trait marker for BD. Increased variability of specific inflammatory mediators in specific disease active states suggests that a subset of subjects with BD may exhibit elevated inflammation as part of a manic or depressive episode.