Levels of Soluble Interleukin 6 Receptor and Asp358Ala Are Associated With Cognitive Performance and Alzheimer Disease Biomarkers

BACKGROUND AND OBJECTIVES: Alzheimer disease (AD) is a neurodegenerative disease process manifesting clinically with cognitive impairment and dementia. AD pathology is complex, and in addition to plaques and tangles, neuroinflammation is a consistent feature. Interleukin (IL) 6 is a multifaceted cytokine involved in a plethora of cellular mechanisms including both anti-inflammatory and inflammatory processes. IL6 can signal classically through the membrane-bound receptor or by IL6 trans-signaling forming a complex with the soluble IL6 receptor (sIL6R) and activating membrane-bound glycoprotein 130 on cells not expressing IL6R. IL6 trans-signaling has been demonstrated as the primary mechanism of IL6-mediated events in neurodegenerative processes. In this study, we performed a cross-sectional analysis to investigate whether inheritance of a genetic variation in the IL6R gene and associated elevated sIL6R levels in plasma and CSF were associated with cognitive performance. METHODS: We genotyped the IL6R rs2228145 nonsynonymous variant (Asp(358)Ala) and assayed IL6 and sIL6R concentrations in paired samples of plasma and CSF obtained from 120 participants with normal cognition, mild cognitive impairment, or probable AD enrolled in the Wake Forest Alzheimer's Disease Research Center's Clinical Core. IL6 rs2228145 genotype and measures of plasma IL6 and sIL6R were assessed for relationships with cognitive status and clinical data, including the Montreal Cognitive Assessment (MoCA), modified Preclinical Alzheimer's Cognitive Composite (mPACC), cognitive domain scores obtained from the Uniform Data Set, and CSF concentrations of phosphoTau(T181) (pTau181), β-amyloid (Aβ) Aβ40 and Aβ42 concentrations. RESULTS: We found that inheritance of the IL6R Ala(358) variant and elevated sIL6R levels in plasma and CSF were correlated with lower mPACC, MoCA and memory domain scores, increases in CSF pTau181, and decreases in the CSF Aβ42/40 ratio in both unadjusted and covariate-adjusted statistical models. DISCUSSION: These data suggest that IL6 trans-signaling and the inheritance of the IL6R Ala(358) variant are related to reduced cognition and greater levels of biomarkers for AD disease pathology. Follow-up prospective studies are necessary, as patients who inherit IL6R Ala(358) may be identified as ideally responsive to IL6 receptor–blocking therapies.