BackgroundStudies in humans and experimental models highlight a role of interleukin-6 (IL-6) in cardiovascular disease. Indirect evidence suggests that inhibition of IL-6 signaling could lower risk of coronary artery disease. However, whether such an approach would be effective for ischemic stroke and other cardiovascular outcomes remains unknown. MethodsIn a genome-wide association study (GWAS) of 204,402 European individuals, we identified genetic proxies for downregulated IL-6 signaling as genetic variants in the IL-6 receptor (IL6R) locus that were associated with lower C-reactive protein (CRP) levels, a downstream effector of IL-6 signaling. We then applied two-sample Mendelian randomization (MR) to explore associations with ischemic stroke and its major subtypes (large artery stroke, cardioembolic stroke, small vessel stroke) in the MEGASTROKE dataset (34,217 cases and 404,630 controls), with coronary artery disease in the CARDIoGRAMplusC4D dataset (60,801 cases and 123,504 control), and with other cardiovascular outcomes in the UK Biobank (up to 321,406 individuals) and in phenotype-specific GWAS datasets. All effect estimates were scaled to the CRP-decreasing effects of tocilizumab, a monoclonal antibody targeting IL-6R. ResultsWe identified 7 genetic variants as proxies for downregulated IL-6 signaling, which showed effects on upstream regulators (IL-6 and soluble IL-6R levels) and downstream effectors (CRP and fibrinogen levels) of the pathway that were consistent with pharmacological blockade of IL-6R. In MR, proxies for downregulated IL-6 signaling were associated with lower risk of ischemic stroke (Odds Ratio [OR]: 0.89, 95%CI: 0.82-0.97) and coronary artery disease (OR: 0.84, 95%CI: 0.77-0.90). Focusing on ischemic stroke subtypes, we found significant associations with risk of large artery (OR: 0.76, 95%CI: 0.62-0.93) and small vessel stroke (OR: 0.71, 95%CI: 0.59-0.86), but not cardioembolic stroke (OR: 0.95, 95%CI: 0.74-1.22). Proxies for IL-6 signaling inhibition were further associated with a lower risk of myocardial infarction, aortic aneurysm, atrial fibrillation and carotid plaque. ConclusionsWe provide evidence for a causal effect of IL-6 signaling on ischemic stroke, particularly large artery and small vessel stroke, and a range of other cardiovascular outcomes. IL-6R blockade might represent a valid therapeutic target for lowering cardiovascular risk and should thus be investigated in clinical trials. CLINICAL PERSPECTIVEO_ST_ABSWhat is newC_ST_ABSO_LIWe identified genetic proxies for downregulated IL-6 signaling that had effects on upstream and downstream regulators of the IL-6 signaling pathway consistent with those of pharmacological IL-6R blockade C_LIO_LIGenetically downregulated IL-6 signaling was associated with a lower risk of ischemic stroke, and in particular large artery and small vessel stroke C_LIO_LISimilar associations were obtained for a broad range of other cardiovascular outcomes C_LI What are the clinical implicationsO_LIInhibition of IL-6 signaling is a promising therapeutic target for lowering risk of stroke and other cardiovascular outcomes and should be further investigated in clinical trials C_LI

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