To investigate the causal relationship between immune cell phenotype, blood metabolites, and myocarditis in a Mendelian randomization study.

Myocarditis is a self-limiting inflammatory disease with a high prevalence of severe arrhythmogenic cardiomyopathy and sudden death. Although studies have emphasized the strong association between myocarditis and immune cells and blood metabolites, the establishment of a causal relationship between these associations has yet to be clarified. Data on 731 immune cells (N = 3757), 1400 blood metabolites (N = 8299), and myocarditis were derived from the genome-wide association studies catalog database. Two-way two-sample Mendelian randomization studies first assessed genetic associations between exposures and outcomes. Mendelian randomization (MR) analyses were performed primarily using inverse variance weighting supplemented with MR-Egger, weighted median, weighted mode and simple models, using Cochran Q-test to detect heterogeneity and MR-Egger intercept to assess horizontal pleiotropy, in addition to leave-one-out for sensitivity analyses to ensure robust results. Finally, we used a two-step mediation analysis to identify pathways from immune cells to myocarditis mediated by blood metabolites. MR analysis identified genetic relationships between 27 immune cells and myocarditis, with no strong evidence that myocarditis can affect 25 circulating immune cells. In addition, 50 blood metabolites were screened for causal associations with myocarditis, and finally 19 mediating associations were obtained by two-step MR, of which 5 immune cells mediated strong causal associations between 6 blood metabolites and myocarditis. Notably, levels of 4-ethylphenyl sulfate and methyl-4-hydroxybenzoate sulfate mediated the association between circulating CCR2 on monocytes and FSC-A on NKT cells with myocarditis, with the effect ratio of 4-ethylphenyl sulfate at 19.4% (P = .003) and that of methyl-4-hydroxybenzoate sulfate at 13.9% (P = .007). Our results clarify the causal relationship between 13 circulating immune cells and 15 blood metabolites and myocarditis. This provides new biological markers for early prevention, diagnosis and treatment of myocarditis. It also facilitates real-time monitoring of the metabolic status of immune responses in myocarditis patients, which contributes to individual precision therapy.