A bidirectional two-sample Mendelian randomization study of immune responses against Epstein-Barr virus nuclear antigen 1 and multiple sclerosis in individuals of European ancestry

Background: Observational studies suggest an association between immune responses to Epstein–Barr virus nuclear antigen 1 (EBNA1) and multiple sclerosis (MS) risk. Nevertheless, a causal effect cannot be established, as confounding and reverse causation cannot be excluded. Methods: We performed a bidirectional two-sample Mendelian randomization (MR) to test whether higher anti-EBNA1 IgG levels causally influence MS risk, or vice versa. Genetic associations for anti-EBNA1 IgG were obtained from 7972 UK Biobank participants, and for MS from 115,803 individuals in the International MS Genetics Consortium. Given that most anti-EBNA1 IgG instruments lie in the pleiotropic major histocompatibility complex, we applied a robust cis-MR framework, sensitivity analyses, and external validation using genetic association from anti-EBNA1 IgG levels obtained from 914 French individuals. Results: A one standard deviation increase in genetically predicted anti-EBNA1 IgG levels was causally associated with a 69% higher MS risk (OR = 1.69 [95% CI: 1.28; 2.23], p < 0.001). Results were consistent across sensitivity analyses and showed no directional pleiotropy. External validation supported these findings. Reverse MR provided no evidence that MS influences anti-EBNA1 IgG levels (p > 0.05). Conclusion: These results support a unidirectional causal effect of elevated anti-EBNA1 IgG levels on MS risk, reinforcing the role of EBV-related immune responses in MS pathogenesis.