Background: Epstein–Barr Virus (EBV) and its clinical manifestation, infectious mononucleosis (IM), are strongly linked to MS risk. A recent in vitro study suggests that HLA‐E*01:03, in contrast to HLA‐E*01:01, may protect against MS through more effective immune responses against EBV‐infected B cells. However, the role of HLA‐E*01 in MS remains unclear.Methods: We investigated if HLA‐E*01:01 was significantly associated with higher MS risk in individuals with a history of IM diagnosis, using 487,144 individuals from the UK Biobank's cohort. We estimated the interaction between HLA‐E*01:01 and IM using Cox proportional hazard models, adjusting for demographics, smoking, childhood body size, older siblings, and MS‐related HLA alleles (e.g., HLA‐DRB1*15:01).Results: HLA‐E*01:01 allele alone was not significantly associated with IM or MS (p > 0.05). However, a significant interaction between HLA‐E*01:01 and IM history was observed in relation to MS risk (p < 0.001). Specifically, MS risk was significantly higher in both HLA‐E*01:01 heterozygotes (HR = 1.74 [95% CI: 1.36, 1.97], p < 0.001) and homozygotes (HR = 3.01 [95% CI: 1.81, 3.88], p < 0.001) with IM history, compared to HLA‐E*01:03 homozygotes. Conversely, these associations were non‐significant in individuals without IM history (p > 0.05). The estimated proportion of the combined risk attributable to interaction effects was 40% in HLA‐E*01:01 heterozygotes and 65% in HLA‐E*01:01 homozygotes.Conclusions: HLA‐E*01:01 carriers diagnosed with IM are at significantly increased risk of MS, independently from other MS‐related HLA alleles. This supports the hypothesis that HLA‐E*01:01 may contribute to MS susceptibility due to weaker immune control over EBV infection. Incorporating HLA‐E*01:01 into existing MHC‐based MS risk models could then enhance personalized risk assessments in individuals with IM history.

Background: Previous investigations into multiple sclerosis (MS) risk factors predominantly relied on retrospective studies, which do not consider different follow-up times and assume a constant risk effect throughout lifetime. Objective: We aimed to evaluate the impact of genetic and early life factors on MS diagnosis by employing a time-to-event analysis in a prospective cohort. Methods: We used the UK Biobank data, considering the observation period from birth up to 31 December 2022. We considered genetic risk, using a multiple sclerosis polygenic risk score (MS-PRS), and various early life factors. Tobacco smoking and infectious mononucleosis diagnosis were also considered as time-varying variables along the follow-up. Using a Cox proportional hazards model, we examined the associations between these factors and MS diagnosis instantaneous risk. Results: We analyzed 345,027 participants, of which 1669 had an MS diagnosis. Our analysis revealed age-dependent effects for sex (females vs males) and higher MS-PRS, with greater hazard ratios observed in young adults. Conclusion: The age-dependent effects suggest that retrospective studies could have underestimated sex and genetic variants’ risk roles during younger ages. Therefore, we emphasize the importance of a time-to-event approach using longitudinal data to better characterize age-dependent risk effects.

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.

Objectives: Multiple sclerosis (MS) patients often experience a higher incidence of clinical conditions before diagnosis, suggesting a prodromal phase. However, their predictive value and temporal trajectories remain underexplored. We investigated these aspects using the large UK Biobank's population‐based cohort, which provided clinical history through ICD‐10 diagnosis codes.Methods: We assessed associations between 600 clinical conditions and MS risk in 477,421 individuals using Cox models adjusted for demographics, smoking, and MS polygenic risk score (MS‐PRS). Statistically significant conditions were included in a LASSO Cox regression (five‐fold cross‐validation on 70% training set) to identify key predictors, with performance assessed by the C‐index and age‐dependent area under the curve (AUC) in the 30% test set. Lastly, temporal trajectories of MS‐associated conditions were analyzed in MS cases.Results: We identified 192 conditions associated with MS, of which only ~20% were onset symptoms. Integrating these conditions into a predictive model already including demographics and smoking, improved the C‐index from 0.65 to 0.71. Among the 30 model‐selected best predictors, ~25% were prodromal conditions, including neuromuscular diseases, thromboembolism, and depression which typically occurred more than five years before MS diagnosis. Including MS‐PRS further increased the C‐index to 0.78, with an age‐dependent AUC exceeding 0.80 in individuals over 50 years. Trajectory analysis highlighted migraine as a common early diagnosis, often followed by hypertension, depression, and dorsalgia.Interpretation: Our findings highlight early conditions and diagnostic trajectories of MS, supporting the existence of a prodromal phase. These insights could improve MS prediction and facilitate earlier detection, particularly for late‐onset cases.

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Background/Objectives: Multiple sclerosis (MS) is a chronic immune-mediated neurological disorder for which few modifiable risk factors are established. Vitamin C, due to its antioxidant and neuroprotective properties, has been hypothesized to reduce MS risk, but epidemiological evidence remains inconsistent. Methods: We conducted a prospective cohort study using UK Biobank data, including 486,908 adults aged 37–70 years free of neurological disease. Regular vitamin C supplementation was self-reported at recruitment. Incident MS cases were identified during a median follow-up of 13.5 years. Propensity score weighting was used to balance a wide range of demographic, lifestyle, and health-related confounders. Weighted Cox proportional hazards models were used to estimate hazard ratios (HRs). Effect modification by polygenic risk score (PRS) for MS was assessed, and multiple sensitivity analyses were performed. Results: During follow-up, 452 participants were diagnosed with MS. Vitamin C supplementation was reported by 8.8% of participants (missingness = 1.5%) and was associated with a lower risk of incident MS (HR = 0.51, [95%CI: 0.32; 0.82], p = 0.004). The estimate was consistent across multiple sensitivity analyses. The association varied according to genetic susceptibility, with significant nonlinear multiplicative interaction (p = 0.004) and additive interaction (p < 0.001). Specifically, significant associations were observed only among those with average or high MS-PRS. Conclusions: Vitamin C supplementation was associated with a lower risk of incident MS, with the association varying across levels of genetic susceptibility. Although residual confounding cannot be excluded, sensitivity analyses did not identify similar associations for overall supplement use or multivitamin use, providing some reassurance against a generalized healthy-user effect. Replication in independent cohorts is warranted.