Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system of unknown etiology. We tested the hypothesis that MS is caused by Epstein-Barr virus (EBV) in a cohort comprising more than 10 million young adults on active duty in the US military, 955 of whom were diagnosed with MS during their period of service. Risk of MS increased 32-fold after infection with EBV but was not increased after infection with other viruses, including the similarly transmitted cytomegalovirus. Serum levels of neurofilament light chain, a biomarker of neuroaxonal degeneration, increased only after EBV seroconversion. These findings cannot be explained by any known risk factor for MS and suggest EBV as the leading cause of MS.

IMPORTANCE Unrecognized demyelinating events often precede the clinical onset of multiple sclerosis (MS). Identification of these events at the time of occurrence would have implications for early diagnosis and the search of causal factors for the disease.OBJECTIVE To assess whether serum neurofilament light chain (sNfL) levels are elevated before the clinical MS onset. DESIGN, SETTING, AND PARTICIPANTSNested case-control study among US military personnel who have serum samples stored in the US Department of Defense Serum Repository. Serum samples were collected from 2000 to 2011; sNfL assays and data analyses were performed from 2018 to 2019. We selected 60 case patients with MS who either had 2 samples collected before onset (mean follow-up, 6.3 years) or 1 sample collected before and 1 after onset (mean follow-up, 1.3 years), among 245 previously identified case patients. For each case, we randomly selected 1 of 2 previously identified control individuals matched by age, sex, race/ethnicity, and dates of sample collection. The sample size was chosen based on the available funding.EXPOSURES Serum NfL concentrations measured using an ultrasensitive single-molecule array assay (Simoa). MAIN OUTCOMES AND MEASUREMENTSLog-transformed sNfL concentrations in case patients and control individuals compared using conditional logistic regression and linear mixed models.RESULTS Mean age at baseline was 27.5 years, and 92 of 120 participants (76.7%) were men. Serum NfL levels were higher in case patients with MS compared with their matched control individuals in samples drawn a median of 6 years (range, 4-10 years) before the clinical onset (median, 16.7 pg/mL; interquartile range [IQR], 12.6-23.1 pg/mL vs 15.2 pg/m; IQR, 10.3-19.9 pg/mL; P = .04). This difference increased with decreasing time to the case clinical onset (estimated coefficient for interaction with time = 0.063; P = .008). A within-person increase in presymptomatic sNfL levels was associated with higher MS risk (rate ratio for Ն5 pg/mL increase, 7.50; 95% CI, 1.72-32.80). The clinical onset was associated with a marked increase in sNfL levels (median, 25.0; IQR, 17.1-41.3 vs 45.1; IQR, 27.0-102.7 pg/mL for presymptomatic and postonset MS samples; P = .009). CONCLUSIONS AND RELEVANCEThe levels of sNfL were increased 6 years before the clinical MS onset, indicating that MS may have a prodromal phase lasting several years and that neuroaxonal damage occurs already during this phase.

RRMS may start years prior to clinical presentation, and disease processes in PPMS could start decades prior to first apparent progressive symptoms. Cognitive problems could be present in both MS forms before apparent symptoms. Apart from potential implications for clinical practice and research, these findings challenge our thinking about the disease. Ann Neurol 2016;80:616-624.

Ritonavir shows a dose and concentration effect relationship of CYP3A inhibition. In addition, a proposed auto-inhibition of ritonavir metabolism resulted in a non-linear exposure of ritonavir with sub-proportional concentrations at low doses. A time-dependent CYP3A activity may result when inhibitors of CYP3A with short elimination half-lives are used.