Role of the HLA system in the association between multiple sclerosis and infectious mononucleosis.

OBJECTIVE: To determine whether multiple sclerosis (MS) and infectious mononucleosis (IM) share common HLA associations. DESIGN: A prospective cohort study was conducted from October 1, 1999, through September 30, 2003. SETTING: University of Edinburgh Richard Verney Health Centre, Edinburgh, Scotland. PATIENTS: Participants included 179 individuals who underwent asymptomatic Epstein-Barr virus seroconversion and 175 patients who developed IM. INTERVENTION: Genotyping for 5 classical HLA loci (HLA-A, HLA-B, HLA-C, HLA-DRB1, and HLA-DQB1). MAIN OUTCOME MEASURE: Diagnosis of IM and allele frequency. RESULTS: Allelic analysis showed that HLA-DRB1*01:01 was significantly associated with the development of IM (odds ratio, 3.2; P < .001). Patients with IM and HLA-DRB1*01:01 had a lower Epstein-Barr virus viral load compared with those without the allele (median, 783 vs 7366 copies/10(6) peripheral blood mononuclear cells; P = .03). CONCLUSION: HLA-DRB1*01:01 is protective against developing MS; thus, a common genetic basis between IM and MS is not supported