Integrated determinants of multiple sclerosis susceptibility: Genetics, environment, infection and the microbiota.

Multiple sclerosis (MS) arises from the convergence of polygenic immune susceptibility, environmental exposures, and infectious determinants, none of which alone is sufficient for disease expression. More than 200 genetic variants contribute to MS risk, with the HLA-DRB115:01 haplotype providing the strongest effect within a broader network of immune-regulatory loci. Functional genomic evidence shows that these variants primarily influence antigen presentation and lymphocyte activation, creating an immune landscape that can be further shaped by external factors. This review integrates epidemiological, genomic, and mechanistic data to outline the main determinants of MS susceptibility. Epstein-Barr virus infection emerges as the dominant infectious driver, with seroconversion preceding early neuroaxonal injury and clinical onset. In contrast, cytomegalovirus infection appears protective, likely through immune imprinting that counterbalances EBV-driven B-cell and T-cell activation. Environmental factors including cigarette smoking, adolescent obesity, vitamin D deficiency, circadian disruption, and gut microbiota dysbiosis further modify susceptibility by promoting proinflammatory immune programs and reducing regulatory stability. Many of these exposures interact synergistically with HLA-DRB115:01, amplifying risk beyond additive expectations. These determinants influence shared immunological pathways regulating antigen presentation, lymphocyte differentiation, and immune tolerance. Clarifying these biological interfaces highlights actionable domains including smoking avoidance, metabolic health optimization, vitamin D sufficiency, viral prevention strategies, circadian alignment, and microbiome-targeted interventions that may inform risk-reduction and early identification efforts.