Multiple sclerosis (MS) is an autoimmune disease targeting the central nervous system (CNS) mainly in young adults, and a
breakage of immune tolerance to CNS self-antigens has been suggested to initiate CNS autoimmunity. Age and microbial infection
are well-known factors involved in the development of autoimmune diseases, including MS. Recent studies have suggested that
alterations in the gut microbiota, referred to as dysbiosis, are associated with MS. However, it is still largely unknown
how gut dysbiosis affects the onset and progression of CNS autoimmunity. In this study, we investigated the effects of age
and gut dysbiosis on the development of CNS autoimmunity in humanized transgenic mice expressing the MS-associated MHC class
II (MHC-II) gene, HLA-DR2a, and T-cell receptor (TCR) genes specific for MBP87-99/DR2a that were derived from an MS patient.
We show here that the induction of gut dysbiosis triggers the development of spontaneous experimental autoimmune encephalomyelitis
(EAE) during adolescence and early young adulthood, while an increase in immunological tolerance with aging suppresses disease
onset after late young adulthood in mice. Furthermore, gut dysbiosis induces the expression of complement C3 and production
of the anaphylatoxin C3a, and down-regulates the expression of the Foxp3 gene and anergy-related E3 ubiquitin ligase genes. Consequently, gut dysbiosis was able to trigger the development of encephalitogenic
T cells and promote the induction of EAE during the age window of young adulthood.