Encephalomyelitis-associated antimyelin autoreactivity induced by streptococcal exotoxins

Objective: After implicating Streptococcus pyogenes as causing acute disseminated encephalomyelitis (ADEM) in a child, we wanted to prove that in vivo activation of autoreactive T lymphocytes by superantigens of this Streptococcus contributed to the dramatic demyelination. Background: ADEM is a demyelinating disorder of the CNS sharing many similarities with MS. Demyelination in MS is considered to be the result of an autoimmune process mediated by autoreactive T lymphocytes with specificity for myelin antigens. Methods: Phenotypic analysis and proliferation assays on blood monocytes, as well as isolation of myelin basic protein (MBP)-reactive T-cell lines/clones; and TCR repertorium analysis by PCR-ELISA and cytokine production. Results: 1) The blood T-cell receptor (TCR) repertoire was compatible with in vivo expansion induced by S. pyogenes exotoxins. 2) TCR expression analysis indicated clonal expansion of CD8⁺ MBP-reactive T cells, suggesting in vivo activation. MBP-reactive T cells showed crossreactivity to S. pyogenes supernatant and exotoxins. 3) Cytokine mRNA quantification of the mononuclear cells revealed a Th2-biased profile. Conclusion: In vivo exposure to S. pyogenes may have induced activation of pathogenic myelin reactive T cells, contributing to the dramatic inflammatory demyelination.