Respiratory tract Moraxella catarrhalis and Klebsiella pneumoniae can promote pathogenicity of myelin-reactive Th17 cells

The respiratory tract is home to a diverse microbial community whose influence on local and systemic immune responses is only beginning to be appreciated. The airways have been linked with trafficking of myelin-specific T cells in the pre-clinical stages of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. Th17 cells are important pathogenic effectors in MS and EAE but are innocuous immediately following differentiation. Upregulation of the cytokine GM-CSF appears to be a critical step in their acquisition of pathogenic potential, but little is known about the mechanisms that mediate this process. Here, primed myelin-specific Th17 cells were transferred to congenic recipient mice prior to exposure to various human respiratory tract-associated bacteria and T cell trafficking, phenotype and the severity of resulting EAE monitored. Disease was exacerbated in mice exposed to the Proteobacteria Moraxella catarrhalis and Klebsiella pneumoniae, but not the Firmicute Veillonella parvula , and this was associated with a significant increase in GM-CSF+ and GM-CSF+IFNγ+ ex-Th17-like donor CD4 T cells in the lungs and CNS of these mice. These findings support the concept that respiratory bacteria may contribute to the pathophysiology of CNS autoimmunity by modulating pathogenicity in crucial T cell subsets that orchestrate neuroinflammation. ### Competing Interest Statement The authors have declared no competing interest.