Lung complications are a major cause of rheumatoid arthritis-related mortality. Involvement of gut microbiota in lung diseases by the gut-lung axis has been widely observed， but the underlying mechanism remains mostly unknown. Using an autoimmune arthritis model， we show that a constituent of the gut microbiota， segmented filamentous bacteria (SFB)， distantly provoke lung pathology. SFB induce autoantibodies in lung during the pre-arthritic phase，?and SFB-dependent lung pathology requires the T?helper 17 (Th17) responses. SFB-induced gut Th17 cells are preferentially recruited to lung over spleen due to robust expression in the lung of the?Th17 chemoattractant， CCL20. Additionally， we found that in peripheral tissues， SFB selectively expand dual T?cell receptor (TCR)-expressing Th17 cells recognizing both an SFB epitope and self-antigen， thus augmenting autoimmunity. This study reveals mechanisms for commensal-mediated gut-lung crosstalk and dual TCR-based autoimmunity.